Species and correlations between selenium and mercury in fishpond ecosystems

The chemical species and contents of selenium and mercury in water, sediments, and crucian carps collected from three wild and three aquaculture fishponds in Tianjin, China, were determined, and the interaction between selenium and mercury in water was also investigated by the calorimetry method. The results revealed that the average contents of total selenium (TSe) and total mercury (THg) in each item of the wild areas were higher than in those of the aquaculture areas, and significant differences (95% confidence) were presented for THg both in the sediments and crucian carps. The molar ratios between TSe and THg in all investigated fishponds were far higher than 1, indicating good protective effects of selenium on mercury toxicity. Obviously, negative correlations (r > 0.9993) were found between TSe and THg in water. The antagonism of selenium on mercury in water was confirmed to mainly result from the reaction between selenate and Hg²⁺ to form an insoluble selenium-mercury oxygenated compound, by which the addition of selenate into the water of fishponds would reduce the environmental risk of mercury. PRACTITIONER POINTS: Se and Hg in different fishpond ecosystems were investigated and compared. Direct evidence was provided for the interaction between Se and Hg in water. The addition of Se(VI) into fishpond ecosystems would reduce the environmental risk of Hg.