Hydrothermal Alteration of Diatomite for Removal of Aqueous Cu2+, Pb2+ and Zn2+

Natural unconsolidated diatomite was hydrothermally treated with a strongly alkaline aluminate solution at 100 and 125 °C to obtain high-value material for cation removal from aqueous solutions. Diatomite (D) alteration at 1 and 0.022 M of KOH and Al,
respectively, leads finally to the formation of merlinoite (M). But coupled dissolution and precipitation yield in an intermediate amorphous potassium-rich alumosilica phase (IP), most conspicuous as 100 to 1,000 nm-sized spherules. All alteration products display high removal capacities for Cu2+, Pb2+, and Zn2+ from artificial and natural solutions. Interestingly the presence of the IP creates elevated surface areas and very high cation removal efficiencies up to 99.97 % without significant cation selectivity. In contrast a pronounced cation selectivity for the removal in the order of Pb2+ > Cu2+ > Zn2+ is stimulated by the occurrence of M, where removal
capacities and surface areas are lower compared to alteration products with the dominance of the IP. Thus bio-opal in terms of frequently occurring diatomite can be efficiently transformed to a highly active composite material for Cu2+, Pb2+, and Zn2+ removal by hydrothermal treatment. Tailoring issues are referred to the formation of the IP versus zeolites and the apparent hierarchical micro- to nanostructures of diatomite and the reaction products, respectively