Sorption-Induced Effects of Humic Substances on Mass Transfer of Organic Pollutants through Aqueous Diffusion Boundary Layers: the Example of Water/Air Exchange

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• 作者：Ksenia Ramus ; Frank-Dieter Kopinke ; Anett Georgi
• 刊名：Environmental Science & Technology (ES&T)
• 出版年：2012
• 出版时间：February 21, 2012
• 年：2012
• 卷：46
• 期：4
• 页码：2196-2203
• 全文大小：311K
• 年卷期：v.46,no.4(February 21, 2012)
• ISSN：1520-5851

文摘

This study examines the effect of dissolved humic substances (DHS) on the rate of water鈥揼as exchange of organic compounds under conditions where diffusion through the aqueous boundary layer is rate-determining. A synthetic surfactant was applied for comparison. Mass-transfer coefficients were determined from the rate of depletion of the model compounds by means of an apparatus containing a stirred aqueous solution with continuous purging of the headspace above the solution. In addition, experiments with continuous passive dosing of analytes into the water phase were conducted to simulate a system where thermodynamic activity of the chemical in the aqueous phase is identical in the presence and absence of DHS. The experimental results show that DHS and surfactants can affect water鈥揼as exchange rates by the superposition of two mechanisms: (1) hydrodynamic effects due to surface film formation (鈥渟urface smoothing鈥?, and (2) sorption-induced effects. Whether sorption accelerates or retards mass transfer depends on its effect on the thermodynamic activity of the pollutant in the aqueous phase. Mass transfer will be retarded if the activity (or freely dissolved concentration) of the pollutant is decreased due to sorption. If it remains unchanged (e.g., due to fast equilibration with a sediment acting as a large source phase), then DHS and surfactant micelles can act as an additional shuttle for the pollutants, enhancing the flux through the boundary layer.