This is the submitted version of the journal article: Fernandez Verdejo, David Juan; Sulonen, Mira; Pérez-Trujillo, Míriam; [et al.]. «Electrochemical dehalogenation of dibromomethane and 1,2-dibromoethane to non-toxic products using a carbon fiber brush electrode». Journal of chemical technology and biotechnology, Vol. 96, issue 2 (Feb. 2021), p. 335-340. DOI 10.1002/jctb.6542 This version is available at https://ddd.uab.cat/record/273820 under the terms of the license 1 Electrochemical dehalogenation of dibromomethane 2 and 1,2-dibromoethane to non-toxic products using a 3 carbon fiber brush electrode 4 David Fernández-Verdejo1, Mira Sulonen2, Míriam Pérez-Trujillo3, Ernest Marco-Urrea1, Albert 5 Guisasola 2,*, Paqui Blánquez1 6 1 7 Biorem UAB, Department of Chemical, Biological and Environmental Engineering, School of 8 Engineering, Universitat Autònoma de Barcelona, 08193, Bellaterra, Barcelona, Spain. 2 9 GENOCOV, Department of Chemical, Biological and Environmental Engineering, School of 10 Engineering, Universitat Autònoma de Barcelona, 08193, Bellaterra, Barcelona, Spain. 3 11 Nuclear Magnetic Resonance Service, Universitat Autònoma de Barcelona, 08193, Bellaterra, 12 Barcelona, Spain. 13 14 * Corresponding author: 15 Albert Guisasola Canudas 16 Telephone: 34 93 581 1879 17 Email: albert.guisasola@uab.cat 1 18 Abstract 19 The electrochemical dehalogenation of 1,2-dibromomethane (DBM) and 1,2-dibromoethane 20 (DBA), two brominated contaminants often detected in groundwater, was studied at different 21 cathode potentials (-0.8, -1 and -1.2 V versus Standard Hydrogen Electrode) in aqueous solution 22 using an inexpensive graphite fiber brush electrode. The degradation followed first-order kinetics 23 with respect to the nominal concentration of the brominated compounds and the kinetic constant 24 increased concomitantly with the decrease of the cathode potential. The amount of bromide ion 25 released during electrochemical dehalogenation, for the range of cathode potentials tested, was 26 96% and 99% of the maximum bromide concentration expected if DBM and DBA were fully 27 dibrominated, respectively. In accordance with these results, the non-brominated compounds 28 methane and ethene were the main products detected during DBM and DBA electrochemical 29 degradation. In addition, minor amounts of formic acid and acetic acid were also detected for each 30 contaminant respectively. The non-toxicity of the effluent was confirmed by a Microtox test. These 31 results open the door to the application of a simple and non-toxic electrochemical approach to the 32 treatment of aliphatic brominated compounds from aquifers and other water sources. 33 34 Keywords 35 Dibromomethane; 1,2-Dibromoethane; Electrochemistry; Remediation, Reductive debromination 36 37 38 2 39 1 Introduction 40 Dibromomethane (DBM) and 1,2-dibromoethane (DBA) are brominated volatile compounds, 41 which are utilized in a wide range of industrial applications, such as petrochemical and 42 pharmaceutical processes, and have been used as agricultural pesticides [1,2]. Due to their 43 extensive usage and accidental events, their presence has been detected in natural environments, 44 including groundwater sources [2,3]. Both compounds have adverse effects on human health, and 45 are considered probable/potential carcinogens [4]. Based on a combination of frequency of 46 occurrence, toxicity, and potential for human exposure, DBA has been ranked 39th on the 2017 47 ATSDR Priority List of Hazardous Substances [5]. 48 Several degradation techniques have been studied to degrade these brominated compounds in 49 aqueous media, including chemical reduction or oxidation [6–9] and aerobic or anaerobic 50 biodegradation [4,6]. Electrochemical techniques have been tested within a broad range of 51 halogenated contaminants, but mostly include chloroalkanes and pesticides [10–14]. With regards 52 to the electrochemical degradation of brominated compounds, Radjenović et al. (2012) [15] 53 reported the successful simultaneous electrochemical reduction of 17 contaminants, including 54 several bromoiodomethanes, bromochloromethanes and bromoform, using resin-impregnated 55 carbon-brushes. Electrocatalysis of halogenated compounds can involve either oxidation or 56 reduction reactions, but electrochemical reduction is promising for the degradation of simple 57 halogenated contaminants because enables the cleavage of the carbon – halogen bond and their 58 further transformation to innocuous compounds (i.e. methane and ethane) [16,17]. The main 59 advantages of the electrochemical degradation over biological techniques are i) the short reaction 60 time, ii) the ability to treat streams even with high concentrations of contaminant and iii) the ability 61 to work in complex mixtures of different pollutants in which the growth of microorganisms may 3 62 be inhibited [18]. On the other side, the main drawbacks of electrochemical techniques are i) the 63 high cost of the precious metals (i.e. Ag, Al, Au, Cu, Ni, Pd, Ti) required to catalyze the 64 electrochemical reactions [16,19], ii) the need of a high external energy input, and iii) the need of 65 specific medium compositions (as low pH or non-aqueous electrolytes), which may complicate its 66 field application [16,20]. In any case, there is a need to find cheaper and novel efficient 67 technologies to treat efficiently halogenated pollutants that can be found in groundwater. 68 The aim of this study is to demonstrate the feasibility of the reductive dehalogenation of DBM and 69 DBA in lab-scale electrochemical cells using an inexpensive and untreated graphite brush 70 electrode under non-aggressive electrolytic conditions. The effect of the electrode potential on 71 reaction rates and degradation efficiencies was studied together with the identification of the 72 byproducts and the toxicity of the treated effluent. 73 2 Materials and methods 74 2.1 Electrochemical cells description 75 The electrochemical cells consisted of two 165 mL glass vessels separated by a cation-exchange 76 membrane (CMI-7000, Membranes International INC, USA), with an aperture diameter of 4 cm. 77 The membrane was hydrated prior to its use by maintaining it in a 5% wt sodium chloride solution 78 for 24 hours at room temperature. A titanium sheet was used as the anode. The cathode was a 79 graphite fiber brush (35 mm length x 30 mm diameter, 7.2 µm fiber diameter, Millrose Co., USA) 80 providing a surface area of 0.132 m2 and a ratio of electrode surface : reaction volume of 802 81 m2/m3. A power source (Quad Potentiostat, Whistonbrook Software) was used to obtain the desired 82 cathode potentials. The cathode potential versus an Ag/AgCl reference electrode (RE-1B, BAS 83 Inc., +197 mV vs SHE) was measured with a digital multimeter (Hayoue DT830B). The potential 4 84 values are reported as V vs SHE, if not otherwise stated. The cathode vessel was sealed with a 85 Teflon cap in order to prevent the loss of volatile compounds. A Teflon-coated butyl rubber stopper 86 sealed with an aluminium crimp cap was used as a sampling port for liquid extraction in the 87 cathode. 88 2.2 DBM and DBA electrochemical reduction experiments 89 Before each experiment, anodic and cathodic chambers were filled with an anoxic bicarbonate- 90 buffered aqueous solution (50 mM) with a pH value adjusted to 7 and a conductivity of 3.85 91 mS/cm. The catholyte was spiked with DBM (Sigma-Aldrich, 99% purity) or DBA (Sigma- 92 Aldrich, ≥98% purity) and vigorously mixed with a magnetic stirrer during all the experiment 93 length. The degradation of both pollutants was tested by triplicate at three different cathode 94 potentials (-0.8, -1 and -1.2 V vs SHE). Open circuit experiments with each contaminant in the 95 same experimental conditions were performed to discard spontaneous degradation of both 96 compounds. During the experiments, liquid samples were withdrawn periodically from the 97 catholyte in order to monitor the degradation of the pollutants and the formation of byproducts. 98 Experiments were conducted at room temperature (25±3 °C). 99 The maximum degradation rate for each contaminant at the different cathode potentials was 100 calculated from the contaminant degradation profile during the application of potential. In 101 addition, a first-order kinetic model (equation 1) was fitted to describe the contaminant degradation 102 rate (µM·h-1) under different applied potentials: 103 𝐷𝑒𝑔𝑟𝑎𝑑𝑎𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒 = −𝑘 · [𝐶𝑜𝑛𝑡𝑎𝑚𝑖𝑛𝑎𝑛𝑡] (1) 104 where k is the first order rate constant (h-1) and [Contaminant] is the remaining concentration of 105 DBM or DBA in the cathode. 5 106 2.3 Coulombic efficiency and energy consumption 107 The coulombic efficiency (CE) for each degradation experiment at the cathode was calculated 108 using equation 2: 2·𝑉·𝐹·[𝐵𝑟 − ] 109 𝐶𝐸 = (2) ∫ 𝐼(𝑡)𝑑𝑡 110 where 2 is the number of electrons required to release a mole of bromide ion, V is the cathodic 111 liquid volume (L), F is Faraday’s constant (96485 A·s·mol-1), [Br-] is the concentration of released 112 bromide ions (µM) and ∫I(t)dt is the integration of the measured intensity data along the 113 experimental time. 114 Additionally, the energetic input (EI) required for each experiment was calculated as follows: 𝑉𝐴 ·∫ 𝐼(𝑡)𝑑𝑡 115 𝐸𝐼 = (3) 𝑉·[𝐵𝑟 − ] 116 where VA is the voltage applied in the system (V). 117 2.4 Analytical methods 118 The volatile halogenated contaminants, methane and ethene were identified by static headspace 119 gas chromatography. The liquid samples (1 mL) from the cathode were transferred to 10 mL vials 120 containing 5.5 mL water. The vials were sealed with Teflon-coated butyl rubber stoppers and 121 aluminum crimp caps, placed in a headspace sampler (Agilent 7964) and heated to 85 ºC for 15 122 min. Subsequently, 1 mL headspace gas sample from the vials was injected automatically into an 123 Agilent 6890N gas chromatograph equipped with an Agilent DB-624 column (30 m × 0.32 mm 6 124 with 0.25 μm film thickness) and a flame ionization detector following a method described 125 elsewhere [21]. 126 To identify nonvolatile compounds produced from the electrochemical degradation of DBA and 127 DBM, 400 mL of aliquot derived from these experiments were mixed with 200 mL of D2O (99.96 128 % D, Cortecnet, Voisins-le-Bretonneux, France) in a 5-mm-diameter NMR tube. A Bruker Avance 129 II 600 nuclear magnetic resonance spectrometer (Bruker Biospin, Rheinstetten, Germany) 130 equipped with a 5 mm TBI probe with Z-gradients, operating at a 1H NMR frequency of 600.13 131 and at 298.0 K of temperature, was used for the NMR experiments. 1D 1H NMR spectra were 132 acquired using the noesy1dpr pulse sequence to suppress the H2O signal. An acquisition time of 133 1.70 s and a relaxation delay of 2 s were used. The data were collected into 32 K computer data 134 points, with a spectral width of 9615 Hz and as the sum of 2048 transients. Identification of 135 compounds was done comparing results with those reported in literature and gathered in NMR 136 spectral data bases [22,23]. 137 Bromide ions were quantified from filtered (0.22 µm PVDF filters) liquid samples (1 mL) using 138 ion chromatography with conductivity detection (Dionex ICS-2000 with an Ultimate 3000 139 Autosampler) as described elsewhere [21]. 140 2.5 Toxicity Analysis 141 A Microtox bioassay was performed before and after the electrochemical treatment to detect 142 possible toxic intermediates. A Microtox model 500 analyser (Azur Environmental) was used and 143 the 81.9% basic test (adequate for samples with expected low toxicity) was selected. The 144 bioluminescence of the model bacterium Aliivibrio fischerii was measured 5 and 15 minutes after 145 exposition to experiment samples and compared to the bioluminescence produced by the control 7 146 tests (i.e without exposition to the contaminant). The tubes were sealed with a rubber septum to 147 prevent the loss of volatile contaminants during the assay. 148 149 3 Results and discussion 150 3.1 DBM and DBA electrochemical reduction experiments and identification of byproduts 151 The electrochemical reduction of DBM and DBA was evaluated at different cathode potentials (- 152 0.8, -1 and -1.2 V vs SHE). The degradation of DBM and DBA was only observed when cathodes 153 were poised at the desired value and not under open circuit conditions (Fig. 1), indicating that 154 DBM and DBA degradation was electrochemically mediated. 155 In the case of DBM, the time needed to obtain a contaminant decrease higher than 95% was 7, 3 156 and 1 h for the potentials of -0.8, -1 and -1.2 V, respectively (Fig. 1A). Time-course degradation 157 experiments showed a correlation between the DBM degradation rate and the cathode potential 158 (Fig. 1A), the degradation rate being the highest (786.7 ± 50.6 µM·h-1) at the lowest cathode 159 potential (-1.2 V) tested. At cathode potentials of -1.0 and -0.8 V, degradation rates of 213.5 ± 160 31.7 µM·h-1 and 102.5 ± 26.5 µM·h-1 were obtained, respectively. 161 A lower percent degradation was achieved for DBA for the same time point experiments and 162 cathode potentials tested for DBM (Fig. 1C). Again, the more negative the cathode potential, the 163 higher the maximum degradation rate. The obtained values were 677.0 ± 107.4, 177.1 ± 18.3 and 164 76.3 ± 10.9 µM·h-1 for the potentials of -1.2, -1.0 and -0.8 V, respectively. Although there is a 165 difference among the percentages of degradation between both contaminants, the comparison 166 between the maximum degradation rates obtained for each potential shows a difference lower than 8 167 26%. Electrochemical DBM degradation has not been previously studied and DBA degradation 168 has only been reported with boron-doped diamond and dropping mercury electrodes at cathode 169 potentials around -1.5 V (vs SHE) [17,24]. In our study, less negative cathode potentials were 170 successfully used due to the high surface area per reaction volume of the graphite brush used as 171 the electrode. 172 The chromatographic analyses of headspace samples indicate that DBM and DBA degraded was 173 mostly transformed to methane and ethene, respectively (data not shown). In addition, liquid 174 samples from DBM and DBA at potential of -1.2 V experiments at time point 60 min were 175 analyzed by 1H NMR spectroscopy to identify any possible nonvolatile degradation compound. 176 Spectra from DBM and DBA presented some very low intense peaks, in form of singlets, which 177 differed from the blank (Fig. 2a). This blank consisted in a sample derived from the cathodic 178 chamber that was electrochemically operated for 60 min at the same electric potential but without 179 brominated compounds spiked. These signals were difficult to identify due to their singlet nature 180 and to their low intensity (i.e. compounds in very low concentration), which hampered the 13 181 performance of other NMR experiments like C based experiments. However, two compounds 182 were identified due to their typical resonance frequencies at the experimental conditions used. In 183 sample derived from DBM degradation, a small singlet at 8.45 ppm corresponding to formic acid 184 was observed (Fig. 2c). In DBA sample, which presented a cleaner spectrum than the equivalent 185 for DBM, a very low intense singlet at the resonance frequency of acetic acid, 1.92 ppm, was 186 detected (Fig. 2b) [22,23]. Also, increasing amounts of bromide ions were released during the 187 dehalogenation of both brominated compounds (Fig. 1B and 1D). No product generation was 188 detected in the open circuit experiments, ensuring that the generation of these substances was 189 directly correlated with the contaminant degradation. 9 190 The amount of bromide ion released from DBM degradation was 96.8% of the maximum bromide 191 concentration expected if all the degraded DBM was fully dibrominated (Fig. 3A). Hydrogenolysis 192 of DBM was discarded as a pathway for degradation because no bromomethane was detected by 193 GC analyses during the experiments (detection limit 1 µM). Similarly, DBA degradation yielded 194 a molar ratio of bromide to DBA of 1.995, which means that 99.8% of the expected bromide was 195 released, considering that all the DBA degraded was completely debrominated (Fig. 3B). In 196 contrast, incomplete electrochemical debromination of DBA (ratio between bromide released and 197 DBA degraded of 1.56 and 1.78) was reported at cathode potentials around -1.5 V (vs SHE) in 198 previous studies [17]. Tokoro et al. (1986) hypothesized that carbanion was generated as reaction 199 intermediate and it was further transformed to bromoethane [17]. In our study, neither vinyl 200 bromide nor bromoethane (formed during dehydrohalogenation or hydrogenolysis of DBA, 201 respectively) were detected in the catholyte by GC analyses (detection limit 1 µM). 202 The Microtox toxicity bioassay was applied to evaluate the toxicity of the electrolyte before and 203 after treatment. Our results showed that the samples analyzed were not sufficiently toxic to produce 204 any effect on the bioluminescence from A. fischeri, indicating that electrochemical degradation of 205 DBM and DBA did not result on the accumulation of major toxic products. 206 3.2 Coulombic efficiency and energy consumption 207 The values of coulombic efficiency and energy consumption were calculated for each experiment 208 based in the measured intensities and voltages (Table 1). All the potentials tested showed similar 209 values of coulombic efficiency for both contaminants (between 60 and 77%). No correlation was 210 found among the different potentials. Alternative cathodic reactions presumably lowering the 211 coulombic efficiency include e.g. the reduction of oxygen (leaking to the cathode from the anode 212 through the cation exchange membrane) and electrochemical hydrogen production. Despite this 10 213 similarity, the energetic consumption adopted higher values in the -1.2 V due to the more negative 214 voltages applied. These results show that the use of carbon brush electrodes can grant high 215 efficiency values despite its simplicity when they are involved in the cleavage of the bromide- 216 carbon bond. 217 Table 1. Coulombic efficiencies (CE) and Energetic Inputs (EI) for DBM and DBA degradation 218 at different potentials. Applied Contaminant potential Voltage (V) CE (%) EI (kWh·mol-1) (V vs SHE) -0.8 -1.9 69.8 ± 3.6 0.15 ± 0.01 DBM -1.0 -2.3 76.3 ± 0.8 0.16 ± 0.01 -1.2 -4.0 68.6 ± 1.8 0.31 ± 0.01 -0.8 -1.9 65.3 ± 8.3 0.16 ± 0.02 DBA -1.0 -2.3 67.2 ± 0.5 0.18 ± 0.01 -1.2 -4.0 60.7 ± 14.7 0.37 ± 0.10 219 220 221 3.3 Kinetics of transformation 222 The degradation of DBM and DBA followed first-order kinetics with respect to the nominal 223 concentration. The rate constants were estimated by fitting the data to a first-order kinetic model 224 (Fig. S1), obtaining R2 values ranging from 0.92 to 0.97. The dependence of the rate constants for 225 both brominated contaminants on the cathode potential is illustrated in Fig. 4. For both compounds, 226 the first order kinetic constant statistically increases when the electric potential adopts more 11 227 negative values. These results are in agreement with the maximum degradation rates discussed in 228 the previous results section. 229 In a previous study reporting electrochemical reduction of DBA at -1.5 V (vs SHE) with a boron 230 doped diamond electrode and 22 mL of a 30 % methanol containing aqueous medium, a kinetic 231 constant value of 0.44 h-1 was obtained [24]. This value is 80 % lower than the one obtained in this 232 study at potential of -1.2 V, while our experiments were performed in a 7.5-fold higher reaction 233 volume. This higher degradation rate is mainly due to the high ratio between electrode surface area 234 and catholyte volume provided by the graphite brush, being two orders of magnitude higher. This 235 indicates that an efficient degradation can be reached with cheaper electrode materials and simpler 236 medium compositions. Further research is needed to assess the optimal conditions and the 237 suitability of the degradation process in a larger scale setup. The kinetic constants obtained in other 238 reports on electrochemical degradation of similar compounds as Radjenović et al. (2012) [15] are 239 difficult to compare due to the complexity of their systems or the use of several simultaneous 240 contaminants. 241 4 Conclusions 242 This study demonstrates the feasability of electrochemical systems to fully debrominate DBA and 243 DBM using inexpensive and environmentally friendly carbon fibers as electrode materials. The 244 short operation time did not lower the coulombic efficiency of the process, maintaining similar 245 efficiency values even in the experiments with higher energetic inputs. The first order kinetic 246 model proposed fits acurately the obtained data and allows to statistically confirm a relationship 247 between the applied cathode potential and the rate of contaminant degradation. The treated effluent 248 was not toxic according to the Microtox bioassay, which is in accordance to the nonhalogenated 12 249 byproducts identified during the electrochemical degradation of DBM and DBA by NMR.. Further 250 investigation is required to assess the application of this methodology on environmental 251 contaminated sites. 252 253 Acknowledgements 254 This work has been funded by the Spanish Ministry of Science, Innovation and Universities project 255 CTM2017-91879-EXP. D. Fernández-Verdejo acknowledges a predoctoral grant from UAB (PIF 256 2017-2018). M. Sulonen acknowledges the financial support from Maj and Tor Nessling 257 Foundation (Project no. 201800101). 258 The authors declare no conflict of interest. 259 260 References 261 [1] J. Mei, Z. Qu, S. Zhao, X. Hu, H. Xu, N. Yan, Promoting effect of Mn and Ti on the structure 262 and performance of Co 3 O 4 catalysts for oxidation of dibromomethane, J. Ind. Eng. Chem. 263 57 (2018) 208–215. doi:10.1016/j.jiec.2017.08.025. 264 [2] S.H. Douglas, B. Dixon, D. Griffin, Assessing the abilities of intrinsic and specific 265 vulnerability models to indicate groundwater vulnerability to groups of similar pesticides: 266 a comparative study, Phys. Geogr. 39 (2018) 487–505. 267 doi:10.1080/02723646.2017.1406300. 268 [3] N.S. Abbai, B. Pillay, Analysis of hydrocarbon-contaminated groundwater metagenomes as 13 269 revealed by high-throughput sequencing, Mol. Biotechnol. 54 (2013) 900–912. 270 doi:10.1007/s12033-012-9639-z. 271 [4] P.B. Hatzinger, J.F. Begley, D.R. Lippincott, A. Bodour, R. 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Bars indicate deviation for triplicate experiments. 364 Figure 2. 1D 1H NMR spectra of liquid samples taken after 60 min of the electrochemical 365 degradation process of DBA (Panel B) and DBM (Panel C) at -1.2 V (vs SHE). Panel A shows the 366 1D 1H NMR spectra of a blank that accounted for an electrochemical cell without brominated 367 compounds. The spectra were acquired with suppression of the water signal (noesy1dpr), at 25 ºC 368 and at a magnetic field of 600 MHz. 369 Figure 3. Fitted curve of bromide released in the liquid medium per DBM (a) or DBA (b) 370 degraded. Values were obtained from the experiments plotted in Figure 1. 371 Figure 4. Dependence of first-order kinetic constants for DBM and DBA reductions on cathode 372 potential using a graphite fibre brush cathode. Shown are means for triplicate assays ± standard 373 deviations. 374 19