Immobilization of cadmium in sediment by treatment with an aluminum salt and freeze-thaw dehydration

Laboratory-prepared silica suspensions, with a known amount of sorbed cadmium, and a cadmium-contaminated lake sediment were treated with aluminum nitrate at pH 9.5. The aluminum-to-solids ratio was varied from 1.6 to 40 mmol aluminum per gram of dry solids (0.05-0.2 mmol m^-2 for the silica suspension). Timed chemical extractions of cadmium were conducted with both 1 M ammonium acetate (NH₄OAc) at pH 8.0 and dilute HNO₃ at pH 5.0 to determine the extent and the mechanisms of cadmium retention by the Al-treated solids. The results indicate that the amount of cadmium extracted from both silica and sediment can be significantly reduced by Al treatment. The NH₄OAc-extractable cadmium in the sediment treated with 1.6 mmol aluminum per gram of sediment was 80% less than the extractable cadmium in the untreated sediment. Both enmeshment of contaminated particles in aluminum hydroxide precipitate and enhanced sorption of cadmium on Al-treated silica seem to be important in reducing cadmium mobility. Freeze-thaw treatment of the Al-treated sediment significantly reduced the volume of the settled solids and did not affect the tendency for the treatment to enhance cadmium retention. The sorption of cadmium by silica can be greatly enhanced by the addition of an amount of aluminum that is much less than that required for monolayer coverage of the silica surface.