Partition coefficient desorption

Over the last decade, some research has indicated that (1) partition coefficients (i. e.,Kd) between solid and solution phase are not measured at true equilibrium [51,59-61], (2) the use of equilibrium rather than kinetic expressions for sorption in fate and effects models is questionable [22-24,60,61], and (3) sorption kinetics for some organic compounds are complex and poorly predictable [22 - 24,26]. This is mainly due to what has recently been discussed as slow sorp-tion/desorption of organic compounds to natural solid phase particles [107, 162-164,166-182]. The following is a summary of some important points supporting this hypothesis [1,66,67,170-183] ... 

Predicting sorption coefficients and hence the mobility of organic pollutants in aqueous-solid systems requires complete knowledge and analysis of various physical and chemical properties of such pollutants. This includes properties such as solubility, equilibrium vapor pressure, Henry s law constant, partition coefficient, as well as pKa and pKb values. Such properties can initially help determine the sorption-desorption behavior of organic pollutants once they are released, directly and/or indirectly, to the aqueous environment and then are in direct contact with solid phases. The following sections briefly summarize these properties. 

Girvin DC, Sklarew DS, Scott AJ, Zipperer JP (1990) Release and attenuation of PCB congeners, measurements of desorption kinetics and equilibrium sorption partition coefficients. GS6875, Electric Power Research Institute, Palo Alto, CA 94304... 

Bowman, B.T. and Sans, W.W. Adsorption, desorption, soil mobility, aqueous persistence and octanol-water partitioning coefficients of terbufos, terbufos sulfoxide and terbufos sulfone, J. Environ. Sol. Health, B17(5) 447-462, 1982. 

Liang C, Pankow JF (1996) Gas/particle partitioning of organic compounds to enviromnental tobacco smoke partition coefficient measurements by desorption and comparison to urban particulate material. Environ Sci Technol 30 2800-2805... 

In a sediment system, the hydrolysis rate constant of an organic contaminant is affected by its retention and release with the sohd phase. Wolfe (1989) proposed the hydrolysis mechanism shown in Fig. 13.4, where P is the organic compound, S is the sediment, P S is the compound in the sorbed phase, k and k" are the sorption and desorption rate constants, respectively, and k and k are the hydrolysis rate constants. In this proposed model, sorption of the compound to the sediment organic carbon is by a hydrophobic mechanism, described by a partition coefficient. The organic matrix can be a reactive or nonreactive sink, as a function of the hydrolytic process. Laboratory studies of kinetics (e.g., Macalady and Wolfe 1983, 1985 Burkhard and Guth 1981), using different organic compounds, show that hydrolysis is retarded in the sohd-associated phase, while alkaline and neutral hydrolysis is unaffected and acid hydrolysis is accelerated. 

Liang, C., and J. F. Pankow, Gas/Particle Partitioning of Organic Compounds to Environmental Tobacco Smoke Partition Coefficient Measurements by Desorption and Comparison to Urban Particulate Material, Environ. Sci, Technol., 30, 2800-2805 (1996). 

The behavior and health effects of environmental tobacco smoke (ETS) depends upon how certain compounds (e.g., PAHs, nicotine, carbazole) are distributed between the gas and the particulate phases. Using a desorption technique, Liang and Pankow (1996) have determined gas/ETS particle partition coefficients for a series... 

Abiotic soil desorption measurements are probably the most important tests to precede measurement of microbial degradation. Desorption tests measure the site-specific soil/water partition coefficients for the contaminants of interest. Several experimental protocols are available for measuring partition coefficients (Wu Gschwend, 1986 Rogers, Tedaldi Kavanaugh, 1993). At the two extremes of bioavailability and biodegradability, contaminants can either be detected near their solubility limit or can be undetectable in the aqueous phase. Measurements of aqueous phase and soil phase concentrations in equilibrium may... 

Girvin, D. C., Skalrew, D. S., Scott, A.J. Zipperer, J. P. (1990). Release and Attenuation of PCB Congeners Measurement of Desorption Kinetics and Equilibrium Sorption Partition Coefficients. Palo Alto, CA. Electric Power Research Institute, EPRI GS-6875. 

The rate of H2Oz consumption and the OH production were directly related to total iron concentration. The concentrations of hydroxyl radical produced were controlled by the rate of reaction with dissolved constituents. Rate constants for adsorption (ka) and desorption (kd) of PCBs from particles were calculated by regression of data from 1.5 to 5 hr. Adsorption rate constants were estimated from Equation (6.130) assuming that the partitioning rate constants between 2 and 5 hr without OH could be used for calculation of equilibrium partition coefficients Ky)... 

Hence, under the quasi-steady approximation, the movement of the species is dictated by a macroscopic convection-diffusion-reaction equation with an instantaneous adsorption/desorption source term. A notable consequence of the three-scale approach is the double-averaging representation for the partition coefficient A which is defined as... 

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