Matthew A. Petersen

Long-term potential of in situ chemical reduction for treatment of polychlorinated biphenyls in soils

Polychlorinated biphenyls (PCBs) are well-known for being hydrophobic and persistent in the environment. Although many treatment approaches have been demonstrated to result in degradation of PCBs in water or water/cosolvent systems, few examples exist where such approaches have been applied successfully for PCB degradation in soil-water systems. A possible explanation for the limited treatment of PCBs in soil-water systems is that reactants that are capable of degrading PCBs in the aqueous phase are unlikely to persist long enough to achieve meaningful treatment of slowly-desorbing PCBs associated with the soil phase. To investigate this explanation, laboratory studies were conducted to evaluate chemical reductants, including zero valent metals, palladium (Pd) catalyst, and emulsified zero valent iron (EZVI), for dechlorination of PCBs in the presence and absence of soil. In the absence of soil, Pd-catalyzed treatments (Pd with electrolytic ZVI or iron/aluminum alloy) achieved rapid destruction of a model PCB congener, 2-chlorobiphenyl, with half-lives ranging from 43 to 110 min. For treatment of soils containing Aroclor 1248 at an initial concentration of approximately 1,500 mg kg(-1), Pd-catalyzed treatments achieved no measurable enhancement over the background PCB depletion rate (i.e., that measured in the untreated control) of 5.3 mg kg(-1)week(-1). In the presence of soils, EZVI was the only approach evaluated that resulted in a clear enhancement in PCB dechlorination rates. EZVI achieved PCB concentration reductions of greater than 50% at an average rate of 19 mg kg(-1)week(-1). The results suggest that slow PCB desorption limits treatment effectiveness in soils.

ELECTROLYTIC REACTIVE BARRIERS FOR CHLORINATED SOLVENT REMEDIATION

The e-barrier is an emerging technology that applies fundamental electrochemical principles to a permeable reactive barrier (PRB). The e-barrier consists of closely spaced (e.g., 1 centimeter [cm]) permeable electrodes installed in a trench that intercepts a plume of contaminated groundwater (Figure 17.1). Low-voltage direct current (DC) sufficient to drive the degradation reactions of interest is applied to the electrodes. If sufficient electrical potential is applied, oxidizing conditions develop at the anode (positive electrode) and reducing conditions develop at the cathode (negative electrode). Since a complete electrical circuit is present, the dissolved contaminants are subject to sequential oxidation-reduction or reduction-oxidation, depending on the sequence of charges applied to the electrode set. This sequence can be altered depending on the contaminant of interest and the chemistry of the local groundwater. Through sequential oxidation-reduction (or reduction-oxidation), an aqueous phase chlorinated compound is degraded into thermodynamically favored carbon dioxide or methane and chloride.