Impact of electrode sequence on electrochemical removal of trichloroethylene from aqueous solution

Abstract

The electrode sequence in a mixed flow-through electrochemical cell is evaluated to improve the hydrodechlorination (HDC) of trichloroethylene (TCE) in aqueous solutions. In a mixed (undivided) electrochemical cell, oxygen generated at the anode competes with the transformation of target contaminants at the cathode. In this study, we evaluate the effect of placing the anode downstream from the cathode and using multiple electrodes to promote TCE reduction. Experiments with a cathode followed by an anode (C→A) and an anode followed by a cathode (A→C) were conducted using mixed metal oxide (MMO) and iron as electrode materials. The TCE removal rates when the anode is placed downstream of the cathode (C→A) were 54% by MMO→MMO, 64% by MMO→Fe and 87% by Fe→MMO sequence. Removal rates when the anode is placed upstream of the cathode (A→C) were 38% by MMO→MMO, 58% by Fe→MMO and 69% by MMO→Fe sequence. Placing the anode downstream of the cathode positively improves (by 26%) the degradation of aqueous TCE in a mixed flow-through cell as it minimizes the influence of oxygen generated at the MMO anode on TCE reduction at the cathode. Furthermore, placing the MMO anode downstream of the cathode neutralizes pH and redox potential of the treated solution. Higher flow velocity under the C→A setup increases TCE mass flux reduction rate. Using multiple cathodes and an iron foam cathode up stream of the anode increase the removal rate by 1.6 and 2.4 times, respectively. More than 99% of TCE was removed in the presence of Pd catalyst on carbon and as an iron foam coating. Enhanced reaction rates found in this study imply that a mixed flow-through electrochemical cell with multiple cathodes up stream of an anode is an effective method to promote the reduction of TCE in groundwater.

Electrochemical degradation of trichloroethylene in aqueous solution by bipolar graphite electrodes

“…Reduction via hydrodechlorination (HDC) is favorable degradation path for TCE from groundwater [23, 25, 26, 32–34]. HDC occurs via a reaction of chlorinate substance with atomic hydrogen that forms at the cathodes and is the main reduction mechanism at hydrogen formation potentials.…”

“…The sequential oxidation and reduction processes can be utilized for the transformation of COCs and lead to the creation of less toxic by-products [27, 34, 38]. Authors previously reported that shifts between oxidation and reduction, via electrode polarity reversal, improve TCE removal [39].…”

“…Authors previously reported that shifts between oxidation and reduction, via electrode polarity reversal, improve TCE removal [39]. However, due to the highly oxidized nature of TCE, inducing HDC as a primary mechanism is valuable and can be supported by placing the cathode upstream of the anode [40]. …”

The influence of cathode material on electrochemical degradation of trichloroethylene in aqueous solution

“…A new system was designed to promote electrochemical reduction, which is the primary degradation mechanism of highly oxidized chemical compounds such as TCE. The design, introduced by our laboratory, adopts a flow-through cell with the cathode placed in front of the active anode with respect to the flow direction, to minimize the effect of oxygen on HDC of TCE (Rajic et al, 2014, 2015a, 2015b, 2015c). …”

Modeling of bubbles hydrodynamics and mass transfer in electrochemical gas-evolving systems

“…Further, the ohmic potential is affected directly by the gap distance between the electrodes; the drop of ohmic potential is proportional to the gap distance between electrodes [156,157]. Thus, an optimized design between the cell cross section (gap distance between electrodes) and bulk flow rate velocity should be considered not only to enhance the bubble release from the electrode surface, but also the avoid having a significant voltage drop in the system.…”