Concentration-dependant selective removal of Cr(III), Pb(II) and Zn(II) from aqueous mixtures using 5-methyl-2-thiophenecarboxaldehyde Schiff base-immobilised SBA-15

Abstract

5-Methyl-2-thiophenecarboxaldehyde (MTC)-derived Schiff base was immobilised over SBA-15 (TS-SBA-15) via a two-step modification procedure. Initially, (3-aminopropyl)triethoxysilane (APTES) was heterogenised over SBA-15 (NH2-SBA-15), and the Schiff base formation was followed by a reaction with MTC in hot methanol. X-ray diffraction and transmission electron microscopy showed that the SBA-15, NH2-SBA-15 and TS-SBA-15 materials had mesoscopically ordered, hexagonal symmetry and well-defined morphologies. 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy revealed a change in the silicon environment during modification. The N2 sorption experiments showed that the material has a large surface area (623 m2 g−1), acceptable pore diameter (6.5 nm) and reasonable pore volume (1.23 cm3 g−1) to accommodate the guest molecules inside the pore channels. Organic functionalisation was determined successfully by Fourier transform infrared and 13C cross-polarisation magic-angle spinning NMR spectroscopy. The optimal condition for the removal of Cr(III), Pb(II) and Zn(II) from water was explored by varying the parameters, such as pH, stirring time, initial concentration and amount of adsorbent. The results suggested that TS-SBA-15 can achieve the adsorption maximum within 6 h at pH 6.5. The adsorbent exhibited moderate-to-good adsorption ability towards the three metal ions under these experimental conditions. The selective adsorption performance of TS-SBA-15 from binary mixture was examined by keeping equal concentrations of metal ions in the mixture. TS-SBA-15 was more selective towards Cr(III) ions from both the binary and ternary mixtures at pH 6.5. The effect of the metal ion concentration over mole adsorption selectivity in the ternary mixture was examined by doubling the concentration of at least one metal ion in the mixture. The result was compared with the ternary mixture containing equal concentrations of metal ions. The reusability test showed that TS-SBA-15 can be recycled for at least six consecutive cycles without losing its adsorption ability.