Mobility and Retention of Rare Earth Elements in Porous Media

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.

Automated summary

There is growing concern about the increasing use of rare earth elements (REEs) in new technologies and products and how this may lead to their release into the environment as contaminants. This study conducted a set of experiments to investigate the adsorption and transport of three representative REEs (La, Gd, and Er) in quartz sand media. The results show that the mobility and retention of REEs are influenced by the formation of REE-HA complexes in the solution, which increases with higher concentrations of humic acid (HA) and solution pH. Gadolinium was found to be the most mobile among the REEs, followed by Er and La. Increasing HA concentrations inhibited the retention of REEs in both batch adsorption and column experiments. The co-adsorption of REEs and HA on the sand was observed, with the attachment of elongated regions containing high concentrations of REEs and HA. Modelling of the column experiments revealed that attachment over time was dominant at high HA concentrations, while both time-dependent and spontaneous attachments played equal roles at lower HA concentrations.