Assessment of ²²⁶Ra and U colloidal transport in a mining environment

The colloidal transport of trace (Fe, Al, Ba, Pb, Sr, U) and ultra-trace (²²⁶Ra) elements was studied in a mining environment. An original approach combining 0.45 μm filtered water sampling, the Diffusive Gradient in Thin films (DGT) technique, mineralogical characterization, and geochemical modelling was developed and tested at 17 sampling points. DGT was used for the truly dissolved fraction of the elements of interest, while the 0.45 μm filtration includes both colloidal and truly dissolved fractions (together referred to as total dissolved fraction). Results indicated a colloidal fraction for Al (up to 50%), Ba (up to 86%), and Fe (up to 99%) explained by the presence of submicrometric grains of kaolinite, barite, and ferrihydrite, respectively. Furthermore, the total dissolved ²²⁶Ra concentration in the water samples reached up to 10-25 Bq/L (1.2-3.0 10^-12 mol/L) at 3 sampling points, while the truly dissolved aqueous ²²⁶Ra concentrations were in the mBq/L range. Such high total dissolved concentrations are explained by retention on colloidal barite, accounting for 95% of the total dissolved ²²⁶Ra concentration. The distribution of ²²⁶Ra between the truly dissolved and colloidal fractions was accurately reproduced using a (Ra_x,Ba_1-x)SO₄ solid solution, with values of the Guggenheim parameter a₀ close to ideality. ²²⁶Ra sorption on ferrihydrite and kaolinite, other minerals well known for their retention properties, could not explain the measured colloidal fractions despite their predominance. This illustrates the key role of barite in such environments. The measured concentrations of total dissolved U were very low at all the sampling points (<4.5 10^-10 mol/L) and the colloidal fraction of U accounted for less than 65%. U sorption on ferrihydrite could account for the colloidal fraction. This original approach can be applied to other trace and ultra-trace elements to complement when necessary classical environmental surveys usually performed by filtration on 0.45 μm.