Selenium Removal by Constructed Wetlands: Role of Biological Volatilization
Selenium-laden effiuents from oil refineries are polluting San Francisco Bay, California. One environmentally-friendly way of cleaning up selenium (Se) from effiuents is by plant and microbial Se volatilization using constructed wetlands. Using mesocosms, we investigated the role of biological Se volatilization in a 36-hectare constructed wetland located adjacent to San Francisco Bay. The constructed wetland was highly effective in removing Se from selenite-contaminated oil refinery wastewater: 89% of the Se was removed. Inflow Se concentrations of 20-30 ug L-1 decreased to < 5 ug L-l in the outflow. Most of the Se was removed by immobilization into sediments and plant tissues where Se concentrations reached ~5 and ~15 mg kg-1, respectively. Biological volatilization may have accounted for as much as 10 to 30% of the Se removed. The highest mean rates of Se volatilization for vegetated sites were 190, 180 and 150 p.1gSe m-2 day-1 (rabbitfoot grass, cattail and saltmarsh bulrush, respectively). Rates for the most dominant species, saltmarsh bulrush, varied during the year: the mean rates were 150, 70 and 25 ug Se m-2 day-1 in February, June and October, respectively. We conclude that biological Se volatilization is a significant pathway of Se removal in wetlands.