URANIUM(VI) SORPTION TO SEDIMENTS UNDER HIGH PH AND IONIC-STRENGTH CONDITIONS

A number of potential sources for uranium pollution have background ch emistries of high ionic strengths (>0.5 M) and pH (>pH 10), including the liquids from many storage tanks and leachates from vitrified-, sal tstone-, and grout-waste forms. The effect of these background chemica l conditions on the tendency of U(VI) to sorb to natural sediments is unknown. To evaluate these factors, a series of batch-type experiments were conducted to determine the effects of U(VI) concentration, pH, a nd ionic strength on U(VI) sorption to a natural sediment containing c arbonate mineral phases. The results show that U(VI) adsorption remain ed constant between 3.3 and 100 mu g/L UO22+ at pH 8.3 and ionic stren gth of 0.02 M, suggesting that the simple distribution coefficient (K- d) model could be used to describe adsorption between this concentrati on range. Uranium(VI)-K-d and solubility values did not change as the ionic strength was increased with NaClO4, to 14 mM. Uranium(VI)-K-d va lues essentially doubled from 1.07 to 2.22 mL/g as the pH of the syste m increased from pH 8.3 to 9.3. Above pH 10.3, precipitation of U(VI)- containing solids occurred, resulting in apparent K-d values of >400 m L/g. Precipitation did not occur unless a sediment was present. This s uggests that heterogeneous precipitation occurred. Thus, in carbonate systems with a pH greater than about 10.5, U(VI) mobility may be much less than in near-neutral pH systems. These results have important ram ifications for performance- and risk-assessment calculations involving the transport of U(VI) from highly alkaline sources.