Removal of cadmium(ii) from aqueous solution by agricultural waste cashew nut shell

Transcription

1 Koran J. Chm. Eng., 29(6), (2012) DOI: /s INVITED REVIEW PAPER Rmoval of cadmium(ii) from aquous solution by agricultural wast cashw nut shll Ponnusamy Snthil Kumar*, Subramaniam Ramalingam**, Vasanthakumar Sathyaslvabala***, Slvaraj Dinsh Kirupha***, Arukkani Murugsan*** and Subramanian Sivansan***, *Dpartmnt of Chmical Enginring, SSN Collg of Enginring, Chnnai , India **Dpartmnt of Chmical Enginring, Univrsity of Louisiana at Lafaytt, LA 70504, Unitd Stats ***Dpartmnt of Chmical Enginring, AC Tch, Anna Univrsity, Chnnai , India (Rcivd 28 Octobr 2010 accptd 5 Octobr 2011) Abstract Cashw nut shll (CNS) is a low cost adsorbnt that has bn usd for th rmoval of cadmium(ii) from an aquous solution. Th ffcts of various paramtrs such as solution ph, CNS concntration, contact tim, initial cadmium(ii) concntration and tmpratur wr xamind. Th CNS was ffctiv for th quantitativ rmoval of cadmium(ii) ions in acidic conditions and quilibrium was achivd in 30 min. Th xprimntal data wr analyzd by two-paramtr (Langmuir, Frundlich, Tmkin and Dubinin-Radushkvich) and thr-paramtr modls (Rdlich- Ptrson, Kobl-Corrigan, Toth and Sips) by nonlinar rgrssion analysis. Th charactristic paramtrs for ach isothrms and rlatd corrlation cofficints hav bn dtrmind by using MATLAB 7.1. Thrmodynamic paramtrs such as G o, H o and S o hav also bn valuatd, and it was found that th sorption procss was fasibl, spontanous and xothrmic. Psudo-first-ordr, psudo-scond-ordr, Elovich kintic and intraparticl diffusion modls wr slctd to follow th adsorption procss. Th rsults of th kintic study show that th adsorption of cadmium(ii) could b dscribd by th psudo-scond ordr quation, suggsting that th adsorption procss is prsumably chmisorption. A singl-stag batch adsorbr was dsignd for diffrnt adsorbnt dos-to-fflunt volum ratios using th Frundlich quation. Th rsults indicat that th cashw nut shll could b usd to ffctivly adsorb cadmium(ii) from an aquous solution. Ky words: Cadmium(II), CNS, Isothrms, Kintics, Thrmodynamics INTRODUCTION Havy mtals ar toxic pollutants rlasd into th nvironmnt as a rsult of industrial, mining and agricultural activitis [1,2]. Cadmium is a havy mtal that is rgardd as an lmnt of high toxicity. Cadmium ions ar not biodgradabl and can b accumulatd asily in living tissu, thus can b radily adsorbd into th human body throughout th food chain [3,4]. An xcssiv lvl of cadmium ions in th watr can affct th bio-systms and b a thrat to human bings. Cadmium toxicity causs advrs halth ffcts such as bon lsions, cancr, lung insufficincy and hyprtnsion [5-8]. Th drinking watr guidlin valu rcommndd for this lmnt by th World Halth Organization (WHO) and Amrican Watr Works Association (AWWA) is mg/l [9,10]. Hnc th rmoval of cadmium ions from th watr and wastwatr is highly important. Th traditional mthods to rmov th havy mtals from aquous nvironmnt includ prcipitation, filtration, ion xchang, vaporation, rvrs osmosis, solvnt xtraction, lctrochmical tratmnt and mmbran tchnologis. Howvr, ths traditional mthods ar ithr infficint or xpnsiv whn havy mtals xist in lowr concntrations [11,12]. Consquntly, it is ssntial to find nw mthods for ffctiv rmoval of havy mtals from watr and wastwatr. Compard with othr traditional mthods, adsorption is quit popular du to its simplicity and high fficincy, as wll as th availability of a wid rang of adsorbnts [13-18]. Activatd carbon is To whom corrspondnc should b addrssd. sivansh1963@gmail.com th most widly usd adsorbnt for th rmoval of havy mtals from th aquous mdia. But its high cost and loss during th rgnration rstrict its applications [3,19]. Whn compard to activatd carbon, th usag of agricultural wasts or industrial by-product as a low-cost altrnativ adsorbnt has rcivd considrabl attntion. Many matrials such as RCS [3], AGCS [3], bal tr laf powdr [20], ric husk [21], sugarcan bagass [22], grap stalk [23], corncob [24], saw dust [25], ta-industry wast [26], oliv cak [27], hazlnut shll [28], bagass fly ash [29], low-grad phosphat [30], hmatit [31], what bran [32], prtratd Asprgillus nigr [33], prtratd Mucor rouxii [34], liv Mucor rouxii [34], Chins hrb Pang Da Hai [35], Gracilaria salicornia (rd) [36], Ciba pntandr hulls carbon [37], oliv ston carbon [38], oak wood char [39], pin bark char [39], oak bark char [39], carbon F-400 [39], calgon F-400 [40], pur smctit [41], lwatit S100 [41], Hypna valntia biomass [42], and banana pls [43] hav bn tstd for cadmium(ii) rmoval from watr and wastwatr systms. Bcaus of th low cost and high availability of ths matrials, it is not ssntial to undrtak a complicatd rgnration procss. Th prsnt rsarch is to dvlop inxpnsiv and ffctiv adsorbnt from th agricultural wasts, such as cashw nut shll (CNS), to rplac th xisting commrcial matrials. In th prsnt study, cashw nut shll from which th cashw nut shll liquid (CNSL) was xtractd [44] was usd as an adsorbnt and it was xamind for sorption proprtis towards cadmium(ii) ions. Th ffct of various xprimntal paramtrs such as solution ph, CNS concntration, contact tim, initial cadmium(ii) concntration and tmpratur hav bn invstigatd. Adsorption isothrms and kintics wr in- 756

2 Rmoval of cadmium(ii) from aquous solution by agricultural wast cashw nut shll 757 Tabl 1. Proprtis of th cashw nut shlls Proximat analysis (wt%) Ultimat analysis (wt%) Ash chmical composition (wt%) Volatil mattr Carbon Silica Moistur Hydrogn Iron oxid Ash Oxygn Aluminium oxid Fixd carbon Nitrogn Calcium oxid Sulphur Nil Magnsium oxid Moistur Sodium oxid Ash vstigatd and diffrnt adsorbnt modls wr usd to valuat th xprimntal data and to lucidat th possibl adsorption mchanism. Thrmodynamic studis wr also conductd to stimat th standard fr nrgy ( G o ), nthalpy chang ( H o ) and ntropy chang ( S o ). EXPERIMENTAL 1. Adsorbnt Th raw CNS was collctd from Karaikudi, Sivagangai District, Tamilnadu, India and th tratd CNS was collctd from th CNSL rcovry unit and it was usd as an adsorbnt. This agricultural wast was thoroughly rinsd with watr to rmov dust and solubl matrial; thn it was allowd to dry at room tmpratur. Th abov drid wast was ground to a fin powdr in a still mill. Th rsulting matrial was sivd in th siz rang of msh particl siz. Th proximat and ultimat analysis of cashw nut shlls ar shown in Tabl Adsorbat All th chmicals usd wr of analytical ragnt (AR) grad. Stock solution of 100 mg/l of cadmium(ii) was prpard from 3Cd(SO 4 ) 8H 2 O (Mrck, India) using doubl distilld watr. Dsird tst solutions of cadmium(ii) ions wr prpard using appropriat subsqunt dilutions of th stock solution. Diffrnt concntrations of cadmium(ii) ions wr prpard from standard solution varid btwn 10 to 50 mg/l. Bfor mixing th adsorbnt, th ph of ach tst solution was adjustd to th rquird valu with 0.1 M NaOH or 0.1 M HCl. 3. Analysis Th concntrations of cadmium in th various solutions bfor and aftr quilibrium wr dtrmind by AA6300 Atomic absorption spctromtr (Shimadzu, Japan). Th ph of solution was masurd with a Hanna ph mtr using a combind glass lctrod. FTIR analysis was usd to idntify th diffrnt functional groups prsnt in th CNS adsorbnt. An FTIR spctromtr (PE IR SPECTRUM ASCII PEDS 1.60) was usd for th IR spctral studis of CNS adsorbnt (4,000 to 450 cm 1 ). For FTIR studis, 1 mg of sampl was mixd and ground with 100 mg of KBr and mad into a pllt. Th background absorbanc was masurd by using pur KBr pllt. Th surfac morphology of th CNS adsorbnt was analyzd with a Lo Gmini 1530 scanning lctron microscop at an acclrating voltag of 10 kv, and with th working distanc of 100 µm for th CNS adsorbnt. 4. Adsorption Studis A sris of batch adsorption xprimnts wr conductd to study th adsorption mchanism, adsorption isothrms and adsorption kintics by shaking th flasks at 120 rpm using a rotary shakr. Th systmatic procss for th rmoval of cadmium(ii) ions from aquous solutions by th us of CNS in a batch systm was studid in th prsnt work. Th data obtaind in batch studis wr usd to calculat th prcntag rmoval of cadmium(ii) ions by using th following mass balanc rlationship: C % o C Cadmium( II) rmoval = C o Whr, C o and C ar th initial and quilibrium concntration (mg/ L) of th cadmium(ii) ion solutions, rspctivly Effct of Solution ph on Cadmium(II) Adsorption Th ffct of solution ph on th adsorption capacity of CNS was invstigatd using a 100 ml solution of 20 mg/l of cadmium(ii) ion for a ph rang of 2.0 to 8.0 at 30 o C. Th xprimnts could not b prformd at highr ph valu du to hydrolysis and prcipitation of cadmium(ii) ions. Th flasks wr agitatd on a rotary shakr for 30 min to nsur that th quilibrium was rachd. Th mixturs wr thn filtrd through Whatman 42 filtr papr and th concntration of cadmium(ii) in th filtrats was analyzd using an AA6300 Atomic absorption spctromtr. Each dtrmination was rpatd thric and th rsults obtaind wr thir avrag valu Effct of CNS Concntration Th batch adsorption xprimnts wr conductd at a diffrnt CNS concntration from 0.1 g to 0.6 g at a 100 ml solution of 20 mg/l of cadmium(ii) ion at ph 5.0, for a contact tim of 30 min at 30 o C. Th sampls wr thn agitatd and filtrd and th filtrats wr analyzd as mntiond bfor Effct of Contact Tim Th batch adsorption xprimnts wr conductd at diffrnt contact tims (5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60 min) for an initial concntration of cadmium(ii) ion in th rang of mg/ L at ph 5.0, with th CNS concntration is 0.3 g in 100 ml solution in 250 ml conical flask at 30 o C. Th sampls wr thn agitatd and filtrd and th filtrats wr analyzd as bfor Effct of Initial Cadmium(II) Ion Concntration Th batch xprimnts wr carrid out by contacting 0.3 g of CNS with 100 ml of cadmium(ii) ion solution of diffrnt initial concntrations (10, 20, 30, 40, 50 mg/l) at ph valu of 5.0 at 30 o C. A sris of such conical flasks wr shakn for 30 min at a spd of 120 rpm. Th sampls wr filtrd and th filtrats wr analyzd as bfor Effct of Tmpratur Th batch xprimnts wr prformd at diffrnt tmpraturs (1) Koran J. Chm. Eng.(Vol. 29, No. 6)

3 758 P. S. Kumar t al. of 30, 40, 50 and 60 o C for th initial cadmium(ii) ion concntrations in th rang of mg/l at constant CNS dos of 3 g/l and ph of 5. A sris of such conical flasks wr shakn for 30 min at a spd of 120 rpm. Th sampls wr filtrd and th filtrats wr analyzd as bfor. 5. Adsorption Isothrms A sris of solutions containing diffrnt initial cadmium(ii) ions concntrations wr prpard and th batch adsorption studis wr undrtakn at 30 o C to chck th applicability of th two-paramtr (Langmuir, Frundlich, Tmkin and Dubinin-Radushkvich) and thr-paramtr adsorption isothrm modls (Rdlich-Ptrson, Kobl- Corrigan, Toth and Sips) undr th spcifid conditions: initial solution ph of 5, contact tim of 30 min, adsorbnt dos of 3 g/l and an initial cadmium(ii) ion concntration rang of mg/l. Th cadmium(ii) ion concntration of th various solutions was analyzd by atomic absorption spctromtr mthod. Th data obtaind in batch quilibrium studis wr usd to calculat th quilibrium cadmium(ii) adsorptiv quantity by using th following mass balanc rlationship: ( C o C )V q = m Whr q is th amount of cadmium(ii) ion adsorbd (mg/g), V is th volum of solution tratd (L), C o is th initial concntration of cadmium(ii) ion (mg/l), C is th quilibrium cadmium(ii) ion concntration (mg/l) and m is th mass of th adsorbnt (g). 6. Adsorption Kintics Kintic studis wr conductd in a 250 ml conical flask at room tmpratur (30 o C). Thus, th adsorbnt dos of 0.3 g of CNS was contactd with 100 ml of a cadmium(ii) ion solution of diffrnt known concntrations (10-50 mg/l). Th solution mixtur was shakn continuously and th sampls wr takn at prdtrmind tim intrvals from th mixtur and wr immdiatly filtrd. Th analysis of cadmium(ii) ion concntration of th various solutions was dtrmind by atomic absorption spctromtr mthod. Th amount of cadmium(ii) adsorbd at tim t, q t (mg/g), was calculatd by th following xprssion: ( C o C t )V q t = m Whr C t is th concntration of cadmium(ii) ion solution at any tim t (mg/l). 7. Dsorption Studis Dsorption studis wr conductd to chck th rvrsibility of th adsorption procss. Dsorption of cadmium(ii) ions from th spnt CNS was studid with th varying concntrations of hydrochloric acid (HCl) solution. Th spnt CNS was transfrrd to stopprd conical flasks which consist of 100 ml of HCl solution. Th bottls wr shakn at 30 o C using a rotary shakr, and th CNS was thn rmovd by filtration. Th concntrations of cadmium(ii) in th aquous solutions wr dtrmind by using AA6300 Atomic absorption spctromtr. RESULTS AND DISCUSSION (2) (3) Fig. 1. FT-IR spctrum of cashw nut shll. 1. Charactrization of CNS 1-1. FTIR Studis FTIR spctroscopy is a usful tool to idntify th diffrnt functional groups prsnt in an adsorbnt. Th adsorption capacity of CNS dpnds upon th porosity as wll as th chmical ractivity of functional groups at th surfac. This ractivity crats an imbalanc btwn th forcs at th surfac whn compard to thos within th body, thus lading to a molcular adsorption by th van dr Waals forc. Knowldg of surfac functional groups givs an insight into th adsorption capability of th CNS. Th FTIR spctrum of th CNS sampl is shown in Fig. 1. Th intns broad band at 3,399 cm 1 is assignd to th O-H strtching vibration of watr and th alcoholic groups. Th prsnc of watr is confirmd by its bnding vibration at 1,630 cm 1. Th prsnc of th alkyl groups is vidnt by th intns paks at 2,925 and 2,854 cm 1 du to th -CH 2 - vibrations. Thir prsnc is also vidnt by thir -CH 2 - bnding vibrations at 1,374 and 1,454 cm 1. A shouldr clos to 1,740 cm 1 is assignd to th C=O vibration of th strs/kto groups. Th prsnc of th str groups is partly vidnt by thir -COO- vibration at 1,233 cm 1. Th intns broad pak at 1,034 cm 1 is assignd to th C-O strtching vibration of th alcoholic groups. Th -CH 2 - rocking mor is sn at 708 cm 1. So from this IR spctrum th CNS is vrifid to hav a larg proportion of alkyl groups, alcoholic groups and smallr amount of str/kto groups. Th prsnc of a larg amount of watr indirctly stablishs many of th alcoholic groups. Thus, th FTIR spctrum of CNS rvals that thr ar svral functional groups in CNS that can bind cadmium(ii) ions. Ths rsults agr with th surfac chmistry of othr agricultural by-products, such as rubbr wood saw dust [45] and hartwood of Arca catchu powdr [46]. Th spcific surfac ara and por structur of th CNS wr dtrmind by using surfac ara and por siz analyzr (Quantachrom, Autosorb-I) on nitrogn adsorption at 77 K. Th spcific surfac ara was calculatd by BET quation [47]. It was found that th BET surfac ara, por volum, avrag por diamtr and bulk dnsity of th CNS wr 395 m /g, cm /g, 5.89 nm and g/cm 3, rspctivly Scanning Elctron Microscopy Th SEM imags of th CNS and cadmium loadd CNS ar shown in Fig. 2(a) and (b). Th surfac of th adsorbnt appars to b irrgular and porous (Fig. 2). On th basis of this fact, it can b concludd that th adsorbnt has an adquat morphology for mtal ion adsorption. Aftr mtal adsorption th porous surfac on th CNS gts filld up by th cadmium(ii) ion. This obsrvation indicats that th mtal ion was adsorbd to th functional groups prsnt insid th wall of th CNS surfac. Fig. 2(b) shows that th mtal ions wr Jun, 2012

4 Rmoval of cadmium(ii) from aquous solution by agricultural wast cashw nut shll 759 Fig. 3. Effct of ph for th adsorption of cadmium(ii) onto CNS (th initial cadmium(ii) ions concntration=20 mg/l, CNS concntration=3 g/l and contact tim=30 min). Fig. 2. (a) SEM imag of cashw nut shll, (b) SEM imag of cadmium loadd CNS. havily loadd in CNS and adsorption of mtal ion occurs insid th wall of th CNS surfac. So, th morphological study of mtal loadd CNS confirms that th adsorption taks plac insid th wall of th CNS surfac. 2. Effct of Solution ph on Sorption Efficincy Th hydrogn ion concntration in th adsorption procss is considrd as on of th most important critical paramtrs that influncs th adsorption bhavior of mtal ions in aquous solutions. It affcts th solubility of mtal ions in th solution, rplacs som of th positiv ions found in th activ sits and affcts th dgr of ionization of th adsorbat during th raction [48,49]. Th ffct of solution ph on th adsorption of cadmium(ii) ions onto CNS was valuatd within th ph rang of Th studis byond ph 8.0 wr not tstd bcaus of th prcipitation of th cadmium ions forming hydroxids [48]. From Fig. 3, it could b sn that th cadmium adsorption incrass with th incras in th solution ph. Th ph dpndncy of adsorption fficincy could b xplaind by th functional groups involvd in mtal uptak and mtal chmistry. Th FT-IR spctroscopic analysis shows that th CNS has a varity of functional groups, such as carboxyl and hydroxyl groups, which ar involvd in almost all th potntial binding mchanisms. Morovr, dpnding on th solution ph, th functional groups participat in mtal ion bindings. At low ph valus, H + ions occupy most of th adsorption sits on th CNS surfac and lss cadmium ions could b sorbd bcaus of lctric rpulsion with H + ions on CNS surfac. Whn th valu of ph incrasd, CNS surfacs wr mor ngativly chargd and th adsorption of cadmium(ii) ions (positiv charg) incrasd and rachd quilibrium at ph 5.0. Th dcras in adsorption fficincy at highr ph (>5.0) is du to th formation of solubl hydroxylatd complxs of th cadmium ions and thir comptition with th activ sits; as consqunc, th rtntion would dcras again. Th ffct of ph on cadmium(ii) rmoval can also b xplaind in trms of point of zro charg (PZC) of th CNS. Th adsorbnt surfac is positiv whn solution ph is blow PZC of th CNS. Morovr, at solution ph<pzc, th prdominant mtal spcis will b positivly chargd [M n+ and M(OH) (n 1)+ ]; thus, th uptak of mtal ions in th ph rang blow PZC is a H + - M n+ or M(OH) (n 1)+ xchang procss. Th incras in solution ph abov PZC will show a slight incras in adsorption as long as th mtal spcis ar still positivly chargd or nutral vn though th CNS surfac is ngativly chargd. Whn both CNS surfac charg and mtals spcis charg bcom ngativ, thn th adsorption procss will dcras significantly. 3. Effct of CNS Concntration on Sorption Efficincy Th numbr of availabl sits and xchanging ions for th adsorption dpnds upon th amount of adsorbnt in th adsorption procss. Th CNS concntration is an important paramtr bcaus this dtrmins th capacity of th CNS for a givn initial cadmium(ii) ion concntration. Th ffct of CNS concntration on th cadmium(ii) rmoval fficincy is prsntd in Fig. 4. Th cadmium rmoval was found to incras rapidly with incrasing concntration of CNS up to an adsorbnt dos of 3 g/l. Howvr, byond this CNS concntration th incras in rmoval fficincy was marginal and this may b du to rduction in concntration gradint. Th maximum adsorption fficincy of cadmium(ii) ion onto CNS was found to b 75.35% at CNS concntration of 3 g/l. Th incras in th rmoval fficincy can b attributd to th incrasd numbr of sits and xchangabl sits wr availabl for adsorption [48]. 4. Effct of Contact Tim on Sorption Efficincy Th ffct of contact tim on fficincy of cadmium(ii) adsorp- Koran J. Chm. Eng.(Vol. 29, No. 6)

5 760 P. S. Kumar t al. Fig. 4. Effct of CNS concntration on cadmium(ii) rmoval (th initial cadmium(ii) ions concntration=20 mg/l, ph=5.0, and contact tim=30 min). Fig. 6. Effct of initial cadmium(ii) ions concntration for th adsorption of cadmium(ii) onto CNS (th initial cadmium(ii) ions concntration=10 to 50 mg/l, ph=5.0, CNS concntration=3 g/l and contact tim=30 min). Fig. 5. Effct of contact tim for th adsorption of cadmium(ii) onto CNS (th initial cadmium(ii) ions concntration=10 to 50 mg/l, ph=5.0 and CNS concntration=3 g/l). Fig. 7. Effct of tmpratur on cadmium(ii) rmoval by CNS (th initial cadmium(ii) ions concntration=10 to 50 mg/l, ph= 5.0, CNS concntration=3 g/l and contact tim=30 min). tion on CNS was invstigatd to study th rat of cadmium(ii) rmoval and th rsults ar shown in Fig. 5. Th data obtaind from th adsorption of cadmium(ii) ions onto th CNS showd that th adsorption incrass with incras in contact tim. Th adsorption of cadmium(ii) ions onto CNS was rapid for th first 25 min and quilibrium was narly rachd aftr 30 min for fiv diffrnt initial cadmium(ii) ion concntrations. A furthr incras in th contact tim has a ngligibl ffct on th fficincy of cadmium(ii) adsorption. Th adsorption fficincy of cadmium(ii) rmoval is highr in th bginning du to a largr surfac ara of th CNS bing availabl for cadmium(ii) adsorption. Latr, as th adsorbd cadmium(ii) forms a monolayr, th capacity of th CNS gts xhaustd and thn th uptak rat is controlld by th rat at which th cadmium(ii) ions ar transportd from th xtrior to th intrior sits of th CNS particls [50]. It is also rlvant that, sinc activ sorption sits in a systm hav a fixd numbr and ach activ sits can adsorb only on ion in a monolayr, th mtal uptak by th sorbnt surfac will b rapid initially and slow down as th comptition for th dcrasing availability of activ sits intnsifis by th mtal ions rmaining in th solution [51]. 5. Effct of Initial Cadmium(II) Ion Concntration on Sorption Efficincy Th ffct of initial cadmium(ii) concntration (10-50 mg/l) was invstigatd at ph of 5.0 in th prsnc of 3 g/l CNS and th rsults ar illustratd in Fig. 6. Th plots rval that an incras in th cadmium(ii) concntration from mg/l rsults in a dcras in prcntag cadmium(ii) rmoval from 80.13% to 66.57%, and an incras in adsorption capacity from to mg/g. Th dcras in th prcntag of cadmium(ii) rmoval can b attributd to th saturation of availabl activ sits on th CNS abov a crtain concntration of cadmium(ii). Th incras in adsorption capacity may b du to th highr adsorption rat and th utilization of all availabl activ sits for adsorption at highr cadmium(ii) concntration. 6. Effct of Tmpratur on Sorption Efficincy and Thrmodynamic Paramtrs Th adsorption of cadmium(ii) ion on CNS was invstigatd as a function of tmpratur and th maximum rmoval of cadmium(ii) ion was obtaind at 30 o C. Th batch adsorption xprimnts wr prformd at diffrnt tmpraturs of 30, 40, 50 and 60 o C for th initial cadmium(ii) ion concntrations in th rang of mg/l at constant CNS concntration of 3 g/l and an optimum ph valu of 5. Th adsorption dcrasd from to 74.32%, to 69.21%, to 65.82% and to 60.24% for th initial cadmium(ii) ion concntration in th rang of mg/l, rspctivly, with th ris in tmpratur from 30 to 60 o C (Fig. 7). This is mainly du to th dcras in surfac activity, suggsting that adsorption btwn Jun, 2012

6 Rmoval of cadmium(ii) from aquous solution by agricultural wast cashw nut shll 761 Fig. 8. Thrmodynamic study. cadmium(ii) and CNS is xothrmic. Thrmodynamic paramtrs such as fr nrgy ( G o ), nthalpy ( H o ) and ntropy ( S o ) chang of adsorption can b valuatd from th following quations: C A K = C (4) G o = RTlnK c (5) G o = H o TDS o (6) S o H o logk c = R 2.303RT (7) Whr K c is th quilibrium constant, C is th quilibrium concntration in solution (mg/l) and C A is th amount of cadmium(ii) adsorbd on th adsorbnt pr litr of solution at quilibrium (mg/ L). G o, H o and S o ar changs in Gibbs fr nrgy (kj/mol), nthalpy (kj/mol) and ntropy (J/mol/K), rspctivly. R is th gas constant (8.314 J/mol/K), T is th tmpratur (K). Th valus of H o and S o ar dtrmind from th slop and th intrcpt from th plot of log K c vrsus 1/T (Fig. 8). Th G o valus wr calculatd using Eq. (5). Adsorption of cadmium(ii) ion on CNS dcrasd, whn th tmpratur was incrasd from 303 to 333 K is shown in Fig. 8. Th procss is thus xothrmic. Th plots wr usd to comput th valus of thrmodynamic paramtrs (Tabl 2). Th valu of nthalpy chang ( H o ) and th ntropy chang ( S o ) rcordd from this work ar prsntd in th Tabl 2. Th Gibbs fr nrgy G o is small and ngativ but incrass with incras in tmpratur, indicating that th adsorption procss lad to a dcras in Gibbs nrgy. Th ngativ G o valu indicats that th procss is fasibl and spontanous in th natur of adsorption. Ngativ H o valu Fig. 9. (a) Th two-paramtr nonlinar adsorption isothrms for cadmium(ii) with CNS at 30 o C, (b) Th thr-paramtr nonlinar adsorption isothrms for cadmium(ii) with CNS at 30 o C. suggsts th xothrmic natur of adsorption and th S o can b usd to dscrib th randomnss at th CNS-solution intrfac during th sorption. Th thrmodynamic paramtrs indicat th adsorption procss can b usd for th rmoval of cadmium(ii) ions by CNS. 7. Adsorption Isothrm Modls Th analysis of th adsorption isothrms data by fitting thm into diffrnt isothrm modls is an important stp to find th suitabl Tabl 2. Thrmodynamic paramtrs for th adsorption of cadmium(ii) ions onto CNS Initial cadmium(ii) conc. (mg/l) H o (kj/mol) S o (J/mol/K) G o (kj/mol) 30 o C 40 o C 50 o C 60 o C Koran J. Chm. Eng.(Vol. 29, No. 6)

7 762 P. S. Kumar t al. modl that can b usd for dsign procss. It was found that adsorption quilibrium tim of cadmium(ii) onto CNS was 30 min. Fig. 9(a) and (b) show th plot of q vrsus C for th adsorption isothrms of cadmium(ii) onto CNS at 30 o C. Ths curvs ar convx upward throughout and ar dsignatd as favorabl typ [52,53]. Th xprimntal data wr applid to th two-paramtr isothrm modls: Langmuir [54], Frundlich [55], Tmkin [56] and Dubinin- Radushkvich [57], and thr-paramtr isothrm modls: Rdlich- Ptrson [58], Kobl-Corrigan [59], Toth [60] and Sips [61]. Th constant paramtrs of th isothrm quations for this adsorption Tabl 3. Th valu of paramtrs for two-paramtr isothrm modls usd in th studis Isothrm modl Paramtr R 2 Equation Langmuir q m = C K L = C SSE=0.263 RMSE=0.296 Frundlich K F =1.741 n=1.511 SSE=0.098 RMSE=0.181 Tmkin A=0.863 B=1.698 SSE=1.676 RMSE= C ln( 0.863C ) ε Tabl 4. Th valu of paramtrs for thr-paramtr isothrm modls usd in th studis Isothrm modl Paramtr R 2 Equation Dubinin- q m =10.60 Radushkvich β= SSE=8.211 RMSE=1.433 Rdlich- Ptrson Kobl-Corrigan modl Toth K R = α R =1.096 β=0.461 SSE=0.084 RMSE=0.205 a= b=0.024 n=0.739 SSE=0.081 RMSE=0.201 f= g=4.528 d=0.406 SSE=0.098 RMSE=0.222 Sips Q max =68.17 K S =0.023 n=1.353 SSE=0.081 RMSE= C C C C 100C [ ( C ) ] C C procss wr calculatd by nonlinar rgrssion analysis using MAT- LAB 7.1. Th constant paramtrs, corrlation cofficints, sum of rror squard (SSE) and root man squar rror (RMSE) ar summarizd in Tabls 3 and Two-paramtr Isothrm Modls Langmuir Isothrm Th thortical Langmuir sorption isothrm is basd on th assumption that th maximum adsorption occurs whn a saturatd monolayr of solut molculs is prsnt on th adsorbnt surfac, th nrgy of adsorption is constant and thr is no migration of adsorbat molculs in th surfac plan. Th non-linar quation of Langmuir isothrm modl is xprssd as follows: q m K L C K L C Whr C is th quilibrium concntration in solution (mg/l), q m and K L ar th Langmuir constants, rprsnting th maximum adsorption capacity for th solid phas loading and th nrgy constant rlatd to th hat of adsorption rspctivly. Th maximum monolayr adsorption capacity was found to b mg/g for CNS. Th ssntial charactristics of th Langmuir isothrm paramtrs can b usd to prdict th affinity btwn th sorbat and sorbnt using sparation factor or dimnsionlss quilibrium paramtr, R L, xprssd as in th following quation [62]: 1 R L = K L C o Whr K L is th Langmuir constant and C o is th initial concntration of cadmium(ii) ion. Th valu of sparation paramtr R L provids important information about th natur of adsorption. Th valu of R L indicats th typ of Langmuir isothrm to b irrvrsibl (R L =0), favorabl (0<R L <1), linar (R L =1) or unfavorabl (R L > 1). Th R L was found to b to for concntration of mg/l of cadmium(ii). Thy ar in th rang of 0-1, which indicats favorabl adsorption [63] Frundlich Isothrm Th Frundlich isothrm modl is th wll known arlist rlationship dscribing th sorption procss. Th modl applis to adsorption on htrognous surfacs with th intraction btwn adsorbd molculs, and th application of th Frundlich quation also suggsts that sorption nrgy xponntially dcrass on compltion of th sorptional cntrs of an adsorbnt. This isothrm is an mpirical quation that can b mployd to dscrib htrognous systms and is xprssd as follows: K F C 1/n (8) (9) (10) Whr K F is th Frundlich constant (l/g) rlatd to th bonding nrgy. 1/n is th htrognity factor and n is a masur of th dviation from linarity of adsorption. This valu indicats th dgr of non-linarity btwn solution concntration and adsorption as follows: if n=1, thn adsorption is linar; if n<1, thn adsorption is a chmical procss; if n>1, thn adsorption is a physical procss. Th n valu in Frundlich quation was found to b for CNS. Sinc n lis btwn 1 and 10, this indicats th favorabl adsorption of cadmium(ii) onto CNS Tmkin Isothrm Th Tmkin isothrm modl contains a factor that xplicitly taks Jun, 2012

8 Rmoval of cadmium(ii) from aquous solution by agricultural wast cashw nut shll 763 into account adsorbing spcis-adsorbat intractions. This modl assums th following facts: (i) th hat of adsorption of all th molculs in th layr dcrass linarly with covrag du to adsorbntadsorbat intractions, and thn (ii) th adsorption is charactrizd by a uniform distribution of binding nrgis, up to a maximum binding nrgy. Th drivation of th Tmkin isothrm assums that th fall in th hat of sorption is linar rathr than logarithmic, as implid in th Frundlich quation. Th Tmkin isothrm has commonly bn applid in th following form: Bln(AC ) (11) Whr, B=(RT)/b, b is th Tmkin constant rlatd to hat of adsorption (J/mol), A is th Tmkin isothrm constant (L/g), R is th univrsal gas constant (8.314 J/(mol K)) and T is th absolut tmpratur (K). Th Tmkin isothrm constants A and B wr found to b and 1.698, rspctivly Dubinin-Radushkvich Isothrm Th Dubinin-Radushkvich isothrm is applid to find th adsorption mchanism basd on th potntial thory assuming a htrognous surfac. Dubinin-Radushkvich isothrm is xprssd as follows: Whr g is th Toth isothrm constant and d is th Toth xponnt (0<d 1). It is obvious that for d=1 this Toth isothrm rducs to th Langmuir quation. Th Toth isothrm constant g and xponnt d valus wr found to b and , rspctivly Comparison of Adsorption Isothrms Th xprimntal data on th ffct of an initial concntration of cadmium(ii) on th CNS surfac of th tst mdium wr fittd to th two-paramtr and thr-paramtr isothrm modls using MAT- LAB 7.1, and th graphical rprsntations of ths modls ar prsntd in Fig. 9(a) and (b). It was obsrvd that th bst fittd adsorpq = q m βε2 (12) Whr q m is th Dubinin-Radushkvich monolayr capacity (mg/ g), β is a constant rlatd to sorption nrgy, and ε is th Polanyi potntial which is rlatd to th quilibrium concntration as follows: 1 ε = RTln C (13) Whr R is th gas constant (8.314 J/mol K) and T is th absolut tmpratur. Th constant β givs th man fr nrgy, E, of sorption pr molcul of th sorbat whn it is transfrrd to th surfac of th solid from infinity in th solution and can b computd using th rlationship; E = (14) 2β Th final form of th Dubinin-Radushkvich isothrm is xprssd as follows: q m xp( β(rt ln(1+1/c )) 2 ) (15) Dubinin-Radushkvich isothrm constants, monolayr capacity (q m ) and sorption nrgy (β) wr found to b 10.6 mg/g and mg 2 /J 2, rspctivly. Th magnitud of E is usd to dtrmin th typ of adsorption mchanism. Whn on mol of ions is transfrrd to th CNS surfac, its valu is lss than 8 kj/mol, which indicats physical adsorption [64]; th valu of E is btwn 8 and 16 kj/mol, indicats th adsorption procss which follows ion-xchang [65]; whil its valu in th rang of kj/mol indicats chmisorption [66]. Th calculatd valu of E for th prsnt study is blow 8 kj/mol, which indicats th adsorption of cadmium(ii) onto CNS surfac is following physical adsorption typ Thr-paramtr Isothrm Modls Rdlich-Ptrson Isothrm Th Rdlich-Ptrson isothrm approximats Hnry s law at lowr sorbat concntrations, and at highr concntrations it bhavs lik th Frundlich adsorption isothrm. Th Rdlich-Ptrson isothrm is xprssd as follows: K R C β 1+ α R C (16) Whr K R is Rdlich-Ptrson isothrm constant (L/g), α R is Rdlich- Ptrson isothrm constant (L/mg) and β is th xponnt which lis btwn 0 and 1. Th Rdlich-Ptrson isothrm constants, K R and α R, for th adsorption of cadmium(ii) onto CNS wr found to b L/g and L/mg, rspctivly. Th xponnt β valu was calculatd as Kobl-Corrigan Isothrm Th Kobl-Corrigan isothrm is anothr thr-paramtr mpirical modl which dpnds on th combination of th Langmuir and Frundlich isothrm quations in on non-linar quation for rprsnting th quilibrium adsorption data. It is commonly xprssd as follows: ac n 1+ bc (17) Whr a and b ar th Kobl-Corrigan constants and n is th Kobl- Corrigan modl xponnt. Th Kobl-Corrigan isothrm constants, a and b, for th adsorption of cadmium(ii) onto CNS wr found to b and , rspctivly. Th xponnt n valu was calculatd as Sips Isothrm Th Sips isothrm is a combination of th Langmuir and Frundlich isothrm modls and it is xpctd to dscrib th htrognous surfacs much bttr. At lowr sorbat concntration it rducs to th Frundlich isothrm, whil at highr sorbat concntrations it approachs to Langmuir isothrm to prdict a monolayr adsorption capacity. Th sips isothrm has bn usd in th following form: 1/n Q max K S C /n 1+ K S C (18) Whr Q max is th Sips maximum adsorption capacity (mg/g) and K S is th Sips constant rlatd with affinity constant (mg/l) 1/n. According to th Sips isothrm, th sips monolayr adsorption capacity valu was found to b mg/g for th adsorption of cadmium(ii) onto CNS. Th xponnt 1/n valu for CNS was closr to unity. It mans that cadmium(ii) adsorption data for CNS is mor of th Langmuir isothrm form rathr than that of th Frundlich isothrm Toth Isothrm Th Toth isothrm is a Langmuir-basd isothrm that considrs a continuous distribution of sit affinitis: fc [ g + ( C ) d 1/d ] (19) Koran J. Chm. Eng.(Vol. 29, No. 6)

9 764 P. S. Kumar t al. Tabl 5. Comparison of maximum monolayr adsorption of cadmium(ii) ions onto various adsorbnts Adsorbnts q m (mg/g) Rfrnc AGCS [3] Cashw nut shll This study Ric husk [21] Corncob 19.3 [24] Ciba pntandr hulls carbon [37] Gracilaria salicornia (rd) [36] Chins hrb Pang Da Hai [35] Hypna valntia biomass [42] Ta-industry wast [26] Oliv cak [27] Liv Mucor rouxii 8.46 [34] Prtratd Mucor rouxii [34] Carbon F [39] Low-grad phosphat 7.54 [30] Calgon F [40] Banana pls 5.71 [43] Hazlnut shll 5.42 [28] Oak bark char 5.40 [39] Prtratd Asprgillus nigr [33] Pur smctit 3.87 [41] RCS [3] Lwatit S [41] Oliv ston carbon [38] Bagass fly ash [29] What bran [32] Oak wood char 0.37 [39] Pin bark char 0.34 [39] Hmatit [31] tion isothrms considring th corrlation cofficint wr obtaind for th two-paramtr isothrms to b in th ordr of prdiction prcision: Frundlich>Langmuir>Tmkin>Dubinin-Radushkvich and for thr-paramtr isothrms: Rdlich-Ptrson, Kobl-Corrigan, Sips>Toth isothrms. Thr-paramtr isothrms hav highr corrlation cofficints than that of th two-paramtr isothrms. Comparing th corrlation cofficints, sum of rror squard (SSE) and root man squar rror (RMSE) wr obtaind from th adsorption isothrms, which shows th bst fits to th xprimntal valus using th diffrnt isothrm quations, and wr also gnrally vry good for all th thr-paramtr isothrm quations. Th applicability of all isothrm quations to th cadmium(ii)-cns systm indicats that both th monolayr adsorption and htrognous surfac conditions xist undr th studid xprimntal conditions. Th adsorption of cadmium(ii) onto th CNS surfac is thus complx, involving mor than on mchanism [63]. Th comparison of maximum monolayr adsorption capacity of cadmium(ii) onto various adsorbnts is prsntd in Tabl 5. It shows that th CNS studid in this work has largr adsorption capacity. This is du to its high surfac ara (395 m 2 /g). 8. Adsorption Kintics Various kintic modls, namly psudo-first-ordr, psudo-scondordr, Elovich modls and intraparticl diffusion hav bn usd for thir validity with th xprimntal adsorption data for cadmium(ii) onto CNS. Th study of adsorption kintics dscribs th solut uptak rat, and vidntly this rat controls th rsidnc tim of adsorbat uptak at th solid-solution intrfac, including th diffusion procss. Th mchanism of adsorption dpnds on th physical and chmical charactristics of th adsorbnt as wll as on th mass transfr procss. Th rsults obtaind from th xprimnts wr usd to study th kintics of mtal ion adsorption. Th rat kintics of cadmium(ii) ion adsorption on CNS was analyzd using psudofirst-ordr [67], psudo-scond-ordr [68], Elovich modls [69] and intraparticl diffusion [70] Th Psudo-first-ordr Kintic Modl Th adsorption of cadmium(ii) ion from an aquous solution to CNS can b considrd as a rvrsibl procss with quilibrium bing stablishd btwn th solution and CNS. Adsorption phnomna can b dscribd as th diffusion control procss assuming a nondissociation molcular adsorption of cadmium(ii) on CNS as follows: A+S AS (20) If initially no cadmium(ii) ion is prsnt on th CNS (i.., C A S o =0 at t=0), and assuming th first-ordr rat kintics thn, th fractional uptak of th cadmium(ii) by th CNS can b xprssd as: Whr q t is th adsorption capacity at tim t (mg/g) and k ad (min 1 ) is th rat constant of th psudo-first ordr adsorption, which was applid to th prsnt study of cadmium(ii) adsorption. Th rat constant, k ad and corrlation cofficints of th cadmium(ii) undr diffrnt concntrations wr calculatd from th linar plots of log (q q t ) vrsus t (Fig. 10) and listd in Tabl 6. Th corrlation coffiq --- =1 xp k A C S t q Equation can b transformd as log q q t Whr log q t =X At and q =X A k A k S ( ) = q k A k S k ad = k A C S k ad t (21) (22) (23) Fig. 10. Psudo-first-ordr kintic fit for adsorption of cadmium(ii) onto CNS at 30 o C. Jun, 2012

10 Rmoval of cadmium(ii) from aquous solution by agricultural wast cashw nut shll 765 Tabl 6. Kintic modls and othr statistical paramtrs at 30 o C and at ph 5.0 Kintic modl Paramtrs Concntration of cadmium(ii) solution 10 mg/l 20 mg/l 30 mg/l 40 mg/l 50 mg/l Psudo-first-ordr k ad (min 1 ) quation q, cal (mg/g) R Psudo-scond-ordr k (g mg 1 min 1 ) quation q, cal (mg/g) h (mg g 1 min 1 ) q, xp (mg/g) R Elovich quation α (mg/g.min) β (g/mg) R Intraparticl diffusion k p (mg/g min 1/2 ) C R cints for th psudo-first-ordr kintic modl ar low. Morovr, a larg diffrnc in quilibrium adsorption capacity (q ) obtaind from th xprimntal and calculatd valus was obsrvd, indicating a poor psudo first-ordr fit to th xprimntal data Th Psudo-scond-ordr Kintic Modl Th kintic data wr furthr analyzd using Ho s psudo-scond-ordr kintic modl. This modl is basd on th assumption that th sorption follows scond ordr chmisorption: dq t = k q q t dt ( ) 2 Intgrating Eq. (24) and applying th boundary conditions, givs: q q t = kt q Eq. (25) can b rarrangd to obtain a linar form: t = t h q t q (24) (25) (26) Whr h=kq 2 (mg g 1 min 1 ) can b rgardd as th initial adsorption rat as and k is th rat constant of psudo-scond-ordr adsorption (g mg 1 min 1 ). Th plot of t/q t vrsus t (Fig. 11) should giv a straight lin if psudo-scond-ordr kintics is applicabl and q, k and h can b dtrmind from th slop and intrcpt of th plot. At all initial cadmium(ii) concntrations, straight lins with xtrmly high corrlation cofficints (>0.99) wr obtaind. In addition, th calculatd q valus also agr with th xprimntal data in th cas of psudo-scond-ordr kintics. This suggsts that th adsorption data ar wll rprsntd by psudo-scond-ordr kintics, and it supports th assumption [68] that th rat-limiting stp of cadmium(ii) ion onto CNS may b chmisorption. From Tabl 6, th valus of th rat constant k dcras with incras in initial cadmium(ii) concntration for th CNS. Th rason for this bhavior can b attributd to th lowr comptition for th sorption surfac sits at lowr concntration. At highr concntrations, th comptition for th surfac activ sits will b high, and consquntly lowr sorption rats ar obtaind Th Elovich Kintic Modl Th Elovich kintic modl is also considrd whr α is th initial adsorption rat in mg/(g min), and β (g/mg) is th dsorption constant rlatd to th xtnt of th surfac covrag and activation nrgy for chmisorptions. Both kintic constants (α and β) will b stimatd from th slop and intrcpt from th plot of q t vrsus ln t. Fig. 11. Psudo-scond-ordr kintic fit for adsorption of cadmium (II) ions onto CNS at 30 o C. Fig. 12. Elovich kintic modl for adsorption of cadmium(ii) ions onto CNS at 30 o C. Koran J. Chm. Eng.(Vol. 29, No. 6)

11 766 P. S. Kumar t al. 1 1 q t = -- ln( αβ) + -- lnt β β (27) Cadmium(II) ion adsorption kintics onto CNS was also tstd with th Elovich kintic modl by plotting q t vrsus ln t as shown in Fig. 12. Rcordd R 2 valus ar low, which indicats that th xprimntal data do not fit th Elovich kintic modl and cadmium(ii) ions rmoval using CNS undr study cannot b dscribd using this modl Th Intraparticl Diffusion Modl Taking into account that th kintic rsults wr fittd vry wll to a chmisorption modl, th intraparticl diffusion modl was plottd in ordr to vrify th influnc of mass transfr rsistanc on th binding of cadmium(ii) to th CNS. Th kintic rsults wr analyzd by th Wbr and Morris intraparticl diffusion modl to lucidat th diffusion mchanism; th modl is xprssd as: Fig. 14. A singl-stag batch adsorbr. q t =k p t 1/2 +C (28) Whr C is th intrcpt and k p is th intraparticl diffusion rat constant, (mg/gmin ), which can b valuatd from th slop of th 1/2 linar plot of q t vrsus t (1/2) as shown in Fig. 13. Th intrcpt of th plot rflcts th boundary layr ffct. Th largr th intrcpt is, th gratr will b th contribution of th surfac sorption in th ratcontrolling stp. Th calculatd intraparticl diffusion cofficint k p valus ar listd in Tabl 6. If th rgrssion in th plot of q t vrsus t (1/2) is linar and passs through th origin, thn intraparticl diffusion is th sol rat-limiting stp. Howvr, th linar plots at ach concntration did not pass through th origin. This dviation from th origin may prhaps b du to th diffrnc in th rat of mass transfr in th initial and final stags of adsorption. This indicats som dgr of boundary layr control and furthr shows that th intraparticl diffusion was not only th rat-limiting stp, but also b th rat controlling of sorption or all may b oprating simultanously. Thus basd on th high co-rlation cofficint valus (Tabl 6), it can b infrrd that adsorption of cadmium(ii) onto CNS followd psudo-scond-ordr modl than that of intraparticl diffusion modl, psudo-first-ordr modl and Elovich kintic modl. 9. Dsigning Batch Adsorption from Equilibrium Data Adsorption isothrms can b usd to prdict th dsign of singlstag batch adsorption systm [71]. Th schmatic diagram of singl-stag batch adsorption systm is shown in Fig. 14. Th dsign objctiv is to rduc th cadmium(ii) ion concntration from C o (mg/l) to C 1 (mg/l) of solution volum V (L). Th amount of adsorbnt is M (g) and th cadmium(ii) ion loading changs from q o (mg/g) to q 1 (mg/g). At tim t=0, q o =0 as tim procds th mass balanc quats th cadmium(ii) ion rmovd from th aquous solution to that pickd by th CNS. Th mass balanc for th singlstag batch adsorption systm can b givn as: V(C o C 1 )=M(q 1 q o )=Mq 1 (29) At quilibrium conditions C 1 C and q 1 q Sinc adsorption isothrm studis fit wll in Frundlich adsorption isothrm, it is usd for q 1 in quation batch adsorption dsign. Rarranging Eq. (29) w gt M ( C ---- o C 1 ) ( C = o C ) ( C = o C ) = /n V q 1 q K F C (30) Fig. 15. shows th plot btwn th calculatd amount of CNS rquird to rmov cadmium(ii) ion solution of initial concntration 20 mg/l for 50%, 60%, 70% and 80% cadmium(ii) rmoval at diffrnt solution volums (1 to 10 L) for a singl-stag batch adsorption systm, for which th dsign procdur is outlind. For xampl, th amount of CNS rquird for th 80% rmoval of cadmium(ii) ion from an aquous solution of 20 mg/l concntration was 3.671, Fig. 13. Intraparticl diffusion modl for adsorption of cadmium (II) ions onto CNS at 30 o C. Fig. 15. Dsign plot gnratd using Frundlich isothrm for adsorbnt mass against volum of fflunt tratd for various prcntag of cadmium(ii) rmoval at 20 mg/l initial cadmium(ii) ion concntration. Jun, 2012

12 Rmoval of cadmium(ii) from aquous solution by agricultural wast cashw nut shll 767 Tabl 7. Dsorption of cadmium(ii) ions from spnt CNS using HCl Initial cadmium(ii) concntration (mg/l) Rmoval fficincy Prcntag rcovry of cadmium(ii) ions 0.05 M 0.1 M 0.15 M 0.2 M , , , , , , , and g for cadmium(ii) ion solution volums of 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 L, rspctivly. 10. Dsorption Studis Th dsorption study is to lucidat th natur of adsorption and rcycling of th spnt adsorbnt and th cadmium(ii) ions. Th us of thrmal activation mthod to rgnrat th adsorbnt could rquir high nrgy and 5-10% adsorbnt loss in ach cycl. Hnc, dsorption studis wr attmptd to us th chmical rgnration for cadmium(ii) ions dsorption from th spnt CNS. Dsorption rsults ar rportd in Tabl 7. Th rsults show that th concntration of HCl incrass with th incras in dsorption rat, but it is rachd a constant valu with 0.15 M HCl. CONCLUSIONS Th prsnt study valuats th cadmium(ii) rmoval by th us of agricultural wast such as cashw nut shll. Th oprating paramtrs such as solution ph, CNS concntration, contact tim, initial cadmium(ii) ion concntration and tmpratur, wr ffctiv in th adsorption fficincy of cadmium(ii) ions. Th maximum cadmium(ii) adsorption was obtaind at ph 5.0. Th monolayr adsorption capacity was found to b mg/g. Th adsorption of th cadmium(ii) ion on CNS rachd quilibrium in 30 minuts. Th two-paramtr (Langmuir, Frundlich, Tmkin and Dubinin- Radushkvich) and thr-paramtr isothrm modls (Rdlich-Ptrson, Kobl-Corrigan, Toth and Sips) wr usd for th mathmatical dscription of th adsorption of cadmium(ii) ions onto CNS and th constants wr valuatd from ths isothrms using MATLAB 7.1. Considring th two-paramtr and thr-paramtr isothrm modls, Langmuir, Frundlich, Rdlich-Ptrson, Kobl-Corrigan, Toth and Sips isothrm modls bst dscrib th adsorption of cadmium(ii) ion onto CNS. 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