In situ organic Fenton-like triggered by anodic polymeric intermediates for electrochemical water purification

Dan-Ni Peia,1, Chang Liua,1, Ai-Yong Zhanga,b,2, Xiao-Qiang Pana, and Han-Qing Yua,2

aChinese Academy of Sciences Key Laboratory of Urban Pollutant Conversion, Department of Environmental Science and Engineering, University of Science and Technology of China, 230026 Hefei, China; and bDepartment of Municipal Engineering, School of Civil and Hydraulic Engineering, Hefei University of Technology, 230009 Hefei, China

Edited by Alexis T. Bell, University of California, Berkeley, CA, and approved October 27, 2020 (received for review March 17, 2020) Organic Fenton-like catalysis has been recently developed for wa- can serve as the metal ligand and reductant to enhance ter purification, but -active compounds have to be ex situ transition-metal redox cycling, and also be involved in the envi- added as oxidant activators, causing secondary pollution problem. ronmental geochemistry of natural organic matters (21–30). Electrochemical oxidation is widely used for pollutant degrada- Moreover, quinonelike moieties and persistent organic radicals tion, but suffers from severe electrode fouling caused by high- can directly serve as an organic activator to initiate organic resistance polymeric intermediates. Herein, we develop an in situ Fenton-like catalysis for environmental remediation (31–40). organic Fenton-like catalysis by using the redox-active polymeric Thus, these redox-active anodic polymeric intermediates are intermediates, e.g., benzoquinone, hydroquinone, and quinhy- likely to trigger organic Fenton-like catalysis. drone, generated in electrochemical pollutant oxidation as H2O2 Inspired by above analyses, we constructed and validated activators. By taking phenol as a target pollutant, we demonstrate in situ organic Fenton-like catalysis for electrochemical water that the in situ organic Fenton-like catalysis not only improves purification at low bias before oxygen evolution (Scheme 1). pollutant degradation, but also refreshes working electrode with Phenol, a model chemical widely present in environments, and 1 · a better catalytic stability. Both O2 nonradical and OH radical are other typical halogenated and nonhalogenated aromatic com- generated in the anodic phenol conversion in the in situ organic pounds were selected as target pollutants. felt (CF), a Fenton-like catalysis. Our findings might provide a new opportu- model material with high activity and low cost, and other typical nity to develop a simple, efficient, and cost-effective strategy for dimensionally stable anodes were selected as target electrodes. electrochemical water purification. Reaction systems were named in the form of “EO + ex situ added reagent + cathode,” as their anodes were identical. Pol- in situ organic Fenton-like catalysis | electrochemical water purification | 1 lutant degradation and electrode antifouling performances were redox-active polymeric intermediates | ·OH radical | O nonradical 2 evaluated under various conditions. After the major reactive oxygen species were identified using a suite of testing methods, he efficient generation of reactive oxygen species is essential for pollutant degradation in water purification. The metal- T Significance mediated Fenton catalysis has been widely used for several de- cades owning to its high efficiency, low cost, and easy operation (1). However, it has several technical drawbacks to largely limit Organic Fenton catalysis is one promising water purification further applications, e.g., harsh pH, metal-rich sludge, secondary process, but it needs ex situ added organic activators and pollution, and poor stability (1). Alternatively, the metal-free suffers from secondary pollution. Herein, we develop in situ Fenton catalysis has recently attracted increasing interests. organic Fenton-like catalysis free of secondary pollution by Redox-active compounds serve as the oxidant activator to de- utilizing the redox-active polymeric intermediates generated from compose pollutants via radical and/or nonradical pathways (2–5). electrochemical pollutant degradation. We elucidate its mecha- These pathways depend highly on the atomic and electronic nisms with electrochemical, photochemical, and spectrographic structures and molecular configurations of compounds and their approaches. H2O2 activation mediated by nonhalogenated ar- molecular interactions with oxidants (6–18). So far organic ac- omatic compounds can be regulated from nonradical to radical tivators are ex situ introduced and cause secondary pollution, pathway by anodic potential to refine the catalytic properties although the performance can be largely improved (2–18). Such in the organic Fenton-like catalysis. This work demonstrates an intrinsic drawback greatly restricts its practical applications. the great potential of in situ organic Fenton-like reaction in Thus, in situ organic Fenton-like catalysis without secondary green catalysis and provides a conceptual advance to design pollution is greatly desired for clean and safe water purification. clean, robust, and cost-effective catalytic systems for water Electrochemical oxidation (EO) at low bias is widely used for purification. pollutant degradation owning to its high current efficiency and Author contributions: A.-Y.Z. and H.-Q.Y. designed research; D.-N.P., C.L., and X.-Q.P. low energy consumption, but largely suffers from electrode performed research; H.-Q.Y. contributed new reagents/analytic tools; D.-N.P., C.L., fouling (19, 20). Such fouling is mainly caused by anodic poly- A.-Y.Z., X.-Q.P., and H.-Q.Y. analyzed data; and D.-N.P., C.L., A.-Y.Z., and H.-Q.Y. wrote meric intermediates with large molecular size, low geometric the paper. polarity, and high structural stability, thus anodic oxidation is The authors declare no competing interest. thermodynamically terminated at this stage (19, 20). How to This article is a PNAS Direct Submission. remove polymeric intermediates is essential for electrochemical Published under the PNAS license. water purification. It is interesting to note that anodic polymeric 1D.-N.P. and C.L. contributed equally to this work. = intermediates usually contain quinonelike moieties (C O) and 2To whom correspondence may be addressed. Email: [email protected] or hqyu@ustc. persistent organic radicals, as the electrons in nucleophilic C-OH edu.cn. can be readily transferred to generate C-O· and C = O (19, 20). This article contains supporting information online at https://www.pnas.org/lookup/suppl/ Quinonelike moieties are redox-active because of their high doi:10.1073/pnas.2005035117/-/DCSupplemental. electron density and strong electron-donating properties, thus First published November 23, 2020.

30966–30972 | PNAS | December 8, 2020 | vol. 117 | no. 49 www.pnas.org/cgi/doi/10.1073/pnas.2005035117 Downloaded by guest on September 26, 2021 and accumulation of high-resistance polymeric intermediates (19, 20). Less than 20% of phenol was degraded in the EO-Ti system. The phenol degradation continuously improved with anodic bias, but oxygen evolution began to occur (Scheme 1, Fig. 1 A–F, and SI Appendix, Figs. S1–S3). When 3.0 mM H2O2 was ex situ added to construct the EO/H2O2-Ti system (Scheme 1), the phenol removal was substantially accelerated, which was not caused by the physical adsorption or atmosphere condition or H2O2 itself (Fig. 1 and SI Appendix, Figs. S1–S7). The syn- ergistic effects between EO and H2O2 were bias-dependent and 2+ pH-insensitive compared to the Fe /H2O2 benchmark (Fig. 1 and SI Appendix, Figs. S8–S10). H2O2 dosage played an impor- tant role because of the self-scavenging effect and anodic de- composition (SI Appendix, Figs. S11 and S12) (1). Thus, the ex situ dosed H2O2 was efficient to refine the EO-Ti system for Scheme 1. Scheme diagrams of the EO-Ti, EO/H2O2-Ti, and EO/O2- phenol degradation (Fig. 1 and SI Appendix, Figs. S1–S3). CF systems. Electrode fouling is the main bottleneck of EO system at low bias (19, 20). Electron transfer, hydrogen abstraction, and radical complexion were main pathways to generate polymeric inter- and the potential role of trace transition metals, especially iron mediates on electrode surface and in bulk solution. Severe and copper, was examined, the possible molecular mechanism of electrode fouling and activity decrease were observed in the EO- the in situ organic Fenton-like catalysis was proposed. Ti system, but they were substantially alleviated and even exhibi- ted an improved performance in the EO/H2O2-Ti system (Fig. 2 Results and Discussion and SI Appendix, Figs. S13–S18). For the first, second, and third Nonspecific Feasibility of the In Situ Organic Fenton-like Catalysis. runs of the degradation tests in the EO/H2O2-Ti system, the Aromatic pollutants can be degraded by direct electron transfer Langmuir–Hinshelwood first-order reaction rate constant in- − or surface-bound reactive oxygen, accompanied by the generation creased from ∼0.030 to ∼0.080 min 1, but it decreased from ∼0.080 CHEMISTRY

Fig. 1. Phenol degradation in the EO-Ti, EO/H2O2-Ti, and EO/O2-CF systems at different external bias: +1.1 V (A), +1.2 V (B), +1.3 V (C), +1.4 V (D), +1.5 V (E), −1 and +1.6 V (F). Testing conditions: phenol (100.0 mg·L ), bias (+1.1 ∼ +1.6 V/SCE), H2O2 (3.0 mM), pH (natural, ∼6.5), Na2SO4 (0.1 M), solution volume 2 2 2 (100.0 mL), anode (carbon felt, 10.0 cm ), cathode 1 (Ti, 10.0 cm ), cathode 2 (CF, 10.0 cm ), electrode gap (2.0 cm), reference electrode (SCE), O2 bubbling (10.0 mL min−1), stirring rate (500 rpm), and reaction time (3.0 h).

Pei et al. PNAS | December 8, 2020 | vol. 117 | no. 49 | 30967 Downloaded by guest on September 26, 2021 rings could react with their own quinones to generate quinhy- drone (QHQ) via radical complexion (SI Appendix, Figs. S25 and S26) (41). These intermediates with a plenty of redox-active quinonelike moieties bound strongly onto the CF electrode to deactivate surface reactive sites (Reaction 1) (19, 20). H2O2 could be activated by the anodic intermediates via radical and 1 nonradical pathways to generate reactive species, e.g., O2, ·OH, surface complexes, etc. (Reactions 2–4, Fig. 3 and SI Appendix, Figs. S27–S29) (42–45). In turn, polymeric intermediates were also self-activated to favor further oxidation with reduced energy barriers (Reactions 5 and 6) (19, 20, 42–45). Such an in situ organic Fenton-like catalysis could not only antifoul electrode, but also improve EO performance (Fig. 2 and Scheme 2).

− phenolads − ne → intermediatesads, [1]

intermediatesads 1 + H2O2 → intermediatesads@H2O2 → productsads+ O2, [2]

Fig. 2. Cyclic phenol degradation on the CF electrode in the catalytic sys- intermediatesads tems of EO/H2O2-Ti (A and B) and EO-Ti (C and D). Testing conditions: phenol −1 + H2O2 → intermediatesads@H2O2 → productsads + · OH, (100.0 mg·L ), bias (+1.4 V/SCE), H2O2 (3.0 mM), pH (natural, ∼6.5), Na2SO4 (0.1 M), solution volume (100.0 mL), anode (CF, 10.0 cm2), cathode (Ti, [3] 10.0 cm2), electrode gap (2.0 cm), reference electrode (SCE), stirring rate (500 rpm), and reaction time (3.0 h). intermediatesads + H O → intermediates @H O → products + 2OH−, ∼ −1 2 2 ads 2 2 ads to 0.015 min when no H2O2 was added for the fourth run [4] (Fig. 2B). In comparison, the EO-Ti system exhibited unstable performance (Fig. 2C and SI Appendix,Figs.S13–S15), but it 1 was largely improved when H2O2 was added in the fourth run productsads + · OH/ O2 → ···→ CO2 + H2O, [5] (Fig. 2D). Redox-active intermediates with rich quinonelike moieties − products − ne → ···→ CO + H O. [6] played governing roles in the in situ organic Fenton-like catalysis. ads 2 2 Their generation, accumulation, and regulation from the anodic phenol conversion were of considerable interests. CF was se- lected due to its good anodic activity at low bias. Key reaction parameters, i.e., anodic bias and pH, were finely regulated for phenol conversion and intermediate distribution (Fig. 1 and SI Appendix, Figs. S3, S9, S11, S12, and S19). The significant change of electrode surface chemistry was essential (SI Appendix, Figs. S16–S18). The CF electrode was deactivated quickly with in- creasing oxygen content after phenol degradation; the fouled electrode even exhibited a Fenton-like activity (SI Appendix,Fig. S19C). Electrode fouling and antifouling played a governing role in the in situ organic Fenton-like catalysis (Scheme 2). In situ organic Fenton-like catalysis was also established in the anodic conversion of other typical aromatic pollutants with and without halo substitution or phenol group in molecular structure. Furthermore, the electrochemical treatment of the real waste- water samples containing phenols and the surface water samples containing natural organic matters (SI Appendix, Table S1 and Figs. S20–S23) was also performed. In addition to CF, other electrodes like PbO2, Sb-SnO2, and boron-doped diamond also exhibited similar promoting effects (SI Appendix, Fig. S24). These results demonstrate the substrate- and electrode- nonspecific feasibility of the in situ organic Fenton-like cataly- sis for electrochemical water purification. Scheme 2. Mechanism of the in situ organic Fenton-like catalysis mediated by the anodic polymeric intermediates generated in the electrochemical Substrate-Specific Mechanisms of the In Situ Organic Fenton-like phenol oxidation for water purification. The organic Fenton-like system is Catalysis. A rapid and exhaustive phenol conversion occurred in in situ constructed based on the as-generated redox-active polymeric prod- the EO/H O -Ti system (Fig. 1 and SI Appendix, Figs. S1–S3). ucts from the anodic phenol oxidation and the cathodic H2O2 formation 2 2 from the selective two-electron reduction of dissolved oxygen. Anodic The first step was the anodic conversion to dihydroxylated rings, polymeric intermediates might trigger a dual Fenton-like activation mech- e.g., catechol, resorcinol, and hydroquinone (HQ) (Reaction 1), anism of selective nonradical and nonselective radical pathways for pollut- producing intermediates such as ortho- and parabenzoquinone ant degradation and electrode antifouling in electrochemical water (SI Appendix, Figs. S1, S2, S25, and S26) (41). Dihydroxylated purification.

30968 | www.pnas.org/cgi/doi/10.1073/pnas.2005035117 Pei et al. Downloaded by guest on September 26, 2021 CHEMISTRY

1 1 Fig. 3. ESR and fluorescent tests for O2 and ·OH generation in organic solutions, benchmark systems, and EO/H2O2-Ti in the phenol degradation: TEMP/ O2 1 1 −1 −1 (A), TEMP/ O2/NaN3 (B), TEMP/ O2/D2O(C), DMPO/·OH (D), DMPO/·CH3 (E), and 3-CCA/·OH (F). Testing conditions: phenol (100.0 mg·L ), HQ (100.0 mg·L ), −1 −1 −1 BQ (100.0 mg·L ), HQ+BQ (50.0 + 50.0 mg·L ), H2O2 (3.0 mM), peroxydisulfate (PDS) (3.0 mM), BiOCl (0.5 g·L ), FeCl2 (10.0 mM), TEMP (100.0 mM), NaN3 (10.0 mM), D2O (100.0 mM), DMPO (100.0 mM), DMSO (10.0 mM), 3-CCA (10.0 mM), bias (+1.4 V/SCE), pH (natural, ∼6.5), Na2SO4 (0.1 M, 24.0-h Chelex- pretreated), solution volume (100.0 mL), anode (CF, 10.0 cm2), cathode (Ti, 10.0 cm2), electrode gap (2.0 cm), reference electrode (SCE), stirring rate (500 rpm), and reaction time (0.5 h). Sample with 3-CCA as the ·OH chemical probe was excited from 340 to 400 nm, and the resulting fluorescence was measured from 350 to 550 nm, respectively.

Both radical and nonradical mechanisms were involved in the The ·OH-mediated organic Fenton-like catalysis was demon- phenol conversion in the in situ organic Fenton-like catalysis strated to occur mainly between H2O2 and halogenated quinoid (Fig. 3, Scheme 2, and SI Appendix, Figs. S27–S29). ·OH and compounds, i.e., tetrachloro-1,4-benzoquinone (2–4). No halo- 1 O2 directly affected the degradation of pollutants, and such a genated quinoid compound was produced and only non–halo- decomposition process was substantially inhibited by the added quinoid intermediates were detected (SI Appendix, Figs. S1, S2, 1 CH3OH and NaN3, respectively (SI Appendix, Fig. S30). O2 was and S19). The promotion of anodic system and the complexity of found to be the main reactive oxygen species (Fig. 3). Consider- polymeric intermediates might be responsible for the radical ing molecular structures and good redox-cycling proper- activation of H2O2 (SI Appendix, Scheme S2). Under anodic ties, H2O2 might be activated by the intermediates to generate polarization, the atomic and electronic structures and molecular 1 O2 as the major nonradical pathway in the in situ organic configurations of the non–halo-quinoid intermediates and their Fenton-like catalysis, via the nucleophilic addition, displacement, molecular interactions with H2O2 could be electrochemically and decomposition with the key dioxirane intermediates as the regulated. They became more thermodynamically favorable to SI Appendix rate-limiting step ( , Scheme S1 and Figs. S31 and undergo the nucleophilic addition reaction by H2O2 to form the S32) (8, 9). However, because of the very poor stability and im- reactive intermediates and trigger the homolytic decomposition mediate consumption of the as-generated reactive dioxirane in- reaction on benzene ring to generate ·OH (SI Appendix, Scheme termediate, its molecular structure and aqueous concentration S2) (42–45). The reaction between semiquinone radical and could not be detected with the currently available tandem mass H2O2 is spin-restricted and impossible to produce ·OH under · spectrometry and liquid chromatography. OH was proven as the usual conditions (2). Thus, in the EO/H2O2-Ti system no organic minor reactive species in such a system. The role of trace tran- Fenton reaction of the semiquinone-mediated one-electron re- sition metals (especially iron and copper) in either the acid- duction occurred. Instead, the organic Fenton-like reaction of pretreated CF electrode or the Chelex-pretreated supporting the quinone-mediated nucleophilic addition and homolytic de- electrolyte in the formation of ·OH was excluded by adding the composition took place (SI Appendix, Scheme S2). Fe/Cu chelating agents into the EO/H2O2-Ti system in the rad- Mechanisms of the in situ organic Fenton-like catalysis were ical detection and phenol degradation tests (SI Appendix, Figs. substrate-specific in the pollutant conversion with and without S33–S37 and Table S2). molecular halo substitution. With the nonhalogenated substrates,

Pei et al. PNAS | December 8, 2020 | vol. 117 | no. 49 | 30969 Downloaded by guest on September 26, 2021 1 i.e., phenol, atrazine, and roxarsone, O2 served as the main re- active oxygen species and the nonradical activation of H2O2 was the major pathway (SI Appendix, Figs. S30 and S38–S40). In comparison, with the halogenated substrates, i.e., dichlorophenol, tetrachlorphenoxide, dibromophenol, and diiodophenol, ·OH acted as the main reactive oxygen species and the radical activa- tion of H2O2 was the major pathway (SI Appendix, Figs. S30 and S38–S40). The anodic promoting effect was also substrate-specific for the ·OH generation in the in situ organic Fenton-like catalysis. A medium amount of ·OH was generated in the anodic degra- dation of the nonhalogenated phenol, while only a trace amount of ·OH was formed in the anodic degradation of the nonhaloge- nated atrazine and roxarsone (Fig. 3 and SI Appendix,Figs. S38–S40). Both the radical and nonradical mechanisms in the in situ organic Fenton-like catalysis promoted electrochemical water purification (Fig. 1 and SI Appendix, Figs. S20–S24).

Self-Feeding In Situ Organic Fenton-like Catalysis. Ex situ added H2O2 was rapidly decomposed in the initial 30 min via a non- radical pathway under anodic polarization in the EO/H2O2-Ti system, regardless of the presence of phenol or not (Fig. 4 A and B). Thus, a fully self-feeding EO/H2O2-Ti system was developed by in situ generating H2O2 for sustainable organic Fenton-like catalysis (Scheme 2 and SI Appendix,Figs.S41–S44). The self-feeding EO/ O2-CF system exhibited a superior capacity and a good feasibility as evidenced by a higher phenol removal and a faster degradation rate (Figs. 1 and 3 and SI Appendix, Figs. S45 and S46). A significant two-stage reaction was observed, especially at +1.1 and +1.2 V (Fig. 1 A and B), indicating a sustainable synergistic effect between EO and H2O2 at the final stage, which was mainly attributed to the effective accumulation of H2O2 (Fig. 4C and Scheme 2). The potential-promoted nucleophilic addition reaction by H2O2 to form the reactive dioxirane intermediates and trigger the hemolytic de- composition reaction to generate ·OH occurring on different electrodes with various nonhalogenated aromatic compounds (SI Appendix,Fig.S47A–F). The potential threshold was highly sub- strate- and electrode-dependent, and a high anodic activity of working electrode and the simple molecular structure of the non- halogenated compound could favor ·OH generation (SI Appendix, Fig. S47 G and H). For electrochemical water purification, the process selectivity and efficiency are of considerable interest for the decontami- nation performance and energy consumption (1). Anodic current distribution between pollutant degradation and water decom- position is a direct indicator. The EO/O2-CF system exhibited a larger net current from pollutant degradation, inet, indicating its higher process efficiency and reaction selectivity with lower en- ergy consumption (SI Appendix, Fig. S48). The negative inet in the EO/H2O2-Ti system suggests that the anodic H2O2 decom- position was the dominant reaction in the initial minutes, if its high aqueous concentration was taken into account (Fig. 4). In principle, the great superiority of the EO/O2-CF system in the current distribution could well explain its excellent water puri-

fication and electrode antifouling performances (Fig. 1). Fig. 4. H2O2 decomposition and accumulation properties in the EO/H2O2-Ti The self-feeding EO/O2-CF system exhibited an excellent sta- system with and without phenol addition (A and B) and in the self-feeding · −1 bility with a sustainable promoting effect on the in situ organic EO/O2-CF system (C). Testing conditions: phenol (100.0 and 0.0 mg L ), bias ∼ Fenton-like catalysis (Fig. 5 and SI Appendix, Figs. S49–S51). The (+1.3, +1.4, and +1.5 V/SCE), pH (natural, 6.5), Na2SO4 (0.1 M), solution volume (100.0 mL), anode (CF, 10.0 cm2), cathode 1 (Ti, 10.0 cm2), cathode 2 redox-active HQ, benzoquinone (BQ), and QHQ were mainly 2 (CF, 10.0 cm ), electrode gap (2.0 cm), reference electrode (SCE), O2 bubbling − responsible for the H2O2 activation by serving as an electron (10.0 mL·min 1), stirring rate (500 rpm), and reaction time (3.0 h). mediator, and they were much stronger than phenol for reactive oxygen generation (Fig. 3). Considering their distinct formation, the different reactive oxygen generations could well be explained (SI Appendix,Fig.S52). Furthermore, the guest- and self- CONCLUSIONS modifications of the self-feeding in situ organic Fenton-like ca- Redox-active anodic intermediates from phenol conversion talysis, i.e., nitrogen-doped CF cathode (NCF), exhibited a much were in situ utilized to initiate organic Fenton-like catalysis for higher O2 reduction, H2O2 generation, and phenol removal in the electrochemical water purification. Phenolic contaminants are EO/O2-NCF system (SI Appendix, Figs. S3, S53, and S54). widely present in water environments, but their electrochemical

30970 | www.pnas.org/cgi/doi/10.1073/pnas.2005035117 Pei et al. Downloaded by guest on September 26, 2021 CHEMISTRY

1 Fig. 5. Continuous-wave ESR spectra for TEMP/ O2 and DMPO/·OH generation in the phenol degradation in the different systems: EO-Ti (A and D), EO/H2O2- −1 Ti (B and E), and EO/O2-CF (C and F). Testing conditions: phenol (100.0 mg·L ), H2O2 (3.0 mM), TEMP (100.0 mM), DMPO (100.0 mM), bias (+1.4 V/SCE), pH 2 2 2 (natural, ∼6.5), Na2SO4 (0.1 M, Chelex-pretreated), solution volume (100.0 mL), anode (CF, 10.0 cm ), cathode 1 (Ti, 10.0 cm ), cathode 2 (CF, 10.0 cm ), −1 electrode gap (2.0 cm), reference electrode (SCE), O2 bubbling (10.0 mL·min ), stirring rate (500 rpm), and reaction time (3.0 h).

∼ · −1 oxidation is thermodynamically terminated by the formation of Na2SO4 aqueous solution (0.1 M) containing 50.0 300.0 mg L phenol was polymeric intermediates. In situ organic Fenton-like system could electrolyzed in 120.0-mL electrochemical cell, which was continuously stirred provide a new way to resolve this problem. The synergy among by a magnetic stirrer at 500 rpm and the applied bias was controlled in the ∼ electroassisted adsorption, anodic oxidation, and organic Fenton- range of +1.1 +1.7 V (versus SCE). Solution samples of 2.0 mL were regu- larly collected and rapidly microfiltered for analysis. All degradation tests like endows a high catalytic capacity, low energy consumption, and were carried out in duplicate, and the mean values with SD are presented. good cyclic stability. Considering that quinonelike moieties are the important components of natural organic matters with conjugated Electron Spin Resonance and Fluorescent Tests. Reactive oxygen radicals

molecular networks, there might be a great possibility to stabilize formed by phenol/H2O2, intermediates/H2O2 and reaction solution were EO-generated persistent free radicals for in situ organic Fenton- monitored by electron-spin resonance (ESR) with 5,5-dimethyl-1-pyrroline like catalysis in natural environments. N-oxide (DMPO) and 2,2,6,6-tetramethyl-4-piperidone (TEMP) as spin- trapping agents with and without dimethyl sulfoxide (DMSO) or sodium

Experimental and Methods azide (NaN3) or deuterium oxide (D2O). They were also characterized by

Construction of the EO-Ti, EO/H2O2-Ti, and EO/O2-CF Systems. Three anodic fluorescence with coumarin-3-carboxylic acid (3-CCA) and terephthalic acid systems were established on electrochemical workstation (CHI 760d, Chenhua (TPA) as chemical probes. For the ESR tests, the basic system consisted of · −1 Co.) (Scheme 1). The EO-Ti system was constructed in a cylindrical three- 100.0 mg L organics, 3.0 mM H2O2, 100.0 mM DMPO or TEMP, 10.0 mM electrode single-compartment cell with CF, Ti, and saturated calomel elec- DMSO or NaN3 or 100.0 mM D2O. For the fluorescent tests, 10.0 mM 3-CCA trode (SCE) as working electrode, counterelectrode, and reference electrode, or 3.0 mM TPA in Chelex-treated Na2SO4 aqueous electrolyte (0.1 M, pH respectively. CF electrode was thermally pretreated in 0.1 M HCl aqueous ∼6.5) was used at ambient temperature. ESR spectra were recorded either solution at 100 °C for 2.0 h, then ultrasonically cleaned in methyl , immediately after interacting with H2O2, or at given time intervals on a acetone, and distilled water in sequence. The working and counterelectrodes Bruker spectrometer (A300) with the following settings: center field = 3,512 were both 2.0 × 6.0 cm2 in size, with electrode gap of ∼2.0 cm. No additional G, microwave frequency = 9.86 GHz, and power = 6.36 mW. Fluorescence

external resistance or iR compensation was used. The EO/H2O2-Ti system was detection was performed on a spectrofluorometer (RF-5301PC, Shimadzu constructed by dosing H2O2 into the EO-Ti system, while the EO/O2-CF system Co.). The samples with 3-CCA and TPA were excited from 340 to 400 nm and was established based on the EO-Ti system with two changes: 1) replacing Ti at 312 nm, and the resulting fluorescence was measured from 350 to 550 nm

cathode by CF cathode to in situ generate H2O2 from O2 reduction, and 2) and at 426 nm, respectively. aerating the supporting electrolyte by continuous O2 flow under CF cathode. Analysis. Pollutants were determined by high performance liquid chroma- Phenol Degradation Test. Phenol degradation tests were carried out in the EO- tography (HPLC)y (HPLC-1100, Agilent Inc.) with a Hypersil-ODS reversed-

Ti, EO/H2O2-Ti, and EO/O2-CF systems for comparison. Typically, 100.0 mL phase column and detected at 254 nm using a VWD detector. Mobile

Pei et al. PNAS | December 8, 2020 | vol. 117 | no. 49 | 30971 Downloaded by guest on September 26, 2021 phase was a mixture of water and methanol (30:70) delivered at a flow rate Data Availability. All study data are included in the article and SI Appendix. of 1 mL·min−1. Mineralization was determined from total organic carbon (TOC) (Vario TOC cube, Elementar Co.). Intermediates were identified by gas ACKNOWLEDGMENTS. This work is supported by the National Natural chromatography mass spectrometry (GCT Premier, Waters Inc.) and liquid Science Foundation of China (21590812, 21876040, 22076036, 51538011 chromatography mass spectrometer (LCMS-2010A, Shimadzu Co.). H2O2 was and 51821006), the National Key Research and Development Program of monitored using iodometric titration method. Electrode potential was China (2018YFC0406303), and the International Partnership Program of measured using a commercial multimeter (UT39A, UNIT Inc.). Chinese Academy of Sciences (GJHZ1845).

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