Aquatic chemistry of selenium: evidence of biomethylation

Abstract

The chemical species of dissolved selenium were examined in surface waters from three sites in the San Joaquin and Imperial Valleys of California. Six dissolved selenium species were identified: the inorganic species selenate and selenite; nonvolatile organic selenides, including seleno amino acids and a dimethylselenonium ion; and the volatile methylated forms dimethyl selenide and dimethyl diselenide. The occurrences of methylated selenium species in the aquatic environment has important implications regarding the biogeochemical behavior of selenium in natural aqueous systems. Laboratory studies indicate that the nonvolatile dimethylselenonium ion can be transformed into volatile dimethyl selenide at neutral pH, providing a pathway for the in situ production of dimethyl selenide in natural waters. Geochemical flux calculations indicate that outgassing of dimethyl selenide may be an important removal mechanism for dissolved selenium from aqueous systems. 22 references, 7 figures, 1 table.