Co-transport of biochar nanoparticles (BC NPs) and rare earth elements (REEs) in water-saturated porous media: New insights into REE fractionation.

The present study investigated the co-transport behavior of three REEs (La, Gd, and Yb) with and without biochar nanoparticles (BC NPs) in water-saturated porous media. The presence of REEs enhanced the retention of BC NPs in quartz sand (QS) due to decreased electrostatic repulsion between BC NPs and QS, enhanced aggregation of BC NPs, and the contribution of straining. The distribution coefficients (K) in packed columns in the co-transport of BC NPs and three REEs were much smaller than in batch experiments due to the different hydrodynamic conditions. In addition, we, for the first time, found that REE fractionation in the solid-liquid phase occurred during the co-transport of REEs in the presence and absence of BC NPs. Note that the REE fractionation during the co-transport, which is helpful for the tracing application during earth surface processes, was driven by the interaction of REEs with QS and BC NPs. This study elucidates novel insights into the fate of BC NPs and REEs in porous media and indicates that (i) mutual effects between BC NPs and REE should be considered when BC was applied to REE contaminated aquatic and soil systems; and (ii) REE fractionation provides a useful tool for identifying the sources of coexisting substances.