Irreversible adsorption of hydrocarbons to natural and surrogate sediments
Hunter, Margaret Ann
Tomson, Mason B.
Doctor of Philosophy
The remediation of soils contaminated by hydrophobic organic chemicals is severely limited by the existence of an irreversibly bound fraction causing long-term low level release. Many mechanisms have been proposed but, the complexity of soils and sediments has made it difficult to determine specifically what causes the bound fraction. To reduce the heterogeneous nature of soils, a surrogate sediment was developed by coating non porous anatase with the surfactant sodium dodecyl benzene sulfonate. The system was verified by comparing the adsorption/desorption properties of the surrogate to those of a natural sediment. The partition coefficient for adsorption of PCB, and naphthalene to both sediments were similar and were in the range of literature reported values. The driving force for the adsorption process was determined to be hydrophobic in nature. Bimodal desorption with a labile reversible phase and a resistant phase has been observed to exist for both a natural and surrogate sediment. The desorption could not be explained by artifacts of the procedure or commonly used kinetic models. The characteristics of the bound fraction were determined by repetitively exposing the sediment to contaminant. The irreversible compartment has a finite and fixed size and appeared to fill in a stepwise manner which is in proportion to the solution phase concentration. The size of the irreversible compartment for naphthalene sorbed to Lula, PCB sorbed to Lula and PCB sorbed to surrogate were 10 $\mu$g/g, 0.41 $\mu$g/g and 0.36 $\mu$g/g, respectively. Desorption from the irreversible compartment reached an equilibrium concentration which was lower than predicted by hydrophobically driven equilibrium equations. The equilibrium concentration of desorption from the irreversible compartment for naphthalene from Lula, PCB from Lula and PCB from the surrogate were 2-5 $\mu$g/l, 0.2-0.6 $\mu$g/l and 0.5-0.8 $\mu$g/l, respectively. Field observations appeared to be consistent with the laboratory observations where the contaminants on soil and sediment resisted desorption and the irreversible compartment is equilibrated at sub ppb residual solution concentrations in the native water above the solids. Irreversible adsorption may have a significant impact on regulatory, modeling and remediation activities.
Environmental science; Organic chemistry; Environmental engineering