Keywords: Biomass; Adsorption; Equilibrium; Isotherms; Kinetics; Thermodynamics; Fixed bed

Transcription

1 BATCH AND FIXED BED ADSORPTION STUDIES OF LEAD (II) CATIONS FROM AQUEOUS SOLUTIONS ONTO GRANULAR ACTIVATED CARBON DERIVED FROM MANGOSTANA GARCINIA SHELL Zaira Zaman Chowdhury, a, *Sharifuddin Mohd. Zain, a Rashid Atta Khan, a Rahman Faizur Rafiqu, b and Khalisanni Khalid c Th fasibility of granular activatd carbon (GAC) drivd from Mangostn (Mangostana garcinia) fruit shll to rmov lad, Pb 2+ cations was invstigatd in batch and fixd bd sorption systms. Batch xprimnts wr carrid out to study quilibrium isothrms, kintics, and thrmodynamics by using an initial lad (Pb 2+ ions) concntration of 50 to 100 mg/l at ph 5.5. Equilibrium data wr fittd using Langmuir, Frundlich, and Tmkin linar quation modls at tmpraturs 30 C, 50 C, and 70 C. Langmuir maximum monolayr sorption capacity was mg/g at 30 C. Th xprimntal data wr bst rprsntd by psudo-scond-ordr and Elovich modls. Th sorption procss was found to b fasibl, ndothrmic, and spontanous. In column xprimnts, th ffcts of initial cation concntration (50 mg/l, 70 mg/l, and 100 mg/l), bd hight (4.5 cm and 3 cm), and flow rat (1 ml/min and 3 ml/min) on th brakthrough charactristics wr valuatd. Brakthrough curvs wr furthr analyzd by using Thomas and Yoon Nlson modls to study column dynamics. Th column was rgnratd and rusd conscutivly for four cycls. Th rsult dmonstratd that th prpard activatd carbon was suitabl for rmoval of Pb 2+ from synthtic aquous solution using batch, as wll as fixd bd sorption systms. Kywords: Biomass; Adsorption; Equilibrium; Isothrms; Kintics; Thrmodynamics; Fixd bd Contact information: a: Dpartmnt of Chmistry, Faculty of Scinc, Univrsity Malaya, Kuala Lumpur 50603, Malaysia; b: Dpartmnt of Civil and Environmntal Enginring, Kumoh National Institut of Tchnology (KIT), Gumi, South Kora; c: Malaysian Agricultural Rsarch and Dvlopmnt Institut (MARDI), Slangor, Malaysia *Corrsponding author: zaira.chowdhury76@gmail.com, zaira_chowdhury@liv.com INTRODUCTION Th prsnc of mtallic contaminants in aquous fflunts manating from diffrnt industris is currntly on of th most crucial nvironmntal issus bing scrutinizd. Th addition of havy mtals into th nvironmnt affcts aquatic lif, human halth, and th ovrall cosystm advrsly du to thir non-biodgradability, complx stat of xistnc, and vry low concntration in a larg volum of surfac watr (Aklil t al. 2004; Srivastava and Hasan 2011; Tofan t al. 2011). Diffrnt mtals such as lad (Pb), coppr (Cu), mangans (Mn), mrcury (Hg), arsnic (As), and zinc (Zn), among othrs, ar known to b highly toxic (Ahluwalia and Goyal 2007). Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

2 Among th havy mtals, divalnt cations of lad (Pb 2+ ions) ar considrd as a priority pollutant (Abdl-Halim t al. 2003). Many industris, spcially acid battry manufacturing, cramic and glass industris, ammunition, mtal plating, and finishing discharg lad-contaminatd watr without sufficint tratmnt to th nvironmnt. Evn at low concntration, lad can damag th kidnys, nrvous systm, and rproductiv systm of popl. Th Environmntal Protction Agncy (EPA) has allowd a maximum prmissibl limit for lad concntration of 0.05 mg/l in wast watr (Gol t al. 2005). For rmdiation of lad contamination in wast strams, svral mthods including prcipitation, ph adjustmnt with alkali hydroxid or lim, ion xchang, rvrs osmosis, and coagulation-sdimntation can b utilizd (Pan t al. 2005; Kobya t al. 2005). It has bn rportd arlir that th prcipitation or alkali complxation procss producs toxic sludg and is lss fficint in diminishing th pollutants concntration to a satisfactory lvl (Gol t al. 2005). Adsorption by using agricultural biomass can b rgardd as on of th most popular mthods for th rmoval of havy mtals including Pb 2+ from th wastwatr du to its abundant availability, biodgradability, and high rmoval fficincy (Zuorro and Lavcchia 2010; Chowdhury t al. 2011a). But th applicability of ths matrials has bn found to b rstrictd up to a crtain xtnt owing to thir small surfac ara and laching of som organic substancs into th aquous solution. To ovrcom such problms, a physio-chmical activation mthod to produc granular-activatd carbon from wast biomass of mangostn fruit shll and thrby nhanc its uptak capacity has bn undrtakn. Adsorption proprtis of numrous wast biomass such as coconut shll (Skar t al. 2004), ta wast (Amarasingh and Williams 2007), Za mays (Zvinowanda t al. 2010), Spirogyra nglcta (Modhr t al. 2009), oil palm ash (Chowdhury t al. 2011a), modifid lignin (Dmirabas 2004), and ric husk (Wong t al. 2003) hav bn rportd for th rmoval of lad from wast watr. Th prsnt rsarch aims to rmov Pb 2+ from synthtic wast watr by using mangostn fruit shll basd activatd carbon (MFSAC), and to invstigat th influnc of all th physicochmical paramtrs involvd during th batch adsorption procss. Txtural charactristics of th prpard activatd carbon ar dtrmind to hav insight about th sorption mchanism. Th rsarch focuss on adsorption isothrm, kintics, and thrmodynamic charactristics of th prpard adsorbat-adsorbnt systm. Furthr information on th fficincy of th prpard activatd carbon for continuous flow adsorption has bn gathrd to study th fixd bd sorption systm.th charactristics of th brakthrough curv has bn analyzd to ascrtain th practical applicability of th adsorbnt for fixd bd sorption systm. EXPERIMENTAL Prparation of Granular Activatd Carbon Raw mangostn fruit shlls wr collctd from th local markt of Malaysia. Th shlls wr washd with hot distilld watr to rmov dust-lik impuritis and inorganic mattrs from th surfac. Aftr washing, th shlls wr drid at 105 C for 24 hours to rmov moistur contnt. Th drid sampls wr asy to cut into small pics Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

3 and wr sivd to th siz of 2 to 3 mm. Granular-activatd carbon was prpard by using a two-stp physiochmical activation mthod from th drid sampl. Drid fruit shlls (50 grams) wr placd insid a tubular ractor. Th flow rat of purifid nitrogn gas and th hating rat wr maintaind at 150 cm 3 /min and 10 C/min, rspctivly. Th tmpratur was incrasd from room tmpratur to 400 C and hld constant for 2 hours to carry out th smi-carbonization stp. Th char producd aftr smi-carbonization was mixd up with a prcis amount of KOH pllts to maintain a ratio 1:1. Dionizd watr (250 ml) was addd to th mixtur of char and KOH in a 500 ml bakr with occasional stirring to nsur sufficint mixing of char with KOH and to dissolv all th KOH pllts. Th mixtur was drid in an ovn. Finally, it was activatd undr CO 2 gas at a flow rat of 150 cm 3 /min. Th tmpratur was incrasd slowly from room tmpratur and kpt constant at 750 C for 2 hours. Th prpard activatd carbon was washd with hot dionizd watr svral tims until th ph rang was 6 to 7. Th final product (MFSAC) thus obtaind was drid and stord in an air-tight containr for furthr application in batch and fixd bd sorption systms. Physiochmical Charactrization of th Granular Activatd Carbon Surfac ara, volum of th por, and por diamtr of th prpard activatd carbon adsorbnt (MFSAC) wr masurd by Quantachrom Autosorb1, an automatd gas sorption systm. Th avrag bulk dnsity was dtrmind by watr displacmnt of a dfinit amount of activatd carbon. Thrmal Gravimtric analysis was carrid out using TGA quipmnt (Modl Prkin Elmr TGA7, US) to invstigat th prsnc of lmnts of moistur, volatil mattrs, fixd carbon, and ash rsidus of th activatd carbon. Prparation of Adsorbat Solution Stock solution of Pb 2+ adsorbat was prpard by dissolving 1.6 grams of Pb (NO 3 ) 2, in a 1000 ml volumtric flask followd by dilution up to th mark by addition of d-ionizd watr. Th concntration of lad in th stock solution was 1000 mg/l. Th tst solutions having concntration of 50 mg/l, 60 mg/l, 70 mg/l, 80 mg/l, 90 mg/l and 100 mg/l wr prpard through frsh dilution of th stock solution prior to ach adsorption study. Th initial ph was adjustd to 5.5 by adding analytical grad dilut hydrochloric acid or sodium hydroxid solution using a ph mtr (Mttls Toldo, Modl: Ross FE 20, USA). Batch Adsorption Studis Th batch xprimnt was carrid out by adding 0.2 grams of activatd carbon with 50 mg/l, 60 mg/l, 70 mg/l, 80 mg/l, 90 mg/l, and 100 mg/l solution of Pb 2+ ions. Th mixtur was agitatd at a spd of about 150 rpm until quilibrium was rachd in a watr bath shakr at a tmpratur of 30 C, 50 C, and 70 C. Th rmaining concntration of th cations was analyzd aftr a st intrval of tim until quilibrium by using an atomic absorption spctrophotomtr (PrkinElmr Modl 3100). Th amount of Pb 2+ ion adsorption at quilibrium, q (mg/g), was calculatd using Equation 1 for a batch sorption systm. Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

4 q ( C0 C ) V W (1) In this xprssion, q (mg/g) is th amount of ion adsorbd at quilibrium. C o and C (mg/l) ar th liquid-phas concntrations of Pb 2+ ions at initial and quilibrium conditions, rspctivly. V (L) is th volum of th solution, and W (g) is th mass of activatd carbon usd. Fixd Bd Adsorption Studis Fixd bd adsorption studis wr conductd in a column mad of pyrx glass having an innr diamtr of 4.5 cm and a hight of 25 cm. A siv mad up of stainlss stl was placd at th bottom of th column. Ovr th siv, a layr of glass wool was placd to prvnt loss of adsorbnt. A pristaltic pump (Modl Mastrfix, Col-Parmr Instrumnt Co., US) was usd to pump th fd upward through th column at a dsird flow rat. Th influnt solution was pumpd upward to prvnt channling du to gravity. Column dynamics wr studid at diffrnt conditions of influnt concntration, bd hight, and flow rat. To dtrmin th ffct of concntration on th brakthrough charactristics, 50 mg/l, 70 mg/l, and 100 mg/l inlt concntrations of Pb 2+ cations wr passd through th column at 4.5 bd hight using flow rat of 1 ml/min. Howvr, 4.5 cm and 3 cm bd hight and 1 ml/min and 3 ml/min flow rats wr usd to analyz th brakthrough curvs using th highst concntration of 100 mg/l. Column Rgnration Studis Column rgnration was carrid out by using 1M HNO 3 acid solution at flow rat 3 ml/min, slightly highr than th influnt flow rat to provid sufficint xchangabl H + ions for 16 hours. Aftr ach cycl, th adsorbnt was washd with hot distilld watr and thn packd insid th column. Th rgnration fficincy (RE%) was calculatd for a bd hight of 4.5 cm, flow rat of 1 ml/min, and initial concntration of 100 mg/l. RESULTS AND DISCUSSION Physiochmical Charactrization of th Granular Activatd Carbon Th adsorption prformanc of th activatd adsorbnt is highly influncd by its physical and chmical proprtis. Th surfac ara, por volum, and diamtr with othr physiochmical charactristics of th smi-carbonizd char and prpard activatd carbon ar listd in Tabl 1. It was obsrvd that, aftr th activation procss, BET surfac ara and total por volum incrasd significantly. This might b du to involvmnt of both chmical and physical activating agnts of KOH and CO 2, rspctivly, during th activation procss. Howvr, KOH is acting hr as a strong dhydrating agnt. During activation at a highr tmpratur, it can produc K 2 O which can furthr ract with CO 2 to yild K 2 CO 3 by watr shift raction (Salman and Hamd 2010). Thus, intrcalation of mtallic potassium insid th carbon matrix of th char causd drastic xpansion of surfac ara, por volum, and diamtr of th prpard activatd carbon (MFSAC). Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

5 According to IUPAC classification th prpard sorbnt is of msoporous typ (IUPAC Manual 1982). Tabl 1. Physio-Chmical Charactristics of Raw Fruit Shll and Granular Activatd Carbon (MFSAC) for Sorption of Pb (II) Cations Physio-Chmical Charactristics Smi-Carbonizd Char Granular Activatd Carbon (MFSAC) BET surfac ara m 2 /g m 2 /g Micropor Surfac ara (DR mthod) m 2 /g m 2 /g Total por volum (DR mthod) cc/g cm 3 /g Avrag por diamtr A 28.9 A Cumulativ adsorption surfac ara (BJH mthod) 1.66 m 2 /g m 2 /g Bulk dnsity g/ml Moistur Volatil Mattr Fixd Carbon Ash Th chmical charactristics of th char along with th activatd carbon wr analyzd by TGA analysis and listd in Tabl 1. From Tabl 1, it was obsrvd that th fixd carbon contnt incrasd whil th volatil mattr dcrasd significantly aftr activation of th char. Th bulk dnsity of th prpard sorbnt (MFSAC) obtaind was g/cm 3. Batch Adsorption Studis Effcts of contact tim and concntration Th ffcts of th initial mtal ion concntration in th rang of 50 mg/l to 100 mg/l for sorption wr invstigatd with rspct to contact tim and ar illustratd in Fig. 1. It is vidnt that th quilibrium uptak, q t (mg/g), of Pb 2+ incrasd proportionally with th initial concntration of th solution. Th incras in initial concntration nhancs th intraction btwn Pb 2+ and activ sits of MFSAC. Highr concntration provids a rlativly larg driving forc to ovrcom th rsistanc to th mass transfr of th sorbat btwn solid and liquid phas, rsulting in a gratr uptak capacity. Howvr, prcntag rmoval of Pb 2+ showd an opposit trnd. As th concntration incrasd, th rmoval prcntag dcrasd. Th prcntag of Pb 2+ rmoval was found to b % for th 50 mg/l sampl whil % was found for th 100 mg/l solution at 30 C. Th adsorption was vry fast at th bginning until 60 minuts. With incrasing tim, th sorption rat bcam mor or lss stabl for all th concntration rangs, bing studid until th systm rachd quilibrium at 60 minuts. Th rapid rat of sorption at th initial stag is du to th incrasd concntration gradint btwn th prsnc of Pb 2+ in liquid phas and onto th solid surfac of MFSAC. At th initial stag, thr wr a larg numbr of vacant activ sits which wr progrssivly occupid by Pb 2+ cations. With th laps of tim, whn th systm was approaching quilibrium, th uptak rat was slowr (Amarasingh and Williams 2007; Srivastava and Hasan 2011). Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

6 Fig. 1. Effct of initial mtal ion concntration and contact tim (min.): C 0 = 50 mg/l to 100 mg/l, Tmpratur = 30 C, ph = 5.5 Equilibrium kintics studis Th kintics of adsorption dscribs th rat of adsorbat uptak (Pb 2+ ) on th adsorbnt at a prdtrmind intrval and quilibrium contact tim. Th kintic paramtrs ar ssntial for th calculation of adsorption rat, providing significant information for dsigning and modling th full scal batch procss (Kalavathy t al. 2005). Th psudo-first-ordr kintic modl has bn widly usd to prdict sorption kintics (Srivastava and Hasan 2011). Th modl is dfind by Equations (2) and (3), k1 log( q qt ) log q t (2) h k1q cal (3) whr, q and q t ar th amounts of adsorbat adsorbd (mg/g) at quilibrium and at tim t (minut) rspctivly, h (mg/g-min) is th initial rat of sorption, and k 1 (min -1 ) is th adsorption rat constant. Figur 2 illustrats th linar plots of ln (q -q t ) vrsus t (minuts) for all th concntrations bing studid. First ordr rat constant k 1, and thortical uptak, q (mg/g) wr calculatd from th slop and intrcpts and listd in Tabl 2. Th linar form of psudo-scond-ordr quation basd on quilibrium Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

7 adsorption is xprssd by (Ho and McKay, 1998) Equations (4) and (5), t q t 1 1 t (4) 2 k q q 2 h k (5) 2 2q cal whr k 2 (g/mg-min) is th rat constant of scond-ordr adsorption and h (mg/g-min) is th initial rat of sorption. Th linar plots of t/q t, vrsus t (mins.) givs l/q as th slop and 1/k 2 q as th intrcpt and is shown by following Fig. 3. Fig. 2. Psudo-First-Ordr Plots: C 0 = 50 mg/l- Fig. 3. Psudo-Scond-Ordr Plots: 100 mg/l, Tmpratur = 30 C and ph = 5.5 C 0 = 50 mg/l-100 mg/l, Tmpratur = 30 C and ph =5.5 Fig. 4. Intra-particl Diffusion Plots: Fig. 5. Elovich Equation Plots: C 0 = 50 mg/l-100 mg/l, Tmpratur = 30 C, C 0 = 50 mg/l-100 mg/l, Tmpratur = 30 C, and ph = 5.5 and ph =5 Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

8 Basd on rlativly low cofficints of corrlation and larg diffrncs btwn xprimntal and calculatd quilibrium uptak capacity, it was obsrvd that psudofirst-ordr kintics was not applicabl for th systm undr invstigation. In contrast, psudo-scond-ordr plots (Fig. 3) showd good linarity indpndnt of concntration rang whn compard to psudo-first-ordr plots. Th calculatd uptak capacitis from psudo-scond-ordr kintics modl agrd vry wll with th xprimntal data with high corrlation cofficints. Both facts suggstd that th adsorption of Pb +2 cations by MFSAC followd psudo-scond-ordr kintics, rflcting that chmisorptions might b th rat-limiting stp in th sorption procss. Tabl 2. Comparison of Psudo-First-Ordr and Psudo-Scond-Ordr Adsorption Rat Constant for Diffrnt Initial Concntration at 30 C Tmpratur Psudo First Ordr Psudo Scond Ordr C 0 q, (xp) q, (cal) k 1 h R 2 q, (cal) k 2 h R 2 (mg/l) (mg/g) (mg/g) (min -1 ) (mg/g) (min -1 ) Th intraparticl diffusion modl is usd to tst th rol of diffusion as th ratlimiting stp in th adsorption procss and is dfind by Equation 6 (Kalavathy t al. 2005), q 0.5 t Kidt (6) whr, K id (mg/g h) is th intraparticl diffusion rat constant obtaind from th slop of th straight lin of q t (mg/g) vrsus t 0.5 (hour), and C is obtaind from intrcpts. Th valu of C givs an ida about th thicknss of th boundary layr (Kannan and Sundaram 2001). Figur 4 shows th intraparticl diffusion plots, and th modl paramtrs ar listd in Tabl 3. Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

9 Tabl 3. Intraparticl Diffusion Modl Rat Constant for Diffrnt Initial Concntration at 30 C Tmpratur Initial Concntration (mg/l) C K dif (mg/gh 0.5 ) R 2 q, cal (mg/g) Two rgions wr obsrvd hr for intraparticl diffusion plots (Fig. 4). At th initial stag, a sharpr rgion was obsrvd du to th instantanous adsorption or xtrnal surfac adsorption procss. Th scond rgion was th gradual adsorption stag whr intraparticl diffusion was th rat-limiting stp. Th straight lins dviatd from th origin, indicating that por diffusion, as wll as othr mchanisms wr involvd in th rat-controlling stp (Mall t al. 2006). Th Elovich quation was furthr implmntd for analyzing th xprimntal data to lucidat th chmisorption natur of th quilibrium systm undr invstigation as sn by Equation 7, 1 1 q t Ln( ab) Lnt (7) b b whr a (mg/g-h) rprsnts initial sorption rat, and b (g/mg) dpicts activation nrgy and xtnt of surfac covrag for chmisorption. Th valu of 1/b Ln (ab) is th sorption quantity whn t (tim) quals to zro, that is t is 1 hour; whras 1/b indicats th numbr of sorption sits for binding. Th linar plots of th Elovich modl ar shown in Fig. 5, and th modl paramtrs ar listd in Tabl 4. Th good linarity showd by th lins in Fig. 5 and high corrlation cofficints furthr support chmisorption natur of adsorption procss. Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

10 Tabl 4. Elovich Modl Rat Constant for Diffrnt Initial Concntration at 30 C Tmpratur Initial Concntration (mg/l) 1/b 1/b ln (ab) R 2 q,cal (mg/g) Equilibrium Isothrm Studis Adsorption isothrms provid lmntary physiochmical statistics for dscribing th distribution of adsorbat btwn solid and liquid phass for a spcific adsorbntadsorbat systm undr fixd raction condition. Th rsults obtaind in this study wr fittd with wll known Langmuir, Frundlich, and Tmkin isothrm modls. Langmuir modl was originally dvlopd for adsorption of gass onto th solid adsorbnt and is also known as an idal localizd monolayr modl (Langmuir 1918). Th Langmuir modl is xprssd as Equation 8. q q K m L (8) 1 K L C C Th linar form of th Langmuir isothrm quation can b writtn as Equation 9, C 1 1 C (9) q q K q m L m whr C is th quilibrium concntration of th adsorbat (mg/l), q is th amount of adsorbat adsorbd pr unit mass of adsorbnt (mg/g), and q m (mg/g) and K L (L/mg) ar th Langmuir constants rlatd to th maximum monolayr adsorption capacity and rat of adsorption, rspctivly. Whn C /q is plottd against C, a straight lin with a slop of 1/q m and intrcpt of 1/q m K L is obtaind (figur not shown). Th ssntial trm of th Langmuir isothrm or sparation factor, R L is rprsntd by Equation 10, R L 1 (10) 1 K C L 0 whr, C 0 is th adsorbat highst initial concntration (100 mg/l). Langmuir sparation factor R L is a dimnsionlss constant which is dscribd by Tabl 5 (Chowdhury t al. 2012). Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

11 Tabl 5. Dscription of Langmuir Constant R L Valus of R L Typs of Isothrm R L > 1 Unfavorabl R L = 1 Linar 0 < R L < 1 Favorabl R L = 0 Irrvrsibl Th Frundlich modl is an mpirical quation that ncompasss th htrognity of th surfac or surfac sustaining sits of wid-ranging affinitis (Chowdhury t al. 2011b). It is basd on xponntial distribution of activ sits and thir nrgis. Th Frundlich isothrm (Frundlich 1906) is xprssd by Equation 11. q K C (11) F 1/ n Th wll-known logarithmic form of Frundlich isothrm is givn by Equation 12. Lnq 1 LnK F LnC (12) n Hr, C (mg/l) is th quilibrium concntration of th adsorbat, q (mg/g) is th amount of adsorbat adsorbd pr unit mass of adsorbnt (mg/g), and K F and n ar Frundlich constants. K F (mg/g (L/mg)) can b dfind as th adsorption or distribution cofficint and rprsnts th quantity of adsorbat adsorbd onto th adsorbnt for a unit quilibrium concntration, and n rprsnts th intnsity factor of sorption. Th Tmkin isothrm is basd on a factor that xplicitly taks into account th adsorbnt-adsorbat intractions (Tmkin and Pyzhv 1940). This modl assums that th hat of adsorption of all th molculs in th layr would dcras linarly with covrag du to adsorbnt-adsorbat intractions. Th adsorption is charactrizd by a rgular distribution of binding nrgis, up to som maximum binding nrgy. Th Tmkin modl is xprssd as: RT q ( ) Ln( KTC ) b (13) Th linar form of Tmkin Isothrm is q BLnK T BLnC (14) RT B (15) b Hr, R is th gas constant (8.314 J/mol K), T is th absolut tmpratur ( K), and RT/b=B. A plot of q vrsus in LnC yilds a linar lin with B as th slop and B (lnk T ) Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

12 as th intrcpt (Figur not shown). K T (L/g) rprsnts th quilibrium binding constant, and B is a hat rlatd constant. Howvr, th isothrm modl paramtrs wr valuatd at 30 C, 50 C, and 70 C to study th ffct of tmpratur on th sorption procss. Th modl paramtrs ar providd in Tabl 6. Tabl 6. Isothrm Modl Paramtrs at 30 C, 50 C, and 70 C Tmpratur Tmp. ( C) q max (mg/g) Langmuir Isothrm Frundlich Isothrm Tmkin Isothrm K L (L/mg) R L R 2 K F (mg/g) (L/mg) 1/n 1/n R 2 B K T (L/mg) From Tabl 5, it was obsrvd that th xprimntal data followd th Langmuir modl significantly bttr than th Frundlich or Tmkin isothrm modls for all th tmpratur rangs studid, rflcting th homognous natur of MFSAC. It also suggstd that th adsorption procss of Pb 2+ onto MFSAC was a monolayr (Pruksathorn and Vitidsant 2009), and adsorption of ach cation had qual activation nrgy. Maximum monolayr sorption capacity, q m (mg/g) incrasd with an incras in tmpratur, dmonstrating ndothrmic natur of sorption. Th Langmuir sparation factor, R L valus obtaind wr btwn 0 to1 for all th tmpratur indicating that th adsorption procss of Pb 2+ onto MFSAC was favorabl. Frundlich xponnt, 1/n ranging btwn 0 and 1 is a masur of adsorption intnsity or surfac htrognity. A valu of 1/n blow on rflcts favorabl adsorption of Pb 2+ onto MFSAC (Chowdhury t al. 2011a,b; 2012). With th incras in tmpratur, th valus of K F also incrasd. Th xprimntal data wr furthr analyzd by Tmkin isothrm, which showd a highr rgrssion cofficint, rflcting strong affinity for adsorption of Pb 2+ onto MFSAC (Kalavathy t al. 2005). Thrmodynamics studis Thrmodynamic paramtrs dscribing G, Gibbs fr nrgy, H, chang in nthalpy of raction, and S, chang in ntropy for adsorbat-adsorbnt intraction can b calculatd by using Equations 16 and 17. R 2 ln K L S R H RT (16) G RTLnK L (17) In ths quations, K L (L/mg) is th Langmuir Isothrm constant at diffrnt tmpraturs; R is th univrsal gas constant (8.314 j/ mol. K), and T is absolut Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

13 tmpratur in Klvin. Th valus of H and S can b dtrmind from th linar plot of lnk L vrsus 1/T (Figur not shown). Howvr, th valus of thrmodynamic paramtrs wr dtrmind and listd in Tabl 7. Tabl 7. Thrmodynamic Paramtrs of Pb (II) Sorption onto Granular Activatd Carbon (MFSAC) Tmpratur, K G (kj-mol -1 ) H (kj-mol -1 ) S (jk -1 mol -1 ) R Th nthalpy chang obtaind hr was (kj-mol -1 ), which was consistnt with Langmuir monolayr sorption capacity, q m (mg/g), rflcting th ndothrmic natur of sorption. Th rsult was also consistnt with a succssiv incras in Frundlich affinity factor, K F with an incras of tmpratur. Th positiv valu of ntropy, S rprsntd an incras in th dgr of frdom of th adsorbat. Th positiv valu of S also rflctd that som changs took plac in th intrnal structur of MFSAC during th adsorption procss. Similar typs of obsrvation wr rportd prviously for rmoval of lad from wastwatr by using activatd palm ash (Chowdhury t al. 2011a). Th magnitud of Gibbs fr nrgy chang ( G ) obtaind was ngativ, dmonstrating that th adsorption took plac spontanously for th tmpratur rang bing studid. Th ngativ valu of G rflctd fasibility of sorption. Fixd Bd Adsorption Studis Effct of adsorbat inlt concntration Th ffct of adsorbat concntration (Pb 2+ ions) on th column prformanc was studid by varying th inlt concntration of lad btwn 50 mg/l, 70 mg/l, and 100 mg/l whil th sam adsorbnt bd hight of 4.5 cm and fd flow rat of 1 ml/min wr usd. Th brakthrough curv is illustratd in Fig. 6. Fig. 6. Brakthrough curvs for adsorption of Pb (II) onto MFSAC for diffrnt initial concntration (Flow rat 1 ml/min, ph 5.5, Tmpratur 30 C) Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

14 As can b obsrvd from th plots (Fig. 6), th activatd carbon bds wr xhaustd fastr at highr adsorbat inlt concntration for 100 mg/l. That is, an arlir brakthrough point was rachd at highr concntration, lading to a rlativly stpr curv whn compard to th lowr concntration. Th brakthrough tim, t b (minuts.) was found to dcras with incrasing adsorbat inlt concntration as th binding sits bcam mor quickly saturatd in th column. A dcras in inlt concntration gav an xtndd brakthrough curv, indicating that a highr volum of solution could b tratd. This is du to th fact that a lowr concntration gradint would caus a slowr transport rsulting in a dcras in diffusion cofficint or mass transfr cofficint (Taty-Costods t al. 2005). Effct of activatd carbon bd hight Figur 7 shows th brakthrough curv obtaind for adsorption of Pb 2+ on MFSAC for two diffrnt bd hights of 3 cm and 4.5 cm ( 3.56 and 4.86g of MFSAC) at constant adsorbat fd flow rat of 1 ml/min and adsorbat inlt concntration of 100 mg/l. As can b sn from Fig. 7, both th brakthrough tim, t b (minuts) and xhaustion tim, t (minuts) wr found to incras with incrasing bd hight. Th plots indicat th shap and gradint of th brakthrough curvs wr slightly diffrnt with variations in bd dpth, which was xpctd. Fig. 7. Brakthrough curvs for adsorption of Pb (II) onto MFSAC for diffrnt bd hight (concntration 100 mg/l, flow rat 1 ml/min, ph 5.5, tmpratur (30 C) A highr uptak was obsrvd at highr bd hight du to th incras in th spcific surfac ara of th activatd carbon, which providd mor fixation of th cations with activ binding sits for th adsorption procss to procd. Th incras in bd hight would incras th mass transfr zon. Th mass transfr zon in a column movs from th ntranc of th bd and procds toward th xit. Hnc, for th sam influnt Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

15 concntration and fixd bd systm, an incras in bd hight would crat a longr distanc for th mass transfr zon to rach th xit, subsquntly rsulting in an xtndd brakthrough tim. For highr bd dpth, th incras of adsorbnt mass would provid a largr srvic ara lading to an incras in th volum of th tratd solution (Taty-Costods t al. 2005). Effct of fd flow rat Th ffct of fd flow rat on th adsorption of Pb 2+ on MFSAC was invstigatd by varying th fd flow rat (1 and 3 ml/min) at a constant adsorbnt bd hight of 4.5 cm and inlt adsorbat concntration of 100 mg/l, as shown by th brakthrough curv in Fig. 8. Th trnd of th curvs showd that at highr flow rat, th front of th adsorption zon quickly rachd th top of th column. This implis that th column was saturatd arly. Lowr flow rat rsultd in longr contact tim, as wll as a shallow adsorption zon. Highr flow rats ar sn by th stpr curv with rlativly arly brakthrough and xhaustion tim; thy rsultd in lss adsorption uptak (Sarin t al. 2006). Fig. 8. Brakthrough curvs for adsorption of Pb (II) onto MFSAC for diffrnt flow rats (concntration 100 mg/l, ph 5.5, tmpratur (30 C) Application of Thomas modl Thomas modl is basd on th assumption that th procss follows Langmuir kintics of adsorption-dsorption with no axial disprsion. It dscribs that th rat driving forc obys 2 nd ordr rvrsibl raction kintics. Th linarizd form of th modl is givn by Equation 18 (Chun t al. 2009). (18) Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

16 In this xprssion k Th, (ml/mg min) is th Thomas rat constant, q 0 (mg/g) is th quilibrium adsorbat uptak, and m is th amount of adsorbnt in th column (Thomas 1944). Th xprimntal data wr fittd with th Thomas modl to dtrmin th rat constant (k th ) and maximum capacity of sorption (q 0 ). Th k th and q 0 valus wr calculatd from th slop and intrcpts of linar plots of ln [(C 0 /C t )-1] against t (mins.) using valus from th column xprimnts (Figur not shown). Th modl paramtrs ar listd in Tabl 8. Tabl 8. Thomas Modl Paramtrs for Pb(II) at Diffrnt Conditions using Linar Rgrssion Analysis Initial Concntration (mg/l) Bd Hight Flow Rat k th (ml/min-mg) x 10-4 (cm) (ml/min) (mg/g) q o R 2 As th concntration incrasd, th valu of k th dcrasd, whras th valu of q o showd a rvrs trnd, i.., incrasd with an incras in concntration. Th bd capacity (q o ) incrasd and th cofficint (k th ) dcrasd with an incras in bd hight. Similarly, q o valus dcrasd and k th valus incrasd with an incras in th flow rat. A similar trnd has also bn obsrvd for sorption of Cr(VI) by activatd wd-fixd bd column (Baral t al. 2009) and azo dy rmoval onto bamboo-basd activatd carbon (Ahmad and Hamd 2010). Exprimntal data fittd th Thomas modl, indicating that th xtrnal and intrnal diffusion is not th limiting stp, and no axial disprsion is prsnt. Application of th Yoon-Nlson modl A simpl thortical modl dvlopd by Yoon-Nlson was applid to invstigat th brakthrough bhavior of Pb 2+ ions on MFSAC. Th linarizd modl for a singl componnt systm is xprssd by Equation 19, whr k YN (min -t ) is th rat constant and τ is th tim rquird for 50% adsorbat brakthrough (Yoon and Nlson 1984). (19) Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

17 Tabl 9. Yoon-Nlson Modl Paramtrs for Pb (II) at Diffrnt Conditions using Linar Rgrssion Analysis Initial Concntration (mg/l) Bd Hight (cm) Flow Rat (ml/min) (l/min) ζ (min) Th valus of k YN and τ wr stimatd from slop and intrcpts of th linar graph btwn ln [C t /(C o - C t )] vrsus t (mins.) at diffrnt flow rats, bd hights, and initial cation concntration (Figurs ar not shown). From Tabl 8, it was obsrvd that valus of k YN wr found to dcras with incrass in bd hight, and th corrsponding valus of τ incrasd with incrasing bd hight. With an incras in initial cation concntration, th k YN incrasd but τ valus dcrasd (Ahmad and Hamd 2010). With an incras in flow rats, th valus of k YN and τ dcrasd. A similar trnd was found by Cd (II) on coir pith for column mod sorption (Bharathi t al. 2011). Column Rgnration Studis MFSAC wr tstd for four cycls aftr th initial application, using 1 M HNO 3 as an luting agnt at a flow rat of 3 ml/min for 16 hours. It was obsrvd that th amount of dsorption was ngligibl aftr 570 minuts. Basd on th Yoon Nlson modl, th amount of adsorbat bing adsorbd in a fixd bd column is half of total adsorbat ntring within 2ζ priod (Bharathi t al. 2011). Thus, th sorption capacity of a column, q org or q q (mg/g) is calculatd from Equation 20 and tabulatd in Tabl 10 for ach cycl. C0r Capacity, qq (20) 1000m Hr, C 0 is th initial concntration, r is flow rat, and m is mass of th activatd carbon in fixd bd. Th rgnration fficincy (RE%) was calculatd for bd hight of 4.5 cm, flow rat 3 ml/min, and initial concntration of 100 mg/l by using Equation 21. k YN R 2 Howvr, th brakthrough tim, t b (minuts), complt xhaustion tim, t (minuts),and rgnration fficincy for diffrnt conditions wr dtrmind and listd in Tabl 10. (21) Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

18 Tabl 10. Rgnration of Fixd Bd Column Adsorbat Cation Cycl No Brakthrough Tim, t b (minut) Column Uptak Capacity (mg/g) Complt Bd Exhaustion Tim, t (minut) Rgnration Efficincy original Lad (II) (%) It was found that th fixd bd sorption systm prformd bttr at a lowr flow rat and concntration of th influnt using highr bd hight. Ths obsrvations wr in agrmnt with th rsarch rportd prviously on various column mod sorption systms (Al-Qodah and Lafi 2003; Padmsh t al. 2006; Malkoc t al. 2006). Ovrall, th sorption capacity obsrvd for fixd bd systm was lowr than th batch systm for th sam initial concntration. This might b du to th insufficint contact tim btwn th adsorbat and adsorbnt in th fixd bd systm than th stirrd batch vssls (Al-Qodah and Lafi 2003). CONCLUSIONS 1. Th physico-chmical activation tchniqu was usd to prpar th adsorbnt (MFSAC) from wast biomass of mangostn fruit shll. This approach was succssful in crating sufficint porositis and surfac ara for adsorption of Pb 2+ from synthtic wast watr. 2. Adsorption data fittd th Langmuir modl bttr than th Frundlich and Tmkin adsorption isothrm modls. 3. Batch quilibrium data followd psudo-scond-ordr kintics, and th Elovich quation rflctd chmisorption as th rat-controlling stp. 4. Equilibrium uptak incrasd with an incras in tmpratur, rflcting ndothrmic natur of sorption which was furthr confirmd by thrmodynamic charactrizations of th systm. 5. Adsorption capacity dtrmind for 50 mg/l, 70 mg/l, and 100 mg/l fd concntration of Pb 2+ at diffrnt conditions wr lss than th batch mthod for th sam intial concntration usd. This may b causd by th probabl irrvrsibility of th sorption procss and th diffrnt approachs to adsorption quilibrium in batch and continuous flow systms in column mod sorption. 6. Rmoval of Pb 2+ onto th prpard adsorbnt (MFSAC) dpnds on an inlt fd concntration, bd hight, and flow rat for fixd bd sorption systm. Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

19 7. Fixd bd sorption data has shown a good agrmnt with wll stablishd column modls, namly th Thomas and Yoon Nlson modls. 8. Th adsorbd Pb 2+ ions can b ffctivly lutd with th us of 1 M HNO 3 acid with th rgnration fficincy of 53.19% up to thr cycls. On th basis of batch and bnch scal column studis, it can b concludd that th adsorbnt (MFSAC) can b usd rpatdly without significant loosing of sorption capacity rflcting its fasibility for commrcial application. ACKNOWLEDGMENTS Th authors ar gratful for th financial support of this projct by Rsarch Grant (UMRG SUS) of Univrsity Malaya, Kumoh National Institut, South Kora (KIT) and Malaysian Agricultural Rsarch and Dvlopmnt Institut (MARDI), Malaysia for thir continuous ncouragmnt. REFERENCES CITED Abdl-Halim, S. H.,Shhata, A. M. A., and El-Shahat, M. F. (2003). Rmoval of lad ions from industrial wast watr by diffrnt typs of natural matrials, Watr Rs. 37, Ahmad, A. A., and Hamd, B. H. (2010). Fixd bd adsorption of azo dy onto granular activatd carbon from prpard from wast, J. Hazard. Matr.175, Ahluwalia, S. S., and Goyal, D. (2007). Microbial and plant drivd biomass for rmoval of havy mtals from wastwatr, Biorsour. Tchnol. 98, Aklil, A., Moutflih, M., and Sbti, S. (2004). Rmoval of havy mtal ions from watr by using calcind phosphat as a nw adsorbnt, J. Hazard. Matr. A112, Al-Qodah, Z., and Lafi., W. (2003). Adsorption of ractiv dys using shal oil ash in fixd bds, Journal of Watr Supply: Rsarch and Tchnology 52, Amarasingh, B. M. W. P. K., and Williams, R. A. (2007), Ta wast as low cost adsorbnt for th rmoval of Cu and Pb from wast watr, Chm. Eng. J. 132, Baral, S. S., Das, N., Ramulu, T. S., Sahoo, S. K., Das, S. N., and Chaudhury, G. R. (2009). Rmoval of Cr(VI) by thrmally activatd wd Salvinia cucullata in a fixd bd column, J. Hazard. Matr. 161, Bharathi, K. S., Badabhagni, N., Nidhsh, P. V., Gandhimathi, R., and Ramsh, S. T. (2011). Brakthrough data analysis of adsorption of Cd(II) on coir pith column, EJEAFCh.10(8), Chowdhury, Z. Z., Zain, S. M., and Rashid, A. K. (2011a). Equilibrium isothrm modlling, kintics, and thrmodynamics study for rmoval of lad from wast watr, E. J. Chm. 8(1), Chowdhury, Z. Z., Zain, S. M., Rashid, A. K., and Ahmd A. A. (2011b). Equilibrium kintics and isothrm studis of Cu (II) adsorption from wast watr onto Alkali Activatd Oil Palm Ash, Am. J. Applid Sci. 8(3), Chowdhury t al. (2012). Lad adsorption on carbon, BioRsourcs 7(3),

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