Technical NoteEvaluation of the gas stripping technique for calculation of Henry’s law constants using the initial slope method for 1,2,4,5-tetrachlorobenzene, pentachlorobenzene, and hexachlorobenzene
Highlights
► Henry’s law constant (HLC) reference uncertainty is high. ► HLCs assessed for 3 chlorobenzenes via gas stripping. ► Volatilization rates decreased over time according to three slope regions. ► Use of entire stripping data set leads to biased HLCs. ► Chlorobenzene HLCs differed from wide reference ranges.
Introduction
Henry’s law constant (HLC) is used to describe mass transfer of chemical between water and air. HLC is an important factor used in environmental risk assessment and fate and transport models (ten Hulscher et al., 2006). Additionally, HLC may also be used in fugacity models with standard partition coefficients to calculate fugacity capacity (Mackay and Paterson, 1981). Despite the need for accurate determination of HLC, the quality and uncertainty of HLC has recently been questioned (Goss et al., 2004, Brachet et al., 2005, Jantunen and Bidleman, 2006, ten Hulscher et al., 2006, Qian et al., 2011).
Measuring HLC directly by several experimental methods has been assessed and discussed previously (Mackay and Paterson, 1981, Fendinger and Glotfelty, 1988, Lau et al., 2006). In the absence of measured data, vapor pressure (Vp) and aqueous solubility (S) may be used to estimate HLC (HLC = P/S). The current gas stripping method has been used for calculation of HLC for sparingly soluble and semivolatile compounds (Oliver, 1985, Yin and Hassett, 1986, Warner et al., 1987, Dunnivant et al., 1988, ten Hulscher et al., 1992, Drouillard et al., 1998, Jantunen and Bidleman, 2006, ten Hulscher et al., 2006). Determination of HLCs for hydrophobic organic compounds (HOCs), such as chlorobenzenes, may be difficult due to low solubilities (ten Hulscher et al., 2006). Additionally, experimental artifacts related to adsorption of chemical to glass surfaces is a known issue for HOCs. (Brachet et al., 2005, ten Hulscher et al., 2006, Qian et al., 2011).
Chlorobenzenes (CBs) are a suite of varying physicochemical properties (Table 1). Three compounds (1,2,4,5-tetrachlorobenzene – TeCB, pentachlorobenzene – PeCB, hexachlorobenzene – HCB) were chosen for the current study. Each of these chlorobenzenes, especially HCB, have numerous literature HLCs available (Table 2). The goal of the current study is to establish HLCs for the three chlorobenzenes using the gas stripping technique. Given the wide range of literature HLCs, determination of accurate HLCs are valuable in future use of gas stripping in subsequent studies.
Section snippets
Theory
Gas spargers are spiked with HOCs and allowed to reach equilibrium with sparger surfaces overnight. Nitrogen gas (N2) is bubbled through the column, allowing equilibrium between gas and water. HLC can be determined via (Drouillard et al., 1998):where V is the sparged water volume (L), R is the gas constant, T is temperature (K), F is the gas flow rate (L h−1), and kv is the first-order volatilization rate constant (h−1). The volatilization rate constant (kv) can be found via (Dunnivant
Chemicals and reagents
1,2,4,5-Tetrachlorobenzene (98% pure), pentachlorobenzene (98% pure), hexachlorobenzene (99% pure), 1,3,5-tribromobenzene (TBB; 98% pure) and Amberlite XAD2 (20–60 mesh) were purchased from Sigma–Aldrich (Canada). Stock solutions of 10 mg L−1 TBB (internal standard) and a mixture TeCB, PeCB and HCB (30, 10, and 10 mg L−1, respectively) were prepared in hexane and methanol, respectively, and stored at 4 °C.
XAD2 resin trap preparation and extraction
XAD2 resins were cleaned prior to use using a flow-through column and distilled water,
Theory validation
The first assumption of the gas stripping technique is the stripping gas must reach equilibrium with the water phase. The column height equilibration assumption has been extensively studied (Mackay et al., 1979, Matter-Mueller et al., 1981, Yin and Hassett, 1986, Dunnivant et al., 1988, ten Hulscher et al., 1992, Drouillard et al., 1998). For the current chlorobenzenes, a 40 cm column was determined to be adequate for equilibrium (ten Hulscher et al., 1992); therefore, the current 60 cm water
Conclusions
Adsorption of analytes onto sampling vessels should be considered in calculation of HLC and the lack of this consideration has subsequently led to questioning of the validity of published data for nonpolar compounds (Brachet et al., 2005). Further, Jantunen and Bidleman (2006) expressed the need for validation of differences in measurement techniques and methodologies which may be responsible for observed variations in values and large uncertainty. Current research, including this study, has
Acknowledgements
This study was supported by the Natural Sciences and Engineering Research Council Canadian Graduate Scholarship (to K.N.M.) and Discovery grant (to R.S. and K.G.D.). The authors would like to thank Bill Middleton for his assistance in analytical work.
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