Enhancement of anammox coupled arsenate reduction by coexisting iron-bearing minerals

Anaerobic ammonium oxidation is a critical pathway for nitrogen (N) loss in terrestrial environments, and its coupled interactions with As(V) reduction has recently been recognized. While Fe-bearing minerals are suspected to regulate this coupling, the underlying mechanisms remain elusive. This study demonstrates that Fe-bearing minerals (ferrihydrite, goethite, montmorillonite, illite, and kaolinite) actively mediate anammox-coupled As(V) reduction by facilitating electron transfer and promoting Fe redox cycling. The presence of these minerals increases As(V) reduction by 1.2-2.4-fold and NH₄⁺ removal by 1.65-3.15-fold, with ferrihydrite exhibiting the strongest effect. In addition to accelerating reaction rates, these minerals reshape N partitioning and microbial community structure: clay minerals primarily influence early-stage NH₄⁺ dynamics via adsorption, whereas Fe (hydr)oxides sustain long-term coupling by suppressing nitrite oxidizers, limiting NO₃^- accumulation, enriching anammox and Fe-reducing bacteria, and enhancing microbial network stability. Redox-equivalent analysis further shows that these minerals increase the electron utilization efficiency of NH₄⁺ oxidation coupled with Fe(III) and/or As(V) reduction from 17% to 19-75%. Furthermore, As(V) reduction rates correlate strongly with the mineral specific surface area and functional genes including hzsA, omcS, arsC and arrA. This highlights the combined roles of surface-mediated electron transfer, enhanced Fe(III)/Fe(II) cycling, and coordinated regulation of N transformation and respiratory As(V) reduction. These findings identify Fe-bearing minerals as electron-buffering and -channeling phases that organize interfacial electron flow and strengthen N-Fe-As redox couples, providing a mechanistic framework for mineral-driven biogeochemical interactions in As-contaminated soils and sediments.