Adsorption of Metal Ions on Organo Functionalized Silica Gel and Ordered Mesoporous Silica (MCM-41 and SBA-15)

Transcription

1 O O O Si R O O Si R M 2+ O M 2+ O O Si R O O Si R OH OCH 3 OH OCH 3 R=-NH 2,-SH M=Cu,Hg Adsorption of Mtal Ions on Organo Functionalizd Silica Gl and Ordrd Msoporous Silica (MCM-41 and SBA-15)

2 3.1. Introduction Th toxic ffct of havy mtals intak prsnt many risks to humans, particularly in th cas of watr and food. Evn low concntrations of havy mtals (ppb) in natural watr supplis can hav dtrimntal ffcts on wildlif and humans. Many transition mtal ions ar known to b toxic in humans and ar of grat nvironmntal concrn. Th rsarch & dvlopmnt fforts on rmoval of toxic mtal ions from contaminatd watr hav rcivd much attntion. On promising tchniqu for achiving th rmoval of low lvls of mtal ions from watr is to trap thm using donor ligands modifid on th surfac of msostructural matrials [2]. Coppr is an ssntial trac lmnt for humans, plants and animals. It occurs naturally throughout th nvironmnt, in soil, watr, and air. Coppr compounds ar gnrally mployd to trat plant disass, for watr tratmnt and as prsrvativs for wood and lathr. Ditary ingstion of coppr is indispnsabl for good halth. Howvr, high amounts of coppr intak can b harmful, causing irritation of nos and throat, nausa, vomiting, and diarrha. Vry high doss of coppr can caus damag to livr and kidnys. Humans ar commonly xposd to coppr through drinking watr, brathing air, foods ating, or having skin contact with coppr, particulats attachd to coppr, or coppr-containing compounds. Undr th circumstancs th dtrmination of trac amounts of coppr in svral matrics is fundamntal to idntify th mtal contamination in nvironmnt, watr or human body [1, 3, 4]. Mrcury is anothr class of poisons that prsnt th gratst hazard to human halth, and its concntration must thrfor b kpt undr constant control [5]. High amounts of mrcury can b harmful, causing irritation of nos and throat, nausa, and diarrha. Mrcury is of particular toxicological intrst bcaus it occurs naturally in th nvironmnt and xists in svral chmical and physical forms that dtrmin its absorption and mtabolism. To sparat and immobiliz mrcury ions from wastwatr is vry important to nvironmnt protction [6]. Th prparation of bttr adsorbnts that ar capabl of rmoving havy mtal ions from aquous nvironmnts has attractd a grat dal of intrst in wastwatr tratmnt. Among various adsorbnt matrials, msoporous silicas (silica gl, M41S, HMS and SBA) appar to hav svral advantags in various aspcts such as good priodicitis in thir msoporous channls, larg surfac aras, high porositis, narrow distributions of th por siz, and asy prparation as wll as modification [7]. Sinc th silanol groups do not form bonds with most of havy mtal ions, modification of th surfac Manu V. 110 Ph. D. Thsis

3 with functional groups (ligands) allows th adsorption of mtals and thir rmoval. As for th ligands, thiol-functionality is th most promising candidat du to its strong mtal-sulfid intraction. Indd, thiol-functionalizd silicas hav bn xamind as candidats for mrcury, silvr and lad rmdiation. Othr transition mtal ions such as Cu 2+, Zn 2+ and Ni 2+ can b fficintly rmovd by aminofunctionalizd msoporous silicas, providing a good coordination ability of amin donors toward ths ions [2, 8-11]. Highly ffctiv Hg 2+ uptak by mans of mrcaptopropyl-functionalizd monolayr on ordrd msoporous silica or mrcaptopropyl-graftd hxagonal msoporous silica was indd obsrvd, dmonstrating th ability of th adsorbnts to bind mrcury ions quantitativly to ach thiol ligand sit in th matrial bcaus of its uniform and larg framwork structur [12] Adsorption Studis of Mtal Ions on Surfac Functionalizd Msoporous Silica This chaptr discusss th application of amino and thiol functionalizd msoporous silica gl, and surfactant tmplatd ordrd msoporous silica for th adsorption of Cu 2+ and Hg 2+ ions from watr. Effct of surfac concntration of th functional groups on th adsorption bhavior of mtal ions has bn studid Matrials Chmicals usd for th adsorption study of mtal ions on various adsorbnts ar givn in tabl 3.1 Tabl 3.1. Ragnts usd for th study of mtal ions adsorption on functionalizd msoporous silica Chmicals Grad Supplirs Chmical formula Coppr sulphat AR S. d. Fin Chm, Ltd, India CuSO 4 5H 2 O Mrcury chlorid AR S. d. Fin Chm, Ltd, India HgCl 2 Hxamin AR S. d. Fin Chm, Ltd, India C 6 H 12 N 4 Xylnol Orang AR S. d. Fin Chm, Ltd, India C 31 H 32 N 2 O 13 S Ammonia AR S. d. Fin Chm, Ltd, India NH 3 Ethylndiaminttraactic acid AR Qualigns Chm. Ltd. India C 10 H 16 N 2 O 8 Manu V. 111 Ph. D. Thsis

4 Environmntal pollution as a rsult of rapid tchnological dvlopmnt is a srious concrn for cology. Havy mtal ion contamination rprsnts a significant thrat to th cosystm and spcially to human bing and othr living organisms. A wid varity of tchniqus to rmov havy mtals from watr is availabl such as ion xchang, rvrs osmosis and nanofiltration, prcipitation, coagulation/coprcipitation, and adsorption. Amongst ths, adsorption basd tchnologis ar th most prominntly usd tchnologis for th rmoval of mtals bcaus of thir fficint rmoval capacity and ovrall low cost. Adsorbnts also can potntially b rgnratd furthr adding to cost savings. During th last twnty yars nw classs of solid adsorbnts hav bn dvlopd as microporous, msoporous and nanoporous matrials, such as modifid silica gl, ordrd msoporous silica. Ths adsorbnts ar normally prpard by attaching organic and inorganic molculs to thir surfac. Th synthsizd functional matrials can ffctivly b usd to rmov spcific toxic mtal ions from aquous mdia, thus ths proprtis rvivd grat intrst for chmical sparation and nvironmntal clanup applications Kintic Study of Mtal ions on Diffrnt Adsorbnts Th ffct of contact tim btwn M 2+ solution (Cu 2+ and Hg 2+ ) solution and adsorbnt along with th influnc of initial mtal concntrations wr invstigatd in ordr to analyz th adsorption kintics of mtal ions for adsorbnts. Known wight of matrials (~0.1 g) was takn in 50 ml vial and was quilibratd with 25 ml aquous solutions of diffrnt concntration of mtal salts (CuSO 4 5H 2 O and HgCl 2 ). Systms wr kpt at 30 ± 2 ºC for diffrnt tim intrvals in th rang of (15 min to 1440 min) whil shaking intrmittntly. Concntration of Cu 2+ in th solution bfor and aftr quilibrium was dtrmind using ICPOES Prkin-Elmr optical mission spctromtr optima 2000DV. Concntration of Hg 2+ in th solution was dtrmind by EDTA complxomtiric titration [13]. Adsorption of M 2+ (Q ) was calculatd using Eqn. 1. Q ( C C ) 0 V = (1) m Whr, Q = quantity of M 2+ ions adsorbd on th adsorbnt at th quilibrium tim (mg/g); C 0 = initial concntration of M 2+ in aquous solution (mg/dm 3 ); Manu V. 112 Ph. D. Thsis

5 C = final concntration of M 2+ in aquous solution at th tim of quilibrium (mg/dm 3 ); V = volum of th solution (dm 3 ); m = mass of adsorbnts (g) Plots of quantity adsorbd Q t (mg/g) at tim t vs tim t (min) wr prpard. Adsorption rat has bn analyzd by using two common smi mpirical kintic modls, which ar basd on adsorption quilibrium capacity: psudo first ordr and psudo scond ordr quations givn in tabl 3.2 [14]. Tabl 3.2. Psudo-first- and psudo scond ordr rat xprssions Eqn. 2 Q dt Psudo first ordr kintic modl Exprssion intgratd form linar form t K1t = k1(q Q t ) Qt Q (1 ) = ln( Q Q t ) = ln Q k1t Q and Q t ar, th adsorbd amounts of mtal ions at quilibrium and tim t rspctivly, K 1 is th rat constant (min -1 ) Eqn. 3 Psudo scond ordr kintic modl Exprssion intgratd form linar form Q t = k2 ( Q Qt ) = + k 2t dt (Q Q ) Q K 2 is th rat constant (g mg -1 min -1 ) t t Q t 1 = k Q Adsorption Isothrm of M 2+ on Diffrnt Adsorbnts Adsorption study of M 2+ was carrid out by batch xprimnts. Known wight ( ~0.1 g) of adsorbnts was takn in 50 ml vial, quilibratd with 25 ml aquous solutions of diffrnt concntration of mtal salt (CuSO 4 5H 2 O, HgCl 2 ) for 24 h at room tmpratur (30 ± 2 ºC) whil shaking vial intrmittntly. Adsorption of M 2+ (Q ) was calculatd using quation (1). Adsorption isothrms wr obtaind by plotting valus of quantity adsorbd, Q a (mg/g) vs. quilibrium concntration, C (mg/dm 3 ). Th isothrms modls of Langmuir (Eqn. 4), Frundlich (Eqn. 5), Sips, (Eqn. 6) Rdlich-Ptrson (Eqn. 7) and Tamkin (Eqn. 8) wr usd to dscrib th quilibrium adsorption. Th quations of Langmuir, Frundlich, Sips, Rdlich- Ptrson and Tamkin modl isothrms ar givn tabl t Q Manu V. 113 Ph. D. Thsis

6 Tabl Equilibrium modl quations; transfrrd y and x valus, slop and intrcpt for th isothrm quations; and dimnsions and manings of quation constants Eqn. 4 Langmuir Isothrm [1-5] Q QmK LC = 1+K C L C y = Q x = C Slop = 1 Q m 1 Intrcpt = (Q K m L ) Eqn. 5 Frundlich Isothrm [15-19] F 1 n F Q = K C y = ln(q ) = ln(c ) Eqn. 6 Sips isothrm [15, 20, 22] Slop = 1 n x F Intrcpt = ln K F Q = QmK C 1+ a 1/ ns S 1/ ns sc Y = C Q 1/ ns 1/ n x = C S 1 Slop = Qm 1 Intrcpt = (Q K m S ) Eqn. 7 Rdlich Ptrson [15, 18, 22] Q K RPC = 1+ α C β C y = ln K 1 RP Q x = ln(c ) Slop = β Intrcpt = ln(α) Eqn. 8 Tmkin [18, 22, 24] Q = ( RT / bt ) ln( KTC) Y = Q x = ln C RT Slop = b T (RT ln K Intrcpt = b C = Concntration at quilibrium (mg/l); Q = Equilibrium adsorption capacity (mg/g) K L = Langmuir adsorption constant (L/mg); Q m = Maximum adsorption capacity (mg/g) K F = Frundlich constant (L/g); n F = Htrognity factor of adsorption sits(dimnsionlss); K S = Sips constant rlatd with affinity constant (mg /dm 3 ) -1/n ; n S = Sips xponnt (dimnsionlss; K RP = constant that is varid to maximiz th linar corrlation cofficint (r 2 ), (L/g); α = constant(mg/l) -β ; β = Rdlich-Ptrson xponnt (dimnsionlss); K T = quilibrium binding constant (dm 3 /g); b T = is rlatd to th hat of adsorption (j/mol) T T ) Manu V. 114 Ph. D. Thsis

7 In ordr to valuat th rror function of diffrnt modl adsorption modls using Eqn. 9. Th distribution cofficint, K d is a vry important paramtr for stimating th affinity of th adsorbnt for adsorbat in aquous solution, K d can b calculatd using Eqn. 9. Prcntag rmoval of mtal ions from th solution at diffrnt initial concntration was calculatd using Eqn. 10. Evaluation of rror function, distribution cofficint, prcntag rmoval capacity of mtal ions by adsorbnt was valuatd using Eqn. 9, Eqn. 10 and Eqn. 11 shown in tabl 3.4. Tabl 3.4. Equation for dtrmination of diffrnt paramtrs Eq. No. Equation nam Equation Rfrnc 9 Error function F rror = p i (Q cal Q xp/ Q P xp) 2 [19] 10 Distribution cofficint Q K d = x10 C 3 [10,19] 11 Prcntag rmoval % R moval = ( C C ) x100 C 0 0 [10,19] Whr, P = th numbr of data points; Q xp = xprimntal valu of quantity adsorbd (mg/g); Q cal = prdictd valu of quantity adsorbd (mg/g); Q = Quantity adsorbd at quilibrium (mg/g), C = Equilibrium concntration (mg/l); C 0 = initial concntration of in solution (mg/l) 3.3. Rsults and Discussion In th cas of silica gl, two diffrnt approachs wr usd for th synthsis of amino functionalizd silica gl. In th first approach (A) th silica gl was tratd at 600 C to obtain surfac having isolatd silanol groups. This silica gl was functionizd by varring th concntration of APTMS. In th scond approach (B) th concntration of surfac silanol group was varid by calcining th silica gl sampls or 6 h at four diffrnt tmpraturs 150, 300, 450 and 600 ºC and using ths calcind sampls for th surfac functionalization. In this cas th xcss APTMS was usd to saturat th surfac with amino functional groups Adsorption Study of Functionalizd Silica Gl Prpard by Varying th APTMS: Silica Ratio Aftr Calcination at 600 C (Approach A) Silica gl was functionalizd by controlling th APTMS: silica ratio (w/w) viz.0.1 (0.56 mmol/g), 0.2 (1.23 mmol/g) and 0.3 (1.68 mmol/g) and dsignatd as GN1, GN2 and GN3 rspctivly. This matrial was usd for th adsorption of Cu 2+ ions. Txtural Manu V. 115 Ph. D. Thsis

8 proprtis and amino group loadings of silica gl (G) and functionalizd silica gls summarizd in tabl 3.5. Tabl 3.5. Txtural proprtis and amino group loadings of silica gl (G) and functionalizd silica gls (GN1, GN2 and GN3) Nam sampls BET SA # (m 2 /g) TPV # (cm 3 /g) BJH PS # (Å) NH 2 loading (mmol/g) G GN GN GN # SA Surfac ara, TPV- Total por volum, PS- Por siz Coppr adsorption kintics Th kintic of coppr adsorption on silica gl with aminopropyl loading 1.01 mmol/g (GN2) was carrid out for two diffrnt initial concntrations of Cu 2+ ions, 1018 and 65 mg/dm 3. Th xprimntal data (Fig. 3.1a) showd a significant dcras in mtal concntration with tim; and quilibrium is stablishd within 3 h. No significant chang in th concntration of Cu 2+ was obsrvd aftr 3 h of contact tim. Rsults wr analyzd using psudo first ordr (tabl 3.2, Eqn. 2) and psudo scond ordr (tabl 3.2, Eqn. 3) kintic modls. Th linarity of th plot of th quations can validat th applid modal. Psudo first ordr modal was not found suitabl to dscrib th kintic profil as it did not showd apparnt linar bhavior (Fig 3.2). Whras rats of aquous Cu 2+ adsorption is accuratly dscribd by psudo scond ordr kintics (Fig. 3.1b) for both th initial concntration studid, similar rsults for Cu 2+ on monoamin and diamin functionalizd ordrd msoporous silica wr rportd [10,14]. Tabl 3.6 summarizs th calculatd valus of Cu 2+ adsorbd (Q cal ), k 2, and rgrssion cofficint R 2 obtaind from linar fits i.. psudo scond ordr plot, for ach initial coppr concntrations (C 0 ). Manu V. 116 Ph. D. Thsis

9 Figur: 3.1. Kintic study of Cu 2+ on aminopropyl functionalizd silica gls GN2; (a) ffct of contact tim on adsorption of Cu 2+ ; (b) psudo scond ordr plot for th adsorption of Cu 2+ Figur: 3.2 Linar plots of psudo first ordr kintic modl for Cu(II) adsorption on GN2 at two diffrnt initial concntration Ths rsults indicatd good corrlation and xprimntal valus of Cu 2+ adsorbd at quilibrium, (Q xp ) wr comparabl with that of (Q cal ). Psudo scond ordr constants, k 2 dcrasd from to g mmol -1 min -1 whn initial Cu 2+ concntration riss from 65 to 1018 mg/dm 3. Manu V. 117 Ph. D. Thsis

10 Tabl.3.6. Effct of initial Cu 2+ concntration on adsorption kintics C 0 (mg/dm 3 ) Q cal Q (mmol/g) k 2 (g mmol -1 min -1 ) Q xp Chaptr Adsorption Isothrm Adsorption of mtal ions on amino functionalizd silica is ph dpndnt [25]. In th cas of bivalnt mtal salts, concntration of bivalnt cationic spcis is vry high as compard to hydroxyl spcis and thrfor adsorption of Cu 2+ on functionalizd silica gl was carrid out at ph 5 [9]. Th ph of initial Cu 2+ solutions: 1018 mg/dm 3 and 65 mg/dm 3 wr found to b 4.5 and 5.1 rspctivly. Thrfor, in th prsnt invstigation adsorption study was carrid out undr its natural ph. Th ph of th solution in contact with th adsorbnt was also monitord at diffrnt tim intrvals (tabl 3.7). In th cas of lowr concntration, thr was a slight incras in th ph aftr 1 h of contact tim; howvr, aftr 1 h ph was stabilizd to ~5.4. In th cas of highr coppr ion concntration, during first hour an incras in ph was obsrvd but gradually ph was dcrasd to 4.3 aftr 24 h. Howvr, ths valus ar wll within th xprimntal rror. Tabl ph of th CuSO 4 solutions in contact with functionalizd silica gl GN2 at diffrnt tim intrval Tim ph (h) (65 mg Cu 2+ /dm 3 ) (1018 mg Cu 2+ /dm 3 ) 0* *Initial ph of th CuSO 4 solution bfor contacting to th adsorbnt Though th quilibrium stablishd in 3 h for GN2 sampl as can b sn from Fig. 3.2a, quilibrium tim of 24 h was kpt for th adsorption isothrms of all th thr sampls. Th adsorption isothrm of Cu 2+ on GN1, GN2 and GN3 ar shown in Fig Th analysis of quilibrium data is ssntial to dvlop an quation that can b usd to compar diffrnt adsorbnts. To xamin th rlationship btwn sorption and aquous concntration at quilibrium, various sorption isothrm modls ar widly mployd for fitting th data. In th prsnt Manu V. 118 Ph. D. Thsis R 2

11 work, a two-paramtr modls (Langmuir (Eqn. 4) and Frundlich (Eqn.5) in tabl 3.3) and thr paramtrs modals (Sips (Eqn. 6) in tabl 3.3) isothrms wr usd to dscrib th quilibrium btwn th Cu 2+ and functionalizd silica gl sampls in aquous Cu 2+ in th solution. Th valus of diffrnt paramtrs of all th modls usd to dscrib th adsorption isothrms ar prsntd in tabl 3.8. In ordr to valuat which modl is fittd wll, th rror function was calculatd using Eqn. 9 (tabl 3.4). Th lowr is th rror function; th lowr will b th diffrnc of th Q valus (quantity adsorbd masurd xprimntally and calculatd by th modl). Th valus of rror function calculatd using Sips modl for all th isothrms wr found to b lowst indicatd that Sips modl was bst fittd amongst all th thr modls. It can b also obsrvd (tabl 3.8) that th valus of R 2 for all th modls xcpt Frundlich wr about Sips plots for all th thr functionalizd silica gls indicatd a bst mathmatical fit for isothrms and hnc th monolayr capacity Q m for all th isothrms was calculatd using Sips modl, considrd to b th narst to th ral valus of monolayr adsorption capacity. Figur: 3.3. Adsorption isothrms of Cu 2+ on aminopropyl functionalizd silica gls GN1, GN2 and GN3 Points show xprimntal valus, lins show calculatd plot using Sips modl Manu V. 119 Ph. D. Thsis

12 Tabl 3.8. Modal paramtrs for th adsorption of Cu 2+ on functionalizd silica gl Isothrm Modl GN1 GN2 GN3 Langmuir NH 2 (mmol/g) Q max (mmol/g) K L R Error function NH 2 /Cu Frundlich K F n F R Error function Sips Q max (mmol/g) K S n S R Error function NH 2 /Cu Monolayr adsorption capacity of amino functionalizd silica gl GN1 (0.51mmol NH 2 /g), GN2 (1.01mmol NH 2 /g) and GN3 (1.45mmol NH 2 /g) wr found to b 0.51, 0.55 and 1.05 mmol/g rspctivly. Morovr whn th loading of NH 2 group was incrasd from 0.51 to 1.01mmol/g, monolayr adsorption capacity of Cu 2+ rmaind almost sam and th valu of NH 2 /Cu mol ratio for GN2 (1.84) was found to b around two tims gratr than that of GN1 (0.96). Ths rsults suggstd that, in th cas of GN1, at saturation ML 1 typ complx formation might hav occurrd on th surfac of GN1, whr as on th surfac of GN2 at saturation ML 2 typ complx might hav formd btwn amin and Cu 2+. Kudryavtsv t al. [26] rportd that at small dgr of adsorption of Cu 2+ on th amino functionalizd silica ML 2 typ of complx is formd, which, was convrtd to ML 1 on incrasing th dgr of Cu 2+ adsorption. Howvr, in th cas of GN2 silica gl this phnomnon was not obsrvd, as at saturation NH 2 /Cu mol ratio was found to b narly 2. In th cas of GN3, th NH 2 /Cu mol ratio (1.38) is clos to 1.5. Th probabl complx formation on th surfac of GN1, GN2 and GN3 sampls may b xplaind as formation complxs having 1:1, 2:1 and 3:2 NH 2 :Cu 2+ ratio rspctivly, as proposd by Lam t al., for th intraction of amino functionalizd Manu V. 120 Ph. D. Thsis

13 2- MCM-41 with th CuSO 4 systm and ths complxs ar stabilizd by th SO 4 countr ions [7]. Blitz t al. [9] hav graftd aminopropyl group using two silans, viz, 3-aminopropyldimthylmthoxysilan (APDMS) and 3- aminopropyltrithoxysilan (APTS) having on and thr hydrolysabl groups rspctivly; and latr was tratd with watr to hydrolyz th hydrolysabl group to form cross-linkd polymric typ graftd modifid silica gl. At highr concntration of Cu 2+, adsorption of Cu 2+ on latr has bn rportd to b lss than that on sampl modifid with APDMS. Howvr, at lowr concntration, uptak of Cu 2+ on crosslinkd aminopropyl graftd gl was found to b mor than that of APDMS modifid gl. This diffrnc was attributd to th largr amount of th functional groups prsnt at shortr distanc btwn on anothr on th surfac of cross-linkd matrial and hnc acts as bi-dntat ligand. Similar diffrnc in adsorption bhavior btwn GN1 and GN2 has bn obsrvd in th prsnt invstigation (Fig. 3.3). This study indicat that th numbr of NH 2 group pr unit ara (population dnsity) dtrmins th NH 2 /Cu ratio at saturation of Cu 2+ on th functionalizd silica gl and hnc th saturation or monolayr capacity of th adsorbnt. Evaluation of distribution cofficint, prcntag rmoval capacity of mtal ions by adsorbnt wr valuatd using Eqn. 10, and Eqn. 11 shown in tabl 3.4 Distribution cofficints for fiv diffrnt initial concntrations wr calculatd for all th thr adsorbnts using Eqn. 10 and rsults ar summarizd in tabl 3.9. It showd that with an incras in initial concntration of Cu 2+ ion, C 0 thr was a dcras in K d for all th thr functionalizd silica gls. Similar rsults wr rportd for th Cu 2+ adsorption on monoamin functionalizd ordrd msoporous silica [10] and adsorption of CrO , HAsO 4 on mono, di and tri amin functionalizd MCM-41 silica [27]. With th incrasd in th surfac concntration of amino groups, thr was an incras in th valus of distribution cofficint K d. This indicats that th affinity of th gls for Cu 2+ incrass with an incras in loading of aminopropyl groups. Th prcntag rmoval of Cu 2+ also incrasd with an incras in th loading of aminopropyl groups on th surfac of silica. Th natur of adsorption isothrm (Fig. 3.3), distribution cofficint K d, and prcntag rmoval of Cu 2+ indicat that affinity of th adsorbnts for Cu 2+ incrasd with an incras in th loading of aminopropyl group on th surfac of silica gl. It has bn obsrvd that at initial concntration of ~20 mg/dm 3 th prcntag rmoval of Cu 2+ has bn Manu V. 121 Ph. D. Thsis

14 incrasd from 66.8 to 99.8 % (~1.5 tims) whn loading of amino group incrasd from 0.51 to 1.45 mmol/g (~3 tims). Monolayr adsorption capacity of th functionalizd silica gls, GN1, GN2 and GN3 dos not incras linarly with an incras in loading of aminopropyl groups. In th cas of GN2 though th loading of aminopropyl group was two tims than that of GN1, th valus of monolayr capacity for both th adsorbnts wr almost similar (tabl 3.8). In th cas of GN3, whn loading was incrasd by thr fold, 1.4 tims incras in th monolayr capacity was obsrvd. Howvr, th valus of K d incrasd significantly (6 tims) with an incras in th loading of aminopropyl groups from ~0.5 to ~1 mmol and to ~250 tims whn loading was incrasd to ~1.5 mmol for th initial concntration of ~20 mg/dm 3 Cu 2+ solution (tabl 3.9). Tabl 3.9. Prcntag rmoval of Cu 2+ and distribution cofficint K d for th Cu 2+ adsorption on aminopropyl functionalizd silica gls GN1, GN2 and GN3 C 0 (mg/dm 3 ) C (mg/dm 3 ) Q (mg/g) K d (dm 3 /g) % rmoval of Cu GN GN GN Manu V. 122 Ph. D. Thsis

15 This indicats that for th rmoval of mtal ions from vry dilut solution, th adsorbnt with highr population dnsity of functional groups ar mor ffctiv than adsorbnts with lowr population dnsity of functional groups on thir surfac Effct of Thrmal Tratmnt of Silica Gls on thir Amino Functionalization and Subsqunt Adsorption Proprtis for Cu 2+ from Aquous Solution of Coppr Sulfat (Approach B) In this approach th concntration of surfac silanol group was varid by thrmal tratmnt of th silica gl sampl (calcind for 6 h at four diffrnt tmpraturs 150, 300, 450 and 600 ºC) to bfor using thm for amin functionalization. Ths sampls ar dsignatd as GC1.5, GC3, GC4.5 and GC6 rspctivly. Ths sampls wr furthr usd for th surfac functionalization. In this cas th xcss APTMS was usd to saturat th surfac with amino functional groups [28]. Th sampls wr dsignatd as GC1.5N, GC3N, GC4.5N and GC6N rspctivly. This matrial was usd for th adsorption of Cu 2+ ions. Txtural proprtis and loading of silica gl (G) and functionalizd silica gls summarizd in tabl Tabl Txtural proprtis and loading of diffrnt thrmal tratd silica gl and functionalizd silica gls Nam sampls BET SA # TPV # (cm 3 /g) BJH PS # (Å) NH 2 loading (m 2 /g) (mmol/g) GC GC1.5N GC GC3N GC GC4.5N GC GC6N # SA Surfac ara, TPV- Total por volum, PS- Por siz Coppr Adsorption Kintics Th kintic study of coppr adsorption on silica gl with aminopropyl loading of all th sampls was carrid out by taking 1091 mg/dm 3 as initial concntration of Cu 2+ ions. Th xprimntal data (Fig. 3.4) showd a significant incras in coppr ion adsorption on th surfac of adsorbat with an incras in th Manu V. 123 Ph. D. Thsis

16 contact tim; and quilibrium is stablishd within 6 h in th cas of GC6N. In th cas of GC1.5N and GC3N it is 12 h and for GC4.5N it is 24 h. Adsorption rat has bn analyzd by using two common smi mpirical kintic modls, which ar basd on adsorption quilibrium capacity: th psudo first ordr (Eqn. 2 of tabl 3.2) and psudo scond ordr quations (Eqn. 3 of tabl 3.2) [14]. Psudo first ordr modal was not found suitabl to dscrib th kintic profil as it did not showd apparnt linar bhavior, whras rat of aquous Cu 2+ adsorption ovr all th four functionalizd silica sampls was accuratly dscribd by psudo scond ordr quation. [10, 14]. Tabl 3.11 summarizs th valus of k 2, calculatd valus of Cu 2+ adsorbd at quilibrium (q cal ) using psudo scond ordr quation. Valu of rgrssion cofficint R 2 for all th sampls was gratr than 0.99 obtaind from linar fits, indicats th validity of th psudo scond ordr quation. Th rsults of Q ar in good agrmnt with valus Q m (monolayr capacity of adsorbnt) calculatd from Sips modls (vid supra tabl 3.12) usd for th xprssion of adsorption isothrms xcpt for sampl GC6N. Figur: 3.4. Kintic study of Cu 2+ on aminopropyl functionalizd silica gls (GC1.5N, GC3N, GC4.5N and GC6N). (a) Effct of contact tim on adsorption of Cu 2+ ; (b) psudo scond ordr plot for th adsorption of Cu 2+ Manu V. 124 Ph. D. Thsis

17 Tabl Valus of psudo scond ordr kintics of Cu 2+ Sampls C 0 (mg/dm 3 ) Q (mmol/g) # K 2 (g mmol -1 min -1 ) R 2 GC1.5N GC3N GC4.5N GC6N # Calculatd from kintic rsults using psudo scond ordr quation Adsorption Isothrm Adsorption of mtal ions on amino functionalizd silica is ph dpndnt [25]. Blitz t al. [9] hav studid th adsorption of Cu 2+ at ph 5, sinc in th cas of solution of bivalnt mtal salts, concntration of bivalnt cationic spcis is vry high as compard to hydroxyl spcis. Th initial ph of th solution was 4.6. Thrfor, in th prsnt invstigation adsorption study was carrid out undr its natural ph. Th ph of th solution in contact with th adsorbnt was also monitord at diffrnt tim intrvals (15 to 1440 min) and no significant variation in th ph was obsrvd. GC1.5N, GC3N, GC4.5N and GC6N wr mployd as adsorbnt for Cu 2+ rmoval from aquous solution using a batch adsorption procdur. Figur: 3.5. Adsorption isothrms of Cu 2+ on aminopropyl functionalizd silica gls (GC1.5N, GC3N, GC4.5N and GC6N) Points show xprimntal valus, lins show calculatd plot using Sips modl Manu V. 125 Ph. D. Thsis

18 Th Cu 2+ adsorption isothrms of diffrnt amino functionalizd silica gl ar shown in Fig Two-paramtrs modl (Langmuir, Eqn. 4 in tabl 3.3) and thr paramtrs modals (Sips, Eqn. 6 in tabl 3.3) wr usd to dscrib th quilibrium btwn th Cu 2+ and functionalizd silica gl in aquous Cu 2+ solution. Th isothrms constants, valus of rgrssion cofficint (R 2 ) and valus of rror function ar prsntd in tabl Tabl Modal paramtrs for th adsorption of Cu 2+ on functionalizd silica gl Langmuir GC1.5N GC3N GC4.5 N GC6N NH 2 (mmol/g) Q m (mmol/g) NH 2 /Cu R K L (mg/dm 3 ) Error function E-08 Sips Q m (mmol/g) n S NH 2 /Cu R K S (mg/dm 3 ) -1/n Error function Valu of R 2 is ~0.999 for Langmuir and Sips modls, and valu of rror function for all th four sampls is slightly highr than that of Sips. Valus of rror function calculatd using Sips modl for all th isothrms was found to b lowst xcpt GC6N sampl. Thrfor, Sips modl has bn considrd to b bst fittd amongst all th modls. Thrfor th monolayr capacity Q m for all th isothrms was calculatd using Sips modl, considrd to b th narst to th ral valus of monolayr adsorption capacity. Monolayr adsorption capacity for Cu 2+ of amino functionalizd silica gl dcrasd from 1.56 to 0.95 mmol Cu 2+ /g with th dcras in surfac concntration of amino group from 2.34 to 1.65 mmol NH 2 /g. Th dcras in concntration of amino group was du to th incras in th calcination tmpratur of th raw silica Manu V. 126 Ph. D. Thsis

19 gls. Fig. 3.6 shows a linar rlationship btwn calcination tmpratur and monolayr capacity of Cu 2+. In th cas of approach A it has bn obsrvd that monolayr adsorption capacity of Cu 2+ on amino functionalizd silica gl (silica gl calcind at 600 ºC followd by amino functionalization, having 1.45 mmol NH 2 /g) is 1.05 mmol Cu 2+ /g with NH 2 /Cu (tabl 3.8). NH 2 /Cu ratio of GC1.5N, GC3N and GC4.5N adsorbnts is narly 1.5 M/M (tabl 3.12). Th probabl intraction of amino group and Cu 2+ can b nvisagd as thr nighboring amino groups occupid by two Cu 2+ ions [7]. Howvr, in GC6N, NH 2 /Cu mol ratio is Th probabl complx formation on th surfac of GC6N sampls may b xplaind as formation of two typs of complxs having 3:2 and 2:1 NH 2 :Cu 2+ ratio rspctivly as proposd by Lam t al. [7], for th intraction of amino functionalizd MCM-41 with th CuSO 4 systm and ths complxs ar stabilizd by th SO 2-4 as countr ions. Hr 60 % of amino groups ar ngagd in 3:2 typ of complx formation and rmaining 40 % ar 2:1 typ of complx. Cu 2+ adsorbd (mmol/g) R 2 = Exprimntal Calculatd Tmpratur 0 C Figur: 3.6. Effct of calcinations tmpratur of raw silica gls monolayr adsorption capacity for Cu 2+ on functionalizd silica gls (GC1.5N, GC3N, GC4.5N and GC6N) Distribution cofficints for fiv diffrnt initial concntrations of coppr ions wr calculatd using Eqn. 10 (tabl 3.4) for all th four adsorbnts and rsults ar summarizd in tabl Whn surfac concntration of amino groups was dcrasd from 2.34 to 1.65 mmol/g, dcras in th valus of distribution cofficint K d was obsrvd. Similar rsults wr rportd for th Cu 2+ adsorption on monoamin functionalizd ordrd msoporous silica [10] and adsorption of CrO 2-4, Manu V. 127 Ph. D. Thsis

20 2- HAsO 4 on mono, di, and tri amin functionalizd MCM-41 silica [27]. Whn surfac concntration of amino groups was incrasd, incras in th valus of distribution cofficint K d was obsrvd. This indicats that th affinity of th gls for Cu 2+ incrass with an incras in loading of aminopropyl groups. Tabl Prcntag rmoval of Cu 2+ and distribution cofficint K d for th Cu 2+ adsorption on aminopropyl functionalizd silica gls (GC6N, GC4.5N, GC3N and GC1.5N) GC1.5N GC3N GC4.5N GC6N C 0 (mg/dm 3 ) C (mg/dm 3 ) Q (mg/g) Manu V. 128 Ph. D. Thsis K d (ml/g) % rmoval of Cu Rmoval of Cu 2+ from th solution at diffrnt initial concntration was calculatd using Eqn. 11 (tabl 3.4) and valus of prcntag rmoval of Cu 2+ ar givn in th tabl Th prcntag rmoval of Cu 2+ also dcrasd with a dcras in loading of aminopropyl groups on th surfac of silica. Th natur of adsorption isothrm (Fig. 3.5), distribution cofficint K d, and prcntag rmoval of

21 Cu 2+ indicat that affinity of th adsorbnts for Cu 2+ incrasd with an incras in th loading of aminopropyl group on th surfac of silica gl. It has bn obsrvd that at initial concntration of ~60 mg/dm 3, th prcntag rmoval of Cu 2+ has bn dcrasd from 100 % to 62 % (~1.6 tims) with an incras in calcination tmpratur (tabl 3.13). This indicats that for th rmoval of mtal ions from vry dilut solution, functionalizd silica gls with highr surfac concntration of amino groups would b mor dsirabl Adsorption of Mrcury on Thiol Functionalizd Silica Silica gl calcind at 600 ºC was usd for th surfac functionalization of thiol groups. Thr diffrnt thiol functionalizd silica sampls wr prpard by th raction of silica with MPTS using thr diffrnt MPTS: silica ratio (w/w) viz.0.1 (1.00 mmol/g), 0.2 (1.7 mmol/g) and 0.3 (2.00 mmol/g) and sampls wr dsignatd as GS1, GS2 and GS3 rspctivly. Ths matrials wr usd for th adsorption of Hg 2+. Txtural proprtis and loading of silica gl (G) and thiol functionalizd silica gls summarizd in tabl Tabl 3.14.Txtural proprtis and loading of silica gl (G) and thiol functionalizd silica gls Nam sampls BET SA # (m 2 /g) TPV # (cm 3 /g) BJH PS # (Å) SH loading (mmol/g) G GS GS GS # SA Surfac ara, TPV- Total por volum, PS- Por siz Mrcury Adsorption Kintics Th kintic study of mrcury adsorption on silica gl with thiol loading of all th sampls was carrid out by taking 2019 mg/dm 3 as initial concntration of Hg 2+ ions at room tmpratur (30 ± 2 C). Th xprimntal data (Fig. 3.7) showd a significant incras in mrcury ion adsorption on th surfac of adsorbnt with an incras in th contact tim; and quilibrium is stablishd within 24 h. No significant chang in th concntration of Hg 2+ was obsrvd aftr 24 h of contact tim. Adsorption rat has bn analyzd by th psudo first ordr (Eqn. 2 in tabl 3.2) and psudo scond ordr quations (Eqn. 3 in tabl 3.2). Psudo first ordr Manu V. 129 Ph. D. Thsis

22 quation rlats th adsorption rat to th amount of mtal ion adsorbd at tim t. Psudo first ordr modal was not found suitabl to dscrib th kintic profil as it did not showd apparnt linar bhavior, whras rat of aquous Hg 2+ adsorption ovr functionalizd silica sampl was accuratly dscribd by psudo scond ordr quation for th sam initial concntration [10, 14]. Figur: 3.7. Kintic study of Hg 2+ on thiol functionalizd silica gls (GS1, GS2 and GS3). (a) ffct of contact tim on adsorption of Hg 2+ ; (b) psudo scond ordr plot for th adsorption of Hg 2+ Tabl Valus of psudo scond ordr kintics of Hg 2+ Nam of sampls C 0 (mg/dm 3 ) Q,(mmol/g) K 2 (g mmol -1 min -1 ) R 2 GS GS GS Tabl 3.15 summarizs th valus of k 2, calculatd valus of Hg 2+ adsorbd at quilibrium (q cal ) using psudo scond ordr quation. Valus of rgrssion cofficint R 2 for ach all th sampls wr gratr than 0.99 obtaind from linar fits, indicat th validity of th psudo scond ordr quation. Manu V. 130 Ph. D. Thsis

23 Adsorption Isothrm Th distribution of Hg 2+ ions btwn th liquid and solid phas at quilibrium can b xprssd by adsorption isothrms. In ordr to invstigat th mrcury adsorption ability of th thiol-functionalizd silica gl, aquous HgCl 2 adsorption isothrms hav bn mad up at 30 ± 2 ºC by bath study with th initial mrcury concntration ranging from ~90 to 2000 mg/dm 3. Maximum capacitis of mrcury adsorption for vry matrial hav bn dtrmind from th corrsponding isothrms. Th monolayr adsorption capacity of th functionalizd silica gls (GS1, GS2 and GS3) incrasd linarly with an incras in loading of thiol groups. Th modifid surfac of thiol functioalizd silica containd sulfur as basic cntr ar potntially availabl as chlating groups for th complx formation with mrcury from aquous solution of HgCl 2. Figur: 3.8. Adsorption isothrms of Hg 2+ on thiol functionalizd silica gls GS1, GS2 and GS3 Points show xprimntal valus, lins show calculatd plot using Langmuir modl Th adsorption isothrm of Hg 2+ on GS1, GS2 and GS3 ar shown in Fig To xamin th rlationship btwn sorption and aquous concntration at quilibrium, various sorption isothrm modls ar widly mployd for fitting th data. Manu V. 131 Ph. D. Thsis

24 Two-paramtr modls (Langmuir, Frundlich, and Tamkin Eqns. 4, 5 and 8 rspctivly shown in tabl 3.3) and thr paramtrs modals (Sips and RP Eqn. 6 and 7 rspctivly shown in tabl 3.3) isothrms wr usd to dscrib th quilibrium btwn th Hg 2+ and functionalizd silica gl sampls in aquous Hg 2+ in th solution. Valus of diffrnt paramtrs of all th modls usd to dscrib th adsorption isothrms ar prsntd in tabl In ordr to valuat which modl is fittd wll, th rror function was masurd. Valus of rror function calculatd using Langmuir modl for all th isothrms wr found to b lowst; this indicatd that Langmuir modl was bst fittd amongst all th four modls. Langmuir modl plots for all th thr functionalizd silica gls indicatd a bst mathmatical fit for isothrms and hnc th monolayr capacity Q m for all th isothrms was calculatd using Langmuir modl wr considrd to b th narst to th ral valus of monolayr adsorption capacity (Fig. 3.8). Monolayr adsorption capacity of thiol functionalizd silica gl GS1 (1.00 mmol -SH/g), GS2 (1.7 mmol -SH/g) and GS3 (1.94 mmol -SH/g) wr found to b 0.98, 1.10 and 1.25 mmol/g rspctivly (tabl 3.16). Though thr in an incras in concntration of SH by 70 % only ~10 % incras in monolayr capacity was obsrvd with th valu of -SH/Hg mol ratio for GS1 and GS and 1.67 rspctivly. This may b du to th distribution of th SH groups on th surfac of adsorbnts, in th cas of GS2 th distanc btwn two nighboring SH groups naturally should b smallr than that of GS1 and would facilitat th ML 2, and M 2 L 3 typ of association which rsultd in avrag occupancy of 1.7 SH groups pr on Hg 2+ ion, which is 1 for GS1. Monolayr adsorption capacity of GS3 was 1.25 and -SH/Hg mol ratio was In th cas of GS3 it is vry clar that formation of M 2 L 3 typ complx occurrd as discussd for Cu 2+ on amino functionalizd silica gl. Th natur of adsorption isothrms of Hg 2+ on GS2 and GS3 is of Langmuir H (high affinity) typ or H2C typ of adsorption isothrms according to th classification of Gils t al. [29, 30]. This indicatd that th isothrms for both th thiol functionalizd silica gl hav vry high affinity for th Hg 2+ at low initial concntration, 100 % mrcury rmovl for both th sampls was obsrvd, indicating H typ of natur followd by long rising linar branch of C typ Manu V. 132 Ph. D. Thsis

25 dmonstrating partition distribution of mrcury molculs btwn solution and adsorbnts. Tabl Paramtrs for th adsorption of Hg 2+ on thiol functionalizd silica gl Isothrm Modl GS1 GS2 GS3 Langmuir SH (mmol/g) Q max (mmol/g) K L R Error function SH/Hg Frundlich K F n F R Error function Sips Q max (mmol/g) K S n S R Error function SH/Hg Tamkin B K T R Error function Rdlich Ptrson K RP β α R Error function Th distribution cofficint, K d is a vry important paramtr for stimating th affinity of th adsorbnt for adsorbat in aquous solution, K d can b calculatd using Eqn. 10 in tabl 3.4. [8, 28]. Distribution cofficints for fiv diffrnt initial concntrations wr calculatd for all th thr adsorbnts and rsults ar summarizd in tabl It showd that with an incras in initial concntration of Hg 2+ ion, C 0, thr was a Manu V. 133 Ph. D. Thsis

26 dcras in K d for all th thr functionalizd silica gls. Whn surfac concntration of thiol groups was incrasd, incras in th valus of distribution cofficint K d was obsrvd. This indicats that th affinity of th gls for Hg 2+ incrass with an incras in loading of thiol groups. Prcntag rmoval of Hg 2+ from th solution at diffrnt initial concntration was calculatd using Eqn. 11 in tabl 3.4, valus of prcntag rmoval of Hg 2+ ar givn in th tabl Tabl Prcntag rmoval of Hg 2+ and distribution cofficint K d for th Hg 2+ adsorption on thiol functionalizd silica gls (GS1, GS2 and GS3) Sampls C 0 (mg/dm 3 ) C (mg/dm 3 ) Q (mg/g) K d (ml/g) % rmoval of Hg 2+ GS1 GS2 GS Th prcntag rmoval of Hg 2+ also incrasd with an incras in loading of thiol groups on th surfac of silica. Th natur of adsorption isothrm (Fig. 3.8), distribution cofficint K d, and prcntag rmoval of Hg 2+ indicat that affinity of th adsorbnts for Hg 2+ incrasd with an incras in th loading of thiol group on th surfac of silica gl. It has bn obsrvd that at initial concntration of GS3 wr ~290 mg/dm 3 th prcntag rmoval of Hg 2+ has bn 100 %. i.., up to this concntration Hg 2+ can b compltly rmov from th aquous solution of mrcury chlorid. In th cas ~550 mg/dm 3 initial concntration prcntag rmovl of Manu V. 134 Ph. D. Thsis

27 mrcury by GS2 was highr than GS3, this is bcaus of th diffrnc nrgy barrir of both sampls. In th cas of GS2 prcntag of complt rmovl of Hg 2+ was obsrvd at initial concntration ~180 mg/dm 3. At highr concntration of mrcury, GS3 showd th maximum K d and prcntag rmovl compard to GS1 and GS Adsorption of Cu 2+ on Amino Functionalizd MCM-41 Thr diffrnt amino functionalizd MCM-41 sampls wr prpard by raction of calcind MCM-41 with APTMS using thr diffrnt APTMS: silica ratio (w/w) viz.0.2 (0.74 mmol/g), 0.4 (1.49 mmol/g) and 0.8 (2.96 mmol/g) sampls wr dsignatd as MCM-41N1, MCM-41N2 and MCM-41N3 rspctivly. Loading of amino groups on th surfac of functionalizd MCM-41 was found to b 0.71, 1.42 and 1.86 mm/g for MCM-41N1, MCM-41N2 and MCM-41N3 rspctivly. This matrial was usd for th adsorption of Cu 2+. Txtural proprtis and loading of MCM-41 and amino functionalizd MCM-41 summarizd in tabl Tabl Txtural proprtis and loading of MCM-41 and amino functionalizd MCM-41 Nam BET SA # TPV # (cm 3 /g) BJH NH 2 loading sampls (m 2 /g) PS # (Å) (mmolg) MCM MCM-41N MCM-41N MCM-41N # SA Surfac ara, TPV- Total por volum, PS- Por siz Coppr Adsorption Kintics Th kintic study of coppr adsorption on MCM-41 with aminopropyl loading of all th sampls was carrid out by taking 1100 mg/dm 3 as initial concntration of Cu 2+ ions. Th xprimntal data (Fig. 3.9) showd a significant incras in coppr ion adsorption on th surfac of adsorbat with an incras in th contact tim; and quilibrium is stablishd with in 12 h in all th amino functionalizd MCM-41. Adsorption rat has bn analyzd by using smi mpirical kintic modls, which ar basd on adsorption quilibrium capacity: th psudo first ordr and psudo scond ordr quations, Eqn. 2 and Eqn. 3 of tabl 3.2 rspctivly [10]. Manu V. 135 Ph. D. Thsis

28 Figur: 3.9. Kintic study of Cu 2+ on aminopropyl functionalizd MCM-41. (a) Effct of contact tim on adsorption of Cu 2+ ; (b) psudo scond ordr plot for th adsorption of Cu 2+ Psudo first ordr modal was not found suitabl to dscrib th kintic profil as it did not show linar bhavior, whras rat of aquous Cu 2+ adsorption on all th thr functionalizd MCM-41 sampls was accuratly dscribd by psudo scond ordr quation [7, 10]. Tabl 3.19 summarizs th valus of psudo scond ordr constants k 2, calculatd valus of Cu 2+ adsorbd at quilibrium (Q cal) using psudo scond ordr quation. Valu of Q calculatd from this quation incrasd linarly with an incras in th loading of amino group on th surfac of MCM-41(inst Fig. 3.19b). Tabl Valus of psudo scond ordr kintics of Cu 2+ sampls C 0 Q * (mg/dm 3 ) (mmol/g) (g mg -1 min -1 ) k 2 R 2 MCM-41N MCM-41N MCM-41N *Calculatd from kintic rsults using psudo scond ordr quation Valus of rgrssion cofficint R 2 for ach sampl wr gratr than 0.99 obtaind from linar fits, indicat th validity of th psudo scond ordr quation. Manu V. 136 Ph. D. Thsis

29 Psudo scond ordr constants, k 2 dcrasd from to g mg -1 min -1 with an incras in th loading of amino group (0.71 to 1.86), indicating that with an incras in loading of amino group th rat of adsorption incrasd Adsorption Isothrm Adsorption of mtal ions on amino functionalizd silica is ph dpndnt [25], and in th cas of bivalnt mtal ion, M 2+ is th main componnt blow ph 6 as th concntration hydroxyl spcis is vry low at this ph [9]. Studis on adsorption of Cu 2+ was carrid out at th natural ph of th CuSO 4 5H 2 O solutions in th rang of 4.5 to 5 for diffrnt initial concntrations, C 0, usd in this work. Calcind MCM- 41 showd vry low adsorption capacity of Cu 2+, whras amino functionalizd sampls adsorbd significant quantity of Cu 2+ from aquous solution of CuSO 4 5H 2 O. Ths rsults suggst that th original silanol groups hav poor affinity for th Cu 2+ adsorption [7]. Th Cu 2+ adsorption isothrms of MCM-41, MCM-41N1 and MCM-41N2 wr of Langmuir L typ (Fig. 3.10), whras, isothrm of MCM-41N3 was of H typ (high affinity) according to th classification givn by th Gils t al. [29, 30]. Two-paramtr (Langmuir and Frundlich Eqn. 4 and Eqn. 5 of tabl 3.3 rspctivly) and thr paramtrs modals (Sips, Eqn. 6 of tabl 3.3) wr usd to dscrib th isothrms [31] and valus of diffrnt paramtrs of all th modls usd ar prsntd in tabl 3.20 along with valus of rror function. Valus of R 2 for Langmuir and Sips modl wr found to b > 0.99 and th valus of n for all th four sampls ar narly 1, and whn n = 1, Sips quation bcoms Langmuir quation. Th lowr is th rror function; th lowr will b th diffrnc btwn th calculatd Q and xprimntal Q. Valus of rror function for Langmuir modl wr found to b lowst for all th sampls xcpt MCM-41N1. Howvr, thr is no significant diffrnc in valus of rror function and monolayr capacity calculatd by both th modl and thrfor Langmuir modl was considrd as bst fit amongst all th thr modls. Th calculatd isothrms using Langmuir modl ar compard with xprimntal rsults (Fig. 3.10). Th incras in Langmuir constants K L (from to ) with an incras in surfac concntration of amino groups indicat that affinity of th adsorbnt incrasd with an incras in surfac concntration of amino groups on MCM-41. Monolayr adsorption capacity of parnt MCM-41 and MCM-41N1 (0.71 mmol NH 2 /g), MCM-41N2 (1.42 mmol Manu V. 137 Ph. D. Thsis

30 NH 2 /g) and MCM-41N3 (1.86 mmol NH 2 /g) wr 0.13, 0.44, 0.76 and 0.93 mmol/g rspctivly (tabl 3.20). Linar rlationship was obsrvd btwn th surfac concntrations of amino groups with Langmuir monolayr capacity, Q m, with R 2 valu It is worth noting, that th valu of intrcpt of linar plot (Fig. 3.11) and th valu of Langmuir monolayr capacity of raw MCM-41 (tabl 3.20) ar almost similar. In th cas of silica gl calcind at 600 C functionalization, linar rlation btwn NH 2 loading and adsorption capacity of th functionalizd silica gl was not obsrvd (tabl 3.8). Howvr, silica gl sampls tratd at diffrnt tmpratur in th rang of 150 to 600 C followd by amino functionalization to thir maximum loading capacity (saturation) showd linar rlation btwn NH 2 loading and Cu 2+ monolayr capacity (tabl 3.12). Figur: Adsorption isothrms of Cu 2+ on aminopropyl functionalizd MCM- 41. Points show xprimntal valus, lins show calculatd plot using Langmuir modl Ths rsults indicat that bhavior of th Cu 2+ adsorption dpnds on th por gomtry and th dgr of hydroxylation of th silica usd for th surfac functionalization. Th valu of NH 2 /Cu mol ratio incrasd from ~1.5 to ~2.0 with an incras in NH 2 loading from 0.71 mmol/g to 1.42 mmol/g and rmaind ~2 for th loading of 1.86 mmolnh 2 /g. In MCM-41N1, th probabl intraction of amino group and Cu 2+ can b nvisagd as thr nighboring amino groups occupid by two Manu V. 138 Ph. D. Thsis

31 Cu 2+ ions or in othr words th intraction btwn mtal ion (M 2+ ) and surfac ligand (L) may of M 2 L 3 typ. In th cas of MCM-41N2 and MCM-41N3 2:1 NH 2 :Cu 2+ (ML 2 typ) complx formation has bn obsrvd [7]. Tabl Paramtrs for th adsorption of Cu 2+ on functionalizd MCM-41 Isothrm Modl MCM-41 MCM-41N1 MCM-41N2 MCM-41N3 Langmuir NH 2 (mmol/g) Q max (mmol/g) K L R Error function NH 2 /Cu Sips Q max (mmol/g) K S n R Error function NH 2 /Cu Frundlich K F n R Error function Kudryavtsv t al. [26] rportd a ML 2 typ of complx at a small dgr of adsorption of Cu 2+ on th amino functionalizd silica, which on incrasing th dgr of Cu 2+ adsorption was transformd to ML 1. Howvr, in th prsnt invstigation this phnomnon was not obsrvd, as at saturation th NH 2 /Cu mol ratio was found to b narly 1.5 for MCM-41N1 and 2 for MCM-41N2 and MCM- 41N3 rspctivly. Manu V. 139 Ph. D. Thsis