Surfactant Cleaning of PCB Impacted Soil Treatment

Surfactants

• PCBs are poorly soluble in water; they dissolve much better in organic solvents. Surfactants are molecules that lower surface tension. They have two parts -- a part that is water-soluble or hydrophilic and a part that has low water solubility or is hydrophobic. Surfactant molecules can cluster around PCB molecules with the hydrophobic part pointing inward and the hydrophilic part pointing outward; these PCB-containing clusters are much more soluble in water. This process is similar to the way surfactants help dissolve fats and oils.

Bioremediation

• Dissolving PCBs with water and surfactants can make them more readily available to bacteria that can break them down. Ordinarily, the rate at which the PCBs dissolve is limited by their poor solubility, which makes them less available to the bacteria. Surfactants help overcome this limitation. Unfortunately, some surfactants can also have adverse impacts on bacterial growth, and the high surfactant concentrations can slow biodegradation

Extraction

• Another approach is to wash the soil with the surfactants and water, thereby dissolving the PCBs, then extract the PCBs with a solvent such as supercritical CO2 -- carbon dioxide held at pressures and temperatures where it features both liquidlike properties and gaslike properties. The PCBs are more soluble in the supercritical CO2 than in the water, so they end up in the CO2 phase. The extracted PCBs can then be broken down through a chemical process, converting it to biphenyl and chloride.

Photocatalytic Degradation

• Another approach involves the use of surfactants and water to wash the PCB-contaminated soil (just as with the extraction method), followed by breaking the PCBs down with UV light and the aid of a catalyst such as titanium dioxide. The same UV light that provides the energy needed to break up the PCBs, however, can also break down some surfactants. The use of fluorinated surfactants (surfactants containing fluorine atoms) has been proposed as a possible approach that would circumvent this problem.