The effect of water hardness on the toxicity of uranium to a tropical freshwater alga (Chlorella sp.)

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Journal Article
Aquatic Toxicology, 2002, 60 (1-2), pp. 61 - 73
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Uranium (U) derived from mining activities is of potential ecotoxicological concern to freshwater biota in tropical northern Australia. Few data are available on the effects of water hardness (Ca and/or Mg), which is elevated in U mine wastewaters, on the toxicity and bioavailability of U to freshwater biota, particularly algae. This study determined the effect of water hardness (8, 40, 100 and 400 mg CaCO3 l-1, added as calcium (Ca) and magnesium (Mg) sulphate) on the toxicity (72 h growth rate inhibition) of U to the unicellular green alga, Chlorella sp., in synthetic freshwater, at constant pH (7.0) and alkalinity (8 mg CaCO3 l-1), similar in chemical composition to sandy coastal streams in tropical northern Australia. A 50-fold increase in water hardness resulted in a 5-fold decrease (P≤0.05) in the toxicity of U to Chlorella sp. (i.e. the 72 h EC50 increased from 56 to 270 μg U l-1). Possible explanation for the ameliorative effect of water hardness includes: (i) competition between U and Ca and/or Mg for binding sites on the cell surface; and (ii) a change in U speciation, and hence, bioavailability. Results showed that extracellular (cell-surface) and intracellular U concentrations significantly (P<0.05) decreased (2-5-fold) as water hardness increased from 8 to 400 mg CaCO3 l-1. Calculation of U speciation using the geochemical model HARPHRQ showed that there were no significant (P>0.05) differences in the predicted speciation (% distribution) of U amongst the four water hardness levels. The reduction in U toxicity with increasing water hardness was most likely due to competition between U and Ca and/or Mg for binding sites on the algal cell surface. The minimum detectable effect concentrations of U were approximately 3 and 24 times higher (at 8 and 400 mg CaCO3 l-1 hardness, respectively) than the national interim U guideline value (0.5 μg l-1) for protecting aquatic ecosystems. Overall, the results reinforce the need for a more flexible U guideline based on a hardness-dependent algorithm, which may allow environmental managers to relax the national guideline for U on a site-specific basis. © 2002 Elsevier Science B.V. All rights reserved.
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