Effects of water hardness and pH on vanadium lethality to rainbow trout

Seven-day LC₅₀s of vanadium pentoxide for 2.5-g trout showed only a small range, from 1.9 to 6.0 mg V I⁻¹, in tests at all 12 combinations of 30, 100 and 355 mg 1⁻² total hardness with pH 5.5, 6.6, 7.7 and 8.8. Toxicity decreased from low to high hardness by an average factor of 1.8, probably because of regulated vanadium intake at the gill membranes. Hardness did not exert a major effect, probably because vanadium is present in water as various anions and does not complex with carbonates and bicarbonates as do most metals which behave as cations. Toxicity was greatest at pH 7.7, and the predominating ion H₂VO⁻₄ was apparently the most toxic one. HVO²⁻₄ which prevailed at high pH, was calculated to be 60% as toxic as H₂VO⁻₄. Toxicity decreased at pH 6.6 and 5.5, probably because much of the metal was then present as decavanadates, which were found to be half as toxic as H₂VO⁻₄ on the basis of vanadium content. Small fish were more resistant to vanadium than larger ones, at least up to 12 g wet wt. However cumulative mortality continued to increase until 2 weeks of exposure, and incipient lethal levels were probably much the same for fish of different size, at about 60–70% of the 7-d LC₅₀s stated in the paper. The toxicity of vanadium is similar to that of zinc except that it does not show a major change with water hardness.