Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 75:971±976 (2000)

Adsorption of sulfur dioxide onto activatedcarbons prepared from oil-palm shellsimpregnated with potassium hydroxideJia Guo* and Aik Chong LuaDivision of Thermal and Fluids Engineering, School of Mechanical and Production Engineering, Nanyang Technological University,Singapore 639798, Republic of Singapore

(Rec

* CoTech

# 2

Abstract: Adsorption of sulfur dioxide (SO2), a gaseous pollutant, onto activated carbons prepared

from oil-palm shells pre-treated with potassium hydroxide (KOH) impregnation was studied.

Experimental results showed that SO2 concentration and adsorption temperature affected signi®-

cantly the amount of SO2 adsorbed and the equilibrium time. However, sample particle sizes

in¯uenced the equilibrium time (due to effect of diffusion rate) only. Desorption at the same

temperature of adsorption and a higher temperature of 200°C con®rmed the presence of chemisorp-

tion due to pre-impregnation. Impregnation with different activation agents was found to have limited

effect on the inorganic components of the sample. Compared with the activated carbon pre-treated

with 30% phosphoric acid (H3PO4) that had larger BET and micropore surface areas, the sample

impregnated with 10% KOH had a higher adsorptive capacity for SO2, which was closely related to the

surface organic functional groups of the sample. In general, the activated carbon prepared from oil-

palm shell impregnated with KOH was more effective for SO2 adsorption and its adsorptive capacity

was comparable to some commercial activated carbons.

# 2000 Society of Chemical Industry

Keywords: adsorption; activated carbon; oil-palm shell; impregnation; surface functional group

INTRODUCTIONAir pollution by sulfur dioxide (SO2) emission, which

causes acid rain and the formation of ground level

ozone that is linked to various respiratory diseases, has

drawn more and more concerns world-wide. The

sources of SO2 generation include power stations,

kilns, smelters, sulfuric acid production plants and

automobiles.1 Various techniques have been used to

reduce or abate the levels at which these pollutants are

emitted into the atmosphere.2 Amongst them, adsorp-

tion of SO2 by activated carbon is a promising

approach since this dry method involves a cheaper

investment than wet scrubbing which requires a waste-

water treatment system. Moreover, nitrogen oxides

(NOx) can be removed simultaneously with SO2 using

this technology.3,4

Extensive research has been carried out experimen-

tally and theoretically on the adsorption of SO2 onto

activated carbon. Davini investigated the adsorption

and desorption of SO2 on a low-ash commercial

activated carbon in the temperature range between

130 and 170°C.5 It was found that oxygen present at

the adsorption stage played a very important role in the

amount of SO2 adsorbed. Daley et al studied the

adsorption of SO2 onto activated carbon ®bres from

phenolic ®bre, which possessed submicropores (dia-

eived 20 October 1999; revised version received 3 May 2000; accept

rrespondence to: Jia Guo, Division of Thermal and Fluids Engineenological University, Singapore 639798, Republic of Singapore

000 Society of Chemical Industry. J Chem Technol Biotechnol 02

meter less than 1nm).6 The initial rate of SO2

adsorption was inversely related to the pore size; for

longer exposure times, the amount adsorbed was

dependent on both the pore size and pore volume.

Gray simulated the SO2 adsorption process with a

single-particle model based on macropore, micropore

and sorbed-phase diffusion.7 The model prediction

correlated with the experimental data very well.

However, there have been no reports in the literature

on the adsorption of SO2 onto chemically activated

carbons and the relationship between the adsorptive

capacities of the activated carbons and their surface

functional groups.

The aim of this study was to test the adsorptive

capability of SO2 onto activated carbons from oil-palm

shells pre-treated with KOH impregnation. It has been

suggested that oil-palm shells, abundant tropical solid

wastes from palm-oil processing mills, be utilised as

starting materials for the preparation of activated

carbons.8 Chars with suf®cient densities and relatively

high porosity have been produced from oil-palm

shells.9 In this study, chars from oil-palm shells pre-

treated with KOH impregnation were subjected to

CO2 activation for the preparation of activated

carbons, which were then used for SO2 adsorption

tests. The effects of SO2 concentration, temperature,

ed 22 June 2000)

ring, School of Mechanical and Production Engineering, Nanyang

68±2575/2000/$30.00 971

J Guo, AC Lua

sample size and impregnating agent on the adsorptive

capacity were investigated. Desorptions at the same

temperature of adsorption and a higher temperature of

200°C were carried out. The chemical characterisa-

tions of inorganic components and surface organic

functional groups of the activated carbons from oil-

palm shells pre-treated with KOH and H3PO4 were

also analysed. KOH10,11 and H3PO412,13 are two

commonly used activation agents for activated carbon

production. Besides enhancing the pore development,

these activation agents are supposed to modify the

chemical properties of the samples produced.14 In this

study, the adsorptive capacity of the oil-palm shell

activated carbon was also compared with some

commercial activated carbons.

EXPERIMENTALPreparation of activated carbonOil-palm shells from a palm-oil mill in Selangor,

Malaysia were ®rst crushed and sieved to different

sizes, namely, 0.3±1.0mm, 1.0±2.0mm, 2.0±2.8mm

and 2.8±4.0mm. After impregnated with 10% KOH

or 30% H3PO4 at room temperature for 24h, the shells

were ®ltered, washed with hot water and dried.

Thereafter, the sample was carbonised in a vertical

tube furnace (818P, Lenton) under a nitrogen ¯ow

(150cm3 minÿ1). The furnace temperature was raised

at a rate of 10°C minÿ1 from room temperature to

600°C and held at this temperature for 2h. The

resulting chars were activated with carbon dioxide

(¯ow rate 100cm3 minÿ1) at 800°C for 1h to produce

the ®nal products. This activation temperature of

800°C and hold time of 1h were found to be optimal

parameters to derive the best physical characteristics

or the highest BET surface area. The textural charac-

teristics of the raw oil-palm shell and the activated

carbons from oil-palm shells (size 1.0±2.0mm) im-

pregnated with 10% KOH and 30% H3PO4 are shown

in Table 1.

SO2 adsorption testThe uptake of SO2 onto the oil-palm shell activated

carbons was measured by a thermogravimetric analy-

ser (TA-50, Shimadzu). Before the tests, the samples

were dried at 110°C for 2h to remove the moisture

adsorbed during storage. SO2 gas of various concen-

Table 1. Physical characteristics of the starting material and the activated carbons

Sample

Solid

density a

(gcmÿ3)

Apparent

density b

(gcmÿ3)

BET

surface are

(m2gÿ1

Oil-palm shell 1.53 1.47 1.6

Carbon 10% KOH 2.18 0.85 1408

Carbon 30% H3PO4 2.21 0.81 1452

a Measured by an ultra-pycnometer (UPY-1001, Quantachrome).b Measured by a mercury intrusion porosimeter (PoreSizer-9320, Micromeritics).c Measured by an accelerated surface area and porosimetry (ASAP-2000, Micromd Calculated from 1/raÿ1/rs, where ra is the apparent density and rs the solid den

972

trations (balanced by N2) was introduced into the

analyser chamber where a platinum sample holder

containing a sample of about 20mg was suspended.

The subsequent sample weight gain due to adsorbed

SO2 was recorded. Tests were conducted at different

temperatures to understand the temperature effect on

adsorption. Immediately after adsorption, desorptions

in a nitrogen ¯ow at the same temperature of

adsorption and then a higher temperature of 200°Cwere carried out to check whether chemisorption had

occurred.

Chemical characterisationFor chemical characterisation, an X-ray diffractometer

(PW-1830, Philips) was used to investigate the

inorganic components of the activated carbons. The

X-ray patterns were recorded in the scan range

2y=10±70°, at a scan rate of 0.1 degree per minute.

The surface organic functional groups were studied by

Fourier transform infrared spectroscopy (FTIR-2000,

Perkin Elmer). The spectra were recorded from 4000

to 400cmÿ1.

RESULTS AND COMMENTSEffect of SO2 concentrationFigure 1 shows the effects of SO2 concentration on the

amount of SO2 adsorbed at room temperature of 25°Conto the activated carbons from oil-palm shells (1.0±

2.0mm) impregnated with 10% KOH. For a low SO2

concentration of 500ppm, the amount of SO2

adsorbed was relatively small (18mg gCÿ1) and a long

time (102min) was required to reach equilibrium.

However, for the SO2 concentration of 2000ppm, the

amount adsorbed was as high as 76mg per gram of

activated carbon and the equilibrium time was only

54min, suggesting that the adsorption capacity of the

activated carbon is directly proportional to the con-

centration of the adsorbate.15

Effect of temperatureThe amounts of SO2 adsorbed from 2000ppm SO2

¯ow at various temperatures (25±100°C) onto the

activated carbons from oil-palm shells (1.0±2.0mm)

impregnated with 10% KOH are shown in Fig 2. With

increasing of adsorption temperature, the amounts of

SO2 adsorbed decreased signi®cantly, especially at 75

a c

)

Micropore

surface area c

(m2gÿ1)

Total

pore volume d

(cm3gÿ1)

Micropore

volume c

(cm3gÿ1)

0.2 0.03 ±

625 0.72 0.40

690 0.78 0.43

eritics).

sity.

J Chem Technol Biotechnol 75:971±976 (2000)

Figure 2. Effect of temperature on the amount of SO2 adsorbed.

Figure 1. Effect of SO2 concentration on the amount of SO2 adsorbed.

Adsorption of sulfur dioxide onto activated carbons

or even 100°C. This is because during the adsorption

process, the SO2 molecules lose their kinetic energies,

making adsorption an exothermic process.16 There-

fore, low ambient temperature is favourable for the

adsorption process to take place.

Table 2 shows the amount of SO2 desorbed under

different desorption temperatures. Regardless of the

temperature at which adsorption took place, the SO2

adsorbed could be divided into two parts: SO2(I) that

was weakly bonded to the carbon surface and easily

desorbed at the same temperature of adsorption; and

SO2(II) that was strongly bonded to the carbon surface

and only desorbed at a higher temperature of 200°C.

This suggested that some chemisorption reactions

occurred during the adsorption process. It could be

seen that with increasing adsorption temperature from

Table 2. Amount of SO2 desorbedunder different desorptiontemperatures

Adsorption

temperature

(°C)

Amount

adsorbed

(mg gCÿ1)

25 76

50 40

75 17

100 8

a Desorption was carried out at the sb Desorption was carried out at 200

J Chem Technol Biotechnol 75:971±976 (2000)

25°C to 100°C, the proportion of SO2 adsorbed as

SO2(I) decreased by about 10% and therefore the

proportion of SO2 adsorbed as SO2(II) increased. As

mentioned before, a higher temperature was not

favourable for physical adsorption (Table 2, column

3). Therefore, the proportion of physical adsorption

dropped signi®cantly (Table 2, column 4) whilst a

higher proportion of chemisorption (Table 2, column

6) resulted. Nevertheless, the total amount of SO2

adsorbed (attributed to the combined physical adsorp-

tion SO2(I) and chemisorption SO2(II)) declined

progressively with increasing adsorption temperature,

as clearly shown in Fig 2 and the second column of

Table 2.

For a ®xed adsorption temperature and a ®xed SO2

concentration, the rate of adsorption can be expressed

by the following Arrhenius equation:17

d�=dt � K�1ÿ ��n � A exp�ÿE=RT ��1ÿ ��n �1�where K is the rate constant, n the reaction order, Athe frequency factor, E the activation energy, R the gas

constant, T the absolute temperature, t the time and athe fractional weight at time t. a is de®ned in terms of

the change in weight of the sample given by:

� � �Wf ÿW �=�Wf ÿW0� �2�where W0, W and Wf are the initial (activated carbon

only), existing and ®nal (after SO2 adsorption equili-

brium) weights of the sample, respectively. Equation

(1) can be rearranged to the following form:

d�=�1ÿ ��n � K dt �3�Integrating eqn (3), the following expressions can be

obtained.

�1ÿ �1ÿ ��1ÿn�=�1ÿ n� � Kt �for n 6� 1� �4a�or ÿ ln�1ÿ �� � Kt �for n � 1� �4b�

For a certain adsorption temperature Ti, ni and Ki can

be obtained by linear regression using the data from

the adsorption curves. Then, the activation energy Eand frequency factor A can be calculated from a group

of Ti and Ki values using the least square method. The

calculated results are listed in Table 3.

It can be seen from Table 3 that for all the

adsorption temperatures studied here, the reaction

orders are around 1. Increasing adsorption tempera-

SO2(I) a

(mg gCÿ1)

Percentage

(%)

SO2(II) b

(mg gCÿ1)

Percentage

(%)

65 85.5 11 14.5

33 82.5 7 17.5

13 76.5 4 23.5

6 75.0 2 25.0

ame temperature for adsorption.

°C.

973

Table 3. Effects of adsorptiontemperature on the adsorption kineticparameters

Temperature

(°C)

Reaction

order

Rate constant

(minÿ1)

Activation energy

(kJ molÿ1)

Frequency factor

(minÿ1)

Correlation

coef®cient

25 0.97 0.057

50 0.96 0.045

75 0.98 0.037 13.2 3.3�103 0.98

100 1.02 0.016

J Guo, AC Lua

ture signi®cantly diminished the reaction rate. The

activation energy of 13.2kJ molÿ1 and the frequency

factor of 3.3�103 minÿ1 were obtained for the

adsorption of SO2 onto the activated carbon from

oil-palm shell impregnated with KOH. The relatively

small activation energy suggests a feasible and easy

adsorption process for SO2.

Effect of particle sizeFigure 3 shows the SO2 adsorption at 25°C onto the

activated carbons from oil-palm shells of different sizes

impregnated with 10% KOH. For all particle sizes

studied, the same equilibrium value was reached.

However, their equilibrium times were quite different.

The larger the sample particle size, the longer was the

equilibrium time. Table 4 shows that the reaction

orders were almost the same for all particle sizes but

the adsorption rate decreased with increasing sample

particle size. This was due to the effect of sample size

on the SO2 diffusion rate into the particles. Bailey18

and Cheremisinoff and Cheremisinoff2 also observed

that particle size affected both the external mass

(adsorbate) transfer and the internal diffusion rate.

For the diffusion rate, it was found to be inversely

proportionate to the square of the particle diameter.

Effect of impregnating agentFigure 4 shows the amounts of SO2 adsorbed by the

activated carbons impregnated with 10% KOH and

30% H3PO4. The latter activated carbon had larger

BET and micropore surface areas than the former

(refer to Table 1). For the adsorption temperatures of

25 and 50°C, the amount of SO2 adsorbed by the

sample pre-treated with 10% KOH was obviously

larger than that pre-treated with 30% H3PO4. This

Figure 3. Effect of sample particle size on the amount of SO2 adsorbed.

974

suggested that the adsorptive capacity of the chemi-

cally activated carbon was not only proportional to

their surface areas, but also dependent on the

impregnating agent used or the surface chemistry of

the activated carbon.

Inorganic componentsThe X-ray diffraction patterns of the activated carbons

pre-treated with 10% KOH and 30% H3PO4 impreg-

nation are shown in Fig 5. For different impregnating

agents used, CaCO3 as Calcite (C) and Vaterite (V)

were detected as dominant components. The effects of

these two impregnating agents on the surface inor-

ganic components were quite similar. The surface

inorganic components as compared to the surface

organic functional groups, had relatively limited

effects on the adsorptive capacity of the activated

carbon.14

Surface organic functional groupsThe FTIR spectra of the activated carbons pre-treated

with 10% KOH and 30% H3PO4 are shown in Fig 6.

The spectrum of the sample pre-treated with 30%

H3PO4 solution displayed the following bands:

3608cmÿ1, free OÐH stretches; 1725cmÿ1, C=O

stretch in ketones; 1642cmÿ1, C=O stretch in acids;

1506cmÿ1, C=C stretch in aromatic rings; and

1219cmÿ1, CÐOH stretch in alcohols. The main

surface organic functional groups present were pre-

sumed to be phenols and carboxylic acids, typical

acidic functional groups, which were ®rst proposed by

Garten and Weiss.19 These acidic functional groups

are favourable for adsorbing alkaline gases but not

acidic gases such as SO2. From this observation, the

samples impregnated with 30% H3PO4 absorbed a

smaller amounts of SO2 than those treated with 10%

KOH.

On the other hand, the spectrum of the sample

impregnated with 10% KOH displayed different

bands: 1754cmÿ1, C=O stretch in ketones;

Table 4. Effects of sample particle size on the adsorption kinetic parameters

Sample particle

size (mm)

Equilibrium Time

(min)

Reaction

order

Rate constant

(minÿ1)

0.3 ±1.0 39 0.93 0.076

1.0 ±2.0 54 0.97 0.057

2.0 ±2.8 85 0.96 0.026

2.8 ±4.0 114 0.94 0.014

J Chem Technol Biotechnol 75:971±976 (2000)

Figure 4. Effect of impregnating agent on the amount of SO2 adsorbed.

Figure 5. Effect of impregnation on the inorganic components.

Figure 6. Effect of impregnation on the surface organic functional groups.

Adsorption of sulfur dioxide onto activated carbons

1503cmÿ1, C=C stretch in aromatic rings;

1251cmÿ1, CÐO stretches; and 814/695cmÿ1, CÐH

out-of-plane bending in benzene derivatives. These

Table 5. Comparison between oil-palm shellactivated carbon and commercial products

Sample

Oil-palm shell

Oil-palm shell

Microcarb1

Carbochem1

J Chem Technol Biotechnol 75:971±976 (2000)

bands were presumed to be alkaline groups of pyrones

(cyclic ketone) and other keto-derivatives of pyran,

which were proposed by Jankowska et al. 20 These

functional groups contributed to the adsorption of SO2

gas onto the activated carbons impregnated with

KOH. In brief, the adsorptive capacities of chemically

activated carbons are strongly related to their surface

organic functional groups.

Comparison with commercial productsSO2 adsorptive capacities of two commercial activated

carbons prepared from bituminous coal with chemical

treatment were determined so that comparison with

that of the oil-palm shell activated carbon developed in

this study could be possible. One was Microcarb1

activated carbon (Carbon Link Limited; activation

method: steam; grade: AG 138; bulk density:

0.49gcmÿ3), whilst the other was Carbochem1

activated carbon (Carbochem Inc; activation method:

steam; grade: GS-75; bulk density: 0.53gcmÿ3).

Table 5 shows the comparative test results for these

commercial activated carbons and the oil-palm shell

activated carbon for SO2 (2000ppm) adsorption at

25°C. The amount of SO2 adsorbed onto the activated

carbon from oil-palm shell with KOH impregnation

was clearly higher than those of commercial ones. This

might be attributed to the higher surface area of the

activated carbon. However, for the Carbochem1

sample with similar BET surface area, the amount of

SO2 adsorbed onto this sample was lower than that of

the oil-palm shell activated carbon with KOH impreg-

nation due to the use of ZnCl2 as the activation agent,

which possibly resulted in weak-acidic (near neutral)

surface functional groups on the Carbochem1 sample.

In brief, the adsorptive capacity of the activated carbon

from oil-palm shell with KOH impregnation was

comparable to those commercial ones.

CONCLUSIONSExperimental results on the adsorption of SO2 onto

the activated carbons prepared from oil-palm shells

pre-treated with KOH impregnation showed that SO2

concentration and adsorption temperature would

signi®cantly affect the amount adsorbed and the

equilibrium time. Higher SO2 concentration and lower

adsorption temperature were favourable for adsorp-

tion. Sample particle size had only an effect on the

equilibrium time due to the effect of diffusion rate.

Desorption at the same temperature for adsorption

and a higher temperature of 200°C con®rmed the

Activation agent

BET surface area

(m2gÿ1)

SO2 adsorbed

(mggÿ1)

10% KOH 1408 76

30% H3PO4 1452 63

Neutralisation 1195 72

ZnCl2 1380 68

975

J Guo, AC Lua

presence of chemisorption. Comparing the activated

carbon pre-treated with 30% H3PO4 with the sample

pre-impregnated with 10% KOH, the former with

larger BET and micropore surface areas had a lower

adsorptive capacity for SO2 than the latter. Impreg-

nation with different agents had limited effects on the

inorganic components but signi®cant effects on their

surface organic functional groups, which were closely

related to the adsorptive capacity of these chemically

activated carbons. In brief, experimental results

showed that SO2 could be adsorbed effectively by

KOH-impregnated oil-palm shell activated carbons,

whose adsorptive capacities were comparable to those

of some commercial activated carbons.

REFERENCES1 Knoblauch K, Juntgen H and Peters W, The Bergbau±Forschung

process for the desulfurization of ¯ue gases, In Proceeding of 4th

International Clean Air Congress, Tokyo. pp 722±726 (1977).

2 Cheremisinoff NP and Cheremisinoff PN, Carbon Adsorption for

Pollution Control, PTR Prentice Hall, New Jersey (1993).

3 Varma HB, Air Pollution Control Equipment, Springer-Verlag,

New York (1981).

4 Lu GQ and Do DD, `Retention of sulfur dioxide as sulfuric acid

by activated coal reject char'. Sep Technol 3:106±110 (1993).

5 Davini P, `Investigation on the adsorption and desorption of

sulphur dioxide on active carbon in the temperature range

between 130°C and 170°C'. Carbon 29:321±327 (1991).

6 Daley MA, Mangun CL, DeBarr JA, Riha S, Lizzio AA, Donnals

GL and Economy J, `Adsorption of SO2 onto oxidized and

heat-treated activated carbon ®bers (ACFs)'. Carbon 35:411±

417 (1997).

976

7 Gray PG, `A fundamental study on the removal of air pollutants

(sulfur dioxide, nitrogen dioxide and carbon dioxide) by

adsorption on activated carbon'. Gas Sep. Purif 7:213±224

(1993).

8 Tay JH, `Complete reclamation of oil palm wastes'. Res Conserv

Recy 5:383±392 (1991).

9 Guo J and Lua AC, `Characterization of chars pyrolyzed from oil

palm shells for the preparation of activated carbons'. J Anal

Appl Pyrolysis 46:113±125 (1998).

10 Laine J and Calafat A, `Factors affecting the preparation of

activated carbons from coconut shell catalyzed by potassium'.

Carbon 29:949±953 (1991).

11 Hu Z and Vansant EF, `Carbon molecular sieves produced from

walnut shell'. Carbon 33:561±567 (1995).

12 Girgis BS, Khalil LB and Taw®k AM, `Activated carbon from

sugar cane bagasse by carbonization in the presence of

inorganic acids'. J Chem Technol Biotechnol 61:87±92 (1994).

13 Jagtoyen M and Derbyshire F, `Activated carbons from yellow

poplar and white oak by H3PO4 activation'. Carbon 36:1085±

1097 (1998).

14 Bansal RC, Donnet JB and Stoeckli F, Active Carbon, Marcel

Dekker, New York (1988).

15 Cheremisinoff PN, Air Pollution Control and Design for Industry,

Marcel Dekker, Inc, New York (1993).

16 Yang RT, Gas Separation by Adsorption Processes, Butterworths,

Boston (1987).

17 Cheng A and Harriott P, `Kinetics of oxidation and chemisorp-

tion of oxygen for porous carbons with high surface area'.

Carbon 24:143±150 (1986).

18 Bailey A, Application of active carbons for gas separation and

respiratory protection, in Porosity in Carbons, Ed by Patrick JW,

Edward Arnold, London. pp 209±224 (1995).

19 Garten VA and Weiss DE, Australian J Chem 10:309 (1957).

20 Jankowska H, Swiatkowski A and Choma J, Active Carbon, Ellis

Horwood, UK. pp 309±314 (1991).

J Chem Technol Biotechnol 75:971±976 (2000)