Geochemistry, Mineralogy and Microbiology of Cobalt in Mining-Affected Environments

This is the final version. Available on open access from MDPI via the DOI in this recordData Availability Statement: Data sharing not applicable. No new data were created or analyzed in this study. Data sharing is not applicable to this article.Cobalt is recognised by the European Commission as a “Critical Raw Material” due to its irreplaceable functionality in many types of modern technology, combined with its current high-risk status associated with its supply. Despite such importance, there remain major knowledge gaps with regard to the geochemistry, mineralogy, and microbiology of cobalt-bearing environments, particularly those associated with ore deposits and subsequent mining operations. In such environments, high concentrations of Co (up to 34,400 mg/L in mine water, 14,165 mg/kg in tailings, 21,134 mg/kg in soils, and 18,434 mg/kg in stream sediments) have been documented. Co is contained in ore and mine waste in a wide variety of primary (e.g., cobaltite, carrolite, and erythrite) and secondary (e.g., erythrite, heterogenite) minerals. When exposed to low pH conditions, a number of such minerals are known to undergo dissolution, typically forming Co2+(aq). At circumneutral pH, such aqueous Co can then become immobilised by co-precipitation and/or sorption onto Fe and Mn(oxyhydr)oxides. This paper brings together contemporary knowledge on such Co cycling across different mining environments. Further research is required to gain a truly robust understanding of the Co-system in mining-affected environments. Key knowledge gaps include the mechanics and kinetics of secondary Co-bearing mineral environmental transformation, the extent at which such environmental cycling is facilitated by microbial activity, the nature of Co speciation across different Eh-pH conditions, and the environmental and human toxicity of Co.UK Foreign, Commonwealth & Development Office (FCDO

The environmental geochemistry and mineralogy of cobalt in mining-affected environments

Mine wastes can pose environmental and human health risks, especially when they contain high levels of mobile metal(loid)s. In this study, the geochemistry, mineralogy and bioaccessibility of Co in mine-affected environments was investigated. The focus was to characterise the geochemistry and mineralogy of Co, combining with sequential extraction procedures and in vitro bioaccessibility tests, to assess the environmental and health risks of the mine wastes originating from legacy mining environments in Cobalt, Canada, and Cornwall, UK. Bulk geochemical results indicated that Co was elevated up to 5630 mg kg-1 in the Nipissing high grade tailings, and 1230 mg kg-1 in the Nipissing low grade tailings at Cobalt. This concentration was by several orders of magnitude higher than at Poldice, Wheal Unity, and Dolcoath mine sites, where the Co concentration was 40 mg kg-1, 76 mg kg-1, and 59 mg kg-1, respectively. BCR-sequential extraction tests results show that in both settings Co was in an exchangeable phase - suggesting that it is mobile in the environment. Mineralogical analysis indicated that erythrite was the most important secondary Co bearing mineral widespread in the Nipissing tailings. Other secondary minerals to which Co occurred in included arseniosiderite, scorodite, and Fe oxyhydroxides. Primary Co bearing minerals identified included cobaltite and safflorite-skutterudite. Cobalt also occurred in other primary minerals including arsenopyrite, loellingite, pyrite and chalcopyrite. At the sites in Cornwall, however, Co bearing primary and secondary minerals were not identified in the samples. Cobalt occurred in the primary minerals arsenopyrite, pyrite, and chalcopyrite and in secondary minerals such as scorodite, and Fe-Mn oxyhydroxides. The lung bioaccessible fraction of Co was higher when extracted in the artificial lysosomal fluids than in Gamble’s solution, varying from 68-88% for samples collected from Cobalt, whilst 28-37% from the Cornish sites. Similarly, the gastric phase, owing to its low pH (2), had the most Co extracted in the intestinal phase (pH 7). Gastric bioaccessibility percentages varied from 44-88% for the samples collected from Cobalt, whilst 21-38% for the Cornish samples. These results combined with the risk assessment code highlight the highly mobile nature of Co, indicating that this metal(loid)s may pose considerable risk to humans and the environment