Zinc and cadmium mobility in a 5-year-old dredged sediment deposit: Experiments and modelling

Background Landfill deposits of contaminated, dredged sediments are subject to chemical alteration and especially to oxidation processes. Accordingly, sulphides are gradually oxidized leading to the formation of secondary phases and associated metals could become mobile and redistributed among the sediment components, such as carbonates, clay and freshly precipitated (hydr)oxides. Once mobilised, metals could represent a hazard for the environment and especially for drinking water supply facilities. Methods In the present study, leaching experiments have been carried out on a dredged sediment to study metal mobilisation after 5 years of field aging. First, kinetic batch tests allowed one to evaluate the impact of solid-liquid contact time and to determine the kinetic parameters. Secondly, two types of dynamic experiments have been conducted: dynamic flush reactor and column leach test to evaluate the impact of solution renewing by excluding or not excluding the transport processes, respectively. In order to evaluate the impact of calcium on the metal mobilisation, the column leaching test is conducted with pure water and Ca(NO₃)₂ solution, at the beginning and at the end of the injection, respectively. Geochemical and reactive transport modelling of the experiments was performed using the geochemical code CHESS and the reactive transport model HYTEC. Results and Discussion The studied sediment is complex with numerous reactive phases such as sulphides, (hydr)oxides, organic matter, phyllosilicates. All leaching tests highlight that zinc and cadmium are mobilised in significant concentrations and lead remains insoluble. A conceptual geochemical model of the sediment has been built to allow simulations of the whole experiments, based on a single, coherent phase description and parameter set. Simulations of the batch, flush and column experiments were performed taking into account the major reaction-controlling mechanisms including, among others, pH buffering, kinetic dissolution (carbonates, silicates) and ion exchange between calcium and exchangeable metals (Cd, Zn and Mn). The simulation results are in good agreements with the results obtained from the experiments. Conclusions The experiments showed that zinc and cadmium are reversibly bound to the sediment and underline the importance of calcium concentration with respect to their mobility. Modelling confirms that ionic exchange is the main mechanism involved in metal mobilisation for this sediment. Recommendations and Perspectives Metals can be retained as cation exchange species due to the high quantity of clays and organic matter present in dredged sediments. Cation exchange is a reversible mechanism and subsequent remobilisation of metals can be expected during oxidation and leaching phases. To quantify the mobilisation potential of polluted sediment, it is recommended to characterize the CEC properties of the sediment and calcium availability. Indeed, dissolution of minerals such as gypsum during rain events lead to metal mobilisation due to the increase of calcium concentration even for well-buffered sediments.