Oxygen-Vacancies-Mediated BrO3- Electroreduction: Positively Charged Microenvironment Enables Directional Diffusion and Spontaneous Adsorption

Oxygen-Vacancies-Mediated BrO3- Electroreduction: Positively Charged Microenvironment Enables Directional Diffusion and Spontaneous Adsorption | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Oxygen-Vacancies-Mediated BrO3- Electroreduction: Positively Charged Microenvironment Enables Directional Diffusion and Spontaneous Adsorption Huabin Zeng, Xiangbin Huang, Yue Cheng, Zhipeng Luo, Ruiqi Zheng, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4805543/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Ozonation can disinfect the drinking water without producing chlorinated byproducts, but bromate (BrO 3 − ) emerges as a carcinogenic by-product. Although the electroreduction could theoretically convert BrO 3 − into non-toxic bromide (Br − ), the detoxification process was hindered by the electrostatic repulsion of BrO 3 − from the negatively charged cathode. In this work, a commercial Fe foam was employed for electrochemically reducing 1.0 mg/L BrO 3 − via direct electron transfer mechanism ( k 1 , 0.7796 min − 1 ), during which microcrystalline magnetite (Fe 3 O 4 ) was in-situ generated with abundant oxygen vacancies (Ovs). The Ovs could shape neighboring Fe 2+ atoms into positively charged microenvironment for accelerating the directional diffusion of BrO 3 − toward themselves. Compared to negatively charged surface, the positive microenvironment could enhance the BrO 3 − diffusion with coefficient change from 0.0059 m 2 /s to 0.0387 m 2 /s. Furthermore, Ovs activated the neighboring Fe 2+ atom into a highly active site for BrO 3 − adsorption with an adsorption energy (E ads ) of 4.21 eV, in comparison to the energy-demanding adsorption on intact Fe 3 O 4 lattice (E ads , -2.17 eV). Afterwards, BrO 3 − underwent complete detoxification through sequential deoxygenation, with Ovs assisting throughout the whole process from BrO 3 − to Br − . By the in-situ defect engineering, the research pointed out a high-efficient approach to create positively charged microenvironment for enhancing oxyanion electroreduction. Earth and environmental sciences/Environmental sciences/Environmental chemistry Physical sciences/Chemistry/Surface chemistry Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Ozonation has been regarded as a greener alternative disinfection technology to chlorination. The technology significantly diminishes the formation of chlorinated disinfection byproducts, but it concomitantly induces the production of bromate (BrO 3 - ) in the bromide-containing (Br - ) raw water. 1,2 Owing to its carcinogenic potential to humans and living organisms, BrO 3 - has been classified as a group 2B carcinogen by International Agency for Research on Cancer, significantly threatening the public health via oral exposure. 3,4 Therefore, it was necessary to explore an efficient drinking water detoxification technology for removing BrO 3 - . Various lab-scale technologies were developed for BrO 3 - removal recently, including resin adsorption, biological remediation, and catalytic reduction. 5-7 Unfortunately, these technologies faced significant challenges for practical application. For instance, while physical separation could isolate BrO 3 - from drinking water, the concentrated residue remained a significant issue. 8 Bioremediation of BrO 3 - often left behind bacteria and organic matter, necessitating downstream treatment for eliminating potential pathogenic microorganisms. 9,10 Although catalytic reduction could eliminate BrO 3 - pollution while minimizing residual by-products, 11,12 the high cost of catalyst synthesis and post-treatment of used electron-donors have restricted its use on an industrial scale. Electrocatalytic reduction could address these issues, while readily converting BrO 3 - into harmless Br - . 13,14 Especially, electrochemical systems were considered to have significant potential for reducing carbon emissions. In general, the electrocatalytic reduction of substrates can proceed through both indirect mechanism via atomic hydrogen (atomic H*) and direct electron transfer. 15 However, due to the negative charge on the cathode surface, oxyanions (e.g., BrO 3 - , NO 3 - , and PFAS) were electrostatically repelled from the region near the cathode, thereby restraining their electroreduction conversion. 16,17 Only when the oxyanions or electron carriers penetrated the electrostatic repulsion region, can they drive the electrochemical reduction of these anions. 18,19 In the indirect reduction mechanism, atomic H* was first produced near the cathode, then diffused toward the bulk solution for injecting electron into the substrate. As an uncharged particle, its diffusion was not affected by electrostatic repulsion, significantly improving the indirect reduction efficiency of BrO 3 - /NO 3 - . Unfortunately, due to its high reactivity, atomic H* was easily annihilated by water matrix. 20 Therefore, assisting charged oxyanions in overcoming electrostatic repulsion and facilitating direct electron transfer on the electrode surface could be an alternative strategy to enhance the electrocatalytic oxyanion detoxification. By employing a pulse strategy that periodically alters the electrode interface charge, depression of NO 3 - concentration proximal to the working electrode was avoided, significantly enhancing the urea electrosynthesis from electroreduction of NO 3 - and CO 2 . 21 Modification of the cathode surface with a quaternary ammonium surfactant introduced additional positive charges, which weakened electrostatic repulsion and facilitated the diffusion of negatively charged PFAS toward the cathode. 17 As a result, a removal efficiency of up to 99.81% and a defluorination efficiency of 78.67% were achieved at the modified cathode. Similarly, Liu., et. al. pointed out that the in-situ -generated Fe 2+ could exist as specifically adsorbed cations in the inner Helmholtz plane, further electrostatically attracting the NO 3 - around the interface and accelerating the electro-conversion of NO 3 - into NH 4 + . Due that the performance improvements were readily achieved in various oxyanion electroreduction, it was desirable to explore the feasibility of enhancing BrO 3 - electroreduction when similar strategy was employed. In this study, a Fe-foam-based electrochemical system was proposed for BrO 3 - reduction and the electroreduction mechanism was investigated by quenching experiments. The morphological change in Fe foam during the electroreduction process was investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) and in-situ Raman spectroscopy, and the density functional theory (DFT) was employed for reaction approach analysis. Then, diffusion behavior of BrO 3 - was studied by fluorescence microscopy, open circuit potential and computational fluid dynamics (CFD) simulation. Finally, a combined process of BrO 3 - electroreduction with ion exchange resin pretreatment was proposed for increasing the current efficiency and application potential. BrO Reduction on Various Cathodes Initially, we conducted the experiment to investigate the feasibility of BrO 3 − reduction in the electrochemical single-cell system using various commercial electrodes (1 ×1 cm), such as Pt, graphite sheet, and Fe foam ( Fig. 1a 1 − 3 ), 22–24 i.e. , in addition to NaBrO 3 (7.8 µM) as model contaminant and Na 2 SO 4 (10 mM) as electrolyte, the reaction system was deoxygenated by purging N 2 into the solution (pH 7.0). As shown in Fig. 1a 1 - 2 , Pt cathode and graphite cathode exhibited no degradation for BrO 3 − . Only when the cathode was Fe foam ( Fig. 1a 3 ), did 6.6 µM BrO 3 − got degraded quickly within 60 min. Meanwhile, Br − was produced in the process and reached 1.2 µM within the reaction time of 60 min. The addition of BrO 3 − and the resultant Br − concentration equaled the initial concentration of BrO 3 − (7.8 µM), the bromine balance results indicated that the reduced BrO 3 − was all converted into nontoxic Br − . Notably, the applied voltage for the 3 systems was 2.5 V, and the current was around 8.0 mA for the whole electrochemical process (Fig. 1 . b 1 − 3 ), electric quantity (Q) for Pt cathode was 27807 C, 32643 C for graphite cathode and 28446 C for Fe foam cathode. The results indicated similar amounts of electron transfer on the surface of different electrodes. Especially, Pt, as a palladium homologue, it showed no capacity for the reduction of BrO 3 − , indicating that it lacks capacity for both indirect reduction and direct electron transfer. To study the factors that lead to the great difference in the reduction results, the adsorption energy of BrO 3 − on different electrode surfaces was estimated by DFT calculation. Interestingly, although the adsorption energy of BrO 3 − on the graphite cathode was positive, that on the Pt was determined to be -1.18 eV. The negative adsorption energy indicated that the interaction between BrO 3 − and the Pt lattice was a spontaneous process, with a high likelihood of BrO 3 − undergoing direct electron transfer. This suggested that there were other hindrances than electron transfer limitation. Generally, electrochemical reduction could procced via indirect reduction mediated by atomic hydrogen (H*), and direct electron transfer from cathode to BrO 3 − . 14 The indirect reduction was achieved via atomic H* produced near the electrode, exchanging electrons with BrO 3 − due to its diffusion towards the bulk solution. And the direct electron transfer mechanism was initiated by the BrO 3 − diffusion from bulk solution to the electrode surface and absorption, then directly transferring the electron from the electrode surface to BrO 3 − . 25 . To better distinguish the roles of the two mechanisms in the electrocatalytic system where Fe foam was employed as cathode for BrO 3 − reduction, quenching experiments were conducted by increasing dissolved oxygen (DO) or adding TBA, owing to their strong reactivity toward atomic H*. 15,26 The experiments were carried out in a two-compartment electrolytic cell, separated by a proton exchange membrane, to eliminate any interference arising from the oxygen evolution reaction occurring at the anode. The DO levels around the Fe foam cathode could be adjusted by purging various gases. Specifically, the DO decreased when N 2 was purged into the system, whereas it increased under an air or O 2 atmosphere. As shown in Fig. 2 a, the BrO 3 − reduction got inhibited under the air/O 2 atmosphere, compared to the N 2 atmosphere. The kinetic constants ( k 1 ) for the BrO 3 − reduction were calculated based on the first-order kinetic model. 27 k 1 was calculated to be 0.7796 min − 1 when N 2 was aerated. And it dropped to 0.6171 min − 1 and 0.5309 min − 1 , respectively, when air and oxygen were aerated. The decrease in BrO 3 − reduction as a function of DO indicate that the direct electron transfer mechanism played a pivotal role in the BrO 3 − reduction process and the indirect electron transfer via atomic H* was not the dominant route, when Fe foam cathode was employed. The quenching experiment using TBA as scavenger further confirmed the idea. As exhibited in the Fig. 2 b, the BrO 3 − reduction process was slightly inhibited by the increase in the concentration of TBA from 1.0 mM to 10 mM and 100 mM, indicating the dominant role of direct electron transfer mechanism on the Fe foam cathode. For further evaluating the role of two mechanisms in the BrO 3 − reduction process, the data about TBA quenching experiments were collected from previous literature, where BrO 3 − reduction was carried out utilizing different materials as cathodes ( Table SI-1 ). The ratio of k 1 in the presence of TBA ( k 1TBA ) to that in the TBA absence ( k 1 ) was defined as a descriptor, which was higher when the direct electron transfer was predominant. The data was plotted in Fig. 2 c, revealing the reduction property of BrO 3 − when utilizing different cathode. Obviously, these cathodes can be divided into two distinct types based on their reduction mechanism: Pd-based cathode and transitional metal cathode. The Pd-based cathodes exhibited exceptional electrocatalytic activity for effective conversion of H 2 O or H + into atomic H*, 28 which could readily reduce BrO 3 − . Thus, the indirect reduction ratio of Pd/NLSBC was 80.20%, and that of Pd-RGO/GAC was 83.22%. On the other hand, certain transition metals, such as Cu, Fe, or Ru, typically exist in specific oxidation states, serving as active sites for donating electron through variation in their valence. When these electrodes were employed, BrO 3 − reduction tended to procced via direct electron transfer, which account for 98.41% for Cu/CNT electrode, 68.75% for Ru/CNT and 65.2% for Fe foam (this work). Considering cathode as a negatively charged surface, the closer the negative oxyanions were to it, the stronger the electrostatic repulsion on the ions. 18,19 Therefore, BrO 3 − was difficult to break through the electrostatic repulsion region (ERR) and to reach the solid-liquid interface (Fig. 2 d), which was the region where direct electron transfer and BrO 3 − reduction could occur (reaction region, RR). However, in the atomic H* mediated reduction process, H 2 O and H + can be first transformed into atomic H* (Volmer reaction) on the cathode surface; then the uncharged atomic H* can penetrate the ERR without being influenced by electrostatic interactions, finally reducing BrO 3 − in the bulk solution. Therefore, taking the BrO 3 − distribution into consideration, the capacity of atomic H* generation would be the key parameter of BrO 3 − reduction when employing these negatively charged electrode, such as Pd, Pt, or graphite. In terms of material property, the generated atomic H* species on Pt (or graphite) cathode the preferred to bond with each other for subsequent H 2 evolution (Heyrovsky reaction), while Pd lattice was well-known for its ability to maintain atomic H* in an isolated state. 29 That is, Pd-involved materials exhibited high performance in the BrO 3 − decontamination through an indirect reduction mechanism, whereas Pt cathode showed no impact on the process. Strangely, direct electron transfer at the solid-liquid interface has been identified as the primary mechanism for BrO 3 − electrochemical reduction using transition metal electrodes. Nevertheless, the unsatisfactory performance of the Pt cathode in facilitating direct electron transfer highlighted the need for further investigation into the electrochemical reduction of BrO 3 − on transition metals. Especially, how BrO 3 − overcame the repulsive force in the electrostatic repulsion region and reached the reaction region for subsequent electron-accepting reaction (Fig. 2 e). BrO Electroreduction Mediated by Oxygen Vacancies For better investigating the BrO 3 − electroreduction on the Fe foam, the morphology evolution of iron compounds on the electrode surface was analyzed by XPS and XRD spectra for both the fresh and used Fe foam. As shown in Fig. 3 a, XPS spectra of fresh Fe foam exhibited the signal of Fe 2+ (711.0 eV and 724.2 eV) and Fe 3+ (712.7 eV and 725.9 eV), as well as obvious peaks for Fe(0) (707.4 eV and 720.3 eV). 30–32 Afterward, the Fe 2p spectra of used Fe foam exhibited a notable increase in the peak intensity of Fe 2+ and Fe 3+ , with the complete disappearance of the Fe(0) peak (Fig. 3 a). The result indicated that the surface zero-valent iron may be transformed into oxidized states in the BrO 3 − electroreduction process. The XRD pattern further confirmed the disappearance of zero-valent iron and the appearance of iron compounds ( Fig. S1 ), such as Fe 3 O 4 and FeOOH. Although FeOOH was believed to be the key intermediate for transferring electron from the Fe foam to NO 3 in the NO 3 − electroreduction process 16 , it was not detected in the in-situ Raman analysis of the BrO 3 − electroreduction process (Fig. 3 b). Nevertheless, the real-time monitoring on the Fe foam revealed the signal increase of Fe 3 O 4 microstructure at the peak of 670 cm − 1 as the BrO 3 − electroreduction proceeded. 33,34 The result aligned well with the XPS and XRD analyses, indicating that the surface of the Fe foam electrode underwent a transformation into an Fe 3 O 4 microstructure throughout the electroreduction process, which might have facilitated the direct electron-transfer of electrocatalysis process. It should be noted that these oxides didn’t decrease the electrode conductivity and its reduction capacity, as the in-situ generated Fe 3 O 4 microstructure has electrical conductivity similar to metal. 35 Millot and Yan (1997) documented that Ovs on the Fe 3 O 4 lattice were readily formed under the reduction condition, 36 and served as prime sites for the adsorption and subsequent activation of reactant molecules. 37 Therefore, the vacancy defects of Fe 3 O 4 microstructure on the Fe foam electrode were investigated by EPR and XPS analyses, both prior to and post utilization. As shown in Fig. 3 c, the peak-to-valley intensity difference was about 0.0133 a.u. for fresh Fe foam electrode, while it was increased to 0.0356 a.u. after its use, signifying the proliferation of Ovs on the Fe 3 O 4 surface in the BrO 3 − electroreduction process. XPS results also confirmed the conclusion (Fig. S2) . The O 1s spectra of fresh electrode showed two peaks at 529.7 eV and 531.2 eV, which were attributed to lattice oxygen (O L ) and Ov, respectively. After BrO 3 − electroreduction, the relative peak intensity increased, especially the peak attributed to Ov. This significant enhancement pointed to the formation of a substantial number of Ovs, thus providing robust proof supporting the results obtained through EPR analysis. 38 For better illustrating the role of Ovs in the BrO 3 − electroreduction, DFT calculation was conducted to analyze the Gibbs free energy variation (ΔG) of BrO 3 − reduction on the (111) lattice of Fe 3 O 4 microcrystal with or without Ovs. In the analysis, a hyphen sign stood for the states where two units bonded with each other. In the proposed mechanism of direct electron transfer via Fe 3 O 4 with Ovs, BrO 3 − was firstly adsorbed on Fe 3 O 4 with ΔG of -4.21 eV, forming a ≡ Fe-BrO 3 − adduct ( Fig. 3d 1 ). The negative value of ΔG indicated that the reaction could occur spontaneously. 28 Subsequently, injection of electrons from the Fe 3 O 4 into BrO 3 − lengthened the Br-O band to 1.92 Å ( Fig. S4a ), indicating the foreseeable band breakage and subsequent formation of free BrO 2 − (ΔG, + 0.50 eV). Similar adsorption-deoxygenation reactions also occurred to the intermediates, by which BrO 3 − lost its oxygen atom one by one and finally formed non-toxic Br − . During the process, the step of Br − formation was confirmed as the determining step through the reaction channel, due to its highest positive ΔG among these elementary reactions (ΔG ds1 , + 0.67 eV). 39 Only when the system overcame the uphill energy barrier, the reaction proceeded fluently. More specifically, the determining step became the “neck of the funnel” for kinetics of BrO 3 − reduction on the Fe 3 O 4 microcrystal, determining the kinetic constant for the whole process according to the Arrhenius equation. Subsequently, the mechanism of direct electron transfer on the Fe 3 O 4 microcrystal without Ovs was investigated ( Fig. 3d 2 ). In the system, BrO 3 − reduction underwent similar adsorption-deoxygenation approach on the intact Fe 3 O 4 . However, the substrate adsorption process on Fe 3 O 4 microcrystal exhibited higher ΔG, among which ΔG of BrO 2 − adsorption step was highest (ΔG ds2 , + 2.97 eV). The higher ΔG ds of BrO 3 − detoxification on the intact Fe 3 O 4 microcrystal revealed that the presence of Ovs significantly lessened the energy barrier for direct electron transfer (+ 2.97 eV to + 0.67 eV), which was thermodynamically conducive to the occurrence of the reaction. Especially in the formation step of ≡ Fe-BrO 3 − adduct, neighboring Fe atoms exhibited a profound affinity for BrO 3 − , altering the originally energy-intensive adsorption process into a spontaneous reaction. Furthermore, the BrO 3 − electroreduction via the indirect mechanism was also evaluated ( Text. S1 ). Whether the atomic H* was generated on the Fe 3 O 4 with Ovs or not, ΔG ds in this process was determined to be higher than + 3.46 eV, indicating that the BrO 3 − electroreduction via direct electron transfer was more likely to occur than that via indirect mechanism. In addition, the mechanism of Ovs in this process was further revealed through the density of states (DOS) calculation and charge density difference. We figured out that the position of the d-band center in DOS calculation served as an indicator for the stability and robustness of adsorption bonds, namely, the higher energy level of the d-band center was relative to the stronger binding capacities of the intermediates. 40 Fig. 3e 1 − 2 showed the DOS inserted into the center of the d-band when the Fermi level was set as zero. In this study, the calculated d-band center of Fe 3 O 4 was − 1.428eV in the presence of Ovs, while it decreased to -1.618eV in the absence of Ovs. The higher d-band central energy level reflected the stronger adsorption capacity of BrO 3 − intermediates in the presence of Ovs, which ultimately lead to a more fluent electrochemical reduction. For better elucidation of the electronic property of Ovs, charge density difference was employed for displaying the electronic figuration variation with BrO 3 − adsorption on the Fe 3 O 4 microcrystal with/without Ovs. 41 In the study, the blue color was assigned to regions that lost electrons, while the red color was assigned to regions that gained electrons. As shown in Fig. 3f 1 , a clear trend of electron loss can be observed in the Ov region. The positively charged microenvironment induced by electron loss could promote the electron transfer from the neighboring Fe atoms to BrO 3 − , simultaneously lengthening the Fe-O band and Br-O band (1.89 Å and 1.92 Å, Fig. S4a ). Comparatively, the BrO 3 − chemisorption in the intact Fe 3 O 4 surface induced electron transfer from BrO 3 − to Fe 3 O 4 lattice ( Fig. 3f 2 ), accompanied by the same band shrinking to 1.84 Å and 1.82 Å ( Fig. S4b ). To sum up, the emergence of Ovs in the Fe 3 O 4 lattice created a positively charged microenvironment, activating the neighboring Fe atoms for fluent electron injection into BrO 3 − molecule. Directional BrO Diffusion induced by positively charged microenvironment The oxygen escaping from the Fe 3 O 4 lattice probably increased the exposure of neighboring Fe atoms. The exposed Fe 2+ atom could be in-situ monitored by fluorescence microscope with its complexation with FerroOrange, which could form highly visible fluorescent substance when stimulated by yellow-green light at 542–572 nm ( Fig. S5 ). 42 As shown in Fig. 4a 1 , there was only negligible fluorescence intensity on the electrode surface before the reaction. The dispersed red particles visible in the fluorescence image were the target product. After electrochemical reduction, the fluorescence intensity increased with lots of shiny red granules cross the whole picture, suggesting the in-situ generation of Fe 2+ . These Fe 2+ could be ascribed not only to the Fe 2+ exposure around the Ovs (specific adsorption Fe 2+ , SA-Fe 2+ ), but also to leaching Fe 2+ in solution (non-specific adsorption Fe 2+ , non-SA-Fe 2+ ). For better distinguish SA-Fe 2+ and non-SA-Fe 2+ , the used electrode was immersed in a 100 mM NaCl solution for removing the non-SA-Fe 2+ by exchanging them with Na + . As exhibited in Fig. 4a 3 , the overall fluorescence intensity had decreased, but some small regions still maintained a high level of fluorescence, indicating the in-situ generation of non-SA-Fe 2+ . Na + in solution could not replace SA-Fe 2+ , therefore SA-Fe 2+ also occurred in the process. The exposed Fe 2+ sites were generated at a non-negligible level, accompanied by generation of Ovs under the electroreduction condition. Subsequently, for investigating the diffusion behavior of BrO 3 − on the electrode, two distinct charged environments on the electrode surface were postulated: one was negatively charged devoid of Fe 2+ generation, and the other was positively charged with a constant Fe 2+ concentration ( Fig. S6 ). To assess the reduction of BrO 3 − under these conditions, CFD steady-state simulations were employed with the same initial BrO 3 − concentration ( Fig. 4b 1 − 2 ). The result indicated that the reduction of BrO 3 − proceeded at a significantly faster rate in the presence of Fe 2+ . Based on the first-order kinetic model, the kinetic constant ( k ) of BrO 3 − reduction reaction was calculated to be 0.40 min − 1 in the condition of no-Fe 2+ . Meanwhile, it rose to 0.80 min − 1 in the presence of constant Fe 2+ . The enhancement can be attributed to the microenvironment created by the positive charge of Fe 2+ on the electrode, which facilitated the degradation of BrO 3 − by exerting a directional influence on the BrO 3 − diffusion. To quantify the instantaneous change of BrO 3 − diffusion coefficient in a positive microenvironment, the transient concentration changes of BrO 3 − under the different charge densities were simulated to calculate the diffusion coefficient ( Text. S2 ). As shown in Fig. 4c 1 − 2 , two opposite charged environments were simulated, both of which exhibit a charge density ranging from 0 to 1 C/m 2 . In the negatively charged environment, where Fe 2+ was absent, the highest diffusion coefficient of BrO 3 − was calculated to be 0.0079 m 2 /s. Conversely, in a positively charged environment imparted by a constant concentration of Fe 2+ , the highest diffusion coefficient was up to 0.0387 m 2 /s, which was about 5 times that of the former. Therefore, it was deduced that a positively charged microenvironment in the negatively charged electrode surface could attract BrO 3 − diffusion, while BrO 3 − mitigation towards its surrounding areas were electrostatically repelled. That is, in our case, the positively charged microenvironment induced by Ovs was conducive to the BrO 3 − approaching the negatively charged electrode, and the Fe 2+ site adjacent to vacancy had been determined to be most active site for BrO 3 − electroreduction on the electrode. The regular electrolyte composite Na + , could be electrostatically attracted and adsorbed on to the cathode surface. The existence of adsorbed Na + could hinder the directional diffusion of BrO 3 − toward no-SA Fe 2+ site by establishing its own microelectric field. For better comparing the intensity of their microelectric field and attraction toward BrO 3 − , open circuit potentials were employed by testing the electrostatic influence of equivalent amount of Na + and Fe 2+ on the electrode. The results showed that the open circuit potential remained unchanged in the Na + group (Fig. 4 d), while the Fe 2+ group exhibited a significant increase of open circuit potentials with the gradual addition of Fe 2+ (Fig. 4 e). The open circuit potential represents the potential difference between the working electrode and the reference electrode when the current density through the electrode is zero, indirectly reflecting the influence from soluble ions. 43 Abovementioned results obviously differentiate the electrostatic influence of Na + and Fe 2+ , highlighting the directional migration of negatively charged BrO 3 − toward the positively charged microenvironment around the divalent Fe 2+ sites. In general, when the commercial Fe foam was employed as a cathode, and the in-situ generated Fe 3 O 4 microcrystals play a pivotal role in the BrO 3 − electroreduction process, which could be encompassed by four distinct steps (Fig. 4 f). The first step was the in-situ generation of Ovs on the Fe 3 O 4 microcrystals, wherein the oxidized iron present on the electrode surface readily generated Ovs under reducing conditions. The exposed Fe 2+ atoms adjacent to the Ovs weakened the electrostatic repulsion due to the emergence of the positively charged environment surrounding Ovs, thereby increasing the directional diffusion coefficient of BrO 3 − towards the area by 5 times. The third step was spontaneous adsorption. According to the DFT calculation, the presence of Ovs significantly activated the neighboring Fe 2+ sites, transforming the originally energy-intensive adsorption into a spontaneous process (E ads from − 2.17 eV to -4.21 eV). Afterwards, the adsorbed BrO 3 − was gradually deoxidized by breaking the Br-O bond. BrO 3 − could undergo complete detoxification through sequential deoxygenation, with Ovs assisting throughout the whole process from BrO 3 − to Br − . Compared with ΔG ds of BrO 3 − detoxification on the intact Fe 3 O 4 microcrystal up to 2.97 eV, the ΔG ds was determined to be only + 0.67 eV. The electrodes used in this work were commercially available Fe foam electrodes and had low pH-dependence ( Text. S3 ). Hence, it was capable of efficiently degrading BrO 3 − within the pH range typically required by conventional water treatment systems. Given the low concentration of BrO 3 − in drinking water (1 to 10 mg/L), the industrialization of this work faced challenges associated with the need to process large volumes of water and the potential for low current efficiency. Therefore, an up-flow multistage reactor filled with commercial resin was proposed, which served as a pretreatment measure for absorbing BrO 3 − and producing a concentrated solution which can be up to 10949.06 mg/L ( Text. S4 ). The electrochemical reduction experiments conducted on BrO 3 − solutions at various concentrations indicated a direct correlation between enrichment concentration and current efficiency ( Text. S5 ). Specifically, the current efficiency increases with the enrichment concentration. Notably, at a concentration of 10000 mg/L, the current efficiency attained a remarkable value of 80% which essentially met the requirements of industrial applications. Based on the conclusion, resin adsorption was expected to be employed as a pretreatment step to elevate the BrO 3 − concentration in the influent water. Subsequently, commercial Fe foam electrodes with in-situ Fe 3 O 4 microcrystals featuring Ovs are utilized as cathodes for the electrochemical reduction of BrO 3 − . The coupled technology can effectively handle the hazard of BrO 3 − in drinking water. Conclusion In summary, we presented an in-situ defect engineering for enhancing the oxygen-vacancy-mediated electroreduction. During the electrocatalytic reduction system, microcrystalline Fe 3 O 4 with abundant Ovs was in-situ generated on the Fe foam. CFD simulation results indicated that the positively charged microenvironment formed by the exposed Fe 2+ adjacent to the Ovs weakened electrostatic repulsion of BrO 3 - by negatively charged cathode, thereby facilitating the directional diffusion of BrO 3 - towards the electrode. The diffusion coefficient of BrO 3 - increased almost fivefold from 0.0079 m 2 /s to 0.0387 m 2 /s. Furthermore, the Ovs activated neighboring Fe 2+ atoms into highly active sites, transforming the originally energy-intensive adsorption into a spontaneous process (E ads from − 2.17 eV to 4.21 eV). Afterwards, BrO 3 - underwent complete detoxification through sequential deoxygenation, with Ovs assisting throughout the whole process from BrO 3 - to Br - . In the process, the Ovs also reduced the Gibbs free energy change of the determining step through the reaction channel (ΔG ds from + 2.97 eV to 0.67 eV), making the detoxification process of BrO 3 - easier to occur thermodynamically. Finally, we proposed combining the oxygen-vacancy-mediated electroreduction system with a resin enrichment system to enhance its industrial potential. Taking the up-flow multistage reactor in our study as an example, when the BrO 3 - concentration was enriched to 10000 mg/L, the electrochemical reduction system achieved a remarkable Faradaic current efficiency of up to 80%. Methods Chemicals and Materials All reagents and solvents used in the experiments were commercially available and they were used without further isolation or purification. Sodium bromate (NaBrO 3 , 99.8%) was obtained from Shanghai Yien Chemical Technology Co., Ltd. A-62MP resin was produced by Beijing Kehisi Technology Co., Ltd. All other chemicals used in this study were obtained from Sinopharm Chemical Reagent Co., Ltd. China. Fe foam was purchased from Jin Yihe Co., LTD, China. The MMO electrode (RuO 2 -IrO 2 coating on Ti mesh), graphite plate, and stainless-steel plates (201 type, 304 type, 316 type) were supplied by Beijing Hengli Ti Co., China. Deionized water (> 18.2 MΩ cm) was used for all experiments. All electrodes were ultrasonically washed in ethanol and 0.01M hydrochloric acid solution for 5 minutes to remove surface impurities. Subsequently, it was thoroughly rinsed with deionized water to remove any residual impurities. Experimental Setup and Methods Platinum sheet (Pt) and silver chloride electrode were used as the anode and reference electrode, respectively. Pt, graphite sheet, and Fe foam were employed as the cathodes. The electrochemical reduction of BrO 3 − was conducted using a CHI660E electrochemical workstation. A similar reactor (100 mL) separated by a proton exchange membrane was used for each electroreduction process ( Fig. S7 ). An undivided three-electrode system cell (200 mL) was employed for the coupled electrochemical process (Fig. S8) . Unless otherwise specified, 10 mM Na 2 SO 4 was added as the supporting electrolyte for all electrochemical processes. Solutions with different BrO 3 − concentrations (1 − 10000 mg/L) were prepared by adding different amounts of sodium bromate to deionized water. The effects of different gases (N 2 , O 2 , and Air), different voltage ranges (2.0-3.0V), different initial pH values ( 3 – 9 ), different cathodes (stainless-steel plates of 201 type, 304 type, and 316 type), and different initial BrO 3 − concentrations (1-10000 mg/L) on the reduction effect were studied. Quenching experiments were conducted using tert-butyl alcohol (TBA) as a radical scavenger for the electroreduction processes. For the experiment, an up-flow multistage reactor was used to simulate the electrochemical reduction of real wastewater. The wastewater was passed through an ion exchange column filled with A-62MP resin at a rate of 20 BV/H. Subsequently, 1 M Na 2 SO 4 , NaOH or H 2 SO 4 was used as a backwash solution, reversely passing through the resin column at a rate of 4 BV/H to obtain a high concentration of BrO 3 − for the following electrochemical reduction treatment. BrO 3 − and Br − were analyzed by an ionchromatograph (Eco IC, Metrohm) equipped with an IonPac AS 23 (4 × 250 mm) anion column and an IonPac AG 23 (4 × 250 mm) guard column. Mobile-phase eluent for the IC was 3.2 mM Na 2 CO 3 and 1.0 mM NaHCO 3 solution, and the flow rate was 0.7 mL/min. The electrodes were reacted with FerroOrange fluorescent probe working fluid before being observed with the fluorescence microscope. Yellow-green light at 560-580nm wavelength is used as excitation light. The fluorescence images of the electrodes were observed under a 20x objective. Density functional theory (DFT) analysis In this work, Vienna Ab initio Simulation Package (VASP) software was used to perform all calculations within the DFT framework, we employed projected augmented waves (PAW) pseudopotentials and the Perdew-Burke-Ernzerhof (PBE) functional. A cutoff energy of 400 eV and an energy convergence criterion of 10 − 4 eV were used. A vacuum layer of 20 Å was included along the z-direction to avoid interlayer interference. VASPsol 6.3.0 was utilized for static self-consistent simulations in a solvent environment. All systems containing Fe were treated with spin polarization. The free energy was obtained through G(T) = H(T) - TS = ZPE + ΔU(0→T) - TS, where ZPE represents zero-point energy, S is entropy change, T is temperature, and ΔU(0→T) is the internal energy difference between 0 K and T K. The vaspkit code was used for post-processing of the VASP calculated data, with the Gibbs free energy thermodynamic correction temperature set at 298.15 K. The energy of H + + e − pair was approximately regarded as half of H 2 energy. VESTA was employed for plotting and differential charge calculations, VMD and Mutifun software were used for potential map data processing and visualization, and p4vasp was utilized for density of states (DOS) data processing. Computational fluid dynamics (CFD) simulation In this numerical simulation, the planar model was built based on the typical microstructure of the electrode. The electrocatalytic reduction of bromate was simulated by the physical field of transport of diluted species (TDS). In this module, Navier-Stokes equations were used to describe the mass transfer process under neglected inertia. The flow was incompressible and a non-slipping wall was applied. Three species were applied to probe the transfers and reactions (BrO 3 − , Br − , and Fe 2+ ). The research method was based on steady-state and transient studies with a range of 5 s with a step of 1 s. Declarations Author contributions H.B. 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Unified polarizable electrode models for open and closed circuits: Revisiting the effects of electrode polarization and different circuit conditions on electrode-electrolyte interfaces. JOURNAL OF CHEMICAL PHYSICS 157, doi: 10.1063/5.0093095 (2022). Additional Declarations There is NO Competing Interest. Supplementary Files SInw.doc Cite Share Download PDF Status: Under Review Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4805543","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":334420906,"identity":"6204aed9-bdce-44b9-a6cd-35d44ca3dd45","order_by":0,"name":"Huabin 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05:40:16","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4805543/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4805543/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":62580184,"identity":"1f6e6ce1-4cde-4ffd-8f88-898fcacc483e","added_by":"auto","created_at":"2024-08-16 06:05:04","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":42967,"visible":true,"origin":"","legend":"\u003cp\u003eElectrocatalytic BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e reduction on (a\u003csub\u003e1\u003c/sub\u003e) Pt electrode, (a\u003csub\u003e2\u003c/sub\u003e) graphite electrode, (a\u003csub\u003e3\u003c/sub\u003e) Fe foam electrode and (b\u003csub\u003e1-3\u003c/sub\u003e) corresponding current. ([BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e]\u003csub\u003eInitial\u003c/sub\u003e, 7.8 μM; [Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e], 10 μM; solution pH, 7.0; applied potential, 2.5 V)\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-4805543/v1/e805217aee7e1f76b4fe754a.png"},{"id":62580180,"identity":"6ccd54be-6622-4341-819e-705313dab532","added_by":"auto","created_at":"2024-08-16 06:05:04","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":62069,"visible":true,"origin":"","legend":"\u003cp\u003eElectrocatalytic BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e reduction with (a) different atmosphere conditions (b) different concentration of TBA; (c) Reduction mechanism of various electrodes; And\u0026nbsp;reaction mechanism hypothesis of (d) Pd-based cathode, (e) transition metal cathode. ([BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e]\u003csub\u003eInitial\u003c/sub\u003e, 7.8 μM; [Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e], 10 μM; solution pH, 7.0; applied potential, 2.5 V)\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-4805543/v1/7ca72fdd584f39e267dead1d.png"},{"id":62580181,"identity":"350de5ef-50fd-4bf0-b152-d8d0e22add70","added_by":"auto","created_at":"2024-08-16 06:05:04","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":153364,"visible":true,"origin":"","legend":"\u003cp\u003e(a) XPS spectra of Fe 2p , (b) \u003cem\u003ein-situ\u003c/em\u003e Raman analysis; and (c) EPR spectra for the fresh and used Fe foam cathode; And Gibbs free energy barrier variation for electrochemical reduction of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e on Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (d\u003csub\u003e1\u003c/sub\u003e) with, (d\u003csub\u003e2\u003c/sub\u003e) without Ovs; Density of states calculation of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (e\u003csub\u003e1\u003c/sub\u003e) with, (e\u003csub\u003e2\u003c/sub\u003e) without Ovs; Charge density difference of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e on Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (f\u003csub\u003e1\u003c/sub\u003e) with, (f\u003csub\u003e2\u003c/sub\u003e) without Ovs.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-4805543/v1/b656e7f620006fcd85ee6543.png"},{"id":62580962,"identity":"bb3f7711-c483-4ce4-99d5-edc702dc9632","added_by":"auto","created_at":"2024-08-16 06:13:04","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":142821,"visible":true,"origin":"","legend":"\u003cp\u003eFluorescence image of electrode surface (a\u003csub\u003e1\u003c/sub\u003e) before reaction, (a\u003csub\u003e2\u003c/sub\u003e) after reaction and (a\u003csub\u003e3\u003c/sub\u003e) immersed in 100 mM NaCl solution after reaction; CFD steady-state simulations of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, Br\u003csup\u003e−\u003c/sup\u003e concentrations as a function of simulation time under (b\u003csub\u003e1\u003c/sub\u003e) no-Fe\u003csup\u003e2+\u003c/sup\u003e and (b\u003csub\u003e2\u003c/sub\u003e) constant-Fe\u003csup\u003e2+\u003c/sup\u003e ; CFD transient simulations of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e concentration under different charge densities in the simulated time with (c\u003csub\u003e1\u003c/sub\u003e) no-Fe\u003csup\u003e2+\u003c/sup\u003e and (c\u003csub\u003e2\u003c/sub\u003e) constant-Fe\u003csup\u003e2+\u003c/sup\u003e; Open circuit potential curves of Fe foam electrodes with (d) Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e and (e) FeSO\u003csub\u003e4\u003c/sub\u003e ;(f) Reduction mechanism of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e on Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e with Ovs.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-4805543/v1/c2167b667b26f948915d47d4.png"},{"id":62581584,"identity":"3d24a958-64de-4eea-8b97-3ca5ba3f1cf3","added_by":"auto","created_at":"2024-08-16 06:21:04","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1112362,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4805543/v1/977473f9-6aa4-4f76-b7d7-3ceed7405c6c.pdf"},{"id":62580185,"identity":"d59be494-470b-4ad5-9682-62d09daedabc","added_by":"auto","created_at":"2024-08-16 06:05:04","extension":"doc","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":21335624,"visible":true,"origin":"","legend":"","description":"","filename":"SInw.doc","url":"https://assets-eu.researchsquare.com/files/rs-4805543/v1/a5a53215599e13b27f5721db.doc"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Oxygen-Vacancies-Mediated BrO3- Electroreduction: Positively Charged Microenvironment Enables Directional Diffusion and Spontaneous Adsorption","fulltext":[{"header":"Introduction","content":"\u003cp\u003eOzonation has been regarded as a greener alternative disinfection technology to chlorination. The technology significantly diminishes the formation of chlorinated disinfection byproducts, but it concomitantly induces the production of bromate (BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e) in the bromide-containing (Br\u003csup\u003e-\u003c/sup\u003e) raw water.\u003csup\u003e1,2\u003c/sup\u003e Owing to its carcinogenic potential to humans and living organisms, BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e has been classified as a group 2B carcinogen by International Agency for Research on Cancer, significantly threatening the public health via oral exposure.\u003csup\u003e3,4\u003c/sup\u003e Therefore, it was necessary to explore an efficient drinking water detoxification technology for removing BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eVarious lab-scale technologies were developed for BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e removal recently, including resin adsorption, biological remediation, and catalytic reduction.\u003csup\u003e5-7\u003c/sup\u003e Unfortunately, these technologies faced significant challenges for practical application. For instance, while physical separation could isolate BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e from drinking water, the concentrated residue remained a significant issue.\u003csup\u003e8\u003c/sup\u003e Bioremediation of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e often left behind bacteria and organic matter, necessitating downstream treatment for eliminating potential pathogenic microorganisms.\u003csup\u003e9,10\u003c/sup\u003e Although catalytic reduction could eliminate BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e pollution while minimizing residual by-products,\u003csup\u003e11,12\u003c/sup\u003e the high cost of catalyst synthesis and post-treatment of used electron-donors have restricted its use on an industrial scale. Electrocatalytic reduction could address these issues, while readily converting BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e into harmless Br\u003csup\u003e-\u003c/sup\u003e.\u003csup\u003e13,14\u003c/sup\u003e Especially, electrochemical systems were considered to have significant potential for reducing carbon emissions.\u003c/p\u003e\n\u003cp\u003eIn general, the electrocatalytic reduction of substrates can proceed through both indirect mechanism via atomic hydrogen (atomic H*) and direct electron transfer.\u003csup\u003e15\u003c/sup\u003e However, due to the negative charge on the cathode surface, oxyanions (e.g., BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, and PFAS) were electrostatically repelled from the region near the cathode, thereby restraining their electroreduction conversion.\u003csup\u003e16,17\u003c/sup\u003e Only when the oxyanions or electron carriers penetrated the electrostatic repulsion region, can they drive the electrochemical reduction of these anions.\u003csup\u003e18,19\u003c/sup\u003e In the indirect reduction mechanism, atomic H* was first produced near the cathode, then diffused toward the bulk solution for injecting electron into the substrate. As an uncharged particle, its diffusion was not affected by electrostatic repulsion, significantly improving the indirect reduction efficiency of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e/NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e. Unfortunately, due to its high reactivity, atomic H* was easily annihilated by water matrix.\u003csup\u003e20\u003c/sup\u003e Therefore, assisting charged oxyanions in overcoming electrostatic repulsion and facilitating direct electron transfer on the electrode surface could be an alternative strategy to enhance the electrocatalytic oxyanion detoxification. By employing a pulse strategy that periodically alters the electrode interface charge, depression of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e concentration proximal to the working electrode was avoided, significantly enhancing the urea electrosynthesis from electroreduction of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e and CO\u003csub\u003e2\u003c/sub\u003e.\u003csup\u003e21\u003c/sup\u003e Modification of the cathode surface with a quaternary ammonium surfactant introduced additional positive charges, which weakened electrostatic repulsion and facilitated the diffusion of negatively charged PFAS toward the cathode.\u003csup\u003e17\u003c/sup\u003e As a result, a removal efficiency of up to 99.81% and a defluorination efficiency of 78.67% were achieved at the modified cathode. Similarly, Liu., et. al. pointed out that the \u003cem\u003ein-situ\u003c/em\u003e-generated Fe\u003csup\u003e2+\u003c/sup\u003e could exist as specifically adsorbed cations in the inner Helmholtz plane, further electrostatically attracting the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e around the interface and accelerating the electro-conversion of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e into NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e. Due that the performance improvements were readily achieved in various oxyanion electroreduction, it was desirable to explore the feasibility of enhancing BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e electroreduction when similar strategy was employed.\u003c/p\u003e\n\u003cp\u003eIn this study, a Fe-foam-based electrochemical system was proposed for BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e reduction and the electroreduction mechanism was investigated by quenching experiments. The morphological change in Fe foam during the electroreduction process was investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) and \u003cem\u003ein-situ\u003c/em\u003e Raman spectroscopy, and the density functional theory (DFT) was employed for reaction approach analysis. Then, diffusion behavior of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e was studied by fluorescence microscopy, open circuit potential and computational fluid dynamics (CFD) simulation. Finally, a combined process of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e electroreduction with ion exchange resin pretreatment was proposed for increasing the current efficiency and application potential.\u0026nbsp;\u003c/p\u003e\n\u003ch3\u003eBrO Reduction on Various Cathodes\u003c/h3\u003e\n\u003cp\u003eInitially, we conducted the experiment to investigate the feasibility of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction in the electrochemical single-cell system using various commercial electrodes (1 \u0026times;1 cm), such as Pt, graphite sheet, and Fe foam (\u003cb\u003eFig.\u0026nbsp;1a\u003c/b\u003e\u003csub\u003e\u003cb\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;3\u003c/b\u003e\u003c/sub\u003e),\u003csup\u003e22\u0026ndash;24\u003c/sup\u003e \u003cem\u003ei.e.\u003c/em\u003e, in addition to NaBrO\u003csub\u003e3\u003c/sub\u003e (7.8 \u0026micro;M) as model contaminant and Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e (10 mM) as electrolyte, the reaction system was deoxygenated by purging N\u003csub\u003e2\u003c/sub\u003e into the solution (pH 7.0). As shown in \u003cb\u003eFig.\u0026nbsp;1a\u003c/b\u003e\u003csub\u003e\u003cb\u003e1\u003c/b\u003e\u003c/sub\u003e\u003cb\u003e-\u003c/b\u003e\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e, Pt cathode and graphite cathode exhibited no degradation for BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. Only when the cathode was Fe foam (\u003cb\u003eFig.\u0026nbsp;1a\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e), did 6.6 \u0026micro;M BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e got degraded quickly within 60 min. Meanwhile, Br\u003csup\u003e\u0026minus;\u003c/sup\u003e was produced in the process and reached 1.2 \u0026micro;M within the reaction time of 60 min. The addition of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and the resultant Br\u003csup\u003e\u0026minus;\u003c/sup\u003e concentration equaled the initial concentration of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e (7.8 \u0026micro;M), the bromine balance results indicated that the reduced BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e was all converted into nontoxic Br\u003csup\u003e\u0026minus;\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eNotably, the applied voltage for the 3 systems was 2.5 V, and the current was around 8.0 mA for the whole electrochemical process (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003cb\u003eb\u003c/b\u003e\u003csub\u003e\u003cb\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;3\u003c/b\u003e\u003c/sub\u003e), electric quantity (Q) for Pt cathode was 27807 C, 32643 C for graphite cathode and 28446 C for Fe foam cathode. The results indicated similar amounts of electron transfer on the surface of different electrodes. Especially, Pt, as a palladium homologue, it showed no capacity for the reduction of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, indicating that it lacks capacity for both indirect reduction and direct electron transfer. To study the factors that lead to the great difference in the reduction results, the adsorption energy of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e on different electrode surfaces was estimated by DFT calculation. Interestingly, although the adsorption energy of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e on the graphite cathode was positive, that on the Pt was determined to be -1.18 eV. The negative adsorption energy indicated that the interaction between BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and the Pt lattice was a spontaneous process, with a high likelihood of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e undergoing direct electron transfer. This suggested that there were other hindrances than electron transfer limitation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eGenerally, electrochemical reduction could procced via indirect reduction mediated by atomic hydrogen (H*), and direct electron transfer from cathode to BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. \u003csup\u003e14\u003c/sup\u003e The indirect reduction was achieved via atomic H* produced near the electrode, exchanging electrons with BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e due to its diffusion towards the bulk solution. And the direct electron transfer mechanism was initiated by the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e diffusion from bulk solution to the electrode surface and absorption, then directly transferring the electron from the electrode surface to BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. \u003csup\u003e25\u003c/sup\u003e. To better distinguish the roles of the two mechanisms in the electrocatalytic system where Fe foam was employed as cathode for BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction, quenching experiments were conducted by increasing dissolved oxygen (DO) or adding TBA, owing to their strong reactivity toward atomic H*.\u003csup\u003e15,26\u003c/sup\u003e The experiments were carried out in a two-compartment electrolytic cell, separated by a proton exchange membrane, to eliminate any interference arising from the oxygen evolution reaction occurring at the anode. The DO levels around the Fe foam cathode could be adjusted by purging various gases. Specifically, the DO decreased when N\u003csub\u003e2\u003c/sub\u003e was purged into the system, whereas it increased under an air or O\u003csub\u003e2\u003c/sub\u003e atmosphere. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea, the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction got inhibited under the air/O\u003csub\u003e2\u003c/sub\u003e atmosphere, compared to the N\u003csub\u003e2\u003c/sub\u003e atmosphere. The kinetic constants (\u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e) for the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction were calculated based on the first-order kinetic model.\u003csup\u003e27\u003c/sup\u003e \u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e was calculated to be 0.7796 min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e when N\u003csub\u003e2\u003c/sub\u003e was aerated. And it dropped to 0.6171 min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 0.5309 min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively, when air and oxygen were aerated. The decrease in BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction as a function of DO indicate that the direct electron transfer mechanism played a pivotal role in the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction process and the indirect electron transfer via atomic H* was not the dominant route, when Fe foam cathode was employed.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe quenching experiment using TBA as scavenger further confirmed the idea. As exhibited in the Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb, the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction process was slightly inhibited by the increase in the concentration of TBA from 1.0 mM to 10 mM and 100 mM, indicating the dominant role of direct electron transfer mechanism on the Fe foam cathode. For further evaluating the role of two mechanisms in the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction process, the data about TBA quenching experiments were collected from previous literature, where BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction was carried out utilizing different materials as cathodes (\u003cb\u003eTable SI-1\u003c/b\u003e). The ratio of \u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e in the presence of TBA (\u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003e1TBA\u003c/em\u003e\u003c/sub\u003e) to that in the TBA absence (\u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e) was defined as a descriptor, which was higher when the direct electron transfer was predominant. The data was plotted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec, revealing the reduction property of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e when utilizing different cathode. Obviously, these cathodes can be divided into two distinct types based on their reduction mechanism: Pd-based cathode and transitional metal cathode. The Pd-based cathodes exhibited exceptional electrocatalytic activity for effective conversion of H\u003csub\u003e2\u003c/sub\u003eO or H\u003csup\u003e+\u003c/sup\u003e into atomic H*,\u003csup\u003e28\u003c/sup\u003e which could readily reduce BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. Thus, the indirect reduction ratio of Pd/NLSBC was 80.20%, and that of Pd-RGO/GAC was 83.22%. On the other hand, certain transition metals, such as Cu, Fe, or Ru, typically exist in specific oxidation states, serving as active sites for donating electron through variation in their valence. When these electrodes were employed, BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction tended to procced via direct electron transfer, which account for 98.41% for Cu/CNT electrode, 68.75% for Ru/CNT and 65.2% for Fe foam (this work).\u003c/p\u003e \u003cp\u003eConsidering cathode as a negatively charged surface, the closer the negative oxyanions were to it, the stronger the electrostatic repulsion on the ions.\u003csup\u003e18,19\u003c/sup\u003e Therefore, BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e was difficult to break through the electrostatic repulsion region (ERR) and to reach the solid-liquid interface (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed), which was the region where direct electron transfer and BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction could occur (reaction region, RR). However, in the atomic H* mediated reduction process, H\u003csub\u003e2\u003c/sub\u003eO and H\u003csup\u003e+\u003c/sup\u003e can be first transformed into atomic H* (Volmer reaction) on the cathode surface; then the uncharged atomic H* can penetrate the ERR without being influenced by electrostatic interactions, finally reducing BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e in the bulk solution. Therefore, taking the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e distribution into consideration, the capacity of atomic H* generation would be the key parameter of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction when employing these negatively charged electrode, such as Pd, Pt, or graphite. In terms of material property, the generated atomic H* species on Pt (or graphite) cathode the preferred to bond with each other for subsequent H\u003csub\u003e2\u003c/sub\u003e evolution (Heyrovsky reaction), while Pd lattice was well-known for its ability to maintain atomic H* in an isolated state.\u003csup\u003e29\u003c/sup\u003e That is, Pd-involved materials exhibited high performance in the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e decontamination through an indirect reduction mechanism, whereas Pt cathode showed no impact on the process.\u003c/p\u003e \u003cp\u003eStrangely, direct electron transfer at the solid-liquid interface has been identified as the primary mechanism for BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e electrochemical reduction using transition metal electrodes. Nevertheless, the unsatisfactory performance of the Pt cathode in facilitating direct electron transfer highlighted the need for further investigation into the electrochemical reduction of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e on transition metals. Especially, how BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e overcame the repulsive force in the electrostatic repulsion region and reached the reaction region for subsequent electron-accepting reaction (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee).\u003c/p\u003e\n\u003ch3\u003eBrO Electroreduction Mediated by Oxygen Vacancies\u003c/h3\u003e\n\u003cp\u003eFor better investigating the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e electroreduction on the Fe foam, the morphology evolution of iron compounds on the electrode surface was analyzed by XPS and XRD spectra for both the fresh and used Fe foam. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, XPS spectra of fresh Fe foam exhibited the signal of Fe\u003csup\u003e2+\u003c/sup\u003e (711.0 eV and 724.2 eV) and Fe\u003csup\u003e3+\u003c/sup\u003e (712.7 eV and 725.9 eV), as well as obvious peaks for Fe(0) (707.4 eV and 720.3 eV). \u003csup\u003e30\u0026ndash;32\u003c/sup\u003e Afterward, the Fe 2p spectra of used Fe foam exhibited a notable increase in the peak intensity of Fe\u003csup\u003e2+\u003c/sup\u003e and Fe\u003csup\u003e3+\u003c/sup\u003e, with the complete disappearance of the Fe(0) peak (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). The result indicated that the surface zero-valent iron may be transformed into oxidized states in the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e electroreduction process. The XRD pattern further confirmed the disappearance of zero-valent iron and the appearance of iron compounds (\u003cb\u003eFig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/b\u003e), such as Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and FeOOH. Although FeOOH was believed to be the key intermediate for transferring electron from the Fe foam to NO\u003csub\u003e3\u003c/sub\u003e in the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e electroreduction process \u003csup\u003e16\u003c/sup\u003e, it was not detected in the \u003cem\u003ein-situ\u003c/em\u003e Raman analysis of the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e electroreduction process (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). Nevertheless, the real-time monitoring on the Fe foam revealed the signal increase of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e microstructure at the peak of 670 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e as the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e electroreduction proceeded.\u003csup\u003e33,34\u003c/sup\u003e The result aligned well with the XPS and XRD analyses, indicating that the surface of the Fe foam electrode underwent a transformation into an Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e microstructure throughout the electroreduction process, which might have facilitated the direct electron-transfer of electrocatalysis process. It should be noted that these oxides didn\u0026rsquo;t decrease the electrode conductivity and its reduction capacity, as the \u003cem\u003ein-situ\u003c/em\u003e generated Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e microstructure has electrical conductivity similar to metal.\u003csup\u003e35\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eMillot and Yan (1997) documented that Ovs on the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e lattice were readily formed under the reduction condition,\u003csup\u003e36\u003c/sup\u003e and served as prime sites for the adsorption and subsequent activation of reactant molecules.\u003csup\u003e37\u003c/sup\u003e Therefore, the vacancy defects of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e microstructure on the Fe foam electrode were investigated by EPR and XPS analyses, both prior to and post utilization. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec, the peak-to-valley intensity difference was about 0.0133 a.u. for fresh Fe foam electrode, while it was increased to 0.0356 a.u. after its use, signifying the proliferation of Ovs on the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e surface in the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e electroreduction process. XPS results also confirmed the conclusion \u003cb\u003e(Fig. S2)\u003c/b\u003e. The O 1s spectra of fresh electrode showed two peaks at 529.7 eV and 531.2 eV, which were attributed to lattice oxygen (O\u003csub\u003eL\u003c/sub\u003e) and Ov, respectively. After BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e electroreduction, the relative peak intensity increased, especially the peak attributed to Ov. This significant enhancement pointed to the formation of a substantial number of Ovs, thus providing robust proof supporting the results obtained through EPR analysis.\u003csup\u003e38\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eFor better illustrating the role of Ovs in the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e electroreduction, DFT calculation was conducted to analyze the Gibbs free energy variation (ΔG) of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction on the (111) lattice of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e microcrystal with or without Ovs. In the analysis, a hyphen sign stood for the states where two units bonded with each other. In the proposed mechanism of direct electron transfer via Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e with Ovs, BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e was firstly adsorbed on Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e with ΔG of -4.21 eV, forming a\u0026thinsp;\u0026equiv;\u0026thinsp;Fe-BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e adduct (\u003cb\u003eFig.\u0026nbsp;3d\u003c/b\u003e\u003csub\u003e\u003cb\u003e1\u003c/b\u003e\u003c/sub\u003e). The negative value of ΔG indicated that the reaction could occur spontaneously.\u003csup\u003e28\u003c/sup\u003e Subsequently, injection of electrons from the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e into BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e lengthened the Br-O band to 1.92 \u0026Aring; (\u003cb\u003eFig. S4a\u003c/b\u003e), indicating the foreseeable band breakage and subsequent formation of free BrO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e (ΔG, +\u0026thinsp;0.50 eV). Similar adsorption-deoxygenation reactions also occurred to the intermediates, by which BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e lost its oxygen atom one by one and finally formed non-toxic Br\u003csup\u003e\u0026minus;\u003c/sup\u003e. During the process, the step of Br\u003csup\u003e\u0026minus;\u003c/sup\u003e formation was confirmed as the determining step through the reaction channel, due to its highest positive ΔG among these elementary reactions (ΔG\u003csub\u003eds1\u003c/sub\u003e, +\u0026thinsp;0.67 eV).\u003csup\u003e39\u003c/sup\u003e Only when the system overcame the uphill energy barrier, the reaction proceeded fluently. More specifically, the determining step became the \u0026ldquo;neck of the funnel\u0026rdquo; for kinetics of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction on the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e microcrystal, determining the kinetic constant for the whole process according to the Arrhenius equation.\u003c/p\u003e \u003cp\u003eSubsequently, the mechanism of direct electron transfer on the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e microcrystal without Ovs was investigated (\u003cb\u003eFig.\u0026nbsp;3d\u003c/b\u003e\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e). In the system, BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction underwent similar adsorption-deoxygenation approach on the intact Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e. However, the substrate adsorption process on Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e microcrystal exhibited higher ΔG, among which ΔG of BrO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e adsorption step was highest (ΔG\u003csub\u003eds2\u003c/sub\u003e, +\u0026thinsp;2.97 eV). The higher ΔG\u003csub\u003eds\u003c/sub\u003e of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e detoxification on the intact Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e microcrystal revealed that the presence of Ovs significantly lessened the energy barrier for direct electron transfer (+\u0026thinsp;2.97 eV to +\u0026thinsp;0.67 eV), which was thermodynamically conducive to the occurrence of the reaction. Especially in the formation step of \u0026equiv;\u0026thinsp;Fe-BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e adduct, neighboring Fe atoms exhibited a profound affinity for BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, altering the originally energy-intensive adsorption process into a spontaneous reaction. Furthermore, the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e electroreduction via the indirect mechanism was also evaluated (\u003cb\u003eText. S1\u003c/b\u003e). Whether the atomic H* was generated on the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e with Ovs or not, ΔG\u003csub\u003eds\u003c/sub\u003e in this process was determined to be higher than +\u0026thinsp;3.46 eV, indicating that the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e electroreduction via direct electron transfer was more likely to occur than that via indirect mechanism.\u003c/p\u003e \u003cp\u003eIn addition, the mechanism of Ovs in this process was further revealed through the density of states (DOS) calculation and charge density difference. We figured out that the position of the d-band center in DOS calculation served as an indicator for the stability and robustness of adsorption bonds, namely, the higher energy level of the d-band center was relative to the stronger binding capacities of the intermediates.\u003csup\u003e40\u003c/sup\u003e \u003cb\u003eFig.\u0026nbsp;3e\u003c/b\u003e\u003csub\u003e\u003cb\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;2\u003c/b\u003e\u003c/sub\u003e showed the DOS inserted into the center of the d-band when the Fermi level was set as zero. In this study, the calculated d-band center of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e was \u0026minus;\u0026thinsp;1.428eV in the presence of Ovs, while it decreased to -1.618eV in the absence of Ovs. The higher d-band central energy level reflected the stronger adsorption capacity of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e intermediates in the presence of Ovs, which ultimately lead to a more fluent electrochemical reduction. For better elucidation of the electronic property of Ovs, charge density difference was employed for displaying the electronic figuration variation with BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e adsorption on the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e microcrystal with/without Ovs.\u003csup\u003e41\u003c/sup\u003e In the study, the blue color was assigned to regions that lost electrons, while the red color was assigned to regions that gained electrons. As shown in \u003cb\u003eFig.\u0026nbsp;3f\u003c/b\u003e\u003csub\u003e\u003cb\u003e1\u003c/b\u003e\u003c/sub\u003e, a clear trend of electron loss can be observed in the Ov region. The positively charged microenvironment induced by electron loss could promote the electron transfer from the neighboring Fe atoms to BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, simultaneously lengthening the Fe-O band and Br-O band (1.89 \u0026Aring; and 1.92 \u0026Aring;, \u003cb\u003eFig. S4a\u003c/b\u003e). Comparatively, the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e chemisorption in the intact Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e surface induced electron transfer from BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e to Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e lattice (\u003cb\u003eFig.\u0026nbsp;3f\u003c/b\u003e\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e), accompanied by the same band shrinking to 1.84 \u0026Aring; and 1.82 \u0026Aring; (\u003cb\u003eFig. S4b\u003c/b\u003e). To sum up, the emergence of Ovs in the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e lattice created a positively charged microenvironment, activating the neighboring Fe atoms for fluent electron injection into BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e molecule.\u003c/p\u003e\n\u003ch3\u003eDirectional BrO Diffusion induced by positively charged microenvironment\u003c/h3\u003e\n\u003cp\u003eThe oxygen escaping from the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e lattice probably increased the exposure of neighboring Fe atoms. The exposed Fe\u003csup\u003e2+\u003c/sup\u003e atom could be \u003cem\u003ein-situ\u003c/em\u003e monitored by fluorescence microscope with its complexation with FerroOrange, which could form highly visible fluorescent substance when stimulated by yellow-green light at 542\u0026ndash;572 nm (\u003cb\u003eFig. S5\u003c/b\u003e).\u003csup\u003e42\u003c/sup\u003e As shown in \u003cb\u003eFig.\u0026nbsp;4a\u003c/b\u003e\u003csub\u003e\u003cb\u003e1\u003c/b\u003e\u003c/sub\u003e, there was only negligible fluorescence intensity on the electrode surface before the reaction. The dispersed red particles visible in the fluorescence image were the target product. After electrochemical reduction, the fluorescence intensity increased with lots of shiny red granules cross the whole picture, suggesting the \u003cem\u003ein-situ\u003c/em\u003e generation of Fe\u003csup\u003e2+\u003c/sup\u003e. These Fe\u003csup\u003e2+\u003c/sup\u003e could be ascribed not only to the Fe\u003csup\u003e2+\u003c/sup\u003e exposure around the Ovs (specific adsorption Fe\u003csup\u003e2+\u003c/sup\u003e, SA-Fe\u003csup\u003e2+\u003c/sup\u003e), but also to leaching Fe\u003csup\u003e2+\u003c/sup\u003e in solution (non-specific adsorption Fe\u003csup\u003e2+\u003c/sup\u003e, non-SA-Fe\u003csup\u003e2+\u003c/sup\u003e). For better distinguish SA-Fe\u003csup\u003e2+\u003c/sup\u003e and non-SA-Fe\u003csup\u003e2+\u003c/sup\u003e, the used electrode was immersed in a 100 mM NaCl solution for removing the non-SA-Fe\u003csup\u003e2+\u003c/sup\u003e by exchanging them with Na\u003csup\u003e+\u003c/sup\u003e. As exhibited in \u003cb\u003eFig.\u0026nbsp;4a\u003c/b\u003e\u003csub\u003e\u003cb\u003e3\u003c/b\u003e\u003c/sub\u003e, the overall fluorescence intensity had decreased, but some small regions still maintained a high level of fluorescence, indicating the \u003cem\u003ein-situ\u003c/em\u003e generation of non-SA-Fe\u003csup\u003e2+\u003c/sup\u003e. Na\u003csup\u003e+\u003c/sup\u003e in solution could not replace SA-Fe\u003csup\u003e2+\u003c/sup\u003e, therefore SA-Fe\u003csup\u003e2+\u003c/sup\u003e also occurred in the process. The exposed Fe\u003csup\u003e2+\u003c/sup\u003e sites were generated at a non-negligible level, accompanied by generation of Ovs under the electroreduction condition.\u003c/p\u003e \u003cp\u003eSubsequently, for investigating the diffusion behavior of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e on the electrode, two distinct charged environments on the electrode surface were postulated: one was negatively charged devoid of Fe\u003csup\u003e2+\u003c/sup\u003e generation, and the other was positively charged with a constant Fe\u003csup\u003e2+\u003c/sup\u003e concentration (\u003cb\u003eFig. S6\u003c/b\u003e). To assess the reduction of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e under these conditions, CFD steady-state simulations were employed with the same initial BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e concentration (\u003cb\u003eFig.\u0026nbsp;4b\u003c/b\u003e\u003csub\u003e\u003cb\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;2\u003c/b\u003e\u003c/sub\u003e). The result indicated that the reduction of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e proceeded at a significantly faster rate in the presence of Fe\u003csup\u003e2+\u003c/sup\u003e. Based on the first-order kinetic model, the kinetic constant (\u003cem\u003ek\u003c/em\u003e) of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction reaction was calculated to be 0.40 min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the condition of no-Fe\u003csup\u003e2+\u003c/sup\u003e. Meanwhile, it rose to 0.80 min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the presence of constant Fe\u003csup\u003e2+\u003c/sup\u003e. The enhancement can be attributed to the microenvironment created by the positive charge of Fe\u003csup\u003e2+\u003c/sup\u003e on the electrode, which facilitated the degradation of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e by exerting a directional influence on the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e diffusion. To quantify the instantaneous change of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e diffusion coefficient in a positive microenvironment, the transient concentration changes of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e under the different charge densities were simulated to calculate the diffusion coefficient (\u003cb\u003eText. S2\u003c/b\u003e). As shown in \u003cb\u003eFig.\u0026nbsp;4c\u003c/b\u003e\u003csub\u003e\u003cb\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;2\u003c/b\u003e\u003c/sub\u003e, two opposite charged environments were simulated, both of which exhibit a charge density ranging from 0 to 1 C/m\u003csup\u003e2\u003c/sup\u003e. In the negatively charged environment, where Fe\u003csup\u003e2+\u003c/sup\u003e was absent, the highest diffusion coefficient of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e was calculated to be 0.0079 m\u003csup\u003e2\u003c/sup\u003e/s. Conversely, in a positively charged environment imparted by a constant concentration of Fe\u003csup\u003e2+\u003c/sup\u003e, the highest diffusion coefficient was up to 0.0387 m\u003csup\u003e2\u003c/sup\u003e/s, which was about 5 times that of the former. Therefore, it was deduced that a positively charged microenvironment in the negatively charged electrode surface could attract BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e diffusion, while BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e mitigation towards its surrounding areas were electrostatically repelled. That is, in our case, the positively charged microenvironment induced by Ovs was conducive to the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e approaching the negatively charged electrode, and the Fe\u003csup\u003e2+\u003c/sup\u003e site adjacent to vacancy had been determined to be most active site for BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e electroreduction on the electrode.\u003c/p\u003e \u003cp\u003eThe regular electrolyte composite Na\u003csup\u003e+\u003c/sup\u003e, could be electrostatically attracted and adsorbed on to the cathode surface. The existence of adsorbed Na\u003csup\u003e+\u003c/sup\u003e could hinder the directional diffusion of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e toward no-SA Fe\u003csup\u003e2+\u003c/sup\u003e site by establishing its own microelectric field. For better comparing the intensity of their microelectric field and attraction toward BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, open circuit potentials were employed by testing the electrostatic influence of equivalent amount of Na\u003csup\u003e+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e on the electrode. The results showed that the open circuit potential remained unchanged in the Na\u003csup\u003e+\u003c/sup\u003e group (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed), while the Fe\u003csup\u003e2+\u003c/sup\u003e group exhibited a significant increase of open circuit potentials with the gradual addition of Fe\u003csup\u003e2+\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee). The open circuit potential represents the potential difference between the working electrode and the reference electrode when the current density through the electrode is zero, indirectly reflecting the influence from soluble ions.\u003csup\u003e43\u003c/sup\u003e Abovementioned results obviously differentiate the electrostatic influence of Na\u003csup\u003e+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e, highlighting the directional migration of negatively charged BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e toward the positively charged microenvironment around the divalent Fe\u003csup\u003e2+\u003c/sup\u003e sites.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn general, when the commercial Fe foam was employed as a cathode, and the \u003cem\u003ein-situ\u003c/em\u003e generated Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e microcrystals play a pivotal role in the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e electroreduction process, which could be encompassed by four distinct steps (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef). The first step was the \u003cem\u003ein-situ\u003c/em\u003e generation of Ovs on the Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e microcrystals, wherein the oxidized iron present on the electrode surface readily generated Ovs under reducing conditions. The exposed Fe\u003csup\u003e2+\u003c/sup\u003e atoms adjacent to the Ovs weakened the electrostatic repulsion due to the emergence of the positively charged environment surrounding Ovs, thereby increasing the directional diffusion coefficient of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e towards the area by 5 times. The third step was spontaneous adsorption. According to the DFT calculation, the presence of Ovs significantly activated the neighboring Fe\u003csup\u003e2+\u003c/sup\u003e sites, transforming the originally energy-intensive adsorption into a spontaneous process (E\u003csub\u003eads\u003c/sub\u003e from \u0026minus;\u0026thinsp;2.17 eV to -4.21 eV). Afterwards, the adsorbed BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e was gradually deoxidized by breaking the Br-O bond. BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e could undergo complete detoxification through sequential deoxygenation, with Ovs assisting throughout the whole process from BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e to Br\u003csup\u003e\u0026minus;\u003c/sup\u003e. Compared with ΔG\u003csub\u003eds\u003c/sub\u003e of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e detoxification on the intact Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e microcrystal up to 2.97 eV, the ΔG\u003csub\u003eds\u003c/sub\u003e was determined to be only\u0026thinsp;+\u0026thinsp;0.67 eV.\u003c/p\u003e \u003cp\u003eThe electrodes used in this work were commercially available Fe foam electrodes and had low pH-dependence (\u003cb\u003eText. S3\u003c/b\u003e). Hence, it was capable of efficiently degrading BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e within the pH range typically required by conventional water treatment systems. Given the low concentration of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e in drinking water (1 to 10 mg/L), the industrialization of this work faced challenges associated with the need to process large volumes of water and the potential for low current efficiency. Therefore, an up-flow multistage reactor filled with commercial resin was proposed, which served as a pretreatment measure for absorbing BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and producing a concentrated solution which can be up to 10949.06 mg/L (\u003cb\u003eText. S4\u003c/b\u003e). The electrochemical reduction experiments conducted on BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e solutions at various concentrations indicated a direct correlation between enrichment concentration and current efficiency (\u003cb\u003eText. S5\u003c/b\u003e). Specifically, the current efficiency increases with the enrichment concentration. Notably, at a concentration of 10000 mg/L, the current efficiency attained a remarkable value of 80% which essentially met the requirements of industrial applications. Based on the conclusion, resin adsorption was expected to be employed as a pretreatment step to elevate the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e concentration in the influent water. Subsequently, commercial Fe foam electrodes with \u003cem\u003ein-situ\u003c/em\u003e Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e microcrystals featuring Ovs are utilized as cathodes for the electrochemical reduction of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. The coupled technology can effectively handle the hazard of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e in drinking water.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn summary, we presented an \u003cem\u003ein-situ\u003c/em\u003e defect engineering for enhancing the oxygen-vacancy-mediated electroreduction. During the electrocatalytic reduction system, microcrystalline Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e with abundant Ovs was \u003cem\u003ein-situ\u003c/em\u003e generated on the Fe foam. CFD simulation results indicated that the positively charged microenvironment formed by the exposed Fe\u003csup\u003e2+\u003c/sup\u003e adjacent to the Ovs weakened electrostatic repulsion of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e by negatively charged cathode, thereby facilitating the directional diffusion of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e towards the electrode. The diffusion coefficient of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e increased almost fivefold from 0.0079 m\u003csup\u003e2\u003c/sup\u003e/s to 0.0387 m\u003csup\u003e2\u003c/sup\u003e/s. Furthermore, the Ovs activated neighboring Fe\u003csup\u003e2+\u003c/sup\u003e atoms into highly active sites, transforming the originally energy-intensive adsorption into a spontaneous process (E\u003csub\u003eads\u003c/sub\u003e from \u0026minus;\u0026thinsp;2.17 eV to 4.21 eV). Afterwards, BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e underwent complete detoxification through sequential deoxygenation, with Ovs assisting throughout the whole process from BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e to Br\u003csup\u003e-\u003c/sup\u003e. In the process, the Ovs also reduced the Gibbs free energy change of the determining step through the reaction channel (ΔG\u003csub\u003eds\u003c/sub\u003e from +\u0026thinsp;2.97 eV to 0.67 eV), making the detoxification process of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e easier to occur thermodynamically. Finally, we proposed combining the oxygen-vacancy-mediated electroreduction system with a resin enrichment system to enhance its industrial potential. Taking the up-flow multistage reactor in our study as an example, when the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e concentration was enriched to 10000 mg/L, the electrochemical reduction system achieved a remarkable Faradaic current efficiency of up to 80%.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eChemicals and Materials\u003c/h2\u003e \u003cp\u003eAll reagents and solvents used in the experiments were commercially available and they were used without further isolation or purification. Sodium bromate (NaBrO\u003csub\u003e3\u003c/sub\u003e, 99.8%) was obtained from Shanghai Yien Chemical Technology Co., Ltd. A-62MP resin was produced by Beijing Kehisi Technology Co., Ltd. All other chemicals used in this study were obtained from Sinopharm Chemical Reagent Co., Ltd. China. Fe foam was purchased from Jin Yihe Co., LTD, China. The MMO electrode (RuO\u003csub\u003e2\u003c/sub\u003e-IrO\u003csub\u003e2\u003c/sub\u003e coating on Ti mesh), graphite plate, and stainless-steel plates (201 type, 304 type, 316 type) were supplied by Beijing Hengli Ti Co., China. Deionized water (\u0026gt;\u0026thinsp;18.2 MΩ cm) was used for all experiments. All electrodes were ultrasonically washed in ethanol and 0.01M hydrochloric acid solution for 5 minutes to remove surface impurities. Subsequently, it was thoroughly rinsed with deionized water to remove any residual impurities.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eExperimental Setup and Methods\u003c/h3\u003e\n\u003cp\u003ePlatinum sheet (Pt) and silver chloride electrode were used as the anode and reference electrode, respectively. Pt, graphite sheet, and Fe foam were employed as the cathodes. The electrochemical reduction of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e was conducted using a CHI660E electrochemical workstation. A similar reactor (100 mL) separated by a proton exchange membrane was used for each electroreduction process (\u003cb\u003eFig. S7\u003c/b\u003e). An undivided three-electrode system cell (200 mL) was employed for the coupled electrochemical process \u003cb\u003e(Fig. S8)\u003c/b\u003e. Unless otherwise specified, 10 mM Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e was added as the supporting electrolyte for all electrochemical processes. Solutions with different BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e concentrations (1\u0026thinsp;\u0026minus;\u0026thinsp;10000 mg/L) were prepared by adding different amounts of sodium bromate to deionized water. The effects of different gases (N\u003csub\u003e2\u003c/sub\u003e, O\u003csub\u003e2\u003c/sub\u003e, and Air), different voltage ranges (2.0-3.0V), different initial pH values (\u003cspan additionalcitationids=\"CR4 CR5 CR6 CR7 CR8\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e), different cathodes (stainless-steel plates of 201 type, 304 type, and 316 type), and different initial BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e concentrations (1-10000 mg/L) on the reduction effect were studied. Quenching experiments were conducted using tert-butyl alcohol (TBA) as a radical scavenger for the electroreduction processes.\u003c/p\u003e \u003cp\u003eFor the experiment, an up-flow multistage reactor was used to simulate the electrochemical reduction of real wastewater. The wastewater was passed through an ion exchange column filled with A-62MP resin at a rate of 20 BV/H. Subsequently, 1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, NaOH or H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e was used as a backwash solution, reversely passing through the resin column at a rate of 4 BV/H to obtain a high concentration of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e for the following electrochemical reduction treatment.\u003c/p\u003e \u003cp\u003eBrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and Br\u003csup\u003e\u0026minus;\u003c/sup\u003e were analyzed by an ionchromatograph (Eco IC, Metrohm) equipped with an IonPac AS 23 (4 \u0026times; 250 mm) anion column and an IonPac AG 23 (4 \u0026times; 250 mm) guard column. Mobile-phase eluent for the IC was 3.2 mM Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e and 1.0 mM NaHCO\u003csub\u003e3\u003c/sub\u003e solution, and the flow rate was 0.7 mL/min.\u003c/p\u003e \u003cp\u003eThe electrodes were reacted with FerroOrange fluorescent probe working fluid before being observed with the fluorescence microscope. Yellow-green light at 560-580nm wavelength is used as excitation light. The fluorescence images of the electrodes were observed under a 20x objective.\u003c/p\u003e\n\u003ch3\u003eDensity functional theory (DFT) analysis\u003c/h3\u003e\n\u003cp\u003eIn this work, Vienna Ab initio Simulation Package (VASP) software was used to perform all calculations within the DFT framework, we employed projected augmented waves (PAW) pseudopotentials and the Perdew-Burke-Ernzerhof (PBE) functional. A cutoff energy of 400 eV and an energy convergence criterion of 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e eV were used. A vacuum layer of 20 \u0026Aring; was included along the z-direction to avoid interlayer interference. VASPsol 6.3.0 was utilized for static self-consistent simulations in a solvent environment. All systems containing Fe were treated with spin polarization.\u003c/p\u003e \u003cp\u003eThe free energy was obtained through G(T)\u0026thinsp;=\u0026thinsp;H(T) - TS\u0026thinsp;=\u0026thinsp;ZPE\u0026thinsp;+\u0026thinsp;ΔU(0\u0026rarr;T) - TS, where ZPE represents zero-point energy, S is entropy change, T is temperature, and ΔU(0\u0026rarr;T) is the internal energy difference between 0 K and T K. The vaspkit code was used for post-processing of the VASP calculated data, with the Gibbs free energy thermodynamic correction temperature set at 298.15 K. The energy of H\u003csup\u003e+\u003c/sup\u003e + e\u003csup\u003e\u0026minus;\u003c/sup\u003e pair was approximately regarded as half of H\u003csub\u003e2\u003c/sub\u003e energy.\u003c/p\u003e \u003cp\u003eVESTA was employed for plotting and differential charge calculations, VMD and Mutifun software were used for potential map data processing and visualization, and p4vasp was utilized for density of states (DOS) data processing.\u003c/p\u003e\n\u003ch3\u003eComputational fluid dynamics (CFD) simulation\u003c/h3\u003e\n\u003cp\u003eIn this numerical simulation, the planar model was built based on the typical microstructure of the electrode. The electrocatalytic reduction of bromate was simulated by the physical field of transport of diluted species (TDS). In this module, Navier-Stokes equations were used to describe the mass transfer process under neglected inertia. The flow was incompressible and a non-slipping wall was applied. Three species were applied to probe the transfers and reactions (BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, Br\u003csup\u003e\u0026minus;\u003c/sup\u003e, and Fe\u003csup\u003e2+\u003c/sup\u003e). The research method was based on steady-state and transient studies with a range of 5 s with a step of 1 s.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eH.B. Z., X. Y., G. Z., R. L., S. F., and Y.Y. Z supervised the project. X.B. H. and R.Q. Z. conceived the project and designed the experimental procedures. Y. C. and Z.P. L. designed and completed the first-principles calculations. All authors interpreted the results, elaborated the structure for data presentation and developed the research conclusions.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupplementary information\u0026nbsp;\u003c/strong\u003eis available for this paper\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorrespondence and requests for materials\u0026nbsp;\u003c/strong\u003eshould be addressed to\u0026nbsp;H.B. Z.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eSoltermann, F., Abegglen, C., Tschui, M., Stahel, S. \u0026amp; von Gunten, U. Options and limitations for bromate control during ozonation of wastewater. 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JOURNAL OF CHEMICAL PHYSICS 157, doi:\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1063/5.0093095\u003c/span\u003e\u003cspan address=\"10.1063/5.0093095\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2022).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4805543/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4805543/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eOzonation can disinfect the drinking water without producing chlorinated byproducts, but bromate (BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) emerges as a carcinogenic by-product. Although the electroreduction could theoretically convert BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e into non-toxic bromide (Br\u003csup\u003e\u0026minus;\u003c/sup\u003e), the detoxification process was hindered by the electrostatic repulsion of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e from the negatively charged cathode. In this work, a commercial Fe foam was employed for electrochemically reducing 1.0 mg/L BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e via direct electron transfer mechanism (\u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e, 0.7796 min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), during which microcrystalline magnetite (Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) was \u003cem\u003ein-situ\u003c/em\u003e generated with abundant oxygen vacancies (Ovs). The Ovs could shape neighboring Fe\u003csup\u003e2+\u003c/sup\u003e atoms into positively charged microenvironment for accelerating the directional diffusion of BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e toward themselves. Compared to negatively charged surface, the positive microenvironment could enhance the BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e diffusion with coefficient change from 0.0059 m\u003csup\u003e2\u003c/sup\u003e/s to 0.0387 m\u003csup\u003e2\u003c/sup\u003e/s. Furthermore, Ovs activated the neighboring Fe\u003csup\u003e2+\u003c/sup\u003e atom into a highly active site for BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e adsorption with an adsorption energy (E\u003csub\u003eads\u003c/sub\u003e) of 4.21 eV, in comparison to the energy-demanding adsorption on intact Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e lattice (E\u003csub\u003eads\u003c/sub\u003e, -2.17 eV). Afterwards, BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e underwent complete detoxification through sequential deoxygenation, with Ovs assisting throughout the whole process from BrO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e to Br\u003csup\u003e\u0026minus;\u003c/sup\u003e. By the \u003cem\u003ein-situ\u003c/em\u003e defect engineering, the research pointed out a high-efficient approach to create positively charged microenvironment for enhancing oxyanion electroreduction.\u003c/p\u003e","manuscriptTitle":"Oxygen-Vacancies-Mediated BrO3- Electroreduction: Positively Charged Microenvironment Enables Directional Diffusion and Spontaneous Adsorption","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-08-16 06:04:59","doi":"10.21203/rs.3.rs-4805543/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"c51346e6-120b-407d-9f10-533f8fcf0dd6","owner":[],"postedDate":"August 16th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":35415835,"name":"Earth and environmental sciences/Environmental sciences/Environmental chemistry"},{"id":35415836,"name":"Physical sciences/Chemistry/Surface chemistry"}],"tags":[],"updatedAt":"2024-08-16T06:04:59+00:00","versionOfRecord":[],"versionCreatedAt":"2024-08-16 06:04:59","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4805543","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4805543","identity":"rs-4805543","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}