David B. Gent

Electrokinetic remediation of wood preservative contaminated soil containing copper, chromium, and arsenic.

As a result of wood treatment, and the recent banning of the copper, chromium, and arsenic (CCA) treated wood for residential use many CCA treatment facilities have been abandoned or being closed. Soil contamination resulting from CCA is common at these sites. In this study, the feasibility of electrokinetic technique to remove CCA from contaminated soil was investigated. To better understand the ionic mobility within the soil and to detect the generation and advancement of acid front, sampling ports were provided along the longitudinal axis of a test cell. To determine the effect of varying current, three tests were performed at different current densities of 5.9, 2.9, and 1.5mA/cm(2) for a period of 15 days. The initial concentrations of copper, chromium, and arsenic in the soil were 4800, 3100, and 5200mg/kg, respectively. Dilute nitric acid was used as an amendment to neutralize the hydroxyl ions produced at the cathode. Experiments resulted in removal efficiencies as high as 65% for copper, 72% for chromium, and 77% for arsenic. The results also indicated that the advancement of acid front favored desorption of metals from the soil and the metals were mobilized either as free cations or metal complexes. Chromium that was in its +6 valence state was transported as anion prior to its reduction. However, once the chromium was reduced to chromium(III) its transport direction reversed with transport being favored towards the cathode.

Electrokinetic-enhanced bioaugmentation for remediation of chlorinated solvents contaminated clay

Successful bioremediation of contaminated soils is controlled by the ability to deliver bioremediation additives, such as bacteria and/or nutrients, to the contaminated zone. Because hydraulic advection is not practical for delivery in clays, electrokinetic (EK) injection is an alternative for efficient and uniform delivery of bioremediation additive into low-permeability soil and heterogeneous deposits. EK-enhanced bioaugmentation for remediation of clays contaminated with chlorinated solvents is evaluated. Dehalococcoides (Dhc) bacterial strain and lactate ions are uniformly injected in contaminated clay and complete dechlorination of chlorinated ethene is observed in laboratory experiments. The injected bacteria can survive, grow, and promote effective dechlorination under EK conditions and after EK application. The distribution of Dhc within the clay suggests that electrokinetic transport of Dhc is primarily driven by electroosmosis. In addition to biodegradation due to bioaugmentation of Dhc, an EK-driven transport of chlorinated ethenes is observed in the clay, which accelerates cleanup of chlorinated ethenes from the anode side. Compared with conventional advection-based delivery, EK injection is significantly more effective for establishing microbial reductive dechlorination capacity in low-permeability soils.

Environmental Electrokinetics for a sustainable subsurface

Soil and groundwater are key components in the sustainable management of the subsurface environment. Source contamination is one of its main threats and is commonly addressed using established remediation techniques such as in-situ chemical oxidation (ISCO), in-situ chemical reduction (ISCR; most notably using zero-valent iron [ZVI]), enhanced in-situ bioremediation (EISB), phytoremediation, soil-washing, pump-and-treat, soil vapour extraction (SVE), thermal treatment, and excavation and disposal. Decades of field applications have shown that these techniques can successfully treat or control contaminants in higher permeability subsurface materials such as sands, but achieve only limited success at sites where low permeability soils, such as silts and clays, prevail. Electrokinetics (EK), a soil remediation technique mostly recognized in in-situ treatment of low permeability soils, has, for the last decade, been combined with more conventional techniques and can significantly enhance the performance of several of these remediation technologies, including ISCO, ISCR, EISB and phytoremediation. Herein, we discuss the use of emerging EK techniques in tandem with conventional remediation techniques, to achieve improved remediation performance. Furthermore, we highlight new EK applications that may come to play a role in the sustainable treatment of the contaminated subsurface.

Electrolytic Alkaline Decomposition of a Munition Constituent (RDX) Contaminated Groundwater

Combined use of electrolysis and alkaline hydrolysis is explored for in-situ decomposition of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in laboratory experiments with simulated groundwater. Alkaline medium generated by water electrolysis at the cathode under direct electric currents will develop a permeable alkaline barrier for in-situ decomposition of RDX. Pseudo-first order transformation rate coefficients were developed from alkaline hydrolysis and electrolytic batch experiments for RDX decomposition with time. The results provided a target pH of 12 and a current density of 1 mA/cm 2 for treatment of RDX under groundwater flow column experiment. The results from the one-dimensional sand-filled alkaline hydrolysis columns (5-cm ID) were used to develop reaction rate coefficients used in designing a large scale-up one-dimensional column to test the electrolytic generation of hydroxide (10-cm ID). The rate coefficient from the 5-cm alkaline columns (0.46 per hr) was used to calculate a column length (160 cm) for complete removal of RDX and its nitroso-substituted products under groundwater flow rate of 30 cm/day. Effluent RDX concentrations from the 10-cm scale-up column (4,000 µg/L influent) were less than 0.1 µg/L for 36 days of treatment. The study concludes that cathodes placed at the down-gradient of groundwater RDX plume can perform as an effective permeable alkaline hydrolysis barrier for decomposition of RDX to levels below EPA drinking water advisory limits.

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%).