Selenium removal by constructed wetlands: Role of biological volatilization

Selenium-laden effluents from oil refineries are polluting San Francisco Bay, California. One environmentally friendly way of cleaning up selenium (Se) from effluents is by plant and microbial Se volatilization using constructed wetlands. Using mesocosms, we investigated the role of biological Se volatilization in a 36-ha 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 mu g L-1 decreased to <5 mu g L-1 in the outflow. Most of the Se was removed by immobilization into sediments and plant tissues where Se concentrations reached similar to 5 and similar to 15 mg kg(-1), respectively. Biological volatilization may have accounted for as much as 10-30% of the Se removed. The highest mean rates of Se volatilization for vegetated sites were 190, 180, and 150 mu g of Se 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 mu g of 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.