David M. Gilbert

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.

Direct Electrolytic Reduction of Energetic Compounds (hexahydro-1,3,5-trinitro-1,3,5-triazine and 2,4,6-trinitrotoluene) in Groundwater

Electrolytic reactive barriers (e-barriers) consist of closely spaced permeable electrodes installed across a groundwater contaminant plume in a permeable reactive barrier format. Application of sufficient potential to the electrodes results in sequential oxidation and reduction of the target contaminant. The objective of this study was to quantify the mass distribution of compounds produced during sequential electrolytic oxidation and reduction of ordinance related compounds (ORCs) in a laboratory analog to an e-barrier. In this study, a series of column tests were conducted using RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) and TNT (2,4,6-trinitrotoluene) as representative ORCs. The experimental setup consisted of a plexiglass column packed with quart-feldspar sand to simulate aquifer conditions. A single set of porous electrodes consisting of expanded titanium-mixed metal oxide mesh was placed at the midpoint of the soil column as a one-dimensional analog to an e-barrier. Constant current of 20 mA (variable voltage) was applied to the electrode set. Initial studies involved quantification of reaction products using unlabeled RDX and TNT. The results indicated approximately 70% of the influent concentration was transformed, in one pass, through sequential oxidation-reduction for both contaminants. Following the unlabeled studies, 14 C labeled ORCs were introduced to conduct the mass balance. An activity balance of up to 95.5% was achieved for both 14 C-RDX and 14 C-TNT. For both contaminants, approximately 21% of the influent activity was mineralized to 14 CO 2 . The proportion of the initial activity in the dissolved fraction was different for the two test contaminants. Approximately 30% of the initial 14 C-RDX was recovered as unreacted in the dissolved phase. The balance of the 14 C-RDX was recovered as non-volatile, non-nitroso transformation products. None of the 14 C-RDX was sorbed to the column sand packing. For 14 C-TNT approximately 51% of the initial activity was recovered in the dissolved phase, the majority of which was unreacted TNT. The balance of the 14 C-TNT was either sorbed to the sand packing (approximately 23.7 %) or dissolved/mineralized as unidentified ring cleavage products (4.5%).