Heavy metals mobilization from harbour sediments using EDTA and citric acid as chelating agents

In this paper experimental tests of heavy metals mobilization from sediments collected from an Italian harbour are presented and discussed. Batch tests of washing with an aqueous solution of chelating agent were performed at selected liquid/solid (L/S) ratios and chelant concentration (0.01, 0.05 and 0.1M). Two chelating agents were chosen: EDTA and citric acid. The objective of the tests was to investigate metal mobilization and soil mineral dissolution promoted by the washing treatment. The study was focused on the mobilization of Pb, Zn, and Cu: their concentrations in the contact solution, together with Fe and Ca concentration, were monitored along time for 144h. Results of experimental tests performed at different chelant concentration and L/S ratio show EDTA effectiveness in mobilizing metals bond to the sediment. The overall amount of the three selected metals (Cu, Pb and Zn) extracted in the tests with EDTA varied in a quite narrow range (from 2.262 to 2.724mmol), while the higher was the EDTA concentration in the washing solution, the higher the dissolution of other competitive cations, such as Ca and Fe. In addition, a general difficulty in copper extraction from the organic fraction of the sediment was observed, due to the high stability of copper humate. The mechanism of metals extraction was found to involve the initial metals salts dissolution, that was pH-dependant, followed by the development of exchange reactions between the metal-EDTA complexes previously formed. Depending upon the pH of the washing solution an initial high Ca dissolution was observed, followed by dramatic reduction in Ca(2+) content, mainly determined by calcium carbonate precipitation, as a result of the increase of pH during mixing. Metal mobilization was evaluated performing sequential extraction tests: the dissolution of Cu, Pb and Zn from the carbonate fraction of the sediment was almost total, as the mobilization of Pb and Zn from the Fe-Mn oxides and hydroxides fraction. On the contrary, as a consequence of the mobilization induced by washing treatment, about 17% of the remaining Cu and 10% of the remaining Zn resulted in the exchangeable fraction. At the investigated operating conditions, citric acid was unable to extract Pb and Cu from the sediment, while at the strong acidic conditions maintained during the extractions with citric acid, a Zn mobilization up to the 80% of the initial amount was observed, depending upon the operating conditions in the extractions. In addition, because of the great affinity between citric acid and Fe, and the strong acidic conditions maintained during extraction, a high Fe dissolution was observed (up to about the 39% of the total Fe content in the sediment).