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

► Effect of water hardness on U toxicity and accumulation in a freshwater macrophyte was studied. ► A 20-fold increase in water hardness resulted in a 4-fold decrease in U toxicity and accumulation. ► Ca provided a 4-fold greater protective effect against U toxicity and accumulation than Mg. ► Negligible differences in the percentages of key U species with increased water hardness ► Mechanisms of competition and/or electrostatic effects at the cell surface are proposed.