Technical Report NTB 90-27

Poços de Caldas Report No. 9Chemical and physical characterisationof suspended particles andcolloids in waters from the OsamuUtsumi mine and Morro do Ferroanalogue study sites, Poços de Caldas, Brazil

Data are presented on suspended particles and colloids in groundwaters from the Osamu Utsumi mine and the Morro do Ferro analogue study sites. Cross-flow ultrafiltration with membranes of different pore sizes (450 nm to 1.5 nm) was used to prepare colloid concentrates and ultrafiltrates for analyses of major and trace elements and U- and Th-isotopic compositions. Additional characterisation of colloidal and particulate material was performed by ESCA, SEM and X-ray diffraction. The results obtained indicate the presence of low concentrations ( 450 nm) in the waters show the same trends as the colloids with respect to U, Th and REE associations, but elemental concentrations were typically higher by a factor of 1'000 or more. Calculated association ratios (in mI·g-1) are in the order of 104 to 105 for U, 105 to 106 for the REEs and 106 to 107 for Th. In waters with low pH and high sulphate content, these ratios are considerably lower. Due to the low concentrations of suspended particles in groundwaters from the Osamu Utsumi U-mine (< 0.5 mg/l), these particles carry only a minor amount of U and REEs (< 10 % of the total concentration of the unfiltered groundwater), but a significant, usually predominant, fraction of Th (30 – 70 %). The suspended particle load in groundwaters from the Morro do Ferro environment is typically higher than in those from the mine by a factor of 5 to 10. This suggests that U and the REEs could be transported predominantly by particulate matter. However, there is chemical and mineralogical evidence that these particles, composed mainly of Fe(III) oxyhydroxides and humic-like compounds, have a low capacity for migration.