Revealing the toxicity of monovalent and trivalent thallium to medaka fish in controlled exposure conditions

Thallium (Tl) is a rare earth element increasingly being used in high-technology manufacturing. It is also an emerging pollutant with high exposure and toxicity risks to aquatic ecosystems. Tl exists in the environment in a monovalent [thallous, Tl(I)] or trivalent [thallic, Tl(III)] state. Currently, the stability of the two Tl species in natural water is uncertain and the toxicity in algae and daphnia are inconsistent due to lack of robust characterization of Tl species and matrix effects, while studies with fish are sparse. In this study, larvae of medaka fish (Oryzias latipes) were dosed with environmentally relevant concentrations of Tl(I) or Tl(III) spiked into synthetic and natural river water for 7 days to observe the toxic effects of two Tl species on fish. The transformation of Tl(I) and Tl(III) in water was analyzed by high performance liquid chromatography coupled with inductively coupled plasma and mass spectrometry. Analytical and toxicity results showed that Tl(I) is more stable presenting higher mortality and bioconcentration in medaka than Tl(III) in different water matrices. Tl(I)-induced LC50 and body burden in treated fish were highly correlated with its competitive ion, potassium (K), especially in waters containing medium K levels. This study provides reliable evidence regarding the stability and toxicity of Tl(I) and Tl(III) as well as the interaction of aqueous K versus Tl(I) in fish. Such information is useful for justifying water-quality guidelines and ecological risks of Tl pollution in natural water ecosystems.