Removal of heavy metal ions from aqueous solutions using Maltese Coralline limestones and Blue Clay

Abstract

Clays are widely utilized in several applications for the removal of heavy metals from aqueous solutions. They are used as barriers in landfills to prevent contamination of groundwater and sub-soil from leachates, or in the treatment of industrial water contaminated with heavy metals. In this study, Maltese Blue Clay, Upper Coralline limestone and Lower Coralline limestone were investigated with regards to the sorption/removal of seven heavy metal ions - cadmium, chromium, copper, manganese, nickel, lead and zinc using batch technique experiments. Variations of particle sizes of the sorbents, pH, and the effect of organic ligands in the solutions were studied to determine the influence on the removal I sorption. The total capacity of Blue Clay towards the metal ions under investigation was compared to results obtained for other clays around the world. It was shown that limestones are efficient as low-cost media in removing heavy metals from aqueous solutions. However, their removal properties are mainly associated with their capability to buffer the pH to a high value and the metals form insoluble precipitates. On increasing the input pH of the contaminated solution or by increasing the surface area of the adsorbent, removal of the heavy metals increases. Strong ligands hinder the adsorption of these heavy metals which thus remain in solution. The Blue Clay buffers the solution pH to a neutral value and this already has an effect on removing the heavy metals by precipitation, however, another important mechanism for removal in this case was probably by adsorption. In fact clay shows selectivity for the metals in question and this is in the order: Pb2+ < Cu2+ < Ni2 + < Cd2+< Zn2+ < Mn2+ < Cr3+.