Ouyang, and three co-authors, reported in 2019 that 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.
The authors concluded that solved Fe and As species under the influence of microbial activities was investigated using static, column reactor experiments and a PHREEQC reactive transport model. Both experimental and numerical modeling results show the release and then decrease of aqueous Fe and As, the consumption of original Fe(III)-minerals, and formation of secondary minerals. The changes in aqueous ions and mineralogy are summarized in both spatial and temporal aspects. Spatial-wise, most of the dissolution happen near the start of a reactive transport. The released aqueous Fe and As are transported and accumulated at the latter part of the transport, where precipitation of secondary minerals (HFOs such as magnetite, goethite, and ferrihydrite) and aqueous As adsorb to HFOs.
Temporal-wise, major Fe(III)-minerals - jarosite, ferrihydrite and goethite - are dissolved in the first place, and intensive dissimilatory iron reduction happens afterwards. Later, accumulated aqueous Fe precipitate to form secondary minerals (HFOs), which then enhance the adsorption of As to HFOs. In the end when continuous flow removed large quantities of Fe3+ and Fe2+ from the system, HFOs are no longer sufficient for As adsorption, and As become released and transported out of the system. Throughout the reaction, changes in flow rates and reaction time will influence the extent of mineral dissolution and transformation. These experiments showed microbial-mediated iron mineral transformation processes and contaminant transport pathways under buried conditions, and the results provided scientific basis for environmental
protection and restoration.
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