Environ. Sci. Technol. 2007, 41, 3284-3289
Selenium and Arsenic Speciation in Fly Ash from Full-Scale Coal-Burning Utility Plants F R A N K E . H U G G I N S , * ,† CONSTANCE L. SENIOR,‡ PAUL CHU,§ KEN LADWIG,§ AND GERALD P. HUFFMAN† CFFS/CME, University of Kentucky, Lexington, Kentucky 40506, Reaction Engineering International, Salt Lake City, Utah 88234, and Electric Power Research Institute, Palo Alto, California 94580
X-ray absorption fine structure spectroscopy has been used to determine directly the oxidation states and speciation of selenium and arsenic in 10 fly ash samples collected from full-scale utility plants. Such information is needed to assess the health risk posed by these elements in fly ash and to understand their behavior during combustion and in fly ash disposal options, such as sequestration in tailings ponds. Selenium is found predominantly as Se(IV) in selenite (SeO32-) species, whereas arsenic is found predominantly as As(V) in arsenate (AsO43-) species. Two distinct types of selenite and arsenate spectra were observed depending upon whether the fly ash was derived from eastern U.S. bituminous (Fe-rich) coals or from western subbituminous or lignite (Ca-rich) coals. Similar spectral details were observed for both arsenic and selenium in the two different types of fly ash, suggesting that the postcombustion behavior and capture of both of these elements are likely controlled by the same dominant element or phase in each type of fly ash.
Introduction Arsenic and selenium are two of the most volatile and potentially hazardous elements to be released from commercial coal-burning plants (1). Although major fractions of both elements are captured on fly ash particles during coal combustion, both elements can be significant components in fugitive emissions that are released to the environment via stack gases, depending on the use and capture efficiency of environmental control devices at a particular plant (1, 2). Although the concentration of arsenic usually exceeds that of selenium in most coals (2-4), the volatility of selenium is greater than that of arsenic, and its presence in fly ash and fugitive emissions is enhanced (2). For the same reason, the ratio of selenium partitioning between fly ash and bottom ash is typically higher than that of arsenic. There are both similarities and significant differences in how arsenic and selenium occur in coal. Both elements can be present in pyrite, FeS2 (3, 5, 6), in which they substitute directly for sulfur (6). However, whereas pyrite is often the most important primary occurrence of arsenic in coal, * Corresponding author phone: (859)257-4045; fax: (859)257-7215; e-mail:
[email protected]. † University of Kentucky. ‡ Reaction Engineering International. § Electric Power Research Institute. 3284
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 9, 2007
selenium may be found in greater abundance as organoselenium forms (3, 5, 6). In such forms, selenium is believed to occur as analogues of organic sulfides (thioether, thiol) and thiophenes dispersed within coal macerals. Consequently, selenium often has a much higher organic association than arsenic and is less efficiently removed during physical coal cleaning. As a result of post-mining oxidation processes, pyrite-associated arsenic and selenium can be found in coal as arsenates and selenites, respectively. Arsenite and selenate species are not typically observed in coal. Arsenic and selenium are virtually completely volatilized during high-temperature combustion (1, 2) so that the original modes of occurrence of these elements in coal have little bearing on their behavior during combustion and their occurrence in post-combustion ash products. One potential area of concern regarding arsenic and selenium in coal combustion fly ashes is their leachability from fly ash used in soil amendment applications or in disposal in tailings ponds. Although significant concerns have been expressed (7-10) regarding the health risks of the release of both elements from coal fly ash, there are relatively few studies documented in the literature about the release of these elements from coal fly ash upon aqueous exposure. Hower et al. (11) investigated fly ash from 21 Kentucky power plants using the toxicity characteristic leaching procedure (TCLP) (12) and established that all of the TCLP leachates met established Resource Conservation and Recovery Act (RCRA) (13) limits for elemental concentrations, including arsenic and selenium. More recently, Jackson and Miller investigated arsenic and selenium extractabilities from 23 fly ashes collected from power plants in the southeastern U.S. (14). In this study, in which extractability was defined as the fraction of an element solubilized when 1 g of fly ash was exposed to 40 mL of deionized water for 16 h, the extractabilities for arsenic and selenium were found to vary from
