Elsevier

Atmospheric Environment

Volume 42, Issue 10, March 2008, Pages 2349-2357
Atmospheric Environment

Theoretical predictions of arsenic and selenium species under atmospheric conditions

https://doi.org/10.1016/j.atmosenv.2007.12.028Get rights and content

Abstract

Thermochemical properties of arsenic and selenium species thought to be released into the atmosphere during the coal combustion were examined using ab initio methods. At various levels of theory, calculated geometries and vibrational frequencies of the species were compared with experimental data, where available. Through a comparison of equilibrium constants for a series of gaseous arsenic and selenium oxidation reactions involving OH and HO2, five thermodynamically favored reactions were found. In addition, it was determined that all favored reactions were more likely to go to completion under tropospheric, rather than stratospheric, conditions.

Introduction

Globally, coal combustion provides a major source of energy with more than five billion short tons of coal burned each year (Katzer, 2007). In the United States, approximately 52% of electricity comes from coal, an amount expected to increase in the coming decades (Energy Information Administration (EIA), 2005; Katzer, 2007). As the use of coal for power increases, attention turns to the potentially harmful effects associated with its combustion. Along with the well-known problem of CO2 emissions, various trace elements are emitted. When coal combusts, both arsenic and selenium are volatilized and escape the smokestacks either as a gas or enriched in the fly ash (Senior et al., 2000). Both trace elements naturally occur in the earth's crust; however, at high concentrations they can have adverse effects on the environment and human health. By understanding the speciation and lifetime of arsenic and selenium in the atmosphere, technologies can be developed to better deal with these potentially harmful elements.

Arsenic ranks 20th in trace elements found in the earth's crust (National Research Council (NRC), 1997). Natural cycles, such as weathering and volcanic activity, emit arsenic into the atmosphere (Cullen and Reimer, 1989); however, studies have traced nearly 75% of arsenic emissions to anthropogenic sources (Zeng et al., 2001). Arsenic content in coal averages 2 ppm (Commission on Life Sciences (CLS), 1977), making coal combustion a major source of anthropogenic arsenic release (Han et al., 2003). When arsenic species enter drinking water sources humans may suffer arsenic toxicity, symptoms of which include problems of the gastrointestinal tract, circulatory system, immune system, and renal system. Skin lesions and cancers, particularly Bowen's disease, may also result (Duker et al., 2005). To protect drinking water sources it is important to look at the lifetime of arsenic species in the atmosphere, i.e., whether they enter the stratosphere, are flushed out with rainwater, or adsorb onto dust particles and settle into soil. Arsenic exists in two oxidation states, the trivalent As(III) and pentavalent As(V). As(III) molecules, such as AsCl3 and AsH3, tend to be water-soluble (Hudson-Edwards et al., 2005; Efermov et al., 2002). The species As2, As4, and elemental As are also found to be at least partially water-soluble (Young, 1979; Harrington et al., 1997). If these molecules are stable in the atmosphere, they will likely end up in soil or fresh water. Arsenic (V) molecules tend to be prone to adsorption (Frentiu et al., 2007) and will likely make their way into soil by adsorbing onto dust or soot particulates.

Selenium levels in coal average 1 ppm (Shah et al., 2007), making coal combustion the main source of anthropogenic release of selenium (ATSDR, 2003). As with arsenic, selenium uptake by humans can be toxic, but, unlike arsenic, selenium plays an essential role as an antioxidant enzyme (Tinggi, 2003). The lower recommended daily intake of selenium is 55 μg day−1 for adults (ATSDR, 2003). Diet provides most of the necessary selenium, preventing against the health effects of selenium deficiencies such as Keshan and Kashin–Beck diseases, which cause cardiovascular and joint problems, respectively. On the other hand, over-consumption of selenium, i.e., intake levels greater than 400 μg day−1, can lead to selenium toxicity, known as selenosis. Symptoms of selenosis include dermal effects such as loss of hair and nails as well as neurological effects such as numbness and paralysis (ATSDR, 2003). In addition to these issues, high levels of selenium have led to birth defects in birds (Ohlendorf et al., 1986) and fish (Lemly, 1993). Although no selenium overdoses in humans have been directly linked to coal combustion emissions and the effects of selenium inputs into the ecosystem may not be immediately noticeable, selenium inputs from coal emissions need to be monitored since excess selenium can be difficult to remove (Lemly, 1997). The varying species of selenium may have different fates in the environment. For instance, selenium dioxide is water-soluble (Zhang, 2002), and can be dissolved in rainwater when released into the atmosphere. Also, species such as elemental Se and SeO2 can condense at low temperatures and readily adsorb onto particles (Yan, 2004). Through their deposition onto soil or vegetation, they have a direct path into the food chain when such soil or vegetation is consumed. Certain plants, such as Astragalus bisulcatus, are known to accumulate selenium in their leaves, causing toxicities in grazing animals (Pickering et al., 2000). Knowledge of selenium's speciation will allow scientists to monitor selenium by predicting its final form, where it is deposited, and, ultimately, its effects on the environment.

Previous experimental and theoretical work on the arsenic and selenium species that may be released from coal combustion was used to determine the possible species that may exist as reactants in the atmosphere. By combusting coal, Senior and Bool concluded that both arsenic and selenium are vaporized in the combustion process. The vapors then condense onto fly ash particles during cooling or remain in gaseous form (Senior et al., 2000). Yan et al. (1999) completed equilibrium modeling through minimizing the Gibbs free energy of the system likely to determine the composition of arsenic, selenium, and mercury species released from coal combustion. The species, AsO, AsCl3, AsH3, As2, As4, H2Se, SeO, SeO2, SeCl2, and elemental Se were all found to be possible stable species in the flue gases of coal combustion. Yan et al. later performed combustion simulations confirming the existence of Se, SeO, and SeO2 in combustion flue gases (Yan et al., 2001, 2004). Thermodynamic modeling was also carried out by Miller et al. (2003) to determine the speciation of trace elements in coal combustion where As4O10, As4O7, As4O6, As4O8, AsO, and AsO2 were found to be probable emission species. From these predicted species, As, AsO, Se, and SeO were chosen for atmospheric oxidation reaction calculations via OH and HO2 radical species. Although these investigations provide insight into the possible arsenic and selenium species that are released from coal combustion processes, they provide no insight into the atmospheric lifetime of these pollutants once released from the stacks. The current investigation involves an examination of the atmospheric cycle of these species generated from coal combustion flue gases.

Theoretical calculations were carried out to enhance the knowledge of gaseous arsenic and selenium speciation within the atmosphere. Beginning with an investigation of the theoretical thermochemical data for several key reactions allows for increased understanding of the pathway of anthropogenic arsenic and selenium cycling through the biosphere. According to Molina et al. (1996), the free radicals OH and HO2 play a major role in determining the lifetimes of various compounds in the atmosphere. For this reason, this work focuses on the oxidation of As and Se species by OH and HO2 to then determine the relative stabilities of the oxidized forms of these species. Once these oxidized forms are recognized it can be determined how they might cycle through the biosphere, i.e., through rainwater, particulates, soil, freshwater, etc. For instance, understanding the oxidation pathways of As and Se species to the potentially water-soluble forms, As, As(III)O, As(III)O2, and SeO2, will be crucial in the development of effective control technologies.

Section snippets

Computational methodology

Calculations were carried out using the Gaussian 03 suite of programs (Frisch et al., 2004). Basis sets incorporating relativistic effects for the inner electrons were explored through the use of small core relativistic effective core potentials (RECP) for arsenic and selenium. This basis set employs the relativistic ECP28MWB pseudopotential of the Stuttgart group (Martin and Sundermann, 2001) for both arsenic and selenium, with the respective energy-optimized (4s2p)/[3s2p] and (4s5p)/[2s3p]

Basis set justification

To determine the most accurate level of theory, six arsenic and selenium species were evaluated using four methods: CCSD, CCSD(T), QCISD, and QCISD(T) combined with each of the Pople basis sets: 6-311G, 6-311+G, 6-311G*, and 6-311+G*, as well as the Stevens ECP and ECP28MWB pseudopotential of the Stuttgart group. The resulting vibrational frequencies were compared to experimental values in order to find the level of theory with the lowest absolute error. The accurate prediction of the

Results and discussion

Table 3a and b present the reaction enthalpies, entropies, and equilibrium constants for a selection of gas phase reactions involving arsenic and selenium. Five As and Se oxidation reactions were found to be spontaneous, having negative Gibb's free energy changes:As+OH→AsOH,As+HO2→AsO+OH,Se+OH→SeOH,Se+HO2→SeO+OH,SeO+HO2→SeO2+OH.

Of these, all were more likely to go to completion at tropospheric (lower) temperatures than at stratospheric (higher) temperatures. Graphs showing variations in Keq

Conclusions

Overall, the QCISD/6-311G* and QCISD with the ECP28MWB pseudopotential of the Stuttgart group performed best for all species and were used to calculate the energetics. From equilibrium constant calculations, possible oxidation–reduction mechanisms of As and Se compounds in the atmosphere were obtained. In particular, SeO2 is water-soluble and the reaction between SeO and HO2 may present a way for selenium from coal combustion to end up in drinking water. More work on arsenic and selenium

Acknowledgments

This material is based upon work supported by the National Science Foundation under Grant no. 03-577. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

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