Handbook of Solvents - George Wypych - ChemTech - Ventech!

1160 William R. Roy
Figure 17.1.3. The ranges in atmospheric half-life of the solvents in Table 17.1.1 (data from Howard et al. 14 and
ATSDR 21 ).
g/kg, the organic C fraction is not a valid predictor of the partitioning of nonpolar organic
compounds, 27 and other properties such as pH, surface area, or surface chemistry contribute
to or dominate the extent of adsorption. Moreover, pyridine occurs at a cation (pKa = 5.25)
over a wide pH range, and thus it is adsorbed by electrostatic interactions rather than by the
hydrophobic mechanisms that are endemic to using Koc values to predict mobility.
The desorption of solvents from soil has not been extensively measured. In the application
of advection-dispersion models to predict solute movement, it is generally assumed
that adsorption is reversible. However, the adsorption of the solutes in Table 17.1.1 may not
be reversible. For example, hysteresis is often observed in pesticide adsorption-desorption
studies with soils. 28 The measurement and interpretation of desorption data for solid-liquid
systems is not well understood. 29,30 Once adsorbed, some adsorbates may react further to become
covalently and irreversibly bound, while others may become physically trapped in the
soil matrix. 28 The non-singularity of adsorption-desorption may sometimes result from experimental
artifacts. 28,31
17.1.6.3 Degradation
As discussed in 17.1.3.3., Howard et al. 14 also estimated soil half-life values (Figures 17.1.1
and 17.1.2) for the degradation of most of the solvents in Table 17.1.1. Biodegradation was
cited as the most rapid process available to degrade solvents in a biologically active soil.
The numerical values obtained were often the same as those estimated for surface water.