Impact of Mixing on Fluid-solid Reactions

Abstract

International audienceThe concentration of solutes transported in hydrological systems evolves through mixing and biogeochemical reactions. These two processes are coupled as biogeochemical reactions depend non-linearly on local solute concentrations. Flow is known to affect solid-fluid reactions, such as dissolution, complexation or redox reactions, which play an important role on geochemical weathering, contaminant transport and the cycle of geochemical elements, such as carbon and nitrogen. The complex interactions between flow and reactions have been often quantified by residence time distribution approaches, which focus on the effect of flow on the exposure time of solute to mineral in each flow path, while generally neglecting mixing between different flow paths. Yet, mixing processes can have a significant impact on fluid-mineral reactions, as their kinetics depend non-linearly on local solute concentrations, which depend directly on mixing rates. In this presentation, we combine reactive transport models with solute mixing theories to establish effective solid-mineral kinetics, which are linked to mixing rates.Using reactive transport simulations with CrunchFlow, we demonstrate that mixing affects solid-mineral reactions characterized by a non-linear dependency on local concentrations. For reactions that give more weight to large concentrations, mixing decreases the overall reaction rate. On the opposite, for reactions that give more weight to low concentrations, mixing increases the overall reaction rate. To quantify this effect, we develop a new approach based on Probability Densiy Functions that describe the full concentration distribution and its coupling with non-linear reaction kinetics. These results open new perspectives to understand and model coupled mixing and solid-fluid reactions in heterogeneous media