Although the classical redox cascades outline the thermodynamic equilibrium-based zonation of biogeochemistry in aquatic, soil and sediment environments, those redox active metastable phases (RAMPs) introduce much more complications to predict elemental cycling and contaminant fate and transport. Beyond its traditional definition of metastable minerals and NOMs, those RAMP species may more inclusively involve intermediate valences (such as trivalent Mn), intermediate speciations (sulfurized/thiolated compounds) and intermediate structures (nanoscale aggregations and clustering). This presentation will start with dissolved Mn(III) species that the late Jim Morgan asked critical questions about, discuss their thermodynamic stability and redox activities as a function of water chemistry. Then we will present examples of sulfur mediated mineral oxidation and transformation processes involving arsenic and iron. Finally, we will share new insights about photochemical transformations of ferrihydrite, a most representative RAMP mineral, and the accompanied production of reactive oxygen species (ROS).

Tracing the redox cascades: Environmental geochemistry of redox active metastable phases

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ACS Spring 2023

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