Effective kinetics driven by dynamic concentration gradients under coupled transport and reaction

Biogeochemical reaction kinetics are generally established from batch reactors where concentrations are uniform. In natural systems, many biogeochemical processes are characterized by spatially and temporally variable concentration gradients that often occur at scales which are not resolved by field measurements or biogeochemical and reactive transport models. Yet, it is not clear how these sub-scale chemical gradients affect reaction kinetics compared to batch kinetics. Here we investigate this question by studying the paradigmatic case of localized pulses of solute reacting with a solid or a dissolved species in excess. We consider non-linear biogeochemical reactions, representative of mineral dissolution, adsorption and redox reactions, which we quantify using simplified power-law kinetics. The combined effect of diffusion and reaction leads to effective kinetics that differ quantitatively and qualitatively from the batch kinetics. Depending on the nonlinearity (reaction order) of the local kinetics, these effects lead to either enhancement or decrease of the overall reaction rate, and result in a rich variety of reaction dynamics. We derive analytical results for the effective kinetics, which are validated by comparison to direct numerical simulations for a broad range of Damkohler numbers and reaction order. Our findings provide new insights into the interpretation of imperfectly mixed lab experiments, the effective kinetics of field systems characterized by intermittent reactant release and the integration of sub-scale concentration gradients in reactive transport models. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The study explores the impact of sub-scale chemical gradients on reaction kinetics using localized pulses of solute reacting with a solid or a dissolved species in excess as a case study. The combination of diffusion and reaction leads to effective kinetics that differ quantitatively and qualitatively from batch kinetics, resulting in a variety of reaction dynamics depending on the nonlinearity of the local kinetics. The findings offer new insights into interpreting imperfectly mixed lab experiments and integrating sub-scale concentration gradients in reactive transport models for field systems.