Geochemical insights and model optimisation for pilot-scale passive treatment of manganese and zinc in a legacy mine in Japan.

Elevated concentrations of manganese (Mn) and zinc (Zn) in water bodies can disrupt ecosystems and damage aquatic life. However, the mechanisms underlying the removal of Mn and Zn under dynamic conditions and the optimal hydraulic retention time (HRT) for passive treatment plants remain unclear. Here, a pilot-scale passive treatment system for the removal of Mn and Zn from legacy mine drainage in northern Japan is proposed; it was performed at circumneutral pH for 152 days. Comprehensive suspended solid mineralogy analyses and geochemical and numerical modelling were conducted to optimise the passive treatment efficiency. Mn removal (efficiency reaching 98 %) primarily depended on the activity of Mn-oxidising bacteria. Zn removal involved Zn co-precipitation with birnessite combined with adsorption or ion exchange on the birnessite surface. The inverse numerical model successfully determined the Mn oxidation rate constant, Zn mass transfer coefficient, and Zn distribution coefficient. Under dynamic conditions, HRT emerged as a key factor underlying the pilot-scale passive treatment efficiency. An HRT of 0.5 days led to optimal Mn and Zn removal conditions and achieved values lower than the Japanese national effluent limit. The findings provide crucial information for passive treatment strategy development and environmental management, especially when considering real-scale implementation.