Soil-water equilibrium data suggest that the transfer of nonionic chemicals from water to soil may be described in terms of a hypothesis of solute partitioning in the soil organic matter. This concept allows estimation of soil-water distribution coefficients either from solvent-water partition coefficients or aqueous solubilities.
H The movement of some chemicals in soils has been treated theoretically by considering for saturated conditions, the chemical diffusion coefficient, the percolation velocity of the water, the sorbtive properties of the soil, the average particle size of the soil, and the fractional number of sorbing sites. A model has been developed for the movement of chemical in saturated soil, based on Fick's law, conservation of energy, and a sorption isotherm. 'Theoretical curves for two different boundary conditions are given for realistic values of the water velocity in the pores and the measured diffusion coefficient.he fate of herbicides in the soil is currently a prob!em of T great interest. The main parameters involved in the process of herbicide movement in soils are: the soil moisture content, the percolation velocity of the water or chemical solution through the soil voids, the sorbtive properties of the soil, the particle size distribution, the ratio of the chemically active area of the soil particles to the total surface area of soil particles, and biological degradation.Much work has been done on the movement, uptake, and degradation of chemicals which have been applied to the soil (Ashton, 1961; Burneside, Feuster et ul., 1963; Freed, Vernette, et ul., 1962;Harris and Warren, 1962;Hartley, 1964; Lambert, Porter, et ul., 1965;Talbert and Fletchall, 1965). Most of this work has been of a qualitative nature, and though very useful by itself, it does not give the understanding of the processes involved which can be derived from the development and testing of a quantitative physical model. Some quantitative models have been postulated (Burneside, Feuster, et ul., 1963;Hayward and Trapnell, 1964), and most are based on the linear diffusion-type partial differential equation. This type of equation fails to take into account leaching or convection. Several detailed analyses of the movement of chemicals in porous media can be found in the literature. These include the early work on the adsorption of chemicals in chromatography and ion-exchange resins (Kipling, 1965 ;Lapidus and Amundson, 1952; Van Schaik, Kemper, et ul., 1966; Vietter and Sladek. 1965), diffusion in proteins and polymers (Chaoand Hodscher, 1966;Houghton, 1963; Ward and Ho11:; and mixing in chemical reactors (Bischoff, 1966; BischofT and Levenspiel, 1962a, 1962b). These studies have led to several mathematical models. One early model (Kasten, Lapidus el ( I / . . 1952; Lapidus and Amundson, 1952) which has proved very useful in chromatography theory is based on the diffusional plus convective-type partial differential equation :where C is the concentration of chemical in the voids, L' i x the velocity of the carrier flowing through the voids, D is the diffusion coefficient, y is the fractional void volume, and N is moles of solute adsorbed per unit volume of packed bed. Using the same equation, other models have been developed by Houghton (1963) and Chao and Hodscher (1966). However. these models are based on nonlinear adsorption, which mal he unnec...
Laboratory studies on the breakdown of several organophosphate pesticides both in aqueous solution and moist soil were conducted. The hydrolysis rates of phosmet, dialifor, malathion, methyl chlorpyrifos, dicapthon, chlorpyrifos, and parathion were measured at 20 and 37.5 °C (pH 7.4) in an aqueous system. A similar study was carried out at 20 °C and pH 6.1. The half-lives at 20 °C (pH 7.4) range from 7.1 h for phosmet to 130 days for parathion; the corresponding rates at 37.5 °C are approximately five-seven times greater than those at 20 °C. The rate equations at pH 7.4 were calculated from the 20 and 37.5 °C data in an Arrhenius form: k = A exp(-EajRT). In moist soil (pH 6.2), degradation rates were measured at pesticide concentrations of approximately 1.0 and 0.1 ppm in a Willamette clay loam soil at a moisture level of 50% of field capacity. A comparison of the 20 °C half-lives for phosmet and dialifor in water and in moist soil at comparable pH indicates an appreciable increase in persistence for these two compounds, but little for the others in the soil-water system. This study was intended to evaluate the stabilities of several agricultural pesticides.
The gel filtration method of Hummel and Dreyer (1962) has been used for the study of binding of herbicides by water‐soluble organic substances extracted from soil with distilled water. Evidence is offered for the binding of bromacil (5‐bromo‐3‐sec‐butyl‐6‐methyluracil), diuron [3‐(3,4‐dichlorophenyl)‐1,1,dimethylurea], chlorotoluron [3‐(3‐chloro‐4‐methylphenyl)‐1,1‐dimethylurea], simazine (2‐chloro‐4,6‐bis‐ethylamino‐s‐triazine), glyphosate [N‐(phosphonomethyl) glycine], and diquat (1,1‐ethylene‐2,2‐bipyridylium ion) by water soluble soil organic materials (WSSOM). Infrared and gel filtration data showed that the WSSOM consist of compounds with molecular masses in the range of 700 to 5000 daltons and resemble very closely the fulvic acids present in soil and surface waters. The amount of herbicide bound by WSSOM was determined from the elution diagram of the herbicide‐WSSOM complex. The results of this study point to the significance of WSSOM in relation to the fate and behavior of pesticides and pollutants in soil and water. The binding of these chemicals by WSSOM may figure important in the assessment of their mobility and transport in the environment.
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