Effect of octahedrally coordinated aluminum ions on the uptake of Au(III) chloro-hydroxy complexes in Al-Si systems

The uptake of Au(III) complexes (initial [Au] = 25 μmol·dm^-3) by Al-Si-bearing precipitates formed by the hydrolysis of aluminum ions (initial [Al] = 0.04 mol·dm^-3) in the presence of either solid silica or aqueous silicic acid was investigated at pH 6.0, [Cl-] = 0.24 mol·dm^-3, and ambient temperature. A maximum of 40% of the total quantity of Au(III) complexes present in solution was sorbed onto the precipitate after 24 h in the absence of silica. Both solid silica and silicic acid inhibited the gold uptake, and silicic acid was a more effective inhibitor. Analyses of the solids showed that the fraction of octahedrally coordinated aluminum decreased as the bulk atomic Si/Al ratio in the solid increased. This decrease was more rapid in the presence of silicic acid than solid silica owing to the more efficient incorporation of aluminum into the silica structure. The zeta potential of the precipitates decreased as the fraction of octahedrally coordinated aluminum decreased and the atomic Si/Al ratio increased. The gold uptake decreased as the zeta potential of the precipitates decreased, suggesting that negatively charged Au(III) complexes were sorbed on the precipitates via electrostatic interactions. Our findings suggest that the octahedrally coordinated aluminum determined the quantity of the gold uptake by governing the positive charge on the precipitate surface. In addition, desorption was promoted by the presence of solid silica after 5 h, suggesting that silica may promote the release of Au(III) chloro-hydroxy complexes as well as other negatively charged metal complexes from aluminum oxides or iron oxides in soil or sediment.