Article pubs.acs.org/est
Identification and Determination of Selenosulfate and Selenocyanate in Flue Gas Desulfurization Waters Panayot K. Petrov,†,‡ Jeffrey W. Charters,†,§ and Dirk Wallschlag̈ er*,† †
Environmental & Resource Sciences Program and Department of Chemistry, Trent University, 1600 West Bank Drive, Peterborough, Ontario K9J 7B8, Canada ‡ Department of Chemical Measurement and Calibration, LGC, Teddington, Middlesex, TW11 0LY, United Kingdom S Supporting Information *
ABSTRACT: In this work, 13 selenium species in flue gas desulfurization (FGD) waters from coal-fired power plants were separated and quantified using anion-exchange chromatography coupled to inductively coupled plasma mass spectrometry. For the first time, we identified both selenosulfate (SeSO32−) and selenocyanate (SeCN−) in such waters, using retention time matching and confirmation by electrospray mass spectrometry. Besides selenite and selenate, selenosulfate was the most frequently occurring selenium species. It occurred in most samples and constituted a major fraction (up to 63%) of the total selenium concentration in waters obtained from plants employing inhibited oxidation scrubbers. Selenocyanate occurred in about half of the tested samples, but was only a minor species (up to 6% of the total selenium concentration). Nine additional Se-containing compounds were found in FGD waters, but they remain unidentified at this point.
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INTRODUCTION Selenium (Se) is a trace metalloid of high environmental concern, because it may accumulate in aquatic food chains and cause reproductive defects in water fowl and predatory fish.1 Cases of Se contamination are typically associated with discharges from industrial operations processing Se-rich natural resources, such as certain sulfidic ores, crude oils, or coals. During coal combustion, Se is volatilized as elemental Se0 and SeO2,2 and is subsequently captured (largely) in the flue gas desulfurization (FGD) processes used at coal-fired power plants to remove sulfur dioxide (SO2) from the exhaust gas. If wet scrubbers are used, most of the captured SeO2 is initially transferred to the scrubber water in the form of selenite (HSeO3−). From here on, the fate of Se depends strongly on technical design and operating parameters, which vary widely among individual power plants. There are three main types of wet scrubbers (with respect to their hydrochemistry): in forced oxidation systems, which represent the majority of currently operating and nearly all planned FGD wet scrubbers, the captured sulfite is oxidized with air and subsequently precipitated as gypsum (CaSO4). The less common natural oxidation systems operate on a similar basis, while in inhibited oxidation systems sulfite oxidation is prevented by the addition of chemical reductants (typically reduced sulfur compounds). Regardless of the used wet scrubber technology, the scrubber effluents sometimes contain elevated Se concentrations and may not undergo Se-specific treatment before their eventual discharge into the aquatic environment, so the discharged Se concentrations are typically determined by power plant operating parameters (such as coal © 2011 American Chemical Society
type, coal consumption, and scrubber recirculation rate), dilution with water from other process streams, and passive attenuation in holding ponds. In order to minimize Se emissions from coal-fired power plants into the aquatic environment, it is crucial to understand and manage Se chemistry in the scrubbers. It has been assumed traditionally that only two dissolved chemical forms of Se (“species”) occur in FGD waters: the selenite (Se(IV)) scrubbed from the flue gas, and its oxidation product selenate (Se(VI)).3 Of these, selenite can be removed (if done correctly) with the common iron (hydr)oxide coprecipitation process,4 employed in many types of industrial process waters to remove a suite of trace elements. By contrast, selenate is not removed in this process and requires dedicated treatment technologies, typically biological reduction in anaerobic bioreactors5 or anaerobic treatment wetlands.6 Not surprisingly, coal-fired power plants (like other industries) report inconsistent treatment success with respect to reducing total Se concentrations by using iron (hydr)oxide coprecipitation; it is found that Se removal varies over time and with FGD system design and operating factors. Besides the formation of selenate, one potential additional reason for this observation could be the presence of other, previously unrecognized, Se species in FGD waters. Recent analytical results, obtained by coupling liquid chromatographic Received: Revised: Accepted: Published: 1716
July 21, 2011 December 27, 2011 December 28, 2011 December 28, 2011 dx.doi.org/10.1021/es202529w | Environ. Sci. Technol. 2012, 46, 1716−1723
Environmental Science & Technology
Article
Table 1. Selenium Speciation Results and Speciation Mass Balance for 19 FGD Water Samples sample A-1 A-2 B-1 B-2 B-3 B-4 C-1 C-2 C-3 C-4 C-5 C-6 D-1a D-2a D-1b D-1b field duplicate D-2b E-1 F-1 a
wet FGD typea FO IO IO IO IO IO FO FO FO FO FO FO FO FO FO FO FO FO FO
FGD additive none none formic acid formic acid formic acid formic acid dibasic acid dibasic acid dibasic acid none none none none none dibasic acid dibasic acid dibasic acid dibasic acid dibasic acid
HSeO3− 266 2082 1103 403 858 179 30 114 81
