Assessing long-term stability of cadmium and lead in a soil washing residue amended with MgO-based binders using quantitative accelerated ageing.

A soil washing residue (SWR) (containing 90% clay, cadmium (Cd) of 132 mg/kg, lead (Pb) of 3410 mg/kg) was stabilized with MgO (M) and MgO + bioapatite (MB) respectively at a dosage of 5% in w/w. The stability of the metals in original and amended SWRs was assessed after immediate treatment and using a laboratory accelerated ageing method simulating 26, 52, 78 and 104 years in field conditions. The dissolved Cd and Pb from the SWR in Toxicity Characteristic Leaching Procedure (TCLP) leachates significantly reduced (by 96.84-99.06%) by both amendments after immediate treatment. The stabilization remained effective within simulated 26 years as the TCLP leached Cd and Pb kept below regulatory levels. This immobilization was mainly due to the increased non-bioavailable Cd and Pb from sequential extraction tests in SWR by the amendments. At simulated 52 years, the TCLP leached Cd from M and MB exceeded regulatory level by 106% and 1% respectively. Large amounts of Cd and Pb were leached out by 36.74-48.18% regardless of the treatments at simulated 104 years. Although bioapatite can significantly aid the stabilization of metals by MgO, the stabilization effectiveness for both treatments diminished at simulated 52 years and from 52 to 104 years.