Mobility and Retention of Rare Earth Elements in Porous Media

Abstract

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.

Mobility of Rare Earth Elements in Coastal Aquifer Materials under Fresh and Brackish Water Conditions

“…REE mobility was tested at pH 8 with HA concentrations of 10 mg L −1 due to the increased mobility of REEs in an HAcontaining solution at alkaline pH and HA concentrations >5 mg L −1 . 42 The model applies a discrete-ligand approach that assumes that HA has eight proton-binding sites with distinct acid−base characteristics. Further details about the model can be found in Gustafsson 61 and Gustafsson et al 62 Two parameters are needed to calculate REE speciation in the applied model: the intrinsic equilibrium constant for bidentate complexation (log K Mb ) and the distribution term that modifies the strength of complexation sites (ΔL K2 ).…”

“…Column experiments without HA addition to the REE solution were not conducted as REEs were previously shown to be immobile while flowing through quartz sand column. 42…”

“…This, in turn, will increase REE retention, as the retention of REE 3+ is more robust than that of REE−HA molecules at pH 8. 40,42,45…”

“…For example, the changes in salinity and the concentration of organic matter affect REE speciation and retention mechanisms. 41,42 In addition, the composition of the coastal aquifer might change spatially, which results in different retention mechanisms along paths of water flow. 53 The main objective of this study is to explore the mechanisms that control the mobility and retention of REEs in different coastal aquifer materials at variable salinities.…”

“…Humic acid is an important organic component of groundwater, as its presence enhances REE mobility while interacting with an aquifer material. 42…”