Quantification of Mn(III)-Oxide Redox Activity: Integrating Mediated Electrochemistry with Kinetic and Mass-Transport Modelling.

Predicting the outcomes of redox processes in aquatic environments requires quantitative constraints on electron transfer reactions. Advances in electrochemical techniques have significantly improved our ability to quantify and constrain mineral-related biogeochemical redox processes that regulate carbon, nutrient, and contaminant cycling in aquatic environments. Manganese oxides are key redox-active minerals in these systems that influence organic matter transformation, nutrient availability, and contaminant fate. In particular, Mn(III)-oxides occupy a critical role due to their ability to act as both electron acceptors and donors in the redox landscape. However, their redox characteristics are poorly understood due to their intermediate and metastable nature. Here, we apply mediated electrochemical analysis (MEA) as an analytical laboratory technique to study the effect of changing redox conditions and solution chemistry on the redox-activity of two representative Mn(III)-oxides—manganite and hausmannite. We initially use MEA to benchmark the reactivity of these Mn(III)-oxides in “simple” pH-controlled aqueous solutions. To interpret the results from MEA, we use a process-based model that couples interfacial electron transfer kinetics with mass-transport dynamics to simulate how the current response changes as a function of electrochemical driving force. Using this approach, we extract redox parameters that dictate the reactivity of these Mn(III)-oxides as a function of shifting redox conditions. After benchmarking the redox behaviour in controlled conditions, we investigate the effect of solution chemistry by performing MEA experiments in aqueous matrices containing carbonate, organic matter, and environmentally relevant ligands to characterize their effects on mineral reactivity. By providing quantitative constraints on Mn redox reactivity, this work illustrates how advanced electrochemical techniques can potentially  improve predictive understanding of coupled biogeochemical processes and inform models of water quality and ecosystem response under changing environmental conditions.