How do reaction kinetics affect 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 [1]. This is the case of most aqueous-phase homogeneous systems. However, the LCE assumption could exceed the range of validity for heterogeneous reactions, such as precipitation/dissolution [2]. 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 alkalinity produced at the cathode by water electrolysis is neutralized by acid addition at the catholyte and the acid environment generated at the anode is exploited to lower the pH of the system in order to dissolve the contaminant-containing minerals. The progress of the acid front is generally hindered by the presence of buffering minerals, such as e.g., calcite (CaCO3). 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.