The Electron-Yielding Capacity of the Matrix Can Explain Apparent Zero-Order Reduction of Electron-Acceptors in Aquifers at Steady State

Microbially mediated reduction reactions of soluble electron acceptors, such as nitrates, involve solute transport and interphase mass transfer coupled to microbial dynamics. Yet, simple zero-order kinetics can often describe the reduction rates at the aquifer scale. We tackle this discrepancy by deriving the steady-state concentrations of subsurface biomass, electron acceptors, and electron donors for a reactive system. As an example, we model denitrification coupled with the oxidation of dissolved organic carbon stemming from solid-phase natural organic matter. Our closed-form solution demonstrates that the microbially mediated hydrolysis step, releasing the electron donor from the matrix, limits the overall reaction. Neither the steady-state concentration of biomass nor the reduction rate of the electron acceptor depends on the available electron-acceptor concentration. These findings are confirmed by numerical experiments with a more complete description of the system. Our theoretical derivations provide a mechanistic explanation for apparent zero-order reduction rates observed in field and experimental settings at steady state. They apply to various settings in which dissolved electron acceptors react with electron donors released from the matrix. To better understand and quantify the reduction rates of electron acceptors at field sites, we propose investigating the processes and rates controlling the microbial access to solid-phase electron donors.