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Selenium has unique fate and transport through a coal-fired power plant because of high vapor pressures of oxide (SeO2) in flue gas. This study was done at full-scale on a 900 MW coal-fired power plant with electrostatic precipitator (ESP) and wet flue gas desulfurization (FGD) scrubber. The first objective was to quantify the partitioning of selenium between gas and condensed phases at the scrubber inlet and outlet. The second objective was to determine the effect of scrubber operation conditions (pH, mass transfer, SO2 removal) on Se removal in both particulate and vapor phases. During part of the testing, hydrated lime (calcium hydroxide) was injected upstream of the scrubber. Gas-phase selenium and particulate-bound selenium were measured as a function of particle size at the inlet and outlet of the scrubber. The total (both phases) removal of Se across the scrubber averaged 61%, and was enhanced when hydrated lime sorbent was injected. There was evidence of gas-to-particle conversion of selenium across the scrubber, based on the dependence of selenium concentration on particle diameter downstream of the scrubber and on thermodynamic calculations.

The isotope (79)Se may be of great concern with regard to the safe disposal of nuclear wastes in deep geological repositories due to its long half-life and potential mobility in the geosphere. The Se mobility is controlled by the oxidation state: the oxidized species (Se(IV)) and (Se(VI)) are highly mobile, whereas the reduced species (Se(0) and Se(-II)) form low soluble solids. The mobility of this trace pollutant can be greatly reduced by interacting with the various barriers of the repository. Numerous studies report on the oxidized species retention by mineral phases, but only very scarce studies report on the selenide (Se(-II)) retention. In the present study, the selenide retention by coprecipitation with and by adsorption on mackinawite (FeS) was investigated. XRD and SEM analyses of the samples reveal no significant influence of Se on the mackinawite precipitate morphology and structure. Samples from coprecipitation and from adsorption are characterized at the molecular scale by a multi-edge X-ray absorption spectroscopy (XAS) investigation. In the coprecipitation experiment, all elements (S, Fe, and Se) are in a low ionic oxidation state and the EXAFS data strongly point to selenium located in a mackinawite-like sulfide environment. By contacting selenide ions with FeS in suspension, part of Se is located in an environment similar to that found in the coprecipitation experiment. The explanation is a dynamical dissolution-recrystallization mechanism of the highly reactive mackinawite. This is the first experimental study to report on selenide incorporation in iron monosulfide by a multi-edge XAS approach.

Genetic variation in salt (Na(2)SO(4), NaCl) and boron (B) tolerance among four ecotypes of the selenium (Se) hyperaccumulator Stanleya pinnata (Pursh) Britton was utilized to select tolerant genotypes capable of phytoremediating Se from salt, B, and Se-laden agricultural drainage sediment. The few individual salt/B tolerant genotypes were successfully selected from among a large population of highly salt/B sensitive seedlings. The distribution, hyperaccumulation, and volatilization of Se were then examined in selected plants capable of tolerating the high salt/B laden drainage sediment. Salt/B tolerant genotypes from each of the four ecotypes had mean Se concentrations ranging from 2510 ± 410 to 1740 ± 620 in leaves and 3180 ± 460 to 2500 ± 1060 in seeds (μg Se g(-1) DW ± SD), while average daily Se volatilization rates ranged from 722 ± 375 to 1182 ± 575 (μg Se m(-2) d(-1) ± SD). After two growing seasons (∼18 months), we estimated that hyperaccumulation and volatilization of Se by tolerant S. pinnata genotypes and their associated microbes can remove approximately 30% of the total soil Se in 0-30 cm sediment. The salt/B tolerant S. pinnata genotypes selected and characterized herein represent promising new tools for the successful phytoremediation of Se from salt/B and Se-laden agricultural drainage sediments.

Wet chemical methods such as sequential extraction procedures are commonly used to assess selenium fractionation in anoxic environments, allowing an estimation of the mobility and bioavailability of selenium. However, the interpretation can be biased by unselective extraction of targeted species and artifacts introduced during the extraction. Here, the selectivity of the single extraction steps to gain reliable selenium speciation information are scrutinized for the first time by direct, nondestructive X-ray absorption near edge structure (XANES) spectroscopy at the selenium K-edge. The sequential extraction procedures seriously overestimated the elemental selenium fraction, as major parts (58%) of the total selenium were present as metal selenides and organic selenium compounds, although extracted in the elemental fraction. Spectral fitting of the XANES spectra by the least-squares linear combinations utilizing a large set of model compounds, including previously neglected Se(-I) selenides, showed a novel degree of complexity in the speciation of selenium treating anaerobic biofilms, with up to 4 modeled selenium species contributing to the speciation, i.e., different elemental, organic, and metal-bound selenium species. Furthermore, a short exposure (10 min) to ambient air during the sequential extraction procedure induced the oxidation of organic selenium compounds, revealing the fragility of selenium speciation in anaerobic biofilms.

The transport and bioavailability of selenium in the environment is controlled by its chemical speciation. However, knowledge of the biogeochemistry and speciation of Se in streambed sediment is limited. We investigated the speciation of Se in sediment cores from the Blackfoot River (BFR), Idaho using sequential extractions and synchrotron-based micro-X-ray fluorescence (micro-SXRF). We collected micro-SXRF oxidation state maps of Se in sediments, which had not been done on natural sediment samples. Selective extractions showed that most Se in the sediments is present as either (1) nonextractable Se or (2) base extractable Se. Results from micro-SXRF showed three defined species of Se were present in all four samples: Se(-II,O), Se(IV), and Se(VI). Se(-II,O) was the predominant species in samples from one location, and Se(IV) was the predominant species in samples from a second location. Results from both techniques were consistent, and suggested that the predominant species were Se(-II) species associated with recalcitrant organic matter, and Se(IV) species tightly bound to organic materials. This information can be used to predict the biogeochemical cycling and bioavailability of Se in streambed sediment environments.

Three transgenic Indian mustard [Brassica juncea (L.) Czern.] lines were tested under field conditions for their ability to remove selenium (Se) from Se- and boron-contaminated saline sediment. The transgenic lines overexpressed genes encoding the enzymes adenosine triphosphate sulfurylase (APS), gamma-glutamyl-cysteine synthetase (ECS), and glutathione synthetase (GS), respectively. The APS, ECS, and GS transgenic plants accumulated 4.3, 2.8, and 2.3-fold more Se in their leaves than wild type, respectively (P < 0.05). GS plants significantly tolerated the contaminated soil better than wild type, attaining an aboveground biomass/area almost 80% of that of GS plants grown on clean soil, compared to 50% for wild type plants. This is the first report showing that plants genetically engineered for phytoremediation can perform successfully under field conditions.

Boron, chromium, molybdenum, and selenium often occur in high concentrations in fly ash leachates. During the leaching of fly ash in alkaline environments, hydrocalumite (Ca4Al2(OH)12(OH)2 x 6H2O) and ettringite (Ca6Al2(OH)12(SO4)3 x 26H20) form as secondary precipitates. In this study, the removal of B, Cr, Mo, and Se oxyanions from high pH waters by incorporation into hydrocalumite and ettringite was examined. Experiments were performed by precipitating these minerals in solutions containing B, Cr, Mo, and Se oxyanions at conditions relevant to lime-leaching of fly ash as well as to fly ash containing concrete. The uptake of all four anions by hydrocalumite and ettringite was high. Anion uptake by hydrocalumite was larger than that by ettringite, and hydrocalumite was able to reduce anion concentrations to below drinking water standards. Ettringite showed an anion preference in the order of B(OH)4- > SeO4(2-) > CrO4(2-) > MoO4(2-). In contrast, borate was least preferred by hydrocalumite. Coordination, size, and electronegativity are likely the factors that result in the observed differences among the oxyanions.

Management of selenium (Se) -contaminated agricultural drainage water is one of the most important environmental issues in California. To evaluate the feasibility of utilizing constructed wetlands to remediate Se-laden drainage water and the role of biological volatilization in Se removal, 10 flow-through wetland cells were constructed in 1996 in Corcoran, California. The monthly monitoring study from May 1997 to December 1999 showed that the vegetated wetlands were capable of significantly reducing Se from the inflow drainage water; an average of 69.2% of the total Se mass in the inflow was removed. Most of the Se was retained in sediment, and <5% of the Se was accumulated in plant tissues. Selenium volatilization was highest in the rabbitfoot grass wetland cell, where 9.4% of the Se input was volatilized over a 2-year period. Volatilization was greater in spring and summer than in fall and winter. For example, in May and June of 1998, 35 and 48%, respectively, of the Se entering the rabbitfoot grass cell was volatilized, whereas in the winter months, <5% was volatilized. The feasibility of using constructed wetlands for Se remediation, methods for the enhancement of Se volatilization, and the importance of considering potential Se ecotoxicity are discussed.