Microwave induced KOH activation of guava peel carbon as an adsorbent for congo red dye removal from aqueous phase

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1 Indian Journal of Chmical Tchnology Vol. 20, Sptmbr 203, pp Microwav inducd KOH activation of guava pl carbon as an adsorbnt for congo rd dy rmoval from aquous phas Pardp Singh*, Pankaj Raizada, Dpak Pathania, Gaurav Sharma & Pankaj Sharma Dpartmnt of Chmistry, Shoolini Univrsity of Biotchnology and Managmnt Scincs, Solan 73 22, India Rcivd 3 Octobr 202; accptd 28 Fbruary 203 Th adsorption capacity of activatd carbon prpard from guava fruit pl wast has bn studid for congo dy. Th activation procss is carrid out undr microwav radiations in th prsnc of KOH. Th guava pl basd activatd carbon (GPAC) and unactivatd carbon (GPUC) hav bn charactrizd using Fourir transform infrard spctroscopy, scanning lctron microscopy and X-ray diffraction. Th comparativ adsorption bhavior of GPAC, GPUC and commrcially activatd carbon (CAC) has bn studid for th congo rd dy rmoval using batch adsorption xprimnts. Th non-linar Langmuir and Frndulich isothrm modls ar usd to stimulat th quilibrium data. Th quilibrium data is rprsntd by th Langmuir isothrm modl. Th kintic modl shows that psudo scond ordr kintics has bst fit to th xprimntal data (R 2 >0.90). Thrmodynamic studis conclud that adsorption procss is spontanous and ndothrmic in natur. GPAC has mrgd as fficint adsorbnt for congo dy as compard to GPUC and CAC. Furthr, dsorption studis show that % of mtal ions can b rcovrd by dsorption in first cycl and adsorbnt can b rusd. Kywords: Activatd carbon, Adsorption isothrm, Congo rd, Dy rmoval, Guava fruit wast, Microwav activation About 5% of th dystuffs ar lost in th industrial fflunts during manufacturing and procssing oprations,2. Th color of fflunts dischargd into cosystm affcts th aquatic flora and fauna and causs many nvironmntal problms 3,4. Thrfor, it bcoms ncssary to trat ths fflunts for nvironmntal protction 5. Som of th biological or physico-chmical tratmnts mployd for th tratmnt of dy fflunts includ anarobic/arobic tratmnt, coagulation, flocculation, oxidation, mmbran sparation and sorption 6. Howvr, adsorption is considrd to b suprior to othr tchniqus du to its low cost, simplicity of dsign, high dcontamination fficincy and ability to trat dys in mor concntratd forms 7. Adsorption also liminats th nd for hug sludg-handling procsss during wast watr trarmnt 8. Ovr th yars, rsarchs hav bn committd to stablish th rliability of natural and rnwabl lignocllulosic biomass as low cost adsorbnts and altrnativ prcursors for th production of activ carbons 8,9. Howvr, th xtnsiv utilization of activ carbons is hindrd by th high cost pyrolysis and activation procsss. Th long procssing tim, *Corrsponding author. pardpchm@gmail.com; pankajchm@gmail.com high nrgy consumption and impropr hating rat, rsult in a dtrimntal ffct on th quality of th prpard activatd carbons 8,9,0. Thrfor, a rapid and asy rout is ncssary for th prparation of bio-basd activatd carbon. Guava (Psidium guajava) is cultivatd in many tropical and subtropical countris including India. Guava fruits ar usually 4-2 cm long. Ths hav both nutritional and mdicinal valus,2. Its wid scal cultivation and application in food industris rsults in gnration in massiv gnration of pl wast spcially at thir pak of production sason 2. Th gnratd pl wast rquirs handling. Rcntly, microwav radiations hav bn usd to fabricat mtal oxid nanoparticls in a vry short raction tim 3,4. Ponnusami t al. 5 xplord th adsorption capacity of guava laf powdr as adsorbnt for mthyln blu rmoval. Th prsnt work focuss on th prparation of chmically activatd guava pl carbon using microwav hating. Th surfac morphology and functionalitis of unactivatd (GPUC) and activatd carbon (GPAC) ar charactrizd using SEM, FTIR and XRD spctral tchniqus. Th adsorption capacitis of GPAC and GPUC ar compard with chmically activatd carbon for congo rd rmoval. Th adsorption paramtrs, such as adsorbnt doss, initial dy concntration, ph,

2 306 INDIAN J. CHEM. TECHNOL, SEPTEMBER 203 tim and tmpratur will b studid to xplor th fasibility of this rmoval procss. Exprimntal Procdur Chmicals All chmical usd wr of analytical grad. Congo rd dy was purchasd from CDH Company, India and usd without furthr purification. KOH and commrcial activatd carbon (CAC) wr obtaind from SD Fin Company, India. Prparation of unactivatd carbon (GPUC) Guava pl wast was collctd from local markt in Solan, Himachal Pradsh, washd thoroughly with distilld watr to rmov dust and othr impuritis, and drid. Th drid pl wast was milld and sivd to obtain powdr. Th obtaind powdr was labld as GPUC and prsrvd for furthr us. Prparation of GPAC GPUC powdr was soakd in KOH (0. N) solution during 0 min with an imprgnation ratio (GPUC: KOH) of :.5 (wt.%) to obtain thick syrupy solution. Th solution was placd in microwav ovn with an input powr of 600 W and 6 min of irradiation tim. Th activatd sampls wr tratd with 0.5 N HCl to rmov any rmaining KOH and thn filtrd. Finally, activatd carbon (GPAC) was drid at 80 C for 3 h and kpt in a glass bottl until usd. Exprimntal Protocol Th batch adsorption xprimnts wr prformd in a st of 250 ml of Erlnmyr flask containing adsorbnt and 50 ml congo rd dy (CR) solution with various initial concntrations. Th flasks wr agitatd in an isothrmal watr bath shakr at 20 rpm and 30 ± C until th quilibrium was attaintd. Th adsorbnt was sparatd from mothr solution with th hlp of vaccum pump. Dy concntration was dtrmind using doubl bam UV-visibl spctrophotomtr at 497nm λ max. Th ph of th solution was adjustd with N HCl and NaOH solutions. Th prcntag rmoval fficincy and amount of dy adsorbd at quilibrium (q ) (mol/g) wr calculatd using th following quations: % Rmoval = Co C C o () q M ( Co C ) (2) V whr C 0 and C ar th initial and quilibrium concntrations of congo rd dy; M, th mass of adsorbnt (mg/50 ml); and V, th volum of dy solution (ml). Rsults and Discussion Charactrization SEM and XRD analysis SEM micrographs of th GPUC and GPAC ar shown in Figs a and b. It can b obsrvd that surfac of GPUC sampl is dns, planar, constrictd and blockd. Howvr, th GPAC sampl shows a wll pronouncd, vn and highly porous structur, indicating good possibility for th dys to b trappd and adsorbd. Fig. c dmonstrats XRD pattrn of GPUC and GPAC. Th broad paks indicat th amorphous and porous natur of prpard carbon. Th significant broadning of spctra in GPAC confirms th improvmnt in porous natur of microwav-koh activatd carbon. FTIR analysis In FTIR spctrum of GPUC, th broad and intns absorption pak at around 3420 cm - corrsponds to th OH strtching vibration of alcohols, phnols, carboxylic acid as in pctin, cllulos and lignin (Fig. d). This pak signifis th prsnc of fr hydroxyl groups on th adsorbnt surfac 6. Th pak at 2900 cm - is attributd th symmtric and asymmtric C-H strtching vibration of aliphatic acid. Th pak at 688 cm - is th strtching vibration of bond du to non-ionic carboxyl groups ( COOH, COOCH 3 ), and may b assignd to carboxylic acids or thir strs 7. Th pak at 58 cm - is attributd to asymmtric and symmtric strtching vibrations of C=O in ionic carboxylic groups ( COO ) 8. In cas of activatd carbon, changs in FTIR spctrum ar obsrvd. On comparing th spctra of GPAC and CPUC, it can b sn that hydroxyl group is shiftd from 3420 cm - to 343 cm - in activatd carbon (GFAC). Th paks at 2900 cm - and 690 cm - ar shiftd to 2925 cm - and cm -, rspctivly. In GPAC, apparanc of nw paks cm - rgion is du to introduction of nw functionalitis on th surfac of activatd carbon. Rmoval of congo rd dy Effct of adsorbnt and dy concntration Th ffct of GPUC, GPAC and CAC dosags on CR rmoval has bn invstigatd to xplor optimal

3 SINGH t al.: MICROWAVE INDUCED KOH ACTIVATION OF GUAVA PEEL CARBON AS DYE ADSORBENT 307 Fig. Charactrization of GPUC and GPAC (a) SEM imags of GPUC, (b) SEM imag of GPAC, (c) XRD spctra of GPUC and GPAC, and (d) FTIR spctra of GPUC and GPAC bfor and aftr adsorption procss. adsorbnt concntration for dy rmoval. Fig. 2 a shows that th rmoval of CR incrass sharply with th incras in adsorbnt concntration from 0 mg/50 ml to 40 mg/50 ml. This may b du to availability of mor adsorbnt sits as wll as gratr availability of spcific surfacs of th adsorbnts. Aftr 40 mg/50 ml of adsorbnt loading, no significant changs in prcntag of CR rmoval ar obsrvd. Du to conglomration of adsorbnt particls, thr is no significant incras in ffctiv surfac ara 9,20. Th rmoval fficincy is found to incras within initial dy concntration (Fig. 2 b). At low concntration thr ar unoccupid activ sits found on th adsorbnt surfac. Th activ sits rquird for th adsorption of th dy molculs ar lacking at highr concntration 9,20. Thrfor, rmoval fficincy is found to b dcras with th incras in dy concntration byond mol dm -3. Effct of contact tim Th ffct of contact tim on th adsorption of CR has bn studid and th findings ar givn in Fig. 2 c. About 85, 75 and 45 % of dy rmoval is rportd in 80 min for GPAC, CAC and GPUC rspctivly. Th quilibrium is attaind aftr 80 min of contact tim. Th chang in rmoval fficincy might b du to th fact that initially all th adsorbnt sits ar vacant and dy concntration gradint is vry high. Th lowr adsorption rat is du to dcrasd vacant sit of adsorbnt and dy concntration. Th CR monolayr formation on adsorbnt surfac rsults in dcrasd adsorption rat, particularly, towards th nd of xprimnts 20,2. Effct of ph Th rsults of blank dy solution indicats that chang in initial ph of dy solution (3-) has ngligibl ffct on λ max of dy solution. It provs that no chmical chang in dy structur occurs at ph 3-. Solution ph xrts a profound ffct on adsorption procss by govrning th surfac charg and ionization/dissociation of adsorbnt. Prcntag rmoval of CR at diffrnt ph is studid in batch xprimnt using 40 mg/50 ml of adsorbnts at an ambint tmpratur condition (30±0.5 C) for contact tim of 80 min and initial dy concntration of mol dm -3 (Fig. 2 d). CR rmoval fficincy is

4 308 INDIAN J. CHEM. TECHNOL, SEPTEMBER 203 Fig. 2 Effct of (a) adsorbnt dos, (b) CR concntration, (c) contact tim, and (d) ph on CR rmoval [[CR] mol dm -3, [adosrbnt] 40 mg/50 ml, contact tim 80 min, ph 6 and tmpratur 30± C] found to b highr at ph 3. Adsorption capacity of invstigatd adsorbnts dcrass with incras in ph from 3 to. At ph 3, th adsorbnt surfac is positivly chargd du protonation of surfac functionalitis lading to highr adsorption of anionic CR dy. Howvr, highr ph valu rsults in wakning of lctrostatic forc of attraction btwn th oppositly chargd adsorbat and th adsorbnt ultimatly, lading to th rduction in th prcntag of CR rmoval Adsorption isothrm studis Th adsorbat concntration in th bulk and at intrfac can b dtrmind by using adsorption isothrms 22,23. In th prsnt study, quilibrium data ar modld by using Langmuir and Frundlich isothrms. Frundlich isothrm modl is givn by following quation: log q = log K f + n log C (3) whr q is th amount of adsorbat adsorbd pr unit wight of adsorbnt (mg/g); K f (mg/g), th adsorption capacity; and C, th initial amount of adsorbat. Langmuir isothrm is givn by following quation: qobc q = + bc (4) Tabl Langmuir and Frundlich isothrm paramtrs for CR adsorption. [[CR] mol dm -3, [adosrbnt] 40 mg/50 ml, contact tim 80 min, ph 6 and tmpratur 30± C] Paramtr GPAC CAC GPUC Langmuir q max, mg/g b, L/mg R R L Frundlich N K f R whr q is th amount of adsorbat adsorbd pr unit wight of adsorbnt (mg/g); q o, th monolayr adsorption capacity (mg/gm); and b, th Langmuir constant (L/mg). Th fasibility of adsorption procss can b dtrmind in trm of th following dimnsionlss constant sparation trm (R L ): R L = (5) + bc o In th prsnt study, R L valus (0< R L <) favor th adsorption of CR onto adsorbnts (Tabl ). Langmuir and Frundlich isothrms modls ar givn in Tabl. Langmuir isothrm modl xhibits bttr fit to data for CR rmoval by GPAC, CAC and GPUC. This suggsts that monolayr adsorption occurs uniformly on th activ sits of th adsorbnts.

5 SINGH t al.: MICROWAVE INDUCED KOH ACTIVATION OF GUAVA PEEL CARBON AS DYE ADSORBENT 309 Tabl 2 Kintic paramtrs for CR adsorption [[CR] mol dm -3, [adosrbnt] 40 mg/50 ml, contact tim 80 min., ph 6 and tmpratur 30± C] Adsorbnt Psudo first ordr Psudo scond ordr Intraparticl diffusion q k R 2 k 2 h R 2 k id C R 2 GPAC CAC GPUC Onc th adsorbat molculs occupy activ sits, no furthr adsorption can tak plac at th sit 2. Adsorption kintics Th kintic study is important to dscrib th tim dpndnc of systm undr various conditions. Th psudo first ordr rat constant is givn by following quation: k log (q q t ) = log q t (6) whr q t and q ar th adsorption capacitis at instantanous contact tim and quilibrium tim rspctivly; and k, th rat constant for adsorption procss. For psudo scond ordr rat, following rlationship is givn 2-24 : t q = 2 k2q + t q (7) whr k 2 is th rat constant of psudo scond ordr quation (g/mg. min). Psudo scond ordr kintics ar fittd wll with th xprimntal data (Tabl 2). Th corrlation cofficints ar also in good agrmnt with psudo scond ordr kintic modls (R 2 > 0.99). It indicats that chmisorption is th rat-controlling mchanism for th adsorption procss 25. Th possibility of intraparticl diffusion was dtrmind using following quation: qt 2 = d ki dt + C (8) whr C is th intrcpt; and k id, th intraparticl diffusion rat constant (mg/lmin /2 ). According to th intraparticl diffusion modl, a plot of q t vrsus t /2 should b linar if intraparticl diffusion is involvd in th adsorption procss 20, Th rsults of intraparticl diffusion ar shown in Tabl 2. Th dviation of th lins from origin indicats that intraparticl transport is not th rat limiting stp 22. Th adsorption procss dos not oby intraparticl diffusion modl. Tabl 3 Thrmodynamic paramtrs for adsorption of CR onto diffrnt adsorbnts Adsorbnt H 0, kj/mol S 0, J/mol K G 0, kj/mol GPAC CAC GPUC Fig. 3 Effct of tmpratur on adsorption procss [[CR] mol dm -3, [adosrbnt] 40 mg/50 ml, contact tim 80 min, and ph 6] Effct of tmpratur on adsorption quilibrium Th ffct of tmpratur on dy adsorption is prsntd in Fig. 3. Th adsorption rat incrass with incras in tmpratur from 25 C to 45 C. This suggsts that adsorption procss is ndothrmic in natur. Thrmodynamic paramtrs ar obtaind at initial CR concntration of mol dm -3. Th valus of S and H ar calculatd using th following rlationship: S H log K = RT c R (9) whr K c is th quilibrium constant of th adsorption and is dfind as: Cadsorbnt K c = (0) C solution

6 30 INDIAN J. CHEM. TECHNOL, SEPTEMBER 203 Tabl 4 Comparativ adsorption capacity data for diffrnt adsorbnts. Prcursor Activation mthod Activating agnt Activation tim min Adsorption capacity mg/g Rfrncs Apricot ston Convntional hating H 2 SO Ric straw Convntional hating (NH 4 ) 2 HPO Durain pl Convntional hating CO Coff prss cak Convntional hating N Bamboo Microwav hating H 3 PO Cotton stalk Microwav hating KOH CAC Prsnt study Guava pl Unactivatd Prsnt study Guava pl Microwav hating KOH Prsnt study Fig. 4 Rcyclic fficincy of GPAC for congo rd adsoprtion [[CR] mol dm -3, [adosrbnt] 40 mg/50 ml, contact tim 80 min, ph 6 and tmpratur 30± C] Hr C adsorbnt and C solution ar th dy concntrations onto th adsorbnt and rsidual CR concntration at quilibrium. Th fr nrgy of an adsorption procss is obtaind from th givn quation: ( K ) G = RT l n () c whr G is th fr nrgy chang (kj/mol); T, th absolut tmpratur (K); and R, th univrsal gas constant (8.34 J/mol K). Th ngativ valu of Gibbs fr nrgy indicats that th adsorption procss is spontanous in natur and th dgr of spontanity of th raction was incrasd with incrasing tmpratur (Tabl 3). Th ovrall adsorption procss is ndothrmic in natur ( H = 2.55 KJ/mol, 5.56 KJ/mol and.26 KJ/mol). This rsult also indicatd that adsorption mchanism is primarily basd on chmisorption procss. Th positiv valu of S suggsts incras in th randomnss at th solid/solution intrfac du to th rdistribution of nrgy btwn dsorbnt and adsorbat 22,24. Rcovry and rcycling fficincy of usd GPAC Rcycling fficincy is an important paramtr for adsorption procss. Dsorption studis ar carrid out with % of HCl, H 2 SO 4 and NaOH. GPAC (40 mg) saturatd with 50 ml of CR dy solution (7 0-5 mol dm -3 ) was placd in dsorption mdium and thn constantly stirrd on a rotatry shakr at 00 rpm for on hour. NaOH is found to b an fficint dsorption mdium for th rmoval of CR from adsorbnt surfac. 88% of CR is dsorbd in 60 min of dsption xprimnt. Aftr dsorption, no significant changs ar obsrvd in FTIR spctrum of GPAC. Th rcovrd GPAC is than tstd for th rmoval of CR from aquous phas. Aftr six cycls, th adsorption capacity is rducd to 65% from 96% for congo rd dy rmoval (Fig. 4). Tabl 4 rprsnts th comparativ study of GPAC with othr adsorbnts prpard by diffrnt mthod This study rports mg/g of CR adsorption using GPAC. Du to prsnc of mor prdominat functionality(k + ), GPAC sms to b fficint adsorbnt than GPUC and CAC. It is notworthy to mntion that activation tim in prsnt study is vry short as compard to rportd mthod. Howvr, dcisiv comparison of adsorbnts was complicatd du to diffrnt raction conditions such as concntration, ph, tmpratur, adsorbnt dos, tc. Conclusion Th prsnt study shows that GPAC can b usd as an ffctiv altrnativ for convntional adsorbnts. This is found to b low-cost adsorbnt for th rmoval of anionic dy (congo rd) from aquous solutions. Adsorption capacity of th adsorbnts is strongly influncd by ph, contact tim, concntration of adsorbnt and initial dy concntration of raction systm. CR rmoval followd psudo scond ordr kintics modls and adsorption is controlld by

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