New insights about the rare earth elements (REE) mobility in river waters

While the rare earth elements (REE) have largely contributed to the understanding of some internal geological processes, their use as tracers of superficial mechanisms (i.e., weathering and elements transport) remains tricky. This results from the poor knowledge we have of the REE distribution in the different fractions of solutions. Up to recently, the studies carried out on the REE behavior in continental aquatic systems were mainly based on the "particulate" and "solution" fractions separated by an arbitrary filtration through a 0.20 or 0.45 mu m pore size membrane. Some recent work has shown, using ultrafiltration experiments, the REE are mainly contained in the colloidal fraction (i.e., 0.20 mu m-1 nm). However, studies devoted to the control of REE by the colloidal matter, have essentially focused on the organic pool (i.e., humic substances). By contrast, the mineral colloidal pool has received little attention. In order to model these processes, we have developed a combined approach that consider (1) ultrafiltration experiments performed on a series of rivers located in Africa (Cameroon) and south America (Brazil, Venezuela) and (2) speciation calculations. This quantitative approach reveals, for the first time, that more than 60% of the total REE in the solution of river waters is controlled by the mineral colloidal pool.