Mobility and Retention of Rare Earth Elements in Coastal Aquifers

Rare earth elements (REEs) play a crucial role in manufacturing high-tech products and developing various technologies, including those related to the transition to clean energy. Consequently, there has been a significant increase in REE production, which has the potential to contribute to the contamination of groundwater systems that are highly susceptible to industrial pollution. Groundwater REE contamination, specifically in coastal aquifer systems, could affect large populations that rely on that water for drinking and domestic use. In this study, we conducted column transport experiments using five representative coastal aquifer materials to understand better the mechanisms that control REE mobility and retention in coastal aquifers. These experiments were conducted by adding humic acid (HA) to the REE solution under fresh and brackish water conditions. The REEs were shown to be most mobile in sand samples, followed by two types of low-calcareous sandstone and one type of high-calcareous sandstone, and least mobile in red loamy sand. The mobility of REEs, found in solution primarily as REE-HA complexes, was controlled mainly by the retention of HA, which increases with ionic strength. Furthermore, it was found that the presence of carbonate and clay minerals reduces REE mobility due to enhanced surface interactions. The enrichment of middle-REE (Nd-Gd) was observed in the sand samples, while heavy-REE (Tb-Lu) enrichment was observed in the calcareous sandstones and the red loamy sand. This change in REE pattern likely originates from the release of carbonate ions from the carbonate minerals that stabilize heavy-REEs compared to middle-REEs.