Colloids and Surfaces A: Physicochem. Eng. Aspects 322 (2008) 164–169

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Adsorption of dimethyl sulfide vapors by activated carbons∗ t

014, In

imethed cagrou

easeson te am

heat tineticmostl sulfiethy

Meenakshi Goyal , Rashmi Dhawan, Mamta BhagaDepartment of Chemical Engineering & Technology, Punjab University, Chandigarh 160

a r t i c l e i n f o

Article history:Received 1 November 2007Received in revised form 28 February 2008Accepted 29 February 2008Available online 13 March 2008

Keywords:AdsorptionSurface groupsAdsorption kineticsIsosteric enthalpy of adsorption

a b s t r a c t

Adsorption isotherms of dsamples of fibrous activatof carbon–oxygen surfaceadsorption generally incrof carbon–oxygen groupssurface which enhances thgroups are eliminated onsulfide obeys first order kcoverage tending to be althe adsorption of dimethyacidic groups and the dimhydrogen bonding.

1. Introduction

Dimethyl sulfide (DMS) vapors are highly poisonous. The short

term exposure causes lung congestion [1,2], diarrhea, stomachpain and several other body ailments while its high concentrationin the air at inhabited places and in confined places in severalprocess industries can displace oxygen in the air and can causesuffocation and may even lead to death [2]. DMS is derived fromcooking of seafood and petroleum refining and several productionprocesses. It is also a metabolic product of many biosystems and isproduced abundantly by marine algae and is the principal volatilesulfur compound in seawater [3]. Thus, the massive production ofatmospheric DMS over the oceans may have a significant impact onthe Earth’s climate [4,5]. Oceanic DMS emissions account for 15% ofthe total global sulfur emissions of 3.2 Tg S/year. Although anthro-pogenic emissions dominate the global sulphur budget, naturalsulphur emissions are still a significant fraction (∼30%) of the totalsulphur emissions [6]. It is thus essential to develop suitable adsor-bents for DMS removal from working places of several industrialunits and from the inhabited places of the ships over the sea.

The adsorptive removal of DMS has been studied using severaltypes of adsorbents such as silica gel, zeolites and activated carbons.Tanada et al. [7,8] and Miyoshi et al. [9] investigated the adsorp-

∗ Corresponding author Tel.: +91 172 2534910 (O)/2583859 (R);fax: +91 172 2779173.

E-mail address: meenakshi chem@yahoo.co.in (M. Goyal).

0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.colsurfa.2008.02.047

dia

yl sulfide vapors have been studied on two samples of granulated and tworbons having different surface areas and associated with varying amountsps. The adsorption isotherms follow the Langmuir isotherm equation. Thewith increase in surface area and is strongly influenced by the presencehe carbon surface. The adsorption increases on oxidation of the carbonounts of these surface groups and decreases gradually when these surfacereatment in vacuum at 400◦, 650◦ and 950 ◦C. The adsorption of dimethyls. The isosteric enthalpy of adsorption decreases with increase in surfaceconstant at higher surface coverages. The adsorption results suggest thatde by activated carbons involves specific interactions between the surfacel sulfide molecules through chemical or quasichemical forces involving

© 2008 Elsevier B.V. All rights reserved.

tion of DMS and several other organic vapors on silica gel, zeolitesand activated carbons. Both these groups of researchers agreed thatthe adsorption is determined mainly by the pore structure of theseadsorbents rather than by their surface properties. Suzuki et al. [10]found that the adsorption of DMS by activated carbons dependedon the oxygen content of the carbon but these workers did notidentify the type of carbon–oxygen surface groups which may influ-

ence the adsorption. As these surface chemical groups are knownto influence profoundly the adsorption behavior of activated car-bons, the present work has been undertaken. The present workdescribes the adsorption isotherms of DMS on activated carbonshaving different surface areas and associated with varying amountsof carbon–oxygen surface groups.

2. Experimental

2.1. Materials

Two samples of granulated activated carbons GAC-1240 andGAC-R obtained from Norit N. V. Netherlands and one sample ofactivated carbon fiber ACF-307 obtained from Ashland PetroleumCompany of U.S.A. and a sample of activated carbon cloth obtainedfrom HEG Ltd., Bhopal, India, have been used as adsorbents in thesestudies. These samples have been referred to as ‘as-received’ acti-vated carbons in the text.

2.2. Oxidation of the activated carbons

About 5 g of the activated carbon sample was heated with 200 mlof 50% nitric acid in a 500 ml beaker on a water bath maintained at

hysicochem. Eng. Aspects 322 (2008) 164–169 165

M. Goyal et al. / Colloids and Surfaces A: P

about 80 ± 5 ◦C. When about 20 ml of the acid was left, the contentswere cooled, diluted with distilled water and filtered. The carbonsample was then washed exhaustively with hot distilled water untilit was free of nitrate ions. The washed carbon sample was dried firstin air and then in an electric oven and outgassed at 120 ◦C and storedin stoppered glass bottles.

2.3. Degassing of the carbons

The oxidized carbons were degassed at temperatures of 400,650, and 950 ◦C to eliminate varying amounts of the carbon–oxygensurface groups [11,12]. The degassing was carried out by placing5 g of the carbon sample in a temperature controlled tube furnace.The tube furnace was then connected to a Hyvac. Cenco vacuumpump capable of giving vacuum to an order of 3 × 10−3 mm of Hgand its temperature was set to the appropriate level by apply-ing an appropriate voltage. The temperature was allowed to risegradually and before it is raised by another 50 ◦C, complete elim-ination of the gases at the preceding temperature was ensured.After degassing at each temperature, the sample was allowed tocool in vacuum to avoid reformation of the carbon–oxygen surfacegroups. Cooled sample was then transferred to a stoppered bottleflushed with nitrogen. These samples are referred to as ‘degassedsamples’.

2.4. Determination of carbon–oxygen surface groups

The carbon–oxygen surface groups present on the as-received,oxidized and degassed carbon samples were determined byevacuating 1 g portion of each sample at gradually increasing tem-peratures upto 950 ◦C. The carbon sample, dried at 120 ◦C andcontained in a platinum boat is placed in a resistance tube furnace.The temperature of the furnace was allowed to rise gradually insteps of 50 ◦C to ensure complete elimination of the gas at the pre-vious temperature. The carbon–oxygen surface groups decompose

into CO2, CO and water vapor which were measured using usualanalytical procedures. The details of the procedure are publishedelsewhere [11–14].

2.5. Adsorption of DMS vapors

Adsorption of DMS vapors was measured at 290 K using a quartzspring (sensitivity 20.9 cm/g) obtained from M/S Thermal SyndicateLtd., England. The carbon sample ∼100 mg was placed in a quartzboat (also obtained from M/S Thermal Syndicate) which was sus-pended in a borosil reactor. The whole system was housed in anair thermostat maintained at 290 K. The reactor was connected tovacuum system and a manometer. The carbon sample was evac-uated under a vacuum of 10−4 Torr and a certain amount of theDMS vapors introduced into the reactor. Preliminary experimentsshowed that 30 min was sufficient to attain the adsorption equilib-rium. After this period, the extension of quartz spring was measuredwith the help of a cathetometer which could read upto 0.01 mmand the equilibrium vapor pressure was noted on the manometer.Similar more doses of vapors were introduced into the reactor andboth the extension and the equilibrium pressure were determined

Table 1Maximum surface area occupied by dimethyl sulfide molecules for various as-received ac

Carbon sample BET (N2) surface area (m2/g) xm (from Langmuir plots)

Activated carbon cloth 792 189ACF-307 910 213GAC-R 1150 227GAC-1240 1175 256

Fig. 1. Adsorption isotherms of dimethyl sulfide vapors on as-received activatedcarbons.

after each equilibrium. The details of the procedure are publishedelsewhere [17].

3. Result and discussion

3.1. Adsorption isotherms

The adsorption isotherms of dimethyl sulfide (DMS) vaporson the two samples of granulated activated carbons (GAC-1240and GAC-R), a sample of activated carbon fiber (ACF-307) and

a sample of activated carbon cloth are presented in Fig. 1. Allthe granulated and fibrous activated carbons adsorb appreciableamounts of dimethyl sulfide vapors, the amount adsorbed gen-erally depending on the surface area of the carbon. The amountadsorbed increases with increase in surface area but there is nodirect relationship between the amount adsorbed and the surfacearea. GAC-1240, which has about the same surface area as GAC-R (cf.Table 1), adsorbs significantly larger amount of DMS. The adsorp-tion isotherms are Type I of the BET classification showing a sharprise in adsorption at lower relative vapor pressures and tending tobecome more or less constant at higher relative pressures.

The adsorption data fit the Langmuir adsorption isotherm equa-tion. The linearised Langmuir equation can be represented as

p

x= 1

xmb+ p

xm

where x is the amount adsorbed at any given pressure p, xm is themonolayer capacity and b is a constant related to adsorption energy.The linear Langmuir plots are shown in Fig. 2.

The monolayer capacity xm values obtained from these linearplots have been used to calculate the surface area occupied by

tivated carbons

Surface area occupied by DMS molecules (m2/g) Pore volume (cm3/g)

549 0.248619 0.280661 0.207746 0.372

166 M. Goyal et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 322 (2008) 164–169

Fig. 2. Linear Langmuir plots for the adsorption of dimethyl sulfide vapors on as-received activated carbons.

dimethyl sulfide vapors using 0.30 nm2 as the molecular area fordimethyl sulfide. These values along with the BET surface area aregiven in Table 1. It is interesting to note that only a part of the car-bon surface varying between 55 and 70% is occupied by the DMSmolecules. This indicates that the adsorption of DMS takes placeon certain specific sites on the carbon surface. These specific sitescould be the carbon–oxygen surface groups which are invariablypresent on activated carbons. Alternatively some of the microporeson the carbon surface have small entrances (cf. Fig. 3) which inhibitthe entry of larger DMS molecules (molecular diameter 0.556 nm).

Fig. 3. Pore size distribution curves for different carbons.

Fig. 4. Adsorption isotherms of dimethyl sulfide vapors on carbon cloth samplebefore and after oxidation and degassing.

3.2. Influence of carbon–oxygen surface groups on the adsorptionof DMS

It is well known [18–20] that activated carbons are invariablyassociated with certain amounts of chemisorbed oxygen which ispresent in the form of two types of surface groups: one which areevolved on evacuation as CO2 in the temperature range 350–700 ◦C.These surface groups are acidic in character and have been postu-lated as carboxyls or lactones [13–16]. The other carbon–oxygengroups are evolved as CO on degassing in the temperature range550–950 ◦C. These groups have been postulated as quinones [14,15]which tend to make the carbon surface hydrophobic in character.Furthermore, the amounts of these both types of surface groups canbe enhanced by oxidation of the carbon surface.

In order, therefore, to examine the influence of carbon–oxygensurface groups on the adsorption of DMS vapors, two of the carbonsamples namely GAC-1240 and carbon cloth were oxidized with

nitric acid to enhance the amount of these surface groups. The oxi-dized carbon samples were then degassed at 400, 650 and 950 ◦C toeliminate varying amounts of these carbon–oxygen surface groups.The adsorption isotherms on the oxidized and degassed carbonsamples are presented in Fig. 4 for carbon cloth and Fig. 5 for GAC-1240. It is interesting to note that the adsorption of DMS increasesconsiderably on oxidation in case of both the carbon samples. Themaximum amount of adsorption increases from 25.5 to 31.7 % incase of GAC-1240 and from 18.2 to 27.4 % in case of the carbon clothsample.

The amounts of the two types of surface groups present on theoxidized carbons, as determined by evacuation at temperaturesupto 950 ◦C [11,12] are shown in Table 2. It is seen that the oxi-dation of both the carbons results in a considerable increase in theamount of the carbon oxygen surface groups, the increase beinglarger in the case of the GAC-1240 carbon. As the variation in sur-face area of GAC-1240 and activated carbon cloth on oxidation isonly small (∼4–5%), it appears that the adsorption of DMS vapors isinfluenced considerably by these surface groups. Thus, the higheradsorption capacity of the GAC-1240 compared to that of the acti-vated carbon cloth is partly due to its larger specific surface area.

M. Goyal et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 322 (2008) 164–169 167

Fig. 5. Adsorption isotherms of dimethyl sulfide vapors on GAC-1240 before andafter oxidation and degassing.

The adsorption isotherms on the oxidized carbon samples, how-ever, do not show clearly the type of carbon–oxygen surface groupswhich enhance the adsorption of DMS vapors.

The adsorption isotherms on the oxidized carbon samples afterdegassing at gradually increasing temperatures of 400, 650 and950 ◦C (cf. Figs. 4 and 5) clearly show that the adsorption decreasesgradually as these surface groups are eliminated from the carbonsurface. The decrease in adsorption is very small (between 1.6 and2.8%) for both the carbons on degassing at 400 ◦C. However, thedecrease in uptake is appreciably larger for 650◦-degassed carbonsamples. The amounts of CO2-evolving (acidic) and CO-evolving(non-acidic) surface groups present on the degassed samples areincluded in Table 2. It is evident that 400◦-degassed carbon sam-ples have lost only a small portion (∼15%) of the acidic surfacegroups while 650◦-degassed carbons have lost a larger proportion(∼85–90%) of the acidic groups. Both these degassed carbons retainmost of their non-acidic surface groups intact. Furthermore, it isseen that no significant variation in adsorption capacity is observedbetween the 650◦-degassed and 950◦-degassed carbon samples.This corroborates the view that the adsorption is mainly influencedby the carboxylic groups, these being eliminated largely at 650 ◦C.

Table 2BET surface areas and carbon–oxygen surface groups evolved on degassing different carb

Sample identification BET surface area (m2/g)

CO

GAC-1240As-received 1175 2.3HNO3 oxidized 1225 13.

HNO3 oxidized and then degassed at (◦C)400 1190 11.7650 1185 2.5950 1195 Tra

Activated carbon clothAs-received 792 1.9HNO3 oxidized 835 6.5

HNO3 oxidized and then degassed at (◦C)400 810 5.3650 795 1.3950 805 Tra

Fig. 6. Linear Lagergren plots for the adsorption of dimethyl sulfide vapors on as-received carbon cloth at different temperatures.

The above results show clearly that the adsorption of DMSvapors is influenced largely by the presence of acidic surface groups.The adsorption increases when these surface groups are increasedon oxidation and decreases when these surface groups are elimi-nated on degassing.

3.3. Adsorption kinetics

The effect of time on the adsorption of DMS vapors on the acti-vated carbon cloth sample at two different temperatures at a vaporpressure of 0.4 was determined. The adsorption was found to attaina constant value after a certain period of time. This is due to the factthat all the available sites become occupied at this point of time. Theadsorption rate constants have been determined using pseudo firstorder model first used by Lagergren [21]. This Lagergren equationcan be represented as

dq

dt= k(qe − qt)

where qt (mg/g) is the amount adsorbed at time t and qe (mg/g)is the equilibrium adsorption amount and k (s−1) is the first order

on samples at 950 ◦C

Oxygen evolved as (g/100 g)

2 CO H2O Total

5 1.02 1.24 4.6152 6.82 1.21 21.55

4 6.79 1.23 19.762 5.34 1.24 9.10ces Traces Traces Traces

2 3.15 1.31 6.383 7.52 1.29 15.34

4 7.21 1.22 13.779 6.04 1.23 8.66ces Traces Traces Traces

168 M. Goyal et al. / Colloids and Surfaces A: Physico

Fig. 7. Variation of differential enthalpy of adsorption with fractional surface cov-erage for the adsorption of dimethyl sulfide on carbon cloth.

rate constant. The integrated form of the Lagergren rate equationwith boundary conditions qe = qo at t = 0 and qt = qo at t = t can bewritten as

log(qe − qt) = log qe − k

2.303t

The linearity of the Lagergren plots between log(qe − qt) and time tin seconds (cf. Fig. 6) indicates that the adsorption of DMS vaporsobeys first order kinetics. The kinetic data also indicates that therate of adsorption varies considerably when the temperature ofadsorption is varied from 293 to 313 K. The rate constants wereused to calculate the activation energy for adsorption which hasbeen found to be 42 kJ/mol. The low value of activation energy sug-gests that the adsorption does not involve a strong chemical bondbetween the surface groups and the DMS molecule.

3.4. Isosteric enthalpy of adsorption

The differential molar enthalpy of adsorption which is alsocalled the isosteric enthalpy of adsorption is given by the relation-ship(

∂ ln p

∂T

)na

= −�H

RT2

which on integration gives(ln

p1

p2

)na

= −�H

R

(T2 − T1

T1T2

)

where p1 and p2 are the equilibrium vapor pressures of the adsor-bate at temperatures T1 and T2 for a given amount adsorbed na, �His the enthalpy of adsorption and R is the gas constant. This equa-tion helps to evaluate the isosteric enthalpy of adsorption fromthe experimental adsorption isotherms at two temperatures. Theenthalpy of adsorption values at different amounts adsorbed asobtained from such isosters at 293 and 313 K in the case of as-received and 950◦-degassed carbon cloth samples are shown inFig. 7. It is seen that the zero surface coverage isosteric enthalpy of

chem. Eng. Aspects 322 (2008) 164–169

adsorption is considerably higher on the as-received carbon sam-ple than that on the degassed carbon sample, the values varyingbetween 30 and 88 kJ/mol. Since the as-received carbon samplehas a heterogeneous surface due to the presence of carbon–oxygensurface groups compared with the 950◦-degassed sample whichis almost completely free of all these surface groups, it appearsthat the zero surface coverage isosteric enthalpy of adsorptionincreases with surface heterogeneity of the surface. The enthalpyof adsorption decreases with increase in surface coverage and ulti-mately tends to become more or less constant. As the enthalpyof adsorption depends upon the heterogeneity of the surfaceand the lateral interactions between the adsorbed molecules, theresults show that the lateral interactions are either absent or theseare unfavorable (repulsive) so that there is overall decrease inthe heat of adsorption with surface coverage [22]. The decreasein the enthalpy of adsorption with surface coverage can alsobe attributed to a decrease in the availability of carbon–oxygensurface groups for interaction with the DMS molecules. Thesmaller value of zero surface coverage enthalpy of adsorptionin the case of the 950◦-degassed samples can be attributed todecreased interactions between the carbon surface and the DMSmolecules as this sample is almost free of the carbon–oxygensurface groups.

4. Mechanism of adsorption

It has been shown that the adsorption of DMS depends uponthe amount of carbon–oxygen groups available on the carbon sur-face. The amount adsorbed increases when these surface groupsare enhanced on oxidation and decreases gradually when thesesurface groups are eliminated on degassing at increasing temper-ature. These results suggest that the DMS molecules are adsorbedon these specific sites by chemical or quasichemical bonds. Themagnitude of the limiting enthalpy of adsorption is also quite highto be caused by purely physical interactions and too small to becaused by the chemical interactions. This suggests that the spe-cific interactions between the surface acidic groups and the DMSmolecules takes place probably through chemical or quasichem-ical forces involving hydrogen bonding. This may be representedas

hysico

[

[

[

[

M. Goyal et al. / Colloids and Surfaces A: P

The formation of such surface compounds in the case of silicaand alumina gel has been confirmed by the infrared analysis of thegas desorbed on heat treatment of the DMS adsorbed silica andalumina gels [23,24].

5. Conclusions

Adsorption of dimethyl sulfide by activated carbons obeys Lang-muir adsorption equation. The adsorption depends on the surfacearea but is strongly influenced by the presence of carbon–oxygensurface groups. It increases on oxidation of the carbon surfacewhich enhances the amount of these surface groups and decreaseson degassing when these surface groups are eliminated. Theadsorption follows first order rate kinetics. The isosteric enthalpy ofadsorption decreases with increase in surface coverage and tendsto be almost constant at higher surface coverages. The adsorptioninvolves hydrogen bonding between the carbon–oxygen surfacegroups and the sulphur atom in DMS.

Acknowledgements

The authors acknowledge their thanks to Defence ResearchDevelopment Organization, Delhi, for the award of the researchproject (No. ERIP/ER/0403493/M/01/824). The authors are alsothankful to HEG Ltd., Bhopal, India, for the supply of activated car-bon cloth, the Ashland Petroleum Company, USA for the supply ofACF-307 and Norit N.V. Netherlands for the supply of GAC-R andGAC-1240. We are also thankful to Dr. R.C. Bansal for going throughthe manuscript.

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[

[

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[

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[

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[

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