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2007-4-28 EST最新文章和摘要
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Evidence of Bias in Air-Water Henry's Law Constants for Semivolatile Organic Compounds Measured by Inert Gas Stripping Chubashini Shunthirasingham, Ying Duan Lei, and Frank Wania* Department of Chemistry, and Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, Canada M1C 1A4 Received for review December 13, 2006 Revised manuscript received March 23, 2007 Accepted March 28, 2007 Abstract: Accurate knowledge of the air-water Henry's law constant (H) is crucial for understanding an organic compound's environmental behavior. The inert gas stripping (IGS) method, widely used to measure H of semivolatile organic compounds (SOCs), may yield erroneously high values for compounds with a high water surface adsorption coefficient, KIA, because chemical adsorbed to the bubble surface may be transferred to the head space upon bursting at the top of the stripping column. Experiments with alkanols of variable chain length identified a KIA threshold of approximately 10-3 m, above which IGS is susceptible to this artifact. Most SOCs are predicted to have KIA values well above that threshold. IGS-determined H-values for chemicals belonging to various groups of SOCs were evaluated by comparison with H-values either calculated from reliable vapor pressure and solubility data or derived from data compilations that achieve thermodynamic consistency through optimized adjustment of measured physical-chemical property data. The investigated deviations were found to be generally consistent with what would be expected from a surface adsorption artifact. Namely, the apparent bias in IGS-determined H-values, if it occurs, (1) is positive, (2) increases with increasing size of an SOC, and (3) increases with decreasing temperature. It generally is also of a magnitude predicted using estimated KIA values. However, different studies display different KIA threshold values, beyond which the artifact becomes notable, and some studies appear to succeed in avoiding the artifact altogether. Whereas the use of aerosol traps cannot explain the absence of a surface adsorption artifact, it may be related to higher flow rates used by some investigators. For large compounds or those with more than one functional group, the predicted deviation is too large when compared to observations, suggesting that the estimated KIA values for those compounds are too high. A full quantitative understanding of the artifact requires more accurate predictions of the adsorption of SOCs to the water surface. -------------------------------------------------------------------------------- |
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