The effects of humic acid on the chemical and biological properties of selenium in the environment.

To shed light on the causes of Kaschin-Beck disease, which can be prevented by supplementation of the diet with sodium selenite, the interactions between inorganic selenium compounds (selenite and selenate) and humic/fulvic acid were investigated. Selenate was found to be slowly reduced to selenite by humic acid in acidic solution. Selenite was adsorbed on manganese dioxide and iron(III) oxide from solution to a much greater degree than on kaolin, humic acid, Yongshu soil, or silicon dioxide. Feeding mice a diet supplemented with sodium selenite increased the selenium concentration in the kidney, liver, spleen, lung, heart and blood. A diet containing sodium selenite and fulvic acid caused the selenium concentrations in the organs, with the exception of the spleen, to be lower than observed with the selenite-only diet. Selenium and fulvic acid increase the activity of glutathione peroxidase. Sodium selenite and fulvic acid injected in combination into the abdominal cavity of mice and rats were less toxic than either substance alone. Selenite and fulvic acid applied separately enhanced the luminosity of photobacterium phosphoreum T-3 at low concentrations but depressed the luminosity at higher concentrations. Selenite and fulvic acid in combination caused a larger enhancement and a smaller depression of the luminosity than observed with either substance alone. The hypothesis is formulated that Kaschin-Beck disease is caused by selenium levels insufficient to prevent the toxic effects of the organic compounds present in the drinking water of the regions in which the disease is endemic.