The effect of organic and inorganic aqueous uranium speciation on U(VI) bioavailability to an aquatic invertebrate

Abstract:

Growing world-wide demand for uranium (U) as an energy source has raised concerns of the human and ecological risks of U extraction and processing in the United States. Because of limited information on the relationship between U speciation and bioavailability, particularly in aquatic animals, we are characterizing U uptake by a model freshwater invertebrate (the snail Lymnaea stagnalis). This species grazes on biofilms and is thus key in the trophic transfer of contaminants through aquatic food webs. We determined the bioavailability of dissolved U(VI) over a range of water hardness, pH (6 to 8), and the presence of dissolved natural organic matter (NOM) as a competing ligand, to test the effect of aqueous speciation on uptake. Bioavailability was assessed using U uptake rate constants (k_uw) derived from a kinetic bioaccumulation model. Dissolved U (1 to 1000 nM) was bioavailable over the range of geochemical conditions tested with k_uw (L/g/d) decreasing with increasing dissolved Ca and with increasing pH. For example, k_uw decreased from 1.6 to 0.3 as dissolved Ca was increased from 0.04 to 1.5 mM, suggesting competition between bioavailable U(VI) species and strong ternary calcium uranyl carbonato complexes. At pH 7.5 in synthetic moderately hard freshwater, k_uw decreased from 0.22 in the absence of NOM to 0.07 in the presence of a hydrophobic acid NOM isolate of high aromaticity (SUVA = 5) consistent with strong aqueous complexation of U(VI) by the NOM. The co-variance of U uptake and aqueous U species distribution is being analyzed to determine which U species are bioavailable. U speciation in systems with NOM is calculated using conditional U-NOM binding constants derived by equilibrium dialysis ligand exchange methodology. The bioavailability of dietborne U is being tested since dietary metal uptake prevails for many aquatic species. These experiments include addition of ferrihydrite with U sorbed, both in the presence and absence of NOM, and mixed with diet.