Pilot-scale evaluation of native plant-based constructed wetland performance in a cold climate

Global demand for aluminium continues to rise, generating bauxite tailings that release hyperalkaline leachates enriched in metals and metalloids. These leachates accumulate in retention basins and pose environmental risks, while treatment is particularly challenging in cold climates. A passive, nature-based treatment approach for bauxite residue leachate in a humid continental climate was evaluated using locally available substrates and native plants. In an 8-week controlled microcosm experiment, sewage sludge-amended systems planted with Calamagrostis canadensis, Scirpus cyperinus, Salix nigra, or Typha latifolia were irrigated with leachate applied as undiluted (pH 11.5), diluted 1:1 (v/v) with tap water or acidified to pH 10.0. Across measured performance metrics, T. latifolia showed the strongest overall response: aerial biomass did not differ significantly among leachate treatments (38.6-49.1 g DW; - 16% to + 6.5% vs control) and the greatest cumulative leachate volume was processed (37.6-42.4 L microcosm⁻¹), exceeding unplanted microcosms (24.5-25.2 L) and other planted species. Despite higher hydraulic loading, lower pH at harvest was observed in T. latifolia and S. cyperinus microcosms than in unplanted controls (~ 0.1-0.35 units), with pH attenuation driven primarily by the substrate. Similarly, arsenic and fluoride retention was largely substrate-controlled, while plant presence provided additional treatment benefits via rhizosphere processes and increased leachate throughput: relative to unplanted microcosms, retained arsenic increased by ~ 20-110% and fluoride removal by ~ 22-74%, with the largest gains observed for T. latifolia and S. cyperinus. Overall, alkalinity attenuation and contaminant immobilisation were achieved under cold-climate conditions through a low-input plant-substrate system, supporting a cost-effective, scalable, and locally sourced passive treatment option for bauxite-residue leachates.