• cost minimisation
• field scale
• buffering capacity
• dredged sediments
• multispecies transport
• kinetics
• geochemical model
• reactive transport
• heavy metals
• numerical model
• electrokinetics

Dredging activities in harbours are needed for the maintenance of waterways. When dredged sediments are polluted, selection of the best intervention strategy is critical. In most cases, sediment treatment is a necessity prior to their relocation or reuse. Electrokinetic remediation can be applied in such situations, especially when most technologies are expected to fail, as in the presence of heavy metal-contaminated sediments with low hydraulic conductivity. However, due to the complexity of the mechanisms involved, the implementation of this remediation technique is troublesome because tools for reliable process prediction and design are not currently available.
The present research aimed at developing a simulation-based methodology for application to the optimisation of the design of electrokinetic systems. A set of laboratory electrokinetic tests was carried out in order to identify the main parameters and processes affecting the removal of heavy metals from real contaminated sediments. A numerical model was implemented to simulate transport of multiple species and geochemical reactions occurring during treatment and it was applied to reproduce the results of the laboratory tests. The main phenomena described by the model were: (1) species transport by diffusion, electromigration and electroosmosis, (2) pH-dependent buffering of hydrogen ions, (3) adsorption of contaminants onto sediment particle surfaces, (4) aqueous speciation and (5) formation and dissolution of solid precipitates. A constitutive relationship between zeta-potential and pH was calibrated and applied to compute the electroosmotic flow. A good agreement was found between simulations of pH, electroosmotic flow and experimental results. The predicted concentration profiles of residual metals in the sediment were also close to experimental profiles for all of the investigated metals (Pb, Zn and Ni). Some removal overestimation was observed, possibly due to the significant metal content bound to residual fraction.
The model was then applied to simulate electrokinetic processes at field scale. Model geometry and boundary conditions were scaled up to simulate sediment treatment in a 150 m3 demonstrative ex-situ plant. Field measurements demonstrated that a two-dimensional schematisation of the electric field could simulate the actual electric field with enough accuracy. Simulations were performed to reproduce acid front migration and lead transport. The agreement between observed and simulated pH profiles was satisfactory, thus validating the reliability of the adopted modelling approach.
Finally, a parametric study was carried out to evaluate the influence of model parameters on the simulation results. The effects of inter-electrode distance and sediment buffering capacity were studied. Simulations allowed us to calculate lead removal rates as a function of time. After defining consumable costs (i.e., energy expenditure, acid consumption, electrode and pipe costs), cost curves were calculated from simulation results. The resulting curves allowed us to identify the optimum design parameters which minimised the overall costs. The results obtained can serve as a valuable tool to support evaluation and design of electrokinetic remediation systems.