Fipronil application to control ants and other pests around homes results in its release to surface waters via stormwater runoff. It frequently occurs at levels above levels associated with aquatic toxicity in California surface waters. This study describes a system to treat stormwater runoff using an activated carbon-based electrode within a semi-passive electrolysis system. The metal electrodes typically used for environmental applications are challenging because they do not feature significant sorptive capacity for contaminants, such that electrochemical degradation must be achieved over the very short timescale of water passage through the electrode. Activated carbon electrodes permit contaminant sorption, and thus allow degradation to occur over longer electrolysis timescales. The electrodes are constructed of crushed activated carbon bound to a carbon cloth current distributor. Fipronil in stormwater runoff passing over the electrode sorbs to the activated carbon. We demonstrate that application of -1 V vs. SHE to the electrode in direct current (DC) mode results in complete reductive dehalogenation of fipronil, releasing 6 molar equivalents of fluoride and two molar equivalents of chloride. Configuration of two activated carbon electrodes in a parallel plate configuration resulted in significant energy savings. However, oxidation of chloride at the anode generated free chlorine, and the anode began to shed carbon particles. We demonstrate that these problems could be overcome by converting the electrolysis system to the alternating current (AC) configuration wherein the voltage is cycled from -1 to +1 V vs. SHE. At high frequency, fipronil degradation was not achieved, but at low frequency, free chlorine formation became problematic. AC electrolysis also significantly reduced the energy consumption. At an optimized frequency, fipronil degradation was achieved without significant free chlorine generation. We demonstrate the ability to treat fipronil over many cycles within authentic surface water, indicating resistance to electrode fouling. The combination of AC electrolysis with activated carbon-based electrodes has not been explored, but features numerous advantages for environmental applications. For stormwater runoff, the sorption of contaminants to the carbon-based electrodes permits periodic regeneration by, for example, solar panels, suitable for some of the remote locations typical of non-point source contamination.

Capture and dehalogenation of fipronil using a semi-passive black carbon-based electrode

255th National Meeting (2018)

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