Fabrication of (amino)thiol chelating agents on SBA-15 and MCM-41 and applications in the extraction of Cd(II), Pb(II) and Cr(VI) cations from aqueous solutions.

Abstract

Reactions of N¹,N³-bis(2-mercaptoethyl)isophthalamide with (3-aminopropyl)triethoxysilane (3-APTES) afforded the the corresponding Schiff base chelating ligand, (1Z,3Z)-N¹,N³-bis(2-mercaptoethyl)-N'¹,N'³bis(3(triethoxysilyl)propyl) isophthalimidamide (L1) in high yields. Subsequent immobilization of L1 on mesoporous silica supports (SBA-15 and MCM-41) following a divergent method produced the respective fabricated L1@SBA-15 and L1@MCM-41 materials. Characterization of the fabricated chelating agents was accomplished using SEM, TEM, EDX, BET, PXRD, FT-IR, and TGA-DTA/DSC techniques. The fabricated materials were used as adsorbents for the removal of Cd(II), Pb(II) and Cr(VI) metal ions from water. Both the L1@SBA-15 and L1@MCM-41 materials demonstrated high extraction efficiencies for Cd(II) and Pb(II) cations, achieving over 98% and 97% removal, respectively, across a broad pH range (3-9). For Cr(VI) anions, relatively lower optimal extraction efficiencies of 67% and 77% were recorded between pH 3-12, for L1@SBA-15 and L1@MCM-41, adsorbents, respectively. Faster extraction kinetics were observed for Cd(II) and Pb(II) cations, with over 94% removal achieved within 30 min, even at low initial metal concentrations (5 ppm) and adsorbent dosages (10 mg). Kinetic modelling showed that the adsorption process followed a pseudo-second-order kinetics, with high correlation coefficients (R²) for Cd(II), Pb(II), and Cr(VI) of 0.97573, 0.97348, and 0.91017 using L1@MCM-41, and 0.9754, 0.97089, and 0.89411 using L1@SBA-15, respectively. Equilibrium data best fitted the Freundlich isotherm model, with R² values of 0.97952, 0.99892, and 0.97374 for L1@SBA-15 and 0.99879, 0.99999, and 0.90923 for L1@MCM-41, corresponding to Cd(II), Pb(II), and Cr(VI), respectively. In competitive metal ion systems, the extraction selectivity followed the order Cd(II) > Pb(II) > Cr(VI), with L1@SBA-15 exhibiting superior overall performance compared to L1@MCM-41. The extraction mechanism was postulated to occur through the coordination of the metal ions to the nitrogen/sulfur donor atoms in the chelating agents, consistent with the hard and soft acid and base (HSAB) principle. Furthermore, L1@MCM-41 demonstrated notable regeneration potential, retaining Cr(VI) extraction efficiencies over four successive cycles.