In situ immobilization of heavy metals in contaminated sediments by composite additives of hydroxyapatite and oxides

The in situ immobilization performances of Zn, Mn, Pb, and Cd in the contaminated sediments using additives [hydroxyapatite (HAP), CaO, MgO, and HAP + CaO/MgO] were investigated by European Community Bureau of Reference (BCR) extraction test, toxicity characteristic leaching procedure (TCLP), and heavy metal releasing test. The results of immobilization test showed that 40.70% of Zn, 47.47% of Mn, 70.40% of Pb, and 61.86% of Cd were immobilized in forms of the oxidizable and residual speciation in the sediments applied by composite additives (HAP + CaO or MgO) with each additive’s dosage of 10% of the sediment. The results of TCLP indicated that, compared to the blank samples, nearly 80% of Zn, Pb, Cd, and 50% of Mn were reduced in the leaching solutions using the composite additives. In addition, the heavy metal releasing experiment demonstrated that the releasing amounts of heavy metals in the immobilized sediments were much lower than those in raw sediments, and the equilibrium of metal releasing cost 24–31 days. The results of this work demonstrated that 10% HAP coupled with 10% CaO or 10% MgO is feasible in immobilizing heavy metals and decreasing the releasing risk of heavy metals in sediments.