Journal of Analytical and Applied Pyrolysis 101 (2013) 199–208

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Journal of Analytical and Applied Pyrolysis

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Characterization of activated carbons derived from short rotation hardwoodpyrolysis char

Mark Stals a,c, Jens Vandewijngaarden a,c, Iwona Wróbel­Iwaniecb, Grazyna Gryglewiczb,Robert Carleer a, Sonja Schreurs a,c, Jan Yperman a,∗

a Research Group of Applied and Analytical Chemistry, CMK, Hasselt University, Agoralaan Gebouw D, 3590 Diepenbeek, Belgiumb Institute of Chemistry and Technology of Petroleum and Coal, Wroclaw University of Technology, ul. Gdanska 7/9, 50­344 Wroclaw, Polandc NuTeC, Department TIW, XIOS, Agoralaan Gebouw H, 3590 Diepenbeek, Belgium

a r t i c l e i n f o

Article history:

Received 31 August 2012

Accepted 22 January 2013

Available online 29 January 2013

Keywords:

Hardwoods

Pyrolysis char

Activated carbon

Characterization

Adsorption

a b s t r a c t

Chars from fast and flash pyrolysis of different short rotation hardwoods are activated by steam or KOH,

resulting in a variety of activated carbons (ACs). Analysis of nitrogen adsorption isotherm shows that

microporous ACs are obtained. Density functional theory calculation is applied to obtain information

concerning the micropore size distribution. Additionally, methylene blue (MB) and iodine adsorption

experiments are performed to evaluate the AC adsorptive properties. The textural and adsorptive prop­

erties are compared with a commercial AC (Norit). KOH activation yields ACs with very high surface area,

micropore volume and adsorption capacity, while steam activation promotes formation of narrower

micropores but yields less pore volume. The MB adsorption kinetics can be described by the pseudo­

second­order model, and most ACs adsorb at least 70% of their maximum MB adsorption capacity within

15 min.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Bio­energy is regarded as one of the key options to mitigate

greenhouse gas emissions and substitute fossil fuels [1]. A prime

requisite for bio­energy is the avoidance of interference with food

production [2], thus so called “second generation biofuel” from lig­

nocellulosic biomass is preferred [3]. An important technique for

generating large quantities of second generation bio­energy is the

short rotation forestry of woody species [4]. Short rotation forestry

produces large amounts of lignocellulosic biomass, additionally,

the cultivation of the trees does not require intensified agricultural

methods. An interesting possibility of some hardwood species, e.g.

willow or poplar cultivars, is the removal of contaminants such as

heavy metals from the soil [5]. This use of plant species to clean up

contaminated soils is known as phytoremediation [6]. Fast or flash

pyrolysis [7,8] are techniques to convert lignocellulosic biomass

into a gaseous, liquid, and solid form, which are generally consid­

ered more valuable than the biomass as such [9]. One of the key

benefits of fast pyrolysis is the possibility to produce pyrolysis oil

which contains only negligible amounts of heavy metals from heavy

metal contaminated hardwoods from phytoremediation [10,11].

∗ Corresponding author. Tel.: +32 11 268320; fax: +32 11 268301.

E­mail address: jan.yperman@uhasselt.be (J. Yperman).

During fast or flash pyrolysis, char is generated as a by­product,

which is usually burnt for energy. It is anticipated that large

amounts of pyrolysis char will be available [12]. Therefore, a higher

economic value than its fuel value is desired. In the present paper,

fast and flash pyrolysis chars are produced from P. nigra, S. frag­

ilis and P. grimminge (from phytoremediation, except the former).

The chars are activated by steam and KOH in order to enhance the

adsorption capacity. The resultant activated carbons are character­

ized by nitrogen sorption technique and density functional theory

is applied. Besides this, the adsorption isotherms and kinetics of

iodine and methylene blue (MB) are determined. A commercial

activated carbon (Norit GAC) is used as a reference. The influence of

the production route (i.e. mode of pyrolysis and mode of activation)

on the obtained mass and adsorption capacities of the activated

carbon is illustrated via mass balances.

2. Materials and methods

2.1. Hardwoods

Three short­rotation hardwood species are studied, i.e.: Wolter­

son poplar (P. nigra) [4], crack willow (S. fragilis) and Grimminge

poplar (P. grimminge: P. deltoides × (P.trichocarpa × P. deltoides) [5].

All samples are chipped (< 2.0 mm) and oven dried at 110 ◦C.

The proximate and elemental analysis of these hardwood

species are given in Table 1.

0165­2370/$ – see front matter © 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.jaap.2013.01.009

200 M. Stals et al. / Journal of Analytical and Applied Pyrolysis 101 (2013) 199–208

Table 1

Proximate and elemental analysis of hardwood species for activated carbon

production.

S. fragilis P. grimminge P. nigra

Proximate analysis (%)

Volatiles 76 75 76

Fixed carbon 18 22 20

Elemental analysis (%)

C 53.5 52.9 52.8

H 6.4 6.6 6.9

N 1.2 1.0 0.5

S < < <

O 36.6 36.4 37.6

Ash 2.3 3.1 2.2

H/C 1.42 1.46 1.5

O/C 0.51 0.52 0.53

2.2. Carbonization

The carbonization step is carried out by one of the two differ­

ent modes of pyrolyzing the hardwoods: (a) fast pyrolysis and (b)

flash pyrolysis. The resultant char is used as precursor material for

activation.

(a) Fast pyrolysis is done by inserting the hardwood sample (10 g)

in a quartz tube which is placed in a furnace [11] (Nabertherm).

Nitrogen is flushed at a rate of 70 ml/min through the tube to

ensure pyrolysis atmosphere. The sample is heated at 35 ◦C/min

to 450 ◦C and is then held isothermal at 450 ◦C for 15 min. After

cooling down, the char is collected and stored in a dessicator.

(b) Flash pyrolysis is done with a lab scale semi­continuous flash

pyrolysis reactor [13,14]. All pyrolysis are performed with 100 g

of biomass and at 450 ◦C pyrolysis temperature, using sand

as heat transfer medium. The biomass is injected at a rate of

40 g/min into the hot sand inside the reactor. The char is formed

in the sand matrix and after the whole system has cooled down

to room temperature, the char is separated from the sand. A

more extensive description of this setup, the studied hard­

woods and the pyrolytic product streams are discussed earlier

in [14,15].

2.3. Activation

2.3.1. Physical activation with steam

The precursor material, i.e. fast or flash pyrolysis char from hard­

wood biomass is milled to pass a 125 mm DIN sieve. The sieved char

is introduced into a quartz tubular reactor and the position of the

sample is fixed by quartz wool plugs. The quartz tube is flushed

with 50 ml/min N2 and inserted in the furnace which is placed in a

heated environment (110 ◦C). A thermocouple is inserted between

the tube and furnace, and the furnace is programmed via FGH

1000 controller to ramp at 35 ◦C/min to 800 ◦C. The temperature is

isothermal at 800 ◦C during activation. During activation, the Schott

TR 100 is controlled via PC to inject 22.5 ml/min liquid H2O into

the steam generator at 210 ◦C, providing 50 ml/min of steam. The

isothermal period is calculated for each precursor sample, so that

the injected mass of water corresponds to 3/2 of the precursor’s

mass. A schematic of this setup is presented in Fig. 1 [16,17].

2.3.2. Chemical activation with KOH

The precursor material, i.e. fast or flash pyrolysis char (1–2 g)

from hardwood biomass is introduced in an Erlenmeyer flask with

400 g/l KOH solution. The ratio of KOH:sample is 4:1 [18]. The sam­

ple is impregnated with KOH by agitating the Erlenmeyer for 24 h in

a water bath at 20 ◦C. After impregnation, the sample is oven dried

at 110 ◦C. The sample is then introduced in a quartz tube and heated

at a rate of 10 ◦C/min to 800 ◦C, followed by 1 h isothermal period

Tubular furnace

Precursor

Steam ge nerator

MKS 247 -N2

Schott TR100

Hea ted environment

PC FGH 100 0

therm

ocouple

outlet

Fig. 1. Physical activation by steam.

at 800 ◦C. The quartz tube is continuously flushed with 50 ml/min

N2 gas, ensuring only negligible remaining oxygen during activa­

tion. After activation, the excess KOH is washed from the sample

by consecutive portions of dilute (0.1 M) HCl and distilled water,

until a pH of 6.5 and 7.5 is attained. The sample is then dried and

weighed.

It is believed that activation (KOH starts to react at 700 ◦C [19]),

initially consists of a redox reaction, where carbon is oxidized to

CO or CO2, creating porosity and K2CO3 as a by­product [20].

2.4. Characterization of activated carbons

2.4.1. Porous texture analysis

The porous texture parameters of the activated carbons (ACs)

are determined from N2 adsorption–desorption isotherms mea­

sured at 77 K with a ASAP2020 sorption analyzer (Micromeritics).

Prior to measurements, the sample is outgassed for 12 h at

250 ◦C. The specific surface area (SBET) is calculated using BET

method [21]. The amount of nitrogen adsorbed at a relative pres­

sure of p p0−1 = 0.98 is used to evaluate the total pore volume

(VT). The micropore volume (VDR) is determined by applying the

Dubinin–Radushkevich equation up to p p0−1 < 0.15. The Den­

sity Functional Theory (DFT) is applied to assess the pore size

distribution. The average micropore size (L0) is calculated using

Dubinin–Stoeckli equation [22].

2.4.2. Methylene blue adsorption isotherm – experimental

Methylene blue (MB) adsorption is representative for the

adsorption of medium­sized molecules and mesopore volume of

the adsorbent. The MB molecules are accessible to pores with a

width larger than 1.5 nm [23]. MB with certificate of analysis is

purchased from Sigma–Aldrich. Blanks containing no MB were ana­

lyzed with each series of experiments.

20 mg of AC is introduced in an Erlenmeyer flask with 20 ml MB

solution. At least six different initial MB concentrations are used for

each AC. The samples are agitated for 2 h in a water bath at 20 ◦C. The

experiments are done without pH adjustment, since the pH is only

of minor influence between pH 4 and 9 [24]. The samples are filtered

(Whatmann 40) and eventually diluted to suitable concentration

for UV–vis measurement at 665 nm. The UV–vis spectrometer is

calibrated with 6 methylene blue standards ranging from 0.5 to

5.0 mg/l MB.

2.4.3. Adsorption isotherm – models

The adsorption data are fitted with Langmuir and Freundlich

models. The Langmuir model (1) assumes a “monolayer” adsorp­

tion and no change in heat of adsorption with increased surface

coverage, whereas the Freundlich model (2) assumes a “multilayer”

adsorption and a logarithmic decrease of heat of adsorption with

increased surface coverage.

qe =qmKLCe

1 + KLCe(1)

M. Stals et al. / Journal of Analytical and Applied Pyrolysis 101 (2013) 199–208 201

Table 2

Char and AC yields from different biomasses, carbonization methods and activation methods.

Hardwood Pyrolysis Char yield (wt%) Activation AC yield (wt%) Overall AC yield (g per kg hardwood)

S. fragilis Fast 29 Steam 44 128

S. fragilis Fast 29 KOH 42 122

S. fragilis Flash 21 Steam 49 103

S. fragilis Flash 21 KOH 33 69

P. grimminge Fast 31 Steam 47 146

P. grimminge Fast 31 KOH 46 143

P. grimminge Flash 19 Steam 50 95

P. grimminge Flash 19 KOH 41 78

P. nigra Fast 30 Steam 42 126

P. nigra Fast 30 KOH 49 147

P. nigra Flash 20 Steam 44 88

P. nigra Flash 20 KOH 44 88

qe = KF C1/ne (2)

qe is the amount of MB adsorbed per unit of mass of adsorbent

(mg/g) and Ce is the equilibrium concentration of MB (mg/l). KL

and qm are the Langmuir constants and KF and n are the Freund­

lich constants. The Langmuir constants qm and KL are related to

the monolayer capacity of the adsorbent (mg/g) and the energy

of adsorption, respectively. Freundlich constants KF and 1/n are

related to the adsorption capacity and intensity, respectively. These

constants can be derived from the intercept and slope of their

respective linear plots. The Langmuir equation. (1) can be linearized

in different ways, for example, according to Stumm and Morgan

[25] (3) and Weber [26] (4), respectively.

1

qe=

1

qm+

1

qmKL

1

Ce(3)

Ce

qe=

1

qmKL+

Ce

qm(4)

Eq. (4) is used to calculate the Langmuir constants, since the

goodness of fit is found to be better than for Eq. (3).

The Freundlich isotherm is linearized as follows:

log qe = log KF +1

nlog Ce (5)

2.4.4. Kinetic study – experimental

Equal amounts of MB solution are added to 7 Erlenmeyer flasks

containing each 20 mg of the same type of AC. The samples are agi­

tated in a water bath at 20 ◦C. The adsorption of MB is stopped after

different time intervals, i.e. after 15 min, 30 min, 1 h, 2 h, 4 h, 8 h, and

24 h. Because of the much higher MB uptake for KOH activated car­

bons, the KOH AC were tested with a higher initial MB concentration

(500 mg/l) than steam AC (100–200 mg/l), see Table 5.

2.4.5. Kinetic study – model

The kinetics of MB removal is described as follows [27]

k´

S + [MB] [SMB]

k´´

←→

(6)

where S is the number of active sites on the sorbent, MB is

the concentration of methylene blue in the solution and SMB is

the concentration of MB adsorbed by the sorbent. The following

assumptions are made:

• Sorption occurs on localized sites and involves no interaction

between sorbed ions• The energy of adsorption is independent from surface coverage• Monolayer adsorption is assumed• The adsorption mechanism is governed by a second­order rate

equation

• The desorption rate (k′′) is negligible compared to the adsorption

rate (k′).

Since the process is strongly influenced by the amount of adsor­

bate on the surface and the adsorbate adsorbed at equilibrium, the

rate of reaction is proportional to the number of available sites on

the surface of the adsorbent. The pseudo­second­order rate expres­

sion can be written as [12,27]

d(S)t

dt= k2[(S)0 − (S)t]

2 (7)

where (S)0 and (S)t are the number of sites available at initial

time t = 0 and t = t, respectively. Eq. (7) can be rewritten in terms

of adsorbed quantity (8) where qe is the equilibrium amount

adsorbed, and qt is the measured quantity adsorbed at time t

[28,29]:

d(q)t

dt= k2(qe − qt)

2 (8)

Integration of Eq. (8) (qt = 0 when t = 0 and qt = qt when t = t)

provides a linear equation:

t

qt=

1

k2q2e

+t

qe(9)

The reaction rate constant (k2) and equilibrium maximum

adsorption qe can be derived from the slope and intercept of a plot

of t/qt vs. t.

The experimental data are also fitted with the Lagergren pseudo

first­order kinetics equation [30]. However, for several samples, a

lower correlation coefficient (around 0.90) is calculated. Mainly the

data points from short contact time show deviation from linearity.

Therefore, the first order model was not retained.

2.4.6. Iodine adsorption

The iodine number indicates the small pores (< 2 nm) on an AC

and reflects the ability to adsorb small molecules [31]. It is assessed

by the ASTM­D4607 method. The iodine number is defined as the

mass (mg) of iodine adsorbed per gram of AC, after 30 s of con­

tact time with a 0.1 N iodine solution. At least three adsorption

data points between 0.008 and 0.04 N residual iodine solutions are

recorded and log(C30 s) is plotted vs. log(q30 s). This is similar to Eq.

(5). From the regression slope and intercept, the corresponding q30 s

for a 0.02 N concentration is calculated to obtain the iodine number.

3. Results and discussion

3.1. Material balance

The three hardwoods (S. fragilis, P. grimminge and P. nigra) are

subjected to both fast and flash pyrolyses, resulting in six different

chars. Those chars are each used as precursor material for activated

202 M. Stals et al. / Journal of Analytical and Applied Pyrolysis 101 (2013) 199–208

Table 3

Characteristics of the porous texture of ACs determined by N2 sorption at 77 K.

Precursor Pyrolysis SBET(m2/g) VT(cm3/g) VDR(cm3/g) L0(nm) VDR/VT Vmez(cm3/g)

KOH activation

S. fragilis Fast 2767 1.25 0.99 1.64 0.79 0.26

P. grimminge Fast 2854 1.32 1.05 1.86 0.79 0.27

P. nigra Fast 2588 1.28 0.95 2.10 0.74 0.33

S. fragilis Flash 2227 1.03 0.85 1.68 0.82 0.18

P. grimminge Flash 2841 1.41 1.04 1.94 0.74 0.37

P. nigra Flash 2223 1.06 0.85 1.41 0.80 0.21

Steam activation

S. fragilis Fast 794 0.46 0.31 0.89 0.67 0.15

P. grimminge Fast 682 0.34 0.27 0.84 0.79 0.07

P. nigra Fast 810 0.38 0.29 0.87 0.76 0.09

S. fragilis Flash 623 0.35 0.22 1.07 0.63 0.13

P. grimminge Flash 648 0.31 0.24 1.01 0.77 0.07

P. nigra Flash 697 0.32 0.25 0.99 0.78 0.07

Reference AC 1137 0.61 0.43 1.27 0.70 0.18

carbon production by physical activation (steam) and chemical

activation (KOH), resulting in 12 different ACs. The pyrolysis char

yields and AC yields are presented in Table 2, the data are sorted

by descending AC yield per kg of biomass processed. Fast pyrolysis

yields are reproducible within 1% rsd; flash pyrolysis experiments

are reproducible within 3–5% rsd and activation (physical or chem­

ical) yield within approximately 5% rsd.

From Table 2, it can be concluded that the highest AC yields are

obtained from fast pyrolysis. This is because fast pyrolysis is favor­

able over flash pyrolysis, when striving for char production: a lower

heating rate results in a higher amount of char produced [32,33].

In the case of fast pyrolysis, the AC yields from steam activation

do not differ strongly from KOH activation yields. In the case of

flash pyrolysis, steam activation produces higher yields than KOH

activation, except for P. nigra.

3.2. Porous texture of ACs

The nitrogen adsorption–desorption isotherms at 77 K for the

ACs prepared by steam activation and KOH activation are shown in

Fig. 2.

Both series of ACs behave like a type I adsorbent according to

the BDDT classification, reflecting their microporous nature [21].

The nitrogen adsorption capacity of KOH ACs is significantly higher

than that of steam ACs. The profile of isotherms clearly proves that

different microporosity in terms of pore width is developed during

KOH and steam activation. A much wider knee of the isotherm of

KOH ACs compared to steam ACs proves that the latter ACs are char­

acterized by narrower micropores. For a given activation method,

the nitrogen adsorption capacity of AC produced from fast pyrolysis

char is higher than that of flash pyrolysis char. The porous texture

parameters of the ACs calculated from their corresponding adsorp­

tion isotherms are given in Table 3. As expected, KOH activation was

found to be a more efficient method for developing porous texture

than the activation with steam. The BET surface area is between

2223 and 2854 m2/g for KOH ACs whereas the steam ACs show the

SBET in the range of 623–810 m2/g only. The same trend is observed

for the micropore volume VDR. Considering the effect of the char

on the porous texture of resultant AC, it can be observed that fast

pyrolysis char shows a higher propensity for developing porosity

than flash pyrolysis char for both modes of activation. The con­

tribution of mesopores to the total pore volume of ACs is in the

range of 0.18–0.37. Taking into account the lower total pore vol­

ume of steam ACs (Table 3), it is clear that KOH ACs show 3–4

times higher mesopore volume, which can be crucial regarding

the adsorption of large molecules. A large difference in the average

micropore width between both series of ACs is observed. KOH acti­

vation yields ACs with significantly wider micropores compared to

steam ACs (1.41–2.10 vs. 0.84–1.07 nm). The relatively severe con­

ditions of KOH activation used in this study, i.e. the KOH/precursor

ratio of 4:1 and the temperature of 800 ◦C, promote the formation

of wider micropores [34].

Fig. 3a and b shows the pore size distribution determined from

the N2 adsorption data using the DFT method for steam and KOH

Table 4

MB adsorption isotherm models: model parameters and correlation coefficients.

Hardwood Pyrolysis Freundlich Langmuir (Stumm and Morgan) Langmuir (Weber)

Kf (mg/g (l/mg)1/n) n R2 qm (mg/g) KL (l/mg) R2 qm (mg/g) KL (l/mg) R2

Chemical activation

S. fragilis Fast 506 12.65 0.9888 637 9.41 0.7785 727 0.94 0.9996

S. fragilis Flash 563 13.12 0.9339 698 4.32 0.9234 703 4.20 0.9997

P. grimminge Fast 418 9.53 0.9440 567 5.11 0.8991 614 2.23 0.9996

P. grimminge Flash 479 9.86 0.7163 612 7.91 0.9282 616 9.04 0.9999

P. nigra Fast 478 19.10 0.9682 582 3.17 0.8184 647 0.54 0.9988

P. nigra Flash 470 11.10 0.9330 666 2.58 0.8517 710 0.69 0.9993

Physical activation

S. fragilis Fast 87 10.51 0.9672 120 1.29 0.9030 123 1.20 0.9980

S. fragilis Flash 38 4.99 0.9989 62 1.56 0.9283 67 0.88 0.9966

P. grimminge Fast 75 6.03 0.9134 158 0.22 0.7338 164 0.16 0.9965

P. grimminge Flash 170 16.24 0.9219 191 51.24 0.8850 207 3.84 0.9980

P. nigra Fast 218 21.72 0.9780 257 16.34 0.8749 266 1.94 0.9991

P. nigra Flash 156 10.43 0.8509 227 0.7 0.8814 232 0.82 0.9993

Reference AC 247 29.58 0.9725 269 3.70 0.8931 307 0.99 0.9994

M. Stals et al. / Journal of Analytical and Applied Pyrolysis 101 (2013) 199–208 203

0

200

400

600

800

1000

0 0,2 0,4 0,6 0,8 1

VS

TP, c

m3/g

p/po

0

200

400

600

800

1000

0 0,2 0,4 0,6 0,8 1

VS

TP, c

m3/g

p/po

0

200

400

600

800

1000

0 0,2 0,4 0,6 0,8 1

VS

TP, c

m3/g

p/po

0

200

400

600

800

1000

0 0,2 0,4 0,6 0,8 1

VS

TP, c

m3/g

p/po

0

200

400

600

800

1000

0 0,2 0,4 0,6 0,8 1

VS

TP, c

m3/g

p/po

a. S. f ragilis fas t pyrolysisc. P. Grimminge fast py rolysis

e. P. nigra fa st pyrolysis

b. S. f ragilis flash pyrolysis

d. P. Gri mmin ge flas h py rol ysis

f. P. nig ra flash py rolysis

0

200

400

600

800

1000

0 0,2 0,4 0,6 0,8 1

VS

TP, c

m3/g

p/po

(a) (b)

(c) (d)

(e) (f)

Fig. 2. N2 adsorption isotherms at 77 K for the ACs produced by KOH activation (N) and steam activation (d).

ACs, respectively. The histograms of KOH ACs present a similar

relatively broad pore size distribution, except the ACs derived

from flash pyrolysis char of P. grimminge and P. nigra (Fig. 3a–d

and a–f, respectively). The aforementioned samples have a pro­

nounced maximum pore volume at a pore width of 1.3–1.5 nm

and 1.1–1.3 nm, respectively. The steam ACs compared to KOH ACs

show a different pore size distribution, with the predominance of

narrow micropores. It is interesting to note that the kind of biomass

and the mode of pyrolysis have a very small impact on the pore size

distribution of AC produced by activation with steam (Fig. 3b). The

maximum volume is observed at a pore width of 0.7–0.9 nm for all

samples. The experimental work also includes a comparative study

of commercial AC. As can be seen in Table 3, the values of porous

texture parameters are in the middle between steam and KOH ACs.

The N2 sorption isotherm at 77 K (a) and the pore size distribution

(b) of the reference AC are shown in Fig. 4.

The shape of isotherm clearly proves the microporous nature of

reference AC (Fig. 4a). However, in contrast to the produced ACs,

the reference AC is characterized by bimodal micropore size distri­

bution (Fig. 4b). There are two maxima in the histogram, the first

maximum at a width of 0.7–0.9 nm in the range of narrow micro­

pores and the second one at a pore width of 1.1–1.5 nm for wider

micropores.

The two series of ACs produced by KOH and steam activation of

flash and fast pyrolysis chars and the reference AC are character­

ized by adsorption of MB and I2, two molecules of different sizes

to confirm the feasibility of these porous carbons in liquid­phase

adsorption processes.

204 M. Stals et al. / Journal of Analytical and Applied Pyrolysis 101 (2013) 199–208

0

0,05

0,1

0,15

0,2

0,25

0,5 0,7 1,1 1,5 1,9 2,3 2,7 3,1 3,5 3,9 4,3

Pore diameter (nm)

0

0,05

0,1

0,15

0,2

0,25

0,5 0,7 1,1 1,5 1,9 2,3 2,7 3,1 3,5 3,9 4,3

Pore diameter (nm)

0

0,05

0,1

0,15

0,2

0,25

0,5 0,7 1,1 1,5 1,9 2,3 2,7 3,1 3,5 3,9 4,3

Pore diameter (nm)

0

0,05

0,1

0,15

0,2

0,25

0,5 0,7 1,1 1,5 1,9 2,3 2,7 3,1 3,5 3,9 4,3

Pore diameter (nm)

0

0,05

0,1

0,15

0,2

0,25

0,5 0,7 1,1 1,5 1,9 2,3 2,7 3,1 3,5 3,9 4,3

Pore diameter (nm)

0

0,05

0,1

0,15

0,2

0,25

0,5 0,7 1,1 1,5 1,9 2,3 2,7 3,1 3,5 3,9 4,3

Pore diameter (nm)

0

0,02

0,04

0,06

0,08

0,1

0,5 0,7 1,1 1,5 1,9 2,3 2,7 3,1 3,5 3,9 4,3

Pore diameter (nm)

0

0,02

0,04

0,06

0,08

0,1

0,5 0,7 1,1 1,5 1,9 2,3 2,7 3,1 3,5 3,9 4,3

Pore diameter (nm)

0

0,02

0,04

0,06

0,08

0,1

0,5 0,7 1,1 1,5 1,9 2,3 2,7 3,1 3,5 3,9 4,3

Pore diameter (nm)

0

0,02

0,04

0,06

0,08

0,1

0,5 0,7 1,1 1,5 1,9 2,3 2,7 3,1 3,5 3,9 4,3

Po

re v

olu

me

(cm

3/g

)P

ore

vo

lum

e (

cm

3/g

)

Pore diameter (nm)

0

0,02

0,04

0,06

0,08

0,1

0,5 0,7 1,1 1,5 1,9 2,3 2,7 3,1 3,5 3,9 4,3

Pore diameter (nm)

0

0,02

0,04

0,06

0,08

0,1

0,5 0,7 1,1 1,5 1,9 2,3 2,7 3,1 3,5 3,9 4,3

Pore diameter (nm)

a. S. fragilis fast pyrolysisc. P. Grimminge fast pyrolysise. P. nigra fast pyrolysis

b. S. fragilis flash pyrolysisd. P. Grimminge flash pyrolysisf. P. nigra flash pyrolysis

a. S. fragilis fast pyrolysisc. P. Grimminge fast pyrolysise. P. nigra fast pyrolysis

b. S. fragilis flash pyrolysisd. P. Grimminge flash pyrolysisf. P. nigra flash pyrolysis

a

(a) (b)b

c d

e f

a b

c d

e f

Po

re v

olu

me

(cm

3 /g

)

Po

re v

olu

me

(cm

3/g

)

Po

re v

olu

me

(cm

3/g

)

Po

re v

olu

me

(cm

3/g

)

Po

re v

olu

me

(cm

3 /g

)

Po

re v

olu

me

(cm

3/g

)

Po

re v

olu

me

(cm

3/g

)

Po

re v

olu

me

(cm

3 /g

)

Po

re v

olu

me

(cm

3/g

)

Po

re v

olu

me

(cm

3/g

)

Fig. 3. a. DFT pore size distribution of KOH ACs; 3b. DFT pore size distribution of steam ACs.

3.3. Methylene blue adsorption study

The adsorption of MB is commonly studied for the determi­

nation of adsorptive properties of porous carbons [35–39]. MB

sorption experiments with pyrolysis char show that the MB adsorp­

tion capacity is around 40–50 mg/g and the iodine sorption is

around 100 mg/g. These figures indicate that it is desirable to

enhance the porous texture from the char.

The MB adsorption isotherms are shown in Fig. 5: ACs from

fast pyrolysis chars left and ACs from flash pyrolysis chars right.

Each graph contains two isotherms, (i.e. for KOH and steam ACs)

together with their Langmuir approximation. All KOH ACs have

superior MB uptake compared to steam activated samples. In

case of S. fragilis and P. grimminge biomass, the carbonization

mode (fast or flash pyrolysis) is of little importance regarding

the performance of KOH ACs. For P. nigra, flash pyrolysis char

produces a slightly higher performing AC than the fast pyrolysis

char.

In the case of steam activation of P. nigra and S. fragilis, fast pyrol­

ysis char is a better precursor than flash pyrolysis char. In contrast, a

(a) (b)

0

200

400

600

800

0 0,2 0,4 0,6 0,8 1

VS

TP, cm

3/g

p/po

0

0,02

0,04

0,06

0,08

0,1

0,5 0,7 1,1 1,5 1,9 2,3 2,7 3,1 3,5 3,9 4,3

Po

re v

olu

me

(cm

3 g

- 1)

Pore diameter (nm)

Fig. 4. Characteristics of porous texture of the reference AC. a, N2 adsorption isotherm at 77 K; b, pore size distribution determined by DFT method.

M. Stals et al. / Journal of Analytical and Applied Pyrolysis 101 (2013) 199–208 205

Fig. 5. MB adsorption experimental data and Langmuir model for ACs.

higher MB uptake is observed when P. grimminge is flash pyrolysed

in the carbonization step.

In Table 4, the Freundlich and Langmuir constants are pre­

sented together with the model correlation coefficients. In all cases,

Weber’s approximation [26] of the Langmuir adsorption isotherm

provides the best fit. The linearization according to Stumm and

Morgan [25] produces lower correlation coefficients, therefore, the

results obtained using Weber’s linearization will be further dis­

cussed. It appears that the Langmuir model describes better the

MB adsorption process than the Freundlich model for both the KOH

and steam ACs. The correlation coefficients are very high (0.999).

Since the Langmuir model best fits all the isotherm data, we can

assume that the adsorption can be described in a way similar to

a monolayer “chemisorption” process. Compared with reported

data in the literature [40,41], the ACs studied in this work have

a very large adsorption capacity. As seen from Table 4, the high­

est Langmuir monolayer adsorption capacity qm is observed for

KOH AC from S. fragilis, fast pyrolysis (qm = 727 mg/g; reference

qm = 307 mg/g) and P. nigra, flash pyrolysis (qm = 710 mg/g). The low

value of qm found for S. fragilis flash pyrolysis steam AC may be

explained by insufficient and difficult removal of sand from the

char.

No obvious relation between SBET and MB adsorption was estab­

lished with the obtained AC.

3.4. Methylene blue kinetic study

The sorption kinetics of ACs derived from fast pyrolysis char

are studied. Fast pyrolysis produces more char than flash pyrol­

ysis, and flash pyrolysis char requires the removal of sand which

is a difficult task to perform very correctly for each sample.

Therefore, only AC from fast pyrolysis char is chosen for kinetic

studies. The first order model did not supply a satisfactory fit,

therefore the previously described pseudo­second­order model is

applied. The pseudo­second order model (7) shows a good corre­

lation with the experimental data. The model data qe (mg/g), k2

(mg/mg h) and correlation coefficients are given in Table 5, together

with the experimental (measured) qe,exp. In all cases R2 is higher

than 0.999 and the model qe and experimental qe,exp are in good

agreement. The experimental data and model plots are given in

Fig. 6. The results confirm the applicability of the pseudo­second­

order kinetic model on the adsorption process of MB on the studied

ACs. This is also demonstrated by other reports on the removal of

MB by biomass­based adsorbents [38–42].

A prime finding is that the adsorption rate is very high at the

beginning of the process, as is illustrated by the initial sorption

rate h which is approximately 3.5–6.0 mg/mg h for KOH ACs and

0.5–2.0 mg/mg h for steam ACs. The experiments with KOH ACs

start with a higher initial MB concentration than steam ACs. If two

equal samples are tested with a different initial MB concentration,

a higher h is expected for the sample with the highest initial MB

concentration. Thus a direct comparison between the two series of

ACs solely yields an absolute performance comparison. A relative

performance comparison can not be made from h when different

initial concentrations are applied.

A relative performance comparison can be done by comparing

the MB removal percentage after a certain time. All KOH ACs adsorb

MB in greater extent than 70% of qe within the first 15 min. The same

goes for the steam ACs samples, except for P. grimminge. All sam­

ples (chemical and physical AC) reach over 80% of their qe within

206 M. Stals et al. / Journal of Analytical and Applied Pyrolysis 101 (2013) 199–208

Table 5

MB adsorption kinetic study of various AC derived from fast pyrolysis char.

Hardwood Activation C0 (mg/l) qe,exp (mg/g) qe (mg/g) k2 (mg/mg h) h (mg/mg h) R2 % of qe,exp within 15 min

S. fragilis KOH 500 756 758 10.3 5.9 0.9999 82

P. grimminge KOH 500 737 742 6.2 3.4 0.9998 74

P. nigra KOH 500 738 739 8.7 4.7 0.9996 90

S. fragilis Steam 100 168 169 20.4 0.6 0.9995 72

P. grimminge Steam 125 181 183 14.7 0.5 0.9991 57

P. nigra Steam 200 284 286 37.0 2.0 0.9994 82

Reference AC 300 323 330 24.7 2.7 0.9997 85

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