Soil distribution coefficient, organic

Among these factors, organic content determines the soil distribution coefficient and is an important parameter that quantifies the distribution of organic pollutants in soil. The soil distribution coefficient (K. ) is the ratio between the mass of contaminant sorbed by soil (mg/g) to the mass of contaminant in the aqueous phase (mg/mL), or ... 

Table I summarizes some typical distribution coefficients. Sediments become enriched in plutonium with respect to water, usually with a factor of vlO5. Also living organisms enrich plutonium from natural waters, but usually less than sediments a factor of 103 - 101 is common. This indicates that the Kd-value for sediment (and soil) is probably governed by surface sorption phenomena. From the simplest organisms (plankton and plants) to man there is clear evidence of metabolic discrimination against transfer of plutonium. In general, the higher the species is on the trophic level, the smaller is the Kd-value. One may deduce from the Table that the concentration of plutonium accumulated in man in equilibrium with the environment, will not exceed the concentration of plutonium in the ground water, independent of the mode of ingestion.

This equation is widely used to describe adsorption in soil and near-surface aquatic environments. Another widely used linear coefficient is the organic-carbon partition coefficient Koc, which is equal to the distribution coefficient divided by the percentage of organic carbon present in the system as proposed by Flamaker and Thompson.131... 

Distribution of organic chemicals among environmental compartments can be defined in terms of simple equilibrium expressions. Partition coefficients between water and air, water and soil, and water and biota can be combined to construct model environments which can provide a framework for preliminary evaluation of expected environmental behavior. This approach is particularly useful when little data is available since partition coefficients can be estimated with reasonable accuracy from correlations between properties. In addition to identifying those environmental compartments in which a chemical is likely to reside, which can aid in directing future research, these types of models can provide a base for more elaborate kinetic models. 

Szabo, G., Guczi, J., Kodel, W., Zsolnay, A., Major, V., Keresztes, R (1999) Comparison of different HPLC stationary phases for determination of soil-water distribution coefficient, KqC, values of organic chemicals in RP-HPLC system. Chemosphere 39, 431 142. 

The first of these environmentally-important parameters can be expressed as a partition coefficient. In aqueous solution many, but not all pesticide compounds exhibit strong affinity for soil organic matter or concentrate in the lipid phase of soil organisms. Some, notably the cationic group, also exhibit marked affinity for clay or other mineral surfaces. An overall partition (or distribution) coefficient (kD) can be defined ... 

Organic compounds released into the vadose zone exist in four closely interrelated forms free-phase NAPL, attenuated to surface of soil grains, dissolved in water retained on and between the soil particles, and present as a gaseous phase. The mass distribution of each of these phases is controlled by such factors as concentration gradients, distribution coefficients, and Henry s law constants. 

The presence of a residual hydrocarbon phase in soils or sediments has been shown to increase the soil- or sediment-water distribution coefficients of poorly water-soluble organic contaminants [463,464]. Such petroleum-hydrocarbon-based phases have been shown to function as effective partition media for PCB congeners [467]. In general, sorption of contaminants by soils and sediments reduces their bio availability to microorganisms [468,469]. In this fashion, the... 

These data were measured at or extrapolated to ambient temperature and pH values. The data are discussed in the text. NA = not available. b/ Kq = soil water distribution coefficient (K ) divided by the organic carbon content of the soil, cj Whenever possible, half-life for soil dissipation is derived from the field data half-lives described in the text rather than lab data. As such, it may not represent a true first-order process. Value has been estimated from the equation in ref. 20. e/ Hydrolysis of total residues (aldicarb + sulfoxide + sulfone). pK for p -phthalic acid is 3.5. The chlorine atoms of DCPA should lower the pK to about 2. Conditions optimized for soil metabolism. 

Therefore, we may not be too surprised to find that nonionic chemicals show increasing solid-water distribution ratios for soils and sediments with increasing amounts of natural organic matter. This is illustrated for tetrachloromethane (carbon tetrachloride, CT) and 1,2-dichlorobenzene (DCB) when these two sorbates were examined for their solid-water distribution coefficients using a large number of soils and sediments (Fig. 9.7, Kile et al., 1995.)... 

A potential limitation of surfactant-enhanced desorption is the observation that sorbed surfactant molecules can increase the sorption of hydrophobic organic contaminants (Edwards et al. 1994 Sun et al. 1995 Ko et al. 1998). Sun et al. (1995) reported that the nonionic surfactant Triton X-100 increased the sorption of p,p -DDT, 2,2 ,4,4 ,5,5 -PCB, and 1,2,4-trichlorobenzene to a soil (joc= 0.001) at concentrations below CMC. At concentrations above CMC, the distribution coefficients (Kp) of the DDT and PCB studied were reduced to levels below their respective values in pure water. However, at a surfactant concentration of five times CMC, the Kp of 1,2,4-trichlorobenzene was still a factor of three higher than Kp in pure water. Edwards et al. (1994) and Ko et al. (Ko et al. 1998) reported similar results for different groups of surfactants. 

We will address the distribution of organic compounds in the environment by looking at equilibrium partitioning of organic compounds between environmental phases, which include air, water, soil, and biota. Taking these phases pairwise, we can define the various physical and chemical properties that control the partition coefficients between these phases ... 

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