Adsorption soil coefficients

Adsorption-desorption coefficients are determined by various experimental techniques related to the status of a contaminant (solute or gas) under static or continuous conditions. Solute adsorption-desorption is determined mainly by batch or column equilibration procedures. A comprehensive description of various experimental techniques for determining the kinetics of soil chemical processes, including adsorption-desorption, may be found in the book by Sparks (1989) and in many papers (e.g., Nielsen and Biggar 1961 Bowman 1979 Boyd and King 1984 Peterson et al. 1988 Podoll et al. 1989 Abdul et al. 1990 Brusseau et al. 1990 Hermosin and Camejo 1992 Farrell and Reinhard 1994 Schrap et al. 1994 Petersen et al. 1995). 

Because organic nonpolar compounds have stronger attraction to organic matter than to mineral content, the amount of adsorption of an organic contaminant is more dependent on the organic content of the soil. The adsorption partition coefficient is generally used to determine this adsorption amount, as it is empirically related to the organic fraction of the soil (/ ,), and the normalized partition coefficient Koc can be expressed as follows ... 

The important assumptions for the pesticide model are instantaneous, linear, reversible adsorption described by an adsorption partition coefficient, K, and first-order decay described by an overall decay rate, k. Parameters for the pesticide model include universal soil loss equation parameters (if erosion loss is to be modeled), pesticide application information (rate, date, and method of application), K, k, and a dispersion coefficient. 

The purpose of the calibration exercise was to determine appropriate parameters which would result in a best-fit match between model simulations and the soil core field observations. The first step in the procedure was to assign values to field-measured, physically-based, parameters which include all parameters for the water balance and crop development portions of the model. The second step was to calibrate chemically and biologically based parameters which are not easily measured in the field. These include the adsorption partition coefficient, K, and the first-order rate of decay, k, of aldicarb. Using reasonable ranges of and k as defined by the literature, a trial-and-error method was used until model predictions matched field observations. 

The calibrated adsorption partition coefficient was 0.50 in the top 15-cm zone for North Carolina, and 1.00 in the top 15-cm zone in Wisconsin. These are higher than would be calculated based upon the koc aldicarb and the soil organic matter of these field sites, and higher than has been used in published aldicarb modeling exercises. However, the observed data of these and other aldicarb field studies indicate that aldicarb, in fact, does stay near the surface more than would be surmised based on a calculated partition coefficient. The reasons for this are unclear, but four possible explanations were offered 1) Top soil water percolation is overestimated due either to an underestimation of water holding capacity of the soil or an underestimation of surface runoff. 

DETERMINATION OF SOIL ADSORPTION PARTITION COEFFICIENTS OF C-LABELED CARCINOGENIC ORGANIC CHEMICALS BY LIQUID SCINTILLATION... 

Several models have been proposed for estimating volatilization of chemicals incorporated into soil, but not all of them are applicable to a given situation. Moreover, no simple model for the estimating the volatilization rate for chemicals distributed and incorporated into soil, is available. The application of these complex models requires a number of input data, e.g. adsorption isotherm coefficients, diffusion coefficients, environmental properties, which in most cases are not experimentally known nor can be accurately predicted. 

Settling and rainout are important mechanisms of contaminant transfer from the atmospheric media to both surface soils and surface waters. Rates of contaminant transfer caused by these mechanisms are difficult to assess qualitatively however, they increase with increasing soil adsorption coefficients, solubility (for particulate contaminants or those adsorbed to particles), particle size, and precipitation frequency. 

Adsorption Coefficient (K c)—The ratio of the amount of a chemical adsorbed per unit weight of organic carbon in the soil or sediment to the concentration of the chemical in solution at equilibrium. 

LymanWJ. 1990. Adsorption coefficient for soils and sediment. In Handbook of chemical property estimation methods. Environmental behavior of organic compounds. Lyman WJ, Reehl WE, Rosenblatt DH, eds. Washington, DC American Chemical Society. ... 

Soils and vadose zone information, including soil characteristics (type, holding capacity, temperature, biological activity, and engineering properties), soil chemical characteristics (solubility, ion specification, adsorption, leachability, cation exchange capacity, mineral partition coefficient, and chemical and sorptive properties), and vadose zone characteristics (permeability, variability, porosity, moisture content, chemical characteristics, and extent of contamination)... 

Dissolution of gasoline compounds to soil water is a function of each compound s solubility. A highly soluble gasoline substance often has a relatively low adsorption coefficient and also tends to be more readily degradable by microorganisms,19 as shown in Table 18.1. 

This equation is widely used to describe adsorption in soil and near-surface aquatic environments. Another widely used linear coefficient is the organic-carbon partition coefficient Koc, which is equal to the distribution coefficient divided by the percentage of organic carbon present in the system as proposed by Flamaker and Thompson.131... 

Winters and Lee134 describe a physically based model for adsorption kinetics for hydrophobic organic chemicals to and from suspended sediment and soil particles. The model requires determination of a single effective dififusivity parameter, which is predictable from compound solution diffusivity, the octanol-water partition coefficient, and the adsorbent organic content, density, and porosity. 

Lesser tendency to partition to organic matter in soil (soil adsorption coefficient). 

PESTAN (12) is a dynamic TDE soil solute (only) model, requiring the steady-state moisture behavior components as user input. The model is based on the analytic solution of equation (3), and is very easy to use, but has also a limited applicability, unless model coefficients (e.g., adsorption rate) can be well estimated from monitoring studies. Moisture module requirements can be obtained by any model of the literature. 

Model selection, application and validation are issues of major concern in mathematical soil and groundwater quality modeling. For the model selection, issues of importance are the features (physics, chemistry) of the model its temporal (steady state, dynamic) and spatial (e.g., compartmental approach resolution) the model input data requirements the mathematical techniques employed (finite difference, analytic) monitoring data availability and cost (professional time, computer time). For the model application, issues of importance are the availability of realistic input data (e.g., field hydraulic conductivity, adsorption coefficient) and the existence of monitoring data to verify model predictions. Some of these issues are briefly discussed below. 

Table I shows the results of calculating a soil diffusion coefficient and soil diffusion half-lives for the pesticides. The 10% moisture level specified means that the soil is relatively dry and that 40% of the soil volume is air available for diffusion. Complete calculations were not made for methoxychlor, lindane, and malathion because, based on Goring s criteria for the Henry s law constant, they are not volatile enough to diffuse significantly in the gas phase. This lack of volatility is reflected in their low values of X. These materials would move upward in the soil only if carried "by water that was moving upward to replace the water lost through evapotranspiration at the surface. Mirex has a very high Henry s law constant. On the basis of Goring s criteria, Mirex should diffuse in the soil air but, because of its strong adsorption, it has a very large a and consequently a very small soil air diffusion coefficient. The behavior of Mirex shows that Goring s criteria must be applied carefully.

The order of the mobilities of alachlor, butylate, and metolachlor in columns of various soils was metolachlor > alachlor > butylate. This correlates directly with the water solubilities and inversely to the adsorption coefficients and octanol/water partition coefficients of these compounds. Diffusion of these compounds in soil thin-layers was as follows butylate > alachlor > metolachlor, which correlates directly with the vapor pressures of these compounds. Significant soil properties affecting diffusion appeared to be bulk density and temperature. Soil moisture is also probably important, but its effect on the diffusion of these compounds was not determined. 

Freundlich Soil Adsorption Coefficients. Control experiments indicated that all of the compounds were stable in the stock solutions, in the adsorption solutions, and in the soil during these studies. A preliminary adsorption run conducted to determine the time required for equilibration of the herbicides between water and soil indicated that ca. 3 hours shaking was adequate. 

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). 

TABLE IV. Adsorption Coefficients for Butylate, Alachlor, and Metolachlor in Keeton Soil at Various Temperatures Obtained Using the Freundlich Equation. ... 

K = Freundlich adsorption coefficient from TABLE IV. ds = bulk density of soil from TABLE I. 

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