Chemical-soil interaction equation

Transport. The mechanisms responsible for transport are considered to be both physical (convection or mass flow) and chemical (diffusion). When considered simultaneously, these processes have been summarized in the convective-dispersive, or miscible displacement, equation. For a non-interacting solute (such as chloride) under steady state water flow conditions in a homogeneous soil, this equation can be written as (10) ... 

Sorption is most commonly quantified using distribution coefficients (Kd), which simplistically model the sorption process as a partitioning of the chemical between homogeneous solid and solution phases. Sorption is also commonly quantified using sorption isotherms, which allow variation in sorption intensity with triazine concentration in solution. Sorption isotherms are generally modeled using the empirical Freundlich equation, S = K CUn, in which S is the sorbed concentration after equilibration, C is the solution concentration after equilibration, and Kt and 1 In are empirical constants. Kd and K are used to compare sorption of different chemicals on one soil or sorbent, or of one chemical on several sorbents. Kd and K are also commonly used in solute leaching models to predict triazine interactions with soils under various environmental conditions. 

Equation 10.3 describes the velocity of a chemical substance moving through soil relative to the velocity of water moving through soil. Any difference between these two velocities is due to the potential of soil to interact with the chemical substance physically and/or chemically. Answer the following questions using Equation 10.3. 

The analytical solution to Equation 2 for a range of boundary conditions is a model of pesticide fate that has been used under a variety of laboratory situations to study the basic principles of soil-water-pesticide interaction. It is in fact limited to such laboratory cases, as steady state water flow is an assumption used in deriving the equation. As a modeling approach it is useful in those research studies in which careful control of water and solute fluxes can be used to study degradation and adsorption. For example, Zhong et al. (11) present a study of aldicarb in which the adsorption and degradation of aldicarb, aldicarb sulfone and aldicarb-sulfoxide were simultaneously determined from laboratory soil column effluent data. The solution to a set of equations of the form of Equation 2 was used. A number of similar studies for other chemicals could be cited that have provided useful basic information on pesticide behavior in soil (4,12,13). Yet, these equations are not useful in the field unless re-formulated to describe transient water and solute fluxes rather than steady ones. Early models of pesticide fate based upon Equation 2 (14) were constrained by such assumptions, but were... 

In previous chapters, the main modes of interaction between ions and soil mineral colloids have been discussed in Chapters 4 and 5, the principles of ion-surface interactions have been laid down, and in Chapters 8 and 9, the main features of adsorption onto silicate and oxide minerals have been reviewed. As introduced in Chapter 11, two main contributions should be considered, namely, the nonelectrostatic forces composed of physical (van der Waals forces) and/or chemical (specific bonding) interactions and the electrostatic forces arising from the charged nature of both the adsorbate and the surface this is reflected in two contributions (considered independent) to the Gibbs free energy, as in Equation 11.5 ... 

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