Microbial reductive transformation of iron-rich tailings in a column reactor and its environmental implications to arsenic reactive transport in mining tailings

Highlights: • Advective flow with reductive bacteria in tailings induces Fe-minerals transformation and As transport. • Microbial iron reduction decomposes Fe³⁺ minerals in oxidized tailings and releases Fe²⁺ ions. • Jarosite is decomposed while other iron oxides are newly formed in reaction column. • Reactive transport model agrees with lab results of As release, transport and immobilization. -- Abstract: The evolution of iron minerals under buried conditions is one of the most important processes controlling the mineral composition and heavy metal transportation in sediments. Microbial-mediated reduction plays a critical role in iron mineral transformation in natural environment. This study examined the transformation pathways of iron minerals mediated by bacteria and the changes of associated arsenic species in iron-rich mine tailings. Static and column reactions were designed to monitor variations of minerals and released iron and arsenic, a reactive transport model was simulated to support laboratory results. Laboratory experiments showed that major ferric minerals were preferentially dissolved and reduced by dissimilatory iron-reducing bacteria. The released Fe³⁺ in fluid promoted oxidative dissolution of pyrite and arsenopyrite, and precipitation of oxides and carbonates. The arsenic released to fluid was inferred to be immobilized by both pristine ferrihydrite and newly formed hydrous ferric oxides via surface complexation. The reaction system maintained a steady-state of iron mineral transformation and arsenic (im)mobilization. In the latter stage of column reactor experiments, continuous reaction and removal of dissolved Fe³⁺ and Fe²⁺ destabilized the state, leading to arsenic re-location and eventually rising concentration in fluid. The findings implicate that microbial-mediated iron mineral evolution remarkably influence the natural mineral assemblages and the fate of contaminant transport in the environment, and that deposition of iron oxides is essential in environmental protection and pollution recovery.