Capture and Reductive Transformation of Halogenated Pesticides by

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Capture and Reductive Transformation of Halogenated Pesticides by an Activated Carbon-Based Electrolysis System for Treatment of Runoff Yuanqing Li, and William A. Mitch Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05259 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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Environmental Science & Technology

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Capture and Reductive Transformation of Halogenated Pesticides by

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an Activated Carbon-Based Electrolysis System for Treatment of

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Runoff

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Yuanqing Li1 and William A. Mitch1, *

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Department of Civil and Environmental Engineering, Stanford University, 473 Via Ortega, Stanford, California 94305, United States

*Corresponding author: email: [email protected], Phone: 650-725-9298, Fax: 650-723-7058

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ACS Paragon Plus Environment


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Abstract

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This study evaluates an electrochemical system to treat the halogenated pesticides,

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fipronil, permethrin and bifenthrin, in urban runoff. Compared to the poor sorption

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capacity of metal-based electrodes, granular activated carbon (GAC)-based electrodes

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could sorb halogenated pesticides, permitting electrochemical degradation to occur

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over longer timescales than reactor hydraulic residence times. In a dual-cell

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configuration, a cathode constructed of loose GAC containing sorbed pesticides was

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separated from the anode by an ion exchange membrane to prevent chloride transport

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and oxidation to chlorine at the anode. When -1 V was applied to the cathode, fipronil

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concentrations declined by 92% over 15 h, releasing molar equivalents of chloride (2)

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and fluoride (6) suggesting complete dehalogenation of fipronil. An electrode

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constructed of crushed GAC particles attached to a carbon cloth current distributor

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achieved >90% degradation of fipronil, permethrin and bifenthrin within 2 h under the

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same conditions. To evaluate a simpler single-cell configuration suitable for scale-up,

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two of the carbon cloth-based electrodes were placed in parallel without an ion

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exchange membrane. For -1 V applied to the cathode, fipronil degradation was >95%

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over 2 h, and energy consumption declined with closer electrode spacing. However,

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chloride oxidation at the anode produced chlorine, and the anode degraded. Application

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of an alternating potential (-1 to +1 V at 0.0125 Hz) to the parallel-plate electrodes

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achieved >90% degradation of fipronil, bifenthrin and permethrin over 4 h, releasing

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chloride at 50-70% of that expected for complete dechlorination. No loss of

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performance or formation of chlorine or halogenated byproducts was observed over 5 2


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cycles of treating fipronil-spiked surface water.

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Introduction

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The halogenated phenylpyrazole pesticide, fipronil, and the pyrethroid

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insecticides, bifenthrin and permethrin, are widely used to control termites, ants,

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cockroaches, and wasps around houses, and to repel ticks on pets.1 Their use has

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raised concerns about potential adverse impacts on non-target insects, such as

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honeybees,2 and on organisms in surface waters impacted by urban runoff.3 For

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example, although the activity of fipronil is somewhat selective for insects over

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vertebrates,4,5 fipronil exhibits toxicity to many fish, birds and mammals.6-11 Previous

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research has demonstrated the biotransformation of fipronil to fipronil sulfide under

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reducing conditions or fipronil sulfone under oxidative conditions.12-15 However, these

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metabolites exhibit toxicity similar to fipronil.5,10,11,16

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Fipronil and its degradation products, fipronil sulfone and fipronyl desulfinyl,

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were detected with frequencies of 49%, 43% and 33%, respectively, in a California

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statewide survey of >500 urban surface water samples.17 For 46% of the samples,

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fipronil concentrations exceeded the 11 ng/L associated with chronic toxicity to aquatic

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life. Fipronil loadings frequently were associated with stormwater runoff. However,

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at one site fipronil loadings were higher during the dry season, suggesting mobilization

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of fipronil applied at homes by household irrigation systems17. Similarly, the toxicity

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measured in samples of residential runoff and two urban creeks in California to aquatic

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amphipods was attributed to pyrethroids, including bifenthrin.3

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Capturing and degrading these halogenated pesticides in stormwater runoff before

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they reach surface waters could permit their continued use while avoiding these adverse 4


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impacts. Treatment systems for this type of non-point source contamination should be

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cheap, and require minimal maintenance. Activated carbon could be used to capture

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pesticides from stormwater, but the carbon would need to be periodically replaced to

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prevent contaminant breakthrough. To eliminate the cost of activated carbon

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replacement and disposal, it would be desirable to treat the activated carbon in situ to

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destroy the sorbed pesticides. Previously, we demonstrated the capture on granular

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activated carbon (GAC) of halogenated disinfection byproducts (DBPs) from reverse

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osmosis permeate associated with the advanced treatment for the potable reuse of

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municipal wastewater.18 The GAC was periodically converted into a cathode within

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an electrochemical cell to achieve reductive electrolytic dehalogenation of the sorbed

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DBPs, achieving >90% dehalogenation of 13 DBPs after 6 hours of treatment at -1 V

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vs. Standard Hydrogen Electrode (S.H.E.). The GAC was reused over multiple cycles

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of DBP capture and regeneration without loss of degradation efficiency. Similarly, we

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demonstrated the ability of the GAC-based electrolysis system to capture gas phase

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methyl bromide effluents relevant to post-harvest fumigation scenarios19,20. The GAC

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bed was regenerated by submerging it in water and applying a negative potential to the

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carbon to achieve ~80% reductive debromination of methyl bromide over 30 hours at

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-1 V vs. S.H.E.

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For these initial applications, the loose GAC particles in the cathode were wrapped

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with sheet graphite to distribute the current. Recognizing that resistance between the

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loose GAC particles within the bed might limit the efficiency of reductive

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dehalogenation, we developed an improved electrode consisting of finely-ground GAC 5



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particles coated onto carbon cloth as a current distributor.

The 3.3-fold lower

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resistance of this cathode increased the efficiency of methyl bromide debromination to

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~80% over 2 hours at -1 V vs. S.H.E20.

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A long-term goal is to develop an electrolysis system to capture and degrade

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pesticides in urban runoff using GAC-based electrodes. Preparatory to developing a

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pilot-scale system, the objective of this work was to characterize a GAC-based

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electrolysis system capable of achieving reductive dehalogenation of sorbed

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halogenated pesticides without generating undesirable halogen-based oxidants as

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byproducts. The first objective was to demonstrate the degradation of fipronil,

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bifenthrin and permethrin using a cathode constructed of either loose GAC particles

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wrapped in sheet graphite or GAC particles bound to carbon cloth within a two-cell

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electrochemical configuration (Figure S1B). This configuration encompassed the

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GAC-based cathodes and a platinum wire anode in different cells separated by a cation

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exchange membrane to prevent the passage of halides released from the cathode to the

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anode, where oxidation could generate undesirable oxidants such as chlorine. This

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configuration was used to demonstrate the quantitative yield of halides via reductive

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dehalogenation of the 2 chlorine and 6 fluorine functional groups in fipronil. Halogens

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are common functional groups in pesticides and are frequently associated with toxicity

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across a range of environmental contaminants (e.g., DBPs, perfluorinated compounds).

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The second objective of the current study was to evaluate modifications to simplify the

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electrode configuration and render it suitable for pilot-scale systems.

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GAC/carbon cloth electrodes were placed in a close, parallel-plate configuration within 6


Two

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a single cell (Figure S1C) to reduce the full-cell voltage and avoid the cost of the

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platinum anode. The system was operated in a direct current (DC) configuration

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wherein electrochemical reductive dehalogenation was targeted on the cathode by

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application of a constant potential. The third objective was to assess the use of

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alternating current (AC) electrolysis, where the polarity of the electrodes is periodically

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reversed, within the parallel-plate electrode configuration (Figure S1C).

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environmental applications of electrolysis systems have focused on DC systems21,

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wherein only one electrode could participate in reductive dehalogenation. In addition

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to permitting both GAC-based electrodes to participate in reductive dehalogenation, we

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demonstrate that AC electrolysis may avoid the need for the cation exchange

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membrane if the frequency is sufficient to avoid transport and oxidation of halides

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between the electrodes.

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Materials and Methods

Most

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Electrode preparation: GAC particles were ground and sieved to 40-60 mesh size,

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and then rinsed three times with deionized water and oven-dried overnight before

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further use. In this study, two different GAC-based working electrodes were

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constructed. For preparation of a working electrode consisting of loose GAC particles

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wrapped in sheet graphite (GAC/graphite electrodes), 0.8 g of Norit GAC was stirred

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with 12.5 mg/L fipronil in 24 mL deionized water. While this fipronil concentration

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exceeded its solubility, >99% of fipronil partitioned to the GAC within 20 min, as

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verified by measurement of the aqueous phase. The GAC was filtered and split into

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eight equal portions (0.1 g dry carbon equivalent). Two of the portions were extracted 7



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with 4 mL acetonitrile (80% extraction efficiency) to validate the sorbed fipronil

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concentration (375 µg/g or 0.86 µmol/g). Another two of the portions were transferred

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into sheet graphite tubes (~ 0.5 cm × 1 cm) and submerged in 40 mL of deionized water

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buffered at pH 7 with 0.1 M phosphate buffer to serve as controls. The remaining four

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portions of GAC (containing 0.34 µmol fipronil) were transferred into a similar sheet

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graphite tube (~ 0.5 cm × 2 cm) and submerged in 150 mL phosphate buffer at pH 7 to

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serve as the working electrode for electrolysis.

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The GAC/carbon cloth-based electrodes were fabricated using the method

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described by Xie et al.22 Briefly, Norit GAC particles were ground into smaller particles

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via ball milling (planetary ball mill, Nanjing T-Bota Scietech Instruments & Equipment

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Co., Ltd., Nanjing, China, 400 rpm for 10 h). An ink was made by mixing fine GAC

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particles (92% dry weight) and PVDF (8% dry weight) in NMP solvent in a weight

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ratio of 1:5 for GAC/PVDF to NMP and stirring overnight. A piece of carbon cloth (6

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cm × 2 cm × 0.38 mm) was used as the current distributor and coated with the GAC ink

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(Figure S1A). A titanium wire was attached to the carbon cloth to convey current to the

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electrode. The electrodes were dried in a vacuum overnight, resulting in a total GAC

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mass loading of ∼0.2 g/electrode (17 mg/cm2). The electrodes were pre-treated by

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electrolysis at -1.0 V vs S.H.E. for 15 h (see below for electrolysis details) to remove

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surface-associated chloride. The electrode was oven-dried for 4 h at 100°C and then

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stored under vacuum.

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Dual cell DC electrolysis: Electrolysis within a conventional dual cell laboratory

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configuration was conducted similarly to our previous studies (Figure S1B).20,22 The

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system consisted of two 150 mL chambers separated by a cation exchange membrane

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(Ultrex CMI-7000, Membranes International, Ringwood, NJ). For the GAC/carbon 8


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cloth electrode, fipronil was sorbed to the electrode by submerging in deionized water

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containing 1 mg/L (23 µM or 3.5 µmol) fipronil and 1-100 mM phosphate buffer (pH 7)

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or 1 mM chloride within the cathodic chamber; this pH and the lower range of salt

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concentrations approximate those in the freshwaters characteristic of stormwater (see

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Lake Lagunita water characteristics below). After stirring for 2 h, 91% of the fipronil

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(3.2 µmol) was sorbed to the electrode. In a separate experiment, a mixture of 150 µg

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each of fipronil, bifenthrin and permethrin were sorbed from a stirred 100 mM

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phosphate buffer (pH 7) to the GAC/carbon cloth electrode. A Ag/AgCl reference

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electrode (CHI111, porous Teflon tip, CH Instruments, Austin, TX, USA) was placed

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in the cathode chamber within 1 cm of either the GAC/carbon cloth cathode or the

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GAC/graphite cathode (pre-loaded with fipronil), and a platinum wire (CH

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Instruments) was placed in the anode chamber as the counter electrode. A CH-600D

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potentiostat (CH Instruments, Austin, TX) applied constant potentials ranging from

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-0.2 to -1.0 V vs S.H.E. to the cathode via a titanium wire. The cathode solution was

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stirred with a Teflon-coated magnetic stir bar.

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Single cell DC or AC electrolysis: Two planar GAC/carbon cloth electrodes (5 cm

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× 10 cm) were fashioned as described above. For DC electrolysis, one electrode was

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spiked directly by distributing 50 µL of a 10 g/L fipronil stock in acetonitrile (500 µg)

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over the electrode; this electrode served as the cathode. The second electrode served

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as the anode. The cathode and anode were held in plastic frames in a parallel

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configuration using 4 ceramic insulating spacers to maintain specific separation

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distances (1.5-3.0 cm; Figure S1C). The electrodes were placed in 500 mL of 0.1 M

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phosphate-buffered deionized water at pH 7 (corresponding to 1 mg/L fipronil if

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dissolved in the water) within a plastic box. Solutions were stirred with a

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Teflon-coated magnetic stir bar. Controls without the GAC/carbon cloth electrode 9



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indicated no significant sorption of fipronil to the plastic box or frames. Electrolysis

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was conducted by applying a constant voltage of -1 V vs. S.H.E. to the cathode. The

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current delivered to the cathode were measured by the potentiostat.

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voltage was determined by measuring the potential difference between the cathode

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and anode using a multimeter.

The full-cell

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For AC electrolysis, each electrode was spiked with 500 µg of either fipronil,

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bifenthrin or permethrin and the electrodes were assembled in a parallel-plate

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configuration separated by 2.2 cm (Figure S1C). Alternating current with a triangular

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waveform was applied using the cyclic voltammetry scan function of the potentiostat.

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The voltage was scanned between -1 V and 1 V vs. S.H.E. at scan rates ranging from

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0.01-0.1 V/s (i.e., 0.0025-0.025 Hz). To investigate chlorine evolution, similar

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experiments were conducted in the presence of 1 mM chloride; aliquots withdrawn

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periodically from the aqueous phase were measured for total chlorine residual by the

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DPD colorimetric method.23

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For an initial assessment of the impact of natural water constituents on the

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long-term performance of AC electrolysis, 5 cycles of electrolysis were performed

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using a filtered sample collected from Lake Lagunita (pH 7.0, 12.7 mg/L dissolved

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organic carbon, 11.4 mg/L chloride and 65 µg/L bromide), a seasonal lake on the

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Stanford University campus. The single cell AC electrolysis apparatus was loaded

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with Lake Lagunita water (500 mL) spiked with 1 mg fipronil and stirred for 4 h

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without electrolysis, permitting competition for sorption between fipronil and other

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constituents (e.g., organic matter). AC electrolysis was then conducted (0.05 V/s or

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0.0125 Hz) for 4 h.

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fipronil and potential byproducts (e.g., fipronil sulfide) over the 8 h cycle. The

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electrodes were then extracted for analysis of residual fipronil and byproducts. In a

Aqueous samples were periodically withdrawn for analysis of

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separate experiment, this process was repeated, but after the 4 h electrolysis, the

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electrodes were not extracted, and the solution was immediately replaced with a fresh,

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fipronil-spiked aliquot of Lake Lagunita water to commence another cycle of sorption

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and electrolysis.

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residual fipronil and byproducts.

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1 and 5 cycles of sorption/electrolysis to detect performance degradation resulting

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from exposure to natural water constituents.

After 5 such cycles, the electrodes were extracted to measure The electrode performance was compared between

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Results and Discussion

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Text S1 provides materials sources and analytical methods.

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Dual-cell DC electrolysis: Initial experiments compared the performance of the

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GAC/graphite and GAC/carbon cloth cathodes using 100 mM phosphate buffer at pH 7

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within a conventional dual cell laboratory electrolysis configuration, in which the

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cathodes were separated from a platinum wire anode by a cation exchange membrane

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(Figure S1B). For treatment of fipronil with the GAC/graphite electrode, fipronil was

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not measureable in the aqueous phase of the cathode cell throughout the experiment,

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reflecting its strong sorption to the GAC. After application of a constant -1 V vs.

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S.H.E. potential to the cathode for 15 h, the sorbed fipronil declined by 92% from 0.34

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µmol to 0.03 µmol. Fipronil sulfide, a reduction product of fipronil, was not detected,

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but 0.71 µmol chloride (206% molar yield) and 1.97 µmol fluoride (572% molar yield)

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were measured in the aqueous phase, indicating nearly complete dehalogenation of the

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2 chlorine and 6 fluorine substituents in fipronil (Figure 1A). Fipronil loss and

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formation of fipronil sulfide, fluoride and chloride were not observed for the 11



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GAC/graphite cathode control to which no potential was applied.

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Upon application of a constant -1 V vs. S.H.E. potential to the GAC/carbon cloth

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cathode at pH 7, the 3.2 µmol of fipronil initially sorbed to the cathode declined 78%

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within 1 h, 95% within 2 h and 99% within 15 h (Figure 1B). Low levels of the

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reduction product, fipronil sulfide (which retains the 2 chlorine and 6 fluorine

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substituents), were measured on the cathode, reaching 14% yield after 2 h, but declined

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to permethrin > bifenthrin (Figure 1D). At

266

-0.2 V, 90%. The reductive dechlorination of permethrin and bifenthrin are

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discussed later in the context of AC electrolysis experiments. All further experiments

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were conducted at -1 V vs. S.H.E. to ensure efficient pesticide degradation.

270

Single-cell DC electrolysis: It was hypothesized that a single-cell, parallel-plate

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configuration of the GAC/carbon cloth electrodes would increase the degradation

272

efficiency by decreasing the resistance; the rationale for this hypothesis is discussed

273

below. This configuration may also be simpler for pilot-scale treatment of a water

274

stream. Two planar GAC/carbon cloth electrodes were configured in parallel, and 13



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separated by 1.5, 2.2 or 3.0 cm without an intervening cation exchange membrane

276

(Figure S1C); the electrode serving as the cathode had previously been spiked with 500

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µg fipronil. When -1 V vs. S.H.E. was applied to the cathode at pH 7 (100 mM

278

phosphate buffer), fipronil loss after 2 h was >95% and comparable for all three

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electrode spacings (i.e., 97%, 95% and 95% removal for the 1.5, 2.2 and 3.0 cm

280

spacings, respectively). The release of chloride was somewhat higher for the closest

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spacing of the electrodes, but in all cases reached at least 50% of the molar yield

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expected for complete dechlorination within 2 h (Figure 2A). No significant chloride

283

release was detected in the control to which no potential was applied.

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However, there were two significant problems with the single-cell DC

285

configuration. First, GAC particles were observed to shed from the anode, likely

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reflecting the instability of the PVDF binding agent when high potentials were applied

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to the anode; towards the beginning of the electrolysis, the anode potential often

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exceeded +2 V vs. S.H.E. To restore the activity of the electrode, it was re-coated with

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the carbon-based ink after every 5 experiments. Second, when the reaction was

290

conducted using 1 mM NaCl, a concentration relevant to freshwater (35 mg/L),

291

production of free chlorine was observed (Figure 3) resulting from chloride oxidation at

292

the anode (equation 1); based on the full-cell voltage (Figure 2B), the anode potential

293

after ~20 min was ~1-2V vs. S.H.E. depending on the electrode spacing. Although

294

similar, free chlorine formation was somewhat faster during the initial stages of the

295

electrolysis when the 1 mM NaCl was buffered with 100 mM phosphate (Figure S2)

296

than when unbuffered (Figure 3). In the dual-cell configuration, the cation exchange 14


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membrane avoids free chlorine production at the anode by preventing the transport of

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chloride from the water sample to the anode. While the absence of the cation exchange

299

membrane simplifies the single-cell configuration, release of free chlorine produced at

300

the anode into the effluent water would present a risk to organisms in receiving waters

301

and would generate undesirable byproducts of free chlorine reactions with organic

302

matter.

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Cl- + H2O → HOCl + 2 e- + H+

[1]

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Single-cell AC electrolysis: The performance of AC electrolysis was evaluated in the

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parallel-plate (2.2 cm spacing) single-cell configuration (Figure S1C) with each of the

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GAC/carbon cloth electrodes spiked with 500 µg (1.15 µmol) fipronil (i.e., 1 mg total).

307

The potential applied to one electrode (as measured using an adjacent reference

308

electrode) was varied using a triangular waveform from -1 to +1 V vs. S.H.E. at scan

309

rates of 0.01, 0.05, and 0.1 V/s (i.e., frequencies of 0.0025-0.025 Hz). The residual

310

fipronil on the electrodes measured after 4 h of electrolysis indicated 98%, 96% and 93%

311

removal for frequencies of 0.0025-0.025 Hz, respectively. The electrolysis treatment

312

time was twice as long as for the single-cell DC experiments, in recognition of the fact

313

that the electrodes were experiencing a negative potential only half of the time.

314

For all three frequencies, ~60% dechlorination (i.e., chloride liberation relative to

315

the total chlorine in fipronil) was achieved after 4 h (Figure 4), comparable to the

316

dechlorination observed after 2 h of DC electrolysis (Figure 2A). However, the chloride

317

yield rate was slower during the first 90 min for the highest frequency (0.025 Hz). One

15



318

potential explanation for the slower release of chloride at higher frequency is that the

319

shorter duration during which the electrodes were held at negative potentials within

320

each cycle hindered the reduction reaction.

321

When 1 mg of either bifenthrin or permethrin was treated by AC electrolysis (-1 V

322

to 1 V vs. S.H.E. at 0.025 Hz), 91% of bifenthrin and 92% of permethrin were degraded

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after 4 h of treatment, similar to the percent of fipronil removed; no significant decay of

324

bifenthrin or permethrin were observed in the absence of applied voltage. The percent

325

dechlorination was even higher than for fipronil, reaching ~70% for both compounds

326

after 4 h (Figure S3). These results suggest that AC electrolysis could be used as a

327

non-selective technique for the treatment of halogenated pesticides in water.

328

Free chlorine formation from 1 mM chloride solutions was near 4 µM (0.3 mg/L as

329

Cl2) for all cases except the lowest frequency (0.0025 Hz) in the presence of 100 mM

330

phosphate buffer (Figures 3 and S2); chlorate and perchlorate were never observed.

331

These results suggest that the lowest frequency permits a sufficient duration at a

332

positive potential for chloride oxidation to occur. When the AC electrolysis was

333

conducted with 1 mM chloride, but without 100 mM phosphate buffer, a scenario

334

relevant to freshwater ionic strength, significant free chlorine accumulation was not

335

observed (Figure 3). The significant reduction in free chlorine production without the

336

use of an ion exchange membrane is a major advantage to the use of AC electrolysis

337

over DC electrolysis. However, the intermediate frequency (0.0125 Hz) may represent

338

a compromise between high dechlorination rates for fipronil and minimizing free

339

chlorine production. 16


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Another advantage for AC electrolysis is that no significant electrode

341

decomposition was observed during AC electrolysis, perhaps due to the limited

342

duration during which the electrodes were subjected to anodic potentials during each

343

cycle. To further explore the stability of electrode performance, 500 mL of an authentic

344

surface water (Lake Lagunita at Stanford University) spiked with 1 mg (2.3 µmol)

345

fipronil was treated by AC electrolysis. First, the solution was stirred for 4 h and the

346

aqueous concentration of fipronil was measured to monitor sorption of fipronil to the

347

electrodes in competition with other solution components (e.g., 12.7 mg/L DOC). The

348

aqueous fipronil concentration declined by 80% after 4 h (Figures 5 and S4). Then AC

349

electrolysis (2.2 cm electrode spacing and 0.0125 Hz with the applied voltage ranging

350

from -1 to +1 V) was conducted for 4 h, after which the aqueous fipronil concentration

351

had declined to 1% of its initial value (Figure S4). Due to the background chloride

352

concentration in the water sample, we were not able to measure the chloride yield from

353

the electrolytic degradation of fipronil. The residual fipronil extracted from the two

354

electrodes accounted for 3.0 ± 0.5 % and 3.0 ± 0.3% of the initial fipronil mass,

355

indicating a total removal of 92.9% ± 0.8 % of the initial fipronil mass during the 8 h

356

sorption and AC electrolysis cycle.

357

This treatment was then repeated over 5 cycles of sorption and AC electrolysis

358

treatment of spiked 500 mL water samples, but the electrodes were extracted to

359

measure residual fipronil only after the final treatment cycle (Figure 5). The fipronil

360

removal profile was similar across all five cycles, with only 2% of the initial fipronil

361

mass measured in the aqueous phase after AC electrolysis during the fifth cycle. The 17



362

residual fipronil extracted from the two electrodes after the fifth cycle represented 5.0 ±

363

0.4 % and 5.0 ± 1.1 % of the initial fipronil mass added during the fifth cycle.

364

Comparing the residual aqueous fipronil concentrations measured after each cycle and

365

the residual fipronil extracted from the electrodes after the fifth cycle to the total

366

fipronil mass added over all 5 cycles indicates a total removal rate over 5 cycles of 95.9

367

± 0.5%. These results suggest that the performance of the electrode did not decline over

368

time despite the potential for blockage of sorption sites by other matrix constituents,

369

including NOM. The slight increase in residual fipronil sorbed to the electrodes over

370

several cycles likely could be degraded by prolonging the electrolysis duration. The

371

only halogenated DBP measured in the aqueous phase after AC electrolysis treatment

372

of Lake Lagunita water was 0.2 µg/L of 1,1,1-trichloropropanone, indicating that AC

373

electrolysis can mitigate the production of halogenated byproducts.

374

To further test the importance of NOM blockage of GAC pore sites, the two

375

GAC/carbon cloth electrodes were exposed to 100 mg/L of Aldrich humic acid for 72 h.

376

The electrodes were then used to treat 500 mL Lake Lagunita water spiked with 1 mg

377

fipronil over one cycle of AC electrolysis as described above. After the 4 h of sorption

378

and 4 h of AC electrolysis, the overall decay of fipronil retrieved from both the aqueous

379

phase (Figure S5) and the GAC/carbon cloth electrodes was 98%, comparable to that

380

observed without prior exposure to humic acid.

381

Comparison of energy consumption: For the single-cell DC electrolysis configuration,

382

the cathodic current declined rapidly at first and then leveled out (Figure 2C). The

383

initial rapid decline was likely due to the initial electrode polarization driven by 18


Page 18 of 34

Page 19 of 34


384

reduction of functional groups on the carbon surface. This behavior was observed with

385

or without a target contaminant (e.g., fipronil), and has been observed in previous

386

studies of carbon-based electrodes.24 The current profiles and the currents after the first

387

1 h of treatment were fairly similar for the three electrode spacings because the current

388

delivered to the cathode is controlled by the constant applied potential (-1 V vs. S.H.E.)

389

and the prevalence of reducible groups on the cathode, which were similar for the three

390

electrode spacings. However, the decline in current was somewhat slower for the

391

closest spacing of electrodes, concurring with the somewhat greater yield of chloride

392

within the first ~15 min. The higher current likely reflects the decrease in solution

393

resistance with closer spacing of the electrodes, as discussed below.

394

The full-cell voltage also declined over time, but more slowly for the widest

395

spacing of the electrodes (Figure 2B). The full-cell voltage (Vcell) is provided by

396

equation 2.25

397

Vୡୣ୪୪ = V ଴ + ηୟ + ηୡ + iR

[2]

398

where V0 (V vs. S.H.E.) is the equilibrium potential for the reduction reaction, and ηୟ

399

and ηୡ are the overpotentials on the anode and cathode, respectively. The

400

overpotentials are determined by the nature of the electrode and electrolytes. In these

401

experiments, V0, ηa and ηc were the same for the three different electrode spacings. The

402

last term iR represents the solution resistance, where i is the current (A) and R is the

403

ohmic resistance (Ω) of the solution. The magnitude of ohmic resistance largely

404

depends on the electrolyte concentration and the geometry of the electrolysis cell

19



405

406

(equation 3): ௟

R = ρ஺

[3]

407

where l is the distance between electrodes (cm), A is the planar area of one of the

408

parallel electrodes (cm2), and ρ is the ionic resistivity (Ω cm) of the electrolytes, which

409

depends on the electrolyte type and concentration. Since the fipronil reaction,

410

electrodes and electrolyte solution were the same for the three electrode spacings, the

411

full-cell voltage should depend predominantly on the current and the spacing between

412

the electrodes via the solution resistance term. By decreasing the spacing between the

413

anode and cathode, and increasing the interfacial area of the electrodes, the

414

parallel-plate configuration was hypothesized to decrease the ohmic resistance

415

compared to the dual-cell configuration. The slower decline in voltage for the widest

416

electrode spacing (Figure 2B) indicates that the increase in ohmic resistance driven by

417

the wider spacing outstrips the decrease in current.

418

Figure 2D provides the full-cell resistance, calculated by dividing the full-cell

419

voltage by the current. The resistance increased initially and then leveled out after

420

~0.5 h, indicating that the current declined to a greater extent than the full-cell voltage.

421

The average resistance measured between 0.5-2 h (2.7-7.7 Ω) was positively related to

422

the electrode spacing (Figure 2E). The total energy consumption (kWh), calculated by

423

integrating the power (P = IV) over the first 2 h, increased with distance between the

424

electrodes from 2.7 x 10-4 kWh to 7.7 x 10-4 kWh (Figure 2F).

425

For the dual-cell configuration with the GAC/carbon cloth electrode, the average 20


Page 20 of 34

Page 21 of 34


426

resistance measured over 0.5-2 h was 248 Ω, more than an order of magnitude higher

427

than for the single-cell parallel-plate configuration. The total energy consumption for

428

the dual-cell configuration over the first 2 h was 5.3 x 10-4 kWh, calculated using the

429

same procedure described for the single-cell DC configuration. Although the full-cell

430

resistance of the dual-cell configuration was significantly higher, its current was lower,

431

resulting in a similar energy consumption compared to the single-cell DC electrolysis.

432

For AC electrolysis (-1 V to +1 V vs. S.H.E.) at 0.0125 Hz with a 2.2 cm electrode

433

spacing, Figure S6 provides current and power profiles, as measured by the potentiostat.

434

Given the fluctuation in the applied voltage, it was not feasible to use a multimeter to

435

measure an average full-cell voltage or resistance. The energy consumed over 4 h of

436

AC electrolysis was 2.12×10-4 kWh, 52% lower than the energy consumption for 2 h of

437

DC electrolysis using the same electrode spacing. At this frequency, 93% removal of

438

fipronil was observed over 4 h, such that the electrical energy required per log order of

439

fipronil removal (i.e., the EEO) was 1.84×10-4 kWh. The lower energy consumption

440

for AC electrolysis likely reflects the fact that the time during which the applied voltage

441

was held at the extreme values (i.e., -1 V or +1 V) was short.

442

Environmental implications: While electrochemical treatment of contaminants has

443

been evaluated at laboratory-scale, there are very few instances where these

444

technologies are applied at full-scale.21 Two factors hindering implementation of

445

electrochemical technologies at full-scale include drawbacks associated with electrode

446

materials and the complexity of electrochemical treatment configurations. For

447

treatment of stormwater flows, residence times of water within the electrochemical 21



448

reactor likely would be ~10 min. Due to their poor sorptive properties, electrochemical

449

treatment of organic contaminants in stormwater using the metal-based electrodes

450

favored in many laboratory-scale studies would require that contaminant degradation

451

occur during these short hydraulic residence times, which may not be possible. For

452

certain organic contaminants that can sorb to GAC, sorption of the organic

453

contaminants to the GAC-based electrodes may be feasible over these short hydraulic

454

residence times. Degradation of the sorbed fipronil could then occur over longer

455

timescales that are more feasible.

456

contaminants and electrode regeneration need be only intermittent; daily regeneration

457

could be powered by solar panels, an attractive feature for remote stormwater locations.

458

Although it is difficult to compare the energy requirements between GAC-based

459

electrodes and conventional metal-based electrodes due to differences in electrode

460

configurations, the longer timescales permitted for degradation of sorbed contaminants

461

by GAC-based electrodes renders treatment of flowing stormwater feasible.

Indeed, degradation of accumulated organic

462

Laboratory-scale electrochemical treatment configurations, such as our dual-cell

463

system with an intervening ion exchange membrane and platinum wire anode, are

464

difficult to scale up. We envision a full-scale system capable of treating a continuous

465

stormwater flow to consist of a series of parallel plates of GAC-based electrodes

466

operated in an AC configuration without an intervening ion exchange membrane

467

(Figure S1D). Organic contaminants in the stormwater would sorb to the GAC-based

468

electrodes during passage of the stormwater through the electrochemical treatment unit.

469

Application of AC electrolysis, potentially intermittently, would achieve reductive 22


Page 22 of 34

Page 23 of 34


470

dehalogenation of sorbed contaminants, permitting long-term operation of the system.

471

Formation of free chlorine, chlorate, and perchlorate by oxidation of chloride has

472

been observed in previous electrochemical studies involving DC electrolysis with

473

metal-based electrodes.26 An ion exchange membrane would prevent the formation of

474

these undesirable byproducts, but would require the development of a series of

475

alternating cathode and anode chambers, with a separate water supply to the anode

476

chambers. Elimination of the ion exchange membrane would simplify the configuration

477

by permitting the water stream to flow in channels between the anodes and cathodes.

478

Indeed, one of the few instances where full-scale electrochemical treatment is applied

479

involves the in situ generation of free chlorine by oxidation of chloride at the anode,

480

and so does not require the ion exchange membrane. Most laboratory-scale studies of

481

electrochemical treatment focus on DC electrolysis. Our results indicate that free

482

chlorine, chlorate and perchlorate generation can be avoided even without an ion

483

exchange membrane by using AC electrolysis. The frequency was low enough to

484

enable the reductive dehalogenation of sorbed pesticides, but high enough to avoid the

485

transport and oxidation of halides at the anode. AC electrolysis also avoided oxidative

486

degradation of the GAC-based electrode and reduced the energy consumption. It is

487

conceivable that the alternating potential could achieve reductive dehalogenation and

488

then oxidative mineralization of the carbon backbone of sorbed contaminants, but

489

further research would be needed to verify the fate of specific contaminants.

490

Many additional factors need to be evaluated to enable the development of a

491

pilot-scale electrochemical treatment system for stormwater runoff. Some of the most 23



492

important include the following. First, the experiments with Lake Lagunita water

493

suggest that sorption of fipronil occurs over long time-scales (~4 h). However, these

494

experiments involved the submergence of closely-spaced parallel plate electrodes

495

within a larger (500 mL) chamber. In ongoing research, we are developing a

496

flow-through reactor configuration in which the water would pass only within narrow

497

channels between the electrodes. This configuration would enable a more accurate

498

evaluation of the ability of the GAC-based electrodes to sorb organic contaminants

499

within hydraulic residence times relevant to stormwater treatment units. The design of

500

such a system will need to consider the effects of an applied potential on the sorption of

501

contaminants, particularly charged contaminants such as fipronil (which features a

502

primary amine functional group), and the potential for clogging by particles in

503

stormwater. Second, research is needed regarding alternative current distributors,

504

including metals, that can bind activated carbon particles but maintain a homogeneous

505

potential over larger electrodes.

506

non-homogenous distribution of potential at larger scales. Third, our AC electrolysis

507

experiments with Lake Lagunita water did not indicate significant degradation of

508

electrode performance over 5 cycles of sorption and electrolysis. However, to be

509

cost-effective, pilot-scale research is needed to demonstrate the performance of this

510

system over many more cycles. Issues of concern include abrasion of GAC particles

511

and reduced performance resulting from sorption of natural organic matter.

512

Acknowledgements: This research was supported by funding from the California

513

Department of Pesticide Regulation under Contract 15-C0087. We thank Dr. Susan

Carbon cloth is expensive, and may exhibit

24


Page 24 of 34

Page 25 of 34


514

Richardson and Joshua Allen of the University of South Carolina Department of

515

Chemistry for help with product identification.

516

SUPPORTING INFORMATION AVAILABLE

517

The Supporting Information is available free of charge on the ACS

518

Publications website at DOI:

519

Alternating current electrolysis current and power profiles.

520

References

521

1.

522

http://npic.orst.edu/factsheets/fiptech.pdf

523

2.

524

S.; Malaspina, O., Effects of Sublethal Dose of Fipronil on Neuron Metabolic Activity

525

of Africanized Honeybees. Arch Environ Con Tox 2013, 64, (3), 456-466.

526

3.

527

insecticides to the Sacramento-San Joaquin Delta of California. Environ. Sci. Technol.

528

2010, 44, 1833-1840.

529

4.

530

desulfinylation with retention of neurotoxicity. P Natl Acad Sci USA 1996, 93, (23),

531

12764-12767.

532

5.

533

Fipronil Insecticide and Its Sulfone Metabolite and Desulfinyl Photoproduct. Chem.

534

Res, Toxicol. 1998, 11, (12), 1529-1535.

535

6.

536

Neurotoxicity resulting from coexposure to pyridostigmine bromide, DEET, and

537

permethrin: Implications of Gulf War chemical exposures. J Toxicol Env Health 1996,

538

48, (1), 35-56.

539

7.

540

aspects of reproduction in Atlantic salmon (Salmo salar L.). Aquatic Toxicology 2001,

National Pesticide Information Center Fipronil technical fact sheet.

Roat, T. C.; Carvalho, S. M.; Nocelli, R. C. F.; Silva-Zacarin, E. C. M.; Palma, M.

Weston, D.P.; Lydy, M.J. Urban and agricultural sources of pyrethroid

Hainzl, D.; Casida, J. E., Fipronil insecticide: Novel photochemical

Hainzl, D.; Cole, L. M.; Casida, J. E., Mechanisms for Selective Toxicity of

AbouDonia, M. B.; Wilmarth, K. R.; Jensen, K. F.; Oehme, F. W.; Kurt, T. L.,

Moore, A.; Waring, C. P., The effects of a synthetic pyrethroid pesticide on some 25



541

52, (1), 1-12.

542

8.

543

synthetic pyrethroid insecticide cis-bifenthrin. J. Environ. Sci. 2009, 21, (12),

544

1710-1715.

545

9.

546

of neonicotinoids and fipronil on vertebrate wildlife. Environ Sci Pollut R 2015, 22,

547

(1), 103-118.

548

10. Tingle, C. C. D.; Rother, J. A.; Dewhurst, C. F.; Lauer, S.; King, W. J., Fipronil:

549

Environmental fate, ecotoxicology, and human health concerns. Rev Environ Contam

550

Toxicol. 2003, 176, 1-66.

551

11. Australian Pesticides and Veterinary Medicines Authority. Fipronil Part 1. Safety

552

of Fipronil in Dogs and Cats: a review of literature. https://apvma.gov.au/node/15191

553

(retrieved October 7, 2017).

554

12. Jones, W. J.; Mazur, C. S.; Kenneke, J. F.; Garrison, A. W., Enantioselective

555

microbial transformation of the phenylpyrazole insecticide fipronil in anoxic

556

sediments. Environ. Sci. Technol. 2007, 41, (24), 8301-8307.

557

13. Konwick, B. J.; Garrison, A. W.; Black, M. C.; Avants, J. K.; Fisk, A. T.,

558

Bioaccumulation, biotransformation, and metabolite formation of fipronil and chiral

559

legacy pesticides in rainbow trout. Environ. Sci. Technol. 2006, 40, (9), 2930-2936.

560

14. Zhu, G. N.; Wu, H. M.; Guo, J. F.; Kimaro, F. M. E., Microbial degradation of

561

fipronil in clay loam soil. Water Air Soil Poll 2004, 153, (1-4), 35-44.

562

15. Lin, K.; Haver, D.; Oki, L.; Gan, J., Transformation and sorption of fipronil in

563

urban stream sediments. J Agr Food Chem 2008, 56, (18), 8594-8600.

564

16. Zhao, X. L.; Yeh, J. Z.; Salgado, V. L.; Narahashi, T., Sulfone metabolite of

565

fipronil blocks gamma-aminobutyric acid- and glutamate-activated chloride channels

566

in mammalian and insect neurons. J Pharmacol Exp Ther 2005, 314, (1), 363-373.

567

17. Budd, R.; Ensminger, M.; Wang, D.; Goh, K. S., Monitoring Fipronil and

568

Degradates in California Surface Waters, 2008–2013. J Environ Qual 2015, 44, (4),

569

1233-1240.

570

18. Li, Y.; Kemper, J. M.; Datuin, G.; Akey, A.; Mitch, W. A.; Luthy, R. G., Reductive

Wang, C.; Chen, F.; Zhang, Q.; Fang, Z., Chronic toxicity and cytotoxicity of

Gibbons, D.; Morrissey, C.; Mineau, P., A review of the direct and indirect effects

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dehalogenation of disinfection byproducts by an activated carbon-based electrode

572

system. Water Res 2016, 98, 354-362.

573

19. Yang, Y.; Li, Y.; Walse, S. S.; Mitch, W. A., Destruction of Methyl Bromide

574

Sorbed to Activated Carbon by Thiosulfate or Electrolysis. Environ. Sci. Technol.

575

2015, 49, (7), 4515-4521.

576

20. Li, Y.; Liu, C.; Cui, Y.; Walse, S. S.; Olver, R.; Zilberman, D.; Mitch, W. A.,

577

Development of an Activated Carbon-Based Electrode for the Capture and Rapid

578

Electrolytic Reductive Debromination of Methyl Bromide from Postharvest

579

Fumigations. Environ. Sci. Technol. 2016, 50, (20), 11200-11208.

580

21. Radjenovic, J.; Sedlak, D. L., Challenges and Opportunities for Electrochemical

581

Processes as Next-Generation Technologies for the Treatment of Contaminated Water.

582

Environ. Sci. Technol. 2015, 49, (19), 11292-11302.

583

22. Xie, X.; Ye, M.; Liu, C.; Hsu, P.-C.; Criddle, C. S.; Cui, Y., Use of low cost and

584

easily regenerated Prussian Blue cathodes for efficient electrical energy recovery in a

585

microbial battery. Energy & Environmental Science 2015, 8, (2), 546-551.

586

23. Clesceri, L. S.; Greenberg, A. E.; Eaton, A. D., Standard Methods for the

587

Examination of Water and Wastewater, 20th ed. American Public Health Association,

588

American Water Works Association, Water Environment Federation: Washington, DC,

589

1998.

590

24. Sun. M.; Reible, D.D.; Lowry, G.V.; Gregory, K.B. Effect of applied voltage,

591

initial concentration, and natural organic matter on sequential reduction/oxidation of

592

nitrobenzene by graphite electrodes. Environ. Sci. Technol. 2012, 46, 6174-6181.

593

25. Hamann, C. H.; Hamnett, A.; Vielstich, W., Electrochemistry. Wiley-VCH:

594

Weinheim, Germany, 2007.

595

26. Jasper, J.T.; Yang, Y.; Hoffmann, M.R. Toxic byproduct formation during

596

electrochemical treatment of latrine wastewater. Environ. Sci. Technol. 2017, 51,

597

7111-7119.

598

27



Page 28 of 34

599

Figure 1. Dual cell electrolysis experiments. A) Electrolytic degradation of 0.34 µmol

600

fipronil by DC electrolysis (-1 V vs. S.H.E.) using the GAC/graphite electrode after 15

601

h treatment. B) Degradation by DC electrolysis (-1 V vs. S.H.E.) using the GAC/carbon

602

cloth electrode of 3.2 µmol fipronil, formation of fipronil sulfide and C) chloride

603

release. Dechlorination percent was calculated by dividing the mass of chloride in the

604

aqueous phase by the initial mass of chlorine in the fipronil. D) Electrolytic degradation

605

after 2 h of 150 µg each of a mixture of fipronil, bifenthrin and permethrin using the

606

GAC/carbon cloth electrode as a function of applied potential vs. S.H.E. Error bars

607

represent the range of experimental duplicates. Some errors are smaller than the

608

symbols. A)

B)

% 0 0 6

Fipronil Fipronil sulfide

% 0 2

% 0 0 1

C/C0

% 0 3

% 0 0 2

C/C0

% 0 0 1

% 0 0 4

Aqueous Fipronil Sorbed Fipronil ClFFipronil sulfide

% 0 1

% 0 5

% 0

% 0

5 1

2

Control

1

0

Initial

Electrolysis

Time (h)

C)

D) % 0 0 1

Fipronil % 0 6

4

Permethrin

15

Applied potential

Time (h)

609 610 611

28


V 2 . 0 -

V 4 . 0 -

V 6 . 0 -

V 8 . 0 -

10

C/C0 % 0

5

% 0 2

% 0

0 0

% 0 4

% 0 5

Dechlorination

2

[Cl-] (µM)

Bifenthrin

Page 29 of 34


612

Figure 2. Single-cell DC electrolysis (-1 V vs. S.H.E.) of 500 µg (1.15 µmol) fipronil at

613

pH 7.0 with 100 mM phosphate buffer. A) Chloride yield (dechlorination is calculated

614

by dividing the aqueous mass of chloride by the initial mass of chlorine in the fipronil),

615

B) full-cell voltage, C) current, D) full-cell resistance, E) average resistance (0.5 – 2 h)

616

and F) energy consumption for different electrode spacings. Error bars represent the

617

range of experimental duplicates. Some errors are smaller than the symbols.

29



618 619

30


Page 30 of 34

Page 31 of 34


620

Figure 3. Free chlorine formation during single-cell electrolysis using direct current

621

(-1 V vs. S.H.E.) and alternating current (-1 V to +1 V vs. S.H.E. at 0.0025, 0.0125

622

and 0.025 Hz) for a 1 mM chloride solution. Error bars represent the range of

623

experimental duplicates. Some errors are smaller than the symbols.

624 625 626 627

31



628

Figure 4. Chloride yield during AC electrolysis of 1 mg (2.3 µmol) fipronil using a

629

triangular waveform from -1 V to +1 V vs. S.H.E. at different scan rates at pH 7.0 using

630

100 mM phosphate buffer. Dechlorination is calculated by dividing the aqueous mass

631

of chloride by the initial mass of chlorine in the fipronil. Error bars represent the range

632

of experimental duplicates. Some errors are smaller than the symbols. B) 1 mg of

633

permethrin and bifenthrin using a triangular waveform from -1 V to +1 V vs. S.H.E. at

634

0.025 Hz.

635 636

32


Page 32 of 34

Page 33 of 34


637

Figure 5. Aqueous fipronil concentration during the AC electrolysis of Lake Lagunita

638

water spiked with 1 mg (2.3 µmol) fipronil using a triangular waveform from -1 V to +1

639

V vs. S.H.E. at 0.0125 Hz during 5 cycles of 4 h sorption and 4 h AC electrolysis each.

640

Error bars represent the range of experimental duplicates. Some errors are smaller than

641

the symbols. % 0 0 1 % 0 6

AC applied AC applied

AC applied

AC applied

% 0 4

Caq/C0

% 0 8

AC applied

% 0 2 % 0

644 645

33


0 4

643

0 3

0 2

0 1

0

Time (h)

642


646

TOC Art

647 648 649

34


Page 34 of 34

Capture and Reductive Transformation of Halogenated Pesticides by

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