Adsorption refers to the attachment of a chemical to the surface of a soil particle. In soil, this process is exhibited when a molecule of gas, free liquid, or contaminants dissolved in water is attached to the surface of an individual soil particle (often in the form of organic carbon). This surface attachment can be physical (very weak, caused by van der Waals forces), chemical (much stronger, which often requires significant effort to separate) and exchange, i.e. characterised by electrical attraction between the sorbate and the surface (exemplified by ion-exchange processes). Since sorption is primarily a surface phenomenon, its activity is a direct function of the surface area of the solid as well as the electrical forces active on that surface. When a pollutant is adsorbed onto soil, it can be released only when the equilibrium between it and the passing fluid (water or air) is disrupted [109, 115].
The process is complex, for which a delay factor is defined that slows down the transport velocity of the contaminant according to the following expression (1.9):
Vc=VwRd(1.9)
where Vc = contaminant velocity, [L/T]; Vw = water velocity, [L/T]; Rd = retardation factor.
The retardation factor describes the apparent discrepancy between the actual migration rate of aquifer water and that of a dissolved organic chemical (somewhat slower). The difference in travel rates is the result of sorption of the chemical onto the aquifer matrix and release into water by the concentration gradient and time of contact. A general equation used for gross estimation of the retardation factor Rd is (1.10):
Rd=1+ρnKd(1.10)
where Rd = retardation factor, ρ = bulk density of soil, [M/L3]; Kd = partitioning coefficient, [L3/M]; n = effective porosity.
The partitioning coefficient Kd can be calculated by (1.11):
Kd=Kocfoc(1.11)
where Koc [L3/M] is the organic carbon equilibrium coefficient and foc is the fraction of organic carbon.
1.5 Conclusion
Explosive contamination is frequently found in soil and groundwater at military training ranges. In order to prevent contamination, minimise remediation costs and ensure continued operation it is essential to understand the potential SPR linkages at a given site. This chapter has shown how the use of conceptual models based on the training range environment can be augmented by laboratory experiments and computational models to understand and predict explosive fate and transport to support identification of pollutant linkages.
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