EDTA-Enhanced Thermal Washing of Contaminated Dredged Marine Sediments for Heavy Metal Removal

Preliminary analysis on dredged marine sediments from Benoi basin in Singapore was carried out showing elevated concentrations of Zn, Cu, Pb, Cd, Cr and Ni. Ethylenediamine tetraacetic acid (EDTA) thermal washing experiments were conducted for heavy metal extraction at temperature 100 C. Results indicated the significant efficiency of thermal washing to extract Pb, Zn and Ni. However, there was little or no influence in the removal of Cu and Cr and a slight effect to Cd indicating multiple mechanisms. In addition, agitation was found to have great influence on the removal efficiency of heavy metals as experiments without agitation performed lesser or no extraction due to limited contact of the washing solution and the dredged sediment. Sequencing processes of thermal treatment followed by EDTA washing showed limited performance, likely due to thermal stabilization of the contaminants particularly at low liquid-to-soil (L/S) ratio. Furthermore, sequential extraction analysis on the metal speciation was performed before and after thermal washing. It was revealed that metals mainly extracted from fractions bound to carbonates and Fe-Mn oxides, the relative mobile fraction. On the contrary, metals in the residual fraction displayed a considerable stability.