Influence of chemical reaction kinetics on electrokinetic remediation modelling results

The local chemical equilibrium (LCE) assumption has been demonstrated as a suitable approach for modeling reactive-transport processes through porous media in electrokinetic remediation (EKR) treatments. LCE can be assumed when the kinetic rates of reversible chemical reactions (in both direct and reverse directions) are fast compared to transport rates. This occurs in most aqueous-phase homogeneous systems. However, the LCE assumption could exceed the range of validity for heterogeneous reactions, such as precipitation/dissolution. In EKR processes, the rate at which target contaminants are released from their mineral-bound forms is essential. For example, in acid-enhanced EKR treatments, the acid environment generated at the anode is exploited to dissolve the contaminant-containing minerals into mobile compounds. The progress of the acid front is generally hindered by the presence of buffering minerals, such as e.g., calcite (CaCO3). Experimental results suggest that the dissolution of these carbonates, does not take place under LCE conditions. Therefore, to further develop the prediction capability of EKR models and to understand the role of dissolution kinetics on the rate of extraction of contaminants, the kinetic rates of the “slow” reactions have to be taken into account. In this work, an EKR reactive-transport model based on Nernst-Planck (NP) equations was implemented under the LCE assumption, while taking into account the kinetic rates of the main precipitation/dissolution reactions.