Arsenic immobilisation analysis at iron-manganese mineral phases with sustainable bioaugmentation

Alternate wetting and drying (AWD) cycles significantly influence arsenic (As) mobility and sequestration in contaminated soils, yet the mechanisms underlying As immobilisation at iron-manganese (Fe-Mn) mineral interfaces remain poorly characterised. This study investigates As entrapment within Fe-Mn mineral layers and evaluates long-term immobilisation stability when bioaugmented with Bacillus subtilis strain 168, a model arsenic-resistant bacterium. Batch incubation experiments were conducted under simulated AWD conditions, alternating between oxic and anoxic phases, to assess As speciation and mineral phase transformations. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and electron microscopy, coupled with energy-dispersive X-ray analysis, characterised As-binding mechanisms on ferrihydrite and manganese oxide surfaces. B. subtilis 168 biofilms were established on pre-equilibrated Fe-Mn mineral assemblages, and extracellular polymeric substances (EPS) were analysed for their role in As binding and stabilisation. Results demonstrated that Mn oxides catalysed oxidative transformation of As(III) to As(V), which subsequently adsorbed onto iron oxyhydroxides as bidentate binuclear complexes. Biofilm-derived EPS enhanced As immobilisation through multi-phase sorption mechanisms, reducing aqueous As concentration by 73% compared to abiotic controls. During redox fluctuations, bioaugmented systems exhibited significantly enhanced As stability, with <5% mobilisation over six wet-dry cycles, whereas non-bioaugmented treatments released 28% of sequestered As. These findings demonstrate that B. subtilis 168-mediated biofilms provide a sustainable approach to stabilise As immobilisation within Fe-Mn mineral matrices under variable hydrological conditions, with implications for climate-resilient soil remediation strategies.