Plasma-assisted oxidation of benzoic acid

Abstract

Plasma-assisted oxidation of organic compounds is one of the developing technologies for wastewater treatment. Plasmas effectively accelerate degradation processes due to plasma generated reactive species and ultra-violet radiation. Oxidation of BA in aqueous solutions by the atmospheric pressure glow discharge and underwater diaphragm discharge was studied and monitored by fluorescence and spectrophotometric methods. Discharge type and solution pH affect the formation rates of mono- and dihydroxybenzoic acids. Dihydroxyl derivatives were formed only by glow discharge action. The yields of hydroxyl radical were estimated on the kinetics data for the hydroxylation of benzoic acid. The steps of the hydroxylation processes and further oxidation were described.

Key words： atmospheric pressure glow discharge; underwater diaphragm discharge; oxidation; benzoic acid; hydroxyl radical

Introduction

Advanced oxidation processes (AOPs) such as photocatalytic oxidation, ozonation, Fenton oxidation, sonication, electrochemistry as well as non-thermal plasma treatment are being investigated as tools for organic pollutants destruction in water and wastewater [¹–⁴]. A principal pathway of these processes is based on the interaction of highly reactive hydroxyl (OH•) radicals with organic compounds. These radicals, which are formed in aqueous solutions by thermal or photochemical reactions, are strong oxidizers (E = 2.8 V at pH<7 and E = 1.4 V at pH>7). Non-thermal plasma in contact or inside liquids is one of the most attractive AOPs [⁵]. The reactive species (hydroxyl radicals, hydrogen atoms and hydrated electrons) are formed by activation and non-equilibrium dissociation of water molecules. The previous investigations have shown that the plasma in liquids leads to the destruction of organic pollutants such as phenol [⁶–⁸] and dye compounds [⁹–¹²]. The combined technique of discharge generated in liquids with classical AOPs such as ozonation has a synergistic effect on the efficiency of treatment [¹³,¹⁴]. The most popular model compound is the phenol. Oxidation of phenol by different types of electrical discharges has been intensively studied, and mechanisms of destruction were described in detail [¹⁵–¹⁸]. On the other hand, the aromatic acids (terephthalic acid, BA (BA), salicylic acid, and their hydroxyl or halogen derivatives) can be also used as model pollutants.

In the present work, the BA was chosen as the model. Benzoic acid is used in textiles, plastics, chemicals, powders, catalyst, and wood bleaching, and its solution is corrosive, toxic and poisonous [¹⁹]. Because electrical discharges could induce not only destruction processes but also polymerization and synthesis of the organic compounds [²⁰–²²], we supposed that the initial processes of destruction could lead to the formation of some derivatives of the initial compound due to its reactions with reactive species. In the case of benzoic acid, its hydroxyl derivatives such as salicylic acid (2-hydroxybenzoic acid (2-HBA)) may form, and even dihydroxy- and polyhydroxy- derivatives could form by further hydroxylation processes. These compounds can register in a solution by using the fluorescence method. In the experiments with non-thermal plasma in contact or in liquid, BA and its chloro- and hydroxy- derivatives are used as chemical probes for OH radical detection [²³–²⁸]. We suppose that the main mechanism of destruction of BA is interaction with hydrogen peroxide or hydroxyl radicals as the reactants of H₂O₂. From this point of view, the two pH values were used: hydrogen peroxide is more stable in acid solutions and yield of hydroxyl radicals is higher in alkali ones.

The objective of the present study is to compare two types of electrical discharge on oxidation efficiency of benzoic acid. Direct current atmospheric pressure glow discharge (APGD) and underwater alternative current (AC) diaphragm discharge (DD) were selected as two different discharge types. The APGD is one of the plasmas in contact with liquid, which is ignited above a solution surface. The chemical transformations occur at thin solution layer under the cathode spot. The underwater electrical discharge generates the non-thermal plasma in a liquid phase. The underwater AC diaphragm discharge is perspective for the applications in the technological process due to its relative simplicity. It does not need specific equipment, for example, an expensive high voltage pulsed source. In the present work, AC source operated at 50 Hz frequency with 0.8‒4 kV voltage and 20‒500 mA discharge current was used. The novelty of this study is to analyze the chemical pathways of oxidation processes in liquids, which occurred during the discharge treatment. In addition, we estimated yields of hydroxyl radical production.

Materials and methods

Materials

All reagents were used without further purification. Benzoic acid, salicylic acid (2-HBA), 2,3- and 2,5-dihydroxybenzoic acids (diHBAs), pyrocatechol (Cat), hydroquinone (HQ), and p-benzoquinone (p-BQ) were purchased from Sigma-Aldrich (USA). The sodium hydroxide and nitric acid were obtained from Chimmed (Russia). All aqueous solutions were prepared in distilled water. Five mmol/L aqueous solution of BA was prepared and stored at room temperature.

The solution pH was initially adjusted to 3.5 or 10 by adding an appropriate amount of NaOH or HNO₃, respectively. In neutral solutions, the formation of non-stable hydroxycyclohexadienyl radicals occurs due to hydrated electrons and hydrogen atoms presence [²⁹]. The solution pH was measured by pH meter (I-160MI, Russia). No change in pH was observed during the discharge treatment.

Experimental setup

All experiments were conducted in a non-symmetrical glass H-type cell in an open air at the constant volume of a solution (170 mL). The configuration of the cell was the same for both types of plasma discharge and described elsewhere [³⁰,³¹]. In the experiments using glow discharge, the discharge was ignited above the liquid cathode surface at atmospheric pressure. The platinum wires were used as the electrodes. The interelectrode gap was constant (3 mm). Discharge current varied in the range from 10 to 50 mA. In this case, the configuration of the cell can prevent reduction processes caused by molecular hydrogen that formed at the metallic cathode.

In experiments using underwater AC diaphragm discharge, graphite rods with a diameter of 5 mm were used as the metallic electrodes. Two voltage probes were connected to an ADC miniLab 1008. Measurements from the probes were recorded by a PC program “TracerDAQ”. The instantaneous power was calculated using Eq. (1):

(1)P={\displaystyle \frac{\mathrm{1}}{T}}{\displaystyle \underset{\mathrm{0}}{\overset{t}{\int }}I\left(t\right)\cdot U\left(t\right)\mathrm{d}t},

where I(t) is the discharge current (A), U(t) is the applied voltage (V), T is the averaging time (it was decided to average over several periods to have a realistic estimation of the dissipated power), and t is a time of an experiment.

In the case of glow discharge, the input power is estimated as follow: P= iU_c, where i is a discharge current, and U_c is a cathode potential fall. To estimate the cathode potential fall, several experiments of voltage distribution in the inter-electrode gap were carried out by moving anode method [³²]. The voltage of burning discharge was measured at different distances between metal anode and solution surface. The distance was changed from 0.5 to 2.5 mm by moving anode. The obtained data were fitted by a linear function, and the extrapolation to zero distance allows estimating the cathode potential fall value. The obtained results have shown that U_c values do not depend on discharge current for both media. The values are 660±20 and 580±15 V for acid and alkali solutions, respectively. Average values of power for both types of electrical discharge are depicted in Fig. 1.

Fig.1 Average values of input power of (1) glow discharge and (2) underwater AC diaphragm discharge at pH (a) 3.5 and (b) pH 10.

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Analysis

2-HBA, 2,3- and 2,5-diHBAs were analyzed by the fluorescence method. Fluorescence spectra have been registered using spectrofluorimeter Cary Eclipse (Varian-Agilent, USA-Australia) with the xenon lamp as a pulsed radiation source in the range of 300‒600 nm. The width of the slits for excitation and emission were 5 nm in the case of 2-HBA and 10 nm in the case of 2,3- and 2,5-diHBAs. Samples (4 mL) were placed in a standard quartz cuvette with an optical length of 1 cm. 2-HBA has a bright fluorescence with emission at 420 nm and excitation of UV light at 320 nm [³³–³⁶]. 2,3- and 2,5-diHBAs are also fluorophore with emission wavelength at 407 nm and excitation of UV light at 290 nm [³³]. The registration system was calibrated with solutions of 2-HBA, 2,3- and 2,5-diHBAs with known concentrations. Because fluorescence spectra of 2,3- and 2,5-diHBAs are superimposed [³³], we have registered the sum intensity of 2,3-diHBA and 2,5-diHBA (denoted as ∑diHBA).

The oxidation processes were monitored by a photometric method using spectrophotometer SF 103 (Akvilon, Russia) in the wavelength range of 190‒900 nm. Samples (4 mL) were collected from the treated solution and placed in a standard quartz cuvette with an optical length of 1 cm. The concentration of products of further oxidation was determined by absorption at 455 nm from calibration curves of several standard solutions of Cat, HQ, p-BQ and their mixture [³⁷].

The accumulation of hydrogen peroxide was detected using the colorimetric method with titanium oxysulfate at 410 nm [³⁸].

Results and discussion

Formation of mono and diHBAs

The kinetic curves of 2-HBA accumulation in solutions during glow discharge and underwater diaphragm discharge treatments are shown in Fig. 2(a). For the experiments using underwater AC diaphragm discharge, the accumulation curves have a maximum value, suggesting that 2-HBA is a transient product. After 9 min of DD treatment, the consumption rate of 2-HBA becomes larger than its formation rate. These results show that there are optimal conditions for 2-HBA formation. For the experiments with glow discharge, the alkali medium is more appropriate, whereas the acid solution is more suitable for the formation of 2-HBA in the case of DD treatment.

Rates of formation were estimated by initial parts of kinetic curves. The effect of discharge current on the rate of formation is presented in Fig. 2(b). The rates of formation of the 2-HBA are lower by underwater diaphragm discharge treatment than by glow discharge. In our previous work [³¹], it was found that more than 50% of input power (energy) is consumed onto solution heating, whereas 30% energy is consumed on heat content increasing (including solution heating and the Fourier heat losses) in the case of the glow discharge with liquid cathode [³²]. This indicates that the thermal destruction of BA and 2-HBA may occur in the experiments with underwater diaphragm discharge, leading to the lower rates of 2-HBA formation.

It should be noted that the discharge current has a little effect on the changes in the formation rates for all hydroxylated products (as will be shown below).

Fig.2 (a) Accumulation kinetics and (b) rate of formation of the 2-HBA in solution by (1) glow discharge and (2) underwater diaphragm discharge at pH (a) 3.5 and (b) 10.

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The kinetics of the sum 2,3- and 2,5-diHBAs are shown in Fig. 3(a). The experimental results show that the accumulation of diHBAs was detected after the third minute of APGD treatment. It was also found experimentally that the fluorescence of diHBAs was absent when discharge current was less than 20 mA in alkali solution and less than 25 mA in the acid one. The values of SdiHBA rapidly become constant in both media, indicating that the formation rate is equal to the rate of consumption. The consumption can include the decomposition of diHBAs [³⁹] as well as the formation of polyhydroxybenzoic acids.

The accumulation of sum 2,3- and 2,5-diHBAs was not observed in the experiments with underwater diaphragm discharge, but the formation rates for two pHs differ by two orders of magnitude (Fig. 3(b)).

Fig.3 Kinetic curves of (a) accumulation and (b) rate of formation of the ∑diHBA by glow discharge.

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The charged particles in glow discharge lead to the formation of several radicals and reactive species like OH, H, O, O₃, etc., through collisions with water vapor molecules in the gas phase. Further, the main charged particles (mainly H₂O⁺ ions) accelerated by the cathode potential fall react with water molecules in a liquid, leading to the formation of H atoms, OH radicals, and hydrated electrons. Mean free path of H₂O⁺ ions in water is about 0.03 µm [⁴⁰]. The diameter of the cathode spot varies from 2.5 to 3.8 mm and depends on the discharge current [⁴¹], so the active volume of solution varies from 1.38×10⁻⁴ to 3.54×10⁻⁴µL, which is less than that reported by Lee et al. (0.21 µL) [⁴²]. The effective lifetime of reactive species in this volume is 0.16 ms, suggesting that the diffuse length of OH radicals varies from 14 nm to 36 nm and all activation processes occur in active volume directly under the cathode spot. Maximov et al. observed that conventional flows appear from cathode spot to the bulk of solution and back, leading to a mixing of the solution [⁴³]. We could assume that 2-HBA and diHBAs form in a small volume of the solution under the cathode spot and then travel into the bulk of a solution by convention flows.

In the case of underwater AC diaphragm discharge, the plasma is initiated and takes place in a bubble that is formed close to the diaphragm. The mechanism of bubble formation and physical processes in the underwater diaphragm discharge were described in detail in previous reports [³⁰,⁴⁴]. When a bubble is collapsed, the convention flows appear and products of plasma (reactive species) are distributed in a solution volume. Among the reactive species, the oxidative (OH• radicals) and the reductive (solvated electrons (e_aqō), H• atoms) species are formed in a solution [³⁰].

The first step oxidation of BA occurs via attachment of OH• radical to the aromatic ring. As a result, a HBA forms (Scheme 1). The detail processes for the formation of 2-HBA were described by Clayden et al. [⁴⁵]

Fig.4 Scheme 1 Formation of 2-HBA.

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The second oxidation step is the further interaction of OH• radicals with 2-HBA to form diHBAs in solutions as the main products (Scheme 2). Hydroxyl radicals have a strong preference to attack the ortho- and para- positions due to its electrophilic reactivity [⁴⁶].

Fig.5 Scheme 2 Formation of 2,3- and 2,5-diHBAs.

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Further destruction by electrical discharges

Continuous plasma treatment shows that the treated solutions become yellow after 11 min by APGD treatment and after 7 min by DD treatment. The absorbance spectrum has the peak at 455 nm. Figure 4 presents the time-dependence of absorbance at 455 nm. A similar solution color was detected by thermal destruction of the salicylic acid solution [³⁷], during phenol oxidation by Fenton’s reagent [⁴⁷] and by discharge plasma treatment [⁸,⁴⁸]. Bobkova et al. attributed this color to the formation of nitrophenols [⁸]. It was expectable because glow discharge operates in air at atmospheric pressure and nitric oxide penetrates in a liquid from the gas phase. However, these compounds have absorbance peaks at 318 and 350 nm for ortho- and para-nitrophenols, respectively [⁴⁹]. The peak at 455 nm related to isomers of a benzoquinone [³⁷,⁴⁷]. In our experiments, the peaks at 318 and 350 nm did not appear.

Our experimental results show that the mixture of Cat, HQ, and p-BQ form during underwater diaphragm discharge whereas the p-BQ forms during APGD treatment only. All single solutions of these compounds have an absorbance at 455 nm, but intensity values are very low. On the other hand, the absorbance of the mixture is comparable with our experimental data. The maximum concentrations of p-BQ during glow discharge treatment are 5 µmol/L and 3.9 µmol/L at pH 3.5 and pH 10, respectively. In the experiments with underwater diaphragm discharge, the maximum concentrations of Cat, HQ, and p-BQ mixture are 63 µmol/L and 150 µmol/L at pH 3.5 and pH 10, respectively.

As shown in Fig. 5, the formation rates of quinones are comparable with the production rates of 2-HBA (Fig. 2(b)), indicating that the conversion process takes place. Because the formation of quinones was detected in all range of discharge currents, we can assume that this process may occur without the formation of diHBAs.

Fig.6 Time-dependence of solution color changes (absorbance at l = 455 nm) at two pHs by (a) glow discharge and (b) underwater diaphragm discharge.

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Fig.7 Formation rates of p-BQ (1) by glow discharge and mixture of Cat, HQ and (2) by underwater diaphragm discharge at pH (a) 3.5 and (b) 10.

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In the experiments with the underwater diaphragm discharge, the destruction process of 2-HBA proceeds via decarboxylation to phenol by the reductive reactive species (H• atoms in acid medium and solvated electrons (e_aqō) in an alkali solution) (Scheme 3). Further oxidation by OH• radicals induces the formation of HQ and p-BQ. In the experiments with glow discharge, the oxidation process of the 2,5-diHBA proceeds via decarboxylation to HQ, which is further oxidized into colored p-BQ (Scheme 4). Similarly, 2,3-diHBA is oxidized through the catechol formation and transformed to o-benzoquinone (Scheme 5) [⁵⁰].

Fig.8 Scheme 3 Conversion of 2-HBA and formation of hydroquinone and p-benzoquinone.

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Fig.9 Scheme 4 Conversion of 2,5-DiHBA.

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Fig.10 Scheme 5 Transformation of 2,3-DiHBA and formation of o-benzoquinone.

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Detection of H₂O₂

In the experiments with both types of discharge, we detected very low concentration of H₂O₂. The concentrations of H₂O₂ are 0.5 µmol/L and 0.1 µmol/L for glow discharge and diaphragm discharge, respectively. These values are very low in comparison with data for these types of discharges [³¹,⁵¹]. We assume that the reactants for production of H₂O₂ can consume with BA and its hydroxylated derivatives. Several experiments were conducted with H₂O₂ (ratio BA/H₂O₂ was 1:1) without electrical discharge. The obtained results showed that the hydroxylation process did not occur. These data are in good agreement with the reported results that the formation of salicylic acid occurs at a high concentration of H₂O₂ or in the presence of Fe²⁺ or Cu²⁺ ions [⁵²,⁵³]. In our case, this means that most of the reactants for hydrogen peroxide consumed with organic substances.

Chemical pathways

Taking into consideration the reaction schemes presented above, we can propose the stepwise hydroxylation of BA by glow discharge or by underwater diaphragm discharge (Fig. 6). According to the literature [⁵⁴], the yields of 3-HBA and 4-HBA are the same as that of 2-HBA. The yields for dihydroxyl derivatives can vary from 1.5% to 9.5%. Our estimations of the product yields show that the formation of para-benzoquinone or further hydroxylation products is likely at pH 3.5 during the glow discharge treatment. Taking into consideration the low yield of HBA in acid media, we suggest that the oxidation of 2-HBA to p-BQ bypasses the formation of diHBA. In the alkali solutions, however, BA is converted into p-BQ via the formation of mono- and di-HBA due to high yield of OH• radicals.

Fig.11 The stepwise conversion of BA during glow discharge or underwater diaphragm discharge treatment.

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In the experiments with underwater diaphragm discharge, we have the opposite results. The alkali solution is more suitable media for conversion of BA into quinones.

Estimations of yields of the OH• radicals

There are several reports related to quantification of hydroxyl radicals by using benzoic and salicylic acids as the chemical probes (scavengers) [²⁴–²⁸,⁵⁰,⁵⁴,⁵⁵]. For calculation of the yield of the hydroxyl radicals, we considered the assumption that the radical product yield is proportional to the fast reaction product yield at the following condition: k[C]>10⁷ s⁻¹, where k is rate constant and [C] is a concentration of a chemical probe. This is true in the absence of competitive reactions in solution [⁵⁶]. At our experimental condition, k₁[BA] = 4.3 × 10⁹× [5 × 10³] = 2.15 × 10⁷ s⁻¹. Therefore, we can estimate the yield of OH radicals using the data of concentrations of 2-HBA and sum of 2,3-diHBA and 2,5-diHBA.

The kinetics of the first step of hydroxylation of BA is written as:

(2)-{\displaystyle \frac{\mathrm{d}\left[\mathrm{BA}\right]}{\mathrm{d}t}}=k_{\mathrm{1}}\left[\mathrm{OH}\right]\left[\mathrm{B}\mathrm{A}\right]=\upsilon \left(\mathrm{HBA}\right),

and the rate of second stage is:

(3){\displaystyle \frac{\mathrm{d}\left[\Sigma \mathrm{diHBA}\right]}{\mathrm{d}t}}=k_{\mathrm{2}}\left[\mathrm{OH}\right]\left[\mathrm{HBA}\right]=\upsilon \left({\displaystyle \mathrm{\Sigma }\mathrm{diHBA}}\right).

Then, the formation rate for hydroxyl radicals can be written as follow:

(4)\upsilon \left(\mathrm{OH}\right)=\upsilon \left(\mathrm{HBA}\right)+\upsilon \left(\mathrm{\Sigma }\mathrm{diHBA}\right),

Assuming that hydroxyl radicals take part in the formation processes of p-BQ and HQ, the rates of its formation also include in the Eq. (4) for OH radicals.

\upsilon \left(\mathrm{O}\mathrm{H}\right)=\upsilon \left(\mathrm{H}\mathrm{B}\mathrm{A}\right)+\upsilon \left(\mathrm{\Sigma }\mathrm{d}\mathrm{i}\mathrm{H}\mathrm{B}\mathrm{A}\right)+\upsilon \left(p-\mathrm{B}\mathrm{Q}\right)

(5)\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{g}\mathrm{l}\mathrm{o}\mathrm{w}\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r}\mathrm{g}\mathrm{e},

\upsilon \left(\mathrm{O}\mathrm{H}\right)=\upsilon \left(\mathrm{H}\mathrm{B}\mathrm{A}\right)+\mathrm{2}\upsilon \left(\mathrm{C}\mathrm{a}\mathrm{t}+\mathrm{H}\mathrm{Q}+p-\mathrm{B}\mathrm{Q}\right)

(6)\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{p}\mathrm{h}\mathrm{r}\mathrm{a}\mathrm{g}\mathrm{m}\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r}\mathrm{g}\mathrm{e}.

Because the rate of formation depends on various parameters (a type of discharge, applied power, solution volume, etc.), the term “energetic yield” is acceptable for comparison with published data. It is a number of formed reactive species per input power. The energetic yield is calculated by using the following equation:

(7)G={\displaystyle \frac{\upsilon \left(\mathrm{OH}\right)N_{\mathrm{A}}Ve}{P}}\times \mathrm{100}\hspace{0.17em}\left(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{u}\mathrm{l}\mathrm{e}\mathrm{s}/\mathrm{100}\mathrm{e}\right),

where υ(OH) is production rate of OH radicals, M/s, N_A Avogadro’s number (6.02×10²³ molecule/mole), V solution volume (0.17 L), e charge of an electron (1.6×10⁻¹⁹ Coulomb), and P is the input power.

Our calculations show that G values do not depend on the discharge current in the frame of errors under our experimental conditions. But the pH of solution and type of discharge affect yield of OH radicals. Calculated yields are presented in Table 1. For alkali solutions, G values are comparable with our previous data [³⁰,³¹]. In the case of acid solutions, the obtained yields are lower than those obtained early for both types of discharge. In the alkaline solutions, however, the values of energetic yields are comparable with published data [²⁴–²⁸].

Tab.1 Effect of solution pH on energetic yields of hydroxyl radicals (molecules/100 eV)

Solution pH	Glow discharge	Underwater diaphragm discharge
3.5	(6.0±0.9)×10−3	(8.9±0.2)×10−3
10	(6.9±0.4)×10−2	(2.3±0.7)×10−2

The low energetic yields of the OH• radicals in acid media for both types of electrical discharge can connect with the difference of electrical parameters. The presence of alkaline metal facilitates the burning discharge due to less ionization potential (ionization potential for Na is 5.14 eV and for HCl is 12.54 eV). It reflects in values of discharge power (Fig. 1). On the other hand, the high yield of hydroxyl radicals in alkaline solutions can be explained by the instability of hydrogen peroxide.

Conclusions

In this work, oxidation of BA in aqueous solutions by two types of discharge plasma treatment has been studied. The chosen discharges, namely, the glow discharge and diaphragm discharges operated at atmospheric pressure, are often utilized for surface processing and represent high interest for several applications as the chemical processing of unwanted toxic products.

In the experiments with glow discharge, the destruction of BA includes two steps of hydroxylation and formation of the mono- and dihydroxybenzoic acids. In the experiments with underwater diaphragm discharge, only one step of hydroxylation occurs, suggest that underwater electrical discharges could lead to the intensification of the oxidation process without the formation of the intermediates.

The formation rates of oxidation products by the glow discharge and underwater diaphragm discharge treatment were estimated. The formation rates of further oxidation products are comparable with that of the initial hydroxylation of benzoic acid. In the experiments with glow discharge, the yields of the 2-hydroxybenzoic and diHBAs are higher in alkaline solutions than in acid solutions, probably due to the higher yield of hydroxyl radicals at pH 10 than in acid solutions.

Acknowledgments

This work was partially supported by RFBR (project No. 16-33-60061).

References

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