Environmental Engineering Reference
In-Depth Information
of soils/sediments (Costerton et al., 1987). Therefore, the majority of con-
taminant-microbial interactions occur in the biofilm that develops within
macropores on the surfaces of soils/sediments.
This suggests that partitioning of an organic contaminant from the solid
phase of the soil/sediment to the aqueous phase in the larger pore spaces
controls soil/sediment bioavailability. These partitioning mechanisms may
include chemical bonding, surface complex formations, electrostatic interac-
tions, and hydrophobic effects (Schwarzenbach et al., 1993; Stumm, 1992).
For hydrophobic contaminants such as recalcitrant compounds, sorption
increases with the content of the organic matter in the soil/sediment and
the degree of hydrophobicity of the specific PAH. Typically, the rate of
desorption can be attributed to the mass transfer of the sorbate molecules
from sorption sites on and in the soil/sediment. Active bacteria should
correspond to the higher available PAH concentrations, which occurs where
desorption is the most intense.
1.4
The sequestration of recalcitrant compounds
As already discussed, the partitioning of recalcitrant compounds represents
a significant part of the mass transfer process. However, describing this
process is complicated by a combination of diffusion, dissolution, encapsu-
lation, and adsorption phenomena. The combined effect of these factors can
generally be defined as sequestration (Luthy et al., 1997). The complexities
of sequestration can best be understood in terms of examining the different
scales of observation associated with the heterogeneity of soils and sedi-
ments. Shown in Figure 1.1 are mineral surfaces in meso- and micropores
(Luthy et al., 1997), both amorphous and condensed natural sorbent organic
matter (SOM), and anthropogenic organic phase matter. The circled letters
refer to different sorption mechanisms associated with specific domains
within soils and sediments:
Case A represents absorption into amorphous or soft natural organic
matter (e.g., within an organic phase akin to solvent partitioning).
Case B represents absorption into condensed or hard organic polymeric
matter or combustion residue (e.g., into solid-like organic matter).
Case C represents adsorption onto water-wet organic surfaces (e.g., soot).
Case D represents adsorption to exposed water-wet mineral surfaces
(e.g., quartz).
Case E represents adsorption into microvoids or microporous minerals
(e.g., zeolites) with porous surfaces at a water saturation of <100
(Luthy et al., 1997).
These different domains within soils and sediments illustrate how struc-
tural and chemical heterogeneity can significantly affect how recalcitrant
compounds behave. Excluding entrapment due to weathering at interfaces
(e.g., nonaqueous phase liquid (NAPL)/water), case A presumably would
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