Amiel Nitai, Dror Ishai, Berkowitz Brian
Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot 7610001, Israel.
ACS Omega. 2022 Jun 2;7(23):19491-19501. doi: 10.1021/acsomega.2c01180. eCollection 2022 Jun 14.
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) 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.
由于稀土元素（REEs）在新技术和产品中的使用增加，人们越来越担心它们会成为新出现的土壤 - 水污染物，这可能导致不可避免地释放到环境中。为了更好地了解稀土元素的环境行为，在石英砂介质中进行了一系列全面的吸附和柱传输实验。研究了三种代表性稀土元素（La、Gd和Er）在有和没有腐殖酸（HA；5、20和50 mg/L）存在的情况下以及在一系列pH条件（5 - 8）下的保留和迁移情况。结果表明，稀土元素的迁移和保留受溶液中形成的稀土 - 腐殖酸络合物数量的控制，该数量随腐殖酸浓度和溶液pH的增加而增加。在代表性稀土元素中，钆的迁移性最强，其次是铒和镧，这与（计算出的）稀土 - 腐殖酸络合物的数量相对应。在稀土溶液中增加腐殖酸浓度会抑制批次吸附和柱实验中稀土元素的保留。对于较低的腐殖酸浓度（Gd > Er > La）也观察到相同的阻滞趋势。在固定的腐殖酸浓度下，随着溶液pH的增加，腐殖酸和稀土元素的吸附同时减少，表明稀土元素和腐殖酸在沙子上的共吸附。扫描电子显微镜检测到附着在沙子上的细长区域，在那里测量到高稀土和碳（腐殖酸）浓度，进一步表明了稀土 - 腐殖酸络合物的共吸附。对柱实验进行建模表明，在高腐殖酸浓度下，时间依赖性附着占主导，而在低腐殖酸浓度下，时间依赖性和自发附着起同等作用。这些结果提供了稀土元素在砂介质中保留和迁移的定量表征。
