COMPLEXATION OF COPPER WITH POLYMERIC SILICA IN AQUEOUS SOLUTION

Abstract

A series of experiments was conducted to study the uptake of Cu by colloidal SiO2 in aqueous solution. Solutions consisting of 2400 mg l−1 monomeric SiO2 were allowed to polymerize for 24 h at pH values from 4 to 9, producing coexisting populations of monomer and polymer in each solution. These solutions were combined with aqueous CuCl2 solutions in final Cu concentrations of 5–30 mg l−1. After the solutions had equilibrated for 24 h, they were filtered through 0.1 μm Millipore filters and analyzed by ICP-AES and gel filtration chromatography (GFC).Experiments conducted at pH 7 with varying Cu concentrations yielded a critical coagulation concentration of 15±1 mg l−1 Cu. GFC analyses of solutions in which coagulation occurred revealed no SiO2 polymer in the filtrates, suggesting that coagulation is due to the coalescence of SiO2 polymers by Cu. Experiments conducted with 20 mg l−1 Cu exhibited a gradual increase in Cu uptake from pH 5 to 7, followed by a sharp decrease in Cu uptake from pH 7.25 to 7.50. The gradual increase in Cu uptake up to pH 7 may reflect the increasingly negative surface charge of SiO2 polymer with increasing pH. The abrupt decrease in the adsorption of Cu to SiO2 polymer at pH values > 7.25 may be attributed to the decrease in dissociated Cu2+ relative to Cu(OH)2° at higher pH. Although it is traditionally held that the formation of Cu silicates, such as chrysocolla, occurs under acid conditions in supergene Cu deposits, this investigation suggests that Cu silicate formation may be favored in near-neutral solutions in nature.