Soil/water distribution ratio

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,... 

Soil Distribution Ratios, Ka, Kom> The sorption of nonpolar organic compounds on soil usually approximates a linear relation as illustrated in Figure 3.5 The slope of these lines defines a soil-water distribution ratio, K, which is expressed... 

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.)... 

Figure 9.7 Observed increase in solid-water distribution ratios for the apolar compounds, tetrachlo-romethane (0) and 1,2-dichIoro-benzene (A) with increasing organic matter content of the solids (measured as organic carbon, /oc, see Eq. 9-21) for 32 soils and 36 sediments. Data from Kile et al. (1995).

Soil solution to soil ratios also strongly affect distribution of some trace elements such as Zn speciation in arid and semi-arid soils. Fotovat et al. (1997) reported that the proportion of free hydrated Zn2+ to total Zn ranged from 20-65% at field capacity soil water content and decreased with increases in solution to soil ratios, while the proportion of Zn complexed with organic ligands increased dramatically in soils. However, solution to soil ratios do not strongly affect the distribution of Cu speciation in soil solution since Cu primarily occurs as organic complexes in these soil solutions. 

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 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) ... 

The term partitioning constant or coefficient refers to one chemical species in each phase (for example, ionisable chemicals present in water may exist in both neutral and dissociated forms and therefore each would have a separate partitioning coefficient). For well-defined phases, such as pure water or the pure liquid state of the chemical, then partitioning with another equally well-defined phase such as air results in the use of the term partitioning constant. The Henry s Law constant describing chemical partitioning between pure air and water is one example. Distribution ratios on the other hand, such as the soil-water... 

Soil pH measurement is usually affected by a number of factors viz. salt concentration, soil-water ratios, suspension effect etc. The use of O.Ol(M) CaClg solution yields stable readings in pH-measurements. The hydrogen ions in the soil system are distributed between solid and liquid phases as follows. The dynamic equilibrium may be represented as follows. 

Soil distribution coefficient (solid/water concentration ratio)... 

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). 

Environmental fate and pathways photodegradation, stability in water, stability in soil, monitoring data (environment), transport between environmental compartments, distribution, mode of degradation in actual use, biodegradation, BOD5, COD or BOD5/COD ratio, bioaccumulation. 

A number of quick-test techniques have been used widely to estimate the extent of sorption of pesticides to soils, and these estimates are often used in pesticide transport models. The most commonly used technique is to determine the ratio of distribution of a chemical, often at one concentration, between the solution and soil solid phases (K-) or simply the distribution between water and octanol phases CK ). The use of K, as an index of adsorption assumes that the stribution ratio is constant over a range of concentrations of the chemical in the soil. In other words, the amount of chemical adsorbed increases linearly with that remaining in the solution. The linear relationship may be valid over a narrow... 

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