Microbial Selenium Volatilization in Rhizosphere and Bulk Soils from a Constructed Wetland

Abstract

The potential of rhizosphere and bulk soil microbes to volatilize selenate, selenite, and selenomethionine was studied in liquid cultures under controlled conditions. Microbes cultured from the rhizosphere of bulrush (Scirpus robustus) plants showed higher Se volatilization than those from bulk soil of a flow‐through, constructed wetland area contaminated with selenite. The data show that bacteria are the dominant microbes involved in Se volatilization; fungi contribute relatively little to this process. Bactericides significantly decreased both Se volatilization and the number of culturable bacteria in rhizosphere cultures compared to an untreated control. In the absence of added C, Se volatilization was greatest from selenomethionine, then selenite, then selenate. Aeration substantially increased the percentages of Se volatilized from rhizosphere soil cultures to which no C was added. Up to 95, 21, and 3% of the Se was volatilized from selenomethionine, selenite, and selenate, respectively. When both C and aeration treatments were applied to the rhizosphere cultures, the corresponding percentages changed to 20, 57, and 4%, that is, selenomethionine volatilization by rhizosphere microbes decreased when C was added while selenite volatilization substantially increased. Since selenite volatilization was the greatest when rhizosphere microbes were supplied with C and aeration, we suggest that microbes in this selenite‐contaminated wetland are adapted to volatilize Se by using C released from roots, and that Se volatilization may be enhanced by oxygen and environmental conditions provided by the plants.

Selenium Removal by Constructed Wetlands: Role of Biological Volatilization

“…Both plants (7) and microbes (8,9) are able to take up Se in the form of selenate or selenite and metabolize it to volatile forms (e.g., DMSe). This process is termed "biological volatilization".…”

Rhizosphere bacteria enhance the accumulation of selenium and mercury in wetland plants

“…Obviously, roots are important for phytoremediation, and the rhizosphere, which contains a large microbial population with high metabolic activity compared to bulk soil (Anderson et al 1993;Sorensen 1997), is thought to play an important role in phytoremediation processes. Bacteria (but not fungi) in rhizosphere sediments were more important than bulk sediment bacteria in removing Se by volatilization from a constructed wetland receiving selenite-contaminated euent (Azaizeh et al 1997). Rhizosphere bacteria enhanced Se accumulation and volatilization by Brassica juncea (de Souza et al 1999), and Azospirillum brasilense enhanced the accumulation of nitrate, potassium and phosphate in roots of Zea mays and Sorghum bicolor (Lin et al 1983).…”

Phytoremediation: A Novel Approach for Utilization of Iron-ore Wastes

“…Terry et al (1992) Unlike other remediation techniques, once contaminants have been removed via volatilization, there is a loss of control over their migration to other areas. Some authors suggest that addition of phytovolatilized contaminants to the atmosphere would not contribute significantly to the atmospheric pollution pool, because the contaminants are probably subject to more effective or rapid natural degradation processes such as photodegradation (Azaizeh et al 1997). The consequences of releasing metals to the atmosphere must be considered before adopting this method as a remediation tool.…”