Long-term performance of a woodchip-based field-scale biofilter for arsenic removal from neutral mine drainage under...

The long-term performance of a woodchip-based field-scale biofilter (50 m x 57 m x 1 m) treating As-rich neutral mine drainage (As-NMD) at the restored Wood-Cadillac mine site (Québec, Canada) was assessed. Operated for over 22 years, the biofilter showed up to 80 % As removal efficiency, decreasing As influent concentration from 0.35 to < 0.1 mg/L in the effluent, and complying with Canadian regulation. Speciation of As revealed depth-dependent redox processes, with As(V) as dominant As species in the surface layer (0-30 cm), As(III) proportions increased in the middle layer (30-60 cm), while in the deepest layer (60-90 cm), monomethylarsonic acid becomes the predominant methylated form. Isotopic analyses ( [Formula: see text] ) and sulfate concentration profiles suggest localized hotspots of microbial sulfate reduction in anoxic layers, associated with higher [Formula: see text] (> 12.0 ‰) and lower sulfate concentrations (61 to 10 mg/L). These spots aligned with high relative abundances of sulfate-reducing bacteria, suggesting As immobilization via As-S complexes and FeAsS/AsS precipitates. Microbial analyses showed stable α-diversity of prokaryotic communities, but significant variation among fungal populations. β-diversity differed significantly within the vertical profile of the biofilter, suggesting depth-dependent shifts in the composition of both prokaryotic and fungal communities. Functional taxa involved in S, Fe, and As cycling showed depth-dependent distributions, reflecting stratified redox conditions and biogeochemical processes. These findings highlight the role of coupled microbial and geochemical processes in sustaining As immobilization over decades and support the use of engineered organic biofilters as long-term passive treatment systems for As-NMD.