Reductive transformation of birnessite by low-molecular-weight organic acids

Soil biogeochemistry is intrinsically coupled to the redox cycling of iron and manganese. Oxidized manganese forms various (hydr)oxides that may reductively transform and dissolve, thereby serving as electron acceptors for microbial metabolisms. Furthermore, manganese oxides might reduce purely abiotically by oxidation of dissolved Mn²⁺ in a specific route of transformation from birnessite (Mn^IVO₂) into metastable feitknechtite (β-Mn^IIIOOH) and stable manganite (γ-Mn^IIIOOH). In natural soil solutions, however, dissolved Mn²⁺ is not abundant and organic substances such as low-molecular-weight organic acids (LMWOA) may be oxidized and serve as an electron donor for manganese oxide reduction instead. We investigated whether LMWOA would impact the transformation of birnessite at a temperature of 290 ± 2 K under ambient pressure for up to 1200 d. We found that birnessite was reductively transformed into feitknechtite, which subsequently alters into the more stable manganite without releasing Mn²⁺ into the solution. Instead, LMWOA served as electron donors and were oxidized from lactate into pyruvate, acetate, oxalate, and finally, inorganic carbon. We conclude that the reductive transformation of short-range ordered minerals like birnessite by the abiotic oxidation of LMWOA is a critical process controlling the abundance of LMWOA in natural systems besides their microbial consumption. Our results further suggest that the reduction of Mn^IV oxides not necessarily results in their dissolution at neutral and alkaline pH but also forms more stable Mn^III oxyhydroxides with less oxidative degradation potential for organic contaminants.