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Carbon 45 (2007) 330336Adsorption of hydrogen sulphide (H2S) by activated carbonsderived from oil-palm shell

Jia Guo a,*, Ye Luo b, Aik Chong Lua c, Ru-an Chi a, Yan-lin Chen a,Xiu-ting Bao a, Shou-xin Xiang a

a Hubei Key Laboratory of Novel Reactor and Green Chemical Technology, School of Chemical and Pharmaceutical Engineering,

Wuhan Institute of Technology, Wuhan 430073, PR Chinab School of Physical Science, Wuhan Institute of Technology, Wuhan 430073, PR China

c School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore

Received 17 January 2006; accepted 14 September 2006Available online 28 November 2006Abstract

Adsorption of hydrogen sulphide (H2S) onto activated carbons derived from oil palm shell, an abundant solid waste from palm oilprocessing mills, by thermal or chemical activation method was investigated in this paper. Dynamic adsorption in a fixed bed configu-ration showed that the palm-shell activated carbons prepared by chemical activation (KOH or H2SO4 impregnation) performed betterthan the palm-shell activated carbon by thermal activation and a coconut-shell-based commercial activated carbon. Static equilibriumadsorption studies confirmed this experimental result. An intra-particle Knudsen diffusion model based on a Freundlich isotherm wasdeveloped for predicting the amount of H2S adsorbed. Desorption tests at the same temperature as adsorption (298 K) and at an elevatedtemperature (473 K) were carried out to confirm the occurrence of chemisorption and oxidation of H2S on the activated carbon. Surfacechemistries of the palm-shell activated carbons were characterized by Fourier transform infrared spectroscopy and Boehm titration. Itwas found that uptaking H2S onto the palm-shell activated carbons was due to different mechanisms, e.g. physisorption, chemisorptionand/or H2S oxidation, depending on the activation agent and activation method. 2006 Elsevier Ltd. All rights reserved.1. Introduction

Hydrogen sulphide (H2S), a rotten-egg smell gas pro-duced by anaerobic digestion in acid condition fromorganic and inorganic compounds containing sulphur, pre-sents dual problems of its toxicity and foul odour. H2S canbe detected by most people as low as 0.0047 ppm [1].Although no significant physical harm may be caused insuch a low concentration, the exposure to this nuisancecan lead to nausea, loss of appetite, and other negativeeffects. From the point of view of engineering design of apurification system, the odour threshold of H2S is0008-6223/$ - see front matter 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2006.09.016

* Corresponding author. Tel.: +86 27 87194980; fax: +86 27 87194465.E-mail address: guojia@mail.wit.edu.cn (J. Guo).0.18 ppm [2]. Various measures can be applied to reduceor eliminate H2S emissions from different sources, particu-larly sewage treatment facilities. Among these measures,the use of activated carbon is most successful and wide-spread due to its safety and effectiveness of the operationprocess [3]. Usually, activated carbons used for H2Sadsorption are those activated carbons modified with caus-tic chemicals such as KOH and NaOH [4] or oxidativeagents KI and KMnO4 [5] which promote oxidation ofH2S to elemental sulphur. All of these carbon modifica-tions, however, create the hazard of fixed-bed self-ignitionowing to the high heat released by the reactions [6]. More-over, the adsorptive capacity of the activated carbon maybe significantly decreased by the modification process sincethe micropores are not accessible due to the blockage ofpore entrances [7]. For these reasons the use of activated

mailto:guojia@mail.wit.edu.cn

Nomenclature

C gas-phase adsorbate concentration (mol m3)Dk Knudsen diffusion coefficient (m

2 s1)F constant in Freundlich isotherm (dimensionless)L length of the fixed bed (m)LMTZ length of the mass transfer zone (m)m constant in Freundlich isotherm (dimensionless)Ms mass of the adsorbate entered the particle (g)M1 mass adsorbed after infinite time (g)r radial coordinate of the spherical particle of the

adsorbent (m)R radius of the adsorbent particle (m)

Greek letters

e total porosity of the adsorbent (%)s time elapsed (s)

s0.05 breakthrough time, defined as the time when theoutlet concentration at 5% of the inlet concen-tration (s)

s0.95 exhaustion time, defined as the time when theoutlet concentration at 95% of the inlet concen-tration (s)

n tortuosity factor related to the path of the mol-ecule (dimensionless)

Subscriptse effective value of corresponding diffusivityi serial number0 initial state

J. Guo et al. / Carbon 45 (2007) 330336 331carbons prepared by chemical activation, particularly fromcarbon-enriched lignocellulosic precursors, as H2S adsor-bents would be beneficial for environment as well as formunicipal facilities.

In present work, dynamic and static adsorption of H2Son oil-palm-shell activated carbons by different activationmethods as well as on a coconut-shell-based commercialactivated carbon was studied. Oil-palm shell (also calledendocarp) is a cheap and abundant agricultural solid wastein tropical countries such as Malaysia and Thailand. Palmshells have been successfully converted into well-developedactivated carbons used for removal of various gaseous pol-lutants [8,9]. However, there are few publications in the sci-entific literature that report on the preparation of activatedcarbon from palm shell impregnated with KOH and H2SO4and even far fewer on use of such adsorbents for H2Sremoval. The objective of the study is to reveal the reactionmechanisms (e.g. physisorption, chemisorption and/or H2Soxidation) between H2S and the palm-shell activated car-bons prepared by various activation methods.2. Experimental

2.1. Adsorbent preparation

Palm shells from a palm-oil mill in Selangor, Malaysia were dried,crushed and sieved to a particle size fraction of 1.02.0 mm. 10 g of thisprocessed starting material was impregnated with 200 ml of H2SO4(40%) or KOH (30%) at room temperature (298 K) for 24 hours and thendried at 383 K overnight. The mixture was activated in a stainless-steelreactor (550 mm length and 38 mm i.d.) under a stream of nitrogen (N2)gas flowing at 150 cm3/min. The reactor was heated in a vertical tube fur-nace (818 P, Lenton) from room temperature to a pre-set temperature(973 K) for 2 h. After cooling to room temperature, the resulting productswere taken out and leached with distilled water. In another comparativestudy, the preparation of activated carbons by thermal activation with car-bon dioxide (CO2) gas (100 cm

3/min) at 1173 K for 2 h was carried out. Acoconut-shell-based commercial activated carbon by steam (H2O) activa-tion (CAC, Damao Chemicals and Chemical Apparatus, Tanjin) was alsoused for testing. The physical properties of the palm-shell and coconut-shell activated carbons are given in Table 1.2.2. Characterizations

Proximate analysis and ultimate analysis were carried out by a thermo-gravimetric analyzer (TGA-50, Shimadzu) and an elemental analyzer(CHN-932, LECO) respectively. Textural characteristics of samples weredetermined by N2 adsorption at 77 K with an accelerated surface areaand porosimeter (ASAP-2000, Micromeritics). The specific surface areawas calculated from the isotherms by the Brunauer-Emmett-Teller(BET) equation [10]. The DubininRadushkevich (DR) equation was usedto calculate the micropore volume from the N2 adsorption data [11].2.3. Chemical characterization

Boehm titration with sodium hydroxide, sodium carbonate, sodiumbicarbonate, and sodium methoxide solutions was carried out to deter-mine the oxygenated surface functional groups under the assumption thatNaOH neutralizes carboxyl, phenolic and lactonic groups; Na2CO3 neu-tralizes carboxyl and lactonic; and NaHCO3 only carboxyl groups. Forgeneral surface functional groups, the samples were studied by a Fouriertransform infrared spectrometer (FTIR-2000, Perkin Elmer). The spectrawere recorded from 4000 to 400 cm1. By comparing to the standard fre-quency patterns, various characteristic chemical bonds (or stretchings)were determined, from which certain surface functional groups could bederived. X-ray photoelectron spectroscopy (MK-II, Vacuum Generator)was used to examine the change of surface chemistry. It was measuredusing Mg Ka radiation generated at 12.5 kV and 250 W. The 1 s core-levelscan spectra were taken for the surfaces of the activated carbon before andafter adsorption process.2.4. Adsorption and desorption of H2S

Dynamic adsorption of H2S was conducted in a copper column(10.0 mm i.d. and 20 cm long) filled with pre-dried activated carbons,which were retained on a metal mesh at the bottom of the column. Thecolumn was operated in the up-flow mode. H2S adsorbate of 2000 ppm(balanced by pure He gas) from a gas cylinder was introduced to the inletof the adsorption column at a certain volumetric flow rate (90 cm3/min).The flow rate was controlled by a rotameter (MVIP804, MDA Scientific),which was calibrated carefully with the soapfilm burette method [12]. All

Table 1Physical properties of the coconut shell and palm shell activated carbons

Sample Palm shell activatedcarbon by CO2activation

Coconut shell activatedcarbon by H2Oactivation

Palm shell activatedcarbon by KOHactivation

Palm shell activatedcarbon by H2SO4activation

Nitrogen adsorptionPorosity (%) 58.4 67.4 56.3 59.5BET surface area (m2/g) 1062 1183 1148 1014Micropore volume (cm3/g) 0.26 0.35 0.25 0.28Proximate analysis (wt.%)Volatile matter 6.4 4.5 10.6 14.2Fixed carbon 86.1 91.9 83.0 82.0Ash 7.5 3.6 6.4 3.8Ultimate analysis (wt.%)Carbon 87.1 87.5 84.1 83.3Hydrogen 1.0 0.9 0.8 0.4Nitrogen 1.1 2.0 0.6 0.2Oxygen (by difference) 10.8 9.6 14.5 16.1

332 J. Guo et al. / Carbon 45 (2007) 330336the runs were carried out at the laboratory temperature of 298 K and anambient pressure of 1.0 bar. The time-dependent concentration at theexit of the column was continually monitored by a flame photometricdetector (FPD, Fisher-Rosemount). The column was operated until satu-ration, that is, when the outlet concentration became equal to that of theinlet.

Static adsorption of H2S was also carried out in the TGA. H2S(1000 ppm, balanced by He) was introduced into the analyzer chamber,where a platinum pan with about 20 mg of adsorbent was suspended.At room temperature of 298 K, the subsequent sample weight gain dueto the amount of gas adsorbed onto the adsorbent was recorded. Afteradsorption, desorption tests using He gas flushing through the analyzerchamber at the same temperature of adsorption and a higher temperatureof 473 K were carried out to verify the occurrence of chemisorption and/oroxidation of H2S.3. Results and discussion

H2S breakthrough curves for activated carbons pre-pared from coconut-shell by steam activation and palm-shell by different activation methods are shown in Fig. 1.In a fixed bed for adsorption, the part that displays a gra-200 240 280 320 360 400

Time (min)

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

con

cent

ratio

n C

/C0 Palm shell: CO2 activation

Coconut shell: H2O activation

Palm shell: KOH activation

Palm shell: H2SO4 activation

Fig. 1. Breakthrough curves for activated carbons prepared from coconutshell by steam activation and palm shell by different activation methods.dient in adsorbate concentration from zero to equilibriumis called the mass transfer zone (MTZ). This is the activepart of the bed where adsorption actually takes place. Asthe saturated part of the bed increases, the MTZ travelsdownstream and eventually exits the bed. The length ofthe MTZ (LMTZ) may be estimated as follow [13]:

LMTZ Ls0:95 s0:05s0:95 s0:05=2

1Table 2 shows the breakthrough-curve characteristicparameters for the coconut-shell and palm-shell activatedcarbons. Compared to the palm-shell activated carbon bythermal activation and coconut-shell activated carbon,the activated carbons prepared by KOH and H2SO4 activa-tion had better dynamic adsorption performances due tolonger breakthrough times, prolonged exhaustion times,and relatively short MTZs. Therefore, the correspondingadsorption capacities were also larger [14,15]. This suggeststhat the palm-shell activated carbons prepared by chemicalactivation are more capable of H2S adsorption.

In the gas-phase adsorption model, the basic assump-tions are as follows:

1. The activated carbon particles are perfect spheres; diffu-sion taking place along the radial direction (r) on theadsorption sites that are uniformly distributed withinthe particles.

2. External mass convective flux is negligible in compari-son to transport by Knudsen diffusion. Therefore, theexternal adsorbate concentration can be considered tobe uniform (C0).

3. The adsorbent is isothermal (temperature constant) dur-ing the adsorption process.

4. Adsorption at the pore mouth follows a Freundlichequilibrium isotherm.

5. The porosity e and the tortuosity factor n of the particleare regarded as constant during the adsorption process.

6. No chemical reactions occur at the adsorbent surface.

Table 2Breakthrough-curve characteristic parameters for the coconut shell and palm shell activated carbons

Activated carbon Breakthrough time s0.05(min)

Exhaustion times0.95(min)

Length of MTZ LMTZ (cm) Adsorption capacity(mg/g)

Palm shell: CO2 activation 245.6 316.1 5.0 46Coconut shell: H2O

activation270.7 359.4 5.6 53

Palm shell KOH activation 291.5 323.8 2.1 68Palm shell: H2SO4 activation 304.3 334.0 1.9 76

0 10 20 30 40 50 60

Time (min)

0

40

80

120

160

200

Am

ount

of H

2S a

dsor

bed

(mg/

g)

Model prediction

Coconut shell: H2O activation

Palm shell: CO2 activation

Fig. 2. Experimental results and model predictions of amount H2Sadsorbed onto coconut shell and palm shell activated carbons.

J. Guo et al. / Carbon 45 (2007) 330336 333Based on these assumptions, the intra-particle masstransfer can be described by the following equation [16]:

eoCos

1 e oClos

er2

o

orDker2

oCor

2

The first and second terms represent accumulation ofadsorbate molecules in gaseous and solid phases withinthe adsorbent particle respectively, while the term on theright-hand side represents Knudsen diffusion of adsorbatemolecules in the gas phase within the particle. In adsorp-tion of gas pollutant by activated carbon, the adsorbedphase is strongly favoured and so the first term in Eq. (2)can be neglected without loss of accuracy.

The initial condition is

C 0 at s 0 for 0 < r < R 3aThe boundary conditions are

oCor

0 at r 0 for all s 3b

C C0 at r r0 for s > 0 3cIf the equilibrium isotherm follows Freundlich type, therelationship between Cl and C can be expressed by the fol-lowing equation:

Cl FCm 4Combining Eqs. (2) and (4), the following equation can beobtained:

oCos

eDke1 e FmC

1m o2Cor2

5

The coefficient eDke1e Fm C

1m can be regarded as a concentra-tion-dependent diffusion coefficient, which links the degreeof asymmetry in the rates of adsorption to the Freundlichisotherm constant m.

The mass of the adsorbate that has entered the particle,Ms, can be found by integrating the adsorbate adsorbedover the entire adsorbent particle, i.e.

M s 3M1Z R0

r2Cm r; s dr 6

However, an analytical solution for Eq. (5) is not availableand a numerical method has to be employed. The non-lin-ear partial differential Eq. (5) was converted into a set oftime-dependent, non-linear, ordinary differential equationsusing orthogonal collocation [17].

Fig. 2 shows experimental results and model predictionsof amount H2S adsorbed onto the coconut-shell and palm-shell activated carbons. The Freundlich constants wereobtained from experimental data under different adsorp-tion temperatures by curve fitting. In this study, the Fre-undlich constants F of 1.19 103 and m of 0.84 wereused. It could be seen that the predicted results by diffusionmodel based on Freundlich isotherm agreed with the exper-imental data very well. One minor disadvantage of thismodel is the need for a numerical solution, which is moretime-consuming than the analytical integration procedurefor linear isotherm model.

Experimental results and model predictions of amountH2S adsorbed on the palm-shell activated carbons preparedby chemical activation are shown in Fig. 3. For both KOHand H2SO4 used as activation agent, the diffusion modelbased on Freundlich isotherm always under-estimated theamount of H2S adsorbed. The possible reason was that thismodel did not take into account chemical reactions (e.g.chemisorption or H2S oxidation), which are related to thesamples surface chemistry.

The results of desorption at room temperature and473 K are shown in Table 3. For the activated carbons pre-pared by physical activation, all (100%) H2S adsorbedcould be desorbed at room temperature, indicating a purephysisorption process involved. However, besides largepart of H2S that was weakly bonded to the chemically acti-vated carbons and easily desorbed at room temperature,some H2S required a higher temperature (473 K) fordesorption, suggesting that chemisorption occurred. For

0 10 20 30 40 50 60

Time (min)

0

50

100

150

200

250

Am

ount

of H

2S a

dsor

bed

(mg/

g)

Model prediction

Palm shell: H2SO4 activation

Palm shell: KOH activation

Fig. 3. Experimental results and model predictions of amount H2Sadsorbed on palm shell activated carbons prepared by chemical activation. 4000 3400 2800 2200 1600 1000 400

Wave Number (cm-1)

Tra

nsm

ittan

ce (

arbi

trar

yun

its)

H2SO4 activation

KOH activation

CO2 activation

659

1745 1

251

2439

754

1507

1723

1627

1502

1613

1504

3415

1204

Fig. 4. FTIR spectra of activated carbons prepared from palm shell bydifferent activation methods.

334 J. Guo et al. / Carbon 45 (2007) 330336the activated carbon prepared by H2SO4 activation, evenafter heated up to 473 K, there were still some residualweights on the carbon surface, suggesting that some irre-versible chemical reactions occurred and indecomposableproducts attached to the sample.

In order to further investigate their surface chemistries,FTIR spectra of activated carbons prepared from palmshell by different activation methods were measured andthe results are shown in Fig. 4. The spectrum of the acti-vated carbon prepared by 30% KOH impregnation dis-played the following bands: 1754 cm1, C@O stretch inketones; 1507 cm1, C@C stretch in aromatic rings;1251 cm1, CO stretches; and 754 cm1, CH out-of-plane bending in benzene derivatives. The main surfacefunctional groups present on the KOH-impregnated adsor-bent are presumed to be alkaline groups of pyrones (cyclicketone) and other keto-derivatives of pyran. In addition,the alkaline chromenes groups might also be present asproposed by Jankowska [18]. Therefore, the surface ofthe oil-palm-shell adsorbent with KOH impregnation isgiven below.

H O

O O R H

R

O C Table 3Desorption properties of the coconut shell and palm shell activated carbons

Activated carbon Total amountadsorbed (mg/g)

Percentage ofdesorption at 298 K

Palm shell: CO2activation

131 100

Coconut shell: H2Oactivation

162 100

Palm shell KOHactivation

169 79

Palm shell: H2SO4activation

203 83These surface functional groups account for the chemi-sorption of H2S gas.

The spectra of the activated carbon prepared by 40%H2SO4 impregnation in Fig. 4 displayed the followingbands: 3415 cm1: HO stretching in hydroxyl groups;1723 cm1: C@O stretching in carboxylic acids or isolatedcarbonyl groups; 1627 cm1: C@O stretching in quinonesor carboxylic anhydrides; 1502 cm1: C@C stretching inaromatic rings, and 1204 cm1: COC stretching in ethersor ether bridges between rings.

The oxygen functional groups are likely to be phenols,carboxylic acids (or carboxylic anhydrides if they are closetogether) and carbonyl groups (either isolated or arrangedin quinone-like fashion), all of which are typical acidic func-tional groups [19]. The surface structure of the H2SO4-impregnated palm-shell activated carbons is given below.

O HOC

O

HO C C

O O O O

O

(%)Percentage ofdesorption at 473 K (%)

Percentage ofresidue (%)

21

10 7

J. Guo et al. / Carbon 45 (2007) 330336 335For the activated carbon prepared by CO2 thermal acti-vation, the band at 2439 cm1 is attributed to CH stretch-ing vibrations in aliphatic structures, in addition to phenolsand the aromatic rings. In this case, the surface structure ofthe thermally activated carbon is given below:

C

H

H H HOFig. 5. XPS spectra of the activated carbon surfaces before and after H2Sadsorption processes.This agreed with the results of Boehm titration as shownin Table 4. For thermally activated carbons, only phenoland carbonyl groups were detected, whilst lactonic and car-bonyl groups were found on the surface of the activatedcarbon prepared by KOH impregnation. On the surfaceof the activated carbon prepared by H2SO4 impregnation,carboxyl, phenol, lactonic and carbonyl groups weredetected.

During chemical activation with H2SO4 for palm shells,the formation of oxygen functional groups can be repre-sented as follows:

CnHxOy H2SO4 ! H2O Selement CnHxOy3 7

During H2S adsorption, besides physisorption onto thepore surface due to van de Waals forces, chemisorptionby forming hydrogen bonding also occurred such that

CnHxOy3 H2S ! CnHxOy3 H2S 8Hydrogen atoms in H2S could strongly interact with oxy-gen in form of hydroxyl (HO) and carbonyl (C@O)groups due to the high electrostatic attraction. H2S cantherefore preferably adsorb onto active adsorption sitesprovided by oxygen functional groups via hydrogen bondsbesides on pure carbon sites. Other researchers also con-firmed the formation of hydrogen bonds during chemisorp-tion [2022]. In addition, due to the oxygen functionalgroups, H2S was easily oxidised as follows:

CnHxOy3 H2S ! CnHxOy2 Selement H2O 9

This may explain why, for the activated carbon preparedby H2SO4 activation, there were still some residual weightson the carbon surface when desorption was carried out atup to 473 K. Some irreversible chemical reactions (H2S oxi-dization) occurred and indecomposable products (elemen-tal sulphur) attached to the sample. The XPS spectra ofthe activated carbon surfaces before and after H2S adsorp-tion processes were shown in Fig. 5. [23,24]Table 4Number of surface groups (meq/g) obtained from Boehm titration

Activated carbon Carboxyl Lactone Phenol Carbonyl

Palm shell: CO2 activation 0.120 0.285Palm shell KOH activation 0.355 1.167Palm shell: H2SO4 activation 0.089 0.416 0.208 0.6044. Conclusions

From the adsorption of H2S onto the activated carbonsprepared from oil-palm shells with and without pre-impregnation, the following conclusions can be drawn:

(1) Compared to the palm-shell activated carbon by ther-mal activation and coconut-shell activated carbon, theactivated carbons prepared by KOH and H2SO4 acti-vation had better dynamic adsorption performancesdue to longer breakthrough times, prolonged exhaus-tion times, and relatively short MTZs. Therefore, thecorresponding adsorption capacities were also larger.

(2) The Freundlich constants were obtained from exper-imental data under different adsorption temperaturesby curve fitting. In this study, the Freundlich con-stants F of 1.19 103 and m of 0.84 were used. Itcould be seen that the predicted result by diffusionmodel based on Freundlich isotherm agreed withthe experimental data very well.

(3) For both KOH and H2SO4 used as activation agent,the diffusion model based on Freundlich isothermalways under-estimated the amount of H2S adsorbed.The possible reason was that this model did not takeinto account chemical reactions (e.g. chemisorptionor H2S oxidation), which are related to the samplessurface chemistry.

(4) The main surface functional groups present on theKOH-impregnated adsorbent are presumed to bealkaline groups of pyrones (cyclic ketone) and otherketo-derivatives of pyran. The oxygen functionalgroups on the H2SO4-impregnated sample are likelyto be phenols, carboxylic acids (or carboxylic anhy-drides if they are close together) and carbonyl groups(either isolated or arranged in quinone-like fashion),all of which are typical acidic functional groups.

336 J. Guo et al. / Carbon 45 (2007) 330336(5) Besides physisorption, some H2S adsorbed by theH2SO4-impregnated sample required a higher tem-perature for desorption, suggesting that chemisorp-tion via hydrogen bonding occurred. There werestill some residual weights on the carbon surfacewhen desorption was carried out at up to 473 K sinceH2S oxidization resulted in indecomposable productsof elemental sulphur attached to the sample.Acknowledgements

The author (Dr. J. Guo) appreciates the financial sup-port from the Program for New Century Excellent Talentsin University (Contract No. NCET-05-0681) and the Scien-tific Research Foundation for the Returned OverseasChinese Scholars (Contract No. 2005-383), Ministry ofEducation, PR China.

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Adsorption of hydrogen sulphide (H2S) by activated carbons derived from oil-palm shellIntroductionExperimentalAdsorbent preparationCharacterizationsChemical characterizationAdsorption and desorption of H2S

Results and discussionConclusionsAcknowledgementsReferences