REMOVAL OF METHYLENE BLUE FROM AQUEOUS SOLUTION BY ACTIVATED CARBON PREPARED FROM THE PEEL OF CUCUMIS SATIVA FRUIT BY ADSORPTION

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1 REMOVAL OF METHYLENE BLUE FROM AQUEOUS SOLUTION BY ACTIVATED CARBON PREPARED FROM THE PEEL OF CUCUMIS SATIVA FRUIT BY ADSORPTION Santhi Thirumalisamy a* and Manonmani Subbian b Th us of low-cost, locally availabl, highly fficint, and co-frindly adsorbnts has bn invstigatd as an idal altrnativ to th currnt xpnsiv mthods of rmoving dys from wastwatr. This study invstigats th potntial us of activatd carbon prpard from th pl of Cucumis sativa fruit for th rmoval of mthyln blu (MB) dy from simulatd wastwatr. Th ffcts of diffrnt systm variabls, adsorbnt dosag, initial dy concntration, ph, and contact tim wr invstigatd, and optimal xprimntal conditions wr ascrtaind. Th rsults showd that as th amount of th adsorbnt incrasd, th prcntag of dy rmoval incrasd accordingly. Th optimum ph for dy adsorption was 6.0. Maximum dy was squstrd within 50 min of th start of ach xprimnt. Th adsorption of mthyln blu followd th psudo-scond-ordr rat quation and fit th Langmuir, Frundlich, Dubinin-Radushkvich (D-R), and Tmpkin quations wll. Maximum rmoval of MB was obtaind at ph 6 as 99.79% for adsorbnt doss of 0.6 g/ 50 ml and 25 mg/l initial dy concntrations at room tmpratur. Th maximum adsorption capacity obtaind from th Langmuir quation was mg g -. Th rat of adsorption was found to conform to psudo-scond-ordr kintics with a good corrlation (R 2 > ) with intraparticl diffusion as on of th rat-dtrmining stps. Activatd carbon dvlopd from th pl of Cucumis sativa fruit can b an attractiv option for dy rmoval from wastwatr. Kywords: Cucumis sativa; Adsorption; Wastwatr; Mthyln Blu; Kintics; Activatd carbon Contact information a* Dpartmnt of Chmistry, Karpagam Univrsity, Coimbator-6402, India, Phon no , Fax ssnilasri@yahoo.co.in b Dpartmnt of Chmistry, PSG Collg of Arts and Scinc, Coimbator-6404, India INTRODUCTION Th fflunts from th txtil, lathr, food procssing, dying, cosmtics, papr, and dy manufacturing industris ar important sourcs of dy pollution (Bhatnagar t al. 2004). Many dys and thir brakdown products may b toxic for living organisms (Kannan t al. 200). Thrfor, dcolorization of dys is an important aspct of wastwatr tratmnt bfor discharg. It is difficult to rmov th dys from th fflunt, bcaus dys ar not asily dgradabl and ar gnrally not rmovd from wastwatr by convntional wastwatr systms (Kargi t al. 2004). Gnrally, biological arobic wastwatr systms ar not succssful for dcolorization of th majority of dys (Kannan t al. 200). Thirumalisamy and Subbian (200). "Adsorption," BioRsourcs 5(),

2 Color rmoval has bn xtnsivly studid using such physco-chmical mthods as coagulation, ultra-filtration, lctro-chmical adsorption, and photo-oxidation (Kannan t al. 200; Bhattacharyya t al. 2005). Among ths mthods, adsorption is widly usd for dy rmoval from wastwatrs (Bhattacharyya t al. 2005; Walkr t al. 998). Granulatd activatd carbon (GAC) or powdrd activatd carbon (PAC) is commonly usd for dy rmoval (Walkr t al. 998; Chrn and Wu 200; Namasivayam t al. 996). Howvr, ths products ar xpnsiv, and thir rgnration or disposal has svral problms. Thus, th us of svral low-cost adsorbnts has bn studid by many rsarchrs. Thy hav studid th fasibility of using low-cost matrials, such as wast orang pl (Namasivayam t al. 998), banana pith (McKay t al. 986), cotton wast, ric husk (Ramakrishna and Viraraghavan 997), bntonit clay (Dogan t al. 2004), nm laf powdr (Walkr t al. 998), powdrd activatd sludg (Bhattacharyya t al. 2005), prlit (Waranusantigul t al. 2003), bamboo dust, coconut shll, groundnut shll, ric husk, and straw (Kannan t al. 200), duck wd (Otro t al. 2003), and swag sludg (Mc-Kay t al. 985) as adsorbnts for rmoval of various dys from wastwatrs. Cucumis sativa is a local fruit availabl in abundanc throughout th yar. Th pl of Cucumis sativa fruit, which is rmovd bfor consumption, is a wast product that is usually discardd. Local vndors at a railway lvl crossing (nar Eachnari, Coimbator, Tamil Nadu) sll about 600 to 700 kg of th fruit vry day at this on local, and about 3% of this accounts as wast. Sinc th pl of Cucumis sativa fruit is availabl fr of cost, only its carbonization would b rquird for wast watr tratmnt. Thrfor th main objctiv of this study was to valuat th possibility of using drid pl of Cucumis sativa fruit to dvlop a nw low-cost activatd carbon and study its application to rmov mthyln blu from simulatd wastwatr. Th pl of Cucumis sativa fruit was prviously invstigatd to adsorb cationic dys (Santhi t al. 2009). Systmatic valuation was carrid out of paramtrs such as ph, adsorbnt dos, adsorbnt particl siz, initial dy concntration, and tim. MATERIALS AND METHODS Prparation of Activatd Carbon from Pl of Cucumis Sativa Fruit (CCS) Th pl of Cucumis Sativa fruit was obtaind from local vndors at Eachnari railway Gat, Coimbator District (Tamil Nadu). Th drid pl of Cucumis sativa fruit biomass was addd in small portions (.0 kg) to 000 ml of 98% H 2 SO 4 for 2 hours, washd thoroughly with distilld watr until it attaind nutral ph, and soakd in two prcnt NaHCO 3 ovrnight in ordr to rmov any xcss acid prsnt. Thn th matrial was washd with distilld watr and drid at 0±2 0 C. Th dry biomass was crushd into granuls, sivd to diffrnt particl sizs, and thn prsrvd in dsiccators until us. Prparation of synthtic solutions A stock solution of 500 mgl - was prpard by dissolving th appropriat amount of MB (obtaind from s.d. Fin Chmicals, Mumbai, India) in 00 ml and mad to 000 ml with distilld watr. Diffrnt concntrations ranging btwn 25 and 200 mg L - of MB wr prpard from th stock solution. All th chmicals usd throughout this study Thirumalisamy and Subbian (200). "Adsorption," BioRsourcs 5(),

3 wr of analytical-grad ragnts. Doubl-distilld watr was usd for prparing all of th solutions and ragnts. Th initial ph was adjustd with 0. M HCl or 0. M NaOH. All th adsorption xprimnts wr carrid out at room tmpratur (27 ±2 o C). Batch Adsorption Studis Effct of ph on MB adsorption Th ffct of ph on th quilibrium uptak of dys was invstigatd by mploying an initial concntration of MB of 00mg/L and 0.2 g/50 ml of CCS. Th initial ph valus wr adjustd with 0. M HCl or NaOH to form a sris of solutions having ph valus from 2 to 0. Th suspnsions wr shakn at room tmpratur (27 ±2 o C) using an agitation spd of 50 rpm. Th minimum contact tim rquird to rach th quilibrium (0 min) and th amount of MB adsorbd wr dtrmind. Effct of CCS dos on MB adsorption Th ffct of adsorbnt dos on th quilibrium uptak of MB (00 mg L - ) was invstigatd with CCS concntrations of 0.2, 0.4, and 0.6 g/ 50mL. Th xprimnts wr prformd by shaking known MB concntrations with th diffrnt CCS concntrations shown abov to th quilibrium uptak (0 min), and th amount of MB adsorbd was dtrmind. Kintic studis Adsorption studis wr conductd in 250-mL shaking flasks at a solution ph of 6.0. Th CCS (0.2g/50mL) was thoroughly mixd individually with 50mL of MB solution (50, 00, 50, and 200 mg/l), and th suspnsions wr shakn at room tmpratur. Sampls of.0 ml wr collctd from th duplicat flasks at rquird tim intrvals viz.0, 20, 30, 40, 50, 60, 70, 80, 90, 00, and 0 min and wr cntrifugd for 5 min. Th clar solutions wr analyzd for rsidual MB concntration in th solutions. Adsorption isothrm Batch adsorption xprimnts wr carrid out in a rotary shakr at 50 rpm using 250mL-shaking flasks at room tmpratur for 0 min. Th CCS (0.2 and 0.6 g) was thoroughly mixd with 50 ml of MB solutions. Th isothrm studis wr prformd by varying th initial MB concntrations from 25 to 200 mg/l at ph 6.0. Th ph was adjustd using 0. M HCl or 0. M NaOH bfor addition of CCS and maintaind throughout th xprimnt. Aftr shaking th flasks for 0 min, th raction mixtur was analyzd for th rsidual MB concntration. Th concntration of MB in solution was masurd by a dirct UV-vis spctrophotomtric mthod using a Systronic Spctrophotomtr-04 at a wavlngth of 480 nm. All xprimnts wr duplicatd, and only th man valus ar rportd. Th maximum dviation obsrvd was lss than ±4%. Adsorption of MB from simulatd wastwatr was studid using 0.2g / 50 ml of CCS and MB concntrations of 00 mg /L at an initial ph 6.0. Th amount of dy adsorbd at quilibrium onto carbon, q (mg/g), was calculatd by th following mass balanc rlationship: Thirumalisamy and Subbian (200). "Adsorption," BioRsourcs 5(),

4 q = (C o C ) V/W () whr C o and C ( mg/l) ar th initial and quilibrium liquid-phas concntration of MB, rspctivly, V th volum of th solution(l), and W is th wight of th CCS usd(g). RESULTS AND DISCUSSION Effct of Systm ph on MB Uptak Th ph of th systm xrtd a profound influnc on th adsorptiv uptak of adsorbat molculs, prsumably du to its influnc on th surfac proprtis of th adsorbnt and ionization/dissociation of th adsorbat molcul. Figur shows th variations in th rmoval of dy from wastwatr at diffrnt systm ph. From th figur, it is vidnt that th maximum rmoval of MB color was obsrvd at ph 6. A similar trnd of ph ffct was obsrvd for th adsorption of mthyln blu on activatd carbon prpard from fly ash (Tabrz t al. 2009), natural Jordanian tripoli (Alzaydin 2009), and biosolids (Sarioglu and Atay 2006). This rsult may b attributd to th hydrophobic natur of th dvlopd carbon, which ld to absorption of hydrogn ions (H + ) onto th surfac of th carbon whn immrsd in watr, making it positivly chargd. A low ph valu (.0 to 3.0) lads to an incras in th H + ion concntration in th systm, and th surfac of th activatd carbon acquirs a positiv charg by absorbing H + ions. On th othr hand, an incras of th ph valu (6) ld to an incras of th numbr of ngativly chargd sits. As th CCS surfac is ngativly chargd at high ph, a significantly strong lctrostatic attraction appars btwn th ngativly chargd carbon surfac and cationic dy molculs, lading to maximum adsorption of MB (El Nmr t al. 2009) from wast watr. Th lowst adsorption occurrd at ph 2.0, and th highst adsorption occurrd at ph approximatly 6.0. Th adsorbnt's surfac would b positivly chargd up to ph < 4, and htrognous in th ph rang 4-6. Thraftr, it should b ngativly chargd. Morovr, th incrasing adsorption of MB with incrasing of ph valu is also attributd to th attraction btwn cationic dy and xcss OH ions in th solution. Th ph at th zro point of charg (ph zpc) of th CCS is rportd to b 4.0. Effct of Contact Tim and Initial MB Concntration Th rlation btwn rmoval of MB and raction tim was studid. Th rsults of prcntag rmoval of MB at ph 6.0 with incras of contact tim using CCS ar prsntd in Fig. 2. It was found that mor than 50% rmoval of MB concntration occurrd in th first 0 min, and thraftr th rat of adsorption of th MB onto CCS was slow. Th rapid adsorption at th initial contact tim is attributd to th highly ngativly chargd surfac of th CCS for adsorption of cationic MB in th solution at ph 6. Th latr slow rat of MB adsorption is probably du to th lctrostatic hindranc or rpulsion btwn th adsorbd positivly chargd adsorbat spcis onto th surfac of CCS and th availabl cationic adsorbat spcis in th solution, as wll as th slow por diffusion of th solut ions into th bulk of th adsorbnt. Th quilibrium was attaind at 0 min whn th maximum MB adsorption onto CCS was rachd. Thirumalisamy and Subbian (200). "Adsorption," BioRsourcs 5(),

5 Dy adsorbd(%) Initial ph Fig.. Effct of systm ph on adsorption of MB (00 mg L ) onto CCS (0.2 g /50mL) at room tmpratur (27 ± 2 C), agitation spd 50 rpm for th minimum contact tim rquird to rach th quilibrium (0 min). Also, th ffct of initial concntration of MB in th solution on th capacity of adsorption onto CCS was studid and rsults ar shown in Fig. 2. Th xprimnts wr carrid out at a fixd adsorbnt dos (0.2 g/50 ml) in th tst solution at room tmpratur (27 ± 2 C), ph 6 and at diffrnt initial concntrations of MB (25, 50, 75, 00, 25, 50, 75, and 200 mgl ) for diffrnt tim intrvals (0, 20, 30, 40, 50, 60, 70, 80, 90, 00, and 0 min). Figur 2 shows that th prcntag of adsorption fficincy of CCS dcrasd with th incras of initial MB concntration in th solution. Though th prcnt adsorption dcrasd with incras in initial dy concntration, th actual amount of MB adsorbd pr unit mass of adsorbnt incrasd with incras in MB concntration in th tst solution. It is vidnt from Fig. 2 that th amount adsorbd on th solid phas CCS at a lowr initial concntration of MB was smallr than th corrsponding amount whn highr initial concntrations wr usd. Howvr, th prcntag rmoval of MB was gratr at lowr initial concntrations and smallr at highr initial concntrations. Th adsorption capacity for CCS was incrasd from 6.78 to 42.4 mg g as th MB concntration incrasd from 25 to 200 mg L. In th procss of MB adsorption initially dy molculs hav to first ncountr th boundary layr ffct, thn thy hav to diffus from th boundary layr film onto adsorbnt surfac, and thn finally thy hav to diffus into th porous structur of th adsorbnt. This phnomnon will tak rlativly longr contact tim. Thirumalisamy and Subbian (200). "Adsorption," BioRsourcs 5(),

6 MB rmoval% Tim(min) Fig. 2. Effct of contact tim on th rmoval of diffrnt initial concntrations of MB using CCS (0.2 g/50ml) at ph 6.0 Effct of Adsorbnt Mass on MB Adsorption Th adsorption of MB on CCS was studid by changing th quantity of adsorbnt (0.2, 0.3, 0.4, 0.5, and 0.6 g/50 ml) in th tst solution whil kping th initial MB concntration (00 mg L ), tmpratur (27 ± 2 o C), and ph (6.0) constant at contact tims for 0 min (Fig. 3). Th adsorption incrasd from 92.53% to 98.07% as th CCS dos incrasd from 0.2 g to 0.6 g/50 ml at th quilibrium tim (0 min). Maximum MB rmoval was achivd within 0 to 50 min, aftr which MB concntration in th raction solution was almost constant. Th incras in th adsorption with adsorbnt dos can b attributd to incrasd MB surfac ara and availability of mor adsorption sits, whil th unit adsorbd of MB dcrasd with incras in CCS dos. Isothrm Data Analysis Th rlationship btwn th amount of a substanc adsorbd at constant tmpratur and its concntration in th quilibrium solution is calld th adsorption isothrm. Th adsorption isothrm is important from both a thortical and a practical point of viw. In ordr to optimiz th dsign of an adsorption systm to rmov th dy, it is important to stablish th most appropriat corrlations of th quilibrium data of ach systm. Equilibrium isothrm quations ar usd to dscrib th xprimntal adsorption data. Th paramtrs obtaind from th diffrnt modls provid important information on th adsorption mchanisms and th surfac proprtis and affinitis of th adsorbnt. Th most widly accptd surfac adsorption modls for singl-solut systms ar th Langmuir and Frundlich modls. Th corrlation with th amount of adsorption and th liquid-phas concntration was tstd with th Langmuir, Frundlich, Tmpkin, and Dubinin Radushkvich (D R) isothrm quations. Linar rgrssion is frquntly usd to dtrmin th bst-fitting isothrm, and th applicability of isothrm quations is compard by judging th corrlation cofficints. Thirumalisamy and Subbian (200). "Adsorption," BioRsourcs 5(),

7 MB rmoval % CCS Concntration(g/50ml) Fig. 3. Effct of adsorbnt concntration on MB rmovals (C 0 : 00 mg L -, ph 6.0, agitation spd: 50 rpm, tmpratur: 27 ± 2 C) Langmuir isothrm Th thortical Langmuir isothrm (Langmuir 96) is valid for adsorption of a solut from a liquid solution as monolayr adsorption on a surfac containing a finit numbr of idntical sits. Langmuir isothrm modl assums uniform nrgis of adsorption onto th surfac without transmigration of adsorbat in th plan of th surfac (Doğan t al. 2000). Thrfor, th Langmuir isothrm modl was chosn for stimation of th maximum adsorption capacity corrsponding to complt monolayr covrag on th adsorbnt surfac. Th Langmuir non-linar quation is commonly xprssd as follows: q Q K C + K C m a = (2) a In Eq. (2), C and q ar as dfind bfor in Eq. (); Q m is a constant and rflcts a complt monolayr (mg g ); K a is adsorption quilibrium constant (L mg ) that is rlatd to th apparnt nrgy of sorption. Th Langmuir isothrm Eq. (2) can b linarizd into th following form (Kinniburgh 986; Longhinotti t al. 998): C q = K Q a m + Q m C (3) A plot of C /q vrsus C should indicat a straight lin of slop /Q m and an intrcpt of /(K a Q m ). Th rsults obtaind from th Langmuir modl for th rmoval of MB onto CCS ar shown in Tabl. Th corrlation cofficints rportd in Tabl showd strong positiv vidnc on th adsorption of MB onto CCS, following th Langmuir isothrm. Thirumalisamy and Subbian (200). "Adsorption," BioRsourcs 5(),

8 Th applicability of th linar form of th Langmuir modl to CCS was provn by th high corrlation cofficints, R 2 > This suggsts that th Langmuir isothrm provids a good modl of th sorption systm. Th maximum monolayr capacity Q m obtaind from th Langmuir is mg g. Th Frundlich isothrm Th Frundlich isothrm modl (Frundlich 906) is th arlist known quations dscribing th adsorption procss. It is an mpirical quation and can b usd for nonidal sorption that involvs htrognous adsorption. Th Frundlich isothrm can b drivd assuming a logarithmic dcras in th nthalpy of adsorption with th incras in th fraction of occupid sits and is commonly givn by th following non-linar quation, q = K C (4) F / n whr K F is a constant for th systm, rlatd to th bonding nrgy. K F can b dfind as th adsorption or distribution cofficint and rprsnts th quantity of dy adsorbd onto adsorbnt for unit quilibrium concntration. /n is indicating th adsorption intnsity of dy onto th adsorbnt or surfac htrognity, bcoming mor htrognous as its valu gts closr to zro. A valu for /n blow indicats a normal Langmuir isothrm, whil /n abov is indicativ of cooprativ adsorption. Equation (4) can b linarizd in th logarithmic form (Eq.(5), and th Frundlich constants can b dtrmind: log q = log KF + logc (5) n Th applicability of th Frundlich adsorption isothrm was also analyzd, using th sam st of xprimntal data, by plotting log(q ) vrsus log(c ). Th data obtaind from linar Frundlich isothrm plot for th adsorption of th MB onto CCS is prsntd in Tabl. Th corrlation cofficints (>0.98) showd that th Frundlich modl is comparabl to th Langmuir modl. Th /n is lowr than.0, indicating that MB is favorably adsorbd by CCS. Th Tmpkin isothrm Tmpkin adsorption isothrm modl was usd to valuat th adsorption potntials of th CCS for MB. Th drivation of th Tmpkin isothrm assums that th fall in th hat of adsorption is linar rathr than logarithmic, as implid in th Frundlich quation. Th Tmpkin isothrm has commonly bn applid in th following form (Aharoni and Sparks 99; Aharoni and Ungarish 977; Wang and Qin 2005): RT q = ln( AC) (6) b Th Tmpkin isothrm Eq. (6) can b simplifid to th following quation, Thirumalisamy and Subbian (200). "Adsorption," BioRsourcs 5(),

9 q =β ln α+β ln C (7) whr β = (RT)/b, T is th absolut tmpratur in Klvin, and R is th univrsal gas constant, 8.34 J (mol K). Th constant b is rlatd to th hat of adsorption (Akkaya and Ozr 2005; Parc t al. 2003). Th adsorption data wr analyzd according to th linar form of th Tmpkin isothrm quation (7). Examination of th data shows that th Tmpkin isothrm fit wll to th MB adsorption data for CCS. Th linar isothrm constants and cofficints of dtrmination ar prsntd in Tabl. Th corrlation cofficints R 2 obtaind from Tmpkin modl wr comparabl to thos obtaind for th Langmuir and Frundlich quations, which indicats th applicability of Tmpkin modl to th adsorption of MB onto CCS. Th Dubinin-Radushkvich (D-R) isothrm Th D R modl was also applid to stimat th porosity apparnt fr nrgy and th charactristics of adsorption (Dubinin 960, 965; Radushkvich 949). Th D R isothrm dos not assums a homognous surfac or constant adsorption potntial. Th D R modl has commonly bn applid in th following Eq. (8), and its linar form can b shown in Eq. (9), q =Q m xp( Kε 2 ) (8) ln q =ln Q m Kε 2 (9) whr K is a constant rlatd to th adsorption nrgy, Q m is th thortical saturation capacity, and ε is th Polanyi potntial, calculatd from Eq. (0). ε = RT ln( + ) (0) C Th slop of th plot of ln q vrsus ε 2 givs K (mol 2 (kj 2 ) ), and th intrcpt yilds th adsorption capacity, Q m (mg g ). Th man fr nrgy of adsorption (E), dfind as th fr nrgy chang whn on mol of ion is transfrrd from infinity in solution to th surfac of th solid, was calculatd from th K valu using th following rlation (Kundu and Gupta 2006): E = () 2K Th calculatd valus of D R paramtr ar givn in Tabl. Th saturation adsorption capacity Q m obtaind using th D R isothrm modl for adsorption of MB onto CCS is mg g at 0.2g / 50 ml asorbnt dos, which is clos to that obtaind (5.48 mg g ) from Langmuir isothrm modl (Tabl ). Th valus of E calculatd Thirumalisamy and Subbian (200). "Adsorption," BioRsourcs 5(),

10 using Eq. () is kj mol, which indicats that th physico-sorption procss plays th significant rol in th adsorption of MB onto CCS. Tabl. Comparison of th Cofficints Isothrm Paramtrs for MB Adsorption onto CCS Isothrm modl Cucumis Sativa activatd carbon concntrations (g /50mL) 0.2 (g /50mL) 0.6(g /50mL) Langmuir Q m (mg g ) K a (L mg ) R Frundlich /n K F (mg g ) R Tmpkin α (L g ) β (mg L ) b R Dubinin Radushkvich Q m (mg g ) K ( 0-5 mol 2 kj 2 ) E (kj mol ) R Kintic Modls Applid to th Adsorption of MB onto CCS Svral stps can b usd to xamin th controlling mchanism of adsorption procss such as chmical raction, diffusion control, and mass transfr; kintic modls ar usd to tst xprimntal data from th adsorption of MB onto CCS. Th kintics of MB adsorption onto CCS is rquird for slcting optimum oprating conditions for th fullscal batch procss. Th kintic paramtrs, which ar hlpful for th prdiction of adsorption rat, giv important information for dsigning and modling th adsorption procsss. Thus, th kintics of MB adsorption onto CCS wr analyzd using psudo- Thirumalisamy and Subbian (200). "Adsorption," BioRsourcs 5(),

11 first-ordr (Lagrgrn 898), psudo-scond-ordr (Ho t al. 2000), Elovich (Chin and Clayton 980; Sparks 986; Zldowitsch 934), and intraparticl diffusion (Wbr t al. 963; Srinivasan t al. 988) kintic modls. Th conformity btwn xprimntal data and th modl-prdictd valus was xprssd by th corrlation cofficints (R 2, valus clos or qual to ). Th rlativly highr valu is th mor applicabl modl to th kintics of MB adsorption onto CCS. Psudo-first-ordr quation Th adsorption kintic data wr dscribd by th Lagrgrn psudo-first-ordr modl (Lagrgrn898), which is th arlist known quation dscribing th adsorption rat basd on th adsorption capacity. Th diffrntial quation is gnrally xprssd as follows, dqt dt = k ( q q ) (2) t whr q and q t ar th adsorption capacity at quilibrium and at tim t, rspctivly (mg g ), and k is th rat constant of psudo-first-ordr adsorption (L min ). Intgrating Eq. (2) for th boundary conditions t = 0 t and q t = 0 q t givs q k log( ) = t q q t (3) Eq. (3) can b rarrangd to obtain th following linar form: k log( q qt ) = log( q) t (4) In ordr to obtain th rat constants, th valus of log(q q t ) wr linarly corrlatd with t by plotting log(q q t ) vrsus t to giv a linar rlationship from which k and th prdictd q valu can b dtrmind from th slop and intrcpt of th plot, rspctivly (Fig. 4). Th variation in rat should b proportional to th first powr of concntration for strict surfac adsorption. Howvr, th rlationship btwn initial solut concntration and rat of adsorption will not b linar whn por diffusion limits th adsorption procss. Figur 4 shows that th psudo-first-ordr quation fit wll for th first 50 min and thraftr th data dviatd from thory. Thus, th modl wll rprsnts th initial stags whr rapid adsorption occurs but cannot b applid for th ntir adsorption procss. Furthrmor, th corrlation cofficints R 2 wr rlativly low for most adsorption data (Tabl 2). This shows that th adsorption of MB onto CCS cannot b applid, and th raction mchanism is not a first-ordr raction. Thirumalisamy and Subbian (200). "Adsorption," BioRsourcs 5(),

12 log(q-qt) R 2 = R 2 = R 2 = 0.89 R 2 = Tim (min) 50 mg/l 00mg/L 50 mg/l 200 mg/l Linar (50 mg/l) Linar (00mg/L) Linar (50 mg/l) Linar (200 mg/l) Fig. 4. Psudo-first-ordr kintics for MB adsorption onto CCS. Conditions: adsorbnt dosag 0.2 g/50 ml, ph 6.0, tmpratur 27 ± 2 C Psudo-scond-ordr quation Th adsorption kintics also may b dscribd by th psudo-scond-ordr modl (Ho t al. 2000). Th diffrntial quation is gnrally givn as follows: dqt dt = k (5) 2 2 ( q qt ) whr k 2 (g (mg min) ) is th scond-ordr rat constant of adsorption. By intgrating Eq. (5) for th boundary conditions q t = 0 q t at t = 0 t, th xprssion is simplifid and can b rarrangd and linarizd to obtain th following: t ) = + ( t) (6) q k q q ( 2 t 2 Th scond-ordr rat constants wr usd to calculat th initial sorption rat, givn by th following quation: h = k q (7) 2 2 If th scond-ordr kintics is applicabl, thn th plot of t/q t vrsus t should show a linar rlationship. Valus of k 2 and quilibrium adsorption capacity q wr calculatd from th intrcpt and slop of th plots of t/q t vrsus t (Fig 5). Thirumalisamy and Subbian (200). "Adsorption," BioRsourcs 5(),

13 t/qt(min mg/g) R 2 = R 2 = R 2 = R 2 = Tim (min) 50 mg/l 00mg/L 50 mg/l 200mg/L Linar (50 mg/l) Linar (00mg/L) Linar (50 mg/l) Linar (200mg/L) Fig. 5. Plot of th psudo-scond-ordr modl at diffrnt initial MB concntrations, CCS 0.2 g /ml, ph 6.0, tmpratur 27 ± 2 C Th linar plots of t/q t vrsus t showd good agrmnt btwn xprimntal and calculatd q valus at diffrnt initial MB and adsorbnt concntrations (Tabl 2). Th corrlation cofficints for th scond-ordr kintic modl wr gratr than 0.90, which ld us to bliv that th psudo-scond-ordr kintic modl providd a good corrlation for th bioadsorption of diffrnt initial of MB onto CCS. Th valus of initial sorption (h), which rprsnt th rat of initial adsorption, practically incrasd from to.436 mg (g min) with th incras in initial MB concntrations from 50 to 200 mg L onto CCS dos 0.2 g ml (Tabl 2). Tabl 2. Comparison of th First- and Scond-Ordr Adsorption Rat Constants and Calculatd and Exprimntal q Valus for Diffrnt Initial MB and CCS (0.2g /50mL) Paramtr First-ordr kintic modl Scond-ordr kintic modl MB (mg L ) q k q R 2 k 2 q h R 2 (xprimntal) (calculatd) (calculatd) k (min ), k 2 (g (mg min) ), q (mg g ), h (mg (g min) ). Thirumalisamy and Subbian (200). "Adsorption," BioRsourcs 5(),

14 Elovich quation Th Elovich quation is anothr rat quation basd on th adsorption capacity. It is gnrally xprssd in th following form (Chin and Clayton 980; Sparks 986; Zldowitsch 934): dqt dt = B xp( A q ) (8) E E t whr B E is th initial adsorption rat (mg (g min) ) and A E is th d-sorption constant (g mg ) during any xprimnt. This xprssion can b simplifid by assuming A E B E t t and by applying th boundary conditions q t = 0 at t = 0 and q t = q t at t = t. Thn Eq. (8) bcoms: qt = ln( BE AE ) + ln( t) (9) A A E E If MB adsorption by CCS fits th Elovich modl, a plot of q t vrsus ln(t) should yild a linar rlationship with a slop of (/A E ) and an intrcpt of (/A E ) ln (A E B E ) (Fig 6). Thus, th constants can b obtaind from th slop and th intrcpt of th straight lin (Tabl 3). Th initial adsorption rat B E incrasd from 5.88 to mg (g min) with incras of initial MB concntration from 50 to 200 mg L on CCS dos of 0.2 g ml. A similar pattrn as mntiond abov was obsrvd for th initial adsorption rat, h, obtaind from psudo-scond-ordr modl. Th dsorption constant, A E, dcrasd from to g mg with incras in th initial MB concntration from 50 to 200 mg L ovr CCS dos of 0.2 g ml (Tabl 3). qt(mg/g) R 2 = R 2 = R 2 = R 2 = Ln t 50 mg/l 00 mg/l 50 mg/l 200 mg/l Linar (50 mg/l) Linar (00 mg/l) Linar (50 mg/l) Linar (200 mg/l) Fig. 6. Elovich modl plot for th adsorption of MB onto CCS (0.2g /50mL) at diffrnt initial dy concntrations (50, 00,50 and 200 mg L ) Thirumalisamy and Subbian (200). "Adsorption," BioRsourcs 5(),

15 Th intraparticl diffusion modl Th adsorbat spcis ar most probably transportd from th bulk of th solution into th solid phas through an intraparticl diffusion/transport procss, which is oftn th rat-limiting stp in many adsorption procsss, spcially in a rapidly stirrd batch ractor (McKay 983). Sinc th MB molculs ar probably transportd from its aquous solution to th CCS by intraparticl diffusion, so th intraparticl diffusion is anothr kintic modl that should b usd to study th rat of MB adsorption onto CCS. Th possibility of intraparticular diffusion was xplord by using th intraparticl diffusion modl, which is commonly xprssd by th following quation, q t =K dif t /2 +C (20) whr C (mg g ) is th intrcpt and K dif is th intraparticl diffusion rat constant (in mg g min /2 ). Th valus of q t wr found to b linarly corrlatd with valus of t /2 (Fig 7), and th rat constant K dif was dirctly valuatd from th slop of th rgrssion lin (Tabl 3). 45 qt (mg/g) R 2 = R 2 = R 2 = R 2 = Tim /2 (min /2) 50 mg /L 00mg/L 50 mg/l 200 mg/l Linar (50 mg /L) Linar (00mg/L) Linar (50 mg/l) Linar (200 mg/l) Fig. 7. Intraparticl diffusion modl plot for th adsorption of MB onto CCS (0.2 g /50mL) at diffrnt initial dy concntration (50, 00, 50, and 200 mg L ) Th valus of th intrcpt C (Tabl 3) provid information about th thicknss of th boundary layr, whr th rsistanc to th xtrnal mass transfr incrass as th intrcpt incrass. Th constant C was found to incras from to with incras of dy concntration from 50 to 200 mg L, indicating th incras of th thicknss of th boundary layr and dcras of th liklihood of th xtrnal mass transfr and hnc incras of th liklihood of intrnal mass transfr. Th R 2 valus givn in Tabl 3 ar clos to unity, indicating th applicability of this modl. This may confirm Thirumalisamy and Subbian (200). "Adsorption," BioRsourcs 5(),

16 that th rat-limiting stp is th intraparticl diffusion procss. Th intraparticl diffusion rat constants, K dif, wr in th rang of to mg g min /2 and thy incrasd with incrasing initial dy concntration. Th linarity of th plots dmonstratd that intraparticl diffusion playd a significant rol in th uptak of th adsorbat by adsorbnt. Howvr, as still thr is no sufficint indication about it, Ho (2003) has shown that if th intraparticl diffusion is th sol rat-limiting stp, thn it is ssntial for th q t vrsus t /2 plots to pass through th origin, which is not th cas in Fig. 7. So it may b concludd that surfac adsorption and intraparticl diffusion wr concurrntly oprating during th MB and CCS intractions. Tabl 3. Th Paramtrs Obtaind from Elovich Kintic Modl and Intraparticl Diffusion Modl Using Diffrnt Initial MB Concntrations Paramtr Elovich Intraparticl diffusion MB (mg L ) A E B E R 2 K dif C R Tabl 4. Charactristics of CCS Paramtr Valu ph of % solution 6.5 Moistur contnt(%).62 Watr solubl mattr(%) 0.2 Acid solubl mattr(%) 8.07 Dcolorizing powr (mg/g) 47.0 Conductivity 0.92 ph ZPC 4.0 CONCLUSIONS. Th rsults of this invstigation show that activatd carbon dvlopd from Cucumis Sativa has a suitabl adsorption capacity for th rmoval of MB from aquous solutions. 2. Th quilibrium adsorption was practically achivd in 0 min. 3. Th xprimntal rsults wr analyzd by using Langmuir, Frundlich, Tmpkin, and Dubinin Radushkvich isothrm modls, and th corrlation cofficints for Thirumalisamy and Subbian (200). "Adsorption," BioRsourcs 5(),

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