Journal of Hazardous Materials

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1 Jural f Hazardus Materials 185 (2011) Ctets lists available at ScieceDirect Jural f Hazardus Materials jural hmepage: Adsrpti f chrmium (VI) by ethyleediamie-mdified crss-liked magetic chitsa resi: Istherms, kietics ad thermdyamics Xi-jiag Hu a,b, Jig-sg Wag c, Yu-gu Liu a,b,, Xi Li a,b, Guag-mig Zeg a,b, Zheg-lei Ba c, Xia-xia Zeg a,b, A-wei Che a,b, Fei Lg a,b a Cllege f Evirmetal Sciece ad Egieerig, Hua Uiversity, Chagsha , PR Chia b Key Labratry f Evirmetal Bilgy ad Plluti Ctrl (Hua Uiversity), Miistry f Educati, Chagsha , PR Chia c Schl f Urba Cstructi, Uiversity f Suth Chia, Hegyag, Hua , PR Chia article if abstract Article histry: Received 2 July 2010 Received i revised frm 3 September 2010 Accepted 7 September 2010 Available lie 17 September 2010 Keywrds: EMCMCR Chrmium (VI) Adsrpti Equilibrium istherm Kietics The adsrpti f chrmium (VI) is frm aqueus sluti by ethyleediamie-mdified crss-liked magetic chitsa resi (EMCMCR) was studied i a batch adsrpti system. Chrmium (VI) remval is ph depedet ad the ptimum adsrpti was bserved at ph 2.0. The adsrpti rate was extremely fast ad the equilibrium was established withi 6 10 mi. The adsrpti data culd be well iterpreted by the Lagmuir ad Temki mdel. The maximum adsrpti capacities btaied frm the Lagmuir mdel are mg g 1, mg g 1 ad mg g 1 at 293, 303 ad 313 K, respectively. The adsrpti prcess culd be described by pseud-secd-rder kietic mdel. The itraparticle diffusi study revealed that film diffusi might be ivlved i the preset case. Thermdyamic parameters revealed the feasibility, sptaeity ad exthermic ature f adsrpti. The srbets were successfully regeerated usig 0.1 ah slutis Elsevier B.V. All rights reserved. 1. Itrducti Chrmium exists i trivalet [Cr (III)] ad hexavalet [Cr (VI)] state. The hexavalet frm has bee csidered mre hazardus t public health due t its mutageic ad carcigeic prperties [1]. Varius methds f chrmium remval iclude filtrati, chemical precipitati, adsrpti, electrdepsiti ad membrae systems r eve i exchage prcess. Amg these methds, adsrpti is e f the mst ecmically favrable ad a techically easy methd [2]. Chitsa is the deacetylated frm f chiti, which is a liear plymer f acetylami-d-glucse. As chitsa is hydrphilic, bidegradability, harmlessess fr livig thigs ad ease f chemical derivatizati, besides, chitsa has may ami ad hydrxyl grups that ca chelate heavy metal is. Therefre, chitsa presets as a very prmisig startig material fr chelatig resis [3]. Several metals are preferetially adsrbed i acidic media while chitsa ca disslve i acid cditi. T vercme such a prblem, sme crss-likig agets such as glutaraldehyde [4], epichlrhydri [5] ad ethyleeglycl diglycidyl ether [6] are Crrespdig authr at: Cllege f Evirmetal Sciece ad Egieerig, Hua Uiversity, Chagsha , PR Chia. Tel.: ; fax: address: huese@126.cm (Y.-g. Liu). used t stabilize chitsa i acid slutis. But glutaraldehyde is the mst widely used because it des t have much dimiishig adsrpti capacity [5]. This methd is used t esure gd mechaically ad chemically stable beads, but it has bee fud t have egative effect the adsrpti capacity f the chitsa. The mai reas fr the lss f adsrpti capacity is that amie grups are ivlved i the crss-likig reacti [7]. The ability f a material t capture metals is ctrlled i part by the umber f available fuctial grups used fr bidig metals. The ami ad tw hydrxyl grups each glucsamie i the repeatig uit f chitsa ca act as a reactive site fr chemical mdificati. I rder t icrease the adsrpti ability f chitsa, ad t imprve the adsrpti selectivity f metal is, several chemicals have bee used t mdify chitsa such as glycie [8], plydimethylsilxae [9], thiurea [10] ad maleic ahydride [11]. the ther had, after the adsrpti is carried ut, the adsrbets are difficult t be separated frm the sluti usig traditial separati methds such as filtrati ad sedimetati. The magetic techlgy is a gd sluti. Magetic carriers are used as the supprt material ad they ca be easily separated frm the reacti medium ad stabilized i a fluidized bed reactr by applyig a magetic field [12]. I the preset wrk, we prepared ethyleediamie-mdified glutaraldehyde-crssliked magetic chitsa resi (EMCMCR) ad used it t adsrb Cr (VI) is i a batch system. The effects f the prcess parameters such as ph, temperature, dsage f EMCMCR, /$ see frt matter 2010 Elsevier B.V. All rights reserved. di: /j.jhazmat

2 X.-j. Hu et al. / Jural f Hazardus Materials 185 (2011) H H H H 2 H H 2 H H H H H H H H H H 2 Step1 H 2 H 2 Step3 H H H H H H Step2 H H H H 2 Cl H H H H H Cl Fig. 1. Scheme fr the sythesis f EMCMCR: (step1) expected crss-likig mechaism fr chitsa usig glutaraldehyde; (step2) expected mechaism f acti fr crss-liked chitsa usig epichlrhydri; (step3) expected mechaism fr amiated chitsa beads. iitial Cr (VI) ccetrati Cr (VI) remval were ivestigated. I rder t better uderstad the adsrpti characteristic, sme istherm, kietic ad thermdyamic mdels were emplyed t evaluate the srpti prcess. 2. Materials ad methds EMCMCR was prepared by disslvig chitsa i acetic acid sluti with stirrig util cmpletely disslved at 50 C ad the the magetic fluid was added t the sluti slwly. Glutaraldehyde ad ethyleediamie were used t frm the gel. Meawhile, ethyleediamie was itrduced it the mixture t mdify the resi. Fig. 1 shws the preparati sketch f EMCMCR. Fig. 2 shws the phts f the chitsa ad EMCMCR Materials Chitsa (90% acetylati degree) was supplied by Sipharm Chemical Reaget C., Ltd. Glutaraldehyde ad epichlrhydri was prvided by Tiaji Guagfu Fie Chemical Research Istitute, Tiaji, Chia. Ethyleediamie was btaied frm Chagsha Subitersecti Plastic Chemical Factry, Chagsha, Chia. All reagets abve were f Chemical grade. Stck slutis (1000 mg L 1 ) f chrmium (VI) was prepared by disslvig g K 2 Cr 2 7 i 1000 ml distilled water. The slutis f differet ccetratis used i varius experimets were btaied by diluti f the stck slutis Preparati f EMCMCR Magetic fluid was prepared accrdig t frmer study [13]. FeCl 3 ad FeS 4 were mixed i the sluti with additi f sdium hydrxide t frm Fe 3 4 magetic particles. The surfactat ad the ultrasic geeratr were used t disperse the magetic particles Batch adsrpti experimets All batch experimets were carried ut with adsrbet samples i a 250 ml cical flasks with 100 ml Cr (VI) aqueus slutis a rtary shaker at 250 r mi 1. The ccetrati f Cr (VI) is was determied spectrphtmetrically at 540 m usig dipheyl carbazide as the cmplexig aget. The study f the ph ( ) depedecy f Cr (VI) adsrpti t EMCMCR was carried ut i 100 ml Cr (VI) slutis with differet iitial ccetrati f 10, 60 ad 100 mg L 1 at a temperature (30 C). The ph value was adjusted by 1 M ah r 1 M HCl. Istherm studies were cducted by ctactig 0.5 g EMCMCR with 100 ml f Cr (VI) sluti at differet iitial ccetrati (20, 40, 60, 100 ad 200 mg L 1 ) shakig fr 60 mi. The experimets were perfrmed at differet temperature (20, 30 ad 40 C). Srpti istherms are plts f the equilibrium adsrpti capacity (q e ) (accrdig t Eq. (1)) versus the equilibrium ccetrati f the Fig. 2. Phts f chitsa ad EMCMCR.

3 308 X.-j. Hu et al. / Jural f Hazardus Materials 185 (2011) Fig. 3. The SEM characterizati f EMCMCR: (A) befre (5000 ) ad (B) after (5000 ) adsrpti. residual Cr (VI) i the sluti (C e ). q e = (C 0 C e )V (1) W where q e is the equilibrium adsrpti capacity (mg g 1 ), C 0 ad C e are the iitial ad equilibrium liquid phase slute ccetrati (mg L 1 ), respectively. V is the liquid phase vlume (L) ad W is the amut f adsrbet (g). T ivestigate the kietic characteristics f the adsrpti, 0.5 g EMCMCR was added t 100 ml f Cr (VI) sluti with three differet ccetrati (60, 100 ad 200 mg L 1 ), ad the samples were agitated fr desigated time perids (2, 4, 6, 8, 10, 20, 30, 40, 50, 60, 90, 120, 150, 180 mi) Desrpti experimets The adsrpti desrpti cycles were repeated csecutively te times t determie the reusability f srbets. After adsrpti experimets (vlume, 100 ml; absrbet dse, 0.5 g; iitial ccetrati, 300 mg L 1 ; ph value, 2; ctact time, 60 mi; temperature, 30 C; agitati speed, 250 rpm), the EMCMCR adsrbed with Cr (VI) is were separated frm the sluti by filtrati ad the added it 20 ml varius strippig slutis (0.1 M EDTA, 0.1 HCl, ad 0.1 ah, respectively) ad stirred at 250 rpm fr 30 mi at 30 C ad the fial Cr (VI) ccetrati was determied. After each cycle f adsrpti desrpti, srbet was washed with distilled water ad used i the succeedig cycle. The desrpti rati f Cr (VI) is frm EMCMCR was calculated frm the amut f Cr (VI) is adsrbed EMCMCR ad the fial Cr (VI) is ccetrati i the desrpti medium. Desrpti rati was calculated frm the fllwig equati: Cr (VI) i was adsrbed by EMCMCR it its pres ad develped a layer f Cr (VI) substace the surface Effect f ph Cr (VI) adsrpti by EMCMCR The effect f iitial sluti ph Cr (VI) remval by EMCMCR is shw i Fig. 4. The adsrpti f Cr (VI) icreased with ph icreasig frm 1.0 t 2.0. The results idicated that at ph 2.0, the maximum adsrpti capacity f 2.00, 11.88, mg g 1 ccurred at a iitial Cr (VI) ccetrati f 10, 60 ad 100 mg L 1, respectively. Withi the rage f ph values frm 2.0 t 8.0, the adsrpti capacity decreased drastically with icreasig f ph. This idicates that the adsrpti f the adsrbet is clearly ph depedet. Similar tred was als bserved with the remval f hexavalet chrmium usig chitsa derivatives [14], while sme researchers have reprted the differet ptimum ph adsrpti f heavy metals by usig differet kid f mdified chitsa. Fr example, the adsrpti f Cr (VI) CMCB ad CMCF was ph depedet ad maximum adsrpti was btaied at ph 3.0 [15]. The maximum adsrpti capacity f the resi QCS fr Cr (VI) ccurred at a ph value f arud [16]. Desrpti rati = amut f Cr (VI) is desrbed t the desrpti medium amut f Cr (VI) is adsrbed t the EMCECR 100 (2) 3. Results ad discussi 3.1. Scaig electr micrscpe studies Surface mrphlgy f the EMCMCR befre ad after adsrpti was bserved by SEM, ad the SEM images are shw i Fig. 3. Befre adsrpti, irregular surface structure ad may pres i the surface were bserved, which may exist as amie grups. It is als csidered helpful fr mass trasfer f Cr (VI) is t EMCMCR. After absrpti, the adsrbet surface became abrmal ad a great deal f crystal adhered t the surface. These images prve that, Fig. 4. The effect f iitial sluti ph Cr (VI) remval by EMCMCR at varius iitial ccetratis (10, 60 ad 100 mg L 1 ). (Vlume, 100 ml; absrbet dse, 0.5 g; iitial ccetrati, 10, 60 ad 100 mg L 1 ; ph value, 1, 2, 3, 4, 5, 6, 7, 8; ctact time, 60 mi; temperature, 303 K; agitati speed, 250 rpm.)

4 X.-j. Hu et al. / Jural f Hazardus Materials 185 (2011) Table 1 Adsrpti equilibrium cstats btaied frm Lagmuir, Freudlich ad Temki istherms i the adsrpti f Cr (VI) t EMCMCR. (Vlume, 100 ml; absrbet dse, 0.5 g; iitial ccetrati, 20, 40, 60, 100, 200 mg L 1 ; ctact time, 60 mi; temperature, 293, 303, 313 K; agitati speed, 250 rpm.). Temperature ( C) Lagmuir mdel Freudlich mdel Temki mdel q max K L R L R 2 K F q m R 2 a T b T R > It ca be explaied that because the ph f the aqueus sluti affects the speciati f chrmium ad the surface charge f the adsrbet [17,18]. Cr (VI) exists i differet frms i aqueus sluti ad the stability f these frms is depedet the ph f the system. At ph 1.0, the chrmium is exist i the frm f H 2 Cr 4, while i the ph rage f , differet frms f chrmium is such as Cr 2 7, HCr 4,Cr ,Cr cexist while HCr 4 predmiates. As ph icreases, this frm shifts t Cr 4 2 ad Cr [2]. Cr (VI) exists predmiatly as HCr 4 i aqueus sluti belw ph 4.0 ad the ami grups ( H 2 )f EMCMCR wuld be i prtated catiic frm ( H 3 + ) t a higher extet i acidic sluti which result i a strger attracti fr a egatively charged i i the sluti, ad electrstatic iteracti ccurs betwee the srbet ad HCr 4 is resultig i high chrmium remval [19]. Hwever, at ph less tha 2.0 a decrease uptake capacity is bserved due t the chrmium beig preset predmiatly as H 2 Cr 4 ad a strg cmpetiti existed betwee H 2 Cr 4 ad prts fr adsrpti sites. Higher ph decreasig the adsrpti capacity may be explaied by the dual cmpetiti f bth the ais (Cr 4 2 ad H ) t be adsrbed the surface f the adsrbet f which H predmiates [2]. Thus, ph f 2.0 was selected as the ptimum ph value f Cr (VI) sluti fr the fllwig adsrpti experimet Istherm studies Adsrpti istherm studies are imprtat t determie the efficacy f adsrpti. Several adsrpti istherms rigially used fr gas phase adsrpti are available ad readily adpted t crrelate adsrpti equilibria i heavy metals adsrpti. Sme well-kw es are Freudlich, Lagmuir, Temki, Redlich Paters ad Sips equati [20]. I this study, Lagmuir, Freudlich ad Temki mdels were used t determie the adsrpti equilibrium betwee the adsrbet ad metal is. The istherm cstats fr the three mdels were btaied by liear regressi methd ad preseted i Table 1. The Lagmuir mdel assumes that a mmlecular layer is frmed whe adsrpti takes place withut ay iteracti betwee the adsrbed mlecules [21]. The Lagmuir mdel ca be represeted as: q e = q maxk L C e (3) 1 + K L C e where C e is the equilibrium ccetrati (mg L 1 ), q e the amut f metal i srbed (mg g 1 ), q max is q e fr a cmplete mlayer (mg g 1 ), K L is a cstat related t the affiity f the bidig sites (L mg 1 ). The liearized frm f the Lagmuir equati is: C e q e = 1 q max K L + C e q max (4) The experimetal data were pltted as C e /q e versus C e ad shw i Fig. 5. The values f Lagmuir cstats q max ad K L were btaied by liear regressi methd ad shw i Table 1. The experimetal data exhibited high crrelati with Lagmuir mdel withi the studied temperature rage. Bth q max ad K L decreased with icreasig temperature, idicatig the bdig betwee heavy metals ad active sites f the adsrbet weakeed at higher temperature ad the adsrpti prcess is exthermic. This utcme is similar t the studies ivlvig Cr (VI) adsrpti chitsa [19]. But the q max values btaied fr EMCMCR ( mg g 1 )at30 C is remarkably higher tha mg g 1 as reprted by Aydı et al. [19]. The essetial features f a Lagmuir istherm ca be expressed i terms f a dimesiless cstat separati factr r equilibrium parameter, R L which is defied by Hall et al. [22] as: R L = K L C 0 (5) where K L (L mg 1 ) is the Lagmuir cstat ad C 0 (mg L 1 )isthe iitial ccetrati. The calculated values f the dimesiless factr R L are icluded i Table 1. All R L values btaied are greater tha zer ad less tha uity idicates the favrable adsrpti f Cr (VI) by EMCMCR uder csiderati. This meas that the equilibrium istherms ca be well described by the Lagmuir mdel, ad the adsrpti prcess is mlayer adsrpti t a surface with fiite umber f idetical sites, which are hmgeeusly distributed ver the adsrbet surface. The Freudlich istherm is a empirical equati assumig that the adsrpti prcess takes place hetergeeus surfaces ad adsrpti capacity is related t the ccetrati f Cr (VI) at equilibrium. This istherm mdel is defied by the equati [23] belw: q e = K F C e where q e, ad C e are the equilibrium ccetratis f chrmium (VI) i the adsrbed (mg g 1 ) ad liquid phases (mg L 1 ), respectively, K F ad are the Freudlich cstats which are related t adsrpti capacity ad itesity, respectively. This equati ca C e /q e (g.l -1 ) ºC ºC ºC C e (mg.l -1 ) Fig. 5. Lagmuir istherm fr the adsrpti Cr (VI) is EMCMCR. (Vlume, 100 ml; absrbet dse, 0.5 g; iitial ccetrati, 20, 40, 60, 100, 200 mg L 1 ; ctact time, 60 mi; temperature, 293, 303, 313 K; agitati speed, 250 rpm.) (6)

5 310 X.-j. Hu et al. / Jural f Hazardus Materials 185 (2011) be writte i the liear frm give belw: I q e = 1 I C e + I K F (7) T determie the maximum srpti capacity, it is ecessary t perate with cstat iitial ccetrati C 0 ad variable weights f srbet, thus lq m is the extraplated value f lq fr C = C 0 [24]. Accrdig t Halsey [25]: K F = q m (8) 1/ C 0 q e (mg.g -1 ) mg.L mg.L mg.L -1 As see frm Table 1, regressi crrelati cefficiets were very high i the temperature rage studied. Hwever, the maximum srpti capacity cmputed usig Halsey expressi (Table 1) are far higher tha the experimetal adsrbed amuts at equilibrium crrespdig t the plateau f the srpti istherms. This meas that the equilibrium istherms cat be described by the Freudlich mdel. The derivati f the Temki istherm is based the assumpti that the declie f the heat f srpti as a fucti f temperature is liear rather tha lgarithmic, as implied i the Freudlich equati. It ca be described as fllw [26]: q e = RT l (a T C e ) = RT l a T + RT l C e (9) b T b T b T where a T is the equilibrium bidig cstat crrespdig t the maximum bidig eergy (L g 1 ), b T is the Temki cstat related t the heat f srpti (kj ml 1 ), ad R is the gas cstat ( kj ml 1 K 1 ), T is the abslute temperature (K). Thus, the cstats ca be btaied frm the slpe ad itercept f a straight lie plt f q e versus lc e. Temki cstats are give i Table 1. Temki istherm geerates a satisfactry fit t the experimetal data as idicated by crrelati cefficiets. Typical bdig eergy rage fr i-exchage mechaism is reprted t be i the rage f 8 16 kj ml 1 while physisrpti prcesses are reprted t have adsrpti eergies less tha 40 kj ml 1 [26]. Values f b T (0.238, 0.258, kj ml 1 at a iitial ccetrati f 60, 100 ad 200 mg L 1, respectively) btaied i the preset study idicates that the adsrpti prcess seems t ivlve chemisrpti ad physisrpti Kietic studies Cr (VI) remval by adsrbet as a fucti f ctact time with differet iitial ccetrati (60, 100 ad 200 mg L 1 ) is shw i Fig. 6, where the adsrpti rate f metal uptake was very fast ad the maximum uptake was bserved withi 6 10 mi. There was almst further icrease f adsrpti after 10 mi. These results shwed that the actual adsrpti f metal i bidig t the adsrbet was very rapid. Data the adsrpti rates f Cr (VI) is by varius mdified chitsa have shw a wide rage f adsrpti times. Fr example, the cpper ad Cr (VI) adsrpti equilibrium time chitsa derivatives was 2 h [14]. The adsrpti equilibrium time f Cr (VI) the by CMCF ad CMC was 16 h [15]. T ivestigate the mechaism f adsrpti ad its ptetial rate-ctrllig steps that iclude mass trasprt ad chemical reacti prcesses, kietic mdels have bee explited t aalyse the experimetal data. I additi, ifrmati the kietics f metal uptake is required t select the ptimum cditi fr fullscale batch metal remval prcesses [20]. Several kietic mdels such as pseud-first-rder, pseud-secd-rder ad itra-particle diffusi mdel have bee applied t fid ut the adsrpti mechaism. The equati f the three kietic mdels is expressed as fllws: t(mi) Fig. 6. Effect f ctact time Cr (VI) adsrpti. (Vlume, 100 ml; absrbet dse, 0.5 g; iitial ccetrati, 60, 100 ad 200 mg L 1 ; ph value, 2.0; ctact time, 0, 2, 4, 6, 8, 10, 20, 30, 40, 50, 60, 90, 120, 150, 180 mi; temperature, 303 K; agitati speed, 250 rpm.) The first-rder rate equati is give as: lg(q e q t ) = lg q e k t (10) where q e ad q t are the amuts f adsrbed Cr (VI) the adsrbet at equilibrium ad at time t, respectively (mg g 1 ), ad k 1 is the first-rder adsrpti rate cstat (mi 1 ). The liearized frm f the pseud-first rder mdel fr the srpti f Cr (VI) is t EMCMCR at varius iitial ccetratis is give i Fig. 7. The calculated results f the first-rder rate equati are give i Table 2. It is fud that crrelati cefficiets values are lw, shwig the bad quality f liearizati. Additially, the q e value acquired by this methd is ctrasted with the experimetal value. S the reacti cat be classified as first-rder. e suggesti fr the differeces i experimetal ad theretical q e values is that there is a time lag, pssibly due t a budary layer r exteral resistace ctrllig at the begiig f the srpti. This time lag is als difficult t quatify. Fr this reas, it is ecessary t use a trial ad errr methd i rder t btai the equilibrium uptake [27]. lg(q e -q t ) mg.L 100mg.L 200mg.L t(mi) Fig. 7. Pseud-first rder srpti kietics f Cr (VI) t EMCMCR at varius iitial ccetratis. (Vlume, 100 ml; absrbet dse, 0.5 g; iitial ccetrati, 60, 100 ad 200 mg L 1 ; ph value, 2.0; ctact time, 0, 2, 4, 6, 8, 10, 20, 30, 40, 50, 60, 90, 120, 150, 180 mi; temperature, 303 K; agitati speed, 250 rpm.)

6 X.-j. Hu et al. / Jural f Hazardus Materials 185 (2011) Table 2 Kietic parameters btaied frm Lagragia mdels i the adsrpti f Cr (VI) t EMCMCR. (Vlume, 100 ml; absrbet dse, 0.5 g; iitial ccetrati, 60, 100 ad 200 mg L 1 ; ph value, 2.0; ctact time, 0, 2, 4, 6, 8, 10, 20, 30, 40, 50, 60, 90, 120, 150, 180 mi; temperature, 303 K; agitati speed, 250 rpm.). C 0 (mg L 1 ) Pseud-first-rder Pseud-secd-rder k 1 (mi 1 ) q e (mg g 1 ) R 2 k 2 (g mg 1 mi 1 ) q e (mg g 1 ) h (mg g 1 mi 1 ) R > > >0.999 The pseud-secd-rder equati is give as: t = 1 q t k 2 q t (11) e q e where k 2 is the secd-rder adsrpti rate cstat (g mg 1 mi 1 ), ad q e is the adsrpti capacity calculated by the pseud-secd-rder kietic mdel (mg g 1 ). The cstat k 2 is used t calculate the iitial srpti rate h (mg g 1 mi 1 ), at t 0 as fllws [24]: h = k 2 q e 2 (12) The liearized frm f the pseud-secd rder mdel is give i Fig. 8. The values f the crrelati cefficiets were all extremely high ad all greater tha Besides, the calculated q e values agreed very well with the experimetal data. Thus experimet results supprts the assumpti behid the mdel that the rate limitig step i adsrpti f heavy metals are chemisrpti ivlvig valece frces thrugh the sharig r exchage f electrs betwee adsrbet ad metal is. Sme studies the kietics f Cr (VI) adsrpti t varius adsrbets have als reprted higher crrelatis fr pseud-secd rder mdel [2,19,28]. The adsrpti prcess a prus adsrbet will geerally have multi-step prcess. I rder t ivestigate the mechaism f the adsrpti f Cr (VI) t EMCMCR, the experimetal data were tested agaist the itraparticle diffusi mdel t idetify the mechaism ivlved i the srpti prcess. The three csecutive steps i the srpti f a srbate by a prus srbet are: (i) mass trasfer acrss the exteral budary layer film f liquid surrudig the utside f the particle; (ii) adsrpti at a site the surface (iteral r exteral) ad the eergy will deped the bidig prcess (physical r chemical), this step is fte assumed t be extremely rapid; (iii) diffusi f the adsrbate mlecules t a adsrpti site either by a pre diffusi prcess thrugh the liquid filled pres r by a slid surface diffusi mechaism [29]. The itraparticle diffusi mdel ca be described as fllws [30]: q t = k p t 1/2 + C (13) where k p is the itra-particle diffusi rate cstat (mg g 1 mi 0.5 ) ad C f adsrpti cstat is the itercept. Fig. 9 is the plt f q t vs. t 1/2 at differet iitial Cr (VI) ccetrati, which idicated that the plt f q t vs. t 1/2 was multi-liear, implyig that mre tha e prcess affected the srpti. Based these plts, the srpti prcesses f the three elemets are cmprised by tw phases, suggestig that the itraparticle diffusi is t the rate-limitig step fr the whle reacti [31]. The iitial prti f the plt idicated a exteral mass trasfer whereas the secd liear prti is due t itraparticle r pre diffusi [32]. Due t the duble ature f itraparticle diffusi (bth film ad pre diffusi) (Fig. 6), ad i rder t determie the actual ratectrllig step ivlved i the srpti prcess, the kietic data have bee aalyzed usig the mdel give by Byd et al. [24,33] F = 1 6 [ ] 1 2 m 2 exp Di 2 m 2 t r 2 (14) m=1 r F = ( ) 2 m 2 exp m 2 Bt (15) m=1 where F is the fractial attaimet f equilibrium at time t ad is btaied by the expressi: F = q t q e (16) mg.L 100mg.L 200mg.L t/q t (mi.g.mg -1 ) q e (mg.g -1 ) mg.L 100mg.L 200mg.L t(mi) Fig. 8. Pseud-secd rder srpti kietics f Cr (VI) t EMCMCR at varius iitial ccetratis. (Vlume, 100 ml; absrbet dse, 0.5 g; iitial ccetrati, 60, 100 ad 200 mg L 1 ; ph value, 2.0; ctact time, 0, 2, 4, 6, 8, 10, 20, 30, 40, 50, 60, 90, 120, 150, 180 mi; temperature, 303 K; agitati speed, 250 rpm.) t 1/2 (mi 1/2 ) Fig. 9. Itraparticle diffusi kietics f Cr (VI) t EMCMCR at varius iitial ccetratis. (Vlume, 100 ml; absrbet dse, 0.5 g; iitial ccetrati, 60, 100 ad 200 mg L 1 ; ph value, 2.0; ctact time, 0, 2, 4, 6, 8, 10, 20, 30, 40, 50, 60, 90, 120, 150, 180 mi; temperature, 303 K; agitati speed, 250 rpm.)

7 312 X.-j. Hu et al. / Jural f Hazardus Materials 185 (2011) Bt mg.L 100mg.L 200mg.L t (mi) Gibbs free eergy chage(kj.ml -1 ) T(K) Fig. 10. Byd plt fr Cr (VI) srpti t EMCMCR at varius iitial ccetratis (60, 100 ad 200 mg L 1 ). (Vlume, 100 ml; absrbet dse, 0.5 g; iitial ccetrati, 60, 100 ad 200 mg L 1 ; ph value, 2.0; ctact time, 0, 2, 4, 6, 8, 10, 20, 30, 40, 50, 60, 90, 120, 150, 180 mi; temperature, 303 K; agitati speed, 250 rpm.) Fig. 11. plt f Gibbs free eergy chage, G versus temperature, T. (Vlume, 100 ml; absrbet dse, 0.5 g; iitial ccetrati, 20, 40, 60, 100, 200 mg L 1 ; ctact time, 60 mi; temperature, 293, 303, 313 K; agitati speed, 250 rpm.) 3.5. Thermdyamic studies where q t (mg g 1 ) is the amut f srbate take up at time t ad q e (mg g 1 ) is the maximum equilibrium uptake ad B = D i 2 r 2 (17) where B is the time cstat (mi 1 ), D i is the effective diffusi cefficiet f the metal is i the srbet phase (cm 2 mi 1 ), r is the radius f the srbet particle (cm), assumed t be spherical, ad m is a iteger that defies the ifiite series sluti. Bt is give by the equati: Bt = l(1 F) (18) Thus, the value f Bt ca be cmputed fr each value f F, ad the pltted agaist time t cfigure the s-called Byd plts. The liearity plt f Bt vs. t plts (Fig. 10) was emplyed t distiguish betwee srpti ctrlled by film diffusi ad particle diffusi [24]. A straight lie passig thrugh the rigi is idicative f srpti prcesses gvered by particle-diffusi mechaisms. therwise they are gvered by film diffusi [34]. Frm Fig. 10,it was bserved that the plts were either liear r passed thrugh the rigi at varius iitial ccetratis, idicatig the filmdiffusi-ctrlled mechaism. Usually, exteral trasprt is the rate-limitig step i systems, which have pr mixig, dilute ccetrati f adsrbate, small particle size ad high affiity f adsrbate fr adsrbet. I ctrast, the itraparticle step limits the verall trasfer fr thse systems that have high ccetrati f adsrbate, gd mixig, large particle size f adsrbet ad lw affiity f adsrbate fr adsrbet [35]. I ur research, the strg exteral resistace which hider the exteral mass trasfer may due t the pr mixig (agitati speed 250 rpm), lw ccetrati f adsrbate (iitial ccetrati, 60, 100 ad 200 mg L 1, respectively, but lw ccetrati remaied whe the itraparticle-diffusi takes ctrl f the adsrpti rate) ad small particle sizes f EMCMCR (particle sizes belw 300 m). Besides, the adsrbate displays higher affiity fr the EMCMCR, which results i lw iteral resistace. Because the verall rate f srpti will be ctrlled by the slwest step (higher resistace step)[35], s the exteral mass trasprt (film diffusi) maily gvers the itraparticle diffusi i ur study. Similar results were btaied by Aguilar-Carrill et al. [32]. Thermdyamic csideratis f a adsrpti prcess are ecessary t cclude whether the prcess is sptaeus r t. The experimetal data btaied at differet temperatures were used i calculatig the thermdyamic parameters such as Gibbs free eergy chage (G ), ethalpy chage (H ) ad etrpy chage (S ). The Gibbs free eergy chage f the srpti reacti is give by the fllwig [36]: G = RT l K L (19) G = H T S (20) where K L is the equilibrium cstat, which ca be btaied frm Lagmuir istherms at differet temperature, R is the uiversal gas cstat, J ml 1 K 1, ad T is abslute temperature (K). H ad S were btaied frm the slpe ad itercept f the plt f Gibbs free eergy chage, G vs. temperature, T (Fig. 11). The calculated results are reprted i Table 3. The egative values f G cfirm the feasibility f the prcess ad the sptaeus ature f adsrpti with a high preferece fr Cr (VI) t EMCMCR. The stadard ethalpy ad etrpy chages f adsrpti determied frm Fig. 11 were kj ml 1 ad J ml 1 K 1, respectively, with a crrelati cefficiet f The value f H is egative, idicatig that the adsrpti reacti is exthermic. The psitive value f S reflects a icrease i the radmess at the slid/sluti iterface durig the adsrpti prcess [37] Desrpti studies Desrpti was carried ut with EDTA, HCl, ad ah. It was bserved that ly 69.76% f the laded Cr (VI) stripped i 0.1 M EDTA ad 15.19% stripped with 0.1 HCl, while 87.96% stripped with 0.1 ah i the first cycle. Perhaps due t the speciati f chrmium ad the surface charge f the adsrbet have chaged it the alkalie medium which wuld weake the electrstatic iteracti betwee the EMCMCR ad the Cr (VI) is, prmtig desrpti. Hece, further experimets were carried ut ly with 0.1 ah slutis. Cmplete desrpti was t pssible, perhaps due t the ivlvemet f -electrstatic frces betwee the EMCMCR ad the Cr (VI) is [38]. The regeerated EMCMCR was reused fr up t 10 adsrpti desrpti cycles ad the results are illustrated i Fig. 12. It was fud that the adsrp-

8 X.-j. Hu et al. / Jural f Hazardus Materials 185 (2011) Table 3 Thermdyamic parameters fr the adsrpti f Cr (VI) EMCMCR at differet temperatures. (Vlume, 100 ml; absrbet dse, 0.5 g; iitial ccetrati, 20, 40, 60, 100, 200 mg L 1 ; ctact time, 60 mi; temperature, 293, 303, 313 K; agitati speed, 250 rpm.). Temperature ( C) K L (L mg 1 ) G (KJ ml 1 ) H (KJ ml 1 ) S (J K 1 ml 1 ) R Ackwledgemets The authrs wuld like t thak fiacial supprt frm the atial atural Sciece Fudati f Chia (Grat s ad ) ad the Scietific Research Fud f Hua Prvicial Educati Departmet (Grat. 06B082) ad the Hua Uiversity ability traiig prjects by the basic perati csts f cetral Clleges ad Uiversities fr Scietific Research (Grat. hdzy0903). Refereces Fig. 12. Adsrpti desrpti cycles fr EMCMCR. (Adsrpti cditis: vlume, 100 ml; absrbet dse, 0.5 g; iitial ccetrati, 300 mg L 1 ; ctact time, 60 mi; temperature, 303 K; agitati speed, 250 rpm. Desrpti cditis: trippig sluti 0.1 ah; vlume, 20 ml; ctact time, 30 mi; temperature, 303 K; agitati speed, 250 rpm.) ti capacity f the EMCMCR was a 8.87% decrease after the first cycle ad a 15.40% decrease after the secd cycle. It culd still be maitaied at 75.73% level at the 10th cycle. These results shwed that the EMCMCR ca be successfully regeerated ad repeatedly used i Cr (VI) is adsrpti studies withut appreciable lsses i their adsrpti capacities. 4. Cclusis The preset study fcuses adsrpti f Cr (VI) frm aqueus sluti usig the EMCMCR as a effectively adsrbet. Adsrpti f Cr (VI) is fud t be effective i the lwer ph rage ad at lwer temperatures. I additi, the very fast adsrpti ad settlig fr the EMCMCR make this material suitable fr ctiuus flw water treatmet systems. Equilibrium istherm data were fitted usig differet tw-parameter mdels. Amg these mdels, Lagmuir mdel ad Temki mdel are i gd agreemet with the experimetal data with high R 2. 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