Abstract
In multispecies electrolyte solutions, each individual species can migrate according to its specific ionic properties. This process is called electrochemical migration or electro-diffusion and is well-described by the Nernst–Planck equation. The common approach for solving the corresponding mathematical system is based on the null current (NC) assumption, which expresses the electric potential in terms of charges and concentrations of chemical components. This assumption has a great advantage as it eliminates the electric potential from the Nernst–Planck equation. However, the NC assumption has limited capacities in describing electro-diffusion processes when the domain is subjected to an external electric field. The validity of the NC assumption could be questionable, even in the absence of an external electric field. This topic has never been investigated in the past. The main goal of this work is to evaluate the validity of the NC assumption and to understand its effect on the model outputs. Thus, we present a new reactive transport model that allows for a reliable representation of the electrochemical migration process. This model is based on the Nernst–Planck and Poisson (NPP) equations which are solved together. We also implement a model based on the NC assumption. Both models have been validated by comparison with CrunchFlow, based on several benchmarks. The results show that in the case of high sorptivity, the NC assumption is no longer valid. Therefore, in the case of sorption processes, the NPP should be used to simulate coulombic interactions.
Highlights
Mass transfer accompanied by chemical reactions, or reactive transport (RT), in porous media is central to a wide range of applications in the fields of geology, hydrogeology, engineering, and environmental research
We validate the developed models based on common benchmarks and we compare them in different configurations of RT to evaluate the validity of the null current (NC) assumption
Assumption allows for eliminating the electrical from the null current assumption
Summary
Mass transfer accompanied by chemical reactions, or reactive transport (RT), in porous media is central to a wide range of applications in the fields of geology, hydrogeology, engineering, and environmental research. This equation takes into account the ionic interactions by including the electric field generated by such interactions. In the Poisson equation, the electric field is linked to the total ionic concentration of species in the system and the dielectric property of the medium This approach is rarely used in reactive transport codes because it cannot be implemented in existing codes. We validate the developed models based on common benchmarks and we compare them in different configurations of RT to evaluate the validity of the NC assumption
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