Transformation of tetrachloroethylene in a flow-through electrochemical reactor

Electrochemical transformation of harmful tetrachloroethylene (PCE) is evaluated as a method for management of groundwater plumes to protect the drinking water resource, its consumers and the environment. In contrast to previous work that reported transformation of trichloroethylene, a byproduct of PCE, this work focuses on transformation of PCE in a saturated porous matrix and the influence of design parameters on the removal performance. Design parameters investigated were electrode configuration, catalyst load, electrode spacing, current intensity, orientation of reactor and flow through a porous matrix. A removal of 86% was reached in the fully liquid-filled, horizontally oriented reactor at a current of 120 mA across a cathode → bipolar electrode → anode arrangement with a Darcy velocity of 0.03 cm/min (150 m/yr). The palladium load on the cathode significantly influenced the removal. Enhanced removal was observed with increased electrode spacing. Presence of an inert porous matrix improved PCE removal by 9%-point compared to a completely liquid-filled reactor. Normalization of the data indicated, that a higher charge transfer per contaminant mass is required for removal of low PCE concentrations. No chlorinated intermediates were formed. The results suggest, that PCE can be electrochemically transformed in reactor designs replicating that of a potential field-implementation. Further work is required to better understand the reduction and oxidation processes established and the parameters influencing such. This knowledge is essential for optimization towards testing in complex conditions and variations of contaminated sites.

Electrochemical transformation of an aged tetrachloroethylene contamination in realistic aquifer settings

Electrochemical removal of chlorinated ethenes in groundwater plumes may potentially overcome some of the challenges faced by current remediation technologies. So far, studies have been conducted in simplified settings of synthetic groundwater and inert porous matrices. This study is a stepwise investigation of the influence of field-extracted groundwater, sandy sediment and groundwater aquifer temperatures on the removal of an aged partially degraded contamination of tetrachloroethylene (PCE) at a typical groundwater flow rate. The aim is to assess the potential for applying electrochemistry at contaminated sites. At a constant current of 120 mA, pH and conductivity were unaffected downgradient the electrochemical zone. Major groundwater species were reduced and oxidized. Some minerals deposited, others dissolved. Hydrogen peroxide, a strong oxidant, was formed in levels up to 5 mg L^-1 with a limited distribution into the sandy sediment. Trichloromethane was formed, supposedly by oxidation of organic matter in the sandy sediment in the presence of chloride. The more realistic the settings, the higher the PCE removal, bringing concentrations down to 7.8 ± 2.3 μg L^-1. A complete removal of trichloroethylene and cis-1,2-dichloroethylene was obtained. The results suggest that competing reactions related to the natural complex hydrogeochemistry are insignificant in terms of affecting the electrochemical degradation of PCE and chlorinated intermediates.

Electrokinetics applied in remediation of subsurface soil contaminated with chlorinated ethenes - A review

Electrokinetics is being applied in combination with common insituremediation technologies, e.g. permeable reactive barriers, bioremediation and in-situ chemical oxidation, to overcome experienced limitations in remediation of chlorinated ethenes in low-permeable subsurface soils. The purpose of this review is to evaluate state-of-theart for identification of major knowledge gaps to obtain robust and successful field-implementations. Some of the major knowledge gaps include the behavior and influence of induced transient changes in soil systems, transport velocities of chlorinated ethenes, and significance of site-specific parameters on transport velocities, e.g. heterogeneous soils and hydrogeochemistry. Furthermore, the various ways of reporting voltage distribution and transport rates complicate the comparison of transport velocities across studies. It was found, that for the combined EK-techniques, it is important to control the pH and redox changes caused by electrolysis for steady transport, uniform distribution of the electric field etc. Specifically for electrokinetically enhanced bioremediation, delivery of lactate and biodegrading bacteria is of the same order of magnitude. This review shows that enhancement of remediation technologies can be achieved by electrokinetics, but major knowledge gaps must be examined to mature EK as robust methods for successful remediation of chlorinated ethene contaminated sites.

Challenges in electrochemical remediation of chlorinated solvents in natural groundwater aquifer settings

Establishment of electrochemical zones for remediation of dissolved chlorinated solvents in natural settings was studied. An undivided 1D-experimental column set-up was designed for the assessment of the influence of site-extracted contaminated groundwater flowing through a sandy aquifer material, on the execution of laboratory testing. A three-electrode system composed of palladium coated pure iron cathodes and a cast iron anode was operated at 12 mA under varying flow rates. The natural settings added complexity through a diverse groundwater chemistry and resistance in the sand. In addition, significant precipitation of iron released through anode corrosion was observed. Nevertheless, the complex system was successfully modelled with a simple geochemical model using PHREEQC. A ranking of the significances of system parameters on the laboratory execution of electrochemical remediation in natural settings was proposed: Geological properties > anode corrosion > site-extracted contaminated groundwater > the carbonate system > sulphate > hydrology > less significant unidentified parameters. This study provides insight in actual challenges that need to be overcome for in situ electrochemical remediation.