Natural cobalt–manganese oxide nanoparticles: speciation, detection and implications for cobalt cycling

Environmental context Cobalt is a technologically critical element due to its uses in the green energy transition, but its cycling is poorly constrained in surface environments. We determined the form of cobalt in naturally enriched soils and found that it is commonly associated with manganese as mixed oxide nanoparticles. These findings demonstrate that the behaviour of critical elements such as cobalt in the environment is in part governed at the nanoscale. Rationale Cobalt (Co) faces increasing demand for use in batteries and alloys, but its environmental behaviour in terrestrial surface environments is poorly constrained. This study analyses cobalt regolith mineralogy and nanoparticulate phase transitions to address this knowledge gap. Methodology We studied Co-enriched environments across six localities and four distinct deposit types in arid and semi-arid Australian regolith environments to analyse its environmental behaviour. We used a combination of single particle inductively coupled plasma–mass spectrometry (SP ICP-MS) and synchrotron X-ray techniques (fluorescence microscopy, X-ray flouresence microscopy and X-ray absorption spectroscopy). Results We discovered the presence of Co oxide-based nanoparticles in soils surrounding cobalt-rich rocks at all of our studied locations, to our knowledge the first detection of terrestrial Co oxide-based nanoparticles. The extractable concentration of Co in the nanoparticles varied from 0.7 ng of nanoparticulate Co per gram of soil (ng Co g–1), up to 1390 ng Co g–1, the latter soil containing 1 × 109 extractable Co-based nanoparticles per gram of soil. Discussion Nanoparticulate cobalt was typically closely associated with manganese (Mn) in the form of natural Co–Mn oxide phases, with only two of the studied locations not showing a close Co–Mn association. We discuss the environmental drivers that may facilitate formation of Co–Mn oxide nanoparticles. Our study suggests that Co may be more mobile in surface environments than previously thought, with Co–Mn oxide nanoparticles found around all four analysed types of Co-rich outcrops.