Numerical modelling of electrokinetic extraction of heavy metals from harbour sediments

We implemented a numerical model able to simulate transport of multiple species and geochemical reactions occurring during the remediation of metal-contaminated sediments characterized by a heterogeneous solid matrix, high buffering capacity and aged pollution. The main phenomena described by the model were: (1) chemical species transport through the porous matrix by electromigration and electroosmosis, (2) pH-dependent buffering of H+, (3) adsorption of contaminants onto sediment particle surfaces, (4) aqueous speciation and (5) formation/dissolution of solid precipitates. A constitutive relationship between zeta-potential and pH was used to compute the local electroosmotic permeability. The electroosmotic flow was computed by the volume averaging the electroosmotic permeability. The results of three electrokinetic tests, carried out with different treatment durations (32, 63 and 120 days) were used to validate the model. A good agreement was found between the experimental data and model predictions. In particular, pH and electroosmotic flow were predicted with good accuracy. The predicted metals profiles were also close to experiments profiles for all of the investigated metals (Pb, Zn and Ni) but an overestimation of the removal was observed in the regions close to the anode, possibly due to the significant metal content bound to the residual fraction, quantified with sequential extraction technique. These results encourage the use of the discussed modelling approach as an engineering tool for the design, implementation and removal efficiency prediction of electrokinetic technology at the field scale.