Chromate Adsorption by Kaolinite

Chromate (CrO 4 2− ) adsorption was investigated on kaolinite (0.2–2 μ m) saturated with NaClO 4 over a range of pH. Adsorption increased with decreasing pH because of protonation of chromate and/or variable charge sites on kaolinite. Chemical pretreatment to remove noncrystalline and crystalline oxide contaminants affected the magnitude of CrO 4 2− adsorption, but not the pH range over which CrO 4 2− adsorbed. Chromate adsorption at different sorbate and sorbent concentrations increased below the pH zpc for the kaolinite edge, suggesting the formation of weak surface complexes. If CrO 4 2− and SO 4 2− were present at equal concentration (5.0 × 10 −7 M), the two solutes sorbed independently, suggesting binding to separate sites. The presence of excess SO 4 2− (5.0 × 10 −4 M), however, unexplainably enhanced CrO 4 2− adsorption. The adsorption of both Chromate and sulfate can be described in terms of a site-binding model of the kaolinite edge, in which the edge is viewed as composite layers of Al and Si oxide. Surface complexation constants for CrO 4 2− on kaolinite were similar to those for alumina, pointing to the importance of Al-OH edge sites in Chromate adsorption.