Sorption Kinetics in the System Uranium / Sulfate / Humic acid / Ferrihydrite

We studied the influence of sulfate on the kinetics of uranium(VI) sorption onto ferrihydrite both in the absence and presence of humic acid at pH 6.5 under aerobic conditions since seepage waters from uranium mine tailing piles in Saxony and Thuringia (Germany) contain up to 3·10-2 M sulfate. The sequence in which sulfate, uranium and humic acid were added to the ferrihydrite suspensions was varied in order to obtain more detailed information on the sorption mechanisms. The uranium(VI) and humic acid concentration was 1·10-6 M and 5 mg/L, respectively. As humic acid, we applied the 14C-labeled synthetic humic acid 14C-M1. Two different sulfate concentrations were applied (0.005 and 0.02 M Na2SO4). The ionic strength was always 0.1 M.
The influence of sulfate, present at naturally relevant concentrations (0.005 or 0.02 M), on uranium sorption was found to be different in the absence and presence of humic material.
In the absence of humic acid, both the uranium sorption rate and the total uranium sorption in equilibrium decreased with increasing sulfate concentration. The reason is that the number of strong ferrihydrite sorption sites, accessible for uranium, is reduced upon sulfate adsorption. This leads to an increased availability of uranium.
In the presence of humic acid, and especially in the presence of higher amounts of uranyl humate complexes in solution, the influence of sulfate on uranium sorption is smaller. The reducing effect of sulfate on the total uranium sorption is almost counterbalanced by humic acid.