Redox-driven geochemical partitioning of metal(loid)s in the iron-rich anoxic sediments of a recently flooded lignite mine pit: Lake Medard, NW Czechia

Abstract

We evaluated the geochemical partitioning of Fe, Mn, As, V, and REE in sediments of a recently flooded open-cast lignite mine to interpret their response to recently established anoxia, and minor variations in the redox potential of its ferruginous bottom water column. Results from a sequential extraction scheme targeting reactive Fe mineral phases are combined with an assessment of sediment enrichment factors and REE systematics. Across the sediment pile metal(loid)s are being released from pre-existing authigenic phases as minor shifts in redox potential induce elemental immobilisation through co-precipitation with Fe into mineral phases of diverse reactivity. REE systematics confirms that minor oscillations in the water column's redox state can trigger swift changes in the speciation of the redox-sensitive elements. The observed metal(loid)s enrichment in the sediments can either be considered an ecological risk, if a management scenario involving solely recreational purposes is conceived, but it can also be seen as a feasible source of critical metals (e.g., REE and V) that are amenable to sustainable, secondary metal recovery endeavours. By anticipating the fate of redox-sensitive metal(loid)s in our study site, we provide parameters useful to delineate management programmes of this and similar post-mining lakes.