Partition coefficients soil/water system

Chlorine dioxide is a very reactive compound and may exist in the environment for only short periods of time (see Section 6.3.2). Chlorine dioxide is readily soluble as a dissolved gas. However, chlorine dioxide can be easily driven out of aqueous solutions with a strong stream of air. The partition coefficient between water and C102(g) is about 21.5 at 35 °C and 70.0 at 0 °C (Aieta and Berg 1986 Kaczur and Cawlfield 1993 Stevens 1982). Transport and partition of chlorine dioxide in soils and sediments will not be significant. Chlorine dioxide is expected to be reduced to chlorite ions in aqueous systems (see Section 6.3.2.2). 

Measurements of Koc have been taken directly from partitioning experiments in sediment-and soil-water systems over a range of environmental conditions in both the laboratory and the field. Not surprisingly, the Koc values for many organic chemicals are highly correlated with their K0w values. Plots of the two partition coefficients for hundreds of chemicals with widely ranging K0w values yield slopes from about 0.3 to 1, depending on the classes of compounds and the particular methods included. Most fate modelers continue to use a slope of 0.41, which was reported by the first definitive... 

The organic carbon-water partition coefficient is a compound-specific term that allows the user to estimate the mobility of a solvent in saturated-soil water systems if the amount or organic carbon is known. For hydrophilic solvents, values have been measured for many compounds. Other values were derived from empirical relationships drawn between water solubility or octanol-water partition coefficients. ... 

In its simplest form a partitioning model evaluates the distribution of a chemical between environmental compartments based on the thermodynamics of the system. The chemical will interact with its environment and tend to reach an equilibrium state among compartments. Hamaker(l) first used such an approach in attempting to calculate the percent of a chemical in the soil air in an air, water, solids soil system. The relationships between compartments were chemical equilibrium constants between the water and soil (soil partition coefficient) and between the water and air (Henry s Law constant). This model, as is true with all models of this type, assumes that all compartments are well mixed, at equilibrium, and are homogeneous. At this level the rates of movement between compartments and degradation rates within compartments are not considered. 

Monkiedje et al. [10] investigated the fate of niclosamide in aquatic system both under laboratory and field conditions. The octanol/watcr partition coefficient (Kaw) of niclosamide was 5.880 x 10 4. Adsorption isotherm studies indicated that the Freundlich parameters (K, n) for niclosamide were 0.02 and 4.93, respectively, for powder activated carbon (PAC), and 9.85 x 10 5 and 2.81, respectively, for silt loam soil. The adsorption coefficient (Aoc) for the drug was 0.02 for PAC, and 4.34 x 10-3 for the same soil. Hydrolysis of niclosamide occurred in distilled water buffer at pH above 7. No photolysis of the drug was observed in water after exposure to long-wave UV light for 4 h. Similarly, neither chemically volatilized from water following 5 h of sample aeration. Under field conditions, niclosamide persisted in ponds for over 14 days. The half-life of niclosamide was 3.40 days. 

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. 

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. 

Sagar, P.M., Meagher, A., Sobczak, S. Wolff, B.G (1996) Chemical composition and potential hazards of electrocautery smoke. Br. J. Surg., 83, 1792 Sato, A. Nakajima, T. (1979) Partition coefficients of some aromatic hydrocarbons and ketones in water, blood and oil. Br. J. ind. Med., 36, 231-234 Sauer, PA. Tyler, E.J. (1995) Heavy metal and volatile organic chemical removal and treatment in on-site waste water infiltration systems. 1. Catch basirrs and septic tanks. Water Air Soil Pollut., 89, 221-232... 

For air-water systems, this equation is known as Henry s law. For solids-water systems, the equilibrium constant is known as the partition coefficient (Ky) or distribution constant (KA). Partition coefficients are available for many organic chemicals from laboratory and field measurements. As organic carbon (OC) present in water (dissolved organic carbon, or DOC), sediment, or soil is the main sink for hydrophobic organic contaminants, the partition coefficients for these compounds are often adjusted (normalized) with respect to the organic carbon content of these compartments ... 

Thin-layer radiochromatography (radio-TLC) is widely applied for a variety of environmental studies involving radiolabeled pesticides, such as plant uptake from soil, bioaccumulation in fish, dissipation from soil, metabolism in soil, plants, and fish, and environmental fate. The determination of the lipophiUdty of pesticides is important because their bioaccumulation and tendency for degradation and biotransformation are related to lipid solubility. TLC has advantages for lipohilicity studies compared to traditional partition coefficient measurement in an octanol-water system. 

Experimental values of Koc appear to be in excellent agreement with Koc estimated from experimental Kow value. Recalling that log Koc estimates from solubility relations were 6.39 - 5.64 and 6.24 -6.49, it is clear that our experimenatl values for the water solubility, octanol-water partition coefficient, and soil organic carbon-water partition coefficient are internally consistent. We attribute this to having taken steps to minimize losses to unknown parts of the equilibrating system, and to correct for losses by the analysis of dioxin in each of the phases involed in the partition system. 

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