The adsorption isothenns for 2,4,6-trinitrophenol (picric acid) on five samples of coconut-based activated carbons (ACs) with varying. surface area have been studied. The results obtained show thatadsorption depends upon surface area but is not linearly related to it. The adsorption increases on oxidation with ammonium persulphate (NH4)2 S2 08) as well as on degassing at 600 °C. The resultshave been explained on the basis of the existence of surface carbonyl groups, where the 02 of thecarbonyl group interacts with pi electrons of the benzene ring of picric acid

Aggarwal, Preeti

Bansal, R. C.

Bhalla, A. K.

Kapoor, S. K.

Misra, Kshipra

English

Defence Science Journal

Defence Science Journal, Vo148, No 2, April 1998, pp. 219-222@ 1998, DESIDOC;6Effect of ~urface Oxygen Complexes of Activated Carbon on theAdsorption of 2,4,6- TrinitrophenolPreeti Aggarwal, Kshipra Misra, S.K. Kapoor, & A.K. BhallaCentre for Environment & Explosives Safety, Delhi -110 054.andR.C. BansalPanjab University, Chandigarh -160 003.ABSTRACTThe adsorption isothenns for 2,4,6-trinitrophenol (picric acid) on five samples of coconut-basedactivated carbons (ACs) with varying. surface area have been studied. The results obtained show thatadsorption depends upon surface area but is not linearly related to it. The adsorption increases onoxidation with ammonium persulphate «NH 4)2 S2 08) as well as on degassing at 600 °C. The resultshave been explained on the basis of the existence of surface carbonyl groups, where the 02 of thecarbonyl group interacts with pi electrons of the benzene ring of picric acid.phe.nols. Gao and coworkersg also observed changein adsorption .behaviour of phenol with change illthe .surface chemical properties of AC broughtabout by oxidation with HNO3 and reduction withH2.1 INTRODUCTIONThe presence of nitrophenols in waste water isof great environmental concern. These compoundsare usually found in effluent wastes from explosiveindustries, textile industries and by photodegradationef pesticides, etc. Adsorption studies of phenols,nitrophenols and some of their derivatives fromaqueous solutions using different adsorbents arewell-documented1-s. However, adsorption onactivated carbon (AC) is the most widely usedtechnique for the removal of phenolic compoundsfrom waste water. Some workers6 have observedenhancement in the adsorption of phenols in thepresence of combined surface 0] on AC.Coughlin 7, et al. reported negative influence ofchemisorbed surface 0] on the adsorption ofThis paper presents adsorptiol} isotherms ofpicric acid on five different grades of indigenouslyavailable coconut based ACs. The effects of surfacearea and surface 0] complexes created by variousoxidative and degassing treatments have beendiscussed.2. EXPERIMENTAL DETAILS2.1 MaterialsFive grades of commercially availablecoconut-based ACs with surface area rangingReceived 10 Apri11997 , revised 13 August 1997219DEF SCI J. VOL 48. NO 2. APRI!~ 1998from 650 to 1300 m2/g were used during thestudies. ACs were dried at 120 °C for 3 hr beforeuse. All chemicals used: were of AR quality anddouble-distilled water was used for preparingsolutions and rinsing.Table I. Langmuir constants and fraction of surface areautilised for adsorption of trinitrophenol~~-~- -Carbon Fraction Q ma.t btype of area (mg/g)utilised0.180 540.457 1900.465 2100.486 2580.448 269Surfacearea,(m-/g)0.32420.{)3140.03130.03160.0282650900100011501300AC-30AC-40AC-50AC-60AC- 702.2 Modification of Activated CarbonsAs-received C samples were oxidised with(NH 4)28208 and H2O2 and degassed at 400, 600 and1000 °C by methods commonly used for the9purpose .2.3 Study of Adsorption IsothermsCarbon dosages were in the range 0.2-2.0 g/ I,whereas the initial concentration of picric acid waskept at 300 mg/I. Isotherm experiments were carriedout at 30 :t I °c by various methods. .3. RESULTS & DISCUSSIONAdsorption isotherms of picric acid ondifferent samples of ACs having different surfaceareas in the range 650-1300 m2/g with varying Cdoses are shown in Fig.l. The plots are concave toconcentration axis, indicating a rapid rate ofadsorption in the beginning. The adsorption datacorrelate well with Langmuir adsorption modes.Values of Langmuir adsorption constants, Qm, themaximum adsorption at monolayer saturation andthe constant, b, as calculated from linear Langmuirisotherms for different C samples, are given inTable I.It is seen that maxium adsorption of picric acid(269 mg/g) took place on AC-70(1300 m2/g) whichhas the largest surface area: minimum adsorption(54 mg/g) was on AC-30(650 m2/g) having thesmallest surface area (Table 1). However, no linearrelationship between adsorption and surface areawas observed in the present study. Fraction of unitsurface area covered by picric acid at monolayerincreases with increase in BET area up to 1000 m2/gand becomes more or less constant thereafter(Fig. 2). This may be attributed to the moleculardimension of picric acid (9.08 A 0) which restrictsthe entry of picric acid molecules on carbonshaving higher fraction of ultra microporosity.It is well-known that the amount adsorbed isnot entirely controlled by the specific surface areabut is also influenced by the chcmical structure ofAC-70 113Ot-'/gl~~60 1'1SO-'/gl50 I--'/glAC-~O 1900-yg)300~0.0=:uo~i200100O AC-30 (6SO.'/gJ~* 00E~~ C~TJII (8g/l120015001000SURfAa AREA (m2/g~Figure I. Adsorption isotherms for trinitrophenol on activatedcarbons of different surface areas. Temperature30 :t I °c, contact time 24 hr, initial concentration 300 mg/1and carbon dose 0. 2-2. 0 g/l.Figure 2. Fraction of surface area utili.,ed for monolayeradsorption of trinitrophenol on activated carbons.220-'~~~0 6-&AJ .~..c:&AJ~ 0.4u..~~II)'50.2zo~u..c:fE 600AGGARWAL, et al: EFFECT OF SURFACE OXYGEN COMPLEXESF:gure 3. Freundlich adsorption isothenns for trinitrophenolon activated carbon AC-60 (1150 m',g) before andafter outgassing at different temperatures.Temperature 30 :t 1 °c, contact time 24 hr, initialconcentration 300 mg/l, and carbon dose 0.2-2.0 g/i.CO2 evolving complexes inhibits the adsorption ofpicric acid. The decrease in adso"rption of phenolsdue to the acidic groups of carbons was earlier beenreported by Puri11 and Oda12. This finding receivesfurther su-pport from the adsorption isotherm on Csamples oxidised with (NH 4)2 S2OB, which containsa larger portion of CO-evolving complexes(Table 2). An increase in the 1000 °C degassedsample where both CO2 and CO groups arenegligible may be attributed to widening of someof the microcapillary pores on degassing at suchhigh temperature13. In the C sample oxidised withH2O2, whereby the CO-evolving complexesincrease marginally, a slight increase in adsorptionof picric acid has been observed. The increase inadsorption on degassing at 600 °C and on oxidationw,ith (NH 4)S2OB can be explained on the basis ofTable 2. Relationship between oxygen evolved.on outgassingAC-60 (1150 m2/g) (original and modified) at 1000 °cand amount adsorbed at monolayer on adsorption ortrinitriphenol.Oxygen evolved asc surface. To assess their influence, adsorptionisotherms were determined by modifying thecarbon-oxygen surface complexes by oxidationwith (NH4)2S20s and H2O2 and by degassing at 400,600 and 1000 °C. The amounts of carbon-oxygensurface complexes given out as CO) and CO, andthe amount adsorbed are shown in Table 2. It is seenthat the oxidation enhances the amount of 0)surface complexes, whereas degassing reduces it.The adsorption isotherms of picric acid onAC-60(1150 m2/g), original and outgassed atdifferent temperatures are shown in Fig. 3.Total Q"'ax(02)CO2 CO H20(8/1008) (g/1008) (8/1008)C-Typeinteraction between c~rbonyl oxygen on the Csurface and the pi electrons of the benzene ring ofpicric acid11. The increase in adsorption on(NH4)S20s-treated samples may also be attributed13to interaction of negative charge on nitro-groups ofpicric acid with partial positive charge on carbonylor quinonic C atoms (Fig. 4).Studies carried out by some workerslO, haveshown that O2 complexes evolved as CO2 onevacuation at 300- 700 ° C are carboxylic 9r lactonicgroups and those evolved as CO on evacuation at550-950 oC are quinonic or carbonyl groups. It isseen that adsorption of picric acid increases on Csamples outgassed at different temperatures, theincrease in adsorption being maximum in the caseof 600 ° C degassed sample. This can be explainedon the basis that 600 oC degassed sample is devoidof a large portion of its CO2 evolving complexes,while it retains a larger portion of CO-evolvingcomplexeslo. It thus appears that the presence ofIt appears from the results of the present studythat though the adsorption of picric acid dependson the surface area, it is considerably influenced bythe chemisorbed O2 present on the surface of AC.The presence ofCO-evolving surface O2 complexes221DEF SCI VOL 48, NO 2, APRll.. 199830G-1 4. Peel, R.G. & Benedek, A. Attainment ofequilibrium in activated carb<)n isotherm studies.Environ. Sci. Technol. 1980, 14, 66.5. Hederlein, S.B.& Schwarzenbach, R.P.Adsorption of substituted Nitrobenzene andNitrophenols to Mineral Surfaces. Environ. Sci.Technol. 1993,27,316-26.6. Epstein, B. P.; Mone, E. P. & Mattson, I. S.Electrochemical investigations of surfacefunctional groups on isotopic pyrolytic carbon.Carbon,1971, 9, 609-14.D So 100 150EQ~ CIKfNTRATm 18IIl)2OtFigure 4. Freundlich adsorption isotherms of trinitrophenolon activated carbon AC-60 (1150 m2Jg) before andafter surface oxidation. Temperature 30 :t 1 °ccontact time 24 hr, initial concentration 300 mg/land carbon dose 0.2-2.0 g/i.7. Coughlin, R. W. Effect of surface oxides onadsorption of polar molecules from aqueoussolutions. Water Poll. Res. Ser., 1979, 70, 29-31.8.enhances the adsorption of picric acid. while thepresence of acidic CO2-evolving groups reduces itsadsorption. --9.Gao, Shangyu; Abe, Ikuo; Tanada, Seiki &Matsuhara, Yoshinobu. Adsorption behaviour ofphenol and benzenesulphonic acid on modifiedactivated carbon in aqueous solution. LinchanHuaxue Ya Gongye, 1994, 14(3), 29-34.Aggarwal, P.; Kapoor, l.C.; Kapoor, s. K.;Bhalla. A. K. & Bansal, R. c. Adsorption ofnitrobenzene on activated carbon from. diluteaqueous solutions. Ind. J. Chem. .Tech., 1996, 3,187-90.ACKNOWLEDGEMENTThe authors express their gratitude to Dr A.K.Datta, Chief Controller Research and Development,New Delhi, for taking keen interest throughout thisresearch work and for granting permission topublish this paper. 10. Donnet, J. B. The chemical reactivity of carbons:A review. Carbon, 1968, 6, 161.REFERENCES11. Purl, B. R.; Bhard~aj, S. S. & Gupta, Uma.Adsorption of phenol from aqueous solutions bycarbons in relation to their specific surface areas.I. Ind. Chem. Soc., 1976, 53, 1095-98.12. Oda, Hifakazu; Muniharo, Kishida and Chikao,Yokokawa. Adsorption of benzoic acid andphenol from aqueous solution. by activatedcarbons-effect of surface acidity. Carbon, 1981,19(4), 243-48.12.Srivastava, S. K.; Gupta, V. K; Yadav, I. S. &Mohan, D. Removal of 2,4-dinitrophenol usi~gbagasse fly ash-A sugar industry waste material.Fresenius Environ. Bull., 1995, 4, 550-57.Srivastava, S. K &; Tyagi, Renu. Competitiveeffect of surfactants in the adsorption of phenolsby activated carbon developed from the fertilizerwaste slurry. Chem. Engg. World, 1993, 28(11),83-86.13. Aggarwal, Pn:~ti; Mishra. Kshipra; Kapoor, S.K.; Bhalla, A. K. & Bansal, R. C. Adsorption ofstyphnic acid on activated carbon. Ind. I. ChemTech, 1997, 4, 42-44.3. Meghea. Aurelia; Mihalache, Romina; Bumbac,G.H. & Costantinescu, I. Competitive adsorptionof P-nitrophenol and O-cresol from aqueoussolutions on activated c3lbon. Sci. Technol.Environ. Prot. !994, 1(1), 10-17.222

Effect of Surface Oxygen Complexes of Activated Carbon on the Adsorption of 2,4,6- Trinitrophenol

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Effect of Surface Oxygen Complexes of Activated Carbon on the Adsorption of 2,4,6- Trinitrophenol

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