On the localization of chemical reactions in multicontinuum media

Abstract

Reactive transport (RT) through heterogeneous media, may cause chemical heterogeneity if water flux is slow through portions of the medium. In such cases, chemical localization (i.e., the occurrence of reactions that would not occur in well mixed media) may develop, which is especially relevant for biochemical reactions occurring in biofilms. The objective of this work is to study the conditions for chemical localization. We represent the impact of heterogeneity by means of the non-local multirate mass transfer (MRMT) model, which views the porous media as consisting of one mobile and many immobile zones. A dimensional analysis of the governing equations shows that the problem is characterized by reaction times and the distribution of residence times in immobile zones, relative to transport time. To analyze the interplay between them, we simulated simple RT problems in multicontinuum media. Results indicate that immobile zones with residence times much smaller than transport can be lumped together with the mobile zone by modifying the reaction rates, which reduces computations. More importantly, reactions driven by species that are not present in the inflowing water but are the result of previous reactions will take place preferentially in immobile zones, whose residence time is comparable to or larger than reaction times. In fact, daughter species may take a long time and distance to build up. That is, daughter species will not be largest near the inflow, where parent species display largest concentrations, but further downstream at isolated (long residence times) immobile zones.