MODELING OF ADSORPTION ISOTHERMS AND KINETICS OF 2,4,6- TRICHLOROPHENOL ONTO MICROPOROUS ZnCl 2 ACTIVATED COIR PITH CARBON

J. Environ. Eng. Manag., 18(4), 275-28 (28) MODELING OF ADSORPTION ISOTHERMS AND KINETICS OF 2,4,6- TRICHLOROPHENOL ONTO MICROPOROUS ZnCl 2 ACTIVATED COIR PITH CARBON Rajndran Subha and Chinnaiya Namasivayam* Environmntal Chmistry Division, Dpartmnt of Environmntal Scincs Bharathiar Univrsity Coimbator 64146, India Ky Words: 2,4,6-Trichlorophnol, activatd coir pith carbon, kintics, adsorption isothrms and dsorption ABSTRACT Activatd carbon prpard from coir pith using ZnCl 2 was invstigatd to find th fasibility of its application for rmoval of 2,4,6-trichlorophnol (TCP) in aquous solution through adsorption procss. Batch mod kintics and isothrm studis wr carrid out to valuat th ffcts of contact tim, initial concntration, adsorbnt dos, ph, and tmpratur. Lagrgrn, scond ordr, and Banghams modls wr usd to fit th xprimntal data. Langmuir, Frundlich, D-R and Tmkin isothrm modls hav bn mployd to analys th adsorption quilibrium data. Th Langmuir adsorption capacity was found to b 172 mg g -1 of th adsorbnt. ph ffct and dsorption studis showd that ion xchang mchanism was involvd in th adsorption procss. Rmoval of TCP from synthtic wastwatr was also tstd. INTRODUCTION Coir pith, an agricultural solid wast indignously availabl in coir fibr industris, is a light fluffy matrial, which is gnratd in th sparation procss of th fibr from coconut husk. Annual production of coir pith is around 7.5 million tons in India [1]. Raw coir pith consists of 27.1% of cllulos, 28.3% of lignin and 8.1% of solubl tannin lik phnolic compounds [2]. Coir pith carbon showd good adsorption capacity towards anions, havy mtals and dys whn th surfac was modifid with activating agnts (such as ZnCl 2 ) du to high microporosity [3]. Utilization of coir pith for th tratmnt of ground watr would b hlpful not only to th conomy of th tratmnts but also for solving th solid wast disposal problms of th coir industris. Sinc raw coir pith did not show good adsorption capacity for organics, th surfac of coir pith was activatd with ZnCl 2 in ordr to nhanc its capacity towards organics. Chlorophnols ar prsistnt and toxic organic compounds rlasd into watr as wast from plastic, psticid and othr organochmical industris and rsarch cntrs [4]. 2,4,6-trichlorophnol (TCP) is usd as a wood and glu prsrvativ, as an antimildw agnt for txtils [5], in psticid formulations (hrbicids, fungicids, bactricids) and in lathr tanning and fishing industris. It has bn dtctd in th mission from fossil ful combustion, municipal wast incinration, and chlorination of watr containing phnols or crtain aromatic acids with hypochlorit or during th disinfction of drinking watr sourcs [6], rivr watr, landfill lachat, chmical plant and swag tratmnt plant fflunts and in ambint air [7]. TCP causs rspiratory ffcts such as cough, chronic bronchitis, chst whzing, altrd pulmonary function and pulmonary lsions [8]. It displays a pronouncd undsirabl ffct in portabl watr at concntrations as low as.1 mg L -1 [9]. Hnc, th rmoval of ths organic pollutants from watr stram is critical to nsur th safty of watr supplis. A considrabl ffort has bn ddicatd in th past concrning th rmoval of ths compounds from wastwatrs. Svral mthods hav bn proposd and dvlopd (physiochmical, chmical oxidation and biological dgradation) and th most xtnsivly usd mthod is adsorption procss. Adsorption of organic soluts from aquous phas is a vry important application of activatd carbon, which covrs a wid spctrum of applications from drinking watr to wastwa- *Corrsponding author Email: cnamasi@yahoo.com

276 J. Environ. Eng. Manag., 18(4), 275-28 (28) tr tratmnts in th food, bvrag, pharmacutical and chmical industris [1]. Rcntly, many rsarchrs hav trid to dvlop activatd carbons for th rmoval of various pollutants using rnwabl and chapr prcursors which wr mainly industrial and agricultural byproducts such as coconut shll [11], activatd clay [12], coconut husk [13], and pat and soil [14]. Incrasd attntion has bn paid for carbonizd coir pith, an agricultural solid wast for adsorption of pollutants from watr [3]. Th focus of this rsarch was to xplor th fasibility of ZnCl 2 activatd coir pith carbon (ZnCPC) for th adsorption of TCP. Adsorption kintics, isothrms and ffcts of ph and adsorbnt dos wr studid. MATERIALS AND METHODS Prparation and charactrization of ZnCPC has alrady bn rportd [3]. Exprimntal solutions of TCP wr prpard using analytical ragnt grad. Synthtic wast watrs containing TCP of 6 and 14 mg L -1 wr prpard using analytical ragnt grad salts, NaCl and Na 2 SO 4 and thir composition: chlorid (49.6 mg L -1 ); sulfat (58 mg L -1 ) and sodium (59.6 mg L -1 ) [15]. TCP, NaCl and Na 2 SO 4 wr obtaind from Loba chmi, Mumbai. Batch mod adsorption studis wr carrid out by agitating 5 mg of ZnCPC with 5 ml of TCP solution of dsird concntration on a thrmostatd rotary shakr (ORBITEK, Chnnai, India) at 2 rpm, 35 C and at an initial ph 2.. At th nd of adsorption, th solution and adsorbnt wr sparatd by cntrifugation at 1, rpm for 2 min and th concntration of rsidual TCP was dtrmind spctrophotomtrically using UV-visibl spctrophotomtr at 29 nm (Spcord 2, Analytic Jna, Grmany). Effct of contact tim was studid by withdrawing sampls at prdtrmind tim intrvals and thn rsidual concntration was analyzd as bfor. Effct of ph was studid in th ph rang 2. to 11. by adjusting th ph using.1 M HCl and.1 M NaOH solutions by mans of a ph mtr (Elico, Mod LI-17, Hydrabad, India). Effct of adsorbnt dos was studid with diffrnt adsorbnt doss (1-25 mg) for TCP solutions (6-14 mg L -1 ). Effct of tmpratur on adsorption of TCP was studid using 6-14 mg L -1 concntration and 5 mg of th adsorbnt at 35, 4, 5, 6 o C in a thrmostatd rotary shakr. Th adsorbnt (5 mg pr 5 ml) that was usd for th adsorption of 6-14 mg L -1 of TCP solution was sparatd from th solution by suctionfiltration using Whatman filtr papr and washd gntly with watr to rmov any unadsorbd TCP. Thn th spnt adsorbnt was mixd with 5 ml of distilld watr whos ph was adjustd to various valus (2.- 11.) and agitatd at tim intrvals longr than th quilibrium tim. Thn th dsorbd TCP was stimatd as bfor. RESULTS AND DISCUSSION Th physio-chmical charactristics of ZnCPC in comparison with coir pith carbon in th absnc of ZnCl 2 activation has bn alrady rportd [16]. 1. Effct of Raction Tim and Kintics Th amount of TCP adsorbd (mg g -1 ) incrasd with incras in agitation tim and rachd quilibrium. Th quilibrium tim was found to b 3, 4, 7, 8 and 1 min for 6, 8, 1, 12, 14 mg L -1, rspctivly. Th amount of TCP rmovd at quilibrium incrasd from 58 to 127 mg g -1 with incras in TCP concntration from 6 to 14 mg L -1. It shows that th adsorption at diffrnt concntrations was rapid in th initial stags and gradually dcrasd with th progrss of adsorption until th quilibrium was rachd. 2. Adsorption Kintics Adsorption kintic modls corrlat th adsorbat uptak rat with bulk concntration of th adsorbat. Kintic data wr fit into Lagrgrn [17], scond ordr [18] and Banghams por diffusion [19] modls. Th first ordr kintic modl is rprsntd as: log (q -q) = log q k 1 t (1) 2.3 whr q and q ar th amounts of TCP adsorbd (mg g -1 ) at quilibrium and at tim t, rspctivly, and k 1 is th Lagrgrn rat constant of first ordr adsorption (min -1 ). Th scond ordr kintic modl is xprssd as: t 1 t = + (2) 2 q k q 2q whr k 2 is th rat constant of scond ordr adsorption (g mg -1 min -1 ). Th xprimntal q valus wr closr to th calculatd q valus obtaind from th scond ordr kintic plots (R 2 clos to 1) compard to thos of th first ordr kintic plots (Tabl 1). Thrfor, th adsorption kintics can wll b approximatd mor favorably by scond ordr kintic modl for TCP. Similar phnomnon has bn obsrvd in th adsorption of TCP by coconut shll carbon [11]. Bangham s quation was usd to valuat whthr th adsorption is por-diffusion controlld. log C K M = log ( ) ( C qm ) 2.3V + a logt (3) whr C is initial concntration (mm), V is volum of th solution (ml), M is wight of th adsorbnt (g L -1 ), q m is amount of adsorbat rtaind at tim t (mmol g -1 ) and α, K ar constants. Plots of log {log [C /(C -q M )]} vs log t (R 2 =.93

Subha and Namasivayam: Adsorption of 2,4,6-TCP onto ZnCl 2 Activatd Coir Pith Carbon 277 Tabl 1. Comparison of first ordr and scond ordr adsorption rat constants and Banghams constants and calculatd and xprimntal q valus for diffrnt initial TCP concntrations and for diffrnt tmpraturs Paramtr q (xp) (mg g -1 ) First ordr kintics Scond ordr kintics Banghams modl k 1 q (cal) (min -1 ) (mg g -1 R 2 k 2 q (cal) ) (g mg min -1 ) (mg g -1 R 2 k ) o α R 2 Concntration ( mg L -1 )* 6 57.9.71 9.2.96.22 58.5.99 64.5.25.93 8 76.2.97 21.3.98.11 77.5.99 48.4.3.99 1 94.9.61 33.7.98.41 98..99 31.3.32.98 12 112.39 4.2.95.31 112.99 3.9.32.97 14 127.36 57.8.98.16 13.99 25.5.38.96 Tmpratur ( C) + 35 57.9.71 9.2.96.22 58.5.99 64.5.25.93 4 58.5.115 8.4.93.27 59.5.99 65.9.3.97 5 58.7.126 9.9.96.34 59.2.99 66.8.3.95 6 58.9.187 12.3.98.3 6.2.99 65.2.35.97 * Conditions: Adsorbnt dos, 5 mg in 5 ml; ph 2.; Tmp. 35 C. + Conditions: Conc., 6 mg L -1 ; adsorbnt dos, 5 mg in 5 ml; ph 2.. loglog(co/co-qtm).2.1.5 1 1.5 2 2.5 -.1 -.2 -.3 -.4 -.5 -.6 log t 6 mg L -1 8 mg L -1 1 mg L -1 12 mg L -1 14 mg L -1 R 2.9316.9946.986.9769.9617 Fig. 1. Banghams plots for adsorption of TCP onto ZnCPC. to.99) wr found to b linar for diffrnt concntrations at 35 C (Fig. 1) which confirms that th adsorption is por-diffusion controlld. Incras in adsorbnt dos incrasd th rmoval of TCP and quantitativ rmoval occurrd at 7, 1, 15, 2 and 25 mg pr 5 ml adsorbnt dos for 6, 8, 1, 12 and 14 mg L -1 concntration of TCP, rspctivly. Incras in adsorption with adsorbnt dos is attributd to gratr surfac ara and th availability of mor adsorption sits. 3. Adsorption Isothrms Th quilibrium adsorption isothrm is fundamntally vry crucial in dsigning adsorption systms. In this study, adsorption of TCP by ZnCPC was analyzd by wll documntd Langmuir (Eq. 4) [2], Frundlich (Eq. 5) [21], Dubinin-Radushkvich (Eq. 6) [22] and Tmkin (Eq. 7) [23] isothrm modls givn blow, rspctivly (Fig. 2). C q 1 C = + Q b Q (4) q (mg g -1 ) 2 18 16 14 12 1 8 6 4 2 2 4 6 8 1 12 14 C (mg L -1 ) Fig. 2. Adsorption isothrms for TCP onto ZnCPC. Exprimntal q Langmuir Frundlich D-R Tmkin log q = log k f + 1/n log C (5) ln q = ln q m ßє 2 (6) q = B ln A + B ln C (7) whr in Eq. 4, C is th concntration of TCP solution (mg L -1 ) at quilibrium, Q givs th thortical monolayr adsorption capacity (mg g -1 ) and b is rlatd to th nrgy of adsorption (mg L -1 ). In Eq. 5, k f and 1/n ar th constants that can b rlatd to adsorption capacity and th intnsity of adsorption rspctivly. In Eq. 6, q m is th thortical saturation capacity (mol g -1 ), ß is a constant rlatd to th man fr nrgy of adsorption pr mol of th adsorbat (mol 2 J -2 ), and є is th Polanyi potntial. In Eq. 7, A and B ar constants. Langmuir constants, Q and b, wr calculatd from th linar plot of C /q vs C and found to b 172 mg g -1 and.234 L mg -1, rspctivly. Th adsorption capacity of coir pith carbon (in th absnc of ZnCl 2 activation) for TCP was found to b ngligibl [24]. Highr surfac ara and highr por volum of th ZnCPC ar rsponsibl for th good adsorption capac

278 J. Environ. Eng. Manag., 18(4), 275-28 (28) Tabl 2. Langmuir, Frundlich, D-R and Tmkin constants for adsorption of TCP by various adsorbnts rportd in litratur Adsorbnt Q b k f q n m E B Rf. Coir pith carbon (in th prsnc of ZnCl 2 activation) Coir pith carbon (in th absnc of ZnCl 2 activation) (mg g -1 ) (L mg -1 ) (mg 1-1/n L 1/n g -1 ) (mol g -1 ) (kj mol -1 ) 172.24 43 2.2 981 12..9 32. 6 This work a a 1.2.7 b b b [24] Coconut shll 122.4.48 2.5 1.5 b b 25.3 [11] activatd crabon Commrcial grad 112.4.14 2.95 1.67 b b b [11] activatd carbon Coconut husk 191.73 b b b b b b [13] Activatd clay 123.5.22 4.42 1.44 b b b [12] Pat b b 7.8.8 b b b [14] Soil b b 52.6 1.4 b b b [14] a - Dos not follow Langmuir isothrm; b - Not rportd. ity of th carbon for TCP. Th Frundlich constants, k f and n wr valuatd from th linar plots of log q vs log C and found to b 43.1 mg 1-1/n L 1/n g -1 and 2.2, rspctivly. Th valu of n lis btwn 1 and 1 indicating favorabl adsorption [25]. Th D-R constants q m and ß wr valuatd from th linar plots of ln q vs є 2 and found to b 981 mol g -1 and 3 1-9 mol 2 J -2, rspctivly. Th constant ß giv an ida about th man fr nrgy E (kj mol -1 ) of adsorption [22]. Th valu of E (12.9 kj mol -1 ) is btwn 8 and 16 kj mol -1 which shows that th adsorption follows ion xchang mchanism [26]. Th Tmkin isothrm constants A and B wr found to b 1 L mg -1 and 32.6 mg g -1, rspctivly. Langmuir, Frundlich, D-R and Tmkin constants for adsorption of TCP by various adsorbnts rportd in litratur ar prsntd in Tabl 2. Taking into account th cost of othr activatd carbons and ZnCPC; it is obvious that ZnCPC is cost ffctiv, bcaus th raw matrial is an unwantd wast and dos not hav any commrcial valu. Figur 2 prsnts diffrnt adsorption isothrms along with th xprimntal data. In ordr to compar th validity of isothrms, a normalizd dviation, q (%), was calculatd using th following quation [27]: Δq (%) = 1 xp cal xp [ ( q q ) q ] ( n 1) 2 (8) whr suprscripts xp and cal ar th xprimntal and calculatd valus, rspctivly, and n is th numbr of masurmnts. From th rsults it was found that valus of q obtaind from Langmuir [ q (%) = 3.5], and Frundlich [ q (%) = 5], wr lowr compard to D-R [ q (%) = 47] and Tmkin [ q (%) = 45] adsorption isothrm modls. It shows that th quilibrium data fit bttr with Langmuir and Ionization % ionization (%) 12 1 8 6 4 2 TCP conc. 6 mg L -1 8 mg L -1 1 mg L -1 12 mg L -1 14 mg L -1 2 4 6 8 1 12 Final ph Fig. 3. Effct of final ph on ionization of TCP. Frundlich isothrms ovr th ntir rang of concntrations compard to D-R and Tmkin isothrms. Incras of tmpratur hardly incrasd th amount adsorbd at quilibrium, q (Tabl1) which shows that tmpratur had no ffct on adsorption. 4. Effct of ph Effct of ph on spcis distribution and adsorption of TCP wr invstigatd. Concntration of unionizd phnol with ph is rlatd by th quation [28]: CPT CP = (9) (ph pka) 1+ 1 whr CP o is th concntration of unionizd chlorophnol spcis, CP T is th total concntration of chlorophnol takn, ph is th final ph (quilibrium ph) aftr adsorption and pk a is 6.1 for TCP. Using Eq. 9, th concntration of ionizd TCP spcis (chlorophnolat) was calculatd at diffrnt final ph valus and plots of th prcnt ionizd chlorophnol concntration vrsus final ph wr obtaind (Fig. 3). As th

Subha and Namasivayam: Adsorption of 2,4,6-TCP onto ZnCl 2 Activatd Coir Pith Carbon 279 Adsorption (%) % 1 9 8 7 6 5 4 3 2 1 6 mg L -1 14 mg L -1 1 2 4 6 8 1 12 Initial ph Fig. 4. Effct of ph and dsorption on TCP. initial ph was incrasd, th prcnt of unionizd spcis dcrasd and that of th ionizd spcis incrasd. Sinc thr was no lctrostatic rpulsion btwn th unionizd chlorophnol spcis and th positivly chargd adsorbnt surfac at ph 2., th rmoval was highr. At alkalin ph, th adsorbnt surfac is ngativly chargd which showd dcras in adsorption du to lctrostatic rpulsion btwn ngativly chargd surfac and chlorophnolat spcis (Fig. 4). Similar rsults hav bn rportd in th adsorptiv rmoval of chlorophnols from aquous solution by coconut shll activatd carbon [11]. At ph > ph zpc (3.4), th dgr of protonation dos not immdiatly drop to zro. This is vidncd by th obsrvd final ph aftr adsorption which was lss than 7. up to initial ph 8.. Thrfor adsorption of TCP was still possibl. 5. Batch Mod Dsorption Studis Dsorption studis hlp lucidat th mchanism of adsorption and rcovr th prcious TCP and adsorbnt. As th dsorbing ph was incrasd, th prcnt dsorption incrasd from 51% at ph 2. to 84% at ph 11. for 6 mg L -1 (Fig. 4). This is du to th incrasd concntration of OH - which displacd th adsorbd phnolat ions from th adsorbnt. Studis on both ph ffct and dsorption show that ion xchang mchanism was involvd for th rmoval of TCP by ZnCPC. 6. Tsts with Synthtic Wastwatr Effct of ph and adsorbnt dos on th adsorption of TCP from synthtic wast watr is similar to pur TCP solutions. Maximum rmoval of 91 and 81%, rspctivly, was obsrvd at ph 2. for th TCP concntrations 6 and 14 mg L -1 in th wastwatr compard to 96 and 88% in pur TCP solutions. Quantitativ rmoval was obsrvd at an adsorbnt dos of 1 and 3 mg 5 ml -1 for th TCP concntrations of 6 and 14 mg L -1, rspctivly, in th wast watr compard 1 9 8 7 6 5 4 3 2 Dsorption Dsorption (%) % to 7 and 25 mg 5 ml -1 in pur TCP solutions. Lowr prcnt rmoval in ph ffct and highr adsorbnt dos rquirmnt in adsorbnt dos ffct for th rmoval of TCP in wastwatr ar attributd to th prsnc of othr comptiting componnts such as Cl - and SO 4 2- in th wastwatr. CONCLUSIONS Th prsnt study shows that ZnCl 2 activatd carbon dvlopd from an agricultural wast, coir pith, is an ffctiv adsorbnt for th rmoval of TCP from aquous solution. Equilibrium adsorption data fit bttr into Langmuir and Frundlich isothrms compard to D-R and Tmkin isothrms. Adsorption kintics followd scond ordr and Banghams modl. Langmuir adsorption capacity was found to b 172 mg g -1. Adsorption was found to b maximum at ph 2.. Th ph ffct and dsorption studis show that ion xchang mchanism was involvd in adsorption procss. Quantitativ rmoval of TCP from synthtic wastwatr using ZnCPC could also b achivd. ACKNOWLEDGEMENTS Authors ar gratful to DAAD, Grmany for providing th quipmnt grant which facilitatd th work prsntd in this papr. REFERENCES 1. Orlando, U.S., T. Okuda, A.U. Bas, W. Nishijima and M. Okada, Chmical proprtis of anion xchangrs prpard from wast natural matrials. Ract. Funct. Polym., 55(3), 311-318 (23). 2. Vinodhini, S., V.S. Gnanambal and S.N. Padmadvi, Efficacy of dgradd coir pith for th growth of mdicinal plants. Ind. Coconut J., 35(11), 16-17 (25). 3. Namasivayam, C. and D. Sangtha, Rcycling of agricultural solid wast, coir pith: Rmoval of anions, havy mtals, organics and dys from watr by absorption onto ZnCl 2 activatd coir pith carbon. J. Hazard. Matr., 135(1-3), 449-452 (26). 4. Sabhi, S. and J. Kiwi, Dgradation of 2,4- dichlorophnol by immobilizd iron catalysts. Watr Rs., 35(8), 1994-22 (21). 5. Intrnational Agncy for Rsarch on Cancr (IARC), Som Halognatd Hydrocarbons. IARC Monographs on th Evaluation of Carcinognic Risk of Chmicals to Humans. IARC, Lyon, Franc, 2, 69 (1979). 6. Hazardous substancs Data Bank (HSDB),

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