MnO₂ and Oxygen Facilitate Organic Matter Formation via Oxidation and Catalysis in Oxygenated Alkaline Conditions

Organic matter (OM) is prevalent in all aquatic and terrestrial environments, but the interactions between environmental materials─organic chemicals, minerals, and natural oxidants─that transform precursor organic chemicals into OM remain unclear. For four decades, MnO₂ has been regarded as both a catalyst and the primary oxidant in the abiotic oxidative transformation of organic chemicals into OM, while dissolved oxygen (DO) is considered a minor or negligible oxidant when copresent with MnO₂. Here, we investigated a well-known abiotic OM formation process: the reaction of catechol (a polyphenol) with MnO₂ and DO under varying pH conditions. The results demonstrated that oxygenated alkaline conditions could enhance catechol oxidation. Both the DO and pH were important factors governing the transformation rate and structure of OM. Three oxidation routes were involved in the oxidative transformation of catechol, including two direct oxidation routes and a surface-catalyzed route. Specifically, MnO₂ promoted OM polymerization under circumneutral conditions, whereas DO promoted ring cleavage and hydroxylation under alkaline conditions. For the first time, DO regeneration was observed during polyphenol transformation with MnO₂ under oxygenated alkaline conditions. This study advances the understanding of OM transformation in natural environments and has implications for artificial OM production, biomass recycling, carbon cycle management, and climate change mitigation.