An unexplored coupling process enhances dark Hg(II) reduction in mineral-Hg(II)-DOM ternary systems

Photochemical and microbial-mediated reduction of mercury (Hg) are recognized as key processes and pathways for Hg geochemical cycling and atmospheric emissions. However, these processes cannot fully account for the total budget of global elemental Hg(0) production. Here we show that net Hg(0) production is significantly enhanced in a goethite-Hg(II)-DOM ternary system under dark conditions, representing a ubiquitous mineral-organic interface in surface ecosystems. This enhancement arises from the combined effects of preferential adsorption of monodentate [Hg-DOM]⁺ complexes, retention of electron-donating DOM in the aqueous phase, and lower interfacial energy barriers for Hg(II) reduction by goethite. We estimate that ternary interactions, as an underappreciated source of terrestrial Hg(0) emissions, release about 6.3% of topsoil Hg to the atmosphere. These findings underscore the need to integrate mineral-Hg(II)-DOM ternary systems into predictive models of the global Hg cycle and associated ecological risks.