Iron sulfide–driven denitrification: Mechanistic insights into chemical and biological pathways and microbial regulation effects

Iron sulfide–driven autotrophic denitrification has emerged as a promising carbon–free strategy for nitrate (NO3–) removal in anoxic environments. While biological denitrification using iron sulfides as electron donors has been extensively studied, the role and mechanism of abiotic NO3– chemodenitrification remain largely unresolved. This research systematically investigated the transformation of NO3–, product selectivity, and coupled elemental cycling of N, S and Fe in chemical and biological denitrification systems mediated by three representative iron sulfides, FeS, FeS2, and pyrrhotite. The specific objectives of this PhD research were: (1) study of the chemical denitrification of NO3− by iron sulfide under mild conditions and its transformation mechanism; (2) investigation of the effect of the surface structure of iron sulfide on autotrophic denitrification and its role in nitrogen cycle; (3) Elucidation of microbial–induced mechanisms driving the shift in nitrogen reduction products from ammonium (NH4+) to nitrogen gas (N2) in FeS–mediated autotrophic denitrification. FeS was found to chemically reduce NO3– to NH4+ with a high NO3– reduction efficiency of 97.5% and NH4+ product selectivity of 82.6%, whereas FeS2 and pyrrhotite showed negligible reactivity. Electrochemical analyses revealed that FeS exhibited a superior electron release rate. Quenching experiments and density functional theory (DFT) calculations confirmed that surface Fe(II) and sulfur vacancies on FeS played key roles in facilitating NO3– reduction through selective oxygen adsorption and water dissociation, leading to NH4+ formation. In microbial systems, FeS and pyrrhotite supported effective autotrophic denitrification, while FeS2 remained biologically inactive. The differences in reactivity were attributed to their mineral–specific surface structures and Fe–S bond energies: pyrrhotite, with iron vacancies and a lower bond energy (1.35 eV), facilitated microbial electron transfer, whereas FeS2, with strong bonding (1.63 eV), hindered microbial colonization and electron mobility. FeS, with intermediate bond energy (1.39 eV) and sulfur vacancies, supported both abiotic and biotic NO3– transformations. Furthermore, microbial colonization of FeS altered its surface redox structure, including Fe(II) oxidation and sulfur speciation changes, enhancing electron transfer capacity and shifting NO3– reduction from NH4+ production toward N2 formation. This demonstrated the critical role of mineral–microbe interactions in modulating reaction pathways and product outcomes. This PhD research provides new mechanistic understanding of iron sulfide–driven NO3– reduction, highlighting the importance of surface defects and electron transfer dynamics in determining chemical and biological reactivity. Beyond advancing knowledge of nitrogen, sulfur, and iron biogeochemistry, these findings carry practical implications for engineering design and operation of wastewater treatment systems. FeS, with its strong chemical activity and high product selectivity toward NH4+, offers promise as a redox–active medium or cathode material in engineered systems where abiotic NO3– reduction is desirable. In contrast, pyrrhotite demonstrated the highest efficiency in supporting microbial autotrophic denitrification toward N2, making it a suitable candidate for biofilter media or long–term bioreactor operation. The demonstrated influence of microbial colonization on FeS further provides insights into managing reaction selectivity and long–term stability. Overall, this research offers strategies for tailoring reactive media selection and process design to achieve targeted NO3– removal and sustainable nutrient management in next–generation carbon–neutral wastewater treatment technologies.

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University of Galway

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CC BY-NC-ND

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University of Galway Theses (PhD Theses)