Sequential extraction of phosphate- and thermal-treated New York/New Jersey harbor dredged sediments

Contaminated sediments dredged from harbors and waterways can be reused when treated to remove or stabilize contaminants. Phosphate addition followed by calcination at 700°C was shown previously with X-ray absorption spectroscopy to change zinc (Zn) speciation to less labile forms, and to reduce metal leachability. In this study, changes in metal association with sediment phases were investigated with the modified three-step European Community Bureau of Reference (BCR) sequential extraction procedure of the Standard Measurement and Testing (SM&T) Programme. The analysis showed a shift in metal forms from labile associations with exchangeable and carbonates phases to recalcitrant residual forms including spinels, ZnAl₂O₄, ZnFe ₂O₄, NiAl₂O₄, and CuFe ₂O₄; phosphates, Zn₃(PO₄) ₂•4H₂O and Pb₅(PO₄) ₃OH; and, oxides, Mn₂O₃ and Mn ₃O₄. At the same time, destruction of organics through calcination was demonstrated by a reduction in the metal associated with the organic/sulfide phase, for example, from 14 to 3% for Zn. Leaching studies confirmed stabilization of metals with up to an 89% reduction in leachability, consistent with thermodynamic predictions. However, solubility appears to be controlled by reaction kinetics as solutions were slightly undersaturated. These results validate structural changes in phosphate and thermal stabilization of contaminants in sediments and demonstrate how an understanding of metal speciation can complement the simple regulatory leaching tests in predicting leachability.