Selenium Removal by Constructed Wetlands:  Role of Biological Volatilization

Abstract

(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 p.1gSe m-z day" in February, June and October, respectively. We conclude that biological Se volatilization is a significant pathway of Se removal in wetlands.[Abstract figure chosen is IntroductionSelenium (Se) is a metalloid which exists in a variety of oxidation states including selenide (Se 2 "), elemental Se (Seo), selenite (Se 4 +), selenate (Se 6 +), and several organic (e.g., selenomethionine) and volatile Se compounds (e.g., dimethyl selenide, DMSe). The oxidized forms of Se, selenite and selenate, are highly soluble and therefore bioavailable and potentially toxic .. The reduced forms, selenide and elemental Se, are insoluble and much less bioavailable. The presence of Se in aqueous discharges from agriculture and industry is a cause for concern in many areas of the world. In the 1980's, selenate-rich agricultural drainage water was discharged into the Kesterson Reservoir marsh in California. This had very serious environmental consequences and resulted in the death and deformity of wildfowl due to Se poisoning (1). Another significant health concern is the rising Se levels in the San Francisco Bay. Much of the Se inflow comes from urban and industrial sources (2). Six oil refineries discharge into the Bay and Se discharge levels from several of the refineries substantially exceed those permitted under the guidelines established by both state and federal regulators.Constructed wetlands constitute a complex ecosystem, the biological and physical components of which interact to provide a mechanical and biogeochemical filter capable of removing many different types of contaminants from water. They have been used to c1eanupmunicipal wastewaters, stonnwater runoff, and many other types of polluted wastewaters in the USA (3), and in Europe (4). Constructed wetlands are orders of magnitude lower in cost than other treatment systems (5); however, the science of understanding of wetland detoxification mechanisms is in its infancy. The first indication that wetlands might be useful in the removal of Se from wastewaters came from a study of a 36-hectare constructed wetland located adjacent to the San Francisco Bay, California. Analysis of the wetland inlet and outlet waters showed that the constructed wetland was successful in removing at least 70% of the Se from the wastewater passing through it (6). Although effluent levels ofSe were well below the permit requirements, there remained the obvious concern that the Se retained in the wetland may eventually be harmful to wildlife. This concern led to a field study to determine Se partitioning among the sediments, water and plant material. The resulting inventory accounted for only 70% of the Se removed by the wetland (6); the fate of the remainder of the Se removed was unknown.One explanation for the «missing Se" at the constructed wetland is that the Se had been converted to volatile for...

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