Transport of technology critical elements and other heavy metals originating from phosphogypsum effluents in carbonate aquifer rock materials

Groundwater systems contaminated with technology-critical elements and other metals originating from phosphogypsum effluents have been documented globally. However, delineation of the mechanisms governing the mobility and retention of these elements often remains lacking. This work examines the underlying physical and chemical processes that govern the retention and mobility of multiple components within the phosphogypsum solution as it interacts with carbonate aquifer rock. We show the results of batch and column experiments that indicate that the flow of acidic phosphogypsum solution through carbonate rock induces non-uniform dissolution, creating preferential flow paths, reducing rock-solution interactions, and affecting various elements. Furthermore, rock dissolution leads to an elevation in pH levels, impacting element speciation and promoting mineral precipitation. Retention mechanisms and magnitudes vary significantly among elements and reactive surface areas. Some elements (e.g., Tl, Rb, Mo, and Ge) exhibit limited interaction with the rock, leading to high recoveries across all examined grain sizes, while others (e.g., Cr, U, Cd, Zn, and Cs) display grain-size-dependent behavior, with recoveries increasing as the reactive surface area decreases. The retention mechanisms of these elements include retention on the mineral surface and/or co-precipitation of newly formed minerals.