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

<h2>Abstract</h2> Elevated concentrations of manganese (Mn²⁺) and zinc (Zn²⁺) in water bodies can disrupt ecosystems and damage aquatic life. However, the mechanisms underlying the removal of Mn²⁺ and Zn²⁺ 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 Mn²⁺ and Zn²⁺ 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. Mn²⁺ removal (efficiency reaching 98 %) primarily depended on the activity of Mn-oxidising bacteria. Zn²⁺ removal involved Zn²⁺ co-precipitation with birnessite combined with adsorption or ion exchange on the birnessite surface. The inverse numerical model successfully determined the Mn²⁺ 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 Mn²⁺ and Zn²⁺ 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.