Insights into the Photochemical Mechanism of Pyrite and the Crucial Roles of Trace Ni/Co Substitution in Modulating Interfacial Reactions and Redox Active Species Production

Pyrite is a common natural sulfide mineral and hosts many trace elements, which may modulate its photochemical activity, yet the mechanism remains poorly understood. Pyrite was substituted with trace levels of Ni and Co to systematically investigate the influence of their incorporation on interfacial reactions and the generation of redox active species under light irradiation. Photocatalytic degradation of model organic matter and a pollutant in the presence of pyrite was significantly accelerated by the trace Ni/Co substitution. Trace Ni/Co substitution was found to significantly enhance the photochemical reactivity of pyrite through altering the pathways of interfacial redox reactions, accelerating the release of Fe²⁺, sustaining the production of redox active species, and suppressing the oxidation of sulfur. Specifically, Ni substitution increased ^•OH production by 1.80-fold, mainly via a photoelectron-driven reduction that sustains the photo-Fenton process. Co substitution increased ^•OH production by 1.64-fold, primarily through photo-Fenton and ligand-to-metal charge transfer (LMCT) processes. Dual substitution with Ni and Co leads to cooperative enhancement of ^•OH production by 1.97-fold through multiple pathways, including photo-Fenton, LMCT, and reactions involving Fe²⁺/sulfur intermediates and O₂. These findings shed light on the photoactivity of sulfide minerals and their roles in the geochemical cycling of organic matter and organic pollutant degradation.