Sulfur vacancies enhance pyrite-driven autotrophic denitrification: mechanistic insights into electron-supplying pathways.

Pyrite-driven autotrophic denitrification (PAD) is a promising carbon-free strategy for nitrate-contaminated wastewater treatment, yet the role of sulfur vacancies (SVs) remains poorly understood. Herein, we investigated the effects of SVs on denitrification activity, interfacial reactivity, and microbial mechanisms in PAD. Electrochemical characterization and batch tests indicated that SVs introduction boosted electron release from pyrite, achieving 98.1% nitrate removal, 1.59-fold higher than pristine pyrite. By integrating X-ray photoelectron spectroscopy, density functional theory calculations, and metagenomic results, we propose a mechanistic framework in which SVs enhance the electron-supplying capacity of pyrite via two routes: (i) SVs strengthen interactions between pyrite and microbial electron shuttles (e.g., riboflavin and methyl-naphthoquinone), supporting the potential involvement of extracellular electron transfer in enhancing electron availability to denitrifiers; and (ii) SVs are expected to weaken local Fe-S bonding and promote Fe(III)-mediated pyrite oxidation, thereby favoring Fe(II) mobilization and the potential involvement of sulfur intermediates (S0, S2O32-) during PAD. These changes were accompanied by altered surface Fe/S speciation and enrichment of sulfur-oxidizing denitrifiers, particularly Thiobacillus. Additionally, SV-enriched PAD system also exhibited superior resistance to antibiotic and metal stress and achieved continuous nitrogen polishing from real secondary effluent, confirming its strong potential for engineering scalability and practical implementation.