Understanding the biogeochemical mechanisms of metal removal from acid mine drainage with a subsurface limestone bed at the Motokura Mine, Japan

Shigeshi Fuchida; Kohei Suzuki; Tatsuya Kato; Masakazu Kadokura; Chiharu Tokoro

doi:10.1038/s41598-020-78069-9

Understanding the biogeochemical mechanisms of metal removal from acid mine drainage with a subsurface limestone bed at the Motokura Mine, Japan

Sci Rep. 2020 Dec 1;10(1):20889. doi: 10.1038/s41598-020-78069-9.

Authors

Shigeshi Fuchida¹, Kohei Suzuki², Tatsuya Kato¹, Masakazu Kadokura², Chiharu Tokoro³

Affiliations

¹ Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan.
² Graduate School of Creative Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan.
³ Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan. tokoro@waseda.jp.

Abstract

Subsurface limestone beds (SLBs) are used as a passive treatment technique to remove toxic metals from acid mine drainage (AMD). In this study, we investigated the mechanisms and thermodynamics of metal (manganese, copper, zinc, cadmium, and lead) precipitation in the SLB installed at the Motokura Mine. Field surveys in 2017 and 2018 showed that the pH of the SLB influent (initially 5-6) increased to approximately 8 in the drain between 24 and 45 m from the inlet. This increase was caused by limestone dissolution and resulted in the precipitation of hydroxides and/or carbonates of copper, zinc, and lead, as expected from theoretical calculations. Manganese and cadmium were removed within a pH range of approximately 7-8, which was lower than the pH at which they normally precipitate as hydroxides (pH 9-10). X-ray absorption near-edge structure analysis of the sediment indicated that δ-MnO₂, which has a high cation-exchange capacity, was the predominant tetravalent manganese compound in the SLB rather than trivalent compound (MnOOH). Biological analysis indicates that microorganism activity of the manganese-oxidizing bacteria in the SLB provided an opportunity for δ-MnO₂ formation, after which cadmium was removed by surface complexation with MnO₂ (≡ MnOH⁰ + Cd²⁺ ⇄ ≡ MnOCd⁺ + H⁺). These findings show that biological agents contributed to the precipitation of manganese and cadmium in the SLB, and suggest that their utilization could enhance the removal performance of the SLB.