Rare earth elements in the Amazon basin

The rare earth element (REE) concentrations of the dissolved and particulate fractions and bed sediment between Manaus and Santarem in the Amazon River, and in some major tributaries, were evaluated. A very important zone in the Amazon basin, the 'encontro das aguas'area where the Rio Solimoes and the Rio Negro meet, was especially sampled. Different size fractions were isolated by ultrafiltration. Water samples were collected at different stages of the mixing. Three groups of waters are distinguished: group I has a low pH ( 0.2 meq 1 -1 ) and higher pH (>6.5 ) and is represented by the Madeira basin rivers, the Solimoes and the Amazon. The highest dissolved REE concentration is in the Rio Negro and the lowest in the Rio Tapajos (dissolved REEs vary by more than a factor of ten). The solubility of REEs is pH dependent: in river waters with a pH 0.2 μm), 20 to 30% are in the colloidal fraction (0.2 μm to 5000 Da) and less than 10% are in solution (<5000 Da). For the Solimoes the distribution is different. More than 95% of REEs are in the particulate fraction. In the mixing area, the colloidal fraction is enriched in heavy REEs.