There is growing concern that rare earth elements (REEs) will become emerging soil–water contaminants because of their increased use in new technologies and products, which may lead to unavoidable release to the environment. To better understand the environmental behavior of REEs, a comprehensive set of adsorption and column transport experiments was conducted in quartz sand media. The retention and mobility of three representative REEs (La, Gd, and Er) were studied in the presence and absence of humic acid (HA; 5, 20, and 50 mg L –1 ) and under a range of pH conditions (5–8). Results show that REE mobility and retention are controlled by the amount of REE–HA complexes formed in a solution, which increases with increasing HA concentrations and solution pH. Gadolinium is the most mobile among the representative REEs, followed by Er and La, corresponding to the amount of (calculated) REE–HA complexes. Increasing HA concentrations in the REE solution inhibits REE retention in both the batch adsorption and column experiments. The same retardation trend was observed for lower HA concentrations (Gd > Er > La). In a fixed HA concentration, HA and REE adsorption decrease simultaneously as the solution pH increases, indicating the co-adsorption of REEs and HA on the sand. Scanning electron microscopy detection of elongated regions attached to the sand, where high REE and carbon (HA) concentrations were measured, further suggests the co-adsorption of REE–HA complexes. Modeling the column experiments shows that the time-dependent attachment is dominant at high HA concentrations, while at lower HA concentrations, both the time-dependent and spontaneous attachments play equal roles. These results provide a quantitative characterization of REE retention and mobility in sand media.

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<p>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.</p>

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