ABSTRACT

The adsorption of copper(ll), cadmium(ll) and chromium(lll) ions from aqueous solutions by unmodified and Schiff base-modified silica was investigated in this study. The pure silica and the Schiff base immobilized silica i.e., 3-aminopropyl trimethoxysilane (APTS)-modified and Salicylaldehyde-APTS modified silica were characterised using Fourier Transform Infra-Red (FTIR) spectral analysis. The influences of experimental conditions such as pH, contact time, initial metal concentration and temperature were determined in a batch adsorption technique. The amount of metal ions adsorbed by the adsorbents was determined Spectrophotometrically using AAS. Results showed that optimum adsorption of the metal ions occurred at a pH of 7 by the pure silica and percentage amounts of Cu(ll), Cd(ll) and Cr(lll) ions adsorbed were 98.86 %, 97.14 % and 99.60 % respectively. That of 3-aminopropyl trimethoxysilane (APTS)-modified silica occurred at a pH of 5 and percentage amounts of Cu(ll), Cd(ll) and Cr(lll) ions adsorbed were 99.39 %, 96.64 % and 99.91 % respectively while that of Salicylaldehyde-APTS modified silica occurred at a pH 6.0 and percentage amounts of Cu(ll), Cd(ll) and Cr(lll) ions adsorbed were 96.36 %, 97.78 % and 99.92 % respectively. Equilibrium batch adsorption of the metal ions was achieved within 60 min. An increase in the metal ions adsorption with an increase in concentration was observed for the range of concentrations investigated. Removal efficiencies of the adsorbents followed the trend: Salicylaldehyde-APTS modified > APTS-modified > pure silica. An optimum amount of chromium adsorbed by pure silica was 10.75 mg/g; that of APTSmodified silica was 10.77 mg/g and 10.98 mg/g by salicylaldehyde-APTS. For Copper(ll) ions, it was observed that the optimum amount adsorbed by pure silica, APTS-modified and salicyaldehyde-APTS-modified were 9.13 mg/g, 9.44 mg/g and 9.81 mg/g respectively. Cadmium(ll) ions adsorbed by pure silica, APTS-modified silica and salicylaldehyde-APTSmodified silica respectively were 7.91 mg/g, 8.87 mg/g and 9.48 mg/g. Results showed that 99.92% of Chromium(lll), 97.78 % of Copper(ll) and 96.36% of Cadmium(ll) were sequestered by the adsorbents i.e. salicylaldehyde-APTS-modified silica, APTS-modified and pure silica in that order and the trend followed: Cr(lll) > Cu(ll) > Cd(ll). Three equilibrium isotherm models namely: Langmuir, Freundlich and Dubinin-Radushkevich isotherms applied to the experimental adsorption data showed that Langmuir isotherm gave the best fit with regression coefficient, R2 of 0.972. Kinetic study also showed that pseudo second- order equation gave the best fit for the experimental data with correlation coefficients >0.990, indicating that the rate-controlling step for the adsorption process may be chemical reaction. The thermodynamic parameters obtained indicate that the adsorption of the metal ions unto the adsorbents is feasible and exothermic. Desorption studies showed that about 24.33% of the metal ions were desorbed with the eluting agents: acids, bases and neutral solutions. This study therefore reveals that these adsorbents could be good materials for the removal of these heavy metal ions from aqueous solutions and may possibly be useful for treating industrial waste waters.