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PHYTOREMEDIATION OF HEAVY METALS IN MINE
DRAINAGE USING TROPICAL AQUATIC PLANTS
100855B D.M.S.M. Dassanayake
100830U U.B. Prabhanga
100850F K.B.N. Pushpakumara
100836T R.P.S. Sampath
Degree of Bachelor of the Science of Engineering
Department of Earth Resources Engineering
University of Moratuwa
Sri Lanka
May 2015
PHYTOREMEDIATION OF HEAVY METALS IN MINE
DRAINAGE USING TROPICAL AQUATIC PLANTS
100855B D.M.S.M. Dassanayake
100830U U.B. Prabhanga
100850F K.B.N. Pushpakumara
100836T R.P.S. Sampath
Research Project Thesis submitted in partial fulfilment of the requirements for the
degree Bachelor of the Science of Engineering
Department of Earth Resources Engineering
University of Moratuwa
Sri lanka
May 2015
ii
Prepared by,
100855B D.M.S.M. Dassanayake
100830U U.B. Prabhanga
100850F K.B.N. Pushpakumara
100836T R.P.S. Sampath
Supervised by
Dr. C.L. Jayawardena, Mr. I.P. Senanayake
iii
DECLARATION
We declare that this is our own work and this thesis does not incorporate without
acknowledgement any material previously submitted for a Degree or Diploma in any
other University or institute of higher learning and to the best of our knowledge and
belief it does not contain any material previously published or written by another
person except where the acknowledgement is made in the text.
Also, we hereby grant to University of Moratuwa the non-exclusive right to reproduce
and distribute our thesis, in whole or in part in print electronic or other medium. We
retain the right to use this content in whole or part in future works (such as articles or
books)
………………………………… ……………………………..
DMSM Dassanayake KBN Pushpakumara
………………………………… ………………………………
UB Prabhanga RPS Sampath
iv
DECLARATION PAGE OF THE SUPERVISORS
We have supervised and accepted this thesis for the submission of the degree.
Signature of the supervisor:
Date:
Signature of the supervisor:
Date:
v
DEDICATION
We dedicate this work to our parents who have always been with us in every hurdle
we faced in our lives.
vi
ACKNOWLEDGEMENT
Our utmost gratitude is extended to Dr. C.L. Jayawardena and Mr. I.P. Senanayake
for being the project supervisors and supporting admonitory to us throughout the
research project. Without their guidance, encouragement and interactive
communication the research would not have been a success.
Our special appreciation goes to the final year research project coordinator of the
department of Earth Resources Engineering, Dr.H.M.R Premasiri for his kind
guidance in research project. We are also grateful to Mr. S. Weerawarnakula and Dr.
A.M.K.B. Abeysinghe for their advices, suggestions and constructive criticisms. We
are also thankful for Ms. B. S. Nanayakkara, Lecturer from the Department of Botany,
University of Peradeniya for enlightening us on the plants.
Our gratitude extends to Eng. P.V.A Hemalal for enlightening us on the Eppawala
Phosphate mining activities and facilitating with the field visit arrangements. Also, we
are thankful for the management of Eppawala Phosphate for enabling us to visit the
site.
We would like to thank Mrs. N. Zavahir, analytical chemist of the department, for her
immense support given under difficult circumstances. Mrs. M.W.P. Sandamali, the
technical officer and Mr. K.G. Prabhath, the Laboratory Attendant of the Analytical
and Geoenvironmental Engineering laboratory also thanked immensely. We would
like to thank all the unmentioned resource personnel and support received to
accomplish our research project successfully.
vii
ABSTRACT
Mine drainage is a major environmental problem associated with mining activities
throughout the world. Most of the time, Acid Mine Drainage (AMD) prevails as an
unsolved mystery, causing heaps of health and environmental problems, during the
mining and even after the mine has been abandoned, irrespectively to the degree of
reclamation process taken place.
There are numerous ways to treat hazardous mine drainage utilizing both active and
passive systems. The active systems require continuous addition of resources to
sustain the process while passive systems require relatively little resource input once
in operation. Hence, the modern waste management practices favour the use of
passive technologies due to the minimal maintenance required and the cost
/aftermaths involved. For an example, in most instances the wetlands are used as a
passive biological technique. The cost effectiveness and nature friendliness in the
wetlands have opened up new horizons for the researchers to explore broader
dimensions of the technique. The concept of floating wetlands is relatively naval
approach in remediating mine drainage where limited documented studies are
available.
This study, evaluate the potential of treating Acid Mine Drainage through
phytoremediation action of aquatic plants in natural floating wetlands. Four heavy
metals (namely Fe, Cu, Cd and Zn) abundant in mine drainage were chosen to
analyse, using solutions created under laboratory conditions to evaluate the most
applicable plant species for wetland construction to stimulate phytoremediation. The
results show that Eichhornia crassipes was the most efficient in extracting the heavy
metals under given conditions. Eichhornia crassipes plants showed considerable
survivability as a species throughout the laboratory tests even though it could not
handle variable pH levels mostly common in high Fe concentrated mine drainage.
Thus, the natural floating wetlands equipped with those two abundant plant species
can be preliminarily recommended to be deployed for phytoremdiation in local
mining environments with potential AMD issues.
Key Words: Acid mine drainage, passive treatment, phytoremediation, heavy metals,
floating wetlands
viii
TABLE OF CONTENTS
Declaration ................................................................................................................... iii
Declaration Page of the supervisors .............................................................................. iv
Dedication ...................................................................................................................... v
Acknowledgement ........................................................................................................ vi
Abstract ........................................................................................................................ vii
List of figures ................................................................................................................ xi
List of tables ................................................................................................................. xii
1. INTRODUCTION ................................................................................................. 1
1.1 Prologue .......................................................................................................... 1
1.2 Introduction ..................................................................................................... 1
1.3 Problem Statement .......................................................................................... 3
1.4 Aims And Objectives ...................................................................................... 4
1.5 Significance Of The Study .............................................................................. 4
2. LITERATURE REVIEW ...................................................................................... 5
2.1 Introduction ..................................................................................................... 5
2.2 Environmental Impacts Of Mining ................................................................. 5
2.2.1 Acid Mine Drainage (AMD) .................................................................... 6
2.2.2 Weathering of sulphide mine wastes ....................................................... 7
2.2.3 Acid producing reactions ......................................................................... 7
2.2.3.1 Pyrite ................................................................................................. 7
2.3 Common contaminants and environmental consequences ............................ 11
2.3.1 Threshold limits ..................................................................................... 13
2.3.1.1 Iron.................................................................................................. 13
2.3.1.2 Cadmium ........................................................................................ 14
2.3.1.3 Copper ............................................................................................ 14
2.3.1.4 Zn .................................................................................................... 14
2.4 AMD Treating methods ................................................................................ 15
2.4.1 Active abiotic remediation technologies ................................................ 16
2.4.2 Passive abiotic remediation technologies .............................................. 17
2.4.3 Active biological remediation technologies ........................................... 17
ix
2.4.4 Passive biological remediation technologies ......................................... 18
2.4.4.1 Aerobic wetlands ............................................................................ 18
2.4.4.2 Anaerobic wetlands/ compost bioreactors ...................................... 18
2.4.4.3 Permeable reactive barriers ............................................................ 18
2.4.4.4 Packed bed Iron-oxidising bioreactors ........................................... 19
2.4.4.5 Floating Wetlands ........................................................................... 19
2.4.5 Phytoremediation ................................................................................... 19
2.4.5.1 The techniques of phytoremediation .............................................. 21
2.4.5.1.1 Rhizofiltration ................................................................................... 21
2.4.5.1.2 Phytostabilisation .............................................................................. 21
2.4.5.1.3 Phytoaccumulation ............................................................................ 22
2.4.5.1.4 Phytovolatilization ............................................................................ 22
2.4.5.2 Selection of plants for the phytoremediation ................................. 22
3. METHODOLOGY .............................................................................................. 23
3.1 Plant selection for the phytoremediation ....................................................... 23
3.2 Sample preparation ........................................................................................ 25
3.2.1 Sample preparation for acid mine drainage ........................................... 25
3.2.2 Preparation of samples to check the capable concentration limits of the
plant.....................................................................................................................26
3.3 Investigation techniques ................................................................................ 27
3.3.1 Plantation process .................................................................................. 27
3.3.2 Equipment .............................................................................................. 29
3.3.3 Atomic Absorption Spectrophotometer (AAS) .................................. 31
3.3.4 Testing for Fe concentrations using UV visible spectrophotometer
(HACH DR 2800)) ........................................................................................... 34
4. RESULTS ............................................................................................................ 36
4.1 Fe ................................................................................................................... 36
4.2 Cd .................................................................................................................. 37
4.3 Cu .................................................................................................................. 38
5. DISCUSSION ...................................................................................................... 40
6. Conclusions and Recommendations .................................................................... 43
7. REFERENCES .................................................................................................... 44
x
ANNEX I ..................................................................................................................... 55
xi
LIST OF FIGURES
Figure 2.1 Pyrite oxidation mechanism (Banks et al, 1997).......................................... 8
Figure 2.2 AMD generation paPyrite oxidation mechanism (Banks et al, 1997)Pyrite
oxidation mechanism (Banks et al, 1997)thways (Banks et al, 1997) ................... 9
Figure 2.3 Biological and abiotic strategies for remediating acid mine drainage
waters(Johnson and Hallberg, 2005).................................................................... 16
Figure 2.4 Phytoremediation mechanisms (Source: Tangahu et al, 2011) .................. 20
Figure 3.1 Pistiastratiotes ............................................................................................. 24
Figure 3.2 Eichhorniacrassipes .................................................................................... 24
Figure 3.3 Drainage containers filled with equal volumes of prepared drainages ....... 28
Figure 3.4 After the plants were planted in the containers .......................................... 28
Figure 3.5 Micro pipette .............................................................................................. 29
Figure 3.6 SOLAAR Atomic Absorption Spectrometer .............................................. 30
Figure 3.7 The pH meter and the holder ...................................................................... 30
Figure 3.8 UV visible spectrophotometer .................................................................... 31
Figure 3.9 Calibration graph ........................................................................................ 33
Figure 4.1 Variations of the Fe2+
concentrations in the samples ................................. 37
Figure 4.2 Variations of the Cd2+
concentrations ........................................................ 38
Figure 4.3 Variations of the Cu2+
concentrations ........................................................ 39
xii
LIST OF TABLES
Table 3-1 Selected plants for analysis ......................................................................... 23
Table 3-2 Fe2+
ion concentrations for acid mine drainage ........................................... 25
Table 3-3 Fe2+
ion concentrations ................................................................................ 26
Table 3-4 Cu2+
ion concentrations ............................................................................... 27
Table 3-5 Cd2+
ion concentrations ............................................................................... 27
Table 3-6 Standard volumes required to make the calibration .................................... 32
Table 3-7 Fe2+
Concentrations ..................................................................................... 35
Table 4-1 Variations in the Fe2+
concentrations .......................................................... 36
Table 4-2 Variations in the Cd2+
concentrations .......................................................... 37
Table 4-3 Variations in the Cu2+
concentrations .......................................................... 38
1. INTRODUCTION
1.1 Prologue
The dawn of the mankind‘s rise as a formidable species is the era where the
possibility of mining the metals from earth had been discovered. Since then mining
has become an essential industry for the human civilization in every aspect. Through
all the leaps and bounds and the difficulties encountered the mining industry has
become the economic backbone for most of the developed countries, as a source of
wealth, energy and political power. Effective use of mineral resources that has been
extracted from the earth is of prime importance due to the growing competitiveness of
the industry and demands of the modern society. Consequently, hundreds of
thousands metric tonnes of ore deposits are being mined annually and their effects on
the environment may be extensive.
1.2 Introduction
Mining wastes and by-products are mostly hazardous to the environment as well as
the human beings (Hamilton, 2000; Lottermoser, 2010; Plumlee and Morman, 2011).
Often the referrals have been made to the health hazards experienced by the mining
personnel but the consequences that have been brought by the industry to the
environment have been neglected in most of the cases. The problems mining industry
has incurred are often outweighed by the profits it brings and with the improvements
in the production the wastes it created became enormous. Thus the world is at the
moment battling to find ways of remediating the land, water and air contaminated due
to extensive mining activities (Younger et al, 2002).
Mining affects air quality as the particulate matter are released in surface mining
when overburden is stripped from the site and stored or returned to the pit. When the
vegetation is also removed, the soil gets exposed causing particulates to become
airborne mainly through wind and road traffic. Mining also causes physical
disturbances to the landscape, creating eyesores such as waste-rock piles and open
pits. Many of the pre-mining surface features cannot be replaced after mining ceases.
Ground movements of the earth's surface due to the collapse of overlying strata into
voids created by underground mining (subsidence) can cause damage to buildings and
roads.
2
Water pollution caused by mining largely includes; acid mine drainage, metal
contamination, and increased sediment levels in streams. Sources can include active
or abandoned surface and underground mines, processing plants, waste-disposal areas,
haulage roads, or tailing ponds. Sediments, typically from increased soil erosion,
cause siltation or the smothering of streambeds. This siltation affects fisheries,
domestic water supply, irrigation, and other uses of water resources. Hence, attention
should be given to minimize the water pollution and to remediate the environment in
concern if contamination is in place.
In general acid mine drainage (AMD) is the major effluent from the mines which
contaminate the surrounding water bodies significantly. AMD does not occur in every
mine and also the generation of AMD is not restricted to one particular phase of
mining, it can occur in the early mining stage as well as after the mine is abandoned,
ergo the threat which is posed on the echo systems is significant (Banks et al, 1997;
Akcil and Koldas 2006; Younger et al, 2002).
AMD has to be treated before it is to be added to the natural environment. Ergo
several techniques are utilized throughout the world to remediate AMD and its effect
on echo systems (Akcil and Koldas 2006). Generally the preferable axiom in the
context is, ―prevention is better than cure‖ (i.e. source control), but hardly the
prevention is achieved (Johnson and Hallberg, 2005). Due to the ineffectiveness of the
source control techniques, there exist numerous AMD remediation options which are
classified based on the techniques included in the processes.
The remediation of AMD is classified into two classes as active remediation and
passive remediation. The active treatment technologies require external energy to be
supplied in the remediation process, the energy may be supplied physically or
chemically to the system. The passive technologies utilize the natural sources of
energy like sun light, gravity, chemical and bio-chemical reactions within the system
(Kalin et al, 2006). The modern expectations are mostly tallying with the passive
biological treatment systems due to the low maintenance cost and the ability in
continuous operation (Johnson and Hallberg, 2005; Neculitaet al, 2007).
The passive biological treatments mostly include wetland systems (Eger and Wagner,
2003). The ―perfect‖ passive system would require no maintenance and operate
indefinitely. In the wetland treatment systems the general technique in remediating the
3
AMD is the phytoremediation process (Sheoran and Sheoran, 2006).
Phytoremediation comes in several forms. Phytoextraction removes metals or
organics from soils by accumulating them in the biomass of plants. Phytodegradation,
or phytotransformation, is the use of plants to uptake, store and degrade organic
pollutants, rhizofiltration involves the removal of pollutants from aqueous sources by
plant roots. Phytostabilization reduces the bioavailability of pollutants by
immobilizing or binding them to the soil matrix, and phytovolatilization uses plants to
take pollutants from the growth matrix, transform them and release them into the
atmosphere (Salt et al, 1995; McIntyre, 2003; Raskin et al, 1997).
Use of phytoremediation techniques for decontamination of water bodies is well
established and considered as environmental friendly compared to the other
technologies available. A wetland is known as an area that holds water either
temporarily or permanently. Marshes, swamps, fens and bogs are natural water
retention ponds/wetlands that facilitate to filter and purify water, replenish and store
groundwater. Accordingly, constructed wetlands gain much popularity due to their
application on contaminated water ponds, which are common on many mining and
processing sites. Floating wetlands are an emerging variant of constructed wetlands,
which the wetlands plants are grown on floating structures on the ponds, instead of
the pond bottom. The floating concept avoids the shallow water depth requirement of
the conventional constructed wetlands, as the plants grow on surface while their root
systems deal with the contaminated water through Rhizofiltration. Hence, floating
wetlands represent a means of potentially improving the treatment performance of
conventional pond systems.
1.3 Problem Statement
Rapid developments in the Sri Lankan road network and construction industry since
the end of war in 2010, has generated a huge demand for rock aggregates. As a result,
industrial scale quarry sites are in operation almost every part of the island creating
enormous amount of employment opportunities as well as a significant environmental
distraction without much of a notice. Hence, in this particular study the main focus is
given to the contamination of water bodies due to the mining process, in Sri Lankan
context. The number of abandon quarry sites has also given attention where
4
applicable, as most of them are left over to develop large ponds of polluted water,
creating environmental and health hazards.
1.4 Aims And Objectives
The overarching aim of this research project is to assess the applicability of the
floating wetlands to reduce the pollutants in the water bodies associated with local
mining activities from a mining engineering perspective.
This project has been broken down into several specific, achievable objectives, which
include;
1. Selection of suitable plants to use as treatment media based on the
contaminant
2. To evaluate the heavy metal absorbance of the selected plant in the
laboratory scale
3. To establish the use of phytoremediation action of aquatic plants in a
treatment unit of mine drainage/waste water treatment plant
4. Evaluate the potential applicability of the process on natural ponds as a
preventive measure
1.5 Significance Of The Study
Pollution at quarries is mainly produced as a result of exploration, mining and
processing activities. The processes involved in quarrying causes a significant impact
on the onsite water resources due to the activities such as; drilling, blasting and
hauling operations. Siltation due to tailings, mixing of oils and explosive constituents
with the active streams adversely affects the local water resources, unnoticeably.
Most of the quarry sites in Sri Lanka are located in residential neighbourhoods.
Hence, minimizing the adverse effects on water bodies due to quarrying activities
should be a priority, even though it is of limited concern at present. Accordingly,
research into cost effective, environmental friendly engineering solutions for such
significant issues, is a necessity for the sustainable development of the natural
resources in Sri Lankan context.
5
2. LITERATURE REVIEW
2.1 Introduction
Throughout the whole process of mining (i.e. Before, during and after the operations
of mining) the environment can be affected (Salomons, 1995). The operations which
cause the major impacts include; vegetation clearance, infrastructure development,
exploration tracks, creation of large voids, destruction of natural habitats, emission of
harmful chemicals into the surrounding ecosystems (Allan, 1995).
Many authorities have imposed rules and regulations regarding the mining activities
to protect and preserve the natural habitat (Ledin and Pedersen, 1996). Although
Guidelines and regulations have been promulgated to protect the environment
throughout mining activities from start-up to site decommissioning, in particular, the
occurrence of acid mine drainage (AMD), due to oxidation of sulfide mineral wastes,
has become the major area of concern to many mining industries during operations
and after site decommissioning (Kuyucak, 1998).
The general constituents of mine drainage may be Fe, Sulphate ions, Zn and other
heavy metals which are site specific. The mining method used to extract the minerals
may also produce several other chemicals into the mine drainage making both the
mining method and area become a significant in defining the constituents in the mine
drainage(Akcil and Koldas, 2006).For an example in the former goldfields in
Australia, particularly in streams, a considerable mercury contamination can be
observed due to the fact that during 19th century mercury was used to extract placer
gold (Bycroft et al, 1982, Churchill et al, 2004).Many abandoned Cu, Pb, Zn and Sn
mine sites are characterized commonly by acid mine drainage which is a consequence
of mining activities involving sulfide minerals. The main sulfide mineral that causes
acid mine drainage is pyrite (Ashley and Lottermoser, 1999; Blowes et al, 2003).
2.2 Environmental Impacts Of Mining
Mining causes the concentrated ore to be exposed to the atmosphere. Hence the ore
starts to react with the atmospheric air as well as water which come into contact
(Lottermoser, 2010). The dissolution of minerals in water occurs as water comes in
contact with the minerals at mine sites.
6
The applications of water in mining activities are numerous. For dust suppression,
mineral processing, coal washing and hydrometallurgical extraction water is
extensively used. In order to fulfil the demand, water is mined from ground water
acquifers and surface water bodies. As a by-product of the mine dewatering process
also, water can come into the existence. During the mining operation dewatering is
required often due to the fact that open pits and underground mining operations
generally extend below the regional water table. To prevent underground workings
from flooding, when substantial ground water acquifers are encountered, some mines
have to pump more than 100000 litres per minute (Lottermoser, 2010). That is why
the constant dewatering is required throughout the mining cycle.
In the historical mine sites, mine water was discharged into the environment without a
control. In contrast, water is collected and discharged to settling ponds and tailing
dams at modern mine sites. Generally the volume of mine water generated at a mine
site exceeds the volume of solid wastes produced. Thus, in general terms dissolved
and particulate matters are encountered in abundance in mine waters. These waters
generated at mine sites carry dissolved matter to receiving water bodies, lakes,
streams or acquifers (Hedin et al, 1994).
Once the mine water has mixed with the water bodies, undesirable turbidity and
sedimentation can be formed, the temperatures may become altered and the plants and
animals may start to experience toxic effects due to the change in chemical
composition. In U.S.A. for an example, it has been estimated that 19300 km of stream
and 72000 ha of lakes and reservoirs have been seriously damaged by mine effluents
from abandoned coal and metal mines (Kleinmann, 1989).
2.2.1 Acid Mine Drainage (AMD)
In the earth‘s crust sulphide minerals exist as common minor constituents (Ronov and
Yaroshevsky, 1969), but in some geological environments they can be found in major
proportions. The availability of sulphides is found to be high in particular metallic ore
deposits like Cu, Pb, Zn, Ni, U and Fe (Dudka and Adriano, 1997; Lottermoser,
2010). When these highly sulphide containing metallic ores are mined, the sulphides
become exposed to oxygenated environment. The tailing‘s dams, waste rock dumps,
heap leach piles, run of mine, quarries and abandoned mines are the generally
encountered places where large volumes of sulphide minerals are exposed to the
7
environment, once the sulphides are exposed to the atmosphere or oxygenated water,
the oxidation process starts forming an acid water laden with sulphate, heavy metals
and metalloids (Lottermoser, 2010). Pyrite (FeS2) being the most common sulphide
mineral causes the largest and most devastating environmental problem faced by the
industry today (Karpenko and Norris, 2002; Salkield, 2012).
2.2.2 Weathering of sulphide mine wastes
In most silfidic mine wastes, silicates are the most common gangue mineral whereas
the sulphides represent the ore or gangue phases. In general the polymineralic
aggregates which form the sulfidic mine wastes contain a large spectrum of minerals
including oxides, hydroxides, phosphates, halides and carbonates apart from sulphides
and silicates (Lottermoser, 2010). That is why the mineralogy of sulfidic wastes and
ores is significantly heterogeneous and specific for each deposit. The spontaneous
initiation of a series of complex chemical weathering reactions can occur when the
mining operations expose sulfidic minerals to an oxidising environment.
The argument can be made that an individual mineral within a polymineralic
aggregate produces acid by generating H+ ions, participates in acid buffering reactions
through the consumption of H+ ions or does not generate or participate in acid
buffering, when the chemical weathering of the mineral within the polymineralic
aggregate is concerned. The following examples emphasise the above argument.
1. Degradation of pyrite, produces H+ ions making it an acid producing reaction
2. Weathering of calcite is an acid buffering reaction
3. Dissolution of quartz does not consume or generate any sort of acid
Thus the major most reaction of the lot would determine whether the material will
turn acidic and produce AMD (Lefebvreet al, 2001; Lottermoser, 2010; Blowes et al,
2003).
2.2.3 Acid producing reactions
2.2.3.1 Pyrite
The pyrite oxidation has been extensively studied. Pyrite can be found in almost all
types of geologic environments (Evangelou and Zhang, 1995; Keith et al, 2000).
8
When pyrite is exposed to oxygen, the oxidation of pyrite takes place (Rimstidt and
Vaughan, 2003). The oxidation which happens due to the presence of
microorganisms, is termed as a ―biotic process‖. Inorganic chemical oxidation which
occurs without the presence of microorganisms is termed as an ―abiotic process‖.
Both the direct oxidation (i.e. the oxidation caused by oxygen) and indirect oxidation
(i.e. the oxidation caused by oxygen and iron) together or separately results in biotic
and abiotic degradation (Evangelou and Zhang, 1995).
The indirect oxidization of pyrite is a result of iron‘s divalent and trivalent state. The
summary of pyrite oxidation mechanism is given in figure 2.1
Figure 2.1 Pyrite oxidation mechanism (Banks et al, 1997)
In the abiotic and biotic direct oxidation processes, oxygen directly oxidises pyrite,
FeS2 (s) + 7/2 O2 (g) + H2O (l) Fe2+
(aq) + 2SO42-
(aq) + 2H+ (aq) +
energy........................................................................................................................eq.1
However, the generally accepted Pyrite oxidation involves the indirect oxidation of
pyrite by oxygen and ferric (Fe3+
) ions. This indirect oxidation process occurs in three
interconnected steps.
FeS2 (s) + 7/2 O2 (g) + H2O (l) Fe2+
(aq) +2SO42-
(aq) +2H+(aq) + energy.…eq.2
Fe2+
(aq) + 1/4 O2 (g) + H+ (aq) Fe
3+ (aq) + 1/2 H2O (l) + energy………...…eq.3
Oxidation of Pyrite
Biotic Oxidation
Direct Oxidation;
oxidation by oxygene in the presence of microorganisms
Indirect Oxidation;
oxidation by oxygene and iron in the presence of
microorganisms
Abiotic Oxidation
Direct Oxidation;
oxidation by oxygene
Indirect Oxydation;
oxidation by oxygene and iron
9
FeS2 (s) + 14 Fe3+
(aq) + 8 H2O (l) 15Fe2+
(aq) + 2SO42-
(aq) + 16H+(aq) +
energy........................................................................................................................eq.4
(Evangelou and Zhang, 1995; Keith et al, 2000; Lottermoser, 2010).
The indirect pyrite oxidation process is thus exothermic. In neutral and alkaline
waters, the solubility of Fe3+ is very low. With increasing pH values the
concentration of dissolved Fe3+ decreases through the precipitation of ferric
hydroxides (Fe(OH)3) and ferric oxyhydroxides (FeOOH). If, for an example, the
partial neutralization reactions which occur due to the carbonate minerals, increase the
pH to be greater than 3, the following reactions will take place (Lottermoser, 2010).
Fe3+
(aq) + 3 H2O (l) Fe(OH)3 (s) +3H+(aq)................................................eq.5
Fe3+
(aq) + 2 H2O (l) FeOOH (s) + 3H+(aq)................................................eq.6
(Lottermoser, 2010; Banks et al, 1997).
Thus the reaction pathways for pyrite oxidation are illustrated in figure 2.2
Figure 2.2 AMD generation pathways (Banks et al, 1997)
Water is acidified significantly by the reaction given by equation 6 due to the release
of H+ ions through the precipitation of dissolved Fe
3+, but again when the acidity
increase lowering the pH value of the solution, more Fe3+
ions tend to be in the
FeS2+
O2
Fe2+
+S22+
+ O2
SO42-
+ Fe2+
+ FeS2
Fe3+
Fe (OH) 3
10
solution at the dissolved state, enabling the pyrite oxidation (Banks et al, 1997). As
the oxidation process of pyrite prevails, the pH value becomes lowered. The reaction
5 and 6 are termed as hydrolysis. The process in which dissolved cations react with
water molecules is called the ―hydrolysis‖ (Johnson and Hallberg, 2005). The
hydrolysis process causes the H+ concentration to rise through the precipitation of the
Fe3+
ions reducing the pH value of the solution. In contrary to the above phenomenon
it can be stated that the pH value governs the hydrolysis process. When the pH value
drops below 3, Fe3+
ions tend to remain in the dissolved state. Thus the precipitation
of Fe3+
occurs at higher pH values than 3 and the precipitation is often encountered as
a reddish yellow to yellowish-brown stain, gelateneous flocculent or sludge (Gan et
al, 2005) in AMD affected locations.
The following equation summarises the overall chemical reaction.
FeS2 (s) + 15/4 O2 (g) + 7/2 H2O (l) Fe(OH)3(s)+2H2SO4(aq)+ energy…eq.7
(Lottermoser, 2010).
Although the overall literature suggests these arguments regarding the oxidation of
pyrite, the precise reaction mechanism is still on debate (Nordstrom and Alpers,
1999).
The dissolution and the reaction rates of a particular set of particles is mostly
characterised by the particles‘ surface area (Kuechler and Noack, 2007). The
oxidation reactions occur on the surfaces of pyrite particles therefore the pyrite
oxidation is also a surface controlled reaction (Evangelou and Zhang, 1995). The
existence of the trace elements puts strain on the crystal structure and hinders the
resistance of pyrite to oxidation (Hutchison and Ellison, 1992). The reactivity of
pyrite-arsenic combination is high when oxidation is concerned with respect to pyrite-
nickel and pyrite-cobalt combinations (Lehner and Savage, 2008).
A galvanic cell is formed when there is direct physical contact between at least two
different sulphide minerals. Since sulfidic wastes generally contain other kinds of
sulphides apart from pyrite, the galvanic cell formation is inevitable and the sulphide
mineral with the lowest electrode potential is weathered more strongly whereas the
mineral with the highest electrode potentially is protected from oxidation during
weathering (Evangelou and Zhang, 1995; Hita et al, 2008).Ergo, one sulphide mineral
11
is leached preferentially over another and the sulphide minerals‘ oxidation occur
selectively (Abraitis et al, 2004).
The electrode potential is highest when pyrite is concerned in comparison with galena
and sphalerite. Sphalerite has the lowest electrode potential among the three common
sulphide minerals, pyrite, galena and sphalerite (Sato, 1992). Pyrite in direct contact
with other sulphides, because of this phenomenon, does not react as vigorously as it
does in isolation (Cruz et al, 2001).
2.3 Common contaminants and environmental consequences
As elucidated in the table 1, minute mineral inclusions and chemical impurities can be
in existence in pyrite as trace elements.
Table 2-1 Sulfide minerals and their chemical formula with minor and trace
element constituents (Vaughan and Craig, 1978)
Mineral Name Chemical
Formula
Minor & Trace Elements substitution
Arsenopyrite FeAsS
Bronite Cu3FeS4
Chalcocite Cu2S
Chalcopyrite CuFeS2 AG,As.Bi,Cd,Co,Cr,In,Mn,Mo,Ni,Pb,Sb,Se,Sn,T
i,V,Zn
Cinnabar HgS
Cobaltite CoAsS
Covellite CuS
Cubanite CuFe2S3
Enargite Cu3AsS4
Galena PbS Ag,As,Bi,Cd,Cu,Fe,Hg,Mn,Ni, Sb,Se,Sn,Ti,Zn
Mackinawite (Fe,Ni)9S8
Marcasite FeS2 As,Hg,Se,Sn,Ti,TlPb,V
Melnikovite Fe3S4
12
As depicted previously, the pyrite oxidation is exothermic. The heat generated by the
oxidation process is preferred by thermophilic bacteria. These thermophilic bacteria
utilize part of the generated heat for their metabolic processes. The abundance of
gangue minerals with poor heat conductivity and the physical confines of waste
dumps and tailing dams entrap most of the energy released as heat. Thus, the more the
reaction occurs, warmer gets the pyrite wastes. With each 10o C increase in
temperature, the oxidation rate of pyrite gets doubled (Smith and Huyck, 1999).
In every AMD environment, there exist numerous microorganisms like bacteria,
archaea, fungi, algae, yeast and protozoa. It doesn‘t matter whether the conditions are
aerobic or anaerobic, acidic or neutral, some of the microorganisms exists without
being compromised. Being isolated from the AMD environments Archaea, Eukarya
and bacteria exhibit diversity and include acidithiobacillus thiooxidans (i.e.
thiobacillus) (Kelly and Wood, 2000).
Certain bacteria which prefer acidic environments participate in the conversion of
Fe2+
to Fe3+. The oxidation of the sulphide minerals is amplified by these acidophilic
Millerite NiS
Molibdenite MoS2
Orpriment As2S3
Pentlandite (Fe,Ni)9S8
Phyrite FeS2 Ag,As,Au,Bi,Cd,Co,Ga,Ge,Hg,In,Mo,Ni,Pb,Sb,S
e,Sn,Ti,Tl,V
Pyrrhotite Fe1-xS Ag,As,Co,Cr,Cu,Mo,Ni,Pb,Se,Sn,V,Zn
Realgar AsS
Stibnite Sb2S3
Sphalerite ZnS Ag,As,Ba,Cu,Cd,Co,Cr,Fe,Ga,Ge,Hg,In,Mn,Mo,
Ni,Sb,Se,Sn,Ti,Tl,V
Tennantite (Cu,Fe)12As4
S13
Tetrahedrite (Cu,Fe)12Sb4
S13
Violarite FeNi2S4
13
bacteria and arshaea. In fact feeding on the energy exerted in the oxidation process,
these bacteria accelerate the rate of ferrous to ferric oxidation which is considerably a
rather slow process under acidic abiotic conditions (Singer and Stumm, 1970).
The inclusion of acid mine drainage not only pollutes the aquatic environment but
also the echo system which is dependent on the aquatic system is severely affected.
The cultivations which are done using the water from the lakes, streams or water
bodies that are contaminated also accumulate significant amounts of heavy metals and
trace elements present in the mine drainage initially, affecting the food chain causing
lots of health problems (Zhuang et al, 2009). Some poisonous chemicals like mercury,
cadmium and arsenic are also present in the mine drainage. These chemicals
accumulate on the living beings posing major health threats (Ogola et al,
2002).Arsenic contamination in water, especially groundwater, has been recognized
as a major problem of catastrophic proportions and ground water in most scenarios is
contaminated due to the mining activities (Choong et al,2007).
2.3.1 Threshold limits
The compositions of industrial effluents like mine drainage are always monitored by
the governing organizations to set standards and observe whether the given standards
are followed by the industry (Hespanhol and Prost, 1994).
According to the final output, the maximum tolerance limits of chemicals vary. For an
example, thetolerance limits for the discharge of industrial waste water in to inland
surface watersare different from thetolerance limits for industrial waste water
discharged on land for irrigation purpose (investsrilanka, 2011).
2.3.1.1 Iron
It is an essential element in human nutrition. The minimum daily requirement of iron
is ranged from about 10 to 50 mg/day (World Health Organization, 1988). Taste
thresholds of iron in water are 0.1 mg/l for ferrous irons and 0.2 mg/l ferric irons. Iron
in water exceeding these limits gives a bitter or an astringent taste. Water used in
industrial processes usually contain less than 0.2 mg/l iron. Black or Brown swaps
water may contain iron concentration of several mg/l in the presence or the absence of
dissolved oxygen. However, this form of iron has little effect on aquatic lives. The
current aquatic life standard is 1.0 mg/l based on toxic effects (Kumar and Puri,
14
2012). The permissible level of iron in water bodies from which house hold water
supply is 0.1 mg/l (World Health Organization, 1988).
2.3.1.2 Cadmium
Cadmium may function in or may be an etiological factor for various pathological
processes including testicular tumors, renal dysfunction, hypertension,
arteriosclerosis, growth inhibition, chronic diseases of old age, and cancer (Flick et al,
1971).The permissible level of cadmium in water bodies from which house hold water
supply is acquired,is 0.005 mg/l (World Health Organization, 1988). The bio-
accumulation capability of Cd is very high making it a dangerous chemical substance
to be encountered in water bodies even in minor concentrations (Bebbington et al,
1977).
2.3.1.3 Copper
Being a vital component in several enzyme systems, Copper can be considered as an
essential element. Although at high intakes, it can have toxic effects, intakes in minor
amounts, is necessary for human health (Fewtrell et al, 2001). Different drinking
water standards suggest different permissible levels. 1 mg/l is the standard Cu
concentration defined by the World Health Organization (1988), as permissible level
for drinking water.
2.3.1.4 Zn
Zinc is the 23rd most abundant element in the Earth's crust. The dominant ore is zinc
blende, also known as Sphalerite. Zinc is called an ‘essential trace element‗, since
very small amounts of zinc are necessary for human health (Rink, 2011). Even
though, similar to Cu, the elevated levels can cause severe health problems. Some
soils are heavily contaminated with zinc, and these are to be found in areas where zinc
has to be mined or refined. Some fish can accumulate zinc in their bodies, when they
live in zinc-contaminated waterways. When zinc enters the bodies of these fish, it is
able to bio magnify up the food chain (Canli and Atli, 2003). The permissible level of
Zn in water bodies consumed by humans is 5 mg/l (World Health Organization,
1988).
15
2.4 AMD Treating methods
If the generation of the mine drainage water is prevented or minimized, it will also
prevent the problems associated with it. The techniques which preclude the generation
of mine drainage are called as ‗source control‗measures.
By excluding either (or both) oxygen and water, it should be possible to prevent or
minimise AMD production, since both oxygen and water are required for the
formation of AMD. Flooding and sealing abandoned deep mines is one method of
achieving the exclusion. For an example, potential acid producing mine tailings were
disposed into underwater storage with the objective of preventing contact between the
minerals and dissolved oxygen (Li et al, 1997).
Some biocides have been also used in minimizing the AMD generation. However,
only short-term control of the problem can be achieved and repeated applications of
the chemicals were required in this method (Loos et al, 1989).
Given the practical difficulties involved often, the only alternative is to minimise the
impact that this polluting water has on receiving streams and rivers, and the wider
environment (Johnson and Hallberg, 2005). This has led to the application of
migration control measures. The migration control measures are comprised of
‘active‗and ‘passive‘ techniques. Based on the technologies which utilize biological
activities (namely biotic) and which don‘t (i.e. abiotic), the migration control
measures can be again classified as illustrated in Figure 2.3.
16
Figure 2.3 Biological and abiotic strategies for remediating acid mine drainage
waters(Johnson and Hallberg, 2005).
2.4.1 Active abiotic remediation technologies
Most common way, in which active abiotic remediation is done, is the addition of a
chemical-neutralising agent (Coulton et al,2003). Often the process would lead to
newer problems due to the fact that most active abiotic remediation measures create
precipitates or sludgy substances. Multiple-stepped addition of reagents accompanied
by pH control, can result in the selective removal of some AMD components such as
arsenic and molybdenum (Kuyucak, 2002). Various flocculating reagents can be used
to promote aggregation of precipitates, thereby aiding their removal in settling ponds
(Coulton et al,2003). For instance, addition of an alkaline material to AMD, in an
active abiotic process, will raise its pH, accelerate the rate of chemical oxidation of
ferrous ions (for which active aeration, or addition of a chemical oxidising agent such
as hydrogen peroxide, is also necessary), and cause many of the metals present in
solution to precipitate as hydroxides and carbonates. The result is the production of an
iron rich sludge that may can also contain various other metals, depending on the
17
chemistry of the mine water treated and how the process is generally done (Johnson
and Hallberg, 2005).
2.4.2 Passive abiotic remediation technologies
The objective with these systems is often termed as ―armouring‖, as an alkali is added
to AMD while maintaining the iron in its reduced form to avoid the oxidation of
ferrous iron and precipitation of ferric hydroxide on the limestone and the term
―armouring‖ is referred because the ferric hydroxide precipitation, if happened, would
hinder the effectiveness of the neutralizing agent (Kleinmann et al, 1998).
Theoretically ALDs (i.e. an alternative approach for addition of alkalinity to
AMD is the use of Anoxic Limestone Drains) require minimum maintenance once
constructed, their use is considered to be a passive approach to mine water
treatment(Kleinmann et al, 1998).
The build-up of hydroxide precipitates gradually decreases drain permeability, which
may cause failure of the drain within 6 months of construction indicating that the
short-term performance of anoxic limestone drains is better than the long term
performance (Johnson and Hallberg, 2005).
2.4.3 Active biological remediation technologies
The construction and operational costs of these systems are considerably higher than
the other remediation options. These technologies are having three potential
advantages over passive biological remediation mainly.
I. The performance is more predictable as well as controllable
II. The advantage of the selective recoverability of some heavy metals (namely
Cu, Zn in some instances
III. The post-processed waters contain low sulphate concentrations (Boonstra
et al, 1999; Johnson and Hallberg, 2005).
The use of off-line sulfidogenic bioreactors is the pronounced active biological
remediation technology (Boonstra et al, 1999). Sulfidogenic bioreactors utilise the
biogenic production of hydrogen sulphide to generate alkalinity and to remove metals
as insoluble sulphides, which is one of the processes that occurs in compost
bioreactors (Johnson and Hallberg, 2005; Kaksonen and Puhakka, 2007).
18
2.4.4 Passive biological remediation technologies
The majority of treatment systems for AMD are passive, their relatively low
maintenance cost has become the main factor for their selection in mine drainage
remediation and constructed wetlands and compost bioreactors have so far been used
in full scale treatment systems(Johnson and Hallberg, 2005).
2.4.4.1 Aerobic wetlands
These wetlands are relatively shallow systems that are operated by surface flow
generally to maintain the oxidizing conditions. Since the main remediative reaction in
aerobic wetlands is the oxidation of ferrous ions and subsequent hydrolysis of the
ferric ions produced, aerobic wetlands are generally net alkaline (Brodie, 1993).
Macrophytes are planted for aesthetic reasons to regulate water flow (i.e. to prevent
channelling) and to filter and stabilise the accumulating ferric precipitates (ochre).
Macrophytes, also provide additional surface area for precipitation of solid phase
ferric ion compounds and minerals (Johnson and Hallberg, 2005).
2.4.4.2 Anaerobic wetlands/ compost bioreactors
The key reactions that occur in compost bioreactors used to mitigate AMD are
anaerobic. Therefore such systems are often called as ‘compost bioreactors‗(Johnson
and Hallberg, 2005). Anaerobic wetlands/ compost bioreactors may not be described
as wetlands due to the fact that the anaerobic wetlands are enclosed entirely below
ground level and do not support any macrophytes (Demchak et al, 2001; Johnson and
Hallberg, 2005). The microbially catalysed reactions that occur in compost
bioreactors generate net alkalinity and biogenicsulphide. Therefore, these systems
may be used to treat mine waters that are net acidic and metal-rich, such as AMD
from abandoned metal mines (Johnson and Hallberg, 2002).
2.4.4.3 Permeable reactive barriers
A wide range of polluted groundwaters are treated using this method. Those that have
been installed to bioremediate AMD operate on the same basic principles as compost
bioreactors emphasized earlier (Benner et al, 1997). In this method a trench or pit is
dug in the flow path of contaminated groundwater/mine drainage. The voids are filled
with reactive materials (a mixture of organic solids and possibly limestone gravel)
that are sufficiently permeable to allow unimpeded flow of the water. The largest PRB
19
(Permeable Reactive Barrier) yet constructed has been installed to remediate
extremely acidic groundwater emanating from a large pyritic shale waste dump in
Shilbottle, north east England (Younger et al, 2003).
2.4.4.4 Packed bed Iron-oxidising bioreactors
Iron oxidising prokaryotes (bacteria and archaea) accelerate the oxidation of ferrous
ion to ferric in acidic (pH<4) mine waters. Acidithiobacillusferrooxidans is one of the
most well studied such bacteria which is an obligate acidophile that also oxidises a
variety of reduced inorganic sulphur compounds (Johnson and Hallberg, 2002).
Various prokaryotes have different affinities for ferrous ions depending on the
temperature and pH optima, etc. Some studies elucidate that different species of iron
oxidisers would be more appropriate to some situations than others (Gómez and
Cantero, 2003). Biological considerations, rather than immobilisation strategies,
might therefore be more important in optimising iron oxidation in packed bed
bioreactors (Johnson and Hallberg, 2002).
2.4.4.5 Floating Wetlands
Floating Treatment Wetlands (FTWs) or Floating wetlands are an innovative variant
of the more traditional constructed wetland and pond technologies that offer great
potential for treatment of stormwaters. Rather than rooted in the
sedimentsmacrophytes (similar to those used in surface and subsurface flow wetlands)
growing on a mat floating on the surface of the water are used to create floating
wetlands. The risk of the plants becoming inundated and stressed is negated since the
plants are meant to be floating on the surface of the water (Headley and Tanner,
2008). Floating wetlands have been successfully utilized in removing fine particulate
matters like Copper and Zinc from the urban stormwaters (Tanner and Headley,
2008). Floating wet lands have been sought after in treating mine waters, and for
ground water remediation and leachate treatment apart from the excessive use in
urban stormwater treatments (Kadlec and Wallace, 2008). But comprehensive
documented studies about the use of floating wetlands in remediating AMD are
limited.
2.4.5 Phytoremediation
The combination of two words gives birth to the word phytoremediation. In Greek
―phyto‖means plants or something related to plants, in Latin ―remedium‖ means
20
correcting or removing an evil. Therefore the generally accepted description regarding
phytoremediation is, ―the use of plants and associated soil microbes to reduce the
concentrations or toxic effects of contaminants in the environments‖ (Greipsson,
2011).
Figure2.4 Phytoremediation mechanisms (Source: Tangahu et al, 2011)
Not like most of the remediation technologies available in the world at the current
context, phytoremediation is an in situ applicable eco-friendly remediation technology
in which the power required is attained from the solar energy. The applications of this
technology range from removal of heavy metals and radionuclides to eradication of
organic pollutants (in general; pesticides, polynuclear aromatic hydrocarbons and
polychlorinated biphenyls). The positive aspects of the technology encompass the
advantages like efficiency and cost-effectiveness and sustainability (Clemens, 2001;
LeDuc and Terry, 2005;Chehregani and Malayeri, 2007; Odjegba and Fasidi, 2007;
Lone et al, 2008). In many approaches where the remediation is done, the top soil‘s
fertility and utility get reduced or destroyed completely but when phytoremediation is
concerned none of those adverse after effects are evitable. In fact most of the times
the plants which are used in phytoremediation process may improve soil fertility with
inputs of organic matter (Mench et al, 2009).
At the locations or sites where the practical implacability of other remedial
technologies is very low and the cost that may be incurred by such proposed remedial
21
technologies is considerably large, the applicability of the phytoremediation
technology is very much approved and accepted (Garbisu and Alkorta, 2003). The
installation and maintenance cost especially are very low in phytoremediation
technology compared to the other remedial measures (Van Aken, 2009).It is estimated
to cost as less as 5% of alternative clean-up methods, when the cost of the
phytoremediation technology is concerned (Prasad, 2003).
2.4.5.1 The techniques of phytoremediation
The uptake mechanisms of the phytoremediation technology are depicted in the figure
1.1 below. For inorganic contaminants and organic contaminants the mechanisms
differ. phytostabilization, rhizodegradation, rhizofiltration, phytodegradation, and
phytovolatilization are the generally involved mechanisms in up taking the organic
contaminants while the mechanisms which can be involved for inorganics are
rhizofiltration, phytostabilization, phytoaccumulation and phytovolatilization
(Tangahu et al, 2011).
2.4.5.1.1 Rhizofiltration
The use of plants, both terrestrial and aquatic, in adsorbing contaminants from
polluted aqueous sources with low contaminant concentration on their roots and
precipitation of the contaminants onto plant roots is defined as ―Rhizofiltration‖. The
rhizofiltration technology is widely used in many applications mostly to remediate
industrial wastes. Industrial discharge, AMD or agricultural runoff can partially be
treated using the rhizofiltration technology (McIntyre, 2003;Chaudhry et al, 1998;
Erakhrumen, 2007)
2.4.5.1.2 Phytostabilisation
Certain plant species have the ability to immobilize the contaminants in the soil and
groundwater through absorption and accumulation in plant tissues, adsorption onto
roots, or precipitation within the root zone. This ability of the plants in preventing the
migration of contaminants in soil, as well as the movement of contaminants by
erosion and deflation is termed as the ―Phytostabilisation‖ (Erakhrumen, 2007).
22
2.4.5.1.3 Phytoaccumulation
The absorption and translocation of contaminants by plant roots into the above ground
portions of the plants (i.e. shoots) is referred to as ―phytoaccumulation‖ or
―phytoextraction‖ (Erakhrumen, 2007). The shoots can be harvested and burned. The
metal absorbed can be extracted from the ash (Ibeanusi, 2004).
2.4.5.1.4 Phytovolatilization
After the uptake, contaminant is released into the atmosphere as a modified form of
the contaminant or as the contaminant itself (Erakhrumen, 2007). The process of
phytovolatilization occurs as growing trees and other plants take up water along with
the contaminants. Some of these contaminants can pass through the plants to the
leaves and volatilize into the atmosphere at comparatively low concentrations (Merkl
et al, 2005).
2.4.5.2 Selection of plants for the phytoremediation
The performance of heavy metal uptake by plants has been elucidated by several
studies (annex i).The selection of plants must be done based on the site specific
conditions.
23
3. METHODOLOGY
3.1 Plant selection for the phytoremediation
Suitable plant species were selected based on the available information with respect to
the tropical weather and also which have significantly exhibit signs of hyper-
accumulation of heavy metals. Eichhorniacrassipswas selected from a pool of plants
which are given in Table 3.1.
Table 3-1 Selected plants for analysis
Scientific name Family Vernacular
name
Metals
absorbed
Life span
Azollapinnata Salviniaceae
(recently
grouped under
Azollaceae)
Water velvet
(in English)
Cu, Hg, Fe,
Pb, Cr, Zn,
Cd, Ni, U
Multiplies in
2-3 days
Eichhorniacrassipes Pontederiaceae Water
hyacinth (in
English)
Japan-jabara
(in Sinhala)
Cu, As, Ag,
Al, Se, Co,
Hg, Fe, Pb,
Cr, Zn, Cd,
Ni, U
Perennial
Pistiastratiotes Araceae Water lettuce
(in English)
Diyagova (in
Sinhala)
Cr, Cd, Pb,
Cu, Hg, Al,
Zn, Co, Fe,
Mn
Perennial
If the plants used for the process had a short life cycle then the required remediation
would not be achieved since the plants would end their life cycle prior to achieving
the total remediation and also the dead plants would have to be continuously removed.
The long term objective of using phytoremediation technology was reducing the cost
and maintenance required for the remediation of mine drainage therefore if the plants
used had to be continuously removed and re-planted then the desired objective would
24
not be achieved. This implied that the plants required for the project needed to be
perennial therefore the option of using Azollapinnatawas negated.
As only Pistiastratiotes and Eichhornia crassipes were the remaining types of plants in
the pool of potential plants for the remediation process, one had to be selected from
them. The survivability of the plants in mine drainage conditions was tested by
planting them in the containers at laboratory conditions.
Figure 3.1 Pistiastratiotes
Figure 3.2 Eichhorniacrassipes
It was elucidated that Eichhornia crassipes could survive in the laboratory conditions
better than Pistiastratiotes. This observation concluded that Eichhornia crassipes could
be used for the remediation process.
25
Young Eichhorniacrassipesplants (i.e. the age was around 2 to 4 months) were
identified and a pool was created. Plants with healthy dark green appearance were
selected from the pool. The leaves were checked for the healthy appearance and roots
were checked to confirm whether they are lush in appearance and around 15 cm in
length, if those conditions were not satisfied, plants which would satisfy those
conditions were selected from the pool. After that, plants with dark purplish roots
were selected. The lengths of the roots were measured together with the weights of
the plants.
3.2 Sample preparation
3.2.1 Sample preparation for acid mine drainage
The study was conducted in order to assess the applicability of the phytoremediation
technology in remediating acid mine drainage. The initial objective was to use actual
acid mine drainage for the project but due to practical constraints, a laboratory made
acid mine drainage had to be used.
In laboratory conditions acid mine deranges were created based on the detailed study
done by Wildeman et al, (1993). The containers were initially filled with Bolgoda
lake water and 3 sample containers were prepared from each concentration by adding
following amount of FeSO4.7H2O. (Table 3.2) Then pH values of the 3 samples were
changed to 2.5, 5, and 7.5, of each concentration of Fe2+
by adding HNO3 acid.
Table 3-2 Fe2+
ion concentrations for acid mine drainage
Concentrations
required (mg/l)
Fe mass required (mg) to
be added to the 3.5 l
containers
Required FeSO4.7H2O
mass (mg)
0.5 1.75 8.715
10.5 36.75 183.015
20.5 71.75 357.315
30.5 106.75 531.615
40.5 141.75 705.915
26
Within seven days all except the plants which were planted in 7.5 pH and the Iron
concentration of 0.5 mg/l, were dead. Therefore the high acidity and Iron
concentration were toxic for the plants.
3.2.2 Preparation of samples to check the capable concentration limits of
the plant
The constituents of the acid mine drainage significantly vary from source to source
based on the area geology and other factors, therefore a representative drainage
sample had been prepared based on the available data.
Major ions which are abundant in mine drainages and harmful to any echo-system
throughout the world were found to be Fe2+
, Cu2+
, Cd2+
. Different concentrations were
prepared using each ion based on the world health organization recommendations of
allowable limits. The concentrations were increased by a factor of 2 starting from the
maximum permissible level indicated by the WHO.
Fe2+
ion concentrations were prepared using FeSO4.7H2O. Cu2+
ion concentrations
were prepared using CuSO4.5H2O. Cd2+
ion concentrations were prepared using 1000
ppm standard Cd2+
solution.
Table 3-3 Fe2+
ion concentrations
Concentrations
required (mg/l)
Fe mass required (mg) to
be added to the 3.5 l
containers
Required FeSO4.7H2O
mass (mg)
0.3 1.05 5.230
0.6 2.1 10.460
1.2 4.2 20.918
27
Table 3-4 Cu2+
ion concentrations
Concentrations
required (mg/l)
Cu mass required (mg)
to be added to the 3.5 l
containers
Required CuSO4.5H2O
mass (mg)
1 3.5 13.752
2 7.0 27.503
4 14.0 55.006
Table 3-5 Cd2+
ion concentrations
Concentrations
required (mg/l)
Cd mass required (mg) to
be added to the 3.5 l
containers
Required volume of 1000
ppm standard solution
(ϻl)
0.005 0.0175 17.5
0.01 0.035 35
0.02 0.07 70
Water from Bolgoda lake was used to create the drainage samples because the
nutrients required for the growth of the plants were present in the bolgoda lake water.
Initially the Bolgoda lake water was tested for the presence of Cd2+
, Cu2+
and Fe2+
ions and it was discovered that all the values were negligible.
3.3 Investigation techniques
3.3.1 Plantation process
After the plants were selected, the weight of each Eichhornia crassipes plant was
measured using an electronic balance. The total accumulated weight of the selected
plants was divided equally among the number of containers and combinations of one
28
or more plants were used to achieve the required amount of plant weight in each
container, with an accuracy ranging ± 5 grams.
The containers were filled with the prepared drainage solutions. All the containers
were labelled properly indicating the concentration of the heavy metal and the date of
preparation.
Figure 3.3 Drainage containers filled with equal volumes of prepared drainages
All the root sizes of the plants were recorded and labelled. After that the plants were
planted in the containers.
Figure 3.4 After the plants were planted in the containers
29
All the containers were placed in the laboratory under homogeneous conditions.
Samples were collected in two to three days periods from each sample and records of
the each plant‘s appearance were kept.
The tests for the heavy metal concentrations were done to assess the phytoremediation
activity of Eichhornia crassipes in removing the excessive Cd, Cu, Fe concentrations
present.
3.3.2 Equipment
This was a research conducted to examine mine drainage behaviour therefore several
number of water quality parameters had to be analysed. In all circumstances standard
methods were followed. The analytical instruments were adopted in order to reach the
intended accuracy and precision.
Sample collecting bottles
Atomic Absorption Spectrophotometer
pH meter (HACH)
UV visible Spectrophotometer (HACH DR 2800)
Titration equipment
Deep freezer
Figure 3.5 Micro pipette
The concentrations were measured using Flame Atomic Absorption Spectrometer
(AAS) and UV visible spectrophotometer. The AAS gave a high degree of accuracy
in the measurements of ppm level. All the concentrations measured by the AAS were
in the range of 0 ppm to 4 ppm. Nonspecific absorption and scattering of light due to
30
concomitant species in leach solutions may produce erroneously high results.
Instrumental background correction was used to compensate for this interference.
Contamination from laboratory glassware, supplies, and environmental particulate
matter (dust) may cause erroneously high results for solutions that require subsequent
analysis by AAS (Baker and Suhr, 1982). To avoid all sorts of errors which are
encountered, correct laboratory practices were followed.
Figure 3.6 SOLAAR Atomic Absorption Spectrometer
Figure 3.7 The pH meter and the holder
31
Figure 3.8 UV visible spectrophotometer
3.3.3 Atomic Absorption Spectrophotometer (AAS)
Reagents
Nitric buffer solution - Dilute 1ppm Conc. HNO3 adding 1000ml of distilled
water
1000 ppm Cu standard solution
1000 ppm Cd standard solution
1000 ppm Zn standard solution
Procedure
The samples were collected from the respective plant-drainage containers in such a
way that the sampling point stands at the centre of the drainage volume. After
collecting the samples of 15 ml from each container, the samples were preserved until
the test was started.
Standard samples were made using the given volumes in table 1, acquired from the
1000 ppm standard solutions. The standard volumes were added to 100 cm3
volumetric flasks. Until the standard concentrations were achieved (i.e. until the
solution total volume reaches 100 cm3) deionized water was added to the flasks.
Micro scale pipette was used to obtain higher accuracy.
32
Table 3-6 Standard volumes required to make the calibration
Substance Required volume
of 1000 ppm/cm3
Required volume
of deionized water
cm3
Concentration of
standards / ppm
Cu 0.20 99.80 2.0
0.35 99.65 3.5
0.55 99.45 5.5
0.75 99.25 7.5
Cd 0.20 99.80 2.0
0.30 99.70 3.0
0.50 99.50 5.0
0.60 99.40 6.0
The samples, obtained from the drainage containers, were filtered using Whatman 541
filter paper. The filtrations were done in order to derive solutions free of obstructing
particles. If these particles had not been removed, the particles would have corrupted
the AAS system. Before the filtrates were introduced into the AAS system for
analysis, the blank sample was introduced first through the suction tube. After that the
filtrates were introduced to the AAS in such a way that after each insertion of the
filtrate sample through the suction tube of the AAS and the concentrate reading was
done, the suction tube was inserted into 0.1 ppm HNO3 solution. The alternative
insertions of the sample and the 0.1 ppm HNO3 were carried out in all the AAS
sample analysis work.
A calibration curve was plotted against the absorbance versus concentration. Blank
solution (i.e. 0.1 ppm HNO3) was included to allow for any signal due to analyte
33
present in reagents used in sample preparation, once the samples were inserted the
sample concentration can be read from the calibration graph.
A1, A2, A3, A4 are the respective absorbance of the C1, C2, C3, C4 standard
concentrations.
Absorbance
Absorbance
of Samples
Blank Readings
Concentration C1 C2 C3 C4
Concentration
A1
A2
A3
A4
Figure 3.9 Calibration graph
34
3.3.4 Testing for Fe concentrations using UV visible spectrophotometer (HACH
DR 2800))
Reagents
1,10-phenanthroline monohydrate – Dissolve 1 g of 1,10- phenanthroline
monohydrate in 1 L of distilled water (warm if necessary)
Hydroxylamine hydrochloride – Dissolve 10 g of hydroxylamine
hydrochloride in 1 L of distilled water
Sodium acetate – Dissolve 100 g of sodium acetate in 1 L of distilled water
Iron (II) ammonium sulphate – weigh accurately 0.08 g of pure iron (II)
Ammonium sulphate, dissolve in distilled water and transfer the solution into a
1 L volumetric flask. Add 4 ml of sulphuric acid and dilute the solution to the
mark
Procedure
A series of standard samples were prepared using 50 ml volumetric flasks as in table
1. 10 ml from each required sample was taken and tested for the Fe concentration.
35
Table 3-7 Fe2+
Concentrations
Flask Standard iron
(II) solution
(ml)
1,10-
phenanthroline
solution (ml)
Hydroxylamine
hydrochloride
solution (ml)
Sodium
acetate
solution (ml)
Blank 0.00 10 2 8
1 5.00 10 2 8
2 7.00 10 2 8
3 9.00 10 2 8
4 11.00 10 2 8
5 13.00 10 2 8
6 15.00 10 2 8
Sample(10 ml) X 10 2 8
X = unknown Fe concentration of the sample
Each flask was diluted to the mark with distilled water and the absorbance was
measured at 470 nm. In order to find the concentration of the unknown sample, a
calibration curve was plotted against absorbance versus the concentration of iron (II)
in ppm units and the concentration of the given sample was found using the
calibration plot.
36
4. RESULTS
4.1 Fe
The initial concentration of Fe in the experiment tanks was 0.25mg/l, 0.5mg/l and 1
mg/l and then measure concentrations of each sample by taking 10ml within the
considerable time intervals.
Table 4-1 Variations in the Fe2+
concentrations
Concentration
Date 0.3 mg/L 0.6 mg/L 1.2 mg/L
2015.01.20 0.276 0.416 0.915
2015.01.21 0.293 0.443 0.915
2015.01.22 0.27 0.345 0.804
2015.01.23 0.138 0.383 0.887
2015.01.26 0.193 0.204 0.388
2015.01.28 0.095 0 0.495
2015.01.30 0 0 0
2015.02.06 0 0 0
37
Figure 4.1 Variations of the Fe2+
concentrations in the samples
4.2 Cd
The initial concentration of Cd in the experiment tanks was 0.005mg/l, 0.01mg/l and
0.02 mg/l and then measure concentrations of each sample by taking 10 ml within the
considerable time intervals. After 3 or 4 days plant aquatic plants were become
weaker and died within week due to heavy metal concentration.
Table 4-2 Variations in the Cd2+
concentrations
0.416
0.943
0.915
0.333
0.804 0.776
0.693
0.97
0.693
0.915
0.915
0.804
0.887
0.388
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
0 1 2 3 4 5 6 7
0.3 mg/L
0.6 mg/L
1.2 mg/L
Concentrations
Date 0.005 mg/l 0.01 mg/l 0.02 mg/l
2015.01.20 0.005 0.01 0.02
2015.01.21 0.001 0.001 0.002
2015.01.22 0.001 0 0.001
2015.01.23 0 0 0
2015.01.26 0 0 0
38
Figure 4.2 Variations of the Cd2+
concentrations
4.3 Cu
The initial concentration of Cd in the experiment tanks was 1 mg/l, 2 mg/l and 4 mg/l
and then measure concentrations of each samples by taking 10 ml within the
considerable time intervals. After 3 or 4 days plant aquatic plants were become
weaker and died within week due to heavy metal concentration.
Table 4-3 Variations in the Cu2+
concentrations
Concentration
Date 1 mg/L 2 mg/L 4mg/L
2015.01.20 1.02 2.46 4.78
2015.01.21 0.26 1.5083 2.8677
2015.01.22 0 1.602 4.0815
2015.01.23 0 1.2227 4.0028
2015.01.26 0 0.2688 3.7658
0.005
0.001 0.001 0
0.01
0 0
0.02
0.002
0.001 0 0
0.005
0.01
0.015
0.02
0.025
0 1 2 3 4
Co
nce
ntr
ati
on
(m
g/l
)
Time (Days)
0.005 mg/l
0.01 mg/l
0.02 mg/l
39
Figure 4.3 Variations of the Cu2+
concentrations
1.02
0.26 0 0 0
2.46
1.5083 1.602 1.2227
0.2688
4.78
2.8677
4.0815 4.0028 3.7658
0
1
2
3
4
5
6
0 1 2 3 4 5 6 7
Co
nce
ntr
ati
on
(m
g/l
)
Time (Days)
1 mg/L
2 mg/L
4mg/L
40
5. DISCUSSION
Phytoremediation of heavy metals in soil and water has been studied extensively (Salt
et al, 1995). Several comprehensively documented studies have emphasized the
phytoremediation of heavy metals in waste waters, storm waters and industrial
effluents. A study conducted by Kamal et al, (2004), shows the laboratory scale, study
conducted to assess the phytoremediation/phytoaccumulation capabilities of plants.
Different plants exhibit quantifiable variations when it comes to heavy metal
accumulation capabilities (Hemen, 2011). Some plants are preferred over the others
due to their hyper-accumulation capacity of heavy metals (Hemen, 2011).
The bio-accumulation capability of the heavy metals becomes a threatening factor to
the echo systems which are in the proximity of heavy metal discharging sources.
These sources include, mineral processing plants, abandoned mine sites, industrial
work places and other factories. The heavy metals that are being discharged
accumulate with time in the echo systems and the concentrations may reach the
threshold limits in the environment. Especially in the abandoned mines there are lots
of small water bodies creating micro-echo systems with aquatic life. The use of
aquatic plants which have the phytoremediation capability of heavy metals would
help to control the accumulation in the water body itself. Therefore exploring the
capability of aquatic plants, like Eichhornia crassipes, which are not entertained by
most of the species as a food, can be regarded as an effective solution to the matters.
This is the vital fact of using such plants in the bio-remediation process as the food
chain will not be significantly affected.
The Fe2+
removal elucidated by the plant in this study from the three contaminated
samples with different concentrations of contaminant can be described expressed by
the following equations;
For iron concentration 0.3 mg/l
YFe = 0.001X2 - 0.0346X + 0.3009 (R² = 0.8595)………………………eq.8
For iron 0.6 mg/l
YFe = 0.0024X2 - 0.0701X + 0.4855 (R² = 0.9178)………………………eq.9
For iron 1.2 mg/l
YFe = 0.0027X2 - 0.1079X + 1.0141 (R² = 0.8903)…………………..…eq.10
41
Where; YFe is the iron concentration in the water (in ppm),
X is the number of days from start of the experiment.
But actually the graphs indicate a significant incoherency in the absorbance values.
The reasons for those fluctuations may be the physical processes including diffusion,
osmosis, ion exchange, complex formation and so on. Diffusion would shift cations
into the plant cells (Win et al, 2002). Osmosis would move water in the opposite
direction and would hinder the Fe2+
uptake. Since both diffusion and osmosis were
across the cell wall, cell membrane permeability should greatly influence Fe2+
uptake.
Ion exchange would modify the uptake rates and complexation would form large
complex ions that would be prevented from entering the plant cell. These would also
block the cell walls and prevent further uptake of uncomplexed ions. Biological
processes could encourage, hinder, or even push out the Fe2+
, depending on the degree
of plant need for the Fe2+
ion. Thus the alternative uptake and releases can be
observed with time. A rigorous study about the Fe(OH)3 precipitation occurred in the
samples, could not be conducted in the given time frame and there may be some
complications induced by the precipitation to the phytoremediation process.
Similarly, removal of Cu can be expressed as;
For Cu 1 mg/l
YCu = 0.0689X2 - 0.5616X + 0.9149 (R² = 0.9167)…………………..…eq.11
For Cu 2 mg/l
YCu = 0.0178X2 - 0.439X + 2.2878 (R² = 0.9269)…………………..…eq.12
For Cu 4 mg/l
YCu = 0.0421X2 - 0.317X + 4.2392 (R² = 0.1007)…………………..…eq.13
And, Cd extraction can be expressed as;
For Cd 0.005 mg/l
YCd = 0.0003X2 - 0.0025X + 0.0044 (R² = 0.875)……….............…..…eq.14
For Cd 0.01 mg/l
YCd = 0.0007X2 - 0.0054X + 0.0084 (R² = 0.8325)…………………..…eq.15
42
For Cd 0.02 mg/l
YCd = 0.0013X2 - 0.0106X + 0.0169 (R² = 0.8297)…………………..…eq.16
In all the above mentioned equations R2
values greater than 0.8 (closer to 1) indicate a
statistically coherent relationship (Kleinbaum et al, 2013), but the relationships given
are not the universal expression for the absorbance.
However, it is apparent in equation 13 that the high Cu concentration does not give
coherent results. That is mostly due to the fact that high Cu concentrations are
producing toxic environment to the plants. Even though the plants died after the Cd
accumulation the results show that the plants can consistently absorb Cd
concentrations until their death.
The pH plays an important role in determining the plant‘s survivability as well as the
precipitation of Fe3+
ions as hydroxides. Generally the pH of mine drainage is less
than 7 because of the acidity. It was evident that plants could not survive in a pH
value less than 7. This means that plants cannot be used for direct treatment of AMD.
Instead, plants can be used for the treatment once the pH value has been improved to
a value around 7 or 7.5. This will not be an issue for the mine drainage from the
abandoned mines but the mines under operation. Fe3+
precipitation occurs when the
pH value is around 7, forming the reddish brown sludge. The effects of pH variations
should be extensively studied in another study to encompass a comprehensive
knowledge.
The laboratory conditions were consistent throughout the study. Thus the reactions of
the plants with the contaminant may have minimal influence from the environment.
However, there is a reasonable possibility that the rate in which the contaminants are
absorbed may vary with the alternating environmental conditions.
The mixtures of the concentrations could have been produced so that the artificially
created mine drainages would represent the actual mine drainages as accurately as
possible if and only if the allowed time frame for the study had been extended as there
was a significant number of different combinations to be tested. If different
combinations were tested a coherent argument could have been built regarding the
behaviour of certain heavy metals in the presence of other heavy metals and the
competition of the metals in absorbing to the plants when the phytoremediation is
taken place.
43
6. CONCLUSIONS AND RECOMMENDATIONS
The phytoremediation can be used to control the heavy metal accumulation in the
abandoned mine sites as well as the active mining sites where there is a potential risk
of water bodies being contaminated with the drainage containing heavy metals. In the
Sri Lankan context, although an issue related to mine drainage has not been
documented, the potential risk prevails. Especially there is a Cu-Magnetite deposit in
Seruwawila and one day if the mining operations will be started there is a significant
probability that a mine drainage containing heavy metals be discharged from that. The
risk is fairly high as the deposit contains metallic ions which might increase the
dissolution rate of the other heavy metals.
Although the mine drainage treatment plants are traditionally comprised of either
active or passive technique to remediate the discharge, the use of hybrid systems (i.e.
a combination of a passive and active system) has become the emerging trend in the
industry. Thus the phytoremediation technique can be used as a treatment unit in such
a hybrid system to further treat the discharge. Not only mine drainage but also the
waste water and discharges from the other industries can be treated using the
phytoremediation technology but further improvements have to be done to obtain
those goals.
With site specific variations the precipitations may occur and sometimes the
precipitations can be toxic to the plants. To avoid such complications further studies
have to be done.
Due to the time constraints a practical a floating wetland created using the aquatic
plants could not be implanted. The future studies could be conducted to develop such
a floating wetland as there may be various advantages of such a system compared to
the other types of wetlands when treating waters with sudden fluctuations in the water
levels (storm water, processing plant discharges etc…). In a floating wetland created
using aquatic plants, the roots will always be below the water surface and also in case
of a sudden increase the plants will not be affected but remain unharmed on top of the
water surface. If Eichhornia crassipesis used for the remediation process then once
the plant has been cultivated aerial tissues (i.e. stems and leaves) can be used for the
biogas production. The continuous use of the plants in generating biogas will help to
control the obnoxious growth rate as well.
44
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55
ANNEX I
Phyto
rem
edia
tion s
tudy o
n w
ater
med
ium
56
Phyto
rem
edia
tion s
tudy o
n w
ater
med
ium
57
Phyto
rem
edia
tion s
tudy o
n w
ater
med
ium
58
Phyto
rem
edia
tion s
tudy o
n w
ater
med
ium
59
Phyto
rem
edia
tion s
tudy o
n w
ater
med
ium
60
Sourc
e: T
angah
u, B
. V
., S
hei
kh A
bdull
ah, S
. R
., B
asri
, H
., I
dri
s, M
., A
nuar
, N
., &
Mukh
lisi
n, M
. (2
011).
A r
evie
w o
n h
eavy m
etal
s (A
s, P
b,
and H
g)
upta
ke
by p
lants
thro
ugh p
hyto
rem
edia
tio
n. In
tern
atio
nal
Journ
al o
f C
hem
ical
Engin
eeri
ng,2
011.
Phyto
rem
edia
tion s
tudy o
n w
ater
med
ium
