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Tailoring residue-derived carbon
materials for the removal of
wastewater contaminants
Adsorption and surface properties
Mirva Niinipuu
Department of Chemistry
Umeå 2019
This work is protected by the Swedish Copyright Legislation (Act 1960:729) Dissertation for PhD ISBN: 978-91-7855-071-5 Cover: Mirva Niinipuu Electronic version available at: http://umu.diva-portal.org/ Printed by: Lars Åberg, KBC Service center, Umeå University Umeå, Sweden 2019
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Table of Contents
Abstract ..................................................................................................... iii
Sammanfattning på svenska ........................................................................ v
Abbreviations ............................................................................................vii
Definitions .............................................................................................. viii
List of publications .....................................................................................ix
Contributions .............................................................................................. x
Introduction ................................................................................................ 1 Water treatment ............................................................................................................... 1 Resource efficiency and valorization of residues ........................................................... 2 Aim of this thesis ..............................................................................................................3
Hydrochars and hydrochar-based activated carbons .................................... 7 HTC as a carbonization method ....................................................................................... 7 Activated carbons ........................................................................................................... 11 Synthesis of hydrochars and chemically activated carbons .......................................... 12
Selection of the precursor materials ....................................................................... 12 Synthesis of hydrothermal carbons and activated carbons .................................. 13 Chemical activation of hydrochars ......................................................................... 14
Characterization of carbon materials ......................................................... 17 Characterization techniques ........................................................................................... 17
Fourier transformation infrared spectroscopy ...................................................... 18 X-ray photoelectron spectroscopy (XPS) ............................................................... 19 Specific surface area ................................................................................................ 19 Scanning Electron Microscopy (SEM) ................................................................... 20
Surface properties of the studied hydrochars and activated carbons........................... 21 Chemical surface analysis ....................................................................................... 21 Porosity .................................................................................................................... 26 Surface morphology ................................................................................................. 27
Wastewater purification by adsorption ......................................................29 Adsorption ..................................................................................................................... 29 Adsorption testing .......................................................................................................... 31
Batch adsorption ...................................................................................................... 31 Selection of adsorbates .................................................................................................. 33 Quantitative analysis of water ........................................................................................35
UV-vis spectrophotometry .......................................................................................35 Liquid chromatography – Mass spectrometry (LC-MS) .......................................35 Inductively coupled plasma-mass spectrometry (ICP-MS) ................................. 36
Adsorption results .......................................................................................................... 37 Methylene blue removal with hydrochars .............................................................. 37 Organic compound and metal removal with hydrochars .................................... 38 Organic compound and metal removal with activated carbons ......................... 43 What is a realistic approach for studying adsorption? ........................................ 44
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Adsorbent and adsorbate properties and adsorption ................................. 47 How are adsorbent and adsorbate roles studied? ......................................................... 47 Hydrochar and adsorbate interactions ......................................................................... 48 Activated carbons and adsorbate interactions ............................................................. 50 What drives adsorption/removal? ................................................................................. 51
Conclusions and future work ..................................................................... 53
Acknowledgements .................................................................................... 57
References ................................................................................................ 59
Appendix
iii
Abstract
The availability of effective, low-cost wastewater treatment is necessary for
increased water recycling and the prevention of environmental pollution on a
global scale. Adsorption on activated carbons is commonly applied in wastewater
treatment, but the high cost of conventional activated carbons limits the use of
this technique. Several waste streams, such as the residues and by-products of
food processing, agriculture and industrial processes, are currently inefficiently
utilized and could be transformed into value-added carbon materials. Re-
thinking how waste is utilized could reduce waste handling costs and increase
resource efficiency, which would provide both economic and environmental
benefits. Therefore, low-cost carbon materials prepared from renewable low-cost
resources are an attractive alternative to decreasing the costs of wastewater
treatment.
The research underlying this thesis investigated the potential of carbonized
residue materials to remove environmentally relevant concentrations of organic
and inorganic contaminants from wastewater. The research covered in this thesis
included the carbonization of tomato- and olive press wastes, rise husks, horse
manure, municipal wastewater sludge and bio- and fiber sludges from pulp and
paper mills. The effect of carbonization temperature and starting material was
studied in terms of surface properties and contaminant removal to gain
knowledge on which surface features are beneficial for the removal of different
contaminants. The extent to which different chemical activations of carbonized
materials improve the contaminant removal was also studied.
The results demonstrate that carbonized materials are generally quite ineffective
at removing organic compounds from water, which may be due to the low surface
areas of these materials. Carbonization temperature was shown to alter the
surface functionalities of the carbons, more specifically, high carbonization
temperatures decreased oxygen-containing surface functionalities that benefitted
the removal of most contaminants (which was most pronounced for Zn and
trimethoprim). Further experiments investigated the role of the water matrix,
and the results unexpectedly showed higher removal from a complex water
matrix. Chemical activation improved removal efficiencies for all of the studied
compounds, with the most pronounced effects observed for organic compounds.
The activated carbons were able to completely remove fluconazole and
trimethoprim from the landfill leachate water, and also showed high removal
efficiensies (50-96%) of Cu and Zn. Furthermore, the results showed that
adsorbate compounds may interact with the adsorbent surface in diverse ways,
for example, via properties such as porosity and the presence of oxygen-
containing functionalities or minerals. Also, adsorbate hydrophobicity (log Kow)
iv
affected the removal of organic compounds in some of the studied hydrochars.
The research discussed in this thesis has highlighted that future studies should
study the broad range of environmentally-relevant adsorbates through multi-
component adsorption systems that include several complex water matrices.
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Sammanfattning på svenska
För att hushålla med våra vattenresurser och förhindra spridning av
miljöföroreningar krävs att vi har tillgång till effektiv vattenrening. Adsorption
med aktivt kol är en vanlig vattenreningsteknik, men den höga kostnaden hos
konventionella aktiva kol begränsar användningen av denna teknik. Samtidigt
genereras ett stort antal avfallsflöden som idag inte används optimalt, exempelvis
från livsmedelsindustrin, jordbruket och industriella processer. Produktion av
kol från dessa restmaterial skulle förutom att generera adsorbenter för
vattenrening även bidra till att minska kostnaderna för hantering och
kvittblivning, och öka resurseffektiviteten, vilket skulle vara ekonomiskt och
miljömässigt fördelaktigt.
Syftet med detta projekt var att undersöka förmågan hos förkolade restmaterial
att avskilja miljöfarliga organiska och oorganiska föroreningar från industriella
vatten. De material som studerades var kol från tomat- och olivpressavfall,
risskal, hästgödsel, kommunalt avloppsreningsslam samt bio- och fiberslam från
massa- och pappersindustri. Effekten av förkolningstemperatur och typ av
utgångsmaterial utvärderades med avseende på ytegenskaper hos de genererade
kolen och deras kapacitet för att avskilja vattenföroreningar, för att därmed
klargöra vilka ytegenskaper som är av betydelse för adsorptionen. Dessutom
undersöktes olika kemiska aktiveringar av kolmaterialen i syfte att förbättra
deras funktion som vattenreningsadsorbenter.
Resultaten visade att kolmaterialens kapacitet att avskilja organiska
vattenföroreningar överlag var låg, vilket kan bero på den begränsade ytarean hos
dessa material. Förkolningstemperaturen påverkade ytfunktionaliteterna hos
kolmaterialen på så vis att hög temperatur minskade förekomsten av
syreinnehållande funktionella grupper på ytan, och dessa bidrar till avskiljningen
av de flesta av de studerade föroreningarna. Den största effekten observerades
för Zn och trimetoprim. Dessutom utvärderades vattenmatrisens roll, och
resultaten visade oväntat på en effektivare avskiljning från en komplex
vattenmatris. Kemisk aktivering förbättrade avskiljningen av samtliga studerade
föroreningar och den mest uttalade effekten observerades för organiska
föroreningar. De aktiverade kolen klarade av att fullständigt avskilja flukonazol
och trimetoprim från deponilakvatten, och även för Cu och Zn var avskiljningen
hög (50-96%). Resultaten visade också att olika föroreningar kan interagera på
olika sätt med adsorbentytorna, vilket styrs av egenskaper som porositet,
syreinnehållande funktionaliteter, och mineraler. Dessutom påverkade
föroreningarnas hydrofobicitet (log Kow) avskiljningen av organiska föreningar
för några av de studerade kolmaterialen. Forskningen som diskuteras i denna
avhandling har uppmärksammat att framtida studier bör studera ett brett
vi
spektrum av föroreningar med miljörelevans i adsorptionssystem där ett flertal
komponenter tas i beaktande och som omfattar komplexa vattenmatriser.
vii
Abbreviations
APCI Atmospheric pressure chemical ionization
APPI Atmospheric pressure photoionization
ATR Attenuated total reflection
BET Brunauer-Emmet-Teller
BJH Barrett, Joyner and Halenda
DOC Dissolved organic carbon
DRIFTS Diffuse reflectance infrared Fourier transform
spectroscopy
EDS Energy dispersive X-ray spectroscopy
ESI Electrospray ionization
FTIR Fourier transformation infrared
GAC Granular activated carbon
HTC Hydrothermal carbonization
ICP-MS Inductively coupled plasma –mass spectrometry
LC-MS Liquid-chromatography –mass spectrometry
PCA Principal component analysis
PFOA Perfluorooctanoic acid
SEM Scanning electron microscope
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Definitions
Absorption In the context of this thesis, absorption refers to the
physical phenomenon in which the energy of
electromagnetic radiation or particles is converted
into another form of energy during passage
through a medium [1] (used in context of
spectroscopy).
Adsorbate “A substance that is adsorbed on a surface.” [1]
Adsorbent “A substance on the surface to which a substance is
adsorbed.” [1]
Adsorption “The formation of a layer of gas, liquid, or solid on
the surface of a solid or, less frequently, a liquid.”
[1]
Macropore Pore that has a diameter wider than 50 nm [2]
Micropore Pore that has a diameter smaller than 2 nm [2]
Mesopore Pore with a diameter between 2 – 50 nm [2]
ix
List of publications
I. E. Weidemann, M. Niinipuu, J. Fick, S. Jansson, Using carbonized
low-cost materials for removal of chemicals of environmental
concern from water, Environmental Science and Pollution Research
25 (2018) 15793–15801.
II. M. Niinipuu, K.G. Latham, J.F. Boily, M. Bergknut, S. Jansson,
Tailoring the functionality of waste materials using hydrothermal
carbonization for water treatment applications. Manuscript
(submitted to Waste and Biomass Valorization).
III. M. Niinipuu, M. Bergknut, J.F. Boily, E. Rosenbaum, S. Jansson, The
water matrix influences the removal of organic and inorganic
contaminants from landfill leachate by sludge and manure
hydrochars. Manuscript (submitted to ACS Omega).
IV. M. Niinipuu, K.G. Latham, S. Jansson, From Waste to Water
Treatment: Physicochemical and Wastewater Adsorption Properties
of Activated Hydrothermally Carbonized Waste Materials.
Manuscript.
x
Contributions
I. The author was responsible for preparing the hydrochars. The author
actively took part in the interpretation of surface characterization
data and adsorption data as well as significantly contributed to the
writing of the manuscript.
II. The author participated in the planning of the study. The author
prepared the hydrochars, collected BET data and FTIR spectra and
carried out adsorption tests, including the water analysis. The author
contributed to the data interpretation and manuscript preparation.
III. The author was responsible for planning the study. The author
prepared the hydrochars, carried out the adsorption tests, analysis of
organic contaminants in water and data and data analysis. The
author was responsible for writing the manuscript.
IV. The author took the main responsibility of the planning the study.
The author prepared all hydrochars and activated carbons. The
author collected FTIR data and was responsible for adsorption tests
and the analysis of organic and inorganic compounds in water. The
author contributed to manuscript preparation.
1
Introduction
The limited resources available on Earth should be managed efficiently to both
sustain humankind and diverse ecosystems. Today, the resources that are
renewed in one year are consumed in 213 days (global average) and if the world
population spent resources at the same rate as inhabitants in Sweden do, the
resources would last only 94 days [3]. The availability of wastewater treatment
should be improved on a global level to prevent environmental pollution and
increase the recycling of water. On the other hand, by-products and residues from
various industrial processes should be seen as resources rather than waste in
order to reduce the environmental and economic burden of consumption.
Recycling and reusing residue materials would decrease waste handling costs,
while the residue materials could be used to produce value-added products. This
chapter provides a brief background on wastewater treatment and circular
economy, and concludes with the main objectives of the research underlying this
thesis.
Water treatment
Improper wastewater treatment is a global challenge that must be tackled to
reduce contaminant discharge from wastewater to water bodies, which, in turn,
would decrease environmental pollution and increase water recycling in sectors
such as agriculture. On a global scale, water scarcity is an enormous threat, with
estimates warning that by 2050 at least one-fourth of people will live in a country
suffering from a shortage of fresh water [4]. Some municipal wastewater is still
not treated today, and untreated wastewater is discharged directly to water bodies
in countries in Africa, South-America, Asia and Europe (such as Egypt, Mexico
and Croatia) [5,6]. United Nations sustainable development goals include clean
water and sanitation objectives that strive to halve the proportion of untreated
wastewater by 2030 [4].
Sweden has rich fresh water resources and wastewater is treated before
discharging. More than 95 % of urban wastewater goes through biological and
chemical treatment, while households not connected to the municipal sewage
system have small-scale treatment facilities [7]. In addition to municipal
wastewater, the wastewater generated by large industrial facilities is purified
separately from municipal wastewater and then either discharged or further
purified in municipal wastewater plants [8]. These industrial actors include pulp-
and paper mills, mines, waste treatment facilities and landfills, as well as the food
and chemical industries [7,8].
2
The widespread use of various chemicals - such as personal care products,
antibiotics and other pharmaceuticals, pesticides, plastic additives, surfactants,
flame retardants and other industrial chemicals found in consumer products -
present a challenge to municipal wastewater treatment plants that have been
traditionally developed to remove organic materials and suspended solids [9,10].
The removal of these chemicals depends on their chemical properties; therefore,
hydrophobic and biodegradable substances are removed or degraded during the
sewage treatment process while more hydrophilic and persistent compounds pass
through the sewage treatment [9]. As some of these persistent and hydrophilic
compounds (such as pharmaceuticals and endocrine disrupting compounds) may
cause responses in biological organisms, their presence in wastewater could even
cause ecosystem-level effects [11,12]. Moreover, antibiotic-resistant bacteria may
develop if the concentrations of antibiotic compounds in the environment are
increasing [13]. Additionally, contaminants that are not removed or degraded
during the wastewater treatment process may be taken up by plants or animals,
and thereby, enter the food chain [14,15].
When considering the importance of wastewater treatment for the environment,
human welfare and industries, more affordable wastewater treatment techniques
should be available on a global level. This includes both introducing water
treatment systems to locations where they are not in use, as well as implementing
additional treatment steps to improve the removal of contaminants in existing
wastewater treatment facilities (such as municipal wastewater treatment plants).
Possible additional treatment techniques – such as membrane filtration,
oxidation, or adsorption on activated carbon (or combination of these
techniques) – can act as a further purification step and should complement the
biological and chemical treatment steps in the current wastewater system to
improve the removal of various contaminants. [9,16].
Resource efficiency and valorization of residues
The conventional linear resource-to-waste economy is based on the consumption
of products made of primary resources and subsequent disposal after use [17]. A
circular economy, on the other hand, aims to include products that are designed
to be easily repaired/reused/recycled and to be in use for as long as possible. After
their use, the resources from these products are recovered and used to produce
new products. Since the wasteful use of resources cannot continue indefinitely,
increasing resource efficiency in the form of circular economy is on many political
agendas [18]. European waste directive 2008/98/EC introduced a waste
hierarchy for ranking different waste handling strategies. Waste prevention is the
most preferential strategy, followed by the reuse, recycling and recovery of
materials and/or energy. Landfilling is the least preferred action in the hierarchy,
and is to be avoided if handling strategies higher in hierarchy are possible [19].
3
The European Union (EU) later published an action plan to promote the
development of a circular economy [20]. The action plan included the stimulation
of investments and removal of obstacles created by legislation or endorsement.
The European waste directive was also recently amended to improve waste
management and emphasize the circular use of resources [21]. In this regulation,
member states are encouraged to actively create end-of-waste criteria, guidance
documents, and case-by-case decisions, among others, that will ensure that
refined waste materials (after fulfilling the end-of-waste criteria) can cease to be
considered waste, and thus, not be regulated as waste. A precondition for using
waste materials as starting materials for new products is that such a product will
not be classified and regulated as waste. [22].
This significant paradigm shift has pushed industries to consider by-products and
residues as resources rather than disposable waste and, as a result, new fields of
research and green technology have formed to develop industrial processes
towards a circular economy. These fields primarily focus on end-of-life handling
(i.e. using less toxic chemicals, including more non-toxic chemicals, designing
products that are easy to dismantle) as well as optimizing production processes
to provide high yields with lower amounts of toxic chemicals. Other market
players have concentrated on decreasing chemical amounts by shifting their
business model from the sale of materials to service and leasing approaches [17].
In the Swedish context, many industrial residues, such as sludges from sewage
treatment plants and pulp and paper mills, are difficult to manage and, as such,
are not currently efficiently valorized [8,22]. Many waste streams contain
valuable components, such as nutrients, metals and carbon, that could be refined
to produce new products or energy [23–25]. For instance, nutrient recycling from
sludge would be one solution to nutrient depletion [24,26,27], while the recycling
of carbon resources would limit the demand for virgin carbon resources, which is
particularly beneficial if fossil carbon sources can be avoided. Therefore, both
municipalities and industrial players should consider different management
strategies for their waste, as well as evaluate the feasibility of various valorization
possibilities for these materials.
Aim of this thesis
The research underlying this thesis aimed to study the properties of carbonized
residue materials as well as their ability to remove organic and inorganic
contaminants from wastewater. Hydrothermal carbonization (HTC) was used to
convert several wet residue materials (food processing residues, manure and
sludge materials) to hydrochars (Papers I – III), which were subsequently
chemically activated to activated carbons (Paper IV). The research covered in
this thesis also explored how HTC treatment temperature influences the surface
4
features and adsorbent properties of the generated carbon materials, as well as
whether chemical activation enhances removal performance. Finally, the
research presented in this thesis aimed to identify which surface properties are
associated with contaminant removal so that future studies can work to optimize
these specific properties.
The specific objectives of the research papers appended to this thesis were:
- To study the removal efficiency of environmentally relevant organic
compounds by hydrochars and describe how adsorbent surface
properties and adsorbate properties contribute to removal. (Paper I)
- To examine the role of hydrothermal carbonization temperature and
choice of feedstock in terms of surface properties and methylene blue
adsorption capacity. (Paper II)
- To investigate the efficiency with which hydrochars remove organic
compounds and metals from simulated and real wastewater, as well as
describe how the water matrix influences this process. The relationship
between hydrochar properties and removal efficiency was also studied.
(Paper III)
- To identify a link between the chemical activation of hydrochars and
removal of organic and inorganic compounds from wastewater. The
effect of surface properties on removal efficiency was also investigated.
(Paper IV)
5
Figure 1. Outline of the research underlying this thesis.
6
7
Hydrochars and hydrochar-based
activated carbons
Hydrothermal carbonization (HTC) has attracted increasing research attention
during recent decades because of its potential to convert wet materials into
various value-added products. Various activation approaches can be used to
enhance the properties of these carbonaceous materials. This chapter reviews
what has been published about hydrothermal carbonization and the associated
activation processes, as well as describes how the materials used in the research
underlying this thesis were synthesized.
HTC as a carbonization method
HTC is a wet carbonization technique that converts carbon-containing materials
into carbon-rich materials, referred to as hydrochars. The terms hydrothermal
carbons, biochar or biocoal are also occasionally used. Biochar or biocoal may be
used to describe materials produced through both dry and wet carbonization, but
in the scientific literature, biochar (or rarely biocoal) is more often associated
with carbons obtained from dry carbonization methods, such as torrefaction and
pyrolysis. According to another definition, biochar is a carbonized material that
is primarily used as a soil amendment (this distinguishes it from charcoal, which
is primarily used as a fuel) to improve soil properties and sequester carbon [28].
Pyrolysis products are also rarely referred to as pyrochars. Within this thesis, the
term hydrochar refers to the solid product generated in the HTC process,
whereas biochar describes solid products that were produced through
torrefaction and pyrolysis.
The foundation for research on hydrothermal treatment was made by Nobel
laureate Friedrich Bergius who studied the production of H2 by oxidation of
coal/carbonaceous materials at elevated temperatures and pressures in water
[29]. He later investigated how the hydrothermal processing of biomass could
yield solid products, and hydrothermal carbonization was first described in 1913
[30]. This technique was less investigated during the following decades because
the discovery of extensive fossil carbon resources made fuel production from
biomass less attractive [29]. However, hydrothermal carbonization research was
revived at the beginning of this century, and has been growing dramatically ever
since [29,31]. Hydrothermal carbonization occurs in water that is contained in a
pressure-proof vessel and typically heated to 180-250 °C; as such, autogenous
pressure forms inside the vessel [32]. Higher process temperatures and pressures
yield lower amounts of solid products as a higher fraction of liquid and gas
products form as a result of hydrothermal liquefaction (~280-370 ºC) and
hydrothermal gasification (> 350 ºC), respectively (Figure 2) [33,34].
8
Figure 2. Water phase diagram of the pressures and temperatures at which different hydrothermal
conversion processes occur. The water phase is presented in grey, while the distinct processes and
products (in parentheses) are shown in black. Adapted from [35].
HTC is a mild technique compared to pyrolysis, which is typically done at higher
temperatures in an inert atmosphere. Pyrolysis occurs typically between 300-650
°C and torrefaction, a mild type of pyrolysis, between 200-300 °C [36,37]. The
solid product yields of HTC (<66%) and torrefaction (61 – 84%) are higher than
what has been observed for pyrolysis (~30%), while the gas yields of these
methods are significantly lower (< 5%) than those of both dry carbonization
methods (20-40%) [37,38]. Furthermore, HTC provides high carbon yields
(~88%) when compared to slow pyrolysis (~58%) and torrefaction (67-85%) [38].
Biochars contain more aromatic carbon than hydrochars, which are rich in
oxygen- and hydrogen-containing surface functionalities that increase cation
exchange capacity [39]. Some of the inorganic components (especially potassium
and sodium) are extracted from the solid materials to the process water during
HTC, decreasing the ash content of the solid material and enhancing the fuel
properties of the carbonaceous product by decreasing the problems related to
slagging, fouling and corrosion in subsequent thermochemical conversion
processes [40,41]. It is important to state that dry carbonization requires the
materials to be dry prior to carbonization; hence, this process consumes
additional energy when water is evaporated from the feedstock materials. In this
way, a clear advantage of HTC is that the materials do not need to be dry prior to
carbonization. Furthermore, HTC makes water removal after carbonization
easier due to increased hydrophobicity of the produced material and the rupture
of cells walls during the HTC process [42,43]. This is reflected in a significantly
lower electrical power consumption and use of thermal energy (20-50% and
~70%, respectively), relative to dewatering the material by mechanical treatment
in combination with evaporation [43,44]. The potential applications of
hydrochars and hydrochar-based materials include fuel, capacitor materials, soil
9
amendment/carbon sequestration, medical imaging and adsorption of gaseous
and aqueous contaminants [45–53].
A wide scale of residue materials, such as manure from agriculture [54–56], pulp
and paper mill sludges [51,57] sewage sludge [58], food processing and food waste
[33,59,60], municipal waste [61], plant and wood biomass [62–64], and polyvinyl
chloride [65], can be subjected to HTC. Furthermore, pure carbohydrate
precursors, such as sucrose, glucose and cellulose, have been widely studied to
elucidate the reaction mechanisms of hydrothermal carbonization and for their
potential in producing carbon materials with higher purity, e.g. spherical carbons
[66–68].
Temperature is an important parameter for HTC (as well as other carbonization
techniques). In general, solid yield and the O/C- and H/C-ratios decrease as HTC
temperature increases [69]. Different feedstock material components also
respond differently to HTC. At very mild conditions (T > 160 °C), glucose is
dehydrated and polymerized into polyfuranic domains that will further condense
into aromatic carbon above 180 °C, while bulk cellulose conversion mainly occurs
via intramolecular condensation, dehydration and decarboxylation (instead of
hydrolysis to water-soluble monosaccharides) at higher temperatures [68].
Hemicellulose transforms into liquid and gaseous products at low HTC
temperatures (120 – 180 ºC), while cellulose degradation occurs at 220 – 230 ºC
and lignin does not undergo substantial changes within the traditional HTC
temperature range (< 250 ºC) [68,70,71]. In untreated biomass, lignin may
additionally inhibit the conversion of carbohydrates by protecting carbohydrates
from the attack of water [72]. In practice, the carbonization of a mixture of various
components entails polymerization of water-soluble species and solid
condensation of insoluble structures (Figure 3).
Figure 3. Simplified scheme of hydrochar formation from waste biomass.
10
In addition to the HTC treatment temperature, the carbonization severity can be
adjusted by changing the residence time, and it has been shown that the
conversion is mainly occurring during the first four hours of reaction time
[73,74]. The precursor:water ratio may also affect carbonization severity, but this
parameter has often been shown to impart a negligible effect compared to
temperature and residence time [40,75,76]. Carbonization can be further altered
by the addition of reagents that work to adjust the pH or act as dopants and/or
catalysts [77–79]. These additives can be easily introduced to the reaction
mixture – which exists in the water phase – and, as such, highlights another
advantage that hydrothermal carbonization has over dry carbonization methods.
HTC reactor design may vary depending on the reactor scale and application.
Batch reactors are simpler from a technical perspective, while continuous
reactors are more beneficial for industrial use. Batch reactors (in lab scale) vary
from simple autoclaves that are heated in a furnace to more sophisticated models
that provide stirring and cooling (Figure 4).
Figure 4. Schematic of a stirred and water-cooled HTC-reactor design.
Sludge dewatering is a very energy-consuming process and it has been estimated
that 0,1% of the energy consumed in Sweden is used for the dewatering of
municipal wastewater treatment sludge [44]. Paper mill sludges are often
incinerated on-site with very low or negligible energy recovery due to their high
11
moisture content [22]. The application of hydrothermal carbonization would
require less dewatering processes for sludge material treatment, which makes it
an appealing alternative for dewatering sludge wastes [22,44,80]. It has been
estimated that treating paper mill sludges with HTC would save costs associated
with waste handling (if sludges are managed externally) or using additional fuel
sources (if sludges are co-combusted at the paper mill boilers) to an extent that
the investment into an HTC facility would be covered in 2-5 years [22].
Additionally, sludge stabilization would not be needed since the product is
completely sterilized, and this technique could also be directly applied to
undigested sludge [44]. Only a few full-scale HTC systems currently exist around
the world. For instance, plants in Germany, China, and Spain produce biofuels
from sludge [81,82]. In addition to fuel, the HTC process produces nutrients [81]
and clean water [82]. The largest HTC system currently operating in Sweden is a
pilot reactor with a capacity of 1 m3 sludge/day. The company that developed this
facility is currently building a demo facility (capacity 20 000 t sludge/year) in
Finland that will convert biosludge to biofuel (production is planned to start in
summer 2019) [83,84]. As an example of fields other than waste handling, the
hydrothermal processing of sugars can produce 5-hydroxymethylfurfural, a
platform chemical for pharmaceutical, food and fine chemical industries [85].
Activated carbons
Hydrochars generally have very low surface areas, and their properties can be
further improved by physical or chemical activation processes that increase
porosity, adjust pore size or create specific surface functionalities. During
physical activation, the precursor is heated to a high temperature (700 – 900 ºC)
in the presence of a controlled amount of CO2, steam or air [86]. On the other
hand, chemical activation employs chemical reagents such as KOH, NaOH, ZnCl2
and H3PO4 to impregnate the precursor material, which is followed by thermal
treatment in an inert atmosphere [25]. Most of the industrial activation processes
subject bituminous coal, coconut shell, charcoal and/or lignite to physical
activation, but wood and peat are also activated with phosphoric acid [87].
Moreover, a KOH-mediated method for activating carbon has been
commercialized [88]. Even though industrial production prefers physical
activation due to its technical viability and lack of chemical reagents, chemical
activation provides better control of product porosity through variations in
activating reagents and/or reagent concentrations [89]. In addition to activation
processes, several other modifications have also been studied. For instance, the
addition of MnO, iron (oxides), carboxyl, amino and sulfonic groups on the
carbon surfaces has been reported to enhance adsorption and surface properties
and/or produce magnetic carbons [90–95].
12
Tuning the surface area and porosity is important for designing products that are
suitable for their intended purpose. Porosity is often described by pore size. More
specifically, pore sizes are classified to micropores, which are smaller than 2 nm
(of which ultramicropores are < 0.7 nm and supermicropores > 0.7 nm),
mesopores, which are between 2 nm and 50 nm, and macropores, which are
larger than 50 nm [2]. Ultramicropores or micropores may be more suitable for
gas adsorption and the removal of small molecules from aqueous media, while
larger pores may be more beneficial for removing larger molecules [96].
Therefore, improving adsorbent selectivity (by tuning the porosity or the surface
functionality) can effectively prolong its lifetime as the adsorbent surface will not
be saturated by components that do not need to be removed.
Chemical activation is based on carbonizing or volatilizing tarry/volatile
materials from existing pores to enhance porosity and creating new porosity
and/or surface functionalities [97,98]. The mechanisms underlying porosity
development are closely linked to the activation method; as such, different
methods provide distinct yields, porosities and functionalities. NaOH, ZnCl2 and
H3PO4 activation yield similar, or higher, surface areas when compared to
physical activation (~400 – 2600 g/m3), while KOH activation produces highly
porous carbons (up to ~3400 g/m3) [66,89,99]. The chemical agents used in
activation produce uniform porosity at low concentrations, but heterogeneous
porosity and mesopores are evident when reagent concentrations increase [89].
Yields are also heavily dependent on the route of activation, e.g. physical and
KOH activation yields are very low when compared to H3PO4 activation yields
[66,87].
Synthesis of hydrochars and chemically activated carbons
The hydrochars used in the research underlying this thesis were prepared from
residue feedstocks that were relevant for the settings of each study. The following
section describes the HTC and activation processes used in the appended
research papers and briefly describes the produced carbons.
Selection of the precursor materials
The precursors used in Paper I were rice husks, tomato waste, and horse manure
(Figure 5). The research presented in Paper I aimed to evaluate the performance
of carbonized residue precursors that represent common agricultural and food
processing waste in South European or African countries. An additional objective
was to use residue materials from countries suffering from water stress to
produce low-cost adsorbents that could be used in local wastewater treatment
facilities.
13
Figure 5. The precursor materials used in Papers I-IV.
The precursor materials in Papers II-IV (horse manure, sewage sludge and bio-
and fiber sludges) represent waste streams that are relevant to Swedish society as
they are produced in large quantities and inefficiently utilized due to a
combination of high handling costs and low energy recovery (usually explained
by high water content). As described in the previous section, HTC could eliminate
the need for dewatering, which could make the hydrothermal processing of these
materials an attractive management alternative. Sludge from pulp- and paper
industries is currently combusted, which is not optimal when considering the
poor fuel properties of sludge materials [22,100]. Even though sewage sludge has
relatively high metal concentrations, it is still mostly used to fertilize agricultural
land, produce manufactured soil1, or serve as hydraulic barrier material in
landfills [8]. Horse manure that is not used as fertilizer is classified as an animal
by-product, and the new hygienization requirements for composting and
digesting facilities – which came into force in 2011 [101] – have severely limited
the options for horse manure handling [102]. Hence, new valorization
possibilities are needed.
Synthesis of hydrothermal carbons and activated carbons
In the research underlying this thesis, hydrothermal carbonization was carried
out in 1L stainless steel HTC reactors with different designs. In Paper I, a stirred
and water-cooled model (Zhengzhou Keda Machinery and Instrument
Equipment Co., Ltd., Zhengzhou, China) was used, while Papers II and III
employed a non-stirred, air cooled reactor (Amar Equipments Pvt., Ltd.,
Mumbai, India). In the case of both reactors, the ramp time was ~1 h and the hold
time was 2 h. In Paper IV, a non-stirred, water-cooled model (Amar Equipments
Pvt. Ltd.) with higher ramp times (2,5 h – 6-5 h) and a longer hold time (4 h) was
used. The synthesis of the hydrochars is further described in each paper. In
Paper I, the materials were carbonized at 220°C, while Papers II-IV
investigated the effect of temperature by using three different temperatures, 180,
1 according to Swedish specifications in AMA (Guidance for the preparation of particular conditions for Building and Civil Engineering Works and Building Services Contracts) and SPCR 148 certification.
14
220 and 260 °C, to cover a wide carbonization temperature range that provides
relatively high solid product yields.
A visual inspection of the produced materials clearly shows that the original
features (texture, color) of the precursor materials are retained in some of the
low-temperature hydrochars, but are lost as HTC temperature increases (Figure
6).
Figure 6. Temperature series of hydrochars used in Papers II and III.
The yields of each study covered in this thesis are discussed in more detail in
Papers I, II and IV. The yields of all of the studied materials varied between
35% and 90%. The yields of horse manure and sewage sludge hydrochars
(produced at 180, 220 and 260 ℃) were 12-15 percentage points higher in Paper
II compared to the yields in Paper IV. This is likely due to the treatment severity
– longer residence time and ramp time, higher water-to-solid ratio - applied in
Paper IV.
Chemical activation of hydrochars
The activated carbons in Paper IV were prepared by impregnating 0.5 g
hydrochar (horse manure or sewage sludge carbonized at 180, 220 or 260 ℃),
untreated horse manure or sewage sludge with KOH or H3PO4. Control samples
without the addition of a chemical reagent were also prepared. Two different
precursor-to-activating agent ratios – 1:4 and 1:8 - were studied. After careful
mixing of the chemicals and hydrochar, the samples were dried overnight at 105
℃ in an oven and activated at 600 ℃. Any chemical residues were removed by
washing the product with water and vacuum filtration, after which the carbons
15
were dried overnight at 105 ℃ in an oven. Visual inspection of the produced
materials suggests that the KOH-activated sewage sludge lost organic carbon –
yielding only a mineral fraction – as the produced material was light brown in
color when compared to the black carbon-rich products (all of the produced
activated carbons and their precursors are shown in Figure 7).
Figure 7. Chemically activated carbons and their precursors produced in paper IV.
The activated carbon yields reported in Paper IV were clearly affected by the
activation method and varied based on the precursor (Figure 8). However, the
effects of chemical activation were similar for precursor materials subjected to
16
HTC at three different temperatures. For instance, 1:4 H3PO4 activation of horse
manure hydrochars produced at 180, 220 and 260 ℃ produced yields varying
between 35 and 40%. It can be seen that the yields of hydrochars produced by
HTC at lower temperatures are higher as compared to those produced at higher
temperatures. However, during activation, these materials loose mass at
relatively higher proportions as compared to materials produced at higher
temperatures. The influence of the activating reagent on yield can partly be
explained by the reaction mechanisms that will be discussed in the surface
characterization section.
Figure 8. Yields for the obtained materials. HM=horse manure, SS=sewage sludge.
17
Characterization of carbon materials
Carbon material characterization is used to assess properties that are relevant
field of application. The properties that are most relevant for adsorption
applications include porosity, chemical functional groups on the surface, and
elemental composition, among others. This section provides a brief background
of each characterization technique used in the research underlying this thesis as
well as summarizes the characterization of the studied materials.
Characterization techniques
A wide variety of characterization methods are used to evaluate theproperties of
hydrochars and activated carbons. The most commonly used methods are listed
in Table 1. These methods are complementary, as such, data extracted from one
technique will not provide a full understanding of the material properties. The
techniques used in the research covered in this thesis are described in the
following sections.
Table 1. List of characterization techniques used for carbon materials. Techniques in the area with
a light gray background provide information on composition/functionality, whereas those in the
area with a white background are used to determine porosity and adsorption. The methods in the
area with a dark gray background are microscopy techniques.
Technique Purpose Comments Ref
Elemental analysis Bulk composition Quantitative analysis [62,103]
Thermal gravimetric
analysis Thermal decomposition [104]
Fourier-transform-infrared
spectroscopy Chemical structure Functionalities [104]
Raman spectroscopy Chemical structure Carbon structure [105]
Solid state nuclear
magnetic resonance Chemical structure Carbon structure/binding [104]
X-ray diffraction Crystallinity Crystallinity of carbon and minerals [70,106]
X-ray photoelectron spectroscopy
Chemical structure Quantitative analysis of elements and functionalities
[104]
Boehm titration Surface functionalities [104]
Zeta potential Surface charge /point
of zero charge [107]
Gas adsorption/desorption
isotherms
Specific surface area,
porosity
N2 most commonly used gas, CO2
used for micropore determination [108]
Iodide number, methylene blue adsorption
Porosity/adsorption [109,110]
Scattering and diffraction
methods Porosity
More specifically small-angle X-
ray/neutron scattering, wide-angle scattering and diffraction
[108]
Scanning electron
microscope Imaging
Combination with EDS provides
spatial composition data [111]
Transmission electron
microscope Imaging [111]
18
Fourier transformation infrared spectroscopy
Infrared spectroscopy is a versatile tool for characterizing the functional groups
of carbon materials. Infrared spectroscopy measures chemical bond vibrations
that induce a change in dielectric dipole moment (including stretching, bending,
and deformation). These vibrations occur in the infrared range and can be
detected by the absorption of infrared light with vibrational frequencies
characteristic for each bond and vibration type [112].
Two different FTIR modes, Diffuse Reflectance Infrared Fourier Transform
Spectroscopy (DRIFTS) and Attenuated Total Reflectance (ATR), are commonly
used in the analysis of carbon materials [104]. In DRIFTS, the sample is ground
and diluted with KBr powder. An infrared beam is then directed towards the
sample, and the reflected light is collected by mirrors and directed to the detector
(Figure 9). The advantage of this technique is that the sample is easy to prepare,
however, the resulting spectra may be noisy [113]. In ATR measurements, a
carbon sample can be directly pressed on top of an ATR diamond crystal using a
sapphire anvil. The infrared beam penetrates through the sample into the upper
region of the ATR crystal. The analysis depth is in the order of ~micron, and
depends on the wavenumber, crystal refractive index and angle of incidence. At
low wavenumbers, light penetrates deeper, which increases signal intensity. For
this reason, ATR and DRIFTS spectra have different band intensities even though
the band positions are identical. ATR can provide high sensitivity when good
contact between the ATR crystal and sample is achieved [113]. When used to
analyze lignocellulosic biomass, ATR has been found to provide spectra that are
easier to interpret and include less variation than those produced by DRIFT [114].
Figure 9. Schematic figure of ATR and DRIFTS. Modified from [113].
Details on how FTIR spectra were recorded can be found in the appended
manuscripts. DRIFTS was used in Paper I, ATR was employed in Papers II and
IV. Briefly, the samples subjected to DRIFTS analysis were ground with KBr
powder and spectra were collected between 400–5000 cm−1, while in the ATR
19
analyses homogenized carbon samples were directly applied on an ATR crystal
and spectra between 600 – 4000 cm-1 were collected.
X-ray photoelectron spectroscopy (XPS)
XPS provides quantitative information about the elemental composition and
functionalities of the material surface. In this technique, the sample is placed in
an ultra-low vacuum and radiated with X-rays. During irradiation,
photoelectrons only escape from the very top portion of the sample (typically 1.5
– 10 nm), making this technique highly sensitive to the elemental composition,
chemical bonding and even oxidation states of the material surface. The
interaction between the X-rays and sample create photoelectrons that have
kinetic energy, and this kinetic energy is measured in a vacuum. Emitted
electrons are transferred to an electron detector via hemispherical analyzer and
electrostatic transfer lens. The binding energies – which are characteristic of
specific elements and their chemical binding states – can then be calculated from
kinetic energy measurements. Wide spectrum approaches cover the whole range
of measured binding energies, whereas high-resolution spectra are collected and
deconvoluted to gain more information about each element of interest. [104,115]
In Papers I, II and IV, XPS data were collected with a Kratos Axis Ultra DLD
spectrometer (Kratos Analytical, Manchester, United Kingdom) with a AlKα
source. Wide spectra collection used a pass energy of 160 eV, while high
resolution spectra were obtained with a pass energy of 20 eV.
Specific surface area
Specific surface area measurements are based on gas adsorption to a material
surface. Nitrogen gas close to its boiling point (77 K) is routinely used due to its
availability [116]. Typically, the solid sample is first evacuated in a vacuum
chamber, after which gas is successively added and allowed to equilibrate. The
amount of gas (i.e. gas pressure) is measured after each addition. These points
form an adsorption isotherm (amount of gas vs. relative pressure p/p0, where p0
is the saturation pressure at the chosen pressure) until the maximum relative
pressure has been achieved (p/p0 =1). After this, the gas is desorbed from the
adsorbent and the amount of gas is again measured at different relative pressure
values, forming a desorption isotherm. Micropores are filled at low relative
pressures before a monolayer is formed on the external surfaces of the material,
whereas capillary condensation occurs in mesopores after multilayer adsorption
to the walls at high relative pressures. Therefore, the porosity of the material can
be determined from the shape of the isotherm.
Specific surface area is determined using Brunauer-Emmett-Teller (BET) theory,
which is used to calculate the amount of gas that forms a closely packed
20
monolayer on a material surface. The assumptions of multilayer formation are
not applicable to micropores, but this method is nevertheless widely used to
identify potentially microporous carbon materials [108]. The Barrett, Joyner and
Halenda (BJH) method is commonly used to obtain pore size distribution [116],
while the t-plot method can be used to determine micropore volumes. The t-plot
method is based on the statistical calculation of film thickness on a flat surface,
but does not consider surface curvature, which may lead to underestimations of
the micropore volumes of highly microporous samples [117]. More accurate
measurements of micropore volume can be obtained from high resolution data
from the low relative pressure range of CO2 adsorption at 273 K or 298 K. This is
feasible because the saturation pressure of CO2 at these temperatures is
noticeably higher than that of nitrogen gas at 77 K, which enables researchers to
easily reach low p/p0 values. However, this technique is used only when obtaining
micropore data, since higher p/po ranges would require very high pressures [108].
Before measuring adsorption/desorption isotherms, physisorbed species should
be removed from the surface while being careful to avoid any irreversible changes
to the surface during degassing. Degassing is done at elevated temperatures,
either in a vacuum or under gas flow [118]. Vacuum degassing of biochars has
been previously studied and changes to the surface were reported at elevated
temperatures, which is problematic because no standard degassing procedure
that minimizes changes made to the surface currently exists [119]. Furthermore,
changes caused by vacuum and gas flow degassing have also not been discussed
in the literature.
The multipoint N2 adsorption/desorption isotherms presented in Papers I, II
and IV were collected using liquid nitrogen at a temperature of 77 K. In Paper
I, BET surface area was determined by Quarda Sorb analyzer (Quantachrome
Instruments, Boynton Beach, FL, USA, analyses performed by a commercial lab)
after vacuum degassing at 120 ℃, while in Papers II and IV a Micromeritics
TriStar 3000 gas adsorption analyzer (Micromeritics, Norcross, GA) was
employed after nitrogen flow degassing at 120 ℃.
Scanning Electron Microscopy (SEM)
SEM gives visual information about the topography and density of a studied
surface. In this technique, the electron beam is focused on a sample in a vacuum,
which results in two distinct interactions: 1) inelastic scattering, in which the
interaction between the sample and incident electrons results in the ejection of
secondary electrons from the uppermost layer of the interaction volume; and 2)
elastic scattering, in which incident electrons interact with atoms deeper in the
sample and are reflected as backscattered electrons. The depth of the interaction
volume depends on both operation energy and the atomic composition of the
21
sample, as the electron beam passes readily through lighter elements. Secondary
electron images provide information on topography, while backscattered electron
images show the contrast between particles with different densities (i.e. atomic
numbers). Additionally, energy dispersive X-ray Spectroscopy (EDS) collects
spatial information about the chemical composition of a surface. X-ray signals are
collected for obtaining EDS data. These signals are caused by electron relaxation
after incident electrons collide with low-energy level electrons in the specimen.
[120]
In Papers II and IV, surface morphology was examined with scanning electron
microscope (Carl Zeiss Evo, Carl Zeiss AG, Oberkochen, Germany) operated in
low vacuum mode. EDS analysis was carried out to study the spatial distribution
of elements.
Surface properties of the studied hydrochars and activated
carbons
The following section presents the surface characteristics of the hydrochars and
activated carbons that were used in the research covered in this thesis. The
section is separated into parts that focus on surface functionalities, surface
porosity and surface imaging.
Chemical surface analysis
Elemental surface composition
Temperature is a crucial parameter for the transformation of material during
HTC. The precursor materials used in the research underlying this thesis were
composed of components such as cellulose, lignin, hemicellulose, proteins and
minerals, with HTC treatment severity (including temperature, time and
precursor:water ratio) dictating the degree to which the different components in
the studied materials were converted. XPS analysis showed that HTC treatment
temperature had a larger effect on the elemental surface composition of horse
manure and fiber sludge than on that of sewage sludge and biosludge (Figure 10).
The increase in carbon and decrease in oxygen contents in horse manure and fiber
sludge hydrochars between 220 and 260 ºC (Paper II) was likely caused by the
degradation of cellulose [70]. The compositions of sewage sludge and biosludge
were not as dramatically affected, which may be explained by their respective
biopolymer compositions. In these materials, it is likely that less stable
components (hemicellulose, cellulose) have already undergone a fair amount of
decomposition during bacterial digestion, which would enrich the proportion of
more stable components (such as minerals and lignin) that are not highly
responsive to HTC at 260 ºC and below. Some inorganic elements seemed to
22
concentrate in hydrochar (Si, Al), while others (e.g. Ca) were washed out during
HTC treatment.
XPS is used to measure sample surface composition, and therefore, does not
provide data about bulk composition. Previous research has shown that
hydrochar particles on the surface layer have high oxygen contents when
compared to the bulk composition of the same material [103]. This may be one
explanation for why untreated horse manure had higher carbon and lower oxygen
contents than horse manure hydrochar prepared at 180 and 220 ºC.
Activation by KOH and H3PO4 also yielded products with different surface
compositions (Paper IV), (Figure 10). KOH activation increased the carbon
content of horse manure hydrochar, while sewage sludge hydrochar showed a
stark decrease in carbon (from 68% to 9% following KOH activation). It is known
that KOH activation consumes carbon due to the oxidation of carbon to carbon
oxides and/or carbonates when potassium is reduced to elemental form [121].
Also the yields of KOH activated carbons (Paper IV) was very low as discussed
in a previous section. These low carbon yields led to higher concentrations of
minerals in the carbon material. A similar process may have occurred in the
control sample, which was heated without an activation agent. The high
proportion of minerals in the sewage sludge sample may have been involved in
the redox reactions that release carbon in oxidized form when the sample is
heated up. Contrary to KOH activation, H3PO4 activation seemed to dissolve most
of the inorganic species, leaving a solid product rich in carbon, oxygen and
phosphorus. Moreover, phosphorus is not efficiently removed from the activated
carbon due to either inefficient removal in the washing step following activation
or the formation of new stable phosphorus species, e.g. insoluble metal
phosphates [89]. This may explain the findings for sewage sludge, as these
materials had three times the amount of phosphorus than horse manure carbons
after phosphoric acid activation. The retention of phosphorus in the activated
carbons also contributes to the high yields (discussed in the previous chapter) of
these materials after H3PO4 activation.
23
Figure 10. Surface compositions (determined by XPS) of the hydrochars studied in Papers I and
II and the chemically activated carbons in Paper IV. The upper rows of the charts show C, O and
N contents, while the lower rows show the contents of other detected elements.
Surface functionalities
Carbon bonding was studied by deconvoluting C1s lines in the XPS spectra.
Graphitic, aromatic or aliphatic carbon increased with HTC temperature, while
24
the proportion of oxygen-bearing functionalities (mostly –OH) decreased as HTC
temperature increased (Figure 11). Chemical activation further decreased the
proportion of –OH groups, but increased the proportions of double-bonded
oxygen, carboxyls, carbonates and aromatics (indicated by π-π* shakeup
satellites ~291 eV) in horse manure carbons. Activated sewage sludge carbons
showed lower proportions of all organic oxygen functionalities, which may be
explained by transformation into inorganic oxygen species when considering the
dramatic increase in total oxygen content.
Figure 11. Surface functionalities of the hydrochars studied in Papers I and II and the chemically
activated carbons studied in Paper IV.
FTIR spectra further confirmed the trends identified in the XPS data. The ATR-
FTIR spectra showed only subtle changes when the precursor material and
hydrochars were prepared at either 180ºC or 220ºC (Paper II). The most
dramatic changes occurred for horse manure and fiber sludge at 260 ºC, as
cellulose degradation resulted in the loss of –OH stretching bands at ~3300 cm-1
and C-O-C bands at ~1000 cm-1 (as shown for horse manure in Figure 12).
Furthermore, the increase in aromatic functionalities at 1595 and 1505 cm-1 could
25
be attributed to lignin accumulation, as this biopolymer has high thermal
stability. The spectra of sewage sludge and biosludge were barely affected by
increasing HTC temperature, which supports the results of the XPS analysis. For
activated carbons (Paper IV), the activation method had a more pronounced
effect on the surface properties than the amount of reagent used for activation or
the pre-treatment of the precursor materials. The spectra of activated carbons
showed very low amounts (or even a lack) of –OH groups (OH stretch ~3300 cm-
1), as well as the presence of aromatics (C-C stretching between 1450 and 1600
cm-1), and mineral species, such as silicon oxides (~950 - 1000 cm-1). In addition,
many narrow bands between 900 and 1300 cm-1 were detected in the spectra of
phosphoric acid activated materials, which suggests that phosphoric compounds,
such as phosphates and phosphoric acid esters, were present in these materials.
Figure 12. ATR-FTIR spectra for the horse manure hydrochars and activated carbons prepared in
Papers II and IV.
26
Porosity
HTC alone did not significantly increase the porosity of the resulting hydrochars,
even if the surface area slightly increased relative to the precursor material. The
porosity of the precursor materials ranged from 0.5 – 4.0 m2/g, while the surface
areas of the hydrochars ranged from 1.1 – 34 m2/g (Papers II and IV). Sewage
sludge and biosludge developed the highest surface areas, while fiber sludge and
horse manure hydrochars had lower (<5.4 m2/g) surface areas. In Paper I, olive
and tomato waste hydrochars had very low surface areas (<1 m2/g), while rice
husk hydrochar showed a surface area of 17 m2/g. The precursors and hydrochars
were meso/macroporous, and earlier research has reported similar porosities for
biomass-based hydrochars [122,123].
In Paper IV, activated sewage sludge showed low porosity (34 – 340 m2/g) when
compared to horse manure carbons (270 – 1400 m2/g). The porosities measured
for activated carbons were lower or similar to previously reported surface areas
for chemically activated carbons, yet noticeably higher surface areas have been
reported for hydrochar-based activated carbons derived from pure carbohydrates
and sawdust (530 – 3400 m2/g) [66,124]. Nevertheless, the activation of sludge
typically yields lower surface areas (6 – 690 m2/g) [125,126], while KOH
activation of sewage sludge biochar has produced substantially higher surface
areas (up to 1900 m2/g) [127]. This notable difference may be explained by a
biochar:reagent ratio (1:1) that was lower than what was used in the research
covered in Paper IV. Furthermore, the acid wash that Lillo-Ródenas et al.
applied after activation could have dissolved inorganic species and exposed more
surface area. It has also been suggested that inorganic materials can contribute
to the development of porosity due to alkaline fusion [127]. This may have been
the case in Paper IV when considering that nearly all of the carbon was lost
during KOH activation, and that KOH-activated sewage sludge had higher surface
area than H3PO4 activation (up to 340 m2/g vs. 160 m2/g, respectively). The
highest surface areas were measured for KOH-activated horse manure. Higher
precursor:KOH ratio increased the surface area in all of the studied materials
(except for activation of raw horse manure), which was not observed when using
H3PO4.
Adsorption/desorption isotherms showed that KOH activation and the control
(i.e. heating the sample without chemicals) produced microporous carbon while
H3PO4 activation produced mesoporous carbon (these isotherms had a hysteresis
loop) (Figure 13). Microporous samples show high adsorption at low relative
pressures without any significant increase in adsorption at higher relative
pressures (Type I according to the IUPAC classification of isotherms), while the
adsorption of mesoporous carbons increased with relative pressure and
developed a hysteresis loop at high relative pressures due to the capillary
condensation of nitrogen in the mesopores (Type IV) [118]. In KOH-activated
27
materials, micropore volume grew as HTC treatment temperature increased,
while mesopore volume development was less dependent on treatment
temperature.
Figure 13. N2 adsorption/desorption isotherms for activated horse manure hydrochars (220 ºC).
Surface morphology
The surface morphologies described in Papers II and IV reveal that the
precursor structures were broken down into smaller particles. In the studied
materials, both particle size and surface roughness decreased as HTC
temperature increased (Figure 14). Chemical activation (and heat treatment
without an activating agent) further decreased particle size. The SEM images,
taken in backscattered electron mode, show clear density (i.e. atomic weight)
differences between samples. Horse manure hydrochars and activated carbons,
as well as sewage sludge hydrochars, had separate inorganic (dense material
resulting in light color) and organic particles (dark color). An EDS analysis
revealed that these particles consisted mostly of Si, Al and Fe (Appendix Figures
A1-A2). After activation, sewage sludge carbons looked more uniform and
brighter in color due to the relatively high mineral content. This was not the case
for horse manure carbons, as separate inorganic particles still persisted after
chemical activation.
28
Figure 14. Backscattering SEM images (150X magnification) of horse manure and sewage sludge
carbons. The upper, middle and lower parts show untreated materials, hydrochar materials
(Paper II) and activated hydrochar-based carbons (Paper IV), respectively.
29
Wastewater purification by adsorption
This chapter introduces adsorption as a wastewater treatment technique and
summarizes the main findings from the research underlying this thesis.
Adsorption
Adsorption is the accumulation of substances on an interface separating two
phases (e.g. liquid-liquid, solid-liquid, solid-gas). The material adsorbing
substances is referred to as the adsorbent while the substances that are being
adsorbed are called adsorbates. In the context of wastewater treatment by
adsorption, carbon adsorbents interact with adsorbates, retaining the adsorbate
on the adsorbent surface and thus removing it from the aqueous phase.
Adsorption on activated carbon is a common wastewater purification technique
due to its strong ability to remove organic substances even at low concentrations
[128]. Powdered activated carbons can be directly applied to the wastewater
reservoir, while granular activated carbons can be packed into columns through
which wastewater flows continuously [128]. Low-cost carbon materials produced
from residue materials – which can noticeably reduce the cost of water treatment
and provide a valorization route for diverse waste streams – have been studied
during recent decades as another wastewater treatment option [129].
Adsorption involves different interactions and mechanisms, which depend on
adsorbent and adsorbate properties (Figure 15). Neutral, non-polar molecules are
driven to the adsorbent surface by hydrophobic interactions and pore filling
[130–133], whereas polar molecules and ions interact with the surface
functionalities by hydrogen bonding, dipole-dipole or dipole-induced-dipole, π-
π electron donor –acceptor interactions [134–137]. Metal cations may
precipitate, form complexes with negative surface groups or adsorb by ion
exchange [90,138,139], while anions interact with positive surface groups such as
minerals [140,141] or cationic surfactants [142]. Bio/hydrochars are typically rich
in negative surface groups that can bind negative adsorbates by hydrogen
bonding [135]. The strength of these interactions depends on whether a chemical
bond forms (chemisorption), or if weaker physical interaction, physisorption,
which is caused by van der Waals forces, is involved.
pH value can dramatically influence the adsorption of charged adsorbates since
it affects the protonation state of the surface. A higher pH value means that the
surface has more negative charge, which causes repulsions between anions and
the surface [135]. In contrast, low pH values create positive surface charge, which
decreases the removal of metal cations with maximum adsorption between pH 5-
8 and at alkaline pH start to precipitate [143–145]. Other variables have more
30
dubious roles, as while certain studies have found temperature and agitation
speed to affect adsorption [146–148], other research has reported the possible
changes in adsorption to be insignificant [149,150].
Figure 15. Examples of possible adsorption mechanisms on carbonaceous material.
Adsorption studies are traditionally carried out in pure water without other
components than the studied adsorbate(s) [151]. However, a clear understanding
of how water matrix components affect adsorption is crucial when designing
adsorbents for real wastewater treatment applications. Different components in
the water may dramatically affect adsorption. For example, ions and dissolved
organic carbon (DOC) are common matrix components in real wastewater, and
their presence has been shown to affect the removal of adsorbates. DOC may
block adsorbent pores and, as such, reduce removal efficiency [152–154];
however, on the other hand, it may also increase complexation and thus improve
removal [152]. Ions in solution may either compete with the adsorbates, which
will saturate the surface faster [90,155,156], or di- or trivalent ions may act as
bridges to increase adsorption [157]. High ion concentrations may also create a
dense hydrate cloud on the adsorbent surface, which will reduce the access of
organic molecules and, as such, decrease adsorption [158,159].
Other adsorbate compounds may reduce removal due to faster saturation of the
adsorbent surface. Previous studies have shown that large organic compounds
tend to adsorb more effectively than small compounds [160], and that the
interfering effect of an ion increases as its radius or electronegativity grows [156].
31
Adsorption testing
Adsorption is commonly studied in batch and column systems. In batch tests, the
adsorbent is mixed with a pre-determined quantity of water and the removal of
adsorbates is measured either 1) before and during equilibration (several time
points) to collect kinetic data or 2) after equilibrium with different
adsorbate:adsorbent ratios (i.e. by varying the concentration or the carbon dose)
to collect isotherm data (Figure 16). Kinetic data can be used to determine the
equilibrium time and removal efficiency at the studied conditions whereas
isotherm data provide the maximum adsorption capacity for the material. As
many wastewater treatment facilities use water treatment columns, results from
column tests (also known as dynamic or fixed bed systems) would better
represent the practical application of a tested adsorbent. Column tests study
adsorption on an adsorbent column by flushing water through the column and
then measuring the concentration of adsorbates in the effluent as a function of
the time (Figure 16). Rotating bed reactors can be used to speed up the kinetics
of a tested adsorption system. These reactors are packed with adsorbent and the
flow of water through the reactor makes mass transfer more effective than in
traditional batch systems [161]. Continuous processes can also integrate rotating
bed technology [162].
Figure 16. An illustration of data collection by kinetic, isotherm and column adsorption tests. In
kinetic testing and column tests, the water is sampled at different time points (t). In isotherm tests,
different starting concentrations (C) are equilibrated with the adsorbents and the equilibrium
concentration is measured in water.
The research underlying this thesis applied batch adsorption tests (both kinetic
and isotherm studies), which are further described in the following section.
Batch adsorption
Batch adsorption testing, which is commonly used to study carbon adsorbent
materials, is a simple way to investigate adsorption kinetics and adsorbent
capacity [151]. Kinetic and isotherm data often serve as the input for several
models that are further described in the next sections. These models were
32
originally non-linear, but several linear versions have been developed to make it
easier to fit data to the model. However, the use of non-linear models is
recommended for obtaining the most accurate results of the model parameters
[163–165].
Adsorption kinetics
Kinetic tests are used to investigate the time needed to reach equilibrium at set
conditions by sampling a water-carbon mixture at different time points during
the adsorption. To facilitate sampling and ensure that the char:water-ratio does
not change during different sampling occasions, water samples for different time
points are often put in separate vessels (with identical water:char ratios). Kinetic
tests provide information on the equilibrium time for an adsorption process and
removal efficiency/adsorption capacity of the adsorbate at a chosen
concentration. Kinetic data are commonly fit to pseudo-first-order and pseudo-
second-order models, but the Elovich model has also been widely used to study
kinetics [164]. The rate-limiting steps for the adsorption process can be studied
by intraparticle diffusion model [166].
Papers I, III and IV studied removal kinetics, as this is an important parameter
for adsorbent materials when considering the short residence time (15-35 min
[128]) of wastewater in an adsorbent column. Additionally, the kinetics data in
Papers III and IV were fitted to pseudo-first- and pseudo-second-order models.
The following nonlinear pseudo-first-order model [167] was used in Papers III
and IV:
𝑞𝑡 = 𝑞𝑒(1 − 𝑒−𝑘1𝑡)
Where qt (ng g-1 or µg g-1) is the adsorption capacity at time = t, qe (ng/g or µg/g)
is the adsorption capacity at equilibrium, and k1 (1/min) is the pseudo-first-order
rate constant.
The following nonlinear pseudo-second-order model [168] was also used:
𝑞𝑡 =𝑞𝑒
2𝑘2𝑡
1 + 𝑞𝑒𝑘2𝑡
Where k2 (g/(ng min) or g/(µg min)) is the pseudo-first-order rate constant.
Adsorption isotherm
Isotherm testing provides information about the maximum adsorption capacity
of a material. In practice, this means that adsorbents are equilibrated with
33
different adsorbate concentrations until the maximum adsorption capacity is
achieved, i.e. the adsorbent surface is saturated. One benefit of this technique is
that maximum adsorption results from different studies can be easily compared
by looking at the parameters (capacity, non-linearity) obtained from adsorption
isotherms. Different models, such as Freundlich, Langmuir, Sips (also known as
Langmuir-Freundlich model), Redlich-Peterson, and Temkin models, are applied
to fit isotherm data [169]. In Paper II, methylene blue adsorption data were
fitted to Langmuir and Freundlich models.
The Langmuir model assumes that a monolayer will form on a uniform surface
and that there are no interactions between adsorbates [163]. A non-linear form
of the Langmuir equation [170] can be expressed as:
𝑞𝑒 = 𝑄𝑚𝑎𝑥𝐾𝑙𝐶𝑒
1 + 𝐾𝐿𝐶𝑒
Where Qmax is the maximum adsorption capacity (mg/g), KL is the Langmuir
constant, and Ce is the adsorbate concentration in the solution at equilibrium
(mg/l)
The Freundlich model assumes multilayer formation and is applicable to
heterogeneous surfaces. Moreover, this model assumes that adsorption will
increase infinitely as the adsorbate concentrations increases [163]. The non-
linear form of the Freundlich isotherm [171] can be expressed as:
𝑞𝑒 = 𝐾𝑓𝐶𝑒
1𝑛⁄
Where qe is the amount (mg/g) of adsorbate at equilibrium, Kf is the Freundlich
constant ((mg/g)(l/mg)-1/n) (indicating adsorption capacity), and n
(dimensionless) represents the nonlinearity of the fitting.
Selection of adsorbates
In Europe, pollution reduction and control in surface and groundwater is
regulated by the European Union’s Water Framework Directive, which oversees
and regulates emissions of metals, biocides and compounds causing biological
effects (such as carcinogenic, mutagenic and endocrine related effects) to water
systems [172]. The organic and inorganic contaminants studied in Papers I, III
and IV (Figure 17) represent commonly used chemicals that are frequently found
in wastewater influents [173]. These compounds are used as antibiotics
(sulfamethoxazole, trimethoprim, ciprofloxacin), analgesic drugs (paracetamol),
non-steroidal anti-inflammatory drugs (diclofenac), antifungal/antibacterial
34
drugs (fluconazole, octhilinone, triclosan), antihistamines (diphenhydramine),
plastic additives (Bisphenol A), and surfactant/polymers (PFOA). Furthermore,
some of these compounds (such as trimethoprim, fluconazole and PFOA) are not
removed during traditional sewage treatment and are therefore discharged into
the environment [174,175]. It is important to state that in some cases sewage
treatment is quite rudimentary or not available at all (as discussed in Paper I);
research that investigates these types of situations should include a broader range
of chemicals.
In Paper II, methylene blue was used to evaluate adsorption capacity. Methylene
blue is a wastewater contaminant originating from the textile industry [176], but
the adsorption of methylene blue also still remains a frequently used method for
assessing the adsorption properties of carbon materials [110,177].
Figure 17. The set of adsorbate compounds studied in the papers included in this thesis.
35
Quantitative analysis of water
A wide range of qualitative analysis methods are employed to analyze adsorbates.
The selection of analysis technique(s) depends on analyte properties and the
concentration range of interest. Ultraviolet–visible (UV-vis) absorption
spectrophotometry is commonly used when quantifying dyes [178,179] and other
UV-vis absorbing organic compounds [150,180], but also occasionally employed
in the colorimetric determination of nutrients [181,182] and metals [144,146].
Quantitative metal analysis can employ atomic absorption spectroscopy, optical
emission spectroscopy or ICP-MS. The main difference between these techniques
is the concentration range, with ICP-MS enabling analyses of very low
concentrations [183]. Organic analytes are often analyzed by LC-MS or GC-MS,
with the choice of methodology depending on the polarity and volatility of the
target analytes. In the research underlying this thesis, water samples were
analyzed by spectrophotometry, LC-MS and ICP-MS, all of which are further
described in the following sections.
UV-vis spectrophotometry
UV-vis spectrophotometry, which is similar to the FTIR techniques described
before, is a method that analyzes the electromagnetic radiation absorbed by a
sample. UV-vis spectrometry measures the absorbance of wavelengths within a
range of hundreds of nanometers (i.e. ultraviolet and visible light ranges), which
are one or two orders of magnitude shorter than the wavelengths of infrared light.
Consequently, the photons used in UV-vis have more energy than those used in
FTIR techniques. A photon with a wavelength within the UV-vis range can excite
an electron in a molecular electron cloud to move to a higher molecular energy
level. The energy needed for this excitation (i.e. the energy difference between the
relaxed energy level and the excited energy level) is characteristic of shifts in each
molecular electron structure, which results in different spectra [184]. This
technique is used in quantitative analyses of colorful or UV-absorbing
compounds (e.g. dyes), as well as non-absorbing analytes that can be transformed
into UV-vis absorbing compounds through a reaction with a specific reagent (i.e.
colorimetric analysis). However, this technique is limited by its low sensitivity
and selectivity [185].
In Paper II, the concentration of methylene blue was determined with a
Spectronic Unicam Helios Gamma (Thermo Fisher Scientific Inc., Waltham, MA,
USA) spectrophotometer at 668.0 nm.
Liquid chromatography – Mass spectrometry (LC-MS)
Due to its sensitivity and specificity, LC-MS is commonly used to identify the
organic non-volatile and polar compounds in wastewater. The analysis is based
on the separation of sample compounds based on their retention to the stationary
36
phase in an analytical column, and subsequently, their mass-to-charge ratios. The
first part of the instrument, namely LC, consists of the sample introduction
system and the analytical column. The sample is flushed through the analytical
column with a flow of mobile phase, and the analytes are separated in the column
based on different interactions with the stationary phase. The challenge of
coupling LC to MS is transferring a liquid sample from the analytical column to
the mass analyzer in ionized gas form. This challenge has led to the development
of various ionization techniques. Electrospray ionization (ESI) is a mild
ionization technique that is suitable for the ionization of polar compounds, while
atmospheric pressure chemical ionization (APCI) and photoionization (APPI) are
used to ionize non-polar compounds. Once the charged analytes arrive at the
mass analyzer, their speed or trajectory is altered by applying an electric or
magnetic field that separates ions based on their specific mass-to-charge ratios.
Various mass analyzers, such as quadrupoles, ion trap and time-of-flight, are
available for this purpose, and they can be combined to add further stages of mass
analysis and to improve selectivity. For instance, a triple quadrupole MS system
consists of three sets of quadrupoles in series, with the first and third acting as
mass analyzers and the second serving as a collision cell to fragment the analytes
[186].
Solid phase extraction (SPE) is commonly used to concentrate the analytes and
remove matrix compounds. SPE can be manually performed prior to analysis (off-
line SPE) or integrated as a part of the LC run (on-line SPE). In an on-line SPE
analysis, the SPE column precedes the analytical column in an LC system [186].
In Paper I, the samples were run on a Thermo TSQ Quantum Ultra EMR triple
quadrupole instrument (Thermo Fisher Scientific) and ionized with heated ESI
and APPI. A Thermo Quantiva triple quadrupole instrument (Thermo Fisher
Scientific) with heated ESI was used in Papers III and IV. Deuterated or 13C-
labeled compounds were used as internal standards for quantification.
Inductively coupled plasma-mass spectrometry (ICP-MS)
ICP-MS is the most powerful tool for analyzing trace metals in a sample. ICP
consists of copper coil, which at high voltage, excites argon to form plasma. The
sample is passed through a nebulizer, turning it into a fine aerosol. The aerosol is
then introduced to the argon plasma. This fully decomposes the sample into its
constituent atoms, which are subsequently excited and turned into ions by the
plasma. The ions are analyzed in a mass analyzer similar to that described for LC-
MS analysis. A common challenge with this technique is the possible formation
of interfering cluster ions from argon and/or sample species that have masses
similar to those of the target analytes. This can be corrected by either calculating
37
the abundance of the other isotope forms of the analytes, using a high-resolution
analyzer (i.e. sector field instrument) or applying triple quadrupole MS. [183]
In Paper III, metals were analyzed by a commercial lab and nitrogen digestion
was used to extract the metals due to particulate matter formation after the
storage of the samples. For Paper IV, diluted, fresh samples were run directly
on an Agilent 8900 triple quadrupole ICP-MS (Santa Clara, CA, USA) in single
quadrupole mode.
Adsorption results
The research covered in this thesis obtained different types of adsorption data,
more specifically, high-concentration methylene blue adsorption and low-
concentration wastewater adsorption data. Analyses of these data identified
different trends, which are discussed further in the following sections.
Methylene blue removal with hydrochars
Methylene blue adsorption, conducted in Paper II, provided the adsorption
capacities of the studied hydrochars (Figure 18). Biosludge hydrochars had
superior adsorption capacities (42 – 68 mg/l) compared to the other materials (7
– 41 mg/l). Carbonization temperature was also found to substantially influence
removal. The hydrochars prepared at 260ºC showed lower removal (7 and 42
mg/l) than hydrochars produced at 180 ºC and 220ºC (31 – 68 mg/l). The
observed adsorption capacities were similar to what has previously been reported
for bio- and hydrochars [166,187,188]. Commercial activated carbons may have
adsorption capacities as high as a thousand milligrams of methylene blue per
gram of carbon [189], but lower capacities (9.81 – 588 mg/g) have been reported
elsewhere [176]. Activated carbons prepared from waste materials have
demonstrated adsorption capacities that are within a similar range (0.84 - 486
mg/g) [176].
Langmuir and Freundlich models were fitted to the obtained data, and good fits
(R2 > 0.944) were obtained for all materials except horse manure hydrochar
produced at 260 ºC. Horse manure treated at 180 and 220ºC and biosludge
treated at 260ºC had the best fit with the Freundlich model, while the rest of the
materials showed better fits with the Langmuir model. For all models (except for
horse manure treated at 220ºC), Freundlich model nonlinearity n > 1, which
suggests favorable adsorption, i.e. efficient removal of an adsorbate at lower
starting concentrations.
38
Figure 18. Methylene blue adsorption isotherms (Paper II) for materials carbonized at 220°C and
for temperature series of biosludge and horse manure hydrochars. The insert in the upper right
corner shows the water samples after the isotherm tests for each sampling point (from left to right
starting concentrations were 0, 10, 40, 60, 80, 120, 160 and 200 mg/l).
Organic compound and metal removal with hydrochars
The research presented in Paper I used a wide range of organic compounds with
different properties as adsorbates. Bisphenol A and diclofenac were nearly
completely removed by all of the hydrochars, while more than 50% of
ciprofloxacin, octhilinone, diphenhydramine and triclosan were removed by rice
husks and/or horse manure hydrochars (and olive waste hydrochar in the case of
octhilinone, Figure 19). All of the studied hydrochars showed removal
efficiencies under 50% for the rest of the tested compounds. Rice husk and
horse manure hydrochars were clearly more efficient in removing contaminants
39
compared to olive and tomato waste hydrochars. The removal occurred before
one minute of contact time had passed. In contrast to the observations in Paper
I, the removal of different compounds did not vary much between different
hydrochars in Paper III. For instance, Paper I reported trimethoprim removal
rates between 0% and 50%, while materials carbonized at 220ºC (the same
temperature that was used in Paper I) removed between 34% and 56% of the
trimethoprim from landfill leachate (Paper III, Figure 20). On the other hand,
HTC treatment temperature clearly influenced trimethoprim removal, as removal
rates decreased to less than 11% at the highest tested carbonization temperature.
Fluconazole and PFOA removals were low (<20%) in all cases.
Figure 19. Removal efficiencies of ten organic contaminants from water with four different
hydrochar precursor materials (Paper I). Tube wall tests were carried out without adsorbent
material to assess how the tube wall contributed to removal.
Arsenic and zinc removal decreased as HTC treatment temperature increased,
while copper removal was insignificantly affected by HTC treatment temperature
(Figure 20). Sewage sludge hydrochars were the only tested material that could
remove arsenic. When compared to commercial activated carbons, most of the
studied hydrochars removed copper with similar efficiencies, but removed zinc
more efficiently. The observed negative removal of arsenic may be related to a
measurement artifact or could be explained by the release of As from activated
carbon [190,191]. Similarly, the negative removals of fluconazole may have been
caused by measurement artifacts (ion signal suppression in LC-MS analysis due
to the disturbance of water matrix compounds during ionization) [192]. The
fluconazole signal was suppressed in samples treated with hydrochars, which
increases measurement uncertainty and variations in the data (even though 13C-
labeled fluconazole was used as internal standard).
40
Figure 20. Removal of organic and inorganic contaminants (Paper III) by hydrochars and
commercial activated carbon (AC).
Most of the organic contaminants studied in Papers I and IV were insufficiently
removed by the hydrochars. In addition, the removals presented here are difficult
to compare with what has been reported in other studies due to differences in the
water matrices and initial contaminant concentrations. For this reason, it was
important to investigate how the water matrix and contaminant concentrations
affect removal. This is discussed in the following sections.
Influence of the water matrix
To further study how different components (ions, dissolved organic carbon
(DOC)) of wastewater influence removal, contaminant removal was studied in
pure water and in solutions containing either humic acid (model for DOC) or ions.
The DOC and ion concentrations, as well as ion speciation, were adjusted to the
levels that had been measured in landfill leachate. The water matrix had a clear
41
influence on the removal of adsorbates (Paper III), with removal efficiencies of
the studied adsorbates often highest when landfill leachate served as the medium
(as compared to pure water, the synthetic humic acid solution or the synthetic
ion-containing solution). In most cases, adsorption tests that separately
investigated the influence of matrix components, i.e. DOC and ions, could not
explain why the landfill leachate matrix showed better removal efficiencies. Only
trimethoprim removal was similar in the synthetic DOC solution and the landfill
leachate. A synthetic solution containing both humic acid and ions was prepared,
but agglomerates formed immediately and they were removed by filtration with
the same filter size that is used to remove hydrochars from the solution. The
prepared solution differed from landfill leachate in terms of color and formation
of agglomerates, which is likely due to differences in the composition of DOC.
Only filtration removed 85%, 43% and 27% of copper, zinc, and arsenic,
respectively, from the synthetic humic acid and ion-containing solution, which
suggests that the studied compounds may adsorb to the fine particles that formed
in the solution.
Another interesting observation was that hydrochars were leaching organic
species into the water, which further increased DOC concentrations in the water
during the adsorption tests. This was confirmed by measuring the DOC content
(analysis carried out by ALS Scandinavia) of the samples before and after
hydrochar treatment. In the case of horse manure hydrochar (220°C), the DOC
concentration doubled (from 41.0 ± 3.6 mg/l to 84 ± 4.3 mg/l after treatment)
once the hydrochar was added to the landfill leachate. This increase in water
matrix complexity could translate to various possible fates for the adsorbate
compounds. For instance, particles that adsorb contaminants may form in the
water, which, in turn, increases removal efficiency. On the other hand, if the
adsorbates adsorbed on dissolved carbon species instead of the adsorbents, the
adsorbate remains in the water phase. Due to these intricacies, the removal of
contaminants from complex water matrices is only partly explained by adsorption
to adsorbents provided by the hydrochar surface, as other processes also have a
significant role in the removal process. Different hydrochars may also leach
different quantities and types of organic species, but this was not covered in the
research underlying this thesis. This should be further addressed in the future
studies.
The release of DOC into the water is very problematic in the context of water
treatment applications. This is because adsorption is often considered as one of
the last purification steps in the water treatment processes [16], providing “fine
polishing” after excessive amounts of organic species have already been removed.
Therefore, contamination with organic carbon species is problematic for the
process. On the other hand, applying adsorption at earlier stages of the treatment
would also not provide a satisfactory result, as the adsorbents would likely
42
saturate faster at this point than in the end of the process due to the larger amount
of wastewater contaminants at the beginning of the process. Previous studies of
different washing approaches have shown these procedures to benefit removals
by hydro- and biochars [180,193]. The leaching of organic matter from the
studied biochar decreased with washing, but was nevertheless not entirely
eliminated [180].
Influence of concentration
The starting concentrations of the organic contaminants used in Papers I and
III (10 µg/l and 800 ng/l) are environmentally relevant but exceptionally low
compared to most previously published adsorption work [151]. To study the
possible effect of concentration, kinetic adsorption tests were conducted at higher
concentrations. Three different trimethoprim, fluconazole and PFOA
concentrations were used (0.1, 1 and 10 mg/l) in a pure water (Figure 21). The
experiment used horse manure hydrochar produced at 220ºC so that the results
could be compared with findings from Papers I and III. The results showed that
the removal efficiencies for trimethoprim and PFOA further increased when
higher starting concentrations were applied, which implies that the hydrochar is
still not saturated after the highest tested contaminant concentration (10 mg/l)
(Figure 21). Fluconazole removal at the higher concentrations was even lower
than what was observed in the low-concentration tests in Papers I and III, but
saturation was still not reached. This means that even higher starting
concentrations of contaminants should be used to establish the maximum
capacity of the tested hydrochars. The studied materials showed poor removal
efficiencies for the studied compounds over the tested concentration range, which
suggests that the isotherm shape of the studied compounds is linear.
Figure 21. Removal efficiencies of different concentrations (in 0.0008, 0.01, 0.1, 1 and 10 mg/l) of
trimethoprim, fluconazole and PFOA from pure water by horse manure hydrochar.
43
Organic compound and metal removal with activated carbons
The removal of trimethoprim, fluconazole and PFOA from landfill leachate by
hydrochars and chemically activated carbons was studied in Papers III and IV,
respectively. The hydrochars showed low removal efficiencies for these
compounds, but these removals did increase after chemical activation (Figure
22). This is especially relevant when considering that the hydrochar:water ratio
was ten times higher than the activated carbon dosage. Horse manure carbons
showed high removal efficiencies for fluconazole and trimethoprim, while PFOA
removal varied between 26 and 81%, with the phosphoric acid activated materials
showing the lowest removal efficiencies. Neither of the activation methods was
able to influence copper, zinc or arsenic removal, with the removals for these
compounds not varying much between different activation methods (90-96%, 77-
83% and > 1%, respectively). The control sample (which was only subjected to
heat treatment) showed slightly lower removal efficiencies of copper and zinc (83
and 67%, respectively). On the other hand, sewage sludge activated carbons
showed even lower removal efficiencies for all of the tested contaminants - except
arsenic, of which 44% was removed by the KOH activated product (this material
lost most of its carbon content during activation, leaving mainly mineral matter).
In contrast to the hydrochars, which leached DOC, the chemically activated
carbons were able to remove DOC from the leachate. Mesoporous 1:4 H3PO4
activated horse manure carbon removed nearly half of the DOC in the leachate
(untreated 41.0 ± 3.6 mg/l vs treated 23.3 ± 2.4 mg/l), while micro/mesoporous
1:4 KOH activated horse manure removed less DOC (the final concentration was
33.7 ± 2.6 mg/l). This difference may be explained by the higher surface area of
H3PO4 activated materials relative to KOH activated materials (1140 vs. 730
m3/g), but could also stem from different pore size distributions.
Based on the kinetic data, the control horse manure hydrochar seemed to perform
equally well as the chemically activated carbons, suggesting that properties
relevant for wastewater treatment are primarily a result of heat treatment.
However, further studies with column tests or adsorption isotherms should be
performed to evaluate the adsorption capacities of these materials.
44
Figure 22. Removal of organic compounds from landfill leachate with hydrochar (Paper I) and
chemically activated samples (Paper IV). It should be noted that tests with hydrochar were carried
out at carbon dosage 5 g/l, while for rest of the tests dosage was only 0.5 g/l.
What is a realistic approach for studying adsorption?
The previous sections describe the removal of methylene blue and
environmentally relevant wastewater contaminants (organic and inorganic)
using the same set of hydrochars (Papers I and III). The careful assessment of
the presented methylene blue isotherms and kinetic data for organic and
inorganic contaminants can yield different conclusions. The hydrochar precursor
materials varied substantially in their methylene blue removal capacities. These
trends were not observed in the later kinetic studies (Paper III) with organic and
inorganic adsorbates. The only consistent finding between these two studies was
that hydrochars prepared at 260ºC, when compared to the materials prepared at
180ºC and 220ºC, are characterized by low removal of adsorbates. Furthermore,
methylene blue tests showed high removal efficiencies before saturation (i.e. the
isotherm increases steeply at low equilibrium concentration), while trimethoprim
and PFOA removal showed more linear behavior, which implies that these
compounds were not effectively removed over the studied concentration range
(0.8 µg/l – 10 mg/l). Therefore, the use of dyes (such as methylene blue) as
“model adsorbates” may fail to represent adsorption of contaminants present in
real wastewater. As such, predictions based on methylene blue adsorption tests
may not be valid for certain adsorbates and testing conditions. Methylene blue
adsorption continues to be a commonly used adsorption method [176,177] and,
for this reason, researchers should exercise caution when using methylene blue
adsorption capacities as an indication of adsorption affinity. The advantages of
methylene blue testing is that it is a relatively simple analysis and there is a high
abundance of methylene blue capacity data in the literature, which facilitates
comparisons between studies. However, this is not the case for other
environmental contaminants because a diverse set of compounds are categorized
as contaminants and most of these are generally less studied. In addition to using
45
more realistic adsorbates, it is also important to consider performing tests in a
realistic water matrix instead of pure water [194].
For practical applications, isotherm shape is equally important as maximum
capacity because it demonstrates removal at lower concentrations. This is
illustrated in Figure 23 through curves A, B and C. The maximum capacity follows
the order A>B>C, even though curve C has the highest capacity at lower
concentrations (q1), leading to the lowest equilibrium concentration in water (C1).
Therefore, the removal efficiencies (i.e. adsorbed amount:total amount-ratio) for
the curves are the complete opposite of the maximum capacities before the
materials are saturated. A curve that has a steep increase at low concentrations
depicts an effective adsorbent which provides high removal efficiencies until the
saturation point is reached. On the other hand, systems with linear isotherms (or
gradually increasing) are characterized by inefficient removal at lower
concentrations even though they may show high maximum capacity.
Furthermore, since isotherm data are most commonly collected in the mg/l
range, data from lower concentration ranges (µg/l – ng/l) are not often tested.
The danger of this approach is that the model created from the gathered data may
be extended to concentration ranges for which no data exist.
Figure 23. Isotherms A, B and C represent different isotherm shapes. A is linear (Freundlich
isotherm n = 1) and B and C represent different curvatures (n>1, nC>nB)
46
47
Adsorbent and adsorbate properties and
adsorption
Adsorption is a process involving two components – the adsorbent and adsorbate
- and the chemical properties of both dictate the interactions that occur on the
surface of an adsorbent particle. Knowledge of desired surface properties is
crucial to the efficient development of new materials. The interaction between the
adsorbent and adsorbate can complicate the development process, as the desired
surface properties depend on the targeted adsorbate(s). This section discusses the
roles of the adsorbent and adsorbate.
How are adsorbent and adsorbate roles studied?
As shown in the previous chapter, contaminant removal strongly depends on both
the adsorbate and adsorbent as well as other variables such as the water matrix.
Adsorption tests are typically carried out with one or a few adsorbents that are
evaluated using one or a few adsorbent materials [151]. Adsorbent – adsorbate
interactions are often discussed using measurements of chemical bonding
changes (for instance by XPS) on the surface of the adsorbent [90,187] or by
studying adsorption thermodynamics [90,166,195] in specific adsorbent-
adsorbate systems. Comparisons of the dynamics between different adsorbates
and adsorbents are necessary for a broad understanding of the affinities of
different contaminants to different adsorbent surfaces. Due to inconsistencies
between different studies (different carbonization and adsorption parameters,
insufficient reporting of the studied systems), reliable comparisons are difficult
[151]; nevertheless, several extensive reviews summarize the current data
regarding adsorption to low-cost adsorbents and, as such, have been able to draw
some conclusions on the affinities of different low-cost adsorbents [129,196]. On
the other hand, the virtually unlimited amount of possible adsorbates – combined
with the overrepresentation of dye adsorption data and underrepresentation of
other environmental contaminant data [151,197] – makes comparisons between
different environmental contaminant studies even harder.
Another approach for investigating adsorption and its connection to adsorbent
and adsorbate properties entails extensive data collection and comprehensive
data evaluation. This means that the studies should cover several adsorbent
materials with different properties and/or a wide range of adsorbates.
Furthermore, uni- and multivariate regression methods could be used to
summarize and visualize the collected data, as well as confirm any identified
trends. Multivariate data analysis is used to study the relationships between a
large number of variables and observations which would be too work-intensive
for univariate analysis methods. Principal component analysis (PCA) is a
48
commonly used type of multivariate analysis. In PCA, variations in the data are
projected onto a plane, with the direction of the highest variation termed the first
principal component (PC1), the direction of the second highest variation termed
the second principal component, and so on [198]. This method is useful for
investigating grouping and trends in the data, but it does not directly show
correlations between grouped variables. [199]
In Paper I, the role of adsorbate properties was studied by linear regression and
the grouping of different hydrochars was studied by using the DRIFTS spectra as
an input for PCA. In Papers III and IV, the correlations between XPS and BET
data and adsorption data were studied by linear regression.
Hydrochar and adsorbate interactions
Several adsorbent materials and adsorbates were studied in Papers III and IV to
allow for comparisons between these materials in terms of contaminant removal.
Paper I focused on the effect of molecular properties on adsorption. Water
solubility and molecular weight of the adsorbate did not affect adsorption in the
studied system, but the log Kow value of an adsorbate was found to be positively
correlated with removal (Figure 24). Log Kow describes the tendency of a
compound to partition between the hydrophobic phase (octanol) and hydrophilic
phase (water), with a high log Kow value reflecting high hydrophobicity.
Statistically significant trends were confirmed (p < 0.05) for rice husks, tomato
and olive waste hydrochars, with all of these materials showing R2 > 0.48 and
relatively high line steepness (0.30 – 0.34). On the other hand, horse manure
hydrochar was less selective towards adsorbate log Kow, since the fit was
statistically insignificant (p = 0.09) and showed a more gradual slope and low R2
when compared to results for the other materials. This finding demonstrates that
adsorbate properties influence adsorption, but also that different materials have
different selectivities.
49
Figure 24. Linear regression of adsorption and log Kow of the adsorbate.
The correlations between maximum methylene blue adsorption capacities
(Paper II), contaminant removal efficiencies (Paper III) and hydrochar
properties (Paper II) were further explored by linear regression (Figure 25).
Total carbon and aromatic/graphitic carbon contents in hydrochars were found
to be negatively correlated with trimethoprim, fluconazole, Cu and Zn removal,
while total oxygen content and oxygen surface functionalities were positively
correlated with these species. This was an expected observation since positively-
charged metal ions may be adsorbed on the surface by electrostatic interactions
with negatively-charged oxygen functionalities. Trimethoprim and fluconazole
can interact with – and bind to - oxygen functionalities via hydrogen bonding.
Moreover, removal efficiencies of PFOA and As were correlated with the Al, P and
Fe contents in the hydrochars. The presence of iron may facilitate As removal as
previous studies have shown iron to precipitate As [200,201]. A highly significant
correlation between PFOA removal and surface area was also identified. None of
the studied hydrochar properties were found to be correlated with maximum
methylene blue adsorption capacity.
When discussing Cu, As, PFOA and fluconazole removal, it is important to keep
in mind that there was not much variation in the removal efficiencies of these
compounds between different hydrochars. Therefore, a significant correlation
between a hydrochar property and a certain adsorbate will not necessarily have a
large influence on removal efficiency.
50
Figure 25. Summary of linear correlations found for the removal of contaminants by hydrochars
(Papers II and III) and chemically activated carbons (Paper IV).
Activated carbons and adsorbate interactions
The correlations between activated carbon surface properties and contaminant
removal efficiencies (XPS elemental analysis and carbon lines; Paper IV) was
also examined by linear regression analysis (Figure 25). High total carbon,
aromatic/graphitic carbon, oxygen containing functionalities (especially –OH
groups) and the presence of aromatic structures were found to positively correlate
with the removal efficiencies of organic contaminants and Cu. This may be due to
the presence of functionalities that enable adsorbates to bind through
electrostatic interactions, hydrogen bonding and different π-interactions. Both
total oxygen and metal content showed a highly negative correlation with
trimethoprim, fluconazole and Cu removal. Interestingly, a completely opposite
correlation pattern was found for As, suggesting that high content of inorganic
species is connected with As removal. Zn removal was correlated only with the
carbonate XPS line, while no other correlations were found. This may be
explained by a small amount of variation in the Zn removal data. Micropore
volume and surface area were both found to significantly influence PFOA
removal, while a linear relationship could not be established for other
51
compounds. On the other hand, trimethoprim, fluconazole and Cu showed an
exponential relationship with surface area; therefore, this property should be
considered an important variable for the removal of these compounds.
What drives adsorption/removal?
The role of adsorbate surface composition and porosity in the adsorption of
contaminants was discussed in detail in the previous sections. The data from
Paper I suggest that log Kow affected removal efficiency in the studied system,
while water solubility and molecular weight exerted non-significant effects on
adsorbate removal in the studied system. In previous studies, adsorbate
hydrophobicity [130], molecular size, boiling point as well as π-polarity and
hydrogen-bonding acceptability parameters [202] have been connected to
adsorption. Furthermore, the data from Papers III and IV suggest that oxygen
functionalities on the adsorbent surface (both activated and non-activated)
promoted the removal of most of the adsorbates, while As removal was only
achieved by using sewage sludge-based materials with relatively high inorganic
(Si, Al, Fe) contents. Also, additional adsorbate-specific interactions were
discussed, which highlights differences in the removal of each type of adsorbate.
However, low variation in the removal of some of the studied adsorbates (such as
Zn removal by activated carbons) means that the discussed findings are not
conclusive.
Different surface features affected adsorption when hydrochars and activated
carbons were compared. In hydrochars, high carbon content was found to be
negatively correlated with contaminant removal, whereas the same property
promoted contaminant removal in activated carbons. In addition, total oxygen,
metal and P contents in hydrochars were positively correlated with removal
efficiencies, while these contents negatively affected removal in activated carbons
(except for As). These differences could be explained by the fact that there was no
actual causality between these features and removal efficiency, but that the
correlation was rather caused by co-variation with other variables. For instance,
C, O and total metal contents were correlated in both hydrochars and activated
carbons, which may lead to co-variations in the studied data.
Some differences can be anticipated due to contrasting features among the tested
materials; for instance, elemental composition and porosity were very different
between these material types, which may lead to different removal mechanisms.
When considering the small surface areas (<1 – 30 m2/g) and high amounts of
oxygen-containing surface functionalities reported for hydrochars, the
mechanism of removal may differ from that of chemically activated carbons,
which had higher surface areas (55 - 1200 m2/g) and different surface
functionalities (e.g. higher inorganic content, less oxygen functionalities).
52
Therefore, it could be concluded that, in most cases, surface area is a less
important feature for hydrochars than surface functionalities due to the low
surface areas of these materials. On the other hand, swelling of the hydrochar in
water may expose more internal pore structures (which are not measured by BET
surface area measurement in dry materials) and therefore increase the potential
binding sites [203,204]. Correspondingly, the high removals noted for activated
carbons can be partly explained by their high surface areas. If hydrochar
adsorption capacity is expressed as adsorption per unit area, the removal can be
very high. As an example, the maximum methylene blue adsorption capacity
measured for horse manure hydrochar (treated at 180ºC) in Paper II was 35
mg/g hydrochar (surface area 3.2 m2/g), which corresponds to 11 mg/m2. For an
activated carbon with a surface area of 1000 m2/g, the adsorption capacity would
have to be 11 000 mg/g activated carbon (i.e. 11 g/g activated carbon) to achieve
similar adsorbate coverage on the surface. This is more than ten times the
typically reported capacities [176,189], implying that more than ten methylene
blue molecules occupy the same surface area on saturated hydrochar surface as
one molecule occupies on saturated activated carbon surface. This notable
difference may be due to the enhanced adsorption provided by functional groups
on the hydrochar surface. On the other hand, one might speculate that methylene
blue adsorption on small pore sizes may be sterically hindered, which would
reduce the molecular coverage per surface area.
53
Conclusions and future work
In the research underlying this thesis, different precursor materials, namely
tomato waste, olive waste, rice husks, horse manure, sewage sludge from a
municipal wastewater treatment plant, and bio- and fiber sludges from a paper
mill, were used to produce hydrochars and activated carbons that were tested as
wastewater adsorbents. Hydrochars showed low removal of most of the studied
organic contaminants, while these materials were able to remove metals at a
similar, or higher, level than commercial activated carbons (Papers I and III).
The removals noted for rice husks, olive and tomato waste were confirmed to
depend on adsorbate log Kow (Paper I). Hydrochar production temperature and
the precursor material were also found to affect maximum methylene blue
adsorption capacity (Paper II). Furthermore, HTC treatment temperature was
identified as the most important variable for the removal of organic compounds,
which may be explained by the decrease in oxygen functional groups noted at high
HTC treatment temperatures (Paper II and III).
The chemical activation of hydrochars dramatically increased the ability of the
tested materials to remove organic compounds and metals (Paper IV). The
chemical activation methods used in this study, i.e. KOH and H3PO4 activation,
yielded carbons with different porosities and compositions. Horse manure
hydrochars responded better to chemical activation than sewage sludge
hydrochars, which generated mineral-rich products low in carbon when treated
with KOH. Moreover, the control thermal treatment, without any activating
agent, also improved removal efficiencies substantially. Both hydrochars and
chemically activated carbons had surface features (such as the oxygen functional
groups and inorganic contents) that were correlated with the removals of certain
contaminants. These interactions were studied in leachate water, and further
studies should aim to investigate the same trends in other water matrices or with
different adsorbent materials. Additionally, only kinetic studies using
environmentally relevant adsorbate concentrations was covered in the research
underlying this thesis; hence, the adsorption capacities of the materials covered
in this thesis should be studied to provide conclusive evidence about their long-
term use.
The complex water matrix used in the research presented in this thesis was shown
to affect adsorption, but in the case of most compounds, a single water matrix
component could not explain the observed removal efficiency (Paper III).
Future studies should address competitive adsorption and the effects of diverse
water matrix components as most of the published work has evaluated a single
component in pure water [194]. Furthermore, relevant adsorbate compounds
and realistic adsorbate concentrations should be included in future research
54
[151]. Researchers should also take into account that the adsorption capacities
obtained from isotherm studies represent the best-case scenario at high
adsorbate concentrations and at equilibrium. In real-life applications,
wastewater, which has contaminant concentrations that are orders of magnitude
lower than what has been reported in other laboratory studies, is flushed through
a carbon column without reaching equilibrium. Therefore, column tests represent
adsorption capacity in a more realistic way.
If hydrochars are used for adsorption applications, washing is necessary to reduce
the leaching of organic components to the water and enhance contaminant
removal. The washing of hydrochars was found to be insufficient for preventing
leaching in the research covered in this thesis; for this reason, future studies
should concentrate on identifying environmentally sound washing methods that
can reduce leaching and increase removal efficiency. Also, the potential release of
trace contaminants present in hydrochars and activated carbons (such as heavy
metals, dioxins and polycyclic aromatic hydrocarbons [39,123,205]) into water
should be investigated, and the potential toxicity of the leached species should be
assessed.
Studies that strive to evaluate the technical, environmental and economic
feasibility of producing residue-based carbons should investigate different
modification and activation methods in pilot-scale experiments. Also life cycle
assessments of these processes should be included. Similarly, the feasibility of
integrating these materials to standard wastewater treatment processes should
also be investigated through large-scale studies.
Between-study comparisons have traditionally been difficult due to different
experimental parameters and a lack of information about experimental
conditions. To improve the potential for comparisons, future research should
include wider sets of adsorbents and/or adsorbates. This would increase the
amount of samples and researchers could more often apply the design of
experiments (DoE) approach to reduce the amount of experimental work. This
method allows several variables to be studied at the same time, and the resulting
data can be extended to models that will identify the optimal conditions for
getting a certain response, or responses. This approach has been employed for
studying the role of different adsorption parameters [206] and for optimizing
hydrochar and activated carbon properties [207,208].
The research covered in this thesis has highlighted that adsorbent surface
features (e.g. porosity, surface functionalities and the presence of metals) can
have varying effects on the adsorption of certain contaminants (Papers III and
IV). This finding suggests that future research could concentrate on producing
materials with different sets of binding sites. This could be achieved by generating
55
hydrochars and activated carbons from mixtures of different precursor materials.
Alternatively, the removal of contaminants could be improved by subjecting
wastewater to a mixture of different hydrochars and/or activated carbons.
56
57
Acknowledgements
Firstly, I would like to thank my main supervisor Stina Jansson for her endless
kindness, optimism and support. Thank you also for your generosity, especially
in terms of encouraging comments and useful tracked chances/comments in my
manuscripts and thesis (and of course, for the horse manure)! I also want to thank
my co-supervisor Jean-François Boily for all your help with surface
characterization, always having a slightly different perspective, and the good
discussions. I would like to extend my gratitude to my company mentors
Magnus Bergknut and Erik Rosenbaum for good collaboration and your
input to my research. Thank you also for making my internship at MTC such a
nice experience! I would also like to thank the Industrial Doctoral School and
MTC for their financial support of this project.
Many thanks go to my co-authors, Eva Weidemann, Jerker Fick and
Kenneth Latham, I have learned a lot from you about scientific writing and
science in general. I also want to thank Peter Haglund and Tomas Hedlund
for annual evaluation meetings and Patrik Andersson for being such a great
backup last time! Great thanks also goes to Anna Linusson Jonsson, Benkt
Wiklund and cohort -14 at the Industrial Doctoral School for all the fun and
educative times we spent at the courses and workshops.
I also want to thank the previous and current members of our research group,
Lisa, Eva, Qiuju, Mar, Ivan, Alex, Ken, Pierre, and Sofie, for all the
brainstorming, travelling and other fun activities. I also want to acknowledge
Andrey for running my XPS samples and being such a great office neighbor!
Nikki, thank you for running SEM on my samples. Special thanks goes to
Richard, who has always been a great help when I have had to deal with the LC-
MS. I would also like to thank Erik B for teaching me how to use ICP-MS and for
the support with the analysis. Vero, thank you for teaching me the secrets of HTC
and welcome to Umeå! Thanks Andrés for lending your HTC equipment. I also
thank (and apologize at the same time) Cathrin and Daniel for taking the sludge
samples for me! Lastly, I would like to thank Irina for all the help in preparing
the student labs, you really made everything so much easier! I would also like to
thank the previous and current members of the PhD council and FUR. I learned
a lot working with you, but also had fun every now and then!
I would like to thank MKL/TAP people for the good working environment, and
for making lunches and fikas so relaxing and fun! I would also like to acknowledge
the friends I made along the way. Kristin and Artur, I’m very glad that you were
the first friends I found in Umeå. Thank you for being good colleagues, but also
for all the other fun moments outside of the university, legendary memories!
58
Thank you Mar and Qiuju (again, but I think I cannot thank you enough) for
your mental support and thank you for all the nice activities we have been doing
together! I would also like to thank Jana for the endless amount of funny
moments and for bearing with my messiness and my yoga ball! Thank you
Sandra, João, and Christine for your friendship and many good moments
during the past years.
Kiitos Hanna ja Hanna kaikista laulu- juttelu- ja kahvituokioista! Kiitos
Hilkka, Sakari, Minna ja Marika ja muut perheenjäsenet kaikesta avusta ja
yhdessä vietetystä ajasta. Haluan myös kiittää Pohjois-Iitin nuorisoseuraa
sekä Matleenaa, Ilonaa ja Annaa vanhojen muistelusta, juoruilusta ja
enemmän tai vähemmän säännöllisistä näkemisistä. Kiitos Mervi postitse
tulleesta ekstraenergiasta!
Kiitokset tietysti myös saareen äitille, Marialle perheineen, Leilalle ja
Leenalle!
Kiitos Matti joustamisesta ja huolenpidosta erityisesti viimeisten kuukausien
aikana. Ja vielä enemmän kiitos viimeisistä viidestä vuodesta, on ollut tosi
mukavaa!
59
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Appendix
Figure A1. EDS maps spectra and elemental composition for activated horse manure carbons.
Figure A2. EDS maps spectra and elemental composition for activated horse manure carbons.
