Metal Speciation in Phosphate and Thermal Stabilization of Contaminated Dredged Sediments

Sediments dredged from harbors and waterways are often contaminated with metal and organic toxins, therefore, their disposal is not only expensive but unsustainable. Treatment and reuse in construction as an alternative to disposal would reduce costs and conserve resources. While thermal treatment followed by carbon adsorption theoretically addresses organics, the fate of metals is of concern. Phosphate addition followed by thermal treatment at 700 °C was investigated for immobilization of metal contaminants with X-ray absorption spectroscopy (XAS). In sediments from Belgium where Zn concentrations were elevated, Zn was precipitated as ZnCO₃ and adsorbed to hydrous manganese oxide and hydrous iron oxide. Phosphate and thermal treatment resulted in sparingly soluble minerals, hopeite (Zn₃(PO₄)₂.4H₂O), and spinels, gahnite (ZnAl₂O₄) and franklinite (ZnFe₂O₄). Leaching assessments with the U.S. EPA toxicity characteristic leaching procedure (TCLP) confirmed Zn and other heavy metals were immobilized. Results of synthetic precipitation leaching procedure (SPLP) showed compliance with the New Jersey State impact to groundwater criteria.