During laboratory and field studies, a fraction of contaminants in soils or sediments often is observed to be highly resistant to desorption. This desorption-resistant fraction may have significant effects on long-term fate and exposure of soil/ sediment-bound contaminants in particular, causing much reduced availability and contaminant persistence. Previous work by many research groups has indicated that this nonideal desorption behavior could be better predicted with biphasic desorption models. The present study further investigated the release of naphthalene and phenanthrene from sediments during and after cosolvent treatment. Experimental results indicate that release of these two compounds under cosolvent conditions can be accurately predicted with a previously developed, biphasic desorption model when the solubility enhancement effect of cosolvent is accounted for using standard activity coefficient ratios. In addition, desorption of the residual contaminants after cosolvent treatment follows the original biphasic desorption model very well, suggesting that cosolvent treatment increases only the aqueous solubility and has little effect on the nature of the desorption-resistant fraction and that cosolvent desorption is a valuable analytical tool for quickly measuring the magnitude of the desorption-resistant fraction. The present findings might have important implications for the mechanisms controlling resistant desorption of hydrophobic organic compounds and for predicting the availability and long-term fate of contaminants in soils and sediments.
"Sediment is composed of a continuum of pores ranging in size from micropores < 0.1 lm in diameter to macropores > 20 lm (Baldock et al., 2004) and a continuum of compartments ranging from rubbery (i.e., loose, flexible) to glassy (i.e., condensed, rigid) organic matter (Xing and Pignatello, 1997) ordered by their desorption rate constants (Pignatello and Xing, 1996) and degree of sorption (Braida et al., 2004). The release of contaminants from these sites is often considered to occur in biphasic stages: a fast desorbing contaminant fraction in equilibrium with the contaminant in solution and a slow desorbing contaminant fraction that is not in equilibrium with the contaminant in solution (Cornelissen et al., 1997; Chen et al., 2008). The fast desorbing fraction releases from amorphous materials (Huang et al., 1997; Cornelissen et al., 2005) and adsorption sites in the outer regions of the sediment aggregates which are in close contact with the aqueous phase (Cornelissen et al., 1997) allowing for rapid and reversible desorption (Qiu and Davis, 2004). "
"Water-miscible organic cosolvents can be encountered in soils and groundwaters as a result of accidental spills during storage, disposal in landfill and cosolvent mediated remediation processes (Chen et al., 2008; Maturi and Reddy, 2008; Wan et al., 2009). Sorption by soil is an important natural process, which influences the bioavailability, mobility and overall fate of hydrophobic organic chemicals (HOCs) released onto a soil system. "
[Show abstract][Hide abstract] ABSTRACT: The effect of the sorption of phenanthrene and 2,2',5,5'-polychlorinated biphenyl (PCB52) by five differently weathered soils were measured in water and low methanol volume fraction (f(c)0.5) as a function of the apparent solution pH (pH(app)). Two weathered oxisols (A2 and DRC), and moderately weathered alfisols (Toronto) and two young soils (K5 and Webster) were used. The K(m) (linear sorption coefficient) values, which log-linearly decreases with f(c), were interpreted using a cosolvency sorption model. For phenanthrene sorption at the natural pH, the empirical constant (alpha) ranged between 0.95 and 1.14, and was in the order of oxisols (A2 and DRC)<alfisols (Toronto)<young soils (K5 and Webster). Smaller alpha values for highly weathered soils are indicative of smaller solute sorption reduction than those predicted from the increment of the solute's activity coefficient in the solution phase. A similar trend was observed for PCB52 sorption. The K(m) values measured at the range of pH 3-7 also showed an inversely log-linear relationship. The regression slope (alphasigma) calculated from the cosolvency sorption model as a function of pH(app) only varied within <5%, with the exception for phenanthrene sorption by two highly weathered soils, which had 10% greater alphasigma values obtained at acidic pH(app). This phenomenon is a result of the greater acid enhancement effect on phenanthrene sorption by the oxisols, which is reduced with increasing f(c). These results revealed an unexplored relationship between the cosolvent effect on the sorption and the properties of the soil organic matter (a primary sorption domain) as a function of the degree of soil weathering.
[Show abstract][Hide abstract] ABSTRACT: Most of what is known about the environmental fate and transport of contaminants comes from studies of dilute, aqueous solutions. Theoretical and practical aspects of hydrophobic organic contaminants in aqueous mixtures of organic solvents are well developed, but relatively little attention has been paid to those situations where inorganic contaminants such as metals, water, and organic solvents occur simultaneously. The authors discuss changes in solution properties accompanying the addition of organic solvents. They review more than 500 references to available data on ion solvation, solubility, activity, acidity, pairing, and pH in electrolyte and electrolyte-free solvent mixtures and tabulate selected experimental values. Whenever available, equations developed to estimate these properties are given and their applications and limitations described. The implications of the progress in mixed-solvents solution chemistry on contaminant behavior and transport are described. Major limitations to the study of inorganic contamination in mixed solvents include the absence of complete and comprehensive data (e.g., type of ionic species, activity coefficients, solubility constants) that characterize solute and solvent properties in these mixtures and the indefinite number of possible solute and solvent combinations. The generation of models predicting such data from easily measured benchmark solute and solvent properties is a major challenge. The availability of such database and models may open the field for environmental engineers and soil scientists to study the basic chemical processes and mechanisms of metal contamination in complex multicomponent systems.
Critical Reviews in Environmental Science and Technology 02/2011; 41(6):521-621. DOI:10.1080/10643380902945839 · 3.47 Impact Factor
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