Retention and mobility of phosphogypsum constituents in carbonate aquifer rock materials

The disposal of phosphogypsum, an acidic and metal-rich by-product of phosphate fertilizer production, represents a growing environmental challenge due to its potential to contaminate groundwater. Despite numerous reports of phosphogypsum leachate (PGL) impacting aquifers worldwide, the mechanisms governing the mobility and retention of contaminant elements remain poorly understood. To elucidate the underlying physical and chemical mechanisms governing the retention and mobility of the different components of the solution as it interacts with carbonate aquifer rock, we present a set of batch and column experiments using PGL and crushed aquifer rock. The results show that the flow of PGL through the rock induces non-uniform dissolution, creating preferential flow paths and reducing the rock-solution interactions and the retention of related elements. Moreover, rock dissolution causes a pH increase, affecting the speciation of the different elements and promoting precipitation. Element-specific retention was observed with some elements, e.g., Mo, Ge, Tl, and Rb, showing limited interaction and high mobility, raising concerns for groundwater contamination, especially for Tl, given its high toxicity. Other elements, including Al, Cr, B, Co, Ni, Cu, Zn, Cd, Cs, and U exhibited grain-size-dependent retention, with smaller grain sizes providing more surface area for sorption or precipitation. REEs were strongly immobilized across all conditions, indicating negligible mobility in acidic carbonate environments. The retention mechanisms of phosphogypsum-related elements include retention on the mineral surface and/or co-precipitation of newly formed minerals. Overall, these results underscore the need for site-specific assessments of PGL disposal in carbonate settings, given the distinct behavior of different elements and the dynamic nature of flow paths and pH conditions.