Evasion of selenium to the atmosphere via biomethylation processes in the Gironde estuary, France
Résumé
Dissolved volatile selenium species were monitored in the Gironde estuary (south-west France) during three cruises along with hydrological parameters (salinity, temperature, suspended particulate matter, dissolved organic matter fluorescence). Volatile selenium species presented a strong seasonal variability with concentrations ranging from non-detectable levels (< 0.9 pmol/l) in December 1992 to a few pmol/l in the September 1992 and May 1993 cruises. Dimethyl selenide [(CH3)2Se] was the main volatile selenium compound detected (84 to 100%). Low concentrations of dimethyl diselenide [(CH3)2Se2] and methane selenol [(CH3)SeH] were also observed in May 1993 with sensitive equipment. The volatile selenide concentration presented a positive correlation with the water temperature and a negative correlation with the rivers freshwater discharge. This temperature dependence allowed us to obtain, by the use of the Arrhenius plot, the apparent activation energy of the dimethyl selenide formation in estuarine waters (147 ± 51 kJ/mol). The nature of the mechanisms involved in the formation of volatile selenium species are suggested to be both related to microbial and algae's activity in the rivers and estuary environments. The production and release of biomethylated volatile selenium compounds in the estuarine waters seem also strongly dependent on the hydrodynamic stability and the water residence time in the Gironde estuary. Considering the seasonal variability of the volatile selenium species in estuarine waters the selenium fluxes to the atmosphere from the Gironde estuary ecosystem were then estimated to range between 0.6 and 2.6 x 105 g of Se per year.