Adsorption of cadmium ions on silica coated metal organic framework

Abstract

Heavy metal ions are commonly encountered pollutants that are toxic to both marine and human life, even at low concentrations. Adsorption is one of the most suited processes for heavy metal ions removal from aqueous solutions owing to its simplicity and efficacy at even low metal ion concentrations. TMU-5 Metal Organic Framework (MOF) was sonochemically synthesized and coated with a silica layer to improve its hydrophobicity. Cadmium found in landfills, especially from exhausted cell batteries, was chosen as the model solute to demonstrate adsorption with TMU-5. The chosen MOF was found to have a good affinity towards cadmium but leaked zinc ions into the water. Though its adsorption capacity was relatively high, the MOF did not possess water stability especially at low pH. When the silica layer coating was applied to the MOF, the adsorption capacity was reduced to 634 mg/g for cadmium. However, the water stability of the MOF was considerably improved even at moderate pH and beyond the point of zero charge, where the adsorbent was conducive for electrostatically adsorbing the cadmium ions. The silica coated MOF could be effectively used at relatively moderate metal ion concentrations and close to natural pH condition of the contaminated aqueous solution. The coated MOF could be reused without decrease in its adsorption capacity.