Adsorption of Cu2+ on Amino Functionalized Silica Gel with Different

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Ind. Eng. Chem. Res. 2009, 48, 8954–8960

Adsorption of Cu2+ on Amino Functionalized Silica Gel with Different Loading Manu V., Haresh M. Mody,* Hari C. Bajaj,* and Raksh V. Jasra† Discipline of Inorganic Materials and Catalysis, Central Salt & Marine Chemicals Research Institute (CSIR), Gijubhai Badheka Marg, BhaVnagar 364 002, Gujarat, India

Silica gel (G) and amino functionalized silica gel with three different loading of an aminopropyl group viz. 0.51, 1.01, and 1.45 mmol/g, (GN1, GN2, and GN3) were synthesized, characterized, and used as adsorbents for the adsorption of Cu2+ ions from aqueous solution. The specific surface area, pore volume, and BJH pore size of silica gel decreased with an increase in the loading of aminopropyl groups. Kinetic of adsorption of Cu2+ on GN2 followed pseudosecond order. Adsorption isotherms of Cu2+ on functionalized silica samples were best fit by the Sips model for all the three functionalized silica gels, among the four models used to describe the Cu2+ adsorption isotherms. The monolayer copper adsorption capacity for the gel GN1 (0.515 mmol Cu2+/g) and GN2 (0.55 mmol Cu2+/g) were found to be almost similar even when the loading of the amino group increased from 0.51 to 1.01 mmol/g. The NH2/Cu mole ratio was found to be around 1 and 2 for GN1 and GN2, respectively. In the case of GN3 (1.45 mmol NH2/g), the monolayer capacity was found to be 1.05 mmol Cu2+/g with a NH2/Cu mole ratio of 1.38. This study indicates that the population density of NH2 determines the NH2/Cu2+ ratio at saturation of Cu2+ on the functionalized silica gel and the affinity of the gel for the Cu2+. Introduction Heavy metals are very hazardous for living organisms, when they exceed the specific limits. The accumulation of Cu2+ in the human body causes skin, brain, pancreas, and heart diseases.1 A wide variety of techniques to remove heavy metals from water is available such as ion exchange, reverse osmosis and nanofiltration, precipitation, coagulation/coprecipitation, and adsorption. The adsorption processes are the only effective and economic methods.1-5 Over the past two decades new classes of solid adsorbents have been developed as porous and nonporous materials, such as surface modified silica gel/templated ordered mesoporous silica, activated carbon fibers, fullerenes, and heterofullerenes for different applications. Adsorbents are normally prepared by anchoring organic and inorganic molecules to their surface.2 The organo functionalization of the inorganic solid surface is used to introduce basic groups on anchored pendant chains.6,7 These functionalized materials can effectively be used as adsorbents for the removal of specific toxic metal ions and other hazardous chemicals for environmental cleanup applications.2-8 Silica gels have many advantages to be used as supports for the immobilization of great variety of silylating agents and the functionalized silica gel is the most suitable adsorbent because the silica supports do not swell or shrink like polymeric resins.9,10 The surface of silica gel terminates in either siloxane groups (tSisOsSit) or silanol groups (tSisOH),11 and these surface silanol groups can act as an anchoring site for the functional groups. Silica gel covalently reacted with organofunctionalized silane, as represented by the general formula Y3SisRsX, here X groups are found at the end of the organic chain, R is usually composed of three methylenic groups linked to the silicon chain, and Y is normally the alkoxide groups. The reaction of a silylating agent with the surface silanol groups results in a stable SisOsSisC linkage.12 Template synthesized * To whom correspondence should be addressed. E-mail: hcbajaj@ csmcri.org (H.C.B.); [email protected] (H.M.M.). Tel.: +91-02782567760or 2471793. Fax: +91-0278-2567562. † Present address: R & D Centre, Reliance Industries Limited, Vadodara 391 346, Gujarat, India.

ordered mesoporous silica materials have also the same advantages as those mentioned for the silica gels, and the surfaces of these materials can be similarly modified. Various organic functional groups have been covalently grafted by silylation on to the surface of silica gel13 and ordered mesoporous silica using one pot synthesis or postsynthesis grafting15-22 for the removal of metal ions. Mesoporous silicas such as MCM-41, HMS, SBA15, and SBA-1 have been functionalized by functional groups such as sNH2, sSH, and sSs, etc., to make the materials capable of interacting strongly with metal ions like Cu2+, Cd2+, Hg2+, Ni2+, etc.22-25 Adsorption behavior of metal ions on the surface functionalized silica depends on the lateral distribution and concentration of the functional groups on the surface of the adsorbent,13 number of donor groups in the grafted ligand, and pH of the system.14 Selective adsorption of anions20 like PdCl42- from the binary solution of PdCl2 and AuCl3 at pH 1.0 and oxyanions22 like Cr2O72- from the solution of Cu2+ and Cr2O72at pH below 3.5 on amino functionalized MCM-41 has been reported. Lam et al.5 have demonstrated the selective adsorption of metal ions from their binary mixtures based on Pearson’s hard-soft acid base (HSAB) concept by modifying the surface of MCM-41 silica with an alkyl substituted amine group (-RNH) in order to decrease the hardness of the basic group and make it more selective for Ag+ than that for Cu2+. MCM41 grafted with a -NH2 group was found to be more selective for Cu2+ at pH 5 than that for Ag+, and this effect was attributed to the interaction between the hard base (-NH2) and hard acid (Cu2+).25 It has also been reported that, on the surface of amino functionalized silica gel, at small degrees of filling, the Cu2+ ion forms a complex with two grafted ligands. With an increase in the degree of filling, a transition from a 1:2 (metal to ligand ratio) type of complex to the preferential formation of complexes with the composition 1:1 takes place.26 In order to control the spatial proximity of the site of amine functional groups, a strategy called molecular patterning has been used in which amine groups were first protected with a bulky molecule like trityl or benzyl moieties to form imine and

10.1021/ie900273v CCC: $40.75  2009 American Chemical Society Published on Web 09/18/2009

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then reacted with the surface of SBA-15 ordered mesoporous silica, under anhydrous conditions to prevent oligomerization. Interaction of an amino group with the unreacted surface silanol groups was prevented by capping of surface silanol groups with hexamethyldisilazane, followed by deprotection of the amino group by hydrolysis.27,28 In present work, we have attempted to control the population density of hydroxyl groups by calcining the gel at 600 °C. It has been reported that when the gel is calcined at 600 °C, about two-thirds of the surface is dehydroxylated.29 Thus, the possibility of the formation of a hydrogen bond between unreacted silanol and grafted amino groups is minimized. The calcined gel was further functionalized by 3-aminopropyltrimethoxysilane having a different degree of loading to study the effect of surface population density of aminopropyl groups on adsorption of Cu2+. Experimental Section Chemicals. The commercial grade sodium silicate (23.31 SiO2 %; 7.48 Na2O %) was obtained from M/S Kadvani Chemicals Pvt. Ltd., Jamnagar, Gujarat, India; sulfuric acid, toluene, isopropanol, and copper sulfate (s.d. Fine Chem. India) AR grade and 3- aminopropyltrimethoxysilane (APTMS 97%, Aldrich Chemical) were used as received. Synthesis. Silica gel was synthesized under acidic conditions. A known weight of sodium silicate solution (23.31 SiO2 %; 7.48 Na2O %) diluted with deionized water to obtain 12% w/v SiO2 concentration was added to a 10.2 N H2SO4 solution under stirring with peristaltic pump in 12.5 min at room temperature to adjust the SiO2 concentration to 8% w/v, aged for 2 h, and then kept at 100 °C for 112 h in a closed Simax glass bottle. The gel thus obtained was washed until it was sulfate free (BaCl2 test), dried at 100 °C in oven, and then further calcined at 600 °C for 6 h. Samples were cooled under vacuum and stored in a capped bottle over P2O5 in a desiccator. Surface functionalization of the silica gel was carried out by suspending the gel in solution of 3-aminopropyltrimethoxysilane in dry toluene (solid:liquid ) 10:100 w/v) and refluxed at boiling temperature for 24 h.30 Samples were filtered, washed with isopropanol thoroughly, and dried at 100 °C overnight. Three different functionalized silica samples were prepared with APTMS using different APTMS: silica ratios (w/w) viz. 0.1 (0.56 mmol/g), 0.2 (1.23 mmol/g), and 0.3 (1.68 mmol/g) which were designated as GN1, GN2, and GN3, respectively. Characterizations. N2 adsorption-desorption analysis was done using an ASAP 2010C (Micromeritics, USA), under liquid N2 temperature (77 K). The samples were degassed at 100 °C under vacuum for 4 h. The specific surface area was calculated by the BET method. The pore size distribution analysis was done by the BJH method using the desorption branch of the isotherm, and the total pore volume was calculated by the Gurtvitch equation.31Thermogravimetric analyses (Mettler Toledo TGA/SDTA 851e) were carried out to detect the decomposition temperature of the organic moieties grafted on the silica gel. Samples were heated from room temperature to 850 °C at the heating rate of 10 °C/min. The grafting of the functional groups of GN adsorbents were analyzed by Fourier transform infrared spectroscopy (FTIR, Perkin-Elmer GX), using KBr pellets. Elemental analysis (CHNS) was carried out using PerkinElmer CHNS/O analyzer (Series II, 2400). Kinetic Study of Functionalized Silica Gel. The effect of contact time between copper solution and adsorbent along with the influence of two different initial Cu2+ concentrations viz. 1018 and 65 mg/dm3 were investigated in order to analyze the adsorption kinetics of the Cu2+ ion for GN2. A known weight

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of functionalized silica gel (100 mg) was taken into different stoppard vials, and 25 or 250 mL of CuSO4 solution (1018 or 65 mg/dm3 Cu2+) was added to each vial. Systems were kept at 30 ( 2 °C for different time intervals in the range of 15-1440 min while shaking intermittently. Supernatant solution from each vial was analyzed for Cu2+ concentration after different time intervals and adsorption of Cu2+ was calculated using eq 1. During the kinetic study, pH of the system was also monitored at different time intervals. Qe ) (C0 - Ce)V/m

(1)

where Qe ) quantity of Cu2+ adsorbed on the adsorbent at the time of equilibrium (mg/g); C0 ) initial concentration of Cu2+ in aqueous solution of CuSO4 · 5H2O (mg/dm3); Ce ) final concentration of Cu2+ in aqueous solution of CuSO4 · 5H2O at the time of equilibrium (mg/dm3); V ) volume of the solution (dm3); m ) mass of adsorbents (g). Adsorption of Cu2+ on Silica Gel and Functionalized Silica Gel Samples. Adsorption study of Cu2+ was carried out by batch experiments. A known weight (∼0.1 g) of silica gel/ functionalized silica gel sample was taken in a 50 mL vial, equilibrated with 25 mL aqueous solutions of different concentrations of CuSO4 · 5H2O for 24 h at room temperature (30 ( 2 °C) while shaking the vial intermittently. The concentration of Cu2+ in the solution before and after equilibrium was determined by ICPOES Perkin-Elmer optical emission spectrometer optima 2000DV. Adsorption of Cu2+ (Qe) was calculated using eq 1. Isotherm Modeling. The isotherms models of Langmuir, Freundlich, Sips, and Redlich-Peterson were used to describe the equilibrium adsorption. The equations of isotherms are given below Langmuir isotherm/equation Qe )

QmKLCe 1 + KLCe

(2)

where Ce ) concentration of Cu2+ at the time of equilibrium (mg/dm3); Qe ) quantity of Cu2+ adsorbed on the adsorbent at the time of equilibrium (mg/g); Qm ) quantity of Cu2+ adsorbed on the adsorbent at saturation or monolayer capacity (mg/g); KL ) Langmuir constant: (dm3/g) Freundlich isotherm Qe ) KFCe1/n

(3)

where KF is the Freundlich constant related with adsorption capacity [mg/g(mg/dm3)-1/n] and n is the Freundlich exponent (dimensionless). Sips isotherm Qe )

QmKSCe1/n 1 + KSCe1/n

(4)

where KS (g/dm3)-1/n is the Sips constant related with affinity and Qm (mg/g) is the Sips maximum adsorption capacity Redlich-Peterson isotherm Qe )

KRPCe 1 + aRPCeβ

(5)

where KRP (dm3/g) and aRP (mg/dm3)-β are Redlich-Peterson constants and β is the Redlich-Peterson exponent (dimensionless).

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Figure 1. Nitrogen adsorption/desorption isotherms at 77 K (a) and pore size distribution curves (b) of silica gel (G) and aminopropyl functionalized silica gels (GN1, GN2, and GN3). Table 1. Textural Properties of Silica Gel (G) and Functionalized Silica Gels properties

G

GN1

GN2

GN3

BET surface area (m2/g) total pore volume (cm3/g) BJH desorption pore size (Å)

575 0.61 47.2

444 0.58 40.3

363 0.49 39.5

360 0.40 35.8

Figure 2. Thermogravimetric curves of silica gel (G) and aminopropyl functionalized silica gels (GN1, GN2, and GN3).

Table 2. Elemental Analysis of Functionalized Silica Gels sample

C (%)

H (%)

N (%)

C/N (M/M)

NH2 (mmol/g)

GN1 GN2 GN3

2.77 4.16 5.31

1.37 2.01 1.45

0.72 1.46 2.03

4.48 3.3 3.1

0.51 1.01 1.45

The Langmuir, Freundlich, Sips, and Redlich-Peterson isotherms were fitted employing the linear fitting. In addition, the model were also evaluated by average relative error function, which measures the differences of the amount of Cu2+ ion up take by the adsorbent predicted by the models and the actual quantity adsorbed, Q, measured experimentally. Results and Discussion Silica gel and surface functionalized silica gel samples were characterized by N2 adsorption at liquid N2 temperature. Adsorption isotherms and pore size distribution curves are presented in Figure 1. All samples exhibited type (IV) adsorption isotherms, characteristics of mesoporous silica.32 As expected for each functionalized silica gel, the surface area, total pore volume, and pore diameter (Table 1, Figure 1b) decreased as the concentration of added organo silane increased, such decreases in all the three properties have been reported after functionalization of silica gels.12,13 Elemental Analysis. Three different adsorbents were prepared from silica gel by grafting different quantities of aminopropyl groups on the surface of silica gel. An increase in carbon, hydrogen, and nitrogen content (Table 2) was observed with an increase in amount of silylating agent during the functionalization of silica surface. The carbon:nitrogen ratios (C/N) of GN1, GN2, and GN3 were 4.45, 3.3, and 3.1, respectively. The case of GN1 with C/N ∼ 4.5 suggested that an average of 1.5 methoxy groups were hydrolyzed out of three methoxy groups of APTMS. This indicated that for about 50% of APTMS, two methoxy groups hydrolyzed to form two Si-O-Si bonds either with surface silanol group of the silica gel or with the neighboring aminopropylsilane moiety, and in the remaining APTMS, only one methoxy group hydrolyzed to form the Si-O-Si bond. In the case of GN2, the C/N value obtained

Figure 3. Infrared spectra of silica gel (G) and aminopropyl functionalized silica gels (GN1, GN2, and GN3).

was 3.3, i.e., 10% higher than the expected value 3. This means that about 90% of the methoxy groups in the APTMS had reacted to form a bond between the organic amines and the silica surface and/or with aminopropylsilane moiety. In GN3, almost all the methoxy groups in the APTMS are reacted to form a covalent bond between the amino silane group and silica surface and/or with neighboring aminopropylsilane moiety.25 Thermogravimetric Analysis. The samples were heated from room temperature to 850 °C, at a rate of 10 °C/min. Thermogravimetric curves (Figure 2) showed an increase in mass loss of 5.8%, 6.8%, 10.5%, and 14.5% for G, GN1, GN2, and GN3, respectively, as results of increase in the loading of organic functional group on the surface of silica gel. For silica gel (G), weight loss occurred up to 150 °C due to the loss of physically adsorbed water molecule, and at high temperature (150-850 °C), the remaining weight loss was due to the condensation of silanol groups. For GN1, GN2, and GN3, an additional weight loss in the temperature range of 350-650 °C was due to the decomposition of organic moiety bonded to the inorganic silica backbone.12,22 Infrared Spectroscopy. IR spectra (Figure 3) of silica gel sample (G) before functionalization and samples GN1, GN2, and GN3, after functionalization showed bands at 463, 454, 459, and 453 cm-1 attributed to the Si-O bond rocking.12,33 Bands

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Figure 4. Kinetic study of Cu2+ on aminopropyl functionalized silica gels GN2. (a) Effect of contact time on adsorption of Cu2+. (b) Pseudo-second-order plot for the adsorption of Cu2+.

at 805, 806, 799, and 798 cm-1 observed in G, GN1, GN2, and GN3, respectively, assigned to the Si-O-Si symmetric vibrations.15,34 After functionalization of silica gel with an aminopropyl group, new absorption peaks at 1520, 1530, and 1531 cm-1 were attributed to the presence of the -NH2 scissor of GN1, GN2, and GN3, respectively.19 Similarly bands at 1461, 1468, and 1447 cm-1 due to the -CH219 scissor have also been observed in the IR spectra of GN1, GN2, and GN3, respectively, after functionalization silica gel samples. Copper Adsorption. Copper Adsorption Kinetics. The kinetics of copper adsorption on silica gel with aminopropyl loading 1.01 mmol/g (GN2) was carried out for two different initial concentrations of Cu2+ ions, 1018 and 65 mg/dm3. The experimental data (Figure 4a) showed a significant decrease in metal concentration with time, and equilibrium is established within 3 h. No significant change in the concentration of Cu2+ was observed after 3 h of contact time. The adsorption rate has been analyzed using two common semiempirical kinetic models, which are based on adsorption equilibrium capacity: the pseudofirst-order and pseudo-second-order equations.4 The pseudo-firstorder equation relates the adsorption rate to the amount of metal ion adsorbed at time “t” as shown below in eq 6. dQt/dt ) k1(Qe - Qt)

(6)

where Qe and Qt are, the adsorbed amounts of metal ions at equilibrium and time t, respectively, expressed in millimoles per gram; k1 is the pseudo-first-order kinetic constant, expressed in inverse minutes. Equation integration and rearrangement yields the linear form (7). ln(Qe - Qt) ) ln Qe - k1t

(7)

The pseudo-second-order equation may be written in the form: dQt/dt ) k2(Qe - Qt)2

(8)

where k2 (g/(mol min)) is the pseudo-second-order rate constant. The differential equation is usually integrated and transformed in its linear form. t/Qt ) t/k2Qe2 + t/Qe

(9)

Table 3. Effect of Initial Cu2+ Concentration on Adsorption Kinetics Qe (mmol/g) C0 (mg/dm3)

Qecal

Qeexp

K2 (g/(mmol min))

r2

65 1018

0.167 0.581

0.167 0.577

0.642 0.0999

0.9999 0.9998

The linearity of the plot of the equations can validate the applied model. The pseudo-first-order model was not found suitable to describe the kinetic profile as it did not show apparent linear behavior, whereas rates of aqueous Cu2+ adsorption over the GN2 sample are accurately described by the pseudo-secondorder equation (Figure 4b) for both the initial concentrations studied. Table 3 summarizes the calculated values of Cu2+ 2 adsorbed (Qcal e ), k2, and regression coefficient r for each initial copper concentration (C0) obtained from linear fits. These results indicated good correlation, and experimental values of Cu2+ cal adsorbed at equilibrium, Qexp e , were comparable with that of Qe . Pseudo-second-order constants, k2, decreased from 0.642 to 0.0999 g/(mmol min) when initial Cu2+ concentrations rose from 65 to 1018 mg/dm3. The kinetic behavior of adsorption of Cu2+ on monoamine functionalized ordered mesoporous silica15 and diamine functionalized SBA-15 silica4 fit well with the pseudosecond-order model. Adsorption Isotherm. Adsorption of metal ions on amino functionalized silica dependent on the pH of the solution.14 In the case of a solution of bivalent metal salts, as the concentration of bivalent cationic species is very high compared to hydroxyl species, therefore adsorption of Cu2+ on functionalized silica gel was carried out at pH 5.13 The pH values of initial Cu2+ solutions, 1018 and 65 mg/dm3, were found to be 4.5 and 5.1, respectively. The pH of the solution in contact with the adsorbent was monitored at different time intervals (Table 4). In the case of lower concentration, there was a slight increase in the pH after 1 h of contact time; however, after 1 h it was stabilized to ∼5.4. In the case of a higher copper ion concentration, during the first hour an increase in pH was observed but gradually it was decreased to 4.3 after 24 h. Though the equilibrium was established in 3 h for the GN2 sample (Figure 4a), an equilibrium time of 24 h was kept for the adsorption isotherms of all the three samples. The adsorption isotherms of Cu2+ on GN1, GN2, and GN3 are shown in Figure 5. Various sorption isotherm models are widely employed for

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Table 4. pH of the CuSO4 Solutions in Contact with Functionalized Silica Gel GN2 at Different Time Intervals pH time (h) a

0 1 3 6 12 24 a

2+

3

(65 mg Cu /dm ) 5.1 5.4 5.4 5.3 5.4 5.4

Table 5. Model Parameters for the Adsorption of Cu2+ on Functionalized Silica Gel isotherm model

2+

3

(1018 mg Cu /dm ) 4.5 4.6 4.5 4.4 4.4 4.3

Initial pH of the CuSO4 solution before contacting the adsorbent.

GN1

GN2

GN3

0.50 0.020168 0.996 0.0318 1.97

0.85 0.002425 0.9993 0.128 1.7

Langmuir Qmax (mmol/g) KL (dm3/g) R2 function error, Ferror NH2/Cu

0.485 0.074172 0.995 0.0105 1.02

KF (mg/g(mg/dm3)-1/n) n R2 Function error, Ferror

1.416 1.8695 0.9663 0.033634

Qmax (mmol/g) KS (g/dm3)-1/n n R2 function error, Ferror NH2/Cu

0.515 0.050098 1.11 0.9923 0.00751 0.96

Freundlich 5.77 3.818 0.9313 0.02089

17.132 4.67 0.9445 0.0477

Sips 0.55 0.01703 1.1764 0.9923 0.00159 1.84

1.05 0.0645 2.22 0.9984 0.00689 1.38

0.6244 0.9923 0.028449 0.95

0.00211 0.9885 0.127 0.95

Redlich-Peterson KRP (dm3/g) R2 function error, Ferror β

Figure 5. Adsorption isotherms of Cu2+ on aminopropyl functionalized silica gels GN1, GN2, and GN3. Points show experimental values, and lines show calculated plots using the Sips model.

fitting the data. In the present work, a two-parameter model (Langmuir and Freundlich) and three-parameter model (Sips and Redlich-Peterson) isotherms were used to describe the equilibrium between the Cu2+ and functionalized silica gel samples in aqueous Cu2+ in the solution. Values of different parameters of all the models used to describe the adsorption isotherms are presented in Table 5. The values of error function calculated using the Sips model for all the isotherms were found to be lowest; this indicated that Sips model was best fit among all four models. It can be also observed (Table 5) that the values of R2 for all the models except Freundlich were about 0.99. Sips plots for all the three functionalized silica gels indicated a best mathematical fit for isotherms, and hence, the monolayer capacity Qm for all the isotherms was calculated using the Sips model, considered to be the nearest to the real values of monolayer adsorption capacity. The monolayer adsorption capacity of amino functionalized silica gel GN1 (0.51 mmol NH2/g), GN2 (1.01 mmol NH2/g), and GN3 (1.45 mmol NH2/g) were found to be 0.51, 0.55, and 1.05 mmol/g, respectively (Table 5). It has been observed that when the loading of the NH2 group was increased from 0.51 to 1.01 mmol/g, the monolayer adsorption capacity of Cu2+ remained almost same and the value of the NH2/Cu mole ratio for GN2 (1.84) was found to be around two times greater than that of GN1 (0.96). These results suggested that, in the case of GN1, at saturation, an ML1 type complex formed on the surface of GN1, whereas on the surface of GN2 at saturation, an ML2 type complex formed between amine and Cu2+. Kudryavtsev et al.26 reported that at a small degree of adsorption of Cu2+ on the amino functionalized silica ML2 type of complex is formed,

0.061506 0.9922 0.00787 0.95

which on increasing the degree of Cu2+ adsorption was converted to ML1. However, in the present investigation for GN2, this phenomenon was not observed, as at saturation the NH2/Cu mole ratio was found to be nearly 2. In the case of GN3, the NH2/Cu mole ratio (1.38) is close to 1.5. The probable complex formed on the surface of GN1, GN2, and GN3 may be 1:1, 2:1, and 3:2 NH2:Cu2+ ratio, respectively, as proposed by Lam et al.36 for the interaction of amino functionalized MCM-41 with the CuSO4, and these complexes are stabilized by the SO42- counterions. Blitz et al.13 have grafted the aminopropyl group using two different silanes, viz, 3-aminopropyldimethylmethoxysilane (APDMS) and 3-aminopropyltriethoxysilane (APTS) having one and three hydrolyzable groups, respectively, and later was treated with water to hydrolyze the hydrolyzable groups to form cross-linked polymeric type grafted modified silica gel. At higher concentration of Cu2+, adsorption of Cu2+ on the latter has been reported to be less than that on a sample modified with APDMS. However, at lower concentration, uptake of Cu2+ on cross-linked aminopropyl grafted gel was found to be more than that of APDMS modified gel. This difference was attributed to the fact that a larger amount of the functional groups are at a shorter distance between one another on the surface of cross-linked material, and hence, it acts as a bidantate ligand. A similar difference in adsorption behavior between GN1 and GN2 has been observed in the present investigation (Figure 5). This study indicates that the number of NH2 per unit area (population density) determines the NH2/Cu ratio at a saturation of Cu2+ on the functionalized silica gel and hence the saturation or monolayer capacity of the adsorbent. The distribution coefficient, Kd, for estimating the affinity of the adsorbent for adsorbate in aqueous solution can be calculated using eq 10.15,19,35 Kd ) Qe/Ce

(10)

Ce ) concentration of Cu2+ at the time of equilibrium (mg/ dm3); Qe ) quantity of Cu2+ adsorbed on the adsorbent at the time of equilibrium (mg/g); Kd ) distribution coefficient.

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Table 6. Percent Removal of Cu and Distribution Coefficient Kd for the Cu2+ Adsorption on Aminopropyl Functionalized Silica Gels GN1, GN2, and GN3

GN1

GN2

GN3

C0 (mg/dm3)

Ce (mg/dm3)

Qe (mg/g)

Kd (dm3/g)

% removal of Cu2+

20.26 60.78 101.3 202 344.4 20.26 60.78 101.3 202 344.4 20.26 60.78 101.3 202 344.4

6.74 30.69 46.39 122.1 238.5 1.56 9.645 20.39 92.54 214.75 0.04 1.315 6.235 33.18 132.95

3.5334 7.6538 12.378 20.065 23.643 5.0272 11.936 17.680 29.913 30.410 5.2931 14.447 24.603 41.403 51.729

0.5247 0.2494 0.2668 0.1643 0.0991 3.2246 1.2376 0.8669 0.3173 0.1416 132.33 10.987 3.9459 1.2478 0.3891

66.8 49.51 54.2 36.7 30.7 92.31 84.13 79.9 54.3 37.65 99.8 97.84 93.8 83.6 61.4

Distribution coefficients for four different initial concentrations were calculated for all the three adsorbents (eq 10) and results are summarized in Table 6. It showed that with an increase in initial concentration of Cu2+ ion, C0, there was a decrease in Kd for all the three functionalized silica gels. Similar results have been reported for the Cu2+ adsorption on monoamine functionalized ordered mesoporous silica15 and adsorption of CrO42- and HAsO42- on mono-, di-, and triamine functionalized MCM-41 silica.19 With the increase in the surface concentration of amino groups, the distribution coefficient Kd increased, indicating the higher affinity for Cu2+ with an increased loading of aminopropyl groups. Removal of Cu2+ from the solution at different initial concentrations, calculated by eq 11, is given in Table 6. % removal ) (C0 - Ce)/C0 × 100

(11)

C0 ) initial concentration of Cu2+ in aqueous solution of CuSO4 · 5H2O (mg/dm3); Ce ) concentration of Cu2+ at the time of equilibrium (mg/dm3). The percent removal of Cu2+ also increased with an increase in loading of aminopropyl groups on the surface of silica. The nature of the adsorption isotherm (Figure 5), distribution coefficient Kd, and percent removal of Cu2+ indicate that affinity of the adsorbents for Cu2+ increased with an increase in the loading of aminopropyl group on the surface of silica gel. It has been observed that at initial concentration of ∼20 mg/dm3 the percent removal of Cu2+ has been increased from 66.8% to 99.8% (∼1.5 times) when loading of amino groups increased from 0.51 to 1.45 mmol/g (∼3 times). The monolayer adsorption capacity of the functionalized silica gels, GN1, GN2, and GN3, does not increase linearly with an increase in loading of aminopropyl groups. In the case of GN2 though the loading of the aminopropyl group was two times that of GN1, the values of monolayer capacity for both the adsorbents were almost similar (Table 5). In the case of GN3, when loading was increased 3-fold, a 1.4 times increase in the monolayer capacity was observed. However, the values of Kd increased significantly (6 times) with an increase in the loading of aminopropyl groups from ∼0.5 to ∼1 mmol and to ∼250 times when loading was increased to ∼1.5 mmol for the initial concentration of ∼20 mg/dm3 Cu2+ solution (Table 6). This indicates that for the removal of metal ions from a very dilute solution, adsorbent with higher population density of functional

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groups is more effective than adsorbents with a lower population density of functional groups on their surface. Conclusions Adsorption of Cu2+ on three different amino functionalized silica gels having different concentrations of aminopropyl groups on their surface was studied. Monolayer capacity did not increase linearly with an increase in the surface concentration of amino groups on the surface of amino functionalized silica gels. This may be due to the difference in the number of amino groups occupied by Cu2+ in GN1, GN2, and GN3, which was determined by the population density of the amino groups on the surface of the adsorbent. Increase in the concentration of amino groups on the surface of silica gels resulted in an increase in affinity of adsorbents for Cu2+. The adsorbent, for its practical application for adsorptive separation of metal ions from aqueous medium, should have high adsorption capacity at low concentration of the adsorbate. It is also desirable that affinity for adsorbates should be such that it can be easily regenerated by desorption. In the present study, it has been observed that an increase in concentration of amino groups on the surface of silica gels increased affinity significantly but an increase in adsorption capacity was not found to be linear. Studies on the effect of loading on desorption behavior of Cu2+ from the surface of amino functionalized silica gels and the interaction of metal ion with number of amino groups by spectroscopic methods are subject for future investigation. Acknowledgment The authors thank the council of scientific and industrial research (CSIR) New Delhi, India, for the financial support (NWP 0010). The authors are also thankful to Mr. Prasanth. K. P, Mr. Vinod Agarwal, Mr. Mitul, Mrs. Sheetal, and Mr. Viral for analytical support. M.V. thanks CSIR for the SRF fellowship. Literature Cited (1) Veli, S.; Alyuz, B. Adsorption of copper and zinc from aqueous solution by using natural clay. J. Hazard. Mater. 2007, 149, 226. (2) Arakaki, L. N. H.; Augusta Filha, V. L. S.; Espinola, J. G. P.; da Fonseca, M. G.; de Oliveira, S. F.; Arakaki, T.; Airoldi, C. New thiol adsorbent grafted on silica gel: synthesis, characterization and employment for heavy meta adsorptions. J. EnViron. Monit. 2003, 5, 366. (3) Tahir, S. S.; Naseem, R. Removel of Cr (III) from tannery wastewater by adsorption onto bentonite clay. Sep. Purif. Technol. 2007, 53, 312. (4) Aguado, J.; Arsuaga, J. M.; Arencibia, A.; Lindo, M.; Gasco´n, V. Aqueous heavy metals removal by adsorption on amine-functionalized mesoporous silica. J. Hazard. Mater. 2009, 163, 213. (5) Lam, K. F.; Chen, X.; Fong, C. M.; Yeung, K. L. Selective mesoporous adsorbents for Ag+/Cu2+ separation. Chem. Commun. 2008, 2034. (6) Prado, A. G. S.; Arakaki, L. N. H.; Airoldi, C. Adsorption and separation of cations on chemically modified silica gel synthesized via the sol-gel process. J. Chem. Soc., Dalton Trans. 2001, 14, 2206l. (7) Prado, A. G. S.; Arakaki, L. N. H.; Airoldi, C. Adsorption and separation of cations on silica gel chemically modified by homogeneous and heterogeneous routes with the ethylenimine anchored on thiol modified silica gel. Green Chem. 2002, 4, 42. (8) Soliman, E. M.; Mahmoud, E. M.; Ahmed, S. A. Synthesis characterization and structure effects on selectivity of silica covalently bonded diethylenetriamine mono-and bis-salicyaldehyde and naphthaldehyde schiff’s bases towards same heavy metal ions. Talanta 2001, 54, 243. (9) Jacques, R. A.; Benrnardi, R.; Caovila., M.; Lima, E. C.; Pavan, F. A.; Vaghetti, J. C. P.; Airoldi, C. Removel of Cu(II), Fe(II), and Cr(III) from aqueous solution by aniline grafted silica gel. Sep. Sci. Techanol. 2007, 42, 591. (10) Brasil, J. L.; Maratins, L. C.; Ev, R. R.; Dupont, J.; Dias, S. L. P.; Sales, J. A. A.; Airoldi, C.; Lima, E. C. Factorial design for optimization of flow-injection preconcentration of copper (II) determination in natural

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ReceiVed for reView February 18, 2009 ReVised manuscript receiVed July 15, 2009 Accepted August 30, 2009 IE900273V