Evaluation of a combined chemical, electrochemical, and mechanochemical approach for metal extraction from contaminated dredged sediments: preliminary studies

Abstract Purpose This study investigates the efficacy of a combination of chemical, electrical, and mechanical methods for extracting specific metal contaminants from marine dredged sediment. Materials and methods Samples of muddy contaminated sediment from a Spanish harbor were characterized, including the mode of occurrence of heavy metals, using sequential chemical extraction. Desorption tests were conducted using the sediment in its fresh state, in a custom-built cell/reactor filled with an electrolyte—either a solution of 0.25 M citric or acetic acid, or deionized water. Electrical current, ultrasonic energy, and circulating flow were applied in various combinations, and the efficacy of such combinations on the metal desorption was evaluated. After the experiments, the solutions were analyzed using inductively coupled plasma-optical emission spectrometry (ICP-OES). X-ray diffraction (XRD) and thermogravimetric analysis/differential thermal analysis (TG/DTA) were performed on the sediment. Results The sequencial extraction revealed that most metals (excluding Hg, Pb and Zn) were primarily bound to the residual fraction (fraction V), with As and Cu exhibiting the highest concentrations. Cadmium and mercury were preferentially extracted in fraction IV, associated with organic matter. Pb and Zn exhibited their highest percentages in fraction III, bound to Fe–Mn oxides. Regarding desorption, the results indicated that increasing treatment time enhanced metal desorption, with the most significant effect observed during the initial stages. The electrolyte used emerged as the most influential factor. Citric acid proved more effective for As, Cr, and Ni, while acetic acid favored Cu, Pb, and Zn extraction. As, Pb, and Zn exhibited preferential desorption in the presence of ultrasounds, while Cr, Ni, Pb, and Zn desorbed more readily under electrical current. Application of electrolyte circulation had a positive effect on the extraction of all metals. Conclusions Changes in the electrolyte's chemical composition are the most significant factor influencing metal desorption. In addition to the electrolyte used, the application of some form of energy had a slight positive effect on metal desorption. However, at this stage it seemed that the synergistic effect of both electric field and ultrasounds appears to be only relevant for Cu. Electrolyte circulation had a positive impact on metal desorption for all metals tested. The concentration of recalcitrant and refractory organic matter decreased after all the tests, indicating its degradation into more labile matter.