Oxidative Weathering of Neutral Mine Drainage from Gold Mine Tailings Impoundment: Tracking Formation and Transformation of Biogenic Iron Minerals and the Associated Arsenic

Globally, tailing impoundment failures frequently occur, making safely held hazardous materials mobile. The pore water within the tailings can have (near-) neutral pH due to acid neutralization before tailings disposal, leading to the generation of neutral mine drainage (NMD) as it leaks from the impoundment. The biogeochemical evolution of key elements, particularly Fe, and the associated mobility of toxic trace metals, including arsenic (As), in these NMD scenarios remain insufficiently explored. In this study, we investigated the oxidative weathering of NMD originating from a gold mine tailings impoundment in Taojiang City, China, where the pH of the sampled tailings' pore water approached neutrality (pH: 6.82). Our observations revealed distinct helically twisted and tubular-like structures of biogenic Fe minerals in suspended particles within the pore water. The dominant Fe-oxidizing bacteria were identified as belonging to the Crenothrix genus. Further characterization identified these biogenic minerals as ferrihydrite (Fh) and amorphous ferric arsenate (AFA). During oxidative weathering, transformations occurred, converting Fh to hematite/goethite and AFA to scorodite. These transformed minerals accumulated as orange-brown precipitates at the impoundment's toe. Arsenite in the pore water underwent complete oxidation to As(V) outside the impoundment. Arsenic was immobilized by biogenic Fe mineral potential through both surface adsorption and structural incorporation mechanisms. Given the prevalence of NMD in mine-impacted areas, the findings of this case study have broad indications for similar environmental settings. The results not only fill knowledge gaps related to Fe biogeochemical cycling but also provide significant guidance for NMD control.