Contaminants phase distribution equations

FIGURE 14.2 Phase distribution of organic contaminants in the vadose zone. The solid arrows in the three-and four-phase models represent the equilibria taken into consideration in the equations of Table 14.3. 

Basic Equations and Required Data for Calculating the Phase Distribution of Contaminants under Equilibrium Conditions... 

When the SVE technology is applied in a contaminated site, the NAPL is gradually removed. Towards the end of the remediation and when NAPL is no longer present, a three-phase model should be considered to calculate the phase distribution of contaminants (see Table 14.3). In this case, the vapor concentration in pore air (Ca) is calculating using the Henry s Law equation (Equation 14.5), which describes the equilibrium established between gas and aqueous phases ... 

The distribution of a contaminant among the four phases depends on (1) the physical and chemical properties of the compound and (2) the characteristics of the soil, and can be described by relatively simple equations (see Table 14.3). 

Step 2 Calculate the initial distribution oftoluene in the subsurface. The initial distribution of toluene can be calculated using equations 1 to 5 from Table 14.3 and taking into consideration that the organic phase is a pure compound, i.e., X = 1, m = 1, and y = 1. The total quantity of contaminant per unit soil Ct can be estimated from the known amount of spill Afspill and the volume of the contaminated zone ... 

The concentrations and the mass distribution of toluene in the four phases, as calculated from this set of equations, are presented in Table 14.4. As seen in the table, the major part of the toluene, i.e., 68.9%, remains in the vadose zone as free NAPL, 27.6% is adsorbed on the surfaces of solid particles, and only 3.5% is distributed between the aqueous and gas phases. Free NAPL occupies only a small part of the available pore volume, and it is not expected to disturb the movement of air through the contaminated zone. 

Each of the properties of the PCB isomers, listed above (Sect. 3.1.2) and either measured or calculated using various equations presented in Sect. 2.1, plays a role in the environmental distribution of these contaminants, especially at air-solid and water-solid interfaces. From the physical and chemical properties specific for PCBs and their isomers (Table 7, Figs. 2-8), the following information evaluates routes by which PCBs are lost from a particular source, spill or environmental compartment, that includes air-solid or aqueous-solid phase interfaces. These include vaporization (i.e., solid— air process), sorption/desorp-tion and partitioning (i.e., water <- solid processes) and biodegradation (i.e., water <- biosolid interactions). 

The first attempt to account for surface contamination in creeping flow of bubbles and drops was made by Frumkin and Levich (FI, L3) who assumed that the contaminant was soluble in the continuous phase and distributed over the interface. The form of the concentration distribution was controlled by one of three rate limiting steps (a) adsorption-desorption kinetics, (b) diffusion in the continuous phase, (c) surface diffusion in the interface. In all cases the terminal velocity was given by an equation identical to Eq. (3-20) where C, now called the retardation coefficient , is different for the three cases. The analysis has been extended by others (D6, D7, N2). 

The distributed reactivity model explanation for the biphasic rate behavior commonly observed for desorption of HOCs from soils is that the soft-carbon sorbed, or labile fraction of the contaminant desorbs readily and reversibly, whereas the hard-carbon sorbed, or resistant component is released much more slowly. The slow desorption step has been attributed to non-Fickian diffusion into a tightly-knit SOM, polymerization, or entrapment within the SOM matrix. The rate model found in comparative analyses to be the most appropriate for description of such behavior (17) is a two-phase release model which couples first-order rate equations for both the slow, resistant, and rapid, labile fractions, < >r (= 1- (j) ... 

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