Capture and Reductive Transformation of Halogenated Pesticides by an Activated Carbon-Based Electrolysis System for Treatment of Runoff

This study evaluates an electrochemical system to treat the halogenated pesticides, fipronil, permethrin, and bifenthrin, in urban runoff. Compared to the poor sorption capacity of metal-based electrodes, granular activated carbon (GAC)-based electrodes could sorb halogenated pesticides, permitting electrochemical degradation to occur over longer timescales than reactor hydraulic residence times. In a dual-cell configuration, a cathode constructed of loose GAC containing sorbed pesticides was separated from the anode by an ion-exchange membrane to prevent chloride transport and oxidation to chlorine at the anode. When -1 V was applied to the cathode, fipronil concentrations declined by 92% over 15 h, releasing molar equivalents of chloride (2) and fluoride (6), suggesting complete dehalogenation of fipronil. An electrode constructed of crushed GAC particles attached to a carbon cloth current distributor achieved >90% degradation of fipronil, permethrin, and bifenthrin within 2 h under the same conditions. To evaluate a simpler single-cell configuration suitable for scale-up, two of the carbon cloth-based electrodes were placed in parallel without an ion-exchange membrane. For -1 V applied to the cathode, fipronil degradation was >95% over 2 h, and energy consumption declined with closer electrode spacing. However, chloride oxidation at the anode produced chlorine, and the anode degraded. Application of an alternating potential (-1 to +1 V at 0.0125 Hz) to the parallel-plate electrodes achieved >90% degradation of fipronil, bifenthrin, and permethrin over 4 h, releasing chloride at 50-70% of that expected for complete dechlorination. No loss of performance or formation of chlorine or halogenated byproducts was observed over 5 cycles of treating fipronil-spiked surface water.