Distinct Abiotic and Biotic Vanadate Reduction by Pyrite: Mineralogical, Electrochemical, and Isotopic Insights

Vanadate [V(V)] reduction, a key process of vanadium(V) cycling and detoxification, is of geochemical interest and environmental importance, which can be driven by reducing minerals solely or by mediated by microorganisms. However, the discrepancies between abiotic and biotic processes remain poorly understood. This study demonstrated that widespread pyrite (FeS₂) could reduce V(V) both directly and through microbial mediation by Thiobacillus denitrificans (T. denitrificans), with distinct outcomes. The presence of T. denitrificans accelerated V(V) reduction and improved the long-term reactivity of pyrite. XANES analysis showed that abiotic and biotic reduction resulted in an apparent oxidation state of V at 4.82 and 4.41, respectively. Microbial activity further enhanced electron transfer, supported by nicotinamide adenine dinucleotide, riboflavin and the upregulation of cnorB, nosZ, mtoA, and soxB. δ⁵¹V isotope fractionations with enrichment factors of -0.53 ± 0.06‰ (abiotic) and -0.89 ± 0.07‰ (biotic) were determined, with significant difference. Larger δ³⁴S isotope fractionation with only δ¹³C change was observed in the biotic system. Bioaugmentation/biostimulation confirmed the effectiveness of pyrite in V(V) immobilization, while T. denitrificans further enhanced V(V) removal and colonized well in aquifer sediment. This work advances the understanding of couped geochemical cycling of V and pyrite and highlights the vital role of microorganisms in V remediation.