Property-soil Water Partitioning Relationships

Temperature Dependence Values of K(K have usually been measured at temperatures between 20 and 25°C. Temperature caused changes in are expected to be similar to those of Kow. Werth and Reinhard [8] studied the influence of temperature on TCE sorption by natural sediments soils, and aquifer material. In agreement with theoretical considerations, they found small heat effects under conditions when the soil organic matter was assumed to be the dominant sorbent phase. 

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An example of using one predicted property to predict another is predicting the adsorption of chemicals in soil. This is usually done by first predicting an octanol water partition coelficient and then using an equation that relates this to soil adsorption. This type of property-property relationship is most reliable for monofunctional compounds. Structure-property relationships, and to a lesser extent group additivity methods, are more reliable for multifunctional compounds than this type of relationship. 

KoC is an important parameter which describes the potential for movement or mobility of pesticides in soil, sediment and groundwater. Because of the structural complexity of these agrochemical molecules, the above simple relationship which considers only the chemical s hydrophobicity may fail for polar and ionic compounds. The effects of pH, soil properties, mineral surfaces and other factors influencing sorption become important. Other quantities, KD (sorption partition coefficient to the whole soil on a dry weight basis) and KqM (organic matter-water partition coefficient) are also commonly used to describe the extent of sorption. K0M is often estimated as 0.56 KoC, implying that organic matter is 56% carbon. 

The octanol-water partition coefficient, Kow, is the most widely used descriptor of hydrophobicity in quantitative structure activity relationships (QSAR), which are used to describe sorption to organic matter, soil, and sediments [15], bioaccumulation [104], and toxicity [105 107J. Octanol is an amphiphilic bulk solvent with a molar volume of 0.12 dm3 mol when saturated with water. In the octanol-water system, octanol contains 2.3 mol dm 3 of water (one molecule of water per four molecules of octanol) and water is saturated with 4.5 x 10-3 mol dm 3 octanol. Octanol is more suitable than any other solvent system (for) mimicking biological membranes and organic matter properties, because it contains an aliphatic alkyl chain for pure van der Waals interactions plus the alcohol group, which can act as a hydrogen donor and acceptor. 

Pollutants with high VP tend to concentrate more in the vapor phase as compared to soil or water. Therefore, VP is a key physicochemical property essential for the assessment of chemical distribution in the environment. This property is also used in the design of various chemical engineering processes [49]. Additionally, VP can be used for the estimation of other important physicochemical properties. For example, one can calculate Henry s law constant, soil sorption coefficient, and partition coefficient from VP and aqueous solubility. We were therefore interested to model this important physicochemical property using quantitative structure-property relationships (QSPRs) based on calculated molecular descriptors [27]. 

Quantity/intensity relationships are often used to describe soil capacity to buffer phosphorus concentration in soil pore water. The quantity (0 refers to the amount of phosphorus adsorbed on soil surface, whereas intensity (/) refers to the concentration of P in soil pore water. This ratio can also be viewed as partition coefficient (K ), as indicated by liner sorption isotherms. The ratio expressed as either QII or is influenced by various physicochemical properties of soils, including clay content, high concentration of Fe and Al oxides, CaCOj content, organic matter content, pH, and redox potential. 

Since the passive root uptake of organic chemicals is driven by the amount of water transpired, the simple lipid-water equilibrium partitioning approach that is applied so successfiilly to animals and fish is less applicable to plants. Transpiration rates vary diumally and seasonally as plants respond to changes in climatic conditions (e.g., humidity, temperature, sunlight, and wind speed). The simplest and obvious response to these complexities has been to measure chemical concentrations in specific plant fluids or tissues at specific times or over specific intervals and relate them to external exposure concentrations in hydroponic solution, soil or soil pore water. The BCFs and RCFs described earlier are examples of these ratios and have been correlated to properties of the chemical such as Kow- An example of one of earliest and the most widely used plant BCE—chemical properties relationship was derived by Travis and Arms (1988) ... 

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