Photoreactivity of dissolved organic matter drives abiotic vivianite formation at sediment-water interfaces

Vivianite serves as a stable phosphorus sink in anaerobic sediments, playing a decisive role in mitigating internal phosphorus-loading and governing global P geochemical cycling. Typically concentrated at organic-rich sediment-water interfaces, vivianite formation is traditionally viewed through microbial ferric iron reduction, where dissolved organic matter (DOM) acts as a metabolic carbon source or an electron shuttle. However, these paradigms overlook the intrinsic semiconductor-like properties of DOM. Here, we demonstrate a previously unrecognized abiotic pathway whereby a DOM-mediated photoelectric process drives ferric iron reduction and vivianite formation. Ferrous iron rapidly accumulated to 31.74 ± 0.36 mg L^-1 within 24 h, and vivianite eventually accounted for 32.7% of the precipitated phosphorus. Mechanistically, light-excited DOM generates transient reducing equivalents that promote ferric iron reduction and phosphate release, while concomitant reactive oxygen species (ROS) consume part of the reducing power and contribute to DOM self-transformation, thereby regulating reaction kinetics. This light-driven mineralization scales systematically with DOM concentration and light intensity, confirming that the photogenerated electrons provide a previously overlooked abiotic route for vivianite formation. Unlike traditional microbially-driven models that rely on organic carbon bioavailability, this light-triggered abiotic pathway bypasses metabolic constraints and directly links solar energy flux to benthic phosphorus immobilization. Our findings reveal that at sunlit sediment-water interfaces enriched in chromophoric DOM, this photoelectric mechanism represents a previously overlooked but critical driver for internal phosphorus cycling, providing a new theoretical basis for predicting and managing phosphorus fate under changing photic conditions.