Soil distribution coefficient, organic calculate

The availability of reliable measurements or estimates of water solubility, octanol-water partition coefficient, bioconcentration factor, rate constants and the like allows one to make qualitative judgements or, through the use of mathematical simulation models such as EPA s EXAMS (19), quantitative calculations of environmental distribution and persistence. In the qualitative use, Swann and coworkers (20) classified chemical mobility in soil based upon reversed-phase HPLC retention data which in turn is related to S. The approximate water solubility equivalents in this first-estimate classification, with chemical examples, are in Table II. This classification holds for chemicals whose primary adsorption in soil is to organic matter, and excludes those chemicals (such as paraquat) which bind ionically to the soil mineral fraction. A recent tabulation of pesticides found in groundwater had 11 entries, 8 of which represented compounds with water solubilities in excess of 200 ppm with the remaining three falling in the range of 3.5 to 52 ppm (21). 

The value of the distribution coefficient, K, varies with several soil properties, most notably the organic carbon content. This has led to the definition of a normalized sorption coefficient, Koc, which is calculated as (12) ... 

Adsorption to soil organic carbon is measured as the organic carbon/water partition coefficient (Koc), calculated as the distribution among a soil and an aqueous phase, normalised to the organic carbon content of the soil [27]. 

The equilibrium between water and sediments can be described by the equation C = Kf.Cy where Cj = concentration of PCBs in sediments, CjY = concentration of PCBs in water and fCp = equilibrium constant that reflects the character of sediments. To calculate the Kf value, tabulated values of Kqq (soil adsorption coefficient) can be used and Kf values calculated using the equation Kf =Kfy,.TOC (total organic carbon). For a description of the behaviour (or distribution) of individual PCB congeners between water and sediments a linear relationship between K( and tabulated Kq values can be used logfr = 0.85 log Q y -F 0.13. The relationship is valid within the range of values of log Kq = 4.S-7.7. 

Tables 12.2 and 12.3. The effect of vertical variability is shown in Table 12.2, while the lateral spatial variability is shown in Table 12.3. The vertical and lateral spatial variabilities were defined on the basis of either the measured adsorption coefficient K), as generated from adsorption isotherms on soil profiles, or on adsorption coefficients on soil organic matter calculated as adsorption on organic carbon per unit weight of soil. We see that both vertical (Table 12.2) and lateral (Table 12.3) variability of soil affect the adsorption coefficients. A comparison between the bromide (conservative) and the two nonconservative herbicides distributions with depth after about 900mm of leaching is shown in Fig. 12.3. We see that, in the case of bromide, there is a continuous displacement of the center of mass with cumulative infiltration. In contrast, the bulk of the herbicide contaminant mass remains in the upper soil layer, with very little displacement.

The partition coefficient Kq of an organic compound in the 1-octanol/water system is used to assess the bioaccumulation potential and the distribution pattern of drugs and pollutants. The partition coefficient of imidazole and ILs strongly depends on the hydrogen bond formed by these molecules and is less than one due to the high solubility in water. The low value of the 1-octanol/water partition coefficient is required for new substances, solvents, insecticides to avoid bioaccumulation. Kqw is an extremely important quantity because it is the basis of correlations to calculate bioaccumulation, toxicity, and sorption to soils and sediments. Computing the activity of a chemical in human, fish, or animal lipid, which is where pollutants that are hydrophobic will appear, is a difficult task. Thus, it is simpler to measure the 1-octanol/water partition coefficient. This parameter is used as the primary parameter characterizing hydrophobisity. 

The exchange of chemical compounds from the gas phase to a surface, e.g. atmospheric particles, soil, water, vegetation or other surfaces, is controlled by the affinity of the compound to this surface. The ratio of vapour pressure to water solubility can be used as indicator between levels in the atmosphere and water surface (Henry s law H constant). In many model calculations, the ratio between POP levels in octanol and water, the octanol-water partitioning coefficient (Kow), is used as reference for the distribution of POP in organic material [14]. Consequently, the expression ///RT (Cair/Cwalcr) and Kow (Coctanoi/Cwater) provide the octanol-air partitioning coefficient (Koa) ... 

It may also be seen that if the various partition coefficients are known, then concentrations in all three compartments, C, C2, and C3 can be calculated and therefore the overall distribution of the chemical within the environment inferred. For partitioning, for example, into soil, it is assumed that the substance partitions solely into the organic carbon fraction of the soil solids, and since this is a relatively small percentage of the total soil, the mass fraction of organic matter has to be included in the calculation. 

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