Increase in rare earth element concentrations controlled by dissolved organic matter in river water during rainfall events in a temperate, small forested catchment

The distributions of dissolved rare earth elements (REEs) in river water during rainfall events in a forested catchment were studied to contribute to understandings of TRUs behavior. Variations in the concentrations of dissolved REEs and several elements (As, Sb, and V) apparently correlated well with peak intensities in the optical spectra (ultraviolet absorption and fluorescence emission) of the river water. The optical properties were considered to represent humic substances in dissolved organic matter (DOM). Similar hysteretic loops were found for the REE concentrations and optical intensities as a function of the water discharge. This result indicates that the mobile REEs and DOM likely shared flow paths from the soil to the stream during these events. An ultrafiltration study revealed that colloidal REEs present in a size fraction of 10–30 kilodaltons comprised the major component of the dissolved REEs and increased under high water flow. Considering these observations and model interpretations in the precedent literature, complexation of the dissolved REEs with DOM was suggested to occur during the studied rainfall events. A high correlation between the size-fractionated REE concentrations and specific ultraviolet absorbance was observed, suggesting that the aromaticity of DOM plays a role in its complexation with dissolved REEs.