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Iron-mediated autotrophic denitrification for nitrate removal: Product selectivity and microbial succession
Chang, Yating
Chang, Yating
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Publication Date
2026-04-30
Type
doctoral thesis
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Yating Chang (2026). Iron-mediated denitrification mechanisms. PhD thesis, University of Galway.
Abstract
Fe-mediated autotrophic denitrification has emerged as a promising technology for low carbon-to-nitrogen (C/N) wastewater treatment. It utilises solid-phase iron sources (zero-valent iron (Fe0) and iron minerals) and released ferrous ions (Fe2+) as electron donors to reduce nitrate (NO3-) or nitrite (NO2-) to nitrogen gas (N2) by autotrophic denitrifying bacteria, avoiding the high cost and secondary pollution risks associated with external organic matter addition, eliminating the generation of large volumes of sludge, and yielding by-products capable of adsorbing phosphorus and heavy metals. But Fe0 can reduce nitrate to ammonia by abiotic chemical reduction, resulting in NH4+ being retained rather than being permanently removed as N2. The contribution of microorganisms to enhanced iron-driven nitrate reduction and product selectivity remains unclear. Meanwhile, the low bioavailability and limited electron-donating capacity of Fe²⁺, especially when coexisting with more competitive electron donors such as thiosulfate, lead to the regulatory role of iron being overlooked. The objectives of this PhD research were to: (1) provide an up-to-date overview of Fe-mediated autotrophic denitrification; (2) explore the effects of unacclimated activated sludge on nitrate reduction and nitrogen product distribution in Fe and Fe/Biochar systems; (3) elucidate the regulatory role of iron in sulfur-mediated autotrophic denitrification, with a focus on denitrification performance, microbial succession, and metabolic pathways. The results showed that the introduction of unacclimated activated sludge significantly enhanced nitrate removal, reaching up to 100%, and improved N₂ selectivity to 79%. The Fe/BC/M group exhibited superior nitrate removal (ranging from 75% to 95%) across a broader pH range (from 2 to 10), with the preferable Fe/BC dosage set at 60 g/L. Microbial introduction increased electrochemical active surface area (ECSA), reduced electron transfer resistance and lowered corrosion potential, with a higher i0 value facilitating Hads generation, thereby enhancing nitrate reduction and N2 selectivity. The acclimation experiment demonstrated that iron modulated sulfur autotrophic denitrification efficiency, microbial succession, and key pathways. 1 mM Fe2+ enhanced denitrification efficiency to 91.1% while preventing cell encrustation. Metagenomic analysis indicated that phylum Campylobacterota (16.0%) and genus Sulfurimonas (14.4%) were enriched under iron-modulated conditions. Iron modulated nitrate reduction by improving the relative abundance of complete denitrification genes (napA, napB, and nosZ) and stimulating sulfur metabolism through the SOX complex pathway (soxZ and soxY). This PhD research supports the practical implementation of Fe-mediated autotrophic denitrification for low C/N wastewater treatment and contribute to the sustainable development of environmentally friendly biotechnologies for advanced nitrogen control.
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University of Galway
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CC BY-NC-ND