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Mechanism and Process Optimization in the Electrooxidation of Oxalic Acid Using BDD Electrode under Nitric Acid Environment
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Mechanism and Process Optimization in the Electrooxidation of Oxalic Acid Using BDD Electrode under Nitric Acid Environment
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Cite this: ACS Omega 2024, 9, 50, 49839–49848
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https://doi.org/10.1021/acsomega.4c08628
Published December 3, 2024

Copyright © 2024 The Authors. Published by American Chemical Society. This publication is licensed under

CC-BY-NC-ND 4.0 .

Abstract

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Various electrochemical tests were carried out to elucidate the electrolytic oxidation mechanism of oxalic acid on a boron-doped diamond electrode in a nitric acid environment. These included cyclic voltammetry, AC impedance, constant current electrolysis, and electron paramagnetic resonance spectroscopy. The impact of electrode potential, current density, nitric acid concentration, and electrode plate spacing on the oxidation of oxalic acid was investigated. In the electrolysis mechanism, indirect oxidation of· •OH plays a major role and direct oxidation at the electrode plays a minor role. Excessive nitric acid concentration will reduce the electrooxidation rate of oxalic acid. The optimal process conditions for electrolyzing oxalic acid are obtained as follows: the plate spacing is 2 cm, and the current density is 60 mA cm–2. Finally, the BDD electrode can electrolyze the oxalic acid concentration to below 0.001 mol/L, which can meet the process requirements.

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Figure 11 was incomplete in the version that published December 3, 2024. This has been corrected and the revised version re-posted December 4, 2024.

1. Introduction

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During the nuclear fuel reprocessing process, oxalic acid in the plutonium oxalate mother liquor still retains trace amounts of plutonium and oxalic acid. The potent chelating capability of oxalic acid toward plutonium consequently impacts the recovery rate of plutonium in the purification cycle. Hence, the degradation of oxalic acid becomes crucial for achieving an effective purification cycle for plutonium.
In the exploration of oxalic acid degradation, various methods are employed, such as direct chemical reagent destruction, (1) photochemical degradation, (2) hydrogen peroxide oxidation, (3) electrochemical destruction, (4) etc. The electrochemical oxidation of oxalic acid exhibits a rapid decomposition rate, minimal waste generation, safe operation, and thorough destruction. The boron-doped diamond (BDD) electrode is gaining increasing attention for its ability to generate hydroxyl radicals easily, resistance to surface adsorption, wide electrochemical window, and corrosion resistance. (5) This electrode yielded promising results across various wastewater treatment applications. Hence, it can significantly contribute to the post-treatment of oxalic acid in the mother liquid. The remarkable oxidation capability of the BDD electrode relies on hydroxyl radicals, the second strongest oxidant known after fluorine, possessing an exceptionally high standard reduction potential. This property enables the rapid degradation of organic matter. The hydroxyl radical has emerged as a noteworthy player in the degradation of toxic pollutants, owing to its potent oxidizing capabilities and nontoxic properties. Hydroxyl radical (•OH) is characterized as “short-lived and difficult to capture,” and there are two main methods for the detection of •OH. The first is to use electron spin resonance spectroscopy (ESR) in the presence of a spin trap, which is a compound that can selectively react with free radicals to form adduct species that have a lifetime long enough to be detected by ESR. The second approach uses probe compounds that react selectively with •OH and often form adducts that can be analytically detected by conventional chromatographic techniques. ESR is currently the most effective research method for detecting free radicals. It is simple, effective, intuitive, and clear. 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) has become the most commonly used trapping agent in ESR detection of •OH due to its high selectivity for •OH. (6,7) DMPO rapidly reacts with hydroxyl radicals, forming a relatively stable spin addition product, DMPO-OH. The ESR technique can detect the signal of DMPO-OH, generating highly identifiable characteristic spectral lines. The intensities of these signal spectral lines can indirectly indicate the relative production of hydroxyl radicals. However, spin scavengers may lead to false positive detection of •OH, (8−10) which is attributed to reverse spin scavenging, (11,12) or the Forrester–Hepburn mechanism. (9,10) False positive detection of hydroxyl radicals makes it difficult to accurately characterize their concentration. Existing qualitative and quantitative detection methods for hydroxyl radicals have some deficiencies, and their reliability needs to be further improved. Therefore, the ESR method can only qualitatively and semiquantitatively determine hydroxyl radicals.
Mart́ınez-Huitle investigated the electrochemical behavior of oxalic acid on various electrodes. It was observed that the BDD electrode exhibits a very low adsorption capacity and high stability to oxidation on the surface. This characteristic allows reactants and intermediates to undergo reactions in a nonadsorbent state, suggesting that hydroxyl radicals may react with oxalic acid on this electrode. (13) Subsequently, the electro-oxidation reaction of oxalic acid was investigated on the BDD electrode. The Tafel slope was found to be within the range of 200–240 mV decade–1, and the reaction order was slightly less than 0.55. There are also many studies on electro-oxidation on BDD electrodes. For example, some scholars used cyclic voltammetry and bulk electrolysis to study the anodic oxidation of 2-naphthol on a synthetic BDD electrode in acidic media and established a model to explore the electrooxidation mechanism, in which •OH plays an important role. (14) The electrochemical oxidation of iohexol on BDD electrodes has also been studied, and the current efficiency and COD elimination rate are consistent with the theoretical model proposed by Comninellis et al. that assumes mass transfer control. (14−16)
This study marks the initial utilization of the BDD electrode for electrochemical investigations on oxalic acid in a nitric acid medium. ESR spectroscopy was employed to detect the electrogenerated hydroxyl radical on the BDD electrode, and an AC impedance test was carried out; then the process conditions were explored. This study aimed to ascertain the mechanism of the electrooxidation of oxalic acid in a nitric acid medium using a BDD electrode. Through this, the study sought to preliminarily explore the potential application of the BDD electrode in the nuclear fuel reprocessing process.

2. Experimental Section

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2.1. Instruments and Reagents

The electrochemical experiments were conducted using a CHI-760E Electrochemical Workstation and a CHI-1140C Electrochemical Workstation, both from Shanghai Chenhua Instrument Co., Ltd. The analytical balance employed was a GB204 Analytical Balance by METTLER TOLEDO, with an accuracy of 10–4 g. An MS-5000 Spectrometer from Magnettech, Germany, was utilized for electron paramagnetic resonance spectroscopy. The concentration of trace oxalic acid was analyzed using the ICS-5000 ion chromatograph produced by ThermoFisherScientific, USA. Additionally, ultrapure water was obtained using the Direct-Q 3UV Ultra-Pure Water Machine by Millipore Corporation. The chemicals used in the experiments were sourced from reputable suppliers: oxalic acid dihydrate and potassium nitrate were obtained in analytically pure form from Sinopharm Group. Nitric acid with a mass fraction of 65%–68%, classified as analytically pure, was acquired from Sinopharm Group. Concentrated sulfuric acid with a mass fraction of 95%–98%, also analytically pure, was sourced from the Sinophosphate Group. 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO, with a purity of 97%) was procured from Shanghai McLean Reagent and was analytically pure.

2.2. Experimental Methods

The procedure involves quantitatively weighing solid oxalic acid and dissolving it in various nitric acid concentrations to create a mixed solution. Ultrapure water was used in the preparation of the solution. Before use, the BDD electrode was rinsed several times with deionized water. Three electrodes were employed in the experiment: the working electrode, BDD electrode (10 mm × 10 mm), the counter electrode, platinum electrode (10 mm × 10 mm), and the reference electrode is saturated calomel electrode (SCE). Unless stated otherwise, the saturated calomel electrode was utilized as the reference electrode and all electrochemical studies were carried out at room temperature.
The constant-current electrolysis method was implemented by using the CHI-1140C electrochemical workstation to perform the electrolysis of oxalic acid at various current densities. The concentration of H2C2O4 was determined through KMnO4 titration with timed sampling. The electrolytic cell, designed and fabricated in-house, was constructed entirely from an acrylic material. The cell features a groove to accommodate the electrodes.
To minimize DMPO consumption and enhance the •OH concentration in the sample, an electrolytic cell with a plate spacing of 1 cm was employed. A Nafion 117 diaphragm was placed in the middle of the electrolytic cell to prevent interference from materials generated at the cathode. The ESR experimental setup included a working electrode composed of BDD (2 cm × 4 cm), an auxiliary electrode made of graphite (2 cm × 4 cm). And the reference electrode was also a saturated calomel electrode. The BDD and graphite sheet electrodes were partially immersed in the solution, with an immersion area of 2 cm × 3 cm.
Before each experiment, a blank sample was prepared by combining DMPO and the electrolyte in an unpowered state. The mixture, consisting of 6.3 mL of electrolyte, was subjected to ESR testing. This involves using DMPO as a spin trap to capture •OH in the system, adjusting to various electrochemical oxidation reaction conditions.
The measurement parameters of •OH in the X-band were configured as follows: the center field is set at 336.7 mT, the scanning width is 10 mT, the scanning time is 30 s, the microwave frequency is 9.49 GHz, the microwave power is 20 mW, the modulation amplitude is 0.1 mT, and the modulation frequency is 100 kHz.

3. Results and Discussion

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3.1. Electrochemical Response of Oxalic Acid on a BDD Electrode

This experiment utilized a titanium-based boron-doped diamond electrode imported from Beijing BOLTARA Technologies Co., Ltd., Germany. Scanning electron microscopy (SEM) was employed to observe this electrode’s morphology to comprehend its microstructure. Figure 1 illustrates the side and plane views of the BDD electrode.

Figure 1

Figure 1. (a) SEM image of the entire surface of the BDD electrode (scale bar: 500 μm), (b) SEM image of the side view of the BDD electrode (scale bar: 200 μm), (c) SEM image of the local amplification of the BDD electrode side (scale bar: 100 μm), (d) SEM image of the plane of the BDD electrode (scale bar: 200 μm), (e) SEM image of the plane of the BDD electrode (scale bar: 100 μm), (f) SEM image of the plane of the BDD electrode (scale bar: 50 μm), (g) SEM image of the plane of the BDD electrode (scale bar: 5 μm), (h) SEM image of the plane of the BDD electrode (scale bar: 1 μm), and (i) SEM image of the plane of the BDD electrode (scale bar: 500 nm).

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Figure 1a,b show approximately 120-μm-thick boron-doped diamond coatings on both sides of the Ti substrate, displaying high density and uniformity. The magnified image at the junction of Figure 1c further illustrates the tight and compact nature of the coating. In Figure 1 d–i, a flat view is depicted, showcasing a highly dense structure. The presence of “small black pits” on the surface is attributed to spark splashing during laser cutting, leading to surface damage on the BDD electrode.
A cyclic voltammetry curve was generated by varying the oxalic acid concentration, as depicted in Figure 2a, to identify the position of the oxalic acid oxidation peak. It is evident that with the rise in oxalic acid concentration, the increase in the oxidation peak is minimal. A faint oxidation peak is observed at 1.0 V vs SCE, occurring before the onset of the oxygen evolution reaction.

Figure 2

Figure 2. (a) Cyclic voltammograms of 0.20 mol/L HNO3 and different concentrations of H2C2O4. (b) Cyclic voltammograms of 0.020 mol/L H2C2O4 and different concentrations of HNO3.

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In order to investigate the impact of adding nitric acid on the oxidation of oxalic acid, the cyclic voltammetry curve depicting oxalic acid with increasing nitric acid concentration is presented in Figure 2b. As the nitric acid concentration increases, the oxidation current of oxalic acid shows no significant decrease near 1.0 V vs SCE. However, the oxidation decomposition current of water notably decreases near 1.8 V vs SCE. On the BDD electrode, water oxidation and decomposition produce a substantial amount of •OH. (17) The oxidation of organic matter on the BDD electrode relies on the indirect oxidation facilitated by •OH. (18,19) Considering the potent oxidation capacity of •OH, it can oxidize and decompose oxalic acid effectively. Following the reaction H2O → •OH + H+ + e–, an increase in the nitric acid concentration shifts the balance to the left, decreasing the production of •OH. The reduction in current indicates a decrease in the rate of •OH generation. Therefore, the addition of nitric acid is not conducive to the oxidation of oxalic acid at the BDD electrode.

3.2. Study on the Detection and Formation Mechanism of Electrogenic Hydroxyl Radicals on BDD Electrode

3.2.1. Effect of Voltage on the Generation of Hydroxyl Radicals

Until now, limited studies have identified electrically generated •OH by DMPO in electrochemical systems. (20) Research indicates two primary reasons for the challenges encountered in obtaining successful results: (I) Theoretically, to generate •OH, the anode potential should surpass the oxygen evolution potential. However, the direct electron transfer reaction (DET) of DMPO may also inevitably occur on the anode, rendering it unable to capture •OH qualitatively. The oxidation potential of DMPO in aqueous solution is challenging to determine, as only the oxidation potential of DMPO in nonaqueous solution is established at 1.87 V vs SHE; (8) (II) the spin adduction of DMPO-OH is chemically unstable, and subsequent transformations, including dimerization, disproportionation, DET oxidation, and further oxidation, may occur in the strong oxidation environment facilitated by anode polarization. In order to obtain effective spin trapping of •OH and reliable ESR signals of DMPO-OH adducts, side reactions between DMPO and DMPO-OH adducts should be minimized, which can be achieved by adding excessive DMPO during the trapping of electrogenerated •OH. Therefore, all DMPO stock solutions are put into the electrolytic cell, and the amount added is fixed at 50 μL each time.
In the electrolysis process, •OH can be captured using DMPO only when the applied potential is greater than the potential generated by the hydroxyl radical; the standard potential for •OH radical formation is 2.38 V vs SHE. Figure 2 illustrates that the potential of hydroxyl radicals generated during water electrolysis in the presence of 0.20 mol/L HNO3 + 0.020 mol/L H2C2O4 electrolyte on the BDD electrode is 1.8 V vs SCE. The working electrode was the BDD electrode, with a graphite plate as the auxiliary electrode. The electrolyte consisted of 0.20 mol/L HNO3 + 0.020 mol/L H2C2O4 and 50 μL of DMPO. The DMPO stock solution was introduced before initiating electrolysis, and experiments were conducted at varying voltages. The voltage settings for the experiment were 1.6 V vs SCE, 1.8 V vs SCE, 2.0 V vs SCE, 2.2 V vs SCE, and 2.4 V vs SCE, with ESR testing conducted at the fourth minute sampling interval. The outcomes of the experiment are depicted in Figure 3.

Figure 3

Figure 3. ESR signal spectra at different voltages on the BDD electrode.

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In Figure 3, the detected free radical spectrum exhibits a peak height ratio of 1:2:2:1, with a g factor of 2.007 and hyperfine structure parameters of approximately αN = 1.51 mT and αH = 1.51 mT. These parameters closely align with the characteristic peak of •OH, as reported in the literature. (17) Consequently, it is established as the characteristic signal peak of the DMPO-OH adduct.
To facilitate a clearer comparison of the DMPO-OH adduct signal heights at different voltages on the BDD electrode in Figure 3, a curve was generated as depicted in Figure 4. This curve represents the signal value at each voltage when the magnetic field is set at 335.5 mT.

Figure 4

Figure 4. ESR signal values of the BDD electrode at different voltages with a constant magnetic field of 335.5 mT.

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As depicted in Figure 4, when the potential is 1.6 V vs SCE, there is virtually no characteristic signal of DMPO-OH. However, from 1.8 V vs SCE to 2.4 V vs SCE, the signal intensity of DMPO-OH steadily increases in the ESR test. This indicates that the concentration of the DMPO-OH adduct also increases, signifying an elevated production of •OH with increased potential. This observation validates that for •OH to be generated on the surface of the BDD electrode, the anode potential must reach at least +1.8 V vs SCE.

3.2.2. Effect of Nitric Acid Concentrations on the Generation of Hydroxyl Radicals

The concentration of nitric acid plays a crucial role in the electrolytic oxidation of oxalic acid. Consequently, various concentrations of nitric acid were systematically examined here to assess their influence on the production of hydroxyl radicals. The experimental setup involved using the BDD electrode as the working electrode and the graphite sheet as the auxiliary electrode and introducing 0.020 mol/L H2C2O4, 0.20 mol/L HNO3 + 0.020 mol/L H2C2O4, 0.40 mol/L HNO3 + 0.020 mol/L H2C2O4, 1.0 mol/L HNO3 + 0.020 mol/L H2C2O4, 3.0 mol/L HNO3 + 0.020 mol/L H2C2O4, respectively, and 50 μL of DMPO as the electrolyte before initiating the electrolysis. The electrolysis experiments were conducted at a constant current density of 60 mA cm–2. The ESR testing was performed at fourth minute of electrolysis. The outcomes of the experiments are presented in Figure 5.

Figure 5

Figure 5. ESR signal spectra at different nitric acid concentrations on the BDD electrode.

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A curve was generated to facilitate a clear comparison of the DMPO-OH signal height on the BDD electrode under different nitric acid concentrations, as depicted in Figure 6. This curve represents the signal value of each nitric acid concentration when the magnetic field is set at 335.5 mT.

Figure 6

Figure 6. ESR signal values of the BDD electrode at different nitric acid concentrations, measured at 335.5 mT.

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As observed in Figures 5 and 6, the signal intensity of DMPO-OH in 0.020 mol/L H2C2O4 without nitric acid is lower than that in 0.20 mol/L HNO3 + 0.020 mol/L H2C2O4. This suggests that a low nitric acid concentration is favorable for the formation of •OH. Studies have shown that compared with alkaline environments, OH oxidation is stronger under acidic conditions. (21)
With the gradual increase in nitric acid concentration, the signal intensity of DMPO-OH gradually decreases, signifying a gradual reduction in the concentration of •OH. This indicates that the progressive increase in nitric acid concentration inhibits the production of •OH. An excessive amount of H+ shifts the equilibrium of H2O → •OH + H+ + e– to the left, decreasing the amount of •OH produced. It is worth noting that the oxidation of DMPO is also affected by pH, (10,22,23) the pH value also affects the stability of DMPO-OH, so the reasons for such fluctuations in the signal may be caused by many aspects and require further investigation. However, as the acidity gradually increased, the DMPO-OH signal intensity first increased and then decreased, which was consistent with the conclusion presented in Figure 2b, that is, too high of an acidity was not conducive to the electrooxidation of oxalic acid.
Based on the findings in Figure 2a,b and the results of the ESR test, the inferred oxidation mechanism of oxalic acid on the BDD electrode in the nitric acid system is illustrated in Figure 7.

Figure 7

Figure 7. Schematic diagram illustrating the oxidation mechanism of oxalic acid at the BDD electrode.

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The electrooxidation of oxalic acid on the BDD electrode leads to the production of CO2, representing the direct electron transfer step. The indirect oxidation involving •OH is manifested in the generation of •OH through water electrolysis at the BDD electrode, followed by its adsorption on the electrode. An increase in the nitric acid concentration reduces the overall quantity of •OH generated in this reaction. Subsequently, the adsorbed free radical reacts with oxalic acid to produce HCO2, CO2, and H2O. Additionally, HCO2 reacts with the BDD electrode to generate CO2. The reaction of adsorbed •OH with oxalic acid also directly yields CO2 and H2O.

3.3. AC Impedance Characteristics of Oxalic Acid on BDD Electrode in Nitric Acid Medium

The AC impedance test in this experiment was conducted using an electrochemical workstation CHI-760E. A small-amplitude AC positive voltage of 5 mV was the perturbation signal. Moreover, the frequency ranged from 0.01 to 100,000 Hz. The AC impedance of a 0.20 mol/L HNO3 + 0.020 mol/L H2C2O4 solution was measured using the BDD electrodes as working electrodes at the open-circuit voltage.
Figure 8 represents the Nyquist diagram of oxalic acid in a nitric acid medium on the BDD electrode. Typically, AC impedance measurement results are fitted to an equivalent circuit. This circuit represents the physical processes transpiring in the entire electrochemical reaction system, encapsulating the comprehensive electrical characteristics of the electrochemical system. The elements within it can elucidate the properties of the solution. However, a single element in the fitting circuit does not always accurately portray the system’s genuine physical and chemical characteristics. The circuit diagram is illustrated in Figure 9.

Figure 8

Figure 8. Nyquist diagram and fitting diagram for 0.20 mol/L HNO3 + 0.020 mol/L H2C2O4 on the BDD electrode.

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Figure 9

Figure 9. Equivalent circuit diagram corresponding to the Nyquist diagram using 0.20 mol/L HNO3 + 0.020 mol/L H2C2O4 on the BDD electrode.

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In Figure 9, R1 symbolizes the solution resistance in the system, Rct represents the electrochemical reaction charge transfer resistance, Rdm denotes the BDD electrode resistance, Cdm refers to the double-layer capacitance of the BDD electrode, and Cdl signifies the double-layer capacitance of the solution. The fitting results are presented in Table 1, demonstrating close alignment with the actual physical and chemical parameters. The electric double-layer capacitance of the boron-doped diamond electrode is comparable to the values obtained by Yoshimura et al. (24)
Table 1. Simulation Results of the Equivalent Circuit on the BDD Electrode
R1/ΩRct/ΩRdm/ΩCdl/FCdm/F
3.62.631.68.0 × 10–58.2 × 10–5
In the Nyquist diagram, a smaller semicircle diameter and lower Rct correspond to smaller charge transfer resistance for oxalic acid oxidation. Consequently, oxalic acid in a nitric acid medium exhibits a high reaction rate on the BDD electrode. This observation aligns with the mechanism of the BDD electrode oxidizing oxalic acid. The oxidation of oxalic acid on the BDD electrode occurs via two pathways. One is the direct oxidation of oxalic acid on the electrode, playing a secondary role. The other is the oxidation of oxalic acid by hydroxyl radicals generated by the BDD electrode, which plays a primary role.

3.4. Studies on Electrolysis of Oxalic Acid in Nitric Acid Medium

3.4.1. Influence of Current Density on the Electrolysis of Oxalic Acid

The BDD electrode was employed to investigate various factors influencing the electrolysis of oxalic acid in a nitric acid medium. These factors include the current density, initial nitric acid concentration, initial oxalic acid concentration, and different plate spacings. The study aimed to optimize the process conditions for the electrolysis of oxalic acid and provide a preliminary discussion on the electrolytic kinetic characteristics of H2C2O4.
Current density is a crucial factor influencing the electrolytic rate and the operational costs. To examine the impact of oxalic acid electrolysis using the BDD electrode under various current densities, the following settings were applied: 20 mA cm–2, 40 mA cm–2, 60 mA cm–2, and 90 mA cm–2. The electrolytes for all cases were 3.0 mol/L HNO3 + 0.30 mol/L H2C2O4, with other conditions kept constant. The oxalic acid removal rate during electrolysis under different current densities is illustrated in Figure 10. As the current density increases, the destruction rate of oxalic acid gradually rises within the same time frame. Moreover, the higher the current density, the more potent the ability to degrade oxalic acid. At a current density of 20 mA cm–2, the removal of oxalic acid is notably slow. However, at current densities of 60 mA cm–2 and 90 mA cm–2, the removal rates reached 93% and 98%, respectively, after 180 min. Despite the current density of 90 mA cm–2 being 50% higher than that of 60 mA cm–2, the final removal rate of oxalic acid is only 5% higher. This suggests a higher energy loss at a higher current density. Consequently, a current density of 60 mA cm–2 is deemed more suitable as the process condition for subsequent electrolysis.

Figure 10

Figure 10. Effect of Current Densities on the electrolysis of oxalic acid.

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Current density serves as an indicator of the electrochemical reaction rate. Therefore, the reaction kinetics of oxalic acid degradation under different current densities were examined, and the results are depicted in Figure 11. The degradation of oxalic acid by the BDD electrode at different current densities follows a quasi-first-order reaction. The apparent reaction rate constant gradually increases from 0.0035 min–1 at 20 mA cm–2 to 0.0240 min–1 at 90 mA cm–2. The increase in current density leads to higher energy input into the oxalic acid system in the nitric acid medium. Consequently, there is an increase in the total amount of •OH produced through electrolysis on the surface of the BDD electrode. This increase enhances the probability of contact between oxalic acid and •OH, accelerating the reaction rate between •OH and oxalic acid and resulting in a more effective degradation of oxalic acid.

Figure 11

Figure 11. Relation between electrolytic oxalate ln (Ct/C0) and reaction time at different current densities.

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The current efficiency under different current densities is calculated, and the results are presented in Table 2.
Table 2. Current Efficiency of Electrolytic Oxalic Acid at Different Current Densities
Current density/mA cm–2Current efficiency (%)
2048.8
4035.2
6030.5
9021.5
As indicated in Table 2, the current efficiency experiences a gradual decline with the increase of current density.
The first reason for this phenomenon is that under higher current conditions, electrolytic products may not have sufficient time to detach from the electrode surface, and reactants may not diffuse to the electrode promptly, resulting in a decrease in current efficiency. Second, typically at low current densities, there is no noticeable formation of hydroxyl radicals in the electrolytic cell, and the •OH produced primarily reacts with the target for degradation or some intermediate products. However, the surplus •OH does not react with the degradation substance as the current density increases. However, it consumes itself, leading to an oxygen evolution side reaction, (25) as illustrated in eq 1.
2OH•→O2+2H++2e−
(1)
With the escalation of the oxygen evolution side reaction, the proportion of current utilized for the degradation of the target pollutant in the increased current reduces. This decrease results in a reduction in the current efficiency. At a current density of 60 mA cm–2, the current efficiency is only slightly less than 5% lower than that at 40 mA cm–2, but it is 9% higher than the 21.5% observed at 90 mA cm–2. Therefore, a current density of 60 mA cm–2 is more suitable for subsequent electrolysis. Considering the degradation rate, reaction rate, and current efficiency of oxalic acid under different current densities, 60 mA cm–2 was chosen as the suitable current density for the subsequent electrolysis process.

3.4.2. Effect of Nitric Acid Concentration on the Electrolysis of Oxalic Acid

All other conditions remained unchanged at a constant current density of 60 mA cm–2. The impact of different concentrations of nitric acid on the electrolysis of oxalic acid was investigated by using the BDD electrode, and the results are depicted in Figure 12. From the graph, it is evident that there is minimal difference in the curve of oxalic acid destruction rate over time from 0.50 mol/L HNO3 to 3.0 mol/L HNO3. Only the curve of the oxalic acid destruction rate over time at 0.050 mol/L HNO3 is higher than the other three curves after 90 min. Furthermore, the oxalic acid destruction rate exceeds 92% under these four nitric acid concentrations at 180th minute. The curve corresponding to 0.050 mol/L of HNO3 achieves a 98% oxalic acid destruction rate.

Figure 12

Figure 12. Effect of nitric acid concentrations on the electrolysis of oxalic acid.

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Further investigation was conducted into the reaction kinetics of oxalic acid and its degradation under different nitric acid concentrations, and the resulting curve is depicted in Figure 13. Figure 13 reveals that as the concentration of nitric acid in the system increases sequentially from 0.050 to 3.0 mol/L, the corresponding apparent reaction rate constants are 0.0228 min–1, 0.0158 min–1, 0.0156 min–1, and 0.0152 min–1. Consequently, the reaction rate of BDD electrode degradation of oxalic acid in the nitric acid system decreases with the increase of nitric acid concentration. This suggests that an increase in nitric acid concentration hinders the electrolysis of oxalic acid. This observation aligns with the findings from a previous study utilizing cyclic voltammetry on the BDD electrode to investigate the impact of nitric acid on oxalic acid electrolysis. An excess of H+ diminishes the level of production of •OH, leading to a decrease in the reaction rate with oxalic acid. Consequently, increasing nitric acid concentration progressively reduces the BDD electrolytic oxalic acid’s apparent reaction rate constant.

Figure 13

Figure 13. Correlation between ln(Ct/C0) of oxalic acid electrolyzed with different nitric acid concentrations and reaction time.

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3.4.3. Effect of Initial Concentration of Oxalic Acid on the Electrolysis of Oxalic Acid

At ambient temperature, maintaining a constant current density of 60 mA cm–2, electrolysis was conducted in 3.0 mol/L HNO3 solutions with different initial oxalic acid concentrations using BDD electrodes. The results are depicted in Figure 14, revealing variations in the removal rate of oxalic acid with different initial concentrations within the first 90 min, with the destruction rate slowing after 45th minute.

Figure 14

Figure 14. Effect of initial oxalic acid concentrations on the electrolysis of oxalic acid.

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Further investigation was conducted into the reaction kinetics of oxalic acid degradation at different initial concentrations. The resulting curve is depicted in Figure 15. Analysis of Figure 15 reveals that as the initial concentration of oxalic acid in the system increases from 0.050 to 0.30 mol/L, the corresponding apparent reaction rate constants are 0.0222 min–1, 0.0235 min–1, 0.0213 min–1, and 0.0152 min–1. Hence, the reaction rate of oxalic acid degradation in the nitric acid system by the BDD electrode initially increased and then decreased with the increase in initial concentration. This phenomenon could be attributed to the extremely short existence time of •OH in the aqueous phase, lasting approximately 10–9 s. Under specific initial conditions of the electrolytic system, the total amount of •OH produced on the surface of the BDD electrode remains constant for a certain time. When the oxalic acid concentration in the system is low, the probability of •OH molecules coming into contact with oxalic acid molecules is relatively low, within their very short lifetimes. As a result, the reaction rate is limited. However, as the oxalic acid concentration increases slightly, a sufficient amount of •OH becomes available, leading to an increased interaction with oxalic acid and an elevated reaction rate. However, when the concentration of oxalic acid is too high, the total amount of •OH produced on the surface of the BDD electrode becomes significantly lower than that of the oxalic acid. Consequently, there is insufficient •OH to effectively interact with oxalic acid for degradation. In essence, the system’s degradation capability is weakened at high oxalic acid concentrations, leading to a decrease in the reaction rate.

Figure 15

Figure 15. Correlation between electrolytic ln(Ct/C0) of oxalic acid and reaction time at different initial oxalic acid concentrations.

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3.4.4. Effect of Plate Spacing on the Electrolysis of Oxalic Acid

Certainly, the electrode spacing is a crucial parameter in the electrolysis process. Therefore, a BDD electrode was used, and the fixed current density was 60 mA cm–2 at room temperature. Figure 16 displays the curves depicting the changes in the oxalic acid electrolytic destruction rate over time at various plate spacings. Notably, the oxalic acid failure rate is significantly higher for plate spacings of 2 and 3 cm compared to 1 and 4 cm. Additionally, the electrolytic failure rate at a 2 cm plate spacing is superior to that at 3 cm.

Figure 16

Figure 16. Effect of the spacings on the electrolysis of oxalic acid.

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The investigation of the reaction kinetics of oxalic acid degradation at different initial concentrations is further explored, and the resulting curve is depicted in Figure 17. It illustrates that the oxidation degradation of oxalic acid by the BDD electrode adheres to a quasi-first-order reaction under different plate spacings, with varying apparent reaction rate constants. The apparent reaction rate constant peaks at a plate spacing of 2 cm, measuring 0.0208 min–1. In contrast, the apparent reaction rate constants for plate spacings of 1 and 4 cm are consistent, registering at 0.0081 min–1. This suggests that a plate spacing of 2 cm is the most suitable configuration for the electrolysis of oxalic acid. The performance of proton transfer during electrolysis is affected by the spacing between the electrode plates. In general, the smaller the plate spacing of the electrodes, the better the effect achieved by electrolyzing the target substance. However, if the distance between the electrode plates is too close, the treatment effect will be reduced, and in severe cases, the risk of a short circuit between the electrodes may also be caused.

Figure 17

Figure 17. Relationship between the electrolytic ln(Ct/C0) of oxalic acid and reaction time for various plate spacing.

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The most suitable current density and plate spacing for electrolysis were explored in the previous article. In order to explore the extent to which the BDD electrode can destroy oxalic acid, the BDD electrode is used as the working electrode, and the graphite sheet electrode is used as the auxiliary electrode. The submerged area of the two electrodes is consistent with the previous work, and the saturated calomel electrode is used as the reference electrode. The electrolysis current density is set to 60 mA cm–2, the electrode plate spacing is 2 cm, and constant current electrolysis is carried out on 0.040 mol/L KCl + 0.010 mol/L H2C2O4. The oxalic acid concentration in the sample is detected using an ion chromatograph. The results are listed in Table 3.
Table 3. Oxalic Acid Concentration at Different Electrolysis Times
Electrolysis time/minOxalic acid concentration / mol L–1
306.92 × 10–3
601.53 × 10–3
1503.12 × 10–4
It can be seen from Table 3 that the BDD electrode can destroy the oxalic acid concentration to below 0.001 mol/L, which meets the process requirement that the residual oxalic acid concentration in the plutonium oxalate mother liquor is less than 0.001 mol/L.

4. Conclusions

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In order to investigate the feasibility of using electrooxidation to destroy oxalic acid in plutonium oxalate mother liquor, the electrochemical behavior of oxalic acid in a nitric acid medium on the BDD electrode was studied, and combined with ESR method detection of DMPO-OH, the mechanism of electrooxidation of oxalic acid in a nitric acid medium was speculated. It was found that hydroxyl radicals play a major role in oxidizing oxalic acid, and direct oxidation of oxalic acid by the electrode plays a minor role. Moreover, too high of a concentration of nitric acid will reduce the electrooxidation rate of oxalic acid, which is also supported in subsequent electrolysis process optimization research. An AC impedance test was carried out on oxalic acid in a nitric acid medium using a BDD electrode. The electric double-layer capacitance of the BDD electrode was determined to be 8.2 × 10–5 F, and the AC impedance test proved that the BDD electrode electrolyzes oxalic acid quickly. Ion chromatography was used to detect the concentration of trace oxalic acid, and it was found that the BDD electrode could finally destroy the oxalic acid concentration to below 0.001 mol/L, meeting the process requirement. This substantiates the potential application of BDD electrodes in the electrolysis of oxalic acid in the plutonium oxalate mother liquor, offering an efficient means of degrading oxalic acid without the introduction of additional waste products.

Author Information

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  • Corresponding Author
  • Authors
    • Lu Qiao - China Institute of Atomic Energy, Beijing 102413, China;  Orcidhttps://orcid.org/0000-0002-5012-1658
    • Jing Zhao - China Institute of Atomic Energy, Beijing 102413, China
    • Zhijun Cen - China Institute of Atomic Energy, Beijing 102413, China
    • Ting Yu - China Institute of Atomic Energy, Beijing 102413, China
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The paper is sponsored by the National Natural Science Foundation of China (U1867205) and the Special Project on Spent Fuel Reprocessing of the State Administration of Science, Technology and Industry for National Defence (BG222512000403).

References

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    We investigated aq. solns. contg. nitrite ions and DMPO (5,5-dimethyl-1-pyrroline-N-oxide) by ESR in the pH range from 1 to 6. A DMPO-OH signal was obsd. below pH 3.0 in the presence of nitrite ions, whereas in the absence of nitrite ion, an extremely weak signal was obsd. below pH 1.5. Addn. of methanol, a hydroxyl radical scavenger, to this system did not lead to the appearance of a detectable DMPO-CH2OH signal. The possibility of this DMPO-OH signal being due to a genuine spin trapping process with hydroxyl radical was, therefore, ruled out. The reactivities of reactive nitrogen species (RNS) in this system with DMPO have also been investigated by d. functional theory (DFT) at the IEFPCM (water)/B3LYP/6-311+G ** level of theory. On the basis of the pH dependence of the signal intensity and the redox potential E° (vs. SHE) calcd. by DFT theory, we propose that the origin of this signal is "inverted spin trapping" via one-electron oxidn. of DMPO by H2ONO+, followed by the nucleophilic addn. of water. Prevention of these false-pos. results when detecting hydroxyl radical using ESR spin trapping requires an awareness of both the presence of nitrite ions in the soln. and the soln. pH.
  24. 24
    Yoshimura, M.; Honda, K.; Uchikado, R.; Kondo, T.; Rao, T. N.; Tryk, D. A.; Fujishima, A.; Sakamoto, Y.; Yasui, K.; Masuda, H. Electrochemical characterization of nanoporous honeycomb diamond electrodes in non-aqueous electrolytes. Diamond Relat. Mater. 2001, 10 (3–7), 620– 626,  DOI: 10.1016/S0925-9635(00)00381-2
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    Zhou, S.; Bu, L.; Yu, Y.; Zou, X.; Zhang, Y. A comparative study of microcystin-LR degradation by electrogenerated oxidants at BDD and MMO anodes. Chemosphere 2016, 165, 381– 387,  DOI: 10.1016/j.chemosphere.2016.09.057
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    25
    A comparative study of microcystin-LR degradation by electrogenerated oxidants at BDD and MMO anodes
    Zhou, Shiqing; Bu, Lingjun; Yu, Yanghai; Zou, Xu; Zhang, Yansen
    Chemosphere (2016), 165 (), 381-387CODEN: CMSHAF; ISSN:0045-6535. (Elsevier Ltd.)
    This study investigated the electrochem. degrdn. of microcystin-LR (MC-LR) using boron-doped diamond (BDD) anode and mixed metal oxides (MMO, IrO2-Ta2O5/Ti) anode in different medium. In-situ electrogenerated oxidants including hydroxyl radical, active chlorine, and persulfate were confirmed in phosphate, chloride, and sulfate medium, resp. Different from MMO anode, hydroxyl radical was obsd. to play a significant role in chlorine generation at BDD anode in chloride medium. Besides, BDD anode could activate sulfate electrochem. due to its high oxygen evolution potential, and MC-LR degrdn. rate increased with the decrease of soln. pH. The effects of natural org. matters (NOM) and algal org. matters (AOM) on MC-LR degrdn. were evaluated and NOM presented stronger inhibition ability than AOM. Furthermore, the intermediates generated in MC-LR degrdn. in chloride and sulfate medium were identified by LC/MS/MS and possible degrdn. pathways were proposed based on the expts. results. Benzene ring and conjugated diene bonds of Adda group and double bonds of Mhda group were found to be the reactive sites of MC-LR. Overall, this study broadens the knowledge of electrochem. oxidn. in removing microcystins in algae-laden water.

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  • Abstract

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    Figure 1

    Figure 1. (a) SEM image of the entire surface of the BDD electrode (scale bar: 500 μm), (b) SEM image of the side view of the BDD electrode (scale bar: 200 μm), (c) SEM image of the local amplification of the BDD electrode side (scale bar: 100 μm), (d) SEM image of the plane of the BDD electrode (scale bar: 200 μm), (e) SEM image of the plane of the BDD electrode (scale bar: 100 μm), (f) SEM image of the plane of the BDD electrode (scale bar: 50 μm), (g) SEM image of the plane of the BDD electrode (scale bar: 5 μm), (h) SEM image of the plane of the BDD electrode (scale bar: 1 μm), and (i) SEM image of the plane of the BDD electrode (scale bar: 500 nm).

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    Figure 2

    Figure 2. (a) Cyclic voltammograms of 0.20 mol/L HNO3 and different concentrations of H2C2O4. (b) Cyclic voltammograms of 0.020 mol/L H2C2O4 and different concentrations of HNO3.

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    Figure 3

    Figure 3. ESR signal spectra at different voltages on the BDD electrode.

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    Figure 4

    Figure 4. ESR signal values of the BDD electrode at different voltages with a constant magnetic field of 335.5 mT.

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    Figure 5

    Figure 5. ESR signal spectra at different nitric acid concentrations on the BDD electrode.

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    Figure 6

    Figure 6. ESR signal values of the BDD electrode at different nitric acid concentrations, measured at 335.5 mT.

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    Figure 7

    Figure 7. Schematic diagram illustrating the oxidation mechanism of oxalic acid at the BDD electrode.

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    Figure 8

    Figure 8. Nyquist diagram and fitting diagram for 0.20 mol/L HNO3 + 0.020 mol/L H2C2O4 on the BDD electrode.

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    Figure 9

    Figure 9. Equivalent circuit diagram corresponding to the Nyquist diagram using 0.20 mol/L HNO3 + 0.020 mol/L H2C2O4 on the BDD electrode.

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    Figure 10

    Figure 10. Effect of Current Densities on the electrolysis of oxalic acid.

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    Figure 11

    Figure 11. Relation between electrolytic oxalate ln (Ct/C0) and reaction time at different current densities.

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    Figure 12

    Figure 12. Effect of nitric acid concentrations on the electrolysis of oxalic acid.

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    Figure 13

    Figure 13. Correlation between ln(Ct/C0) of oxalic acid electrolyzed with different nitric acid concentrations and reaction time.

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    Figure 14

    Figure 14. Effect of initial oxalic acid concentrations on the electrolysis of oxalic acid.

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    Figure 15

    Figure 15. Correlation between electrolytic ln(Ct/C0) of oxalic acid and reaction time at different initial oxalic acid concentrations.

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    Figure 16

    Figure 16. Effect of the spacings on the electrolysis of oxalic acid.

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    Figure 17

    Figure 17. Relationship between the electrolytic ln(Ct/C0) of oxalic acid and reaction time for various plate spacing.

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  • References

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      Effect of Dissolved Cobalt(II) on the Ozonation of Oxalic Acid
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      Environmental Science and Technology (2002), 36 (19), 4046-4051CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Co2+ was examd. as an ozonation catalyst in lab.-scale batch ozonation expts. run at near-neutral pH and 24°. A OH- probe compd., p-chlorobenzoic acid (pCBA), was also included in the soln. matrix. Batch expts. showed trace amts. of Co2+ accelerated ozonation of oxalate. The rate of oxalate removal increased with decreasing pH from pH 6.7 to pH 5.3. The presence of Co2+ also increased the pCBA removal rate, indicating that OH- generation was a byproduct of Co2+-catalyzed ozonation of oxalate. It was proposed the first step in the catalytic ozonation reaction pathway is formation of a Co(II)-oxalate complex. Co(II)-oxalate is then oxidized by O3 to form cobalt(III)-oxalate. This catalytic cycle is completed with decompn. of the cobalt(III)-complex to form Co2+ and an oxalate radical. Assuming that Co, oxalate, and water were in equil., second-order reaction rate consts. at pH 6 for ozonation of Co(II)-monooxalate and Co(II)-dioxalate species were 30 ± 9 and 4000 ± 500/M-s, resp. These were much greater than the reaction rate const. for ozonation of free oxalate (kO3 ≤ 0.04/M-s).
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      Kim, E. H.; Chung, D. Y.; Kwon S, W.; Yoo, J. H. Photochemical decomposition of oxalate precipitates in nitric acid medium. Korean J. Chem. Eng. 1999, 16 (3), 351– 356,  DOI: 10.1007/BF02707124
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      Sun, J.; Lu, H.; Lin, H.; Du, L.; Huang, W.; Li, H.; Cui, T. Electrochemical oxidation of aqueous phenol at low concentration using Ti/BDD electrode. Sep. Purif. Technol. 2012, 88, 116– 120,  DOI: 10.1016/j.seppur.2011.12.022
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      Electrochemical oxidation of aqueous phenol at low concentration using Ti/BDD electrode
      Sun, Jianrui; Lu, Haiyan; Lin, Haibo; Du, Lili; Huang, Weimin; Li, Hongdong; Cui, Tian
      Separation and Purification Technology (2012), 88 (), 116-120CODEN: SPUTFP; ISSN:1383-5866. (Elsevier B.V.)
      The high quality boron-doped diamond (BDD) film electrodes deposited on titanium (Ti) substrate were prepd. by a microwave plasma-assisted chem. vapor deposition (MP-CVD) method. The products consist of high quality dense grains characterized by Raman spectroscopy and SEM employed. The BDD electrodes have wide electrochem. window (approx. 4.0 V) and low background current (near zero) and the oxidn. peak of phenol appears before the discharge of water, examd. by cyclic voltammetry and linear sweep voltammetry. Compared with the fluorine-doped lead dioxide anodes (Ti/F-PbO2), the Ti/BDD anodes showed higher current efficiency and lower power consumption with degrdn. of low concn. phenol. The exptl. results demonstrated that the BDD electrode on the Ti substrate possesses the excellent electrocatalytic activity in dil. soln.
    6. 6
      Roberts, J. G.; Voinov, M. A.; Schmidt, A. C.; Smirnova, T. I.; Sombers, L. A. The Hydroxyl Radical is a Critical Intermediate in the Voltammetric Detection of Hydrogen Peroxide. J. Am. Chem. Soc. 2016, 138 (8), 2516– 2519,  DOI: 10.1021/jacs.5b13376
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      The Hydroxyl Radical is a Critical Intermediate in the Voltammetric Detection of Hydrogen Peroxide
      Roberts, James G.; Voinov, Maxim A.; Schmidt, Andreas C.; Smirnova, Tatyana I.; Sombers, Leslie A.
      Journal of the American Chemical Society (2016), 138 (8), 2516-2519CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Cyclic voltammetry is a widely used and powerful tool for sensitively and selectively measuring hydrogen peroxide (H2O2). Herein, voltammetry was combined with ESR spectroscopy to identify and define the role of an oxygen-centered radical liberated during the oxidn. of H2O2. The spin-trap reagents, 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and 2-ethoxycarbonyl-2-methyl-3,4-dihydro-2H-pyrrole-1-oxide (EMPO), were employed. Spectra exhibit distinct hyperfine patterns that clearly identify the DMPO•-OH and EMPO•-OH adducts. Multiple linear regression anal. of voltammograms demonstrated that the hydroxyl radical is a principal contributor to the voltammetry of H2O2, as signal is attenuated when this species is trapped. These data incorporate a missing, fundamental element to our knowledge of the mechanisms that underlie H2O2 electrochem.
    7. 7
      Pei, S.; You, S.; Ma, J.; Chen, X.; Ren, N. Electron Spin Resonance Evidence for Electro-generated Hydroxyl Radicals. Environ. Sci. Technol. 2020, 54 (20), 13333– 13343,  DOI: 10.1021/acs.est.0c05287
      7
      Electron Spin Resonance Evidence for Electro-generated Hydroxyl Radicals
      Pei, Shuzhao; You, Shijie; Ma, Jun; Chen, Xiaodong; Ren, Nanqi
      Environmental Science & Technology (2020), 54 (20), 13333-13343CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Electro-generated hydroxyl radicals (•OH) are of fundamental importance to the electrochem. advanced oxidn. process (EAOP). Radical-specific ESR evidence is still lacking in assocn. with the direct electron transfer (DET) reaction of spin trap (e.g., 5,5-dimethyl-1-pyrroline-N-oxide; DMPO) and side reactions of the DMPO-OH adduct in the strongly oxidative environment offered by anodic polarization. Herein, we showed ESR identification of electro-generated •OH in EAOP based on the principle of kinetic selection. Excessive addn. of a DMPO agent and fast spin trapping allowed suitable kinetic conditions to be set for effective spin trapping of electro-generated •OH and subsequent ESR identification. Otherwise, interferential triplet signals would emerge due to formation of paramagnetic dimer via dehydrogenation, DET oxidn., and dimerization reactions of the DMPO-OH adduct. The results demonstrate that •OH formation during spin-trapping on the titanium suboxide (TiSO) anode could be quantified as 47.84 ± 0.44μM at c.d. of 10 mA cm-2. This value revealed a pos. dependence on electrolysis time, c.d., and anode potential. The effectiveness of ESR measurements was verified by the results obtained with the terephthalic acid probe. The ESR identification not only provides direct evidence for electro-generated •OH from a fundamental point of view, but also suggests a strategy to screen effective anode materials.
    8. 8
      Jing, Y.; Chaplin, B. P. Mechanistic study of the validity of using hydroxyl radical probes to characterize electrochemical advanced oxidation processes. Environ. Sci. Technol. 2017, 51 (4), 2355– 2365,  DOI: 10.1021/acs.est.6b05513
      8
      Mechanistic Study of the Validity of Using Hydroxyl Radical Probes To Characterize Electrochemical Advanced Oxidation Processes
      Jing, Yin; Chaplin, Brian P.
      Environmental Science & Technology (2017), 51 (4), 2355-2365CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The detection of hydroxyl radicals (OH•) is typically accomplished by using reactive probe mols., but prior studies have not thoroughly studied the suitability of these probes for use in electrochem. advanced oxidn. processes (EAOPs), due to the neglect of alternative reaction mechanisms. The authors studied the suitability of four OH• probes (coumarin, p-chlorobenzoic acid, terephthalic acid, and p-benzoquinone) for use in EAOPs. Both coumarin and p-chlorobenzoic acid are oxidized via direct electron transfer reactions, while p-benzoquinone and terephthalic acid are not. Coumarin oxidn. to form the OH• adduct product 7-hydroxycoumarin was found at anodic potentials lower than that necessary for OH• formation. D. functional theory (DFT) simulations found a thermodynamically favorable and non-OH• mediated pathway for 7-hydroxycoumarin formation, which is activationless at anodic potentials > 2.10 V/SHE. DFT simulations also provided ests. of E° values for OH• probe compds., which agreed with voltammetry results. Results from this study indicated that terephthalic acid is the most appropriate OH• probe compd. for the characterization of electrochem. and catalytic systems.
    9. 9
      Leinisch, F.; Ranguelova, K.; Derose, E. F.; Jiang, J.; Mason, R. P. Evaluation of the Forrester-Hepburn mechanism as an artifact source in ESR spin-trapping. Chem. Res. Toxicol. 2011, 24 (12), 2217– 2226,  DOI: 10.1021/tx2003323
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      Leinisch, F.; Jiang, J.; Derose, E. F.; Khramtsov, V. V.; Mason, R. P. Investigation of spin-trapping artifacts formed by the Forrester-Hepburn mechanism. Free Radical Biol. Med. 2013, 65, 1497– 1505,  DOI: 10.1016/j.freeradbiomed.2013.07.006
      10
      Investigation of spin-trapping artifacts formed by the Forrester-Hepburn mechanism
      Leinisch, Fabian; Jiang, Jinjie; DeRose, Eugene F.; Khramtsov, Valery V.; Mason, Ronald P.
      Free Radical Biology & Medicine (2013), 65 (), 1497-1505CODEN: FRBMEH; ISSN:0891-5849. (Elsevier B.V.)
      Free radical detection with ESR spin trapping relies on the specific addn. of the radical to nitrone/nitroso compds. It also has been proposed that spin traps can react in biol. systems to give false-pos. results. For nitrone spin traps, the reaction with nucleophiles, first described by Forrester and Hepburn, has been discussed as the most crit. source of artifacts. For artifact identification, the ESR preincubation method may be used, which employs isotopically marked spin traps. Here we investigated the influence of fast sulfite-hydroxylamine equil. chem. on the validity of this assay. Using the (faster) aspiration technique, we found that the Forrester-Hepburn mechanism also contributes to DMPO/•SO3- adduct formation during ferricyanide-mediated sulfite oxidn., but no evidence for artifactual DMPO/•SO3- formation was found if the more potent horseradish peroxidase was used. This is ESR evidence that the Forrester-Hepburn mechanism can occur under mild conditions, depending on the exptl. details. This technique can also be used to test for other artifact mechanisms. We investigated the known ene reaction of DBNBS and tryptophan in more detail. We found that a strong artifact signal is induced by light; however, with atypically long incubations, we found that the artifact is also formed thermally.
    11. 11
      Eberson, L.; Balinov, B.; Hagelin, G.; Dugstad, H.; Thomassen, T.; Forngren, B.; Forngren, T.; Hartvig, P.; Markides, K.; Yngve, U.; Ögren, M. Formation of Hydroxyl Spin Adducts via Nucleophilic Addition Oxidation to 5, 5-DimethyI-1-pyrroline N-Oxide. Acta Chem. Scand. 1999, 53, 584– 593,  DOI: 10.3891/acta.chem.scand.53-0584
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      Cerri, V.; Frejaville, C.; Vila, F.; Allouche, A.; Gronchi, G.; Tordo, P. Synthesis, redox behavior and spin-trap properties of 2, 6-di-tert-butylnitrosobenzene (DTBN). J. Org. Chem. 1989, 54 (6), 1447– 1450,  DOI: 10.1021/jo00267a041
      12
      Synthesis, redox behavior and spin-trap properties of 2,6-di-tert-butylnitrosobenzene (DTBN)
      Cerri, V.; Frejaville, C.; Vila, F.; Allouche, A.; Gronchi, G.; Tordo, P.
      Journal of Organic Chemistry (1989), 54 (6), 1447-50CODEN: JOCEAH; ISSN:0022-3263.
      Oxidn. of 2,6-di-tert-butylaniline yielded the corresponding nitroso compd. (DTBN), whose redox and spin-trap properties were investigated and compared with those of 2,4,6-tri-tert-butylnitrosobenzene (TTBN). DTBN, like TTBN, presented two spin-trapping sites and the regioselectivity of the radical addn. was governed by the bulk of the trapped radical. DTBN's para hydrogen was completely invisible in the ESR spectra of the nitroxide spin adducts while the magnitude of its coupling was about twice that of the meta hydrogens in both the N-alkoxyanilino adducts and the DTBN radical anion. This difference was interpreted in terms of a change from an orthogonal geometry for the former to a planar geometry for the latter.
    13. 13
      Martínez-Huitle, C. A.; Ferro, S.; de Battisti, A. Electrochemical incineration of oxalic acid: Role of electrode material. Electrochim. Acta 2004, 49 (22–23), 4027– 4034,  DOI: 10.1016/j.electacta.2004.01.083
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    14. 14
      Panizza, M.; Michaud, P. A.; Cerisola, G.; Comninellis, C. Anodic oxidation of 2-naphthol at boron-doped diamond electrodes. J. Electroanal. Chem. 2001, 507 (1), 206– 214,  DOI: 10.1016/S0022-0728(01)00398-9
      14
      Anodic oxidation of 2-naphthol at boron-doped diamond electrodes
      Panizza, M.; Michaud, P. A.; Cerisola, G.; Comninellis, C.
      Journal of Electroanalytical Chemistry (2001), 507 (1-2), 206-214CODEN: JECHES ISSN:. (Elsevier Science S.A.)
      The anodic oxidn. of 2-naphthol in acid media was studied at a synthetic boron-doped diamond thin film electrode (BDD) using cyclic voltammetry and bulk electrolysis. The results showed that in the potential region, where the supporting electrolyte is stable, reactions involving simple electron transfer, such as oxidn. of 2-naphthol to naphthoxy radical and 1,4-naphthoquinone occur. Polymeric materials, which lead to electrode fouling, are also formed in this potential region. Electrolysis at high pos. potentials in the region of electrolyte decompn. causes complex oxidn. reactions by electrogenerated hydroxyl radicals leading to the complete incineration of 2-naphthol. Electrode fouling is inhibited under these conditions. The exptl. results were also compared with a theor. model. This model is based on the assumption that the rate of the anodic oxidn. of 2-naphthol is a fast reaction and it is under diffusion control.
    15. 15
      Tissot, G. B.; Anglada, A.; Dimitriou-Christidis, P.; Rossi, L.; Arey, J. S.; Comninellis, C. Kinetic experiments of electrochemical oxidation of iohexol on BDD electrodes for wastewater treatment. Electrochem. Commun. 2012, 23, 48– 51,  DOI: 10.1016/j.elecom.2012.07.006
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    16. 16
      Panizza, M.; Michaud, P. A.; Cerisola, G.; Comninellis, C. Electrochemical treatment of wastewaters containing organic pollutants on boron-doped diamond electrodes: Prediction of specific energy consumption and required electrode area. Electrochem. Commun. 2001, 3 (7), 336– 339,  DOI: 10.1016/S1388-2481(01)00166-7
      16
      Electrochemical treatment of wastewaters containing organic pollutants on boron-doped diamond electrodes: Prediction of specific energy consumption and required electrode area
      Panizza, M.; Michaud, P. A.; Cerisola, G.; Comninellis, Ch
      Electrochemistry Communications (2001), 3 (7), 336-339CODEN: ECCMF9; ISSN:1388-2481. (Elsevier Science B.V.)
      A theor. anal. is presented for the prediction of the specific energy consumption and the required electrode surface for the electrochem. combustion of org. compds. on synthetic B-doped diamond thin film electrodes. The model is formulated for a perfect mixed electrochem. reactor operated as a batch recirculation system under galvanostatic conditions. The anodic oxidn. of orgs. is assumed to be under diffusion control. An exptl. validation with the anodic oxidn. of phenol and under different exptl. conditions is provided.
    17. 17
      Ganiyu, S. O.; El-Din, M. G. Insight into in-situ radical and non-radical oxidative degradation of organic compounds in complex real matrix during electrooxidation with boron doped diamond electrode: A case study of oil sands process water treatment. Appl. Catal., B 2020, 279, 119366,  DOI: 10.1016/j.apcatb.2020.119366
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    18. 18
      Song, H.; Yan, L.; Jiang, J.; Ma, J.; Zhang, Z.; Zhang, J.; Liu, P.; Yang, T. Electrochemical activation of persulfates at BDD anode: Radical or nonradical oxidation. Water Res. 2018, 128, 393– 401,  DOI: 10.1016/j.watres.2017.10.018
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    19. 19
      Marselli, B.; Garcia-Gomez, J.; Michaud, P. A.; Rodrigo, M. A.; Comninellis, C. Electrogeneration of Hydroxyl Radicals on Boron-Doped Diamond Electrodes. J. Electrochem. Soc. 2003, 150 (3), D79– D83,  DOI: 10.1149/1.1553790
      19
      Electrogeneration of Hydroxyl Radicals on Boron-Doped Diamond Electrodes
      Marselli, B.; Garcia-Gomez, J.; Michaud, P.-A.; Rodrigo, M. A.; Comninellis, Ch.
      Journal of the Electrochemical Society (2003), 150 (3), D79-D83CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      The electrogeneration of hydroxyl radicals was studied at a synthetic B-doped diamond (BDD) thin film electrode. Spin trapping was used for detection of hydroxyl radicals with 5,5-dimethyl-1-pyrroline-N-oxide and with salicylic acid using ESR and liq. chromatog. measurements, resp. The prodn. of H2O2 and competitive oxidn. of formic and oxalic acids were also studied using bulk electrolysis. Oxidn. of salicylic acid gives hydroxylated products (2,3- and 2,5-dihydroxybenzoic acids). The oxidn. process on BDD electrodes involves hydroxyl radicals as electrogenerated intermediates.
    20. 20
      Pei, S.; Shen, C.; Zhang, C.; Ren, N.; You, S. Characterization of the Interfacial Joule Heating Effect in the Electrochemical Advanced Oxidation Process. Environ. Sci. Technol. 2019, 53 (8), 4406– 4415,  DOI: 10.1021/acs.est.8b06773
      20
      Characterization of the Interfacial Joule Heating Effect in the Electrochemical Advanced Oxidation Process
      Pei, Shuzhao; Shen, Chao; Zhang, Chenghu; Ren, Nanqi; You, Shijie
      Environmental Science & Technology (2019), 53 (8), 4406-4415CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      The electrochem. advanced oxidn. process (EAOP) has gained popularity in the field of water purifn. During the EAOP, it is in the boundary layer of the anode-soln. interface that org. pollutants are oxidized by hydroxyl radicals (•OH) produced from water oxidn. Applying current to an anode dissipates heat to the surroundings according to Joule's law, leading to an interfacial temp. that is much higher than that of the bulk soln., which is known as the "interfacial Joule heating" (IJH) effect. The modeling and exptl. results show that the IJH effect had an inevitable consequence for the activity of •OH, rate consts., and mass transport within the boundary layer. The interfacial temp. could be increased from 25 to 70.2 °C, a value mostly doubling that of the bulk soln. (33.6 °C) at the end of a 120 min electrolysis (10 mA cm-2). Correspondingly, the •OH concn. available for oxidn. of org. pollutants was much lower than that calcd. at a const. temp. of 25 °C probably due to H2O2 formation via •OH dimerization. The enhanced •OH diffusion resulting from strengthened mol. thermodn. movement and decreased kinematic viscosity of the soln. also drove •OH to move far from the anode surface and thus extended the max. thickness of the boundary layer. The oxidn. rate was pos. correlated to the interfacial temp., the activation energy, and the no. of activated mols., indicated by a 1.57-2.28-fold increase depending on the target org. compds. The finding of the IJH effect prompts a re-examn. of the literature based on a realistic rather than a const. temp. (e.g., 20-30 °C), the case reflected in a no. of prior studies that does not exist virtually, and reconsideration of behaviors that can be attributed to the change in temp. during EAOP.
    21. 21
      Geng, R.; Zhao, G. H.; Liu, M. C.; Lei, Y. Z. In situ ESR Study of Hydroxyl Radical Generation on a Boron Doped Diamond Film Electrode Surface. Acta Phys.-Chim. Sin. 2010, 26 (6), 1493– 1498
      There is no corresponding record for this reference.
    22. 22
      Bilski, P.; Reszka, K.; Bilska, M.; Chignell, C. F. Oxidation of the Spin Trap 5,5-Dimethyl1-pyrroline N Oxide by Singlet Oxygen in Aqueous Solution. J. Am. Chem. Soc. 1996, 118 (6), 1330– 1338,  DOI: 10.1021/ja952140s
      22
      Oxidation of the Spin Trap 5,5-Dimethyl-1-Pyrroline N-Oxide by Singlet Oxygen in Aqueous Solution
      Bilski, P.; Reszka, K.; Bilska, M.; Chignell, C. F.
      Journal of the American Chemical Society (1996), 118 (6), 1330-8CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) is frequently used to identify free radicals that are generated photochem. using dyes as photosensitizers. When O is present in such systems, singlet O (1O2) may be produced and can react with DMPO. The authors have studied the reaction of DMPO with 1O2 in aq. solns. over a wide range of pH, using micellar Rose Bengal (pH 2-13) and anthrapyrazole (pH < 2) as photosensitizers. DMPO quenches 1O2 phosphorescence (kq = 1.2 × 106 M-1 s-1), thereby initiating O consumption that is slow at pH 10 but increases ∼10-fold at pH < 6. This O consumption is a composite process that includes efficient oxidn. of both DMPO and its degrdn. products. The oxidn. products include both products in which the DMPO pyrroline ring remains intact (DMPO/•OH and 5,5-dimethyl-2-oxo-pyrroline-1-oxyl (DMPOX) radicals) and those in which it becomes opened (nitro and nitroso products). The nitroso product itself strongly quenched 1O2 phosphorescence, while (photo)decompn. of the nitroso group, presumably to nitric oxide (NO•), produced nitrite as a minor product. Probably 1O2 adds to the >C:N(O) bond in DMPO, producing a biradical, >C(OO•)-N•(O). This biradical may follow one of two pathways: (i) It may be protonated and rearrange to a strongly oxidizing nitronium-like moiety, which could be reduced to the DMPO hydroperoxide radical DMPO/•O2H while oxidizing another DMPO moiety to ultimately form DMPOX. The DMPO/•O2H could undergo further redox decompn., e.g. via the known Fenton-like reaction, to produce both free •OH radical and the DMPO/•OH radical. (Ii) The biradical >C(OO•)-N•(O) may cyclize to a 1,2,3-trioxide (ozonide), which could open the pyrroline ring to form 4-methyl-4-nitropentan-1-al and 4-methyl-4-nitrosopentanoic acid. Because the oxidn. of DMPO by 1O2 leads to both rapid O2 depletion and the formation of transients and products that might interfere with trapping and identification of free radicals, DMPO should be used with caution in systems where 1O2 is produced.
    23. 23
      Takayanagi, T.; Kimiya, H.; Ohyama, T. Formation of artifactual DMPO-OH spin adduct in acid solutions containing nitrite ions. Free Radical Res. 2017, 51, 739– 748,  DOI: 10.1080/10715762.2017.1369536
      23
      Formation of artifactual DMPO-OH spin adduct in acid solutions containing nitrite ions
      Takayanagi, Tetsuya; Kimiya, Hirokazu; Ohyama, Tatsushi
      Free Radical Research (2017), 51 (7-8), 739-748CODEN: FRARER; ISSN:1029-2470. (Taylor & Francis Ltd.)
      We investigated aq. solns. contg. nitrite ions and DMPO (5,5-dimethyl-1-pyrroline-N-oxide) by ESR in the pH range from 1 to 6. A DMPO-OH signal was obsd. below pH 3.0 in the presence of nitrite ions, whereas in the absence of nitrite ion, an extremely weak signal was obsd. below pH 1.5. Addn. of methanol, a hydroxyl radical scavenger, to this system did not lead to the appearance of a detectable DMPO-CH2OH signal. The possibility of this DMPO-OH signal being due to a genuine spin trapping process with hydroxyl radical was, therefore, ruled out. The reactivities of reactive nitrogen species (RNS) in this system with DMPO have also been investigated by d. functional theory (DFT) at the IEFPCM (water)/B3LYP/6-311+G ** level of theory. On the basis of the pH dependence of the signal intensity and the redox potential E° (vs. SHE) calcd. by DFT theory, we propose that the origin of this signal is "inverted spin trapping" via one-electron oxidn. of DMPO by H2ONO+, followed by the nucleophilic addn. of water. Prevention of these false-pos. results when detecting hydroxyl radical using ESR spin trapping requires an awareness of both the presence of nitrite ions in the soln. and the soln. pH.
    24. 24
      Yoshimura, M.; Honda, K.; Uchikado, R.; Kondo, T.; Rao, T. N.; Tryk, D. A.; Fujishima, A.; Sakamoto, Y.; Yasui, K.; Masuda, H. Electrochemical characterization of nanoporous honeycomb diamond electrodes in non-aqueous electrolytes. Diamond Relat. Mater. 2001, 10 (3–7), 620– 626,  DOI: 10.1016/S0925-9635(00)00381-2
      There is no corresponding record for this reference.
    25. 25
      Zhou, S.; Bu, L.; Yu, Y.; Zou, X.; Zhang, Y. A comparative study of microcystin-LR degradation by electrogenerated oxidants at BDD and MMO anodes. Chemosphere 2016, 165, 381– 387,  DOI: 10.1016/j.chemosphere.2016.09.057
      25
      A comparative study of microcystin-LR degradation by electrogenerated oxidants at BDD and MMO anodes
      Zhou, Shiqing; Bu, Lingjun; Yu, Yanghai; Zou, Xu; Zhang, Yansen
      Chemosphere (2016), 165 (), 381-387CODEN: CMSHAF; ISSN:0045-6535. (Elsevier Ltd.)
      This study investigated the electrochem. degrdn. of microcystin-LR (MC-LR) using boron-doped diamond (BDD) anode and mixed metal oxides (MMO, IrO2-Ta2O5/Ti) anode in different medium. In-situ electrogenerated oxidants including hydroxyl radical, active chlorine, and persulfate were confirmed in phosphate, chloride, and sulfate medium, resp. Different from MMO anode, hydroxyl radical was obsd. to play a significant role in chlorine generation at BDD anode in chloride medium. Besides, BDD anode could activate sulfate electrochem. due to its high oxygen evolution potential, and MC-LR degrdn. rate increased with the decrease of soln. pH. The effects of natural org. matters (NOM) and algal org. matters (AOM) on MC-LR degrdn. were evaluated and NOM presented stronger inhibition ability than AOM. Furthermore, the intermediates generated in MC-LR degrdn. in chloride and sulfate medium were identified by LC/MS/MS and possible degrdn. pathways were proposed based on the expts. results. Benzene ring and conjugated diene bonds of Adda group and double bonds of Mhda group were found to be the reactive sites of MC-LR. Overall, this study broadens the knowledge of electrochem. oxidn. in removing microcystins in algae-laden water.