A mechanistic surface complexation approach for the prediction of rare earth element reactive transport in quartz porous media

Although rare earth elements (REE) are now considered as emerging contaminants, the mechanisms controlling the mobility of REE in geochemical systems remain elusive. The complexity and multi-element characteristics of REE including potential synergistic and antagonistic interactions with environmental surfaces make the prediction of REE fate in nature a challenging task. In this study, a comprehensive set of batch and column transport experiments were conducted to examine the interactions of REE group, as well as some co-occurring or chemically analogous elements (Sc, Y, Th and U), with 100-300 & mu;m quartz sand particles. Results from batch experiments showed that middle REE (MREE) and heavy REE (HREE) are preferentially adsorbed at low and high REE loadings, which showed the occurrence of two different types of binding sites. A surface complexation model has been developed, which successfully predicted sorption of REE, Sc, Y, Th and U. Experimental data and reactive transport modeling evidenced the importance of the strong sites, and highlighted the competitive binding of MREE with other REE and Y for the quartz surface sites. These results may have strong implications for the development of new prediction tools for accurately assessing the reactive transport of REE in natural systems.