STUDY OF THE BEHAVIOR OF METAL ADSORPTION IN ACID SOLUTIONS ON LIGNIN USING A COMPARISON OF DIFFERENT ADSORPTION ISOTHERMS

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1 Latin Amrican Applid sarch 37: (27) STUDY OF THE BEHAVIO OF METAL ADSOPTION IN ACID SOLUTIONS ON LIGNIN USING A COMPAISON OF DIFFEENT ADSOPTION ISOTHEMS N. PÉEZ, M. SÁNCHEZ, G. INCÓN and L. DELGADO Dpto. Procsos y Sistmas, Univ. Simón Bolívar, 18 Caracas, Vnzula. naprz@usb.v Abstract Th adsorption procss of Nickl and dissolvd in acid liquor on lignin is studid in this work. Diffrnt adsorption isothrm modls ar usd to adjust liquid-solid systms: Langmuir, Frundlich and dlich-ptrson. It was obtaind that th mtal adsorption using lignin procds by similar chmical mchanisms. Possibly th Nickl is adsorbd in htrognous monolayrs, contrary to which is adsorbd in homognous ons. sults of th paramtrs obtaind for vry adsorption modl and adsorption systm ar rportd too. Kywords adsorption, havy mtals, lignin, isothrms. I. INTODUCTION Th havy mtals rmoval from liquids is a growing ncssity du to two fundamntal rasons: th ncssity to diminish th fflunt contamination for avoids possibl damag to cosystm and to th halth of human bing, and du to th high cost of ths mtals; prsnting its rmoval as an conomical fasibl way to rcovr it for its futur rutilization (Baily t al., 1999). Many tchnologis hav bn dvlopd with th last propos (Baily t al., 1999), howvr th most of thm rquir procss or sparation agnts that ar xpnsiv. On of th ways has bn th adsorption of ths mtals with activatd carbon, which has many advantags, for xampl, to b a provd tchnology. Howvr th high cost of th activatd carbon and its limitd rgnrativ capacity has difficult th stablishmnt of this tchnology for ths proposs. Vnzula has on of th biggr havy crud oil rsrvs of th world. Ths rsrvs ar charactrizd by th high contnt of impuritis lik sulfur (S), nickl (Ni) and vanadium (V). Bcaus of th problms in th industrial us of th crud and its drivats, is ncssary to find altrnativs to diminish ths impuritis. With this propos th group of Tcnologías Altrnativas limpias (Clan Altrnativ Tchnologis, TECall) of th Simon Bolivar Univrsity has dvlopd a procss that consists in th mtal xtraction from carbonacous matrials using a tratmnt with acid solution and stimulatd by microwav (Chamorro and Kraütr, 2). During th tratmnt, th rmovd mtals ar dissolvd in th acid solution, bcoming this acid liquor th main fflunt of th microwav procss LIM (liquor of microwav irradiation). Howvr, to achiv that this procss to b industrially fasibl, th LIM has to b rcovrd to rus it. In this sns, TECall, propos th us of an industrial byproduct ( lignin ) that is coming from black liquors of th pulp industry lik mtals adsorbnt, fundamntally bcaus its low cost and its obsrvd adsorptiv capacity (Acmioğlu, 24). In this work th rsults obtaind whn two havy mtals, lik Ni and V, ar put in contact with lignin ar ar prsntd. Th xprinc is prformd at laboratory scal with controlld conditions of initial ph, tmpratur, contact tim, and adsorbnt and adsorbat concntrations. Th goal is to valuat th adsorptiv capacity of lignin to adsorb th mtals and to prdict th possibl adsorption mchanism in which is dvlopd th adsorption by mans of an analysis of thr diffrnt adsorption isothrms. II. THEOY Th adsorption of a substanc is its accumulation in crtain surfac. Th rsult is th formation of a liquid or gasous film on th surfac of a solid body. Into a matrial, all th chmical connctions (ionic, covalnt or mtallic) of constitunt atoms ar satisfid. Howvr, by dfinition, th surfac rprsnts a discontinuity of thos connctions. For ths incomplt connctions, it is strongly favorabl to ract with anything that is availabl, and for that rason it taks plac in a spontanous form. Th xact natur of th connction dpnds on th spcimns particularity implid, but th adsorbd matrial gnrally is classifid lik physisorbd or chmisorbd. Th simplst form of adsorption is th physisorption, du to wak attractiv forcs, gnrally Van dr Waals forcs. Bcaus ths forcs ar omniprsnt, any clan surfac xposd to th atmosphr accumulats a layr of physisorbd matrial. Th chmisorption happns whn is formd a chmical connction, in this cas dfind as an lctron intrchang. Th intrchang dgr and its symmtry dpnd on th involvd matrials. In physisorption th adsorbd spcis (physisorbd) consrvs its chmical natur, whras a chmisorbd spcis undrgos a mor or lss intnsiv transformation, to giv a diffrnt spcis. On th othr hand, in a physisorption procss th adsorbat can b adsorbd forming succssiv layrs, whras a chmisorption 157

2 Latin Amrican Applid sarch 37: (27) procss is rstrictd to a monolayr, sinc th solid surfac is compltly covrd with th adsorbd substanc. Th chmical transformation of th chmisorbd spci rquirs a crtain activation nrgy that is not ncssary in th physisorption, rason why th last on usually happns to smallr tmpraturs. Th amount of accumulatd matrial dpnds on th dynamic balanc rachd btwn th adsorption rat of th matrial on th surfac and th dsorption rat, that normally dpnds on tmpratur. Whil adsorption rat is gratr and th dsrtion on is smallr, th availabl surfac fraction that will b covrd by adsorbd matrial at quilibrium will b gratr. Th analysis and dsign of an adsorption procss rquirs th adsorption quilibrium, which is th most important pic of information in th undrstanding of th adsorption procss (Vasanth and Sivansan, 26). Isothrms hlp to dtrmin th adsorptiv capacity of matrial and furthr hlp to valuat th mchanism of prformanc of th adsorption systm. Furthr it allows valuat how an adsorption systm can b improvd (Vasanth and Sivansan, 26). So far, most of th adsorption thoris hav bn dvlopd for gas solid systms bcaus th gasous stat is bttr to undrstand than th liquid. Th solid liquid adsorption procss is influncd by paramtrs such as ph, solubility of solut in th solvnt, solution tmpratur and also th initial solut concntration (Vasanth and Sivansan, 26). Som modls of adsorption isothrms hav bn usd to prdict th wast rmoval from rsidual watr with crtain adsorbnts. Frquntly th Langmuir, Frundlich and dlich-ptrson isothrms ar usd for this propos (Baily t al., 1999). A. Langmuir Isothrm Langmuir in 1916 proposd a thory to dscrib th adsorption of gas molculs ovr mtal surfacs. This modl dpnds on th assumption that intrmolcular forcs dcras rapidly with distanc, and consquntly prdicts th xistnc of a monolayr of th adsorbat at th outr surfac of th adsorbnt. Th isothrm quation furthr assums that adsorption taks placs at spcific homognous sits within th adsorbnt. It is thn assumd that onc a molcul occupis a sit, no furthr adsorption can tak plac at th sit. Morovr, th Langmuir quation is basd on th assumption of an adsorbnt of homognous structur whr all adsorption sits ar idntical and nrgtically quivalnt (Langmuir, 1916). Thorically, th adsorbnt has a finit capacity for th adsorbat. Thn th saturation valu is rachd whn no furthr sorption can tak plac. Th adsorption capacity can b rprsntd by th following quation: qm K L C q = (1) 1 + K C L whr C (mmol/l) is th aquous phas adsorbat concntration at quilibrium, q (mmol/g) is quantity of 158 solut adsorbd at quilibrium, q m is qual to q for th complt monolayr and K L (L/mmol) is th Langmuir isothrm constant (Vasanth and Sivansan, 26). Th ssntial charactristic of th Langmuir isothrm can b rprsntd by a sparation factor (dimnsionlss) calld quilibrium paramtr ( L ) (Acmioğlu, 24), and has th following form: 1 L = (2) 1 + b C whr b is th Langmuir constant (L/mmol) y C is th initial concntration of th adsorbat (mmol/l). Th valu L indicats th isothrm typ. A valu btwn and 1 indicat a favorabl adsorption procss (Acmioğlu, 24). B. Frundlich Isothrm Th Frundlich modl is an mpirical quation mployd to dscrib htrognous systms, it is charactrizd by a htrognity factor 1/n. Whn n = 1/n, th Frundlich quation rducs to Langmuir modl. Th mpirical quation can b dscribd by: F 1/ n q = K C (3) whr C (mmol/l) is th liquid-phas adsorbat concntration at quilibrium, q (mmol/g) th quantity of solut adsorbd at quilibrium, K F (L/(mmol 1-1/n.g)) is th Frundlich constant, and 1/n is th htrognity factor. This isothrm is anothr form of th Langmuir, an approach for adsorption on an amorphous surfac. Th amount of adsorbd matrial is th sum of adsorption on all sits. Th Frundlich isothrm is drivd by assuming an xponntial diminution in th distribution function insrtd into Langmuir quation. It dscribs rvrsibl adsorption and is not rstrictd to th formation of a monolayr (Frundlich, and Hllr, 1939). C. dlich-ptrson Isothrm dlich and Ptrson (1959) incorporatd thr paramtrs into an mpirical isothrm. Th dlich-ptrson isothrm modl combins lmnts from both th Langmuir and Frundlich quations, and th mchanism of adsorption is a hybrid uniqu and dos not follow idal monolayr adsorption. Th dlich-ptrson quation is widly usd as a compromis btwn Langmuir and Frundlich systms. For furthr applications of th isothrm for us in kintic or mass transport modls, it is important to hav th most accurat corrlating quation (dlich and Ptrson, 1959). Th quation for this modl is: K C q = β (4) 1 + a C whr q (mmol/g) is th solid-phas sorbat concntration at quilibrium, C (mmol/l) is th concntration of adsorbat in quilibrium with liquid phas, K (L/g) and a (L/mmol 1-1/β ) ar th dlich-ptrson isothrm constants, and β is th xponnt, which lis btwn 1

3 N. PÉEZ, M. SÁNCHEZ, G. INCÓN, L. DELGADO and. Th application of this quation has bn discussd widly, and its limiting bhavior is summarizd hr: Whn β=1 th quation is rducd to th Langmuir modl: K C q = (5) 1 + a C Whn β= th quation is rducd to Hnry s law: K C q = (6) 1 + a Th linar form of ths quations is obtaind applying logarithms proprtis to both sids of th quations. Th Tabl 1 shows th linar form of Langmuir, Frundlich and dlich-ptrson isothrm. An valuation of th isothrms can b don just plotting th known data and dtrmining th rst using th slop and th intrsction point of th lin. For xampl for th Langmuir cas, has to b plottd th known data as C /q against C, and with th slop is dtrmind th 1/q m valu, and with th intrsction point is obtaind 1/K L.q m. Similarly, ln(q ) is plottd against ln(c ) for th Frundlich isothrm, and with th slop and th intrsction point can b dtrmind 1/n and ln(k F ) rspctivly. Th dlich-ptrson cas is a littl mor complicatd, bcaus thr ar thr unknown paramtrs. Th form to find ths paramtrs is basd in th maximization of 2 from th linar fit to th data. In this way th K valus ar modifid until obtain th bst fit of th data. Tabl 1. Linar form of th isothrms C 1 1 Langmuir = C + q q K q Frundlich ln ( q ) ln( K ) + ln( C ) dlich- Ptrson ln K F C q m L m (7) 1 = F (8) n 1 = ln( a ) + β ln( C ) (9) Lignin lik adsorbnt Th adsorption of mtals onto ths matrials (biosorbnts) can b attributd to th protins, carbohydrats, and phnolic compounds that hav carboxylic groups, hydroxyl, sulphat, phosphat and amin that can coordinat with mtallic ions. Th procss of adsorption of mtals consists in diffrnt mchanisms that quantitativ and qualitativly diffrs according to th mtallic spci in solution and to th origin and procssd of th adsorbnt (Villascusa t al., 24). Th lignin is an amorphous biopolymr, formd from fnolic groups, and containing hydroxyl groups, mtoxyl, thr and carboxylic groups into its polymric nt (Parajuli t al., 25), which ar rsponsibl of th high adsorptiv capacity of th lignin (Baily t al., 1999). In spit of th amount of studis mad during th last fiv dcads, th lignin structur has not bn totally lucidatd, spcially th on coming from hardwood (Capanma t al., 25). Th chmical structur, molcular wight, molcular wight distribution and th cross linking grad dpnd on factors such as: th ag and typ of th plant (hard wood, soft wood, grasss, tc.) and th part of th plant (tail, laf, bark, tc.) from which th lignin is isolatd (Singh t al., 25). Du to its high lvl of complxity and th absnc of a univrsal charactr, has bn vry difficult to achiv a consnsus around its structur. Evn bn in capacity to approach a low molcular wight, th lignin is insolubl in watr. Furthr mor lignin has a strong rsistanc to chmical raction, and a molcular wight that vary from 2 to 15 g/mol, which rval a potntial us lik adsorbnts of havy mtals for its rmoval of wast watrs (Drmibas, 24). It is important to mphasiz that th physisorption dpnds principally on th suprficial ara; and th chmisorption involvs othrs aspcts such as: th lctronic distribution, th por siz, th activ groups and th connction form of th adsorbd molculs (Barrow, 1964), whras th chmisorption involvs a hug varity of chmical ractions that ar not abl to b undrstand with a coupl of simpl procsss. Th coxistnc of both procsss at th sam tim, can xplain that a substanc with a no larg suprficial ara is abl to adsorb an ionic matrial comptitivly and with rlativ simplicity. For this rason, th lignin can b studid as adsorbnt. III. MATEIALS AND METHODS A. Adsorbnt usd Acid prcipitatd basic lignin from th pulp and papr industry s black liquor was usd. In Tabl 2, it is shown th most important suprficial charactristic of this lignin. B. Evaluation of th adsorptiv bhavior To valuat of lignin s adsorbing potntial, LIM solutions of known Ni and V concntrations wr put in contact with th lignin adsorbnt at concntration of 1 g/l of solution during two hours at 25 C, and a 5 ph valu, this valu was kpt constant adding H 2 SO 4 (,5 mol/l). Nxt, th suspnsion was vacuum filtrd and washd with distillat watr, and th Ni and V concntrations of th rsulting solutions wr masurd using an Intgra XL GBC optical ICP spctromtr (inductivly coupld plasma.) Th diffrnc btwn th initial and th final mtal quantitis corrsponds to th adsorbd amount. Tabl 2. Suprficial txtur of th lignin Suprficial charactristics lignin BET ara (m 2 /g) 1.37 Total volum of por (cc/g).18 Avrag diamtr of por (Å)

4 Latin Amrican Applid sarch 37: (27) IV. ESULTS AND DISCUSSION sults of adsorption of th studid mtals on th lignin ar shown in Fig. 1. This figur rprsnts th amount of mtal adsorbd pr amount of lignin (adsorbnt agnt) (q ). It is obsrvd that th Ni adsorption prformanc is bttr than th V on for all th initial concntrations valuatd. In Fig. 1, th four rgions idntifid for an isothrm by Vasanth and Sivansan (26) can b clarly diffrntiatd, which ar ncssary as a guarant of th rliability of th paramtrs obtaind using th isothrm analysis. For th cas of Ni, th final concntration at quilibrium pr amount of absorbnt is ovr.9 mmol/g whras th concntration of V is around.8 mmol/g. Ths rsults wr compard with th rsults obtaind by Gonzálz (25), who obtaind th adsorption curv of Ni and V ovr activatd carbon CAC. Th final concntrations at quilibrium obtaind by Gonzálz (25) for th Ni and V, wr.4 and.14 mmol/g rspctivly. Ths quantitis ar similar to thos obtaind in this work, rason why it is possibl to say that th lignin is a comptitiv adsorbnt, although it has a suprficial ara 1 tims smallr. Carrillo (1999) mntiond that th suprficial ara is not a satisfactory indicator of th adsorptiv capacity for an adsorption in liquid phas, in this cas th rlvant factor is th distribution and siz of th pors. Whn th por siz and th adsorbd molcul siz ar similar th attraction is highr (Martin, 198). In this sns, th lignin has th advantag of an avrag por siz quivalnt to th msopors, this siz is appropriatd for adsorbnts. In ordr to prdict th adsorption mchanism of Ni and V on th lignin, th Langmuir modl was initially usd du to its simplicity; in spit of it would suppos in this cas, th homognity of th lignin surfac usd. Figur 2 shows th Langmuir isothrms for th studid systms. Each on of ths isothrms was fittd using a linar rgrssion obtaind standard dviation valus ( 2 ) highr than.99 in both cass (Tabl 3). It indicats that contrary to th xpctd rsults, a convnint fittd of this modl for th studid cas is rachd. Th xistnc of an adsorption procss by homognous monolayrs would sm improbably du to th knowldg of th,1,8 C/q (g/l) Nickl,1,2,3,4,5,6,7,8 C (mmol/l) Figur 2. Isothrm of Langmuir Tabl 3. Paramtrs of th fit by Langmuir isothrm Nickl C /q (g/l) C C Paramtrs K L (L/mmol) q m (mmol/g) htrognity of th adsorbnt. In addition, paramtrs of th Langmuir modl ar also prsntd in Tabl 3. Th xistnc of a multilayr procss was corroboratd fixing th xprimntal data convnintly in ordr to obtain th Frundlich isothrms, which wr fittd using a linar rgrssion as is shown in Fig. 3. Tabl 4 shows th paramtrs obtaind for th studid systms using th Frundlich isothrms. Th standard dviation valus rsultd lowr than thos obtaind using th Langmuir modl. It suggsts that th adsorption in th studid systms do not sm to carry through multilayrs, thrfor th quantity of adsorbnt could b a limiting in th procss, and whn it is saturatd, rsults impossibl to adsorb mor mtal. On th othr hand, it can b obsrvd that th rgrssions obtaind from Langmuir modl ar bttr than thos from Frundlich modl, this fact rflcts that th adsorption procss is fundamntally chmical, bcaus th Frundlich modl rprsnts principally th cas of physical adsorption whras th Langmuir modl rprsnts th cas of chmical adsorption. -4 q (mmol/g),6,4,2 Nickl,1,2,3,4,5,6,7,8,9 1 C (mmol/l) Figur 1. Adsorption of mtals on th lignin ln(q) Nickl ln(c) Figur 3. Isothrm of Frundlich 16

5 N. PÉEZ, M. SÁNCHEZ, G. INCÓN, L. DELGADO Tabl 4. Paramtrs of th fit by Frundlich isothrm Nickl Ln(q ).171 ln( C ) ln( C ) Paramtrs K F (L/mmol 1-1/n g) /n As it was initially assumd that th adsorption procss would not carry through homognous monolayrs, and th fitting by Frundlich modl was poor for th studid systms, th dlich-ptrson modl (Fig. 4), which rprsnts th adsorption modl as monolayrs that can b homognous, was usd to adjust th procss. Whn th xprimntal data wr fittd using this modl th bst standard dviation valus wr obtaind for th Ni compard with th othr modls prviously usd (around.998), but not for th V (around.984). It is infrrd that as Frundlich modl prdict, th adsorption mchanisms of both mtals sm to b diffrnt, bing th physisorption mor important for th V than for th Ni. It could b xplaind bcaus th V ions ar biggr than th Ni ons and thy has highr possibility to b grabbd in th msopor of th lignin usd, making possibl a high physical adsorption for this cas. Tabl 5 shows th paramtrs obtaind for th dlich-ptrson modl. A diffrnc btwn ths paramtrs for Ni and V is obsrvd. This is an additional vidnc that corroborats th nonquality btwn both mchanisms. V. CONCLUSIONS Som conclusions can b obtaind through th ralization and analysis of th isothrms for Ni and V on th lignin: ln(k.c/q - 1) Nickl ln(c) Figur 4. Isothrm of dlich-ptrson Tabl 5. Paramtrs of th fit by dlich-ptrson isothrm Nickl,8428 ln C + 9,,6391 ln( C ) + 2, 638 ln(k.c /q -1) ( ) 64 2,9977,984 Paramtrs K (L/g) 8,152 a (L/mmol 1-1/β ) 867,6 13,985 Β,8428,6391 Th lignin is an adsorbnt agnt for Ni and V, bing bst for th first on. Th adsorption procss obtaind for both mtals is fundamntally chmical. Th adsorption of Ni could b to carry through th formation of monolayrs of htrognous shaps on th lignin. Th adsorption of V could b to carry through th formation of homognous monolayrs with high possibility to b adsorbd for physical procss. It is rcommndabl to account th adsorption nrgis prsnt in th procss for ach mtal in ordr to corroborat if a physical or chmical mchanism is bing dvlopd. VI. ACKNOWLEDEGMENTS This work was supportd by Simon Bolivar Univrsity s DID through projct DI-CAI-8-5, by FONACIT through projct No , and by th Alfa lignocarb Program (Alfa412FAFI). EFEENCES Acmioğlu B., moval of F(II) ions from aquous solution by Calabrian pin bark wasts, Biorsourc Tchnology, 93, (24). Baily S., T. Olin,. Bricka and D. Adrian, A rviw of potntially low-cost sorbnts for havy mtals, Watr sarch, 33, (1999). Barrow G., Fisicoquímica, Editorial vrté, 1964, Capanma E., M. Balakshin and J. Kadla, Quantitativ Charactrization of a Hardwood Milld Wood Lignin by Nuclar Magntic sonanc Spctroscopy, Journal of Agricultural and Food Chmistry, 53, (25). Carrillo M., Estudio d la ficincia dl carbón activado n la rmoción d MTBE n mustras d aguas contaminadas, Trabajo d grado d Licnciatura n Química. Univrsidad Simón Bolívar. (1999) Chamorro M. and M. Kraütr, Simultanous dmtallization and dsulphuration of carbonacous matrials via microwavs, U. S. Patnt nº , May (2). Dmirbas A., Adsorption of lad and cadmium ions in aquous solutions onto modifid lignin from alkali glycrol dlignication, Journal of Hazardous Matrials, 19, (24). Frundlich, H., and W. Hllr, On Adsorption in Solution, J. Amr. Chm. Soc., 61, 2228 (1939). Gonzálz N., Estudio d la adsorción d níqul y vanadio por ligninas, Trabajo d grado d Licnciatura n Química. Univrsidad Simón Bolívar. (25) Langmuir I., Th constitution and Fundamntal Proprtis of Solids and Liquids, J. Am. Chm. Soc., 38, (1916). Martin., Activatd Carbon Product Slction for Watr and Wastwatr Tratmnt, Ind. Eng. Chm. Prod. s. Dv., 19, (198). Parajuli D., K. Inou, K. Ohto, T. Oshima, A. Murota, M. Funaoka and K. Makino, Adsorption of havy 161

6 Latin Amrican Applid sarch 37: (27) mtals on crosslinkd lignocatchol: a modifid lignin gl, activ and Functional Polymrs, 62, (25). dlich O. and D. L. Ptrson, A Usful Adsorption Isothrm, J. Phys. Chm., 63, 124 (1959). Singh, S. Singh, K. Trimukh, K. Pandar, K. Bastawad, D. Gokhal and A. Varma, Lignin Carbohydrat complxs from sugarcan bagass: Prparation, purification, and charactrization, Carbohydrat Polymrs, 62, (25). Vasanth K. and S. Sivansan, Equilibrium data, isothrm paramtrs and procss dsign for partial and complt isothrm of mthyln blu onto activatd carbon, Journal of Hazardous Matrials, In Prss, Corrctd Proof, Availabl onlin 25 January 26. Villascusa I., N. Fiol, M. Martínz, N. Miralls, J. Poch and J. Srarols, moval of coppr and nickl ions from aquous solutions by grap stalks wasts, Watr sarch, 38, (24). 162 civd: May 24, 26. Accptd for publication: August 11, 26. commndd by Editor. Gómz.