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Adsorption Isotherm, Thermodynamic and Kinetic Study of Arsenic (III) on Iron Oxide Coated Granular Activated Charcoal

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Intrnational Rsarch Journal of Environmnt Scincs ISSN Adsorption Isothrm, Thrmodynamic and Kintic Study of Arsnic (III) on Iron Oxid Coatd Granular Activatd Charcoal Abstract Saikia Ananta *,
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Intrnational Rsarch Journal of Environmnt Scincs ISSN Adsorption Isothrm, Thrmodynamic and Kintic Study of Arsnic (III) on Iron Oxid Coatd Granular Activatd Charcoal Abstract Saikia Ananta *, Banrj Saumn and Vr Vijay fnc Rsarch Laboratory, RO, Post Bag No. 2, Tzpur , INIA Availabl onlin at: Rcivd 17 th cmbr 2014, rvisd 11 th January 2015, accptd 21 st January 2015 In this papr, adsorption of arsnic (As) (III) as arsnit on iron oxid coatd granular activatd charcoal (FOGAC) has bn studid. Prcntag adsorption of As (III) on FOGAC was dtrmind with rspct to ph, contact tim and adsorbnt dos. Th study rvald that th rmoval of As (III) was bst achivd at ph rang from 7.5 to 9.5. Th initial As (III) concntration (100 µg/l) cam down to lss than 10 µg/l at contact tim 90 min with adsorbnt dos of 7.5g/50mL. Th adsorption was studid for Langmuir, Frundlich, Tmkin and ubinin-radushkvich isothrms. Th Langmuir isothrm modl was found to b fittd wll. uring th study th thrmodynamic paramtrs lik nthalpy chang ( H ), fr nrgy chang ( G ), ntropy chang ( S ) and activation nrgy (E a ) wr dtrmind for th bttr undrstanding of th adsorption procss. Th adsorption was found to b kintically psudo-scond ordr controlld procss. Kywords: Arsnic, iron oxid, granular activatd charcoal, adsorption, kintic. Introduction Watr is on of th most ssntial rquirmnts for all lif on arth and it is considrd as vry important rsourc for human civilization. Saf sourc for pur and affordabl watr is crtainly on of th basic humanitarian goals, and is idntifid as a major global challng for th 21 st cntury. Watr contamination is a most important critical issu facing th world du to an incras in population growth and has bcom a srious thrat to nvironmntal and human halth. Thrfor, dcontamination of pollutd watr is bcom on of th most important nvironmntal mattr in th rcnt dcad. Th availability of arsnic in groundwatr spcially in drinking watr, is causing a major nvironmntal and halth issu 1,2 in svral dvloping rgions 3-5. Th minimum lthal dos for adult humans for various compounds has bn rportd to b 100 to 200 mg 6. Various ffcts lik carcinognic, cardiovascular, rspiratory, drmal, gastrointstinal, mutagnic tc. may b occurrd du to short-trm xposur to arsnic and also it may disturb th immun systm of our body. Cancr of th skin, kidny, bladdr, lungs, nasal passags, livr and prostat may b rsultd du to long-trm xposur to arsnic. All ths halth problms occur mainly du to th consumption of drinking watrs containing high dos of arsnic. Arsnic pollution has bn rportd rcntly in USA, China, Chil, Bangladsh, Taiwan, Mxico, Romania, Unitd Kingdom, Argntina, Poland, Canada, Hungary, Nw Zaland, Vitnam, Japan and India 7. In India, NE stats along with Jharkhand, Bihar, Wst Bngal, Madhya Pradsh, Uttar Pradsh, tc. ar badly affctd by groundwatr contaminatd with arsnic Thus, th World Halth Organization (WHO, 2006) th Unitd Stats Environmntal Protction Agncy (USEPA, 2009) and also Turkish Standards Institut (TSE-266, 2005) hav applid th maximum allowabl lvl of arsnic in drinking watr as 10 µg/l (10 ppb). Arsnic contamination of watr systms is mainly occurs from natural and anthropognic sourcs 12,13. Bsid ths, anothr primary contamination sourc of arsnic is th industrial wastwatr. Arsnic in aquous systms xists in both organic and inorganic forms; inorganic arsnic is mostly found in natural watr systms. In gnral inorganic arsnic is mor poisonous to loving systms than organic arsnic 14. Gnrally, in natural aquous systms inorganic arsnic has two diffrnt oxidation stats, i.. trivalnt (+3) and pntavalnt (+5) 15,16. Th mobility of arsnical forms in watrs is vry dpndnt on ph, Eh conditions and prsnc of diffrnt chmical spcis 12 Pntavalnt arsnic [As (V), arsnat] is stabl in oxidativ condition and xists as a monovalnt (H 2 AsO 4 ) or divalnt (HAsO 2 4 ) anion, whil trivalnt arsnic [As (III), arsnit] is stabl in rductiv conditions 17 and xists as an unchargd (H 3 AsO 0 3 ) or anionic spcis (H 2 AsO 3 ). Inorganic As(III) is approximatly tn tims mor toxic than As(V) 18. Rcnt rport 19 rvals that th toxicity ordr of arsnic is as follows: inorganic As 3+ organic As 3+ organic As 5+ inorganic As 5+ 20,21. In th last two dcads hug attntion has gaind for th dvlopmnt of cost ffctiv tchnologis to rmov arsnic along with othr havy mtals from drinking watr. uring this priod svral physiochmical tchniqus such as adsorption, ion-xchang or chlation, lim softning, rvrs osmosis (mmbran tchniqus), coagulation and prcipitation hav bn applid to rmov arsnic as wll as othr havy mtals from aquous systms But ths tchniqus hav svral Intrnational Scinc Congrss Association 64 Intrnational Rsarch Journal of Environmnt Scincs ISSN drawbacks lik incomplt mtal rmoval, high capital and oprational cost or th disposal of th rsidual mtal sludg, and somtims ar not suitabl for small-scal industris. Among ths tchnologis, adsorption procsss ar ffctiv tchniqus and thy hav long bn usd in th watr and wastwatr industris to rmov inorganic and organic pollution for its as of handling, minimal sludg production, cost ffctivnss and its rgnration capability 1,35. Th tchniqu is also popular bcaus of th availability of a wid rang of adsorbnts such as activatd alumina, activatd carbon, fly ash, frric hydroxid, chitosan-coatd biosorbnt, oxids, clay minrals and zro valnt iron Among all typs of convntional and non-convntional adsorbnts, granular or powdrd activatd carbon is th most widly usd adsorbnt for arsnic rmoval 37 bcaus of thir high surfac ara, micro/msoporous structur and significant adsorption capacity. Activatd carbon modifid with iron has bn usd for th rmoval of various contaminants from watr Howvr, som of ths procsss ar not conomic or rquir th control of ph and/or othr paramtrs to achiv th optimum arsnic rmoval; thrfor, a mor ffctiv and conomical tchniqu would b highly dsirabl. Arsnic rmoval from drinking watr using iron compounds has bn rportd by svral authors Elmntary iron 40-42, granular iron hydroxids and frrihydrits hav bn proposd for th arsnic dcontamination of watr. Most of th adsorption procsss invstigatd so far was rportd in th rviw of Mohan and Pittman 28. In th prsnt study xprimnts wr carrid out to valuat th prformanc of iron oxid coatd granular activatd charcoal (FOGAC) for As (III) rmoval. Th procss paramtrs such as ffct of adsorbnt dos, ph, initial arsnic concntration and contact tim wr studid. Four isothrms lik Langmuir, Frundlich Tmkin and ubinin-radushkvich modls wr studid. Various thrmodynamic paramtrs wr also calculatd for th procss to complt th invstigation for fficacy of FOGAC in adsorption of arsnic from contaminatd watr. Th procss was also analysd for kintic modls lik psudo first ordr, psudo scond ordr, Elovich and Intraparticl diffusion modls at diffrnt xprimntal conditions. Matrial and Mthods Matrials: Th granular activatd carbon (GAC) usd in this study was obtaind from Clariant Chmicals. Th GAC, prior to us in this study was backd in an ovn at 200 C for 24 hrs and washd svral tims with dionisd watr. Aftr washing th GAC was compltly drid in ovn at 110 C, coold in dsiccators, and stord at room tmpratur in a covrd glass containr until furthr us. Th As 2 O 3, FCl 3, HCl and NaOH tc. usd wr LR grad from Mrck and usd as such. Stalk solution of 1000 µg/l arsnic solution was prpard by dissolving g of As 2 O 3 in 1000 ml of dionisd watr with a pallt of NaOH. All th instrumnts usd during th xprimnts wr calibratd as pr standard procdur. Th initial ph of th arsnic solutions was adjustd using NaOH (0.1 M) and/or HCl (0.1M) solutions as and whn ncssary and analysd by Cybrscan ph 510 (Eutch) instrumnt. Th dtrmination of concntration of arsnic (III) was don using AA 7000 Atomic Absorption Spctrophotomtr (Labindia) having dtction limit 5µg/L. Adsorbnt Prparation: Iron oxid coatd GAC (FOGAC) was prpard by mixing 200 g of GAC with 100 ml of 2M FCl 3 solution and gradually adding 5M NaOH solution (narly 180 ml) undr slow agitation until th ph of th final mixtur was attaind at around Th mixtur was thn allowd to sttl for fw hours so air trappd in pors could scap. Aftr that th mixtur was compltly drid in an ovn at 95 C, allowd to cool to room tmpratur, and washd svral tims with dionisd watr to rmov xcss F-oxid. Th FOGAC thus obtaind was thn drid in an ovn at 100 C and stord at room tmpratur in air tight glass containr until us. Adsorption Exprimnts: Th xprimnts wr carrid out by batch procss to obtain rat and quilibrium data. Two paralll xprimnts wr conductd using control granular activatd charcoal (GAC) and FOGAC (figur-1) sparatly to compar fw paramtrs. Th raction mixtur consisting of 50 ml known concntration of As (III) solution and known quantity of adsorbnt was shakn in a tmpratur controlld orbital shakr with shakn spd of 175 rpm at thr diffrnt tmpraturs of 298 K, 303 K and 308 K. Th ffct of adsorbnt dos was studid by varying th adsorbnt dos from 2.5 to 10g/50 ml and maintaining ph of th solution at 7.5 with a constant contact tim of 90 min. In this study th spikd watr arsnic concntration was fixd at 100 µg/l. Th study of th ffct of initial ph of th solutions on arsnic adsorption by th FOGAC was don by using 100 µg/l of th adsorbnt at varying ph of th solutions. Th ffct of contact tim was studid with varying contact tim from 30 to 180 minuts kping ph of th solutions and dos of th adsorbnt constant. Th ffct of initial arsnic concntration in fd watr was studid with various adsorbnt doss with varying initial arsnic concntration from 50 to 250 µg/l. Th adsorption isothrm was also prformd by using 7.5g of FOGAC with 50 ml spikd watr at diffrnt initial concntrations of arsnic. Th kintics and thrmodynamic paramtrs for arsnic adsorption on FOGAC wr stablishd by conducting th xprimnts at diffrnt contact tims and at thr diffrnt tmpraturs rspctivly. Figur-1 Imag of FOFAC Intrnational Scinc Congrss Association 65 Intrnational Rsarch Journal of Environmnt Scincs ISSN sorption xprimnts: To dtrmin th rusability of th FOGAC sampls th adsorption/dsorption cycls wr carrid out. Th saturatd FOGAC was prpard by trating 30 g of FOGAC with 200 ml of 250 µg/l arsnic solution undr agitation for about 3 hours at shakn spd of 175 rpm. Thn filtrd th arsnic saturatd FOGAC and drid in ovn 100 C. Th arsnic saturatd FOGAC (7.5 g) was agitatd for about 3 hours at shakn spd of 175 rpm with 100 ml of NaOH solution (0.1, 0.3 and 0.5 M solution wr usd sparatly). Th aquous phass wr thn sparatd and concntrations of arsnic in that phas wr dtrmind. Rsults and iscussion Effct of adsorbnt dos: Th ffct on arsnic (III) rmoval at fixd initial arsnic (III) concntration 100 µg/l by adsorbnt dos is shown in figur-2. It shows that th adsorption fficincy of arsnic (III) from th solution incrass rapidly with incras in adsorbnt dos from g/50 ml; a marginal incras is obsrvd on furthr incras in th adsorbnt dos for both GAC and FOGAC. Th incras in th rmoval fficincy of arsnic (III) may b attributd to th fact that with th incras in adsorbnt dos, availabl surfac for adsorption incrass for th solut to b adsorbd 54,55. % Rmoval of arsnic GAC FOGAC Adsorbnt dos (g/50 ml) Figur-2 Plots of prcnt rmovals of arsnic vrsus adsorbnt dos Effct of ph: Th ffct of ph on th adsorption of arsnic (III) onto FOGAC and GAC wr studid in th ph rang of 5.5 to iffrnt ph solutions wr prpard by adding dilut NaOH or HNO 3 solutions drop wis to achiv ph valus. Th figur-3 rvals that th fficincy of arsnic (III) rmoval incrass with incrasing ph from 5.5 to 7.5 aftr that th marginal incras was obsrvd. In th study highst arsnic rmoval was achivd at ph rang from 7.5 to 9.5. Th rsults suggstd that th rmoval fficincy of FOGAC and GAC wr found to dcras both at lowr and highr than ph 9.5. This may b du to th fact that highly and xtrm low protonatd surfac of FOGAC and GAC ar not favourabl for arsnic (III) adsorption 56. Th variation of adsorption with rspct to th initial solution ph can b undrstood from th zro point charg (phzpc) of th adsorbnt. phpzc is th point of ph, at which th solid surfac of th adsorbnt has th nt zro charg. Th phzpc of th FOGAC is dtrmind by plotting initial ph of th working solution vrsus th final ph of th solution. Th phzpc is th ph corrsponds to th point at which th curv crosss th straight lin of plot Initial ph=final ph (figur-4). Th phzpc was found to b at ph 8.2. At ph phzpc th surfac of FOGAC bcom ngativly chargd and th As(III) is positivly chargd. In this situation th lctrostatic attraction btwn th mtal ions and th adsorbnt surfac incrass rsulting in incrasd adsorption of As(III) on to th FOGAC surfac is obsrvd. But in th cas, at ph phzpc th surfac of FOGAC bcom positivly chargd lading to a dcrasd adsorption of th As(III) du to th highr concntration of th H + ions in th solution that will compt with th positivly chargd As(III) ions for th activ sits. % Rmoval of arsnic ph GAC FOGAC Figur-3 Plots of prcntag rmoval of arsnic vrsus ph Final ph FOGAC phzpc=8.2 Initial ph=final ph Initial ph Figur-4 trmination of point zro charg (phpzc) of FOGAC Intrnational Scinc Congrss Association 66 Intrnational Rsarch Journal of Environmnt Scincs ISSN Effct of contact tim: Effct of duration of agitation tim was studid with various adsorbnt doss by varying th agitation tim using fd watr containing 100 µg/l of arsnic. From th figur-5 it is clar that adsorption fficincy incrass with incrasing th contact tim, raching narly maximum rmoval at contact tim of 90 minuts; thraftr rmoval bcoms narly constant for both th cass of FOGAC and GAC. Th rason may b du to th fact that, initially all adsorbnt sits wr mpty and th arsnic concntration was high. Aftrward, th arsnic (III) adsorption capacity of th adsorbnt was dcrasd significantly du to th dcras in availabl adsorption sits as wll as arsnic concntration 57. It was also obsrvd that th rmoval fficincy incrass with incrasing th adsorbnt dos. Th fact has alrady bn discussd during th discussion of ffct of adsorbnt dos. furthr arsnic (III) rmoval du to which th procss is sms to b cost ffctiv and suitabl for th application in th rural aras. Figur-6 Effct of initial arsnic concntration on th prcntag rmoval of As (III) using various adsorbnt doss Figur-5 Effct of duration of agitation tim on th adsorption of As (III) onto FOGAC using various adsorbnt doss Effct of initial arsnic concntration: Th influnc of initial arsnic concntration in fd watr was studid using various adsorbnt doss by varying initial arsnic (III) concntration. Figur-6 rvals that adsorption fficincy is highr at lowr initial arsnic concntration (50 µg/l) and a slow dcras in arsnic (III) adsorption by FOGAC as wll as GAC wr obsrvd with incrasing initial arsnic (III) concntration. Th rason for th dcras in As (III) rmoval fficincy at highr initial concntration may b du to saturation of th activ sits of th adsorbnt by th arsnic (III) and hnc, furthr incrasing th arsnic (III) concntration not lading to th incras in absorption significantly 58. From th rsult it is rvald that for 50 ml fd watr containing maximum 100 µg/l arsnic th minimum ffctiv adsorbnt dos to rduc th arsnic contnt to lss than 10 µg/l (WHO limit) is 7.5 g. sorption Study: Th xprimntal rsults (tabl-1) from th dsorption study rvald that trnd of dsorption prcntag at diffrnt concntration of NaOH is in th 0.1M 0.3M 0.5M. Th maximum dsorption of arsnic was found to b 71.4 % with 0.5 M NaOH solution. From th study it is rvald that th usd FOGAC can b rgnratd satisfactorily and rusd for Adsorbt adsorbd by adsorbnt at saturatd point (µg/g) Tabl-1 sorption Study of FOGAC Adsorbt Conc. lutd Elunt of from usd lunt adsorbnt (M) (µg/g) Rgnracy of adsorbnt (%) 1.4 NaOH NaOH NaOH Adsorption Isothrms Study: Langmuir isothrm: Th main assumptions of th Langmuir adsorption isothrm ar; i. Th adsorption is mainly du to saturatd monolayr adsorption of adsorbat molculs on th adsorbnt surfac, ii. th nrgy of adsorption is constant and iii. in th plan of adsorbnt surfac, no transmigration of adsorbat molculs. Th Langmuir adsorption isothrm 59 can b xprssd by th quation givn blow: qmbc q = 1+ bc C 1 Or = C q q 1 + m qm b Whr; q (µg/g) = th amount of adsorbd adsorbat pr unit mass of adsorbnt, C (µg/l) = th unabsorbd adsorbrnt concntration in solution at quilibrium condition, q m (µg/g) = th maximum amount of adsorbat adsorbd pr unit mass of adsorbnt to form a complt monolayr on th surfac b (L/µg)= Langmuir constant. Intrnational Scinc Congrss Association 67 Intrnational Rsarch Journal of Environmnt Scincs ISSN Th Langmuir adsorption modl was adoptd for th dtrmination of maximum arsnic uptak (q m ) at diffrnt arsnic (III) concntration (50 µg/l- 250 µg/l). Th linar plot of C /q vrsus C (figur-7) with gratr corrlation cofficint (R 2 ) valu indicats th monolayr adsorption on FOGAC. Th valus of q m and b wr calculatd from th slop and intrcpt rspctivly ar prsntd in tabl-2. From th rsults it was obsrvd that th maximum arsnic uptak (q m ) and th valu of Langmuir constant (b), rlatd to th affinity of th binding sits wr incrasd with th incras of tmpratur. Frundlich isothrm: Both th monolayr (chmisorption) and multilayr adsorption (physisorption) can b studid by th Frundlich adsorption isothrm. Th isothrm considrd th adsorption of th adsorbat onto th htrognous surfac of an adsorbnt 60. Th Frundlich isothrm 61 can b xprssd by a linar quation as: q = K f C 1 n 1 Or log q = log K f + logc n Whr q (µg/g) = th amount of adsorbd adsorbat pr unit mass of adsorbnt, K f [µg/g (L/µg) 1/n ] = a constant, sorption capacity of th adsorbnt, n = favourability of th adsorption procss, constant rlatd to nrgy of intnsity of adsorption. Figur-7 Langmuir isothrms of FOGAC for arsnic adsorption at diffrnt tmpraturs Tabl-2 Adsorption isothrms 298 K 303 K 308 K Adsorption isothrm Adsorption paramtrs Valus Langmuir Frundlich Tmkin ubinin-radushkvich q m (µg/g) b (L/µg) R n K f [µg/g (L/µg) 1/n ] R B A (L/g) b (kj/mol) R q (µg/g) B ( mol 2 /kj 2 ) 3x10-7 3x10-7 4x10-8 E (kj/mol) R Intrnational Scinc Congrss Association 68 Intrnational Rsarch Journal of Environmnt Scincs ISSN Th valu of K f and n (tabl-2) wr obtaind from th linar plot (figur-8) of log q vrsus log C. Th valus of n lying btwn 1 and 10 indicating chmisorptions 62. Isothrms with n 1 ar known as L-typ isothrms indicating a high affinity btwn th adsorbat and th adsorbnt and is dnots th chmisorption 63. Th Frundlich isothrm constant, K f, which is associatd with th adsorption capacity of th adsorbnt, incrasd with tmpratur, rflcting th ndothrmic adsorption procss. Th rlativly high valu of corrlation cofficint (R 2 ) valu indicats th satisfactory fitnss of th data with th Frundlich isothrm modl. Tmkin Isothrm: Th Tmkin isothrm modl is basd on th assumptions that th adsorption nrgy dcrass linarly with th surfac covrag, which is du to th adsorbnt-adsorbat intractions. Th Tmkin isothrm modl 64 is xprssd by th linar quation as; RT q = ( ) log( AC ) b Or q = Blog A + BlogC Whr: RT B = b (J/mol) = Tmkin constant, rlatd to hat of b, sorption, A (L/g) = Tmkin isothrm constant also, calld quilibrium binding constant, R = gas constant (8.314 J/mol.k), T (k) = absolut tmpratur. Th linar plot (figur-9) of q vrsus logc at diffrnt tmpraturs for Tmkin adsorption isothrm, which considr th chmisorptions of an adsorbat onto th adsorbnt 65, fittd satisfactorily with corrlation cofficints (R 2 ) 0.94 (tabl-2). This supports th findings that th adsorption of arsnic onto FOGAC is a chmisorption procss 66. Figur-8 Frundlich isothrms of FOGAC for arsnic adsorption at diffrnt tmpraturs Figur-9 Tmkin Isothrms of FOGAC for arsnic adsorption at diffrnt tmpraturs Intrnational Scinc Congrss Association 69 Intrnational Rsarch Journal of Environmnt Scincs ISSN ubinin Radushkvich (-R) Isothrm: ubinin Radushkvich (-R) isothrm modl prdicts that th adsorption procss follows a por filling mchanism and is xprssd by a smi-mpirical quation. Th assumptions of th modl ar that th adsorption has a multilayr charactr, involvs van dr Waals forcs and is applicabl for physical adsor
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