Coupled redox cycling of Fe and Mn in the environment: The complex interplay of solution species with Fe- and Mn-(oxyhydr)oxide crystallization and transformation

The chemistry of Fe and Mn in natural geochemical systems are closely coupled, as Fe and Mn always co-exist in various forms (e.g., hydrated ions, soluble complexes, and particles) in a variety of surface and near-surface environments (e.g., waters, soils, and sediments) and show strong mutual interactions. The redox cycling of Fe and Mn functions as a “pump” for element cycling and energy flow, assuring the significant roles of Fe and Mn species in environmental system dynamics. Specifically, an increasing number of studies have found that the coupled redox between Fe and Mn can bilaterally affect the crystallization and transformation of the (oxyhydr)oxides of Fe and Mn (i.e., FeOx and MnOx), which are among the most consequential nanominerals and mineral nanoparticles in these environments. In this review, we map the complex reaction networks between Fe and Mn by discussing the reaction characteristics and mechanisms of each coupled system with various co-existing Fe and Mn species (i.e., FeOx-Mn(II), MnOx-Fe(II), and Fe(II)-Mn(II) systems). Due to the higher redox potentials of MnOx/Mn(II) compared to those of FeOx/Fe(II), MnOx can trigger the oxidation of Fe(II) through direct electron transfer, with MnOx undergoing reductive dissolution; while in the FeOx-Mn(II) system, where direct redox reaction between FeOx and Mn(II) is thermodynamically unfavorable, surface-catalyzed oxidation of Mn(II) can be induced by FeOx. In the Fe(II)-Mn(II) system, these species can experience complex homo-and heterogeneous oxidation and crystallization in aerobic environments. The coupled redox cycling of Fe and Mn, which involves crystallization of FeOx and MnOx, dissolution of MnOx substrates, production of reactive oxidants, and the blockage of reactive surfaces of substrates, etc., can exert more complex and significant influences on the biogeochemical processes as compared to individual Fe or Mn.