Batch and Column Studies of Phenol Adsorption by an Activated Carbon Based on Acid Treatment of Corn Cobs

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1 IASIT Intrnational Journal of Enginring and Tchnology, Vol. 7, No. 6, Dcmbr 015 Batch and olumn Studis of Phnol Adsorption by an Activatd arbon Basd on Acid Tratmnt of orn obs Pablo D. Rocha, Adriana S. Franca, and Landro S. Olivira Abstract orn cobs wr valuatd as raw matrial in th production of an adsorbnt for phnol rmoval from auous solutions. Adsorption kintics and uilibrium wr satisfactorily dscribd by th psudo scond-ordr and Frundlich modls, rspctivly. Fixd bd adsorption (brakthrough curv) was satisfactorily dscribd by Bohart-Adams, Yoon-Nlson and Dos-Rspons modls. Adsorption tsts showd that th prpard adsorbnt prsntd highr adsorption capacity than activatd carbons producd from othr agricultural rsidus, confirming that this typ of wast matrial is a suitabl candidat for us in th production of adsorbnts. Indx Trms Adsorption, agricultural rsidus, corn cobs, phnol rmoval. I. INTRODUTION Phnols ar considrd to b priority pollutants, bcaus thy ar harmful to living organisms vn at low concntrations du to thir toxicity and carcinognicity proprtis [1]. Also, th apprciabl solubility of this compound in watr, combind with its high ractivity and rsistanc to biodgradation, mak it an important toxic matrial to b monitord in th auatic nvironmnt. Th US Environmntal Protction Agncy (EPA) rgulations call for lowring phnol contnt in wastwatr tolss than 1 mg/l []. Industrial wasts and agricultural by-products ar classs of matrials valuatd as promising prcursors for th production of low-cost adsorbnts, bcaus thy ar rnwabl, locally availabl in larg uantitis and inxpnsiv [3].Brazil is th third largst corn producr in th world, with a production of 55 million tons in 01. Solid rsidus from corn production such as corn cobs prsnt grat potntial for us as raw matrials in th production of adsorbnts [4]-[6]. Wastwatr tratmnt by adsorption using activatd carbons (A) is uit vry ffctiv and has bn citd by EPA as bing on of th bst availabl nvironmntal control tchnologis [7]. Howvr, th widsprad us of A adsorption is still rstrictd bcaus of th high costs of commrcial carbons. Thus, many rcnt studis hav focusd on th us of rnwabl and chapr prcursors, mainly agricultural wasts and by-products, as raw matrials in th Manuscript rcivd April 10, 014; rvisd July 9, 014. This work was supportd by th following Brazilian Govrmnt Agncis: NP (Grant # /013-8) and FAPEMIG (Grant # EX - APQ and PPM ). Th authors ar with DEME, Univrsidad Fdral d Minas Grais, Av. Antônio arlos 667, Blo Horizont, MG, , Brazil (-mail: pablo.drocha@yahoo.com.br, adriana@dmc.ufmg.br, landro@dmc.ufmg.br). production of activatd carbons [3], [8]. Th basic procsss of turning lignoclluloss matrials (.g. agricultural wasts and by-products) into an activatd carbon can b dividd into physical or chmical activation. Physical activation consists of hat tratmnt of th matrial in a stram of gass, usually carrid out in a two-stp procss:i) carbonization in an inrt atmosphr to produc th charcoal, followd by ii) activation or a scond hat tratmnt in th prsnc of O or stam to incras th porosity of th matrial [9]. hmical activation is carrid out by mans of th imprgnation of th carbon structur with an activating agnt (ithr an acid or bas) followd by hat tratmnt, so both th carbonization and activation stps ar carrid out simultanously. This procss is considrd to b mor advantagous in comparison to physical activation, bcaus usually occurs at lowr tmpraturs, lads to highr carbon yilds, and allows bttr control of th dvlopmnt of microporosity. Acidactivation is gnrally mployd in ordr to oxidiz th porous carbon surfac. It incrass th acidic charactristic, rmovs th minral lmnts and improvs th hydrophilic natur of surfac [10]. Prvious studis hav shown that activation of lignoclulosic rsidus such as ric husks and chrry stons with phosphoric acid provids ffctiv adsorbnts for phnol rmoval [11], [1]. Thus, th objctiv of this study was to valuat th potntial of acid activatd corn cobs as adsorbnts. Givn that adsorption procsss for purification of wastwatrs can b carrid out ithr discontinuously, in batch ractors, or continuously, in fixd-bd columns, th prformanc of th adsorbnt was valuatd in both batch and column tsts. II. METHODOLOGY A. Adsorbnt Prparation Th corn cobs wr ground (particl siz < 0.85mm), tratd with phosphoric acid 85% (1:1 ml acid/g matrial) and submittd to h carbonization in a muffl ovn at 500. Aftrwards, th producd adsorbnts wr washd with NaOH until ph 7 to rmov th xcss acid. Th solids wr drid at 110 o for 1 h and ground to particl diamtrs ranging grom 0.15 to 0.43 mm. B. Adsorption Tsts Batch xprimnts of adsorption wr prformd in 50 ml Erlnmyr flasks, with th flasks bing agitatd on an orbital shakr at 100 rpm. In all sts of xprimnts, th adsorbnt was thoroughly mixd with 100 ml phnol (Ph). Initial Ph concntrations rangd from 100 to 500 mg L -1 at a fixd adsorbnt concntration (10 g L -1 ). All tsts wr prformd in two rplicats. ml aliuots wr takn from DOI: /IJET.015.V

2 IASIT Intrnational Journal of Enginring and Tchnology, Vol. 7, No. 6, Dcmbr 015 th Erlnmyr flasks at pr-spcifid tim intrvals and phnol concntration was dtrmind by a UV Vis spctrophotomtr (Hitachi U-010) at 69 nm. Th amount of phnol adsorbd, (mg/g) was calculatd by: ( ) V W o = (1) whr o and (mg. L -1 ) ar th liuid-phas concntrations of phnol at initial and sampling tims, rspctivly; V is th volum of th solution and W is th mass of dry adsorbnt usd. ontinuous flow adsorption xprimnts wr conductd in a cylindrical stainlss stl column (.5 cm intrnal diamtr and 10 cm hight). At th bottom of th column, a 0.5 mm stainlss stl siv was attachd followd by glass wool. Known uantitis of adsorbnt (1 g) wr placd into th column, yilding bd hights of 5 cm. Phnol solutions of known concntrations wr pumpd downward through th column bd. Sampls wr collctd at th column outlt at diffrnt tim intrvals and wr analyzd for phnol concntration by a UV Vis spctrophotomtr (Hitachi U-010) at 69 nm. Th inlt concntration was varid from 00 to 500 mg/l and th flow rat was varid from 18 to 33 ml/min at a fixd inlt concntration (500 mg/l). III. RESULTS AND DISUSSION A. Effct of ontact Tim Th batch adsorption data shown in Fig. 1 indicat that a contact tim of 6 hours assurd attainmnt of uilibrium conditions for initial phnol concntrations blow 500 mg L -1. Adsorption can b viwd as a two-stag procss, with a rapid initial adsorption, followd aftrwards by a much slowr rat. This is attributd to th high valus of concntration gradint in th bginning of th adsorption procsss, rprsnting a significant driving forc for phnol transfr btwn th solution and th adsorbnt surfac. Such ualitativ bhavior was also obsrvd for phnol rmoval by othr typs of adsorbnts [13], [14]. (mg/g) mg/l 400 mg/l 300 mg/l 00 mg/l 100 mg/l t (min) Fig. 1. Adsorption capacity vs. tim for diffrnt valus of initial phnol concntration. B. Adsorption Kintics Th controlling mchanisms of th adsorption procsss ar usually invstigatd by fitting psudo first and scond-ordr kintic modls to th xprimntal data [15] gnrically rprsntd by th following uation: d t n = kn ( t dt ) (1) whr and t corrspond to th amount adsorbd pr unit mass of adsorbnt (mg g -1 ) at uilibrium and at tim t, rspctivly, k n corrsponds to th rat constant for n th ordr adsorption. Th intgratd forms of th uations ar: First-ordr kintics (n=1): Scond-ordr kintics (n=): t t ( ) k 1 1 = () t t 1 t = + (3) k Evaluation of ach modl s ability to prdict th xprimntal data was basd on both rgrssion corrlation cofficint valus (r ) and diffrnc btwn xprimntal ( t,xp ) and modl-stimatd ( t,st ) valus, valuatd by mans of th rror masur: RMS(%) = 100 [( )/ ] / N t,st t,xp t,xp whr N is th numbr of xprimntal points in ach t vs. t curv. Rsults for th non-linar fits of th kintic modls and thir stimats for uilibrium adsorption capacity ar shown in Tabl I. Th psudo-scond ordr modl providd highr r valus and lowr valus of RMS rror in comparison to th psudo-first ordr modl, thus bing considrd mor aduat for dscription of th adsorption data, for all valuatd tmpraturs. This modl has bn succssfully applid for dscription of adsorption kintics of svraladsorbats, dscribing both chmisorption and ion xchang [15]. It was also th mor aduat modl for dscription of rmoval of phnolic aminoacids for adsorbnts basd on th sam rsidu [5], [16] and also for phnol rmoval by othr typs of adsorbnts such as zolits [1] and activatd carbon basd on tobacco lavs [17]. Th ffct of diffusion as th rat controlling stp in th adsorption was valuatd according to th intra-particl diffusion modl [18]: 1 / t p + (4) = k t (5) whr k p is th intra-particl diffusion rat constant, valuatd as th slop of th linar portion of th curv t vs. t 1/. Rsults ar displayd in Fig.. In thory, th intra-particl diffusion plots can prsnt up to four linar rgions, rprsnting boundary-layr diffusion, followd by intraparticl diffusion in macro, mso, and micro pors. Ths four rgions should thn b followd by a horizontal lin rprsnting th systm at uilibrium [16]. Th plots prsntd in Fig. show two rgions followd by a platau for mor dilutd solutions, indicating that boundary-layr 460

3 IASIT Intrnational Journal of Enginring and Tchnology, Vol. 7, No. 6, Dcmbr 015 diffusion sms to b th rat-controlling stp. Howvr, for mor concntratd solutions ( i >300 mg L -1 ) w can obsrv thr rgions, with th first lin crossing th origin, thus indicating that por diffusion is bcoming mor important. TABLEI: KINETI PARAMETERS FORPHENOL ADSORPTION. Phnol initial concntration (mg L -1 ) (xprimntal) Psudo first-ordr k 1 (h -1 ) (stimatd) (mg g -1 ) r RMS (%) Psudo scond-ordr k (g mg -1 h -1 ) (stimatd) (mg g -1 ) r RMS (%) (mg/g) Fig.. Fitting of intra-particl diffusion modl for phnol adsorption.. Adsorption Euilibrium Th adsorption isothrm is prsntd in Fig. 3. Th shap of th curv indicats favorabl adsorption. Although thr ar many modls for dscription of adsorption isothrms in th litratur, th most widly mployd ar Frundlich and Langmuir. Frundlich s uation is an mpirical modl that dos not account for adsorbnt saturation and has bn associatd to both htrognous and multilayr adsorption, bing dscribd by th following uation: F 1/ n = K (6) whr K F is a constant that indicats th rlativ adsorption capacity (mg 1-(1/n) L 1/n g 1 ) and n is rlatd to th intnsity of adsorption. Langmuir isothrm, on th othr hand, is basd on a thortical modl assuming monolayr adsorption ovr an nrgtically and structurally homognous adsorbnt surfac and taks into account adsorbnt saturation, bing rprsntd by th following uation: K max L = (7) 1 + K L whr (mg g 1 ) and (mg L 1 ) corrspond to th amount adsorbd pr gram of adsorbnt and to th solut t 1/ concntration (mg L 1 ) in th auous solution, rspctivly, aftr uilibrium was rachd. max and K L ar constants rlatd to th maximum adsorption capacity (mg g 1 ) and th adsorption nrgy (L mg 1 ), rspctivly. Modl slction was basd on highst R valus coupld with th lowst diffrnc btwn calculatd and xprimntal valus, valuatd according to th following root man suar rror masur: RMS = [( )/ ] / N,st,xp,xp whr, xp and, st ar th xprimntal and modl stimatd uilibrium adsorbnt amounts, rspctivly, and N corrsponds to th numbr of xprimntal isothrm points. Evaluatd modl paramtrs ar displayd on Tabl II. An valuation of both R and RM S valus show that phnol adsorption was bttr dscribd by th Frundlich modl, indicating htrognous and multilayr adsorption. This modl was also th on that providd th bst dscription for phnol rmoval by zolits [1]. (mg/g) Exprimntal Langmuir Frundlich (mg/l) Fig. 3. Euilibrium isothrm (5 o ). TABLE II: EQUILIBRIUM PARAMETERS FOR PHENOL ADSORPTION. Modl Paramtr valus R RMS Langmuir K L = 0.09 max = Frundlich K F = 3.91 n = D. olumn Studis Th ffcts of adsorbat solution inlt concntrations on a fixd bd of adsorbnts ar prsntd in th brakthrough curvs dpictd in Fig. 4. Notic from Fig. 4 that th brakthrough curvs somwhat follow th idal S shap profil that is charactristic of adsorbats of small molcular sizs and also of adsorbnts comprisd of small sizd particls [19]. An incras in th inlt concntration (Fig. 4a) lad to a shortning of both th brakthrough tim and th bd srvic tim, i.., th adsorbnt was mor uickly saturatd. Th incras in inlt adsorbat concntration also ld to a stpning of th slop of th brakthrough curv or a shortning of th mass transfr zon. This is attributd to a highr concntration gradint causing a fastr transport and mor ffctiv intra-particl diffusion [19]. This corroborats th batch rsults for intra-particl diffusion modls, confirming th incrasd ffct of por diffusion for mor concntratd solutions. (8) 461

4 IASIT Intrnational Journal of Enginring and Tchnology, Vol. 7, No. 6, Dcmbr 015 Th ffcts obsrvd on th brakthrough bhavior whn varying th adsorbat solution flow rat (Fig. 4b)) wr a dcras in both th volum tratd and th brakthrough tim as th flow rat was incrasd. Th brakthrough occurrd fastr at highr flow rats and, thus, th bd srvic tim was shortnd. Th brakthrough curv bcam stpr whn th flow rat was incrasd, i.., th mass transfr zon was shortnd, indicatingmor ffctiv intra-particl diffusion ffcts [0]. Othr studis on fixd bd adsorption [1] hav rportd thatan incras of th flow rat byond a crtain valu will caus th rat of adsorption to dcras bcaus of th dcras in th rsidnc tim of th adsorbat in th column. Evn though th flow rat in this work has not bn incrasd byond such valu, it is clarly sn from Fig. 4(b) that incrasing th flow rat abov 33 ml/min would probably not improv th adsorption prformanc, givn that thr was not much diffrnc btwn th brakthrough curvs for 5.6 and 33.0 ml/min. TABLE III: BOHART-ADAMS, YOON-NELSON, AND DOSE-RESPONSE MODEL PARAMETERS AT DIFFERENT ADSORBATE SOLUTION INLET ONENTRATIONS AND FLOW RATES i (mg/l) Q (ml/min) Modl Bohart-Adams Yoon-Nlson Dos-Rspons K BA 10 4 (L mg 1 min 1 ) (mg g 1 ) R K YN (min -1 ) (mg g 1 ) t 50 (min) R a (-) (mg g 1 ) R Exprimntal 0 (mg g 1 ) t 50 (min) Bohart-Adams, Yoon-Nlson and Dos-Rspons brakthrough modls wr fittd to th xprimntal data and thir rspctiv paramtrs wr stimatd using nonlinar (Statistica 8.0 program) rgrssions. Dtails on stimatd modls kintic paramtrs and th adsorption capacity ar prsntd in Tabl III togthr with th rspctiv xprimntally dtrmind valus. Bohart-Adams modl [] is basd on th assumption that th adsorption rat is proportional to both th rsidual capacity of th solid and th concntration of th adsorbing spcis, andcan b rprsntd by: b rprsntd by: whr t 50 is th tim ruird for 50% brakthrough and Q (L min -1 ) is th volumtric flow rat through th column. i = xp( K BAit ) xp( K BAit ) + xp( K BAN o z / U o ) 1 (9) 0 = N0zS / m (10) 1 = i 1 + xp kyn ( t50 t) (11) iqt 0 = (1) m 50 whr (mg L -1 ) is th adsorbat concntration in th fluid at th column outlt at tim t(s), i (mg L -1 ) is th adsorbat concntration in th fluid at th inlt of th column, N o (mg L 1 ) is th sorption capacity pr unit volum of fixd bd, z (cm) corrsponds to bd dpth, U o (cm min 1 ) is th suprficial vlocity, S (cm ) is th bd cross sction ara, m (g) is th adsorbnt mass and 0 is th adsorption capacity. Yoon and Nlson s modl [3] considrs that th probability of adsorption for ach adsorbat molcul dcrass at a rat that is proportional to both th adsorbat adsorption and adsorbat brakthrough probabilitis. It can Fig. 4. Exprimntal brakthrough curvs for th producd adsorbnt at diffrnt valus of a) inlt phnol concntration and b) flow rat. Th Dos rspons modl was proposd for th dscription of havy mtal biosorption in columns [4]. It can b rprsntd by th following uations: 46

5 IASIT Intrnational Journal of Enginring and Tchnology, Vol. 7, No. 6, Dcmbr = (13) 1 ( / ) i + Qt b b i (14) 0 = m As sn from th data prsntd in Tabl III, th rat constant (k BA ) stimatd from th nonlinar fitting of Bohart-Adams modl dcrasd slightly with an incras in adsorbat inlt concntration, and did not vary significantly with flow rat. Th opposit bhavior was obsrvd for th rat constants of th Yoon-Nlson modl, that incrasd with incrass in inlt concntration and flow rats. All modls prsntd satisfactory fits to th xprimntal data, both in trms of r valus and stimation of adsorption capacity. Th adsorption capacity for th prpard adsorbnts, ranging from 36 to 64 mg/l can b considrd significant whn compard to fixd bd capacity data for othr low-cost adsorbnts such as sugarcan bagass, ~1 mg/l [5], pinus bark, ~0.4 mg/l [6], as wll as adsorbnts producd by phosphoric acid activation of othr rsidus such as spnt coff grounds and Raphanussativus prss cak, ~8-34 mg/l [7].olumn adsorption capacity was highr in comparison to batch systms undr th sam initial phnol concntration, 39.5 mg/g.th lack of corrspondnc btwn batch and column data is usually attributd to th fact that (i) adsorption in fixd-bd columns dos not ncssarily oprat undr uilibrium conditions sinc th contact tim is not sufficintly long for th attainmnt of uilibrium; (ii) granular adsorbnts rarly bcom totally xhaustd in commrcial procsss, and (iii) chmical or biological changs that occur in th adsorbnt cannot b prdictd by th isothrms [8]. IV. ONLUSIONS Exprimnts wr conductd to invstigat th potntial of acid activatd corn cobs, as adsorbnts for phnol rmoval. Euilibrium data dmonstratd favorabl adsorption. Fixd-bd brakthrough curvs prsntd th S profil charactristic of adsorbats of small molcular sizs and also of adsorbnts comprisd of rlativly small sizd particls. Brakthrough modls fittd wll to th xprimntal data with th Dos-rspons modl prsnting a bttr fit than th othrs. Th maximum valu of uptak capacity for this systm was highr in comparison to adsorbnts obtaind by thrmal and/or chmical tratmnt of othr rsidus, confirming th adsorption potntial of th matrial. REFERENES [1] R. I. Yousf, B. E. Eswd, and A. H. A. Muhtasb, Adsorption charactristics of natural zolits as solid adsorbnts for phnol rmoval from auous solutions: Kintics, mchanism, and thrmodynamics studis, hmical Enginring Journal, vol. 171, pp , July 011. [] F. A. Banat, B. A. Bashir, S. A. Ashh, and O. 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6 IASIT Intrnational Journal of Enginring and Tchnology, Vol. 7, No. 6, Dcmbr 015 [6] G. Vázuz, R. Alonso, S. Frir, J. G. Álvarz, and G. Antorrna, Uptak of phnol from auous solutions by adsorption in a Pinuspinastr bark packd bd, Journal of Hazardous Matrials, vol. 133, pp , May 006. [7]. O. Alvs, M. Faustino, A. S. Franca, and L. S. Olivira, omparativ valuation of activatd carbons prpard by thrmo-chmical activation of lignocllulosic rsidus in fixd bd column studis, Intrnational Journal of Enginring and Tchnology, submittd, 014. [8] A. S. Franca and L. S. Olivira, Fixd bd adsorption studis, in Sorption Procsss and Pollution: onvntional and Non-onvntional Sorbnts for Pollutant Rmoval from Wastwatrs G. rini and P.-M. Badot, Ed. Bsançon: Prsss univrsitairs d Franch-omté, 010, pp Pablo D. Rocha was born in 1986 and h was graduatd in chmical nginring from Univrsidad Fdral d Minas Grais (UFMG), Blo Horizont, Brazil, in 011. H is currntly pursuing his mastr s dgr in mchanical nginring at UFMG. Adriana S. Franca was born in 1966 and sh was graduatd in chmical nginring in 1988 and obtaind hr M.Sc. in mchanical nginring in 1991 from Univrsidad Fdral d Minas Grais, Blo Horizont, Brazil. Sh compltd hr Ph.D. in agricultural and biological nginring from Purdu Univrsity, USA, in Sh is currntly working as an associat profssor at th Dpartmnt of Mchanical Enginring from Univrsidad Fdral d Minas Grais and also tachs at th graduat cours in food scincs. Sh has publishd ovr 70 articls in intrnational journals, 14 book chaptrs and has prsntd svral rsarch paprs at various intrnational confrncs hld in Brazil and abroad. Hr rsarch intrsts includ sustainabl uss of agricultural rsidus, coff chmistry, microwavs, FTIR and othrs. Landro S. Olivira was born in H was graduatd in chmical nginring in 1988 and obtaind his M.Sc. in mchanical nginring in 1991 from Univrsidad Fdral d Minas Grais, Blo Horizont, Brazil. H compltd his Ph.D. in agricultural and biological nginring from Purdu Univrsity, USA, in H is currntly working as an associat profssor at th Dpartmnt of Mchanical Enginring from Univrsidad Fdral d Minas Grais and h is currntly th had of th graduat program at that Dpartmnt. Dr. Olivira also tachs at th graduat cours in food scincs. H has publishd 63 articls in intrnational journals, 11 book chaptrs and has prsntd svral rsarch paprs at various intrnational confrncs hld in Brazil and abroad. His rsarch intrsts includ sustainabl uss of solid rsidus, biodisl, coff chmistry and othrs. 464