Silicate-bound iron drives abiotic solar energy conversion in terrestrial environments.

Abstract

Solar energy conversion mediated by natural semiconducting minerals constitutes an abiotic pathway in terrestrial environments, but the drivers and biogeochemical consequences of this process remain poorly understood. Here, based on national-scale field and laboratory measurements, we identify silicate-bound iron, particularly a goethite-kaolinite association, as the primary photoelectrochemical response driver. An S-scheme heterojunction at the goethite-kaolinite interface enhances photogenerated carrier separation, yielding a photocurrent density four times that of pure goethite. A model predicts higher soil photogenerated electron fluxes in warm-humid regions, where intense weathering promotes secondary silicate-bound iron formation. Process-specific theoretical upper-bound estimates suggest that these photogenerated electrons could support alternative metabolic pathways in microorganisms (supplying ~0.01-3.49% of the metabolic electron demand), drive elemental cycling (e.g., donating ~0.16-9.49% of the electrons required for the reductive dissolution of Fe oxides), and promote pollutant degradation (e.g., contributing ~8.20-49.4% of the electrons required for Cr(VI) reduction), highlighting a potential energy flow in surficial Earth systems. These findings reveal a mineral-mediated abiotic pathway for solar energy conversion that fuels biogeochemical processes and sustains ecosystem functions.