Electrokinetic mobilization of inorganic contaminants from dredged sediment

The management of dredged sediment is commonly recognized as a complex procedure in industrialized countries, where sediments are often contaminated by metals and organic contaminants due to the presence of a number of point and diffuse contamination sources (i.e., river and coastal discharges, shipping, yacht maintenance). Due to the huge amounts to be dredged every year due to harbor maintenance or environmental dredging activities, the identification of management options different from landfilling is claimed. The valorization of dredged sediments in application typical of the civil and environmental sector (i.e., road construction, embankments, structural materials formulation) usually needs a pretreatment stage aimed at improving the physical (water content, grain size distribution) and chemical properties (pollutants content, salt concentration) of the sediments. Among the available treatment options, Electrokinetic Remediation (EKR) seems to be particularly suitable due to a number of potential advantages, like the capability of treating fine graded and low-permeability matrices such as sediments, and the ability to remove organic and inorganic contaminants. EKR is based on the application of an electric field, originated from electrodes immersed in aqueous solutions (anodic and cathodic solutions), to a portion of the contaminated sediment so as to promote the mobilization of the contaminants. The application of EK to contaminated porous media induces the mobilization of ions towards the electrodes due to electromigration. electroosmosis, and electrophoresis. In order to improve the remediation process is possible to use enhancement agents like acids, chelating agents, surfactants added in electrode compartments. In particular, for the metals removal the use of the acids seems to be a promising strategy. However, due to the complexity of both the sediment matrix and the contaminants nature, despite the interest demonstrated by the scientific sector in EK, the uncertainties in process yields and control still hinder the full-scale application. The aim of this work consists in the study of lab-scale experiments of EKR carried out on metal contaminated sediment samples, collected in three Italian harbors. The EKR tests were performed using different types of agents, namely acetic, nitric and hydrochloric acid, EDTA and ascorbic acid. Each sample was characterized before the EKR test so as to determine: water content, grain size distribution, pH, Acid Neutralization Capacity (ANC), metal content, metal speciation and electrical resistivity. The results showed that the most critical elements were Cu, Cr, Ni, Cd, Pb and Zn, as they were present at concentrations above the limits defined by the Italian Ministry for the Environment. The EKR tests, performed in plexiglass cells, were conducted using the same conditions in terms of current density, while the duration of the tests and the dosages of the extracting agents were varied, in order to understand the best enhancement conditions on the basis of metals removal and metals mobilization results and to identify the parameters that can hinder the treatment. Furthermore during the treatment the electrical resistivity was monitored in order observe whether this physical parameter could provide information on the progress of decontamination. Main conclusions: The use of acids in order to neutralize the basic front produced at the cathode was found to be the best strategy in enhanced electrokinetics tests on dredging sediments. The tests also evidenced that the process was influenced by several factors: treatment time, properties of the matrix (buffering capacity) and of metals (metal fractionation). It was also observed the nonexistence of a condition of equilibrium between the process fluids and the matrix treated, hence it was not possible apply the prediction models generally used for the description of the chemical reactions. The monitoring of the electrical resistivity shown that through this parameter is not possible evaluate the migration of the metals during the treatment, likely due to the increase of the ionic content in the system caused by the addiction of the extracting agents. If this hypothesis is confirmed, the monitoring of the electrical resistivity could be used to evaluate the migration of the enhancing agents through the specimen and at this regard, further investigations are in progress.