ABSTRACT

This study compared the potential of four adsorbents: pure silica, 3-aminopropyl trimethoxysilane modified silica (APTSMSi), Schiff- base modified silicas APTSMSi-PMAPSA and APTSMSi-4-HPMAPSA in the removal of Pb(II), Cd(II) and Ni(II) ions from aqueous solution. In this study, two Schiff base ligands (N-[(4-{[(Z)-phenylmethylidene]amino}phenyl)sulfonyl]acetamide (PMAPSA) and N-[(4-{[(Z)-(4 hydroxyphenyl)methylidene]amino}phenyl)sulfonyl]acetamide (4-HPMAPSA)) were synthesized and immobilized onto silica support after silanization by 3-aminopropyl trimethoxysilane (APTS) to give APTSMSi-PMAPSA and APTSMSi-4-HPMAPSA. The Pure and modified silica adsorbents- were characterized using Fourier Transform Infra-Red (FTIR) spectroscopy and Scanning Electron Microscopy (SEM). The influences of experimental conditions such as pH, adsorbent dose, initial metal ion concentration, contact time, temperature and co-ions were determined in a batch adsorption technique.  The batch experiments were conducted to evaluate the sorption performance of these adsorbents in the removal of Pb(II), Cd(II) and Ni(II) ions from aqueous solution. Results show that for the four silica-based adsorbents, the optimum pH for adsorption of Pb(II), Cd(II)  and Ni(II) ions occurred at a pH of 4.0, 6.0 and 6.0 respectively and having maximum adsorbed amounts of 19.901, 19.85 and 19.66 mg/g for the metal ions following the original order onto pure silica. Values of 19.92, 19.874 and 19.676 mg/g were obtained for adsorption of Pb(II), Cd(II)  and Ni(II) ions onto APTSMSi. Also, values obtained for adsorption onto that of  APTSMSi-PMAPSA were 19.941, 19.899 and 19.696 mg/g following the original order while that of sorption of the metal ions onto APTSMSi-4-HPMAPSA were 19.96, 19.974 and 19.716 mg/g for Pb(II), Cd(II)  and Ni(II) ions respectively. Studies on the effect of temperature show that the optimum temperature for adsorption of the metal ions onto pure silica and APTSMSi is 50^oC while that of APTSMSi-PMAPSA and APTSMSi-4-HPMAPSA) occurred at 40^oC and beyond these temperatures, there was a decrease in removal efficiency.  Equilibrium batch adsorption of the metal ions was achieved within 120 min. An increase in the metal ions adsorption with an increase in the concentration was observed for the range of concentrations investigated. Removal efficiencies of the adsorbents followed the trend:  APTSMSi-4-HPMAPSA > APTSMSi-PMAPSA > APTSMSi > Pure silica indicating that the modified adsorbents were most effective in the removal of the metal ions.  At the maximum concentration of 60 mg/L employed in the study, the trend of adsorption of the metal ions by the adsorbents followed: Pb(II) > Cd(II) > Ni(II). The kinetics of the sorption process was evaluated using pseudo-first-order, pseudo-second-order and intraparticle diffusion models. Results show that the adsorption kinetics follows pseudo-second-order with (R² = 1) for all the four adsorbents. The transport mechanism process was particle and film diffusion controlled. The sorption equilibrium was analysed with Langmuir, Freundlich, Tempkin, Halsey, Harkins-Jura and Dubinin–Radushkevich (D–R). The best interpretation for equilibrium data was given by Freundlich and Halsey, which suggesting the possible formation of multilayer of the metal ions onto the adsorbents. The negative and low ∆G° values obtained (< 40 kJmol^-1) show that the mode of metal ions- adsorbent bonding followed physisorption and was feasible. Results from desorption studies further supports physisorption as the mode of bonding of the metal ions onto the surfaces. This study therefore reveals that the adsorbents could be promising materials for the removal of these heavy metal ions from aqueous solutions and may be possibly be applied in the treatment of wastewaters.