Equilibrium and Thermodynamic Studies of Cesium Adsorption on Natural Vermiculite and Optimization of Operation Conditions

Transcription

1 Iran. J. Chm. Chm. Eng. Vol. 28, No. 4, 29 Equilibrium and Thrmodynamic Studis of Csium Adsorption on Natural Vrmiculit and Optimization of Opration Conditions Hadadi, Noshin ; Kananpanah, Somaih; Abolghasmi, Hossin* + School of Chmical Enginring, Univrsity Collg of Enginring, Univrsity of Thran, Thran, I.R. IRAN ABSTRACT: Rmoval of csium from synthtic aquous solution through adsorption on vrmiculit, undr batch quilibrium xprimntal condition at six initial valus of ph (3, 4, 6, 9, 11 and 12) and fiv tmpraturs (25, 5, 75, 85 and 95 C) has bn invstigatd. It is ncssary to propos a suitabl modl for a bttr undrstanding of th mchanism of csium adsorption on vrmiculit. For this propos th suitability of th Langmiur, Frundlich and Rdlich-Ptrson (R-P) adsorption modls for quilibrium data wr invstigatd. Th paramtrs in th adoptd adsorption isothrm modls wr dtrmind by Eviws softwar. Th study of quilibrium isothrm shows that th bst modl for analysis of xprimntal data is Rdlich-Ptrson modl with corrlation cofficint highr than.99(both for tmpratur and ph). Th rsults showd that incrasing of ph and tmpratur incrasd th adsorption ability of vrmiculit. Optimum conditions for adsorption wr dtrmind as T=75 C, ph=9, vrmiculit dos=1.5 g and contact tim of 24 hr. Finally th thrmodynamic constants of adsorption phnomna, H and S wr found to b kj/mol and.563 kj/mol K in th rang of C rspctivly. Th ngativ valu of th Gibbs fr nrgy G dmonstrats th spontanous natur of csium adsorption onto vrmiculit. KEY WORDS: Adsorption, Vrmiculit, Csium, Equilibrium. INTRODUCTION Th contamination of watrs by industrial fflunts rich in radionuclid has rcivd much attntion in th last dcads. Th wast watr purification systm most usually mployd by industris is th prcipitating tchniqu. Howvr, in many cass this tratmnt dos not allow raching th lgal limits imposd for radioactiv wastwatrs and vn gnrats a larg amount of sludg difficulty to handl. Mor ffctiv purification systms, such as ion-xchang columns, ar oftn mor xpnsiv and, hnc, unsuitabl for ths industris [1]. For ths rasons, in rcnt yars, many rsarchrs studid th bhavior of natural organic or inorganic matrials having high adsorption capacity and which ar particularly abundant and inxpnsiv in ordr to us * To whom corrspondnc should b addrssd. + hoab@aut.ac.ir /9/3/15 7/$/2.7 29

2 Iran. J. Chm. Chm. Eng. Hadadi, N., t al. Vol. 28, No. 4, 29 thm as low-cost ffctiv sorbnts for th rmoval of radionuclid from various fflunts [2]. Csium is an important radionuclid for svral rasons: it xhibits almost unlimitd solubility; its invntory in radioactiv wast is significant and is xtrmly long-livd. Furthrmor, bcaus of its chmical similarity to K, csium is radily assimilatd by trrstrial and aquatic organisms. Clay minrals ar known to adsorb csium vry wll and thr ar many studis concrning in particular th adsorption of csium on clay minrals, spcially illit, smctit and vrmiculit. [3-6]. Vrmiculit is a clay minral with 2:1 crystallin structurs with high cation xchang capacity, which is a consqunc of surfac and intrlayr ion xchang procsss, as wll as isomorphic substitution. Such proprtis, associatd to high surfac ara, hav mad this matrial th targt of svral adsorption studis aiming for a bttr undrstanding of nvironmntal procss in soils and watrs [7-9]. Th charactristics of adsorption bhavior ar gnrally infrrd in trms of both adsorption kintics and quilibrium isothrms. Thy ar also important tools to undrstand th adsorption mchanism, vis. th thortical valuation and intrprtation of thrmodynamic paramtrs [1, 11]. Equilibrium isothrms Sorption quilibrium is usually dscribd by an isothrm quation whos paramtrs xprss th surfac proprtis and affinity of sorbnt, at a fixd tmpratur and ph [1]. What th adsorption isothrm dscribs is th rlationship btwn th solut concntration q (1/g adsorbnt) in th quilibrium adsorbnt and th solut concntration C in th cas of crtain tmpratur and ph [12]. Th amount of mtal ion sorbd on th vrmiculit, q was computd by th following quation: q υ = (C C ) (1) m Whr C and C ar th initial concntration and th quilibrium concntration of th ions in th liquid phas, rspctivly. Also v and m ar th volum of th solution and wight of th rsin particls, rspctivly. Th quantitativ xplanation for adsorption xprimntal data can b xprssd using diffrnt mathmatical modls, of which Langmuir modl and Frundlich modl ar most widly considrd as th bas for studying adsorption procss and thy ar most commonly usd to dscrib th adsorption charactristics of adsorbnt usd in watr and wastwatr [13]. Th Rdlich-Ptrson quation is a combination of Langmuir and Frundlich modls Langmuir isothrm This modl is obtaind undr th idal assumption of a totally homognous adsorption surfac and it may b rprsntd as follows: q q k adc = (2) 1+ k C ad Th Langmuir constants q and k ad ar rlatd to th adsorption capacity and th nrgy of adsorption, rspctivly. Frundlich isothrm This modl is suitabl for a highly htrognous surfac as wll as multilayr sorption and is xprssd by th following quation: f 1 n q = k C (3) n and k f ar th Frundlich constants. Th fit of data to Frundlich isothrm indicats th htrognity of th sorbnt surfac. Th magnitud of th xponnt 1/n givs an indication of th adquacy and capacity of th adsorbnt/adsorbat systm [14]. Rdlich - Ptrson isothrm Such an isothrm is a combination of two prvious modls [15] which is dfind as follows: K C q = (4) 1+α R β RC Whr R and K R ar th constants of Rdlich-Ptrson and is th xponnt of quation which lis btwn and 1. Whn =1, this quation changs to Langmuir isothrm and whn =, it changs to Hnry quation which is dfind as follows: K RC q = (5) 1+α R 3

3 Iran. J. Chm. Chm. Eng. Equilibrium and Thrmodynamic Vol. 28, No. 4, 29 Tabl 1: Charactrization of vrmiculit. OXIDES SiO 2 Al 2O 3 F 2O 3 TiO 2 MgO CaO K 2O Na 2O H 2O TOTAL PCT N.D Th prsnt work dals with a sris of xprimnts to assss th optimum conditions of vrmiculit for th rmoval of csium from aquous solutions. Th amounts adsorbd at quilibrium sorption wr masurd at diffrnt phs and tmpraturs. Th adsorption isothrms at varying solution tmpraturs and phs wr masurd, and suitabl modl quations wr trid to fit th xprimntal data MATERIALS AND METHODS Adsorbnt Th vrmiculit sampl was supplid by th company, Gilan Mica.Th vrmiculit min is locatd in Amlash, Gilan Provinc, Iran. Th original vrmiculit sampl was charactrizd by X-ray diffraction pattrns. Vrmiculit with a man siz of 1 mm was usd in th ntir study. Th chmical composition of such vrmiculit in wight prcntag is prsntd in tabl 1. Pr-tratmnt of vrmiculit Vrmiculit sampls wr thoroughly washd with distillatd watr to rmov all adhring dirt and thn, wr ovn-drid at 7 C, till achiving constant wight. Th drid vrmiculit was usd without any physical or chmical pr-tratmnt. Solutions Th divalnt cation csium was in chlorid form (Mrck) as ragnt grad was usd without prvious purification. Th rang in concntrations of Cs prpard from stock solution varid btwn and mol.l -1. ADSORPTION EXPERIMENTS Batch adsorption xprimnts wr carrid out by allowing an accuratly wightd amount of vrmiculit to rach quilibrium with Cs solutions of various initial concntrations btwn and mol.l -1. For valuating th influnc of ph, th suspnsion ph was adjustd to 3, 4, 6, 9, 11 and 12 by adding KOH and HNO 3. Th ffct of tmpratur on adsorption procss was also chckd by adjusting th tmpratur of shaking machin on 25, 5, 75, 85 and 95 C. Known wights of vrmiculit (1.5 g) wr addd to 1 ml solution. Th bottls wr shakn in a tmpraturcontrolld shakr at a constant spd of 18 rpm. Prliminary xprimnts showd that adsorption quilibrium was rachd within 24 hr. At th nd of th quilibrium priod th contnts of th bottls wr filtrd. Th uptak of csium was calculatd from th masurd valus of its concntration bfor and aftr quilibration. Th rsults ar xprssd as % Adsorption. C C % Adsorption = 1 (6) C Whr, C and C ar th csium concntration bfor and aftr quilibrium. RESULTS AND DISCUSSION Dtrmination of optimum oprating conditions optimum dosag Th ffct of th adsorbnt dosag on th adsorption of C S on vrmiculit is shown in Fig. 1. Vrmiculit dosag was varid btwn.5 and 2.5 g/1 ml for C S concntration of and quilibratd for 24 hr. It can b rvald from Fig. 1. That th rmoval of csium incras with incras in vrmiculit dosag. Th rmoval of C S at vrmiculit dosag gratr than 1.5 g/1 ml rmains almost constant. Thus th quilibrium concntration is considrd to b 1.5 g/1 ml (93 % Adsorption) for vrmiculit. Optimum contact tim Fig. 2 prsnts th plot of C S rmoval vrsus contact tim for vrmiculit in initial concntrations btwn and mol.l -1 at 25 C. It is rvald that 31

4 Iran. J. Chm. Chm. Eng. Hadadi, N., t al. Vol. 28, No. 4, 29 Cs Rmoval (%) Vrmiculit Dossag (g/1 ml) Fig. 1: Effct of vrmiculit dosag on Cs rmoval (T=298 K; contact tim=24 hr; C = mol L -1 ). Cs rmoval (%) ph Fig. 3: Rmoval of Cs vs. ph (T=25 C; vrmiculit dosag=1.5 g/1 ml). Cs Rmoval (%) Tim (hr) 2 1^-5 mol L-1 4 1^-5 mol L-1 6 1^-5 mol L-1 8 1^-5 mol L-1 Fig. 2: Rmoval of Cs vs. contact tim (T=298 K; vrmiculit dosag=1.5 g/1 ml). thr was no considrabl chang for C S rmoval aftr 24 hr of contact tim for diffrnt initial concntrations. Optimum ph On of th most important paramtrs in th adsorption procss is initial ph of solution. For ach mtal ion, thr is a spcific ph valu at which th maximum adsorption of that ion taks plac. This ph taks plac in basic rgion (i.. at high ph valus) for th cationic mtals; it taks plac as an xcptional cas at th low ph valus for th anionic mtals. Th adsorption of csium on to vrmiculit is dpndnt on th ph of th solution, which affcts th surfac charg of th adsorbnt, dgr of ionization and spciation of th adsorbnt spcis [16-18]. Solutions with valus of ph (3, 4, 6, 9, 11 and 12) and sam concntration (4 1-5 mol/l), containing 1.5 g of vrmiculit wr prpard and placd into shaking machin by adjusting th tmpratur at 25 C. Chang of % Cs rmoval as a function of ph is givn in Fig. 3. This curv shows that incras in th initial ph of th Cs rmoval (%) T ( C) Fig. 4: Rmoval of Cs vs. Tmpratur (ph 9; vrmiculit dosag=1.5 g/1 ml). solution from 3 to 9 causd incrasing in th prcntag of Cs ions adsorption. Adsorption of Cs ions was about 95 % at ph of 9, and it dcrasd to 84 %, 7 % and lowr valus with th incrasing ph valus. This may b xplaind by th influnc of solution ph on th surfac charg distribution which is strongly rlatd with th mtal ions rmoval. At ph 9, du to th xcss amount of OH ions within th mdium, th activ sit on th vrmiculit is ngativly chargd. This causs a strong attraction btwn ths sits and positivly chargd Cs ions. Thrfor ph of 9 is th optimum ph of solution. Optimum tmpratur Th othr ffctiv paramtr is tmpratur of adsorption procss. Effct of tmpratur on adsorption capacity of vrmiculit was invstigatd at mol/l initial concntration by using 1.5 g of vrmiculit at th tmpraturs of 25, 5, 75, 85 and 95 C. th rsults at this xprimnt ar givn in Fig

5 Iran. J. Chm. Chm. Eng. Equilibrium and Thrmodynamic Vol. 28, No. 4, 29 q 1 5 (mol/g) C 1 5 (mol/l) ph=9 ph=6 ph=3 Fig. 5: Effct of initial ph on quilibrium adsorption on vrmiculit(t=298 K). q (mol/g) Exprimnt Fittd C (mol/l) 1-5 Fig. 7: Comparing an xprimntal isothrm curv and its corrsponding Rdlich-Ptrson profil at ph 9. q C 5 C 25 C C 1 5 q (mol/g) Exprimnt modl C (mol/l) 1-5 Fig. 6: Effct of initial tmpratur on quilibrium adsorption on vrmiculit (ph=9). This figur shows that thr is a slight incras in quilibrium concntration whn tmpratur is changd from 25 to 75 C and thn th changs wr almost constant. Kintic nrgis of Cs ions wr low at low tmpraturs. Thrfor T=75 C is th optimum tmpratur for this adsorption systm. It is vry difficult and tim-consuming procss for ions to rach th activ sits on th adsorbnt. Incras in tmpratur causs incras in th mobility of th ions. Equilibrium study Th quilibrium adsorption of C S ion onto vrmiculit at six initial ph solution (3, 4, 6, 9, 11 and 12) and fiv tmpraturs (25, 5, 75, 85 and 95 C) was invstigatd. In ordr to optimiz th dsign of sorption systm to rmov C S ions from fflunts, it is important to stablish th most appropriat corrlation for th quilibrium curv. In this part, basd on th rsults of optimization studis, thr phs (3, 6, 9) and thr tmpraturs (25, 5, 75 C) wr slctd for Fig. 8: Comparing an xprimntal isothrm curv and its corrsponding Rdlich-Ptrson profil at T= 75 C. quilibrium studis. Th rsults wr shown in Figs. 5 and 6. Ths figurs show th adsorption isothrms of C S ions at diffrnt phs and varying solution tmpraturs rspctivly. Also th xprimntal data wr fittd with various isothrm modls. Th adsorption isothrm paramtrs ar calculatd with EViws program. (Listd in tabls 2, 3). Th corrlation cofficints paramtrs in tabls 2 and 3 dmonstrat that all of th modls ar accptabl (R 2 >.9, both in diffrnt phs and tmpraturs). Furthrmor, th paramtrs in th Rdlich-Ptrson modl hav bn found to dscrib th xprimntal data vry wll. Figs. 7 and 8 show a plot comparing an xprimntal isothrm curv and its corrsponding Rdlich-Ptrson profil at optimum ph and tmpratur rspctivly. Dtrmination of thrmodynamic paramtrs Th Gibbs fr nrgy chang G indicats th dgr of th spontanity of adsorption procss. 33

6 Iran. J. Chm. Chm. Eng. Hadadi, N., t al. Vol. 28, No. 4, 29 Tabl 2: Isothrm paramtrs for Cs adsorption on vrmiculit at diffrnt ph. Isothrmal ph Modl paramtrs Langmuir 3 q=.768; k ad=1.973; R 2 =.955; Adjustd R 2 = q=1.75; k ad =.32; R 2 =.998; Adjustd R 2 = q=1.741; k ad =.344; R 2 =.994; Adjustd R 2 =.992 Frundlich 3 k f=.2; n=1.695; R 2 =.974; Adjustd R 2 = k f=.258; n=1.52; R 2 =.987; Adjustd R 2 = k f=.457; n=2.465; R 2 =.999; Adjustd R 2 =.999 Rdlich- Ptrson 3 k r=1.994; r=9.473; =1.555; R 2 =.999; Adjustd R 2 = k r=1.96; r=6.568; =1.393; R 2 =.999; Adjustd R 2 = k r=.463; r=.9; =.62; R 2 =.999; Adjustd R 2 =.998 Tabl 3: Isothrm paramtrs for Cs adsorption on vrmiculit at diffrnt tmpratur. Isothrmal modl Tmpratur (K) Modl paramtrs Langmuir 298 q=.646; k ad =.57; R 2 =.989; Adjustd R 2 = q=.754; k ad =1.14; R 2 =.992; Adjustd R 2 = q=.87; k ad =1.377; R 2 =.983; Adjustd R 2 =.975 Frundlich 298 q=.266; b=.556; R 2 =.999; Adjustd R 2 = q=.332; b=.49; R 2 =.971; Adjustd R 2 = q=.45; b=.443; R 2 =.994; Adjustd R 2 =.991 Rdlich- Ptrson 298 k r=.311; r=.163; =.55; R 2 =.999; Adjustd R 2 = k r=.583; r=.648; =.928; R 2 =.992; Adjustd R 2 = k r=.51; r=.26; =.688; R 2 =.997; Adjustd R 2 =.992 For significant adsorption to occur, th fr nrgy changs of adsorption G must b ngativ. Th Gibbs fr nrgy chang can b calculatd by th following quation: G = RT lnk ad (7) Whr K ad, adsorption quilibrium constant (drivd from Langmuir modl);t, absolut tmpratur, K; R,gas constant , kj/(mol K). Th rlation among K ad and thrmodynamic paramtrs H (nthalpy) as wll as S (ntropy) can b dscribd using Van't Hoff formul. ln K ad = S/R H/RT (8) Th valus of H and S wr obtaind from th slop and intrcpt of ln(k ad ) vrsus l/t plot (Fig. 9). H is th positiv valu, whrby proving that th procss of csium adsorption on vrmiculit is ndothrmic procss. In addition, it can b known from calculatd valu H=2.672 kj/mol that th abov procss is physical adsorption rathr than chmical adsorption. G is th ngativ valus with th gnral trnd apparing to b th ngativ growth, whrby proving that th abov procss is th spontanous procss; and S is th 34

7 Iran. J. Chm. Chm. Eng. Equilibrium and Thrmodynamic Vol. 28, No. 4, 29 Tabl 4: Thrmodynamic paramtrs for adsorption procss of csium onto vrmiculit. Tmpratur (K) K ad G (kjmol -1 ) H (kjmol -1 ) S (kmol -1 K -1 ) R Ln kad /T (1/K) Fig. 9: Van't Hoff straight lin. positiv valus thus, showing that procss is th ntropy incrasing procss. Th valus of thrmo-dynamic paramtrs ar prsntd in tabl 4. CONCLUSIONS Th isothrms for th adsorption of csium from aquous solution by vrmiculit hav bn analyzd. Optimum adsorbnt dos obsrvd as 1.5 g/1 ml of solution for C S oncntration of Th quilibrium btwn vrmiculit and csium was achivd in approximatly 24 hr with 93 % rmoval of csium. Basd on th prsnt invstigation, it can b concludd that vrmiculit xhibits good sorption proprtis for csium ions in aquous solution. Many xprimntal factors influnc th xtnt of C S uptak, such as ph and tmpratur. Rsults showd that ph of 9. and T = 75 C ar th optimal ph and tmpratur for adsorption of Cs on vrmiculit. It is ncssary, hnc, to considr ths factors to ffctivly prdict th bhavior of this sorbnt in purification systms of wastwatrs contaminatd by C S. Rdlich-ptrson modl is th bst modl among thr prsntd quilibrium modls in this study to dscrib Cs/vrmiculit adsorption systm, as it has th maximum R 2 valu. Th ngativ valu of G indicats th fasibility and spontanity of th adsorption procss. Th positiv valu of H suggsts, adsorption on vrmiculit is ndothrmic natur of th adsorption procss. Rcivd : 6 th March 28 ; Accptd : 26 th Sptmbr 28 REFRENCES [1] Vjsada, J., Hradil, D., Adsorption of Csium on Czch Smctit-Rich Clays-A Comparativ Study, Applid Clay Scinc, 3, 53 (25). [2] M.G. da Fonsca, M.M. d Olivira, Natural Vrmiculit as an Exchangr Support for Havy Cations in Aquous Solution, J. Colloid and Intrfac Scinc, 285, 5 (25). [3] Poinssot, Ch., Bayns, B., Exprimntal and Modlling Studis of Casium Sorption on Illit, Gochimica t Cosmochimica Acta, 63, 3217 (1999). [4] Sikalidis, C. A., Misalids, P., Casium Slctivity and Fixation by Vrmiculit in th Prsnc of Various Compting Cations, Environ. Pollut., 52, 67 (1988). [5] Bradbury, M.H., Bayns, B., A Gnralisd Sorption Modl for th Concntration Dpndnt Uptak of Casium by Argillacous Rocks, J. Contaminant Hydrology, 42, 141 (2). [6] Brigatti, M.F., Laurora, A., Adsorption of [Al(Ura)6]3+ and [Cr(Ura)6]3+ Complxs in th Vrmiculit Intrlayr, Applid Clay Scinc, 3, 21 (25). [7] Malandrinoa, M., Abollino, O., Adsorption of Havy Mtals on Vrmiculit: Influnc of ph and Organic Ligands, J. Colloid and Intrfac Scinc, 299, 573 (26). [8] Koning, A., Comans, R. N. J., Rvrsibility of Radiocasium Sorption on Illit, Gochimica t Cosmochimica Acta, 68, 2815 (24). [9] Abat, G., Masini, J. C., Influnc of ph, Ionic Strngth and Humic Acid on Adsorption of Cd(II) 35

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