Co-transport and competitive retention of different ionic rare earth elements (REEs) in quartz sand: Effect of kaolinite.

The increasing excavation and utilization of rare earth elements (REEs) have resulted in an elevated release of these elements into the environment. Therefore, investigating the transport behavior of REEs is critical for a comprehensive understanding of their geochemical cycles and to propose potential pollution control strategies. This study investigated the transport, co-transport, and competitive retention of three REEs: La (a light REE), Gd (a middle REE), and Yb (a heavy REE), as well as the co-transport of REEs and kaolinite (a representative clay mineral) in porous media. Both observed and simulated breakthrough curves and retention profiles demonstrated that all ionic REEs exhibited considerable breakthrough and slight retention with almost uniform shapes in quartz sand (QS) owing to the weak affinity of ionic REEs to QS. The breakthrough of REEs in all experiments followed the order of La > Gd > Yb, indicating that REE breakthrough increased with decreasing atomic number. The same elements exhibited their highest breakthrough during the co-transport of the three REEs, followed by co-transport of two REEs, and finally single transport. Furthermore, mathematical modeling indicated that the retention of REEs in QS was a predominantly kinetic process, whereby competitive blocking was the dominant mechanism for the enhanced breakthrough of REEs during co-transport, as compared to single transport. The co-transport of REEs and kaolinite demonstrated that kaolinite has a slight influence on the transport of REEs in QS under adsorption kinetics. However, REEs inhibited the transport and strongly enhanced the retention of kaolinite in QS due to a decreasing electrostatic repulsion between kaolinite and QS in the presence of REEs, even if the adsorption of REEs onto kaolinite was weak under adsorption kinetics. Therefore, this study increases our understanding of the transport mechanisms of REEs in the environment.