Electrolytic reductive degradation of halogenated contaminants in air and water using activated carbon-based electrodes

A wide array of environmental contaminant classes are halogenated, including persistent organic pollutants, pesticides, industrial solvents, disinfection byproducts and agricultural fumigants. These contaminants occur in air, surface waters, stormwater runoff and drinking water. The treatment of halogenated contaminants is necessary to reduce or mitigate their toxicity and environmental impacts. Traditionally, granular activated carbon (GAC) has been used to capture halogenated contaminants, but disposal of the spent carbon can be expensive and inconvenient since the contaminant has not been destroyed. In this dissertation we introduce a novel GAC-based electrochemical treatment technique to capture and then dehalogenate halogenated contaminants. Halogenated contaminants are captured and sequestered from the gas or liquid phases onto GAC-based electrodes. Reductive dehalogenation is then achieved by applying a negative potential to the electrodes. The ability of the GAC-based electrodes to sequester contaminants is important. The kinetics of reductive dehalogenation of contaminants sorbed to the GAC can be significantly longer than the timescales of contaminant sorption to the GAC. Thus this system enables treatment of higher flowrates of gas of liquid waste streams. Although electrochemical treatment has several advantages including absence of chemical reagents and low sensitivity to pressure and temperature, the application of electrochemistry in environmental engineering is very rare for several reasons. First, most studies focus on oxidation processes that are not optimal for treatment of halogenated contaminants due to the electron-withdrawing nature of halogenated substituents. Second, most studies have used metal-based electrode materials that feature low sorption capacity and thus contaminant degradation must be achieved during the short hydraulic residence times relevant to waste stream treatment. Third, the dual-cell configuration used in traditional lab studies is not feasible to scale up due to its complicated structure. Fourth, oxidation of halides at the anode in single-cell configurations leads to the production of significant levels of halogenated byproducts, including chlorate and perchlorate. This dissertation demonstrates the combination of activated carbon sorption and reductive electrolysis to dehalogenate contaminants containing C-F, C-Cl, C-Br, and C-I bonds in a variety of environmental contexts. These contexts include disinfection byproducts in the final effluents of facilities practicing the potable reuse of municipal wastewater, gas phase fumigants emitted from post-harvest fumigation chambers used to kill insects on produce prior to export, and pesticides in urban runoff. This dissertation also developed several improvements to the electrolytic system configuration that are critical for scale-up. These improvements include the development of an activated carbon-based electrode featuring faster electrochemical reduction kinetics due to higher electrical conductivity and smaller activated carbon particle size. This dissertation also demonstrated that a single-cell, parallel plate configuration of the electrodes could significantly reduce the energy demand while simplifying the reactor configuration. Compared to the conventional use of direct current electrolysis, this dissertation employed alternating current electrolysis to further reduce the energy demand, while reducing the oxidation of halides and associated production of halogenated byproducts, a significant challenge for environmental electrolysis. Chapter 1 introduces background information on halogenated contaminants, the challenges facing application of current electrochemical techniques in environmental engineering and the objectives of this dissertation. Chapter 2 demonstrates the feasibility of converting GAC adsorbent to a cathode using as an example the treatment of 22 halogenated disinfection byproducts (DBPs) in the reverse osmosis permeate within advanced treatment trains for the potable reuse of municipal wastewater. In this study, GAC particles were transferred into a sheet graphite cylinder after adsorption of halogenated DBPs as the working electrode (cathode). A subset of halogenated DBPs (chloropicrin) underwent reductive dehalogenation upon sorption to the GAC, even without electrolysis. Experiments demonstrated that this reactivity of GAC be promoted by proper electrolytic treatment of GAC particles. If a constant voltage of -1.0 V vs. Standard Hydrogen Electrode (S.H.E.) was applied, significant degradation of all 22 DBPs (including compounds with C-Cl, C-Br and C-I bonds) was achieved by such reductive electrolysis in batch systems. The lowest removal efficiency over 6 h electrolysis was for trichloromethane (chloroform; 47%), but removal efficiencies were > 90% for 13 of the 22 DBPs, and the degradation was verified by the production of halides as reductive products, indicating a substantial removal of toxicity. Continuous GAC treatment of the reverse osmosis effluent in an advanced treatment train for potable reuse effectively reduced the concentrations of chloroform, bromodichloromethane and dichloroacetonitrile measured in the column influent to below the method detection limits. Treatment of the GAC by reductive electrolysis at -1 V vs. S.H.E. over 12 h resulted in significant degradation of the chloroform (63%), bromodichloromethane (96%) and dichloroacetonitrile (99%) sorbed to the GAC. The results suggested this strategy is capable of capturing a variety of halogenated contaminants (halogenated DBPs in this case) from a flowing waste stream and then degrade the sorbed contaminants over longer timescales by reductive dehalogenation. Chapter 3 discusses the development of activated carbon-based electrodes to improve their conductivity and reactivity. This improvement was achieved by coating a thin layer of fine GAC particles (1 -- 10 µm) onto carbon cloth as the current distributor. Compared with the electrode consisting of loose GAC particles wrapped in sheet graphite as the current distributor as described in Chapter 2, this new electrode significantly lowered the total cell resistance by combining porous and fine GAC particles with highly conductive carbon cloth. In this chapter we applied this novel electrode to the treatment of methyl bromide (CH3Br) exhaust from post-harvest fumigation chambers. CH3Br is a gaseous fumigant that could cause ozone depletion, and its extensive usage in fumigation chambers requires its capture and degradation over the ~24-hour fumigation cycles typical of these fumigation chambers. With the carbon cloth-based electrode, 96% reductive debromination of CH3Br sorbed at 30% by weight to the GAC was achieved within 15 h at −1.0 V applied potential vs. S.H.E., a time-scale and efficiency suitable for postharvest fumigations. The cathode exhibited stable performance over 50 CH3Br capture and destruction cycles. This chapter also evaluated the cost of this treatment system, which is estimated as ∼

Advisor: Mitch William A. 
Criddle Craig 
Luthy Richard G.