Assessing the fate and toxicity of Thallium I and Thallium III to three aquatic organisms

• IC25s for both Tl species were 100-fold greater than the current Canadian guideline.
• Reducing K concentrations increased toxicity to within environmentally relevant concentrations.
• Over a 7-d period majority of Thallium (III) was converted to Thallium (I).
• In fresh solutions (<24-h old) recovery of Thallium (III) was greater than 80%.

Thallium has been shown to significantly increase in both water and aquatic biota after exposure to metal mine effluent, however, there is a lack of knowledge as to its fate and effect in the aquatic environment. The objectives of this project were to assess (1) fate of thallium by conducting speciation analysis and determining the influence of water quality on toxicity and (2) effects of thallium (I) and (III) on three aquatic species; the algae, Pseudokirchneriella subcapitata, the invertebrate Ceriodaphnia dubia and the vertebrate Pimephales promelas.Speciation analysis proved challenging with poor recovery of thallium (I), however analysis with solutions >125 μg/L revealed that over a 7-d period, recovery of thallium (III) was less than 15%, suggesting that the majority of thallium (III) was converted to Thallium (I). It was only in fresh solutions where recovery of Thallium (III) was greater than 80%.The lowest IC25s generated during our effects assessment for both Thallium (I) and (III) were more than 10-fold greater than the highest concentration recorded in receiving environments (8 μg/L) and more than 100-fold greater than the current guideline (0.8 μg/L).To assess the influence of water quality on thallium toxicity, the concentrations of both potassium and calcium were reduced in dilution water. When potassium was reduced for both C. dubia and P. subcapitata tests, the lowest IC25 generated was 5-fold higher than the current guideline, but within the range of concentrations reported in receiving environments for both Thallium (I) and (III). When calcium was reduced in dilution water, toxicity only increased in the Tl (III) tests with C. dubia; the IC25 for Tl(III), similar to the exposures conducted with reduced potassium, was within the range of total thallium concentrations reported in the receiving environment.Without an accurate, repeatable method to assess thallium speciation at low concentrations it is not possible to draw any firm conclusions as to whether the IC25s for Tl (III) are relevant to concentrations present in receiving environments. Based on the results of our study we recommend that any test, to determine Thallium (III) toxicity, use fresh solutions, made daily, to get good recovery and accurate toxicity results. The results generated in our effects and exposure assessment would indicate that the current guideline of 0.8 μg/L is protective. Special attention should be placed on the concentration of potassium in receiving environments when estimating thallium toxicity.