Water hardness reduces the accumulation and toxicity of uranium in a freshwater macrophyte (Ceratophyllum demersum)

Abstract

There is a lack of good quality data and mechanistic understanding on the effects of true water hardness (calcium (Ca) and magnesium (Mg)) on the bioavailability and toxicity of uranium (U) to freshwater biota. This study determined the effect of true water hardness (20, 75, 150, 275 and 400mgCaCO3L−1) on the cell surface binding affinity (log K), accumulation and toxicity (growth inhibition) of U in a submerged, rootless, macrophyte (Ceratophyllum demersum) in a synthetic freshwater with constant alkalinity (13mgCaCO3L−1) and pH (6.2) over 7days. A 20-fold increase in water hardness resulted in a 4-fold decrease in U toxicity (median effect concentration (EC50)=134μgL−1U at 20mgCaCO3L−1 hardness, increasing to 547μgL−1 U at 400mgCaCO3L−1 hardness), cell surface binding affinity (log K=6.25 at 20mg CaCO3L−1 hardness, decreasing to log K=5.64 at 400mgCaCO3L−1 hardness) and accumulation (the concentration factor decreased from 63 at 20mgCaCO3L−1 hardness to 15 at 400mgCaCO3L−1 hardness) of U. Calcium provided a 4-fold greater protective effect against U accumulation and toxicity compared to Mg. Speciation calculations indicated negligible differences in the percentages of key U species (UO22+, UO2OH+, UO2(OH)2) over the range of water hardness tested. The inhibition of U binding at the cell surface, and subsequent uptake, by C. demersum, with increasing Ca and/or Mg concentration, may be explained in terms of (i) competition between Ca2+/Mg2+ and UO22+ (and/or UO2OH+) for physiologically active sites at the cell surface, and/or (ii) reduced negative charge (electrical potential) at the cell surface, resulting in a decrease in the activity of UO22+ (and/or UO2OH+) at the plant/water interface (boundary layer), and consequently, less U bound to physiologically active cell surface sites. In the absence of a biotic ligand model for U, the results of this study (together with previous work) reinforce the need for a more flexible, hardness-dependent, U guideline for the protection of selected freshwater biota.