Acid mine drainage (AMD) poses a significant environmental hazard due to its high concentration of toxic metals, including manganese (Mn), iron (Fe), copper (Cu), zinc (Zn), arsenic (As), cadmium (Cd), and lead (Pb). Traditional AMD treatment involves adding neutralizing agents like lime, but passive treatment methods using natural geochemical and biological processes offer a cost-effective alternative. Passive techniques such as open limestone channels (OLCs), limestone leach beds (LLBs), and wetlands are employed, each with its advantages and limitations. OLCs and LLBs can suffer from rapid clogging, while wetlands may struggle to effectively remove Mn in the presence of high iron concentrations. Subsurface flow systems, such as anoxic limestone drains (ALDs), address these issues by preventing contact with atmospheric oxygen, promoting limestone dissolution and increasing pH. However, ALDs can be less effective when dealing with high concentrations of Fe, Al, and dissolved oxygen. The Motokura Mine in Japan utilizes a pilot-scale subsurface limestone bed (SLB) for AMD treatment, yet the underlying biogeochemical mechanisms remain unclear. This study aims to elucidate these mechanisms through field monitoring, chemical and biological analysis, and geochemical modeling, ultimately contributing to improved design and management of SLBs for efficient AMD remediation.

Existing literature highlights various passive treatment strategies for acid mine drainage (AMD), each with its strengths and weaknesses. Active treatment methods using neutralizing agents such as lime are effective but costly. Passive systems, including open limestone channels (OLCs), limestone leach beds (LLBs), and wetlands, provide economically viable alternatives by leveraging natural processes. However, OLCs and LLBs can be prone to rapid clogging, reducing their effectiveness over time. Wetlands, while offering biological remediation pathways, may not efficiently remove all metals, particularly manganese (Mn) in iron-rich AMD. Anoxic limestone drains (ALDs) offer a promising approach by preventing oxygen exposure and promoting limestone dissolution, thereby raising pH and facilitating metal precipitation. However, the performance of ALDs can be compromised by high concentrations of Fe and Al. Existing models for ALD sizing often lack detailed consideration of the biogeochemical reactions influencing metal removal, underlining the need for a more comprehensive understanding of these processes for optimal system design and effective AMD treatment. This study builds upon previous research by providing a detailed investigation into the specific biogeochemical mechanisms responsible for metal removal in a subsurface limestone bed (SLB), a relatively less-studied passive treatment system.

The study was conducted at the Motokura Mine in Hokkaido, Japan, which utilizes a three-stage passive treatment system comprising a limestone tank, an oxic wetland, and a subsurface limestone bed (SLB). Field surveys were carried out in May 2017 and May 2018 to collect water and sediment samples from various points along the SLB. Water samples were analyzed for pH, electrical conductivity (EC), dissolved oxygen (DO), oxidation-reduction potential (ORP), and the concentrations of various cations and anions using ICP-MS and ion chromatography. Sediment samples were analyzed using X-ray absorption near-edge structure (XANES) spectroscopy to identify manganese oxide species and real-time PCR to assess the presence of manganese-oxidizing bacteria. Geochemical modeling using PHREEQC software was employed to simulate the changes in chemical composition along the SLB, incorporating reactions such as precipitation, surface complexation, and calcite dissolution. Adsorption experiments were conducted to determine the equilibrium constant for Cd adsorption onto δ-MnO₂. The parameters for kinetic calculations were determined by fitting the model results to the measured values. The reactive surface area of calcite was estimated by fitting the model to the measured Ca concentration.

Field surveys revealed a significant increase in pH from ~5-6 to ~8 within the SLB, primarily attributed to limestone dissolution. This increase in pH led to the precipitation of Cu, Zn, and Pb as hydroxides and/or carbonates, consistent with thermodynamic calculations. However, Mn and Cd were removed at a lower pH (7–8) than expected, indicating the involvement of additional processes. XANES analysis of sediment samples confirmed the predominance of δ-MnO₂ (>99%), a manganese(IV) oxide, which is unusual under the observed conditions. Real-time PCR revealed the presence of manganese-oxidizing bacteria (*Pseudomonas* and *Bosea* species), suggesting a key role in the rapid oxidation of Mn(II) to Mn(IV). This biogenic δ-MnO₂ demonstrated a high capacity for Cd removal through surface complexation at near-neutral pH. The geochemical model, incorporating the biogeochemical reactions, accurately simulated the observed changes in chemical composition along the SLB. The model indicated that a residence time of at least 15.5–18.3 h was necessary to achieve effluent standards, and that roughly half of the SLB's neutralizing capacity was lost after five years due to secondary mineral formation. This indicates that the efficiency of metal removal in the SLB is significantly influenced by the activity of manganese-oxidizing bacteria. The high affinity of biogenic δ-MnO₂ for Cd plays a crucial role in achieving efficient removal even at neutral pH.

The findings address the research question by identifying the biogeochemical mechanisms driving metal removal in the Motokura Mine's SLB. The unexpected removal of Mn and Cd at lower pH is explained by the biological mediation of δ-MnO₂ formation, which facilitates efficient Cd adsorption. This highlights the importance of considering biological processes in the design and management of passive AMD treatment systems. The close agreement between the field observations and geochemical model results validates the model's ability to predict the performance of SLBs under various conditions. The relatively long residence time required suggests that the size and design of SLBs should be optimized based on the specific characteristics of the AMD. The loss of neutralization capacity over time due to secondary mineral formation underscores the need for regular maintenance, such as dredging, to maintain the system's long-term effectiveness. The results contribute to a more nuanced understanding of metal behavior in SLBs, moving beyond purely abiotic chemical reactions.

This study demonstrates the significant role of biological agents, specifically manganese-oxidizing bacteria, in enhancing the efficiency of metal removal in a subsurface limestone bed (SLB) used for acid mine drainage treatment. The formation of biogenic δ-MnO₂ facilitates efficient removal of Mn and Cd even at near-neutral pH. The developed geochemical model, which integrates biogeochemical reactions, accurately predicts SLB performance and informs optimal design parameters. Future research could focus on optimizing SLB design for various AMD compositions, investigating the long-term dynamics of biological communities within SLBs, and developing predictive models that account for changes in biological activity over time and the impacts of secondary mineral formation.

The study focuses on a single SLB at the Motokura Mine, limiting the generalizability of the findings to other sites. The model incorporates simplified kinetic equations, potentially affecting its accuracy in representing complex biogeochemical interactions. While the study addresses many potential limitations of passive treatment, variations in microbial communities and their activity at different sites could influence performance. Future research should examine the effects of varying AMD compositions, seasonal changes, and other environmental factors on the performance of SLBs. Regular monitoring and maintenance are critical for sustained effectiveness. Long-term monitoring of the system's neutralizing capacity and metal removal efficiency is recommended to fully assess its long-term effectiveness.