Multi-scale synchrotron study of critical-metal phases by XRF and XANES spectroscopies

Our world is highly dependent on advanced technology, which places a heavy demand on mineral resources, although they produce large volumes of waste. Regarding non-finite mineral resources, mining waste can now be considered a valuable secondary resource. However, they can also be a potential source of pollution. Large heterogeneities in mineralogical compositions and physical and chemical properties make remediation solutions highly complex.

In France, the remediation of mining residues is always challenging due to the aforementioned issues. The age of the mine heaps, dating back to antiquity and extending to the end of the 20^th century — and the widespread distribution of the waste, contribute to investigate how a non-remediated site (W district - French Massif Central) evolved over time.

This study primarily examines the formation of new minerals resulting from the weathering of mining waste and their significance in natural attenuation processes. Of particular interest are hardpans - indurated iron layers - due to their recognized ability to effectively sequester metal(loid)s. These hardpans develop from the weathering of sulfide-rich phases in mining waste, especially arsenic-rich pyrite. Beyond W, our analysis includes other trace elements present in the waste, such as As, known for its toxicity, and Bi, whose geochemical behavior and toxicity remain poorly understood. Notably, all three elements are designated as critical raw materials.

This work seeks to elucidate the in-situ formation of hardpans and their capacity to retain critical and/or potentially toxic trace elements over the long term, with a particular focus on the sub-micron scale processes. To achieve this, we employed a multiscale approach integrating synchrotron-based XRF and XANES spectroscopy. By analyzing the As, Bi, Fe, S, and W absorption edges, we aim to characterize the speciation and redistribution of these elements following weathering processes.

The mineralogical composition of the various wastes present on site was determined, with a focus on hardpans, as well as the fractionation of metal(loid) elements within the mining waste. Crystal chemistry, substitutions, and competitive effects between ions were studied within metal-bearing phases, particularly sulfates and iron oxides, using natural and synthetic samples. The results provide an overview of the trace elements' new distribution and give insights into the weathering of mineral phases. It also helps in understanding the elemental mobility toward various environmental compartments in the surrounding area.