Soil-water distribution coefficients

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. 

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. 

Twining, J. R., Payne, T. E., and Itakura, T. (2004). Soil-water distribution coefficients and plant transfer factors for Sr and Zn under field conditions in tropical... 

ICF. 1986. Development of soil water distribution coefficients for LLM inorganic chemicals (Draft). Washington, DC. 

Several attempts have been made in order to correlate the values to physico-chemical properties of soils however, not much success has been accomplished. Carlon et al. [26] reported a correlation between pH and soil-water distribution coefficient (K ) for Pb log = 1.99-1- 0.42pH. The EPA [19] collected values for cadmium, cesium, chromium, lead, plutonium, radon, strontium, thorium, tritium and uranium in soils. The variability in values can be many orders of magnitude as shown in Table 2. 

A comparison of measured and predicted values of the HCB soil-water distribution coefficient, AT, for the Tween 80 and Appling soil system is presented in Figure 8. The measured values of (data points) were obtained from the HCB... 

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... 

Sorption of pharmaceuticals onto the surface of particulate matter or their distribution between two phases (water and either sludge, sediment or soil) depends on many factors, the most important being liquid phase pH and redox potential, the stereochemical structure and chemical nature of both the pharmaceutical compound and the sorbent, the lipophilicity of the sorbed molecules (excellent sorption at log Kov > 4, low sorption at log < 2.4), the sludge-water distribution coefficient Kd Kd > 2 L g SS good sorption, < 0.3 L g SS low sorption), the extent of neutral and ioiuc species present in the wastewater and the characteristics of the suspended particles. Moreover, the presence of humic and fulvic substances may alter the surface properties of the sludge, as well as the number of sites available for sorption and reactions, thereby enhancing or suppressing sorption of PhCs [38, 55, 61]. 

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.)... 

The soil-water partition coefficient, soil-water is a conditional and not a fundamental physicochemical compound property. Xsoii-water is included here because of its great practical significance. Its value depends on a number of soil and solution characteristics, such as the organic carbon (OC) or organic matter (OM) content, clay content and type, pore volume, pore size and distribution, and solution conditions. soil—water can be defined as the ratio of the sorbate s mass sorbed per unit volume of soil to the mass dissolved per unit volume of aqueous phase with both phases at equilibrium ... 

The A oii-water partition coefficient is dimensionless. When determined in batch tests sorption is typically indicated as the soil-water distribution coefficeint, Kj, in which the soil is quantified in terms of mass rather than volume ... 

Various coefficients are helpful in measuring the potential of a chemical to partition between the aqueous and solid phases. These parameters are valuable in predicting the potential of a chemical to adsorb to the solid phase. The soil-water distribution ratio, Kd,... 

Figure 4 shows the results of the extraction of water (figure 4a) and dry and wetted soil (figure 4b and 4c, respectively) using pure carbon dioxide, carbon dioxide with 2 mol % benzene, and carbon dioxide with 2 mol % methanol at 15 MPa and 298 K. For the extraction of dry contaminated soil, the distribution coefficient of phenol between the soil and pure carbon dioxide was 0.35, as shown in Figure 4b. The presence of benzene increased the distribution coefficient almost 100 %, while the presence of methanol resulted in the removal of essentially all of the phenol from the soil (within experimental accuracy, K-values > 7 correspond to almost complete removal). For the extraction of wetted soil (10 wt.% water )with pure carbon dioxide the distribution coefficient of phenol was again 0.35. The effect of the entrainers on this system, however.

Fig. 5. Soil organic-water distribution coefficients plotted as a function of the aqueous sol> ubilities of selected neutral organic compounds (27). [Reprinted with permission from Science 206, 831 (1979). Copyright 1979 by the American Association for the Advancement of Science.]...

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.

Results of adsorption experiments for butylate, alachlor, and metolachlor in Keeton soil at 10, 19, and 30°C were plotted using the Freundlich equation. A summary of the coefficients obtained from the Freundlich equation for these experiments is presented in TABLE IV. Excellent correlation using the Freundlich equation over the concentration ranges studied (four orders of magnitude) is indicated by the r values of 0.99. The n exponent from the Freundlich equation indicates the extent of linearity of the adsorption isotherm in the concentration range studied. If n = 1 then adsorption is constant at all concentrations studied (the adsorption isotherm is linear) and K is equivalent to the distribution coefficient between the soil and water (Kd), which is the ratio of the soil concentration (mole/kg) to the solution concentration (mole/L). A value of n > 1 indicates that as the solution concentration increases the sorption sites become saturated, resulting in a disproportionate amount of chemical being dissolved. Since n is nearly equal to 1 in these studies, the adsorption isotherms are nearly linear and the values for Kd (shown in TABLE IV) correspond closely to K. These Kd values were used to calculate heats of adsorption (AH). 

The Level I calculations for environmental pHs of 5.1 and 7 suggest that if 100,000 kg (100 tonnes) of pentachlorophenol (PCP) are introduced into the 100,000 km2 environment, most PCP will tend to be associated with soil. This is especially the case at low pH when the protonated form dominates. Very little partitions into air and only about 1% partitions into water. Soil contains most of the PCP. Sediments contain about 2%. There is evidence of bioconcentration with a rather high fish concentration. Note that only four media (air, water, soil and bottom sediment) are depicted in the pie chart therefore, the sum of the percent distribution figures is slightly less than 100%. The air-water partition coefficient is very low. As pH increases, dissociation increases and there is a tendency for partitioning to water to become more important. Essentially, the capacity of water for the chemical increases. Partitioning to air is always negligible. 

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