Adsorption of Cu 2+ 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 Cu 2+ 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 Cu 2+ on GN2 followed pseudosecond order. Adsorption isotherms of Cu 2+ 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 Cu 2+ adsorption isotherms. The monolayer copper adsorption capacity for the gel GN1 (0.515 mmol Cu 2+ /g) and GN2 (0.55 mmol Cu 2+ /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 NH 2 /Cu mole ratio was found to be around 1 and 2 for GN1 and GN2, respectively. In the case of GN3 (1.45 mmol NH 2 /g), the monolayer capacity was found to be 1.05 mmol Cu 2+ /g with a NH 2 /Cu mole ratio of 1.38. This study indicates that the population density of NH 2 determines the NH 2 /Cu 2+ ratio at saturation of Cu 2+ on the functionalized silica gel and the affinity of the gel for the Cu 2+ . Introduction Heavy metals are very hazardous for living organisms, when they exceed the specific limits. The accumulation of Cu 2+ 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 nano- filtration, precipitation, coagulation/coprecipitation, and adsorp- tion. 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 organo- functionalized silane, as represented by the general formula Y 3 SisRsX, 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 ordered mesoporous silica materials have also the same advan- tages 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 gel 13 and ordered mesoporous silica using one pot synthesis or postsynthesis grafting 15-22 for the removal of metal ions. Mesoporous silicas such as MCM-41, HMS, SBA- 15, and SBA-1 have been functionalized by functional groups such as sNH 2 , sSH, and sSs, etc., to make the materials capable of interacting strongly with metal ions like Cu 2+ , Cd 2+ , Hg 2+ , Ni 2+ , etc. 22-25 Adsorption behavior of metal ions on the surface function- alized 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 anions 20 like PdCl 4 2- from the binary solution of PdCl 2 and AuCl 3 at pH 1.0 and oxyanions 22 like Cr 2 O 7 2- from the solution of Cu 2+ and Cr 2 O 7 2- at 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 Cu 2+ . MCM- 41 grafted with a -NH 2 group was found to be more selective for Cu 2+ at pH 5 than that for Ag + , and this effect was attributed to the interaction between the hard base (-NH 2 ) and hard acid (Cu 2+ ). 25 It has also been reported that, on the surface of amino functionalized silica gel, at small degrees of filling, the Cu 2+ 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 * To whom correspondence should be addressed. E-mail: hcbajaj@ csmcri.org (H.C.B.); hmmody@csmcri.org (H.M.M.). Tel.: +91-0278- 2567760or 2471793. Fax: +91-0278-2567562. † Present address: R & D Centre, Reliance Industries Limited, Vadodara 391 346, Gujarat, India. Ind. Eng. Chem. Res. 2009, 48, 8954–8960 8954 10.1021/ie900273v CCC: $40.75  2009 American Chemical Society Published on Web 09/18/2009