Stabilization of Cu and Cd in the presence of montmorillonite by means of coal fly ash

Coal fly ash pre-treated with NaOH at high temperature added to montmorillonite artificially contaminated with high amounts of Cu or Cd (15 mg/g dry solid matter) was capable of drastically reducing the solubility of both metals. Cu and Cd solubility decreased permanently with increasing aging time and after 6 months their solubility, both at 30 and 60 degreesC, became lower than I ppm. Experiments by EDTA sequential extractions showed that Cu and Cd stability in/on the solid phase increased over time. After 6 months of aging at 30 and 60 degreesC, 16% - 31% of the metals are no longer extractable by EDTA. No stabilization was observed at identical pH values in the absence of coal fly ash. The effects observed support the general mechanism for the formation of amorphous geopolymers originating from pre-treated coal fly ash, which immobilized Cu and Cd inside the solid phase.