Keywords: Poisson-Nernst-Planck system, chemical equilibrium, electrokinetic soil remediation, electrokinetic desalination.

A mathematical model for the electrokinetic phenomena is described. Numerical simulations of different applications of electrokinetic techniques to the fields of civil and environmental engineering are included, showing the versatility and consistency of the model. The electrokinetics phenomena consist of the mobilization of species within the pore solution of porous materials induced by an externally applied electric field. Many physicochemical aspects have to be taken into account when modeling this strongly coupled reactive transport process.

A finite element integration of the Poisson-Nernst-Planck system of equations, accounting for ionic migration, chemical diffusion and advection is used for modeling the transport process. The advection term contributor is studied by including in the system the water transport through the porous media, mainly due to electroosmosis. The pore solution filling the porous materials undergoes an electroosmotic flow subject to externally applied electric fields. Electroosmotic transport makes electrokinetic techniques suitable for the mobilization of non-charged particles within the pore structure, such as the organic contaminants in soil.

Chemical equilibrium conditions are assumed between the aqueous species and the solid matrix for a set of feasible chemical equilibrium reactions defined for each specific application. A module for re-establishing the chemical equilibrium has been developed and included in the system for this purpose. Changes in the porosity along the domain arising from the reactivity of the solid matrix have been taken into account. These changes affect significantly to the transport process of both aqueous species and the water, since the effective transport coefficients depends on the porosity of the media.

The model described here gives good estimations of the results obtained from laboratory and pilot scale electrokinetic treatments. The model can be easily adapted to simulate different enhanced techniques and different chemical systems. Apart from the prediction ability, the detailed definition of the transport process increases the understanding of the main physicochemical aspects affecting the process.

Results from simulations of some test examples are presented, showing the versatility of the model. Two types of enhanced methods are compared: The electrokinetic desalination of a brick sample, using carbonated clay to hinder the acid penetration; and an acid-enhanced electrokinetic soil remediation process, where the basic front is neutralized in order to avoid the precipitation of hydroxides of the target heavy metal.

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