Electrochemical zone for degradation of chlorinated ethenes in aquifers

Bente Højlund Hyldegaard

Research output: Book/ReportPh.D. thesis

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Abstract

The potential of applying electrochemistry as a means for remediation of harmful chlorinated ethene plumes in groundwater aquifers was investigated. The aim of this PhD project was to mature the method towards field-application for protection of the drinking water resource. To simulate the electrochemical concept of a field-application, two undivided flow-through column reactors and one box reactor were designed. These reactors were used for assessment of the influence of various reactor design parameters and field-realistic parameters on the tetrachloroethylene (PCE) removal performance, and induced hydrogeochemical changes. The reactor design parameters assessed included a variety of electrode materials, catalyst loadings, electrode shapes, electrode configurations, constant current intensities, reactor orientations, reactor dimensions, porous matrices and flow rates. The assessment of influence of the reactor design and induced changes in hydrogeochemistry revealed:
-In complex field-realistic systems using electrode rods, the main transformation pathway responsible for removal of PCE was indirect reduction and indirect oxidation via electrochemically generated reactants. This is promising considering field-application where installation of a transect of electrode meshes is not viable.
-Aged contamination of PCE, with degradation products in groundwater, was treated concurrently in complex field-realistic systems when establishing a redox sequence of reduction followed by oxidation, which was also the sequence affecting the surrounding hydrogeochemistry the least.
-At lab scale, significant release of electrolytically generated gases from the electrode surfaces at high current intensity, short electrode spacing, low flow rate and presence of porous matrices obscured the actual performance variation related to these parameters. This challenge would most likely not be present in in situ electrochemical application.
-Advantages of using iron anodes were outperformed by significant corrosion and clogging of porous matrices when applied in field-realistic systems.
-Naturally present chloride and organic matter was oxidized in a multistep reaction, forming trichloromethane, which over time decreased in concentration.

The field-parameters incorporated into the assessment of chlorinated ethene removal performance in field-realistic systems were a realistic flow rate, contaminated groundwater, sandy sediment and groundwater aquifer temperatures. Major findings are:
-Electrochemical removal of an aged contamination of PCE was highest when applied in field-realistic systems, despite of competition by complex chemistry, contrary to simplified systems of synthetic groundwater and an inert porous matrix of glass beads, which represent state of the art for this PhD project.
-PCE concentrations reached from at starting concentration of 50 μg/l were 7.8±2.3 μg/l and the lower chlorinated ethenes were completely removed with no intermediates accumulating and insignificant volatilization.

These findings contribute with knowledge on electrochemical removal of PCE and the performance of electrochemical remediation in complex field-realistic aquifer settings. The low PCE concentrations reached after treatment and the complete removal of intermediates are encouraging in relation to upscaling the method.
Original languageEnglish
PublisherTechnical University of Denmark, Department of Civil Engineering
Number of pages194
ISBN (Electronic)87-7877-522-1
Publication statusPublished - 2019

Bibliographical note

Industrial PhD thesis R-424

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