Kinetics of methylene blue degradation in VUV/UVC irradiation process and the role of hydroxyl radical

doi:10.21203/rs.3.rs-1349045/v1

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Kinetics of methylene blue degradation in VUV/UVC irradiation process and the role of hydroxyl radical

https://doi.org/10.21203/rs.3.rs-1349045/v1

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A series of investigations have been carried out to study the decomposition of methylene blue (MB) by the VUV/UVC based AOPs. However, MB degradation by VUV/UVC direct photolysis has not been reported previously. For the first time, the degradation kinetics and mechanisms of methylene blue by a chemical-free technology VUV/UVC (185 nm/254 nm) irradiation were systematically investigated in bench scale. Firstly, we investigated the impact of reaction conditions (i.e., VUV power, temperature, initial pH, and initial MB concentration on the VUV/UVC irradiation process. We found that MB degradation by VUV/UVC irradiation is an endothermic reaction, and the apparent activation energy was calculated to be 14.72 kJ·mol^-1, which is similar UVC/TiO₂ and electrical discharge plasma system, while higher VUV power and lower initial MB concentration have enhancement effects. The degradation rate constant was a convex function of the solution pH and reached the optimal value at pH 9.19. Compared with VUV/UVC irradiation and UVC/H₂O₂ processes, hydroxyl radical (·OH) plays the dominated role in VUV/UVC irradiation system, while both ·OH and HO₂· contribute to conventional UVC/H₂O₂ process. The degradation pathways were proposed including demethylation and chromophores oxidation, and the possible pathway was confirmed by combining the experimental observations with the density functional theory (DFT) calculation. In summary, this study provides the results that VUV/UVC is an effective and chemical-free technology for MB removal from water.

Vacuum UV

Chemical-free

Methylene blue

Degradation mechanism

Hydroxyl radical

Organic dyes are extensively usage in food processing, paint (Islam et al. 2020), printing (Kecić et al. 2018), textile industries (Badmus et al. 2020) and leather tanning (Xu et al. 2020), etc. Unfortunately, a large quantity of organic dyes wastewater produced during its producing and consuming (Hasanpour &Hatami 2020) and caused serious problems to human health, aquatics and plants (Berradi et al. 2019). Moreover, the presence of organic dyes in industrial effluent has become one of the major challenge of wastewater treatment (Mittal et al. 2018, Tkaczyk et al. 2020). Organic dyes wastewater is difficult to treat satisfactorily using conventional wastewater treatment process due to its complex composition, deep color, strong toxicity, large water quantity, stable nature and high organic content (Singh &Arora 2011). Over the past few decades, hydroxyl radical (·OH)-based advanced oxidation processes (AOPs) are considered as an effective and promising method for the degradation and mineralization of the organic contaminants in wastewaters. A number of AOPs methods have been development to produce ·OH such as UV/H₂O₂ (Zhang et al. 2019a), Fenton (Zárate-Guzmán et al. 2019), ozonation (Wang et al. 2019) electrochemical oxidation (Liang et al. 2015) and photocatalysis (Lin et al. 2016). However, these AOPs process need adding external chemical as oxidant, catalyst, electrolyte, etc., which need a consistent chemical supply (Krichevskaya et al. 2011). Therefore, development of an efficient and chemical-free technology for organic dyes wastewater is always in need.

Recent years, a novel chemical-free AOP that set Vacuum UV (VUV) as the light source has gained increasing attention. VUV AOP was found to be an efficient and economical method compared with traditional UV-based AOPs by effective utilization of the in situ generated reactive oxygen species (Zhang et al. 2021). VUV is a form of electromagnetic radiation with wavelength from 100 nm to 200 nm which corresponds to high photon energy (Zoschke et al. 2014). Under the VUV radiation, ·OH, hydrogen atoms (·H), and solvated electrons (e_aq^-) are generated by the photochemical ionization of water molecule (Eqs. 1 and 2).

\({H}_{2}O\underrightarrow{\lambda \text{=}185nm}{·OH+H}^{+}+{e}_{aq}^{-}\)

Φ(e_aq⁻) = 0.045 [21] (1)

\({H}_{2}O\underrightarrow{\lambda \text{=}185nm}·OH+H·\)

Φ(·OH) = 0.33 [21] (2)

Low pressure mercury lamps (LPUV) is a kind of typical VUV light source which using synthetic silica in the production of the lamp tube (Li et al. 2016). LPUV (known as VUV/UVC) illuminate a limited number of well-defined lines, which concentrate at 253.7 nm (UVC, nominal value: 254 nm) and 184.9 nm (VUV, nominal value: 185 nm) (Zoschke et al. 2014). VUV/UVC are mostly used as VUV light source for eliminating organic pollutants from water. In previous studies, microcystin-LR (Chintalapati &Mohseni 2020), carbamazepine (Zhu et al. 2019), imipramine (Xie et al. 2019), tris(2-chloroethyl) phosphate (Chen et al. 2019) and fluoroquinolone antibiotics (Geng et al. 2020) have been proved to be efficiently removed in VUV/UVC irradiation process. Because of high efficacy and cost-effective, VUV/UV irradiation process is a promising technique for the treatment of waters contaminated with organic pollutants (Moussavi &Shekoohiyan 2016). A series of previous studies have investigated the degradation of methylene blue by VUV/UVC based AOPs, such as VUV/UVC/TiO₂ (Huang et al. 2013), VUV/UVC-photo-Fenton (Li et al. 2016) and VUV/UVC/persulfate (Wen et al. 2020) process. However, the mechanism and kinetics of methylene blue degradation in VUV/UVC direct irradiation process have been no reported so far, and many questions remain to be investigated.

In this study, MB was selected as a target organic dye for investigating the degradation kinetics and mechanisms in VUV/UVC irradiation process. The evaluation of MB degradation efficiency was employed in UVC alone, VUV/UVC alone and UVC/H₂O₂ process. Active oxygen species (ROS) quenching tests and UV-Vis spectrum characterization were used to confirm the degradation mechanism. The effect of important operational factors, including VUV power, temperature, initial solution pH, and initial MB concentration were evaluated on the performance of MB degradation in VUV/UVC process. The major organic and inorganic byproducts were identified, and the degradation pathway of MB was proposed.

2.1. Chemicals

Methylene blue, potash iodide, and ammonium molybdate were purchased from Chron Chemical, China. 1.4-benzoquinone, potassium acid phthalate, and sodium bicarbonate were purchased from Sinopharm Group Co., China. Chemicals used for experiments were reagent grade and without further purification. All solutions were prepared with ultrapure water produced from a Direct-Pure UP water purification system (RephiLe Bioscience, China).

2.2. Experimental procedures

The degradation experiments were conducted in a photo-reactor, shown in Fig. S1, and the description was in Text 2. All the batch experiments for MB degradation were performed in a lab scale annular photo-reactor. A glass column (50mm×250mm) with a working volume of 300 mL, and reaction temperature was adjusted and kept with a thermostatic water bath (THD-1015, Ningbo Tianheng, China). An 8 W low pressure mercury lamp (Beijing Aerospace Hong Da Optoelectronics Technology Co., China) was the experimental light source, which could emit VUV/UVC (10-08100, emitting at both 185 and 254 nm) or UV (10-08000, emitting only at 254 nm) spectrum. The lamps were inserted into a quartz sleeve and placed in the central axes of the glass column. Different intensity of VUV irradiation was achieved from VUV/UVC that release different proportions of VUV (the full release is set at 100%). Initial solution pH was uncontrolled but measured, which remained around 6.23 during the reactions. Hence, initial solution pH was adjusted with 0.1 M HCl and NaOH solution to investigating the influences of pH. For each experiment, samples were withdrawn from the photo-reactor at predetermined time intervals and analyzed timely.

2.3. Analytical methods

The concentration of methylene blue (MB) and UV-Vis absorption spectra of aqueous was analyzed on an UV-Vis spectroscopy (Specord® 200 plus, Analytik Jena, Germany). Total organic carbon of the sample was determined using a TOC analyzer (N/C 3100, Analytik Jena, Germany). Quantitative analysis of HCOO^-, NO₂^-, NO₃^-, and SO₄^2- was conducted by Ion Chromatograph (Metrohm Eco IC, Switzerland), and NH₄⁺ was measured by Salicylic acid spectrophotometry. The hydrogen peroxide generated during VUV/UV photolysis was measured by KI method (Kormann et al. 1988). Solution pH was measured with an FE28 pH meter (Mettler-Toledo, Switzerland). A LC-MS 8040 system (Shimadzu Corporation, Japan) was used for analysis of the degradation products of MB.

3.1 Effect of typical factors on the MB degradation

3.1.1 Effect of VUV power

The influence of VUV power on degradation of MB by VUV/UVC irradiation was investigated. As shown in Fig. 1a, the MB degradation process was conformed to the pseudo-first order reaction. The reaction kinetics constant of MB degradation increased from 0.0235 to 0.1729 min^-1 with the VUV power increasing from 10–100% (VUV full power was set as 100%). In the VUV/UVC system, the energy of photon from VUV irradiation at 185 nm reaches 6.7 eV, it is higher than the threshold energy for photochemical induced splitting of water (Nikogosyan &Görner 1992) and the bond dissociation energy (BDE) of MB molecular (Huang et al. 2013). More photon generated to destroy MB directly increase with the VUV power. The increasing VUV power also enhances the yield of ·OH and HO₂·, resulting in higher degradation efficiency of MB.

3.1.2 Effect of temperature

The effect of temperature on the degradation efficiency of MB was investigated, and results are shown in Fig. 1b. The MB degradation process at various temperature followed pseudo-first-order reaction kinetics, and the constants increased from 0.132 to 0.239 min^-1 when temperature increased from 288 to 318 K. Furatian (Furatian &Mohseni 2018) reported that the 185 nm photolysis of H₂O is fairly insensitive to temperature due to the absorbance of water is negligible. In VUV/UVC irradiation system, two ·OH can merge into one H₂O₂ molecule (Eq. 3). As the previous reported, the rise of temperature results in an enhancement in the generation and reactivity of ·OH together with a decomposition of H₂O₂ (Sanz et al. 2013). Moreover, the quantum yield of ·OH and molecular diffusion higher with the sequential increase of high temperature (Cao et al. 2020). It was attributed to the radiation intensity of the ultraviolet lamp enhanced by the rise of temperature. To the light source, the optimum working temperature range of LPUV is 313 ~ 315 K (40 ~ 42 ℃) (Masschelein 2002) and the efficiency could be increased with the rise of temperature during 288 ~ 318 K. The Arrhenius plot for the MB degradation is displayed in Fig. S2. According to Arrhenius formula, the apparent activation energy of MB degradation was calculated 14.72 kJ·mol^-1. This data is similar to the degradation of MB in the UV/TiO₂ photocatalytic system (14.45 kJ·mol^-1) (Lee et al. 1999) and in the electrical discharge plasma (13.09 kJ·mol^-1) (Benetoli et al. 2011).

3.1.3 Effect of initial pH

To further explore the rule of MB decomposition in VUV/UVC irradiation system at different conditions, degradation of MB was evaluated at different initial pH values. As shown in Fig. 1c, MB was degraded entirely at various initial pH in 30 min, but initial pH has different effects on the degradation rates. When solution pH increased from 3.02 to 9.19, the pseudo-first-order reaction kinetics constant increased from 0.123 to 0.203 min^-1, though to a lesser extent, and then reduced to 0.181 min^-1 as pH increased to 10.98. It should be noted that the effect of pH on MB photodecomposition was not significant except at high pH values. The improvement of MB degradation with increase of pH can be attributed to either consumption of ·OH by OH^- (Eq. 4). As initial pH increased from 3.03 ~ 9.19, few of ·OH were quenched by OH^- (Zhang et al. 2020) and leading to MB degradation. However, as pH increased to 10.98, too much ·OH quenched by OH^- and inhibited the MB degradation. For the oxidizing nature of ·OH, the oxidation potential of ·OH reduced with pH from 2.59 V (pH 0) to 2.18 V (pH 7.0), and ·OH could be further converted to O^·- (E⁰ = 1.64 V) under alkaline conditions (Koppenol &Liebman 1984). In addition, the light screening effect of OH^- (ε₁₈₅ > 3099 M^-1 cm^-1) (Sauer et al. 2004) also reduced the ·OH generation (ε₁₈₅ > 1.8 M^-1 cm^-1) (Weeks et al. 1963).

\(2·OH\leftrightarrow {H}_{2}{O}_{2}\)

k = 5.5 × 10⁹ M^− 1s^− 1(3)

\(\bullet OH+{OH}^{-}\to {H}_{2}O+{O}^{\bullet -}\)

k = 1.2 × 10¹⁰ M^− 1s^− 1(4)

3.1.4 Effect of initial MB concentration

Figure 1d shows the degradation process of MB at the different initial concentration in VUV/UVC irradiation system. Apparently, the MB degradation rate decreased gradually with increased initial MB concentration, the pseudo-first-order reaction kinetics rate constant reduced from 0.554 to 0.058 min^-1 when the initial MB concentrations increased from 10 to 70 mg/L. Yang et al. (Yang et al. 2020) observed the same trends in the degradation of atrazine using VUV/UVC/Fe. In the VUV/UVC system, MB achieved effective degradation at low MB concentration due to the relative abundant photons and ROS. However, as initial concentration of MB increased, MB and its products compete with photons and ROS. Furthermore, the VUV photons was absorbed by MB and its products, and inhibited the generation of ROS by the light screening effect (Li et al. 2017).

3.2 Degradation mechanism of MB in different reaction process

VUV/UVC radiation process is a complex reaction system due to the coexist of radical reaction and photochemical reaction. In addition to VUV induced ROS processes and UVC photolysis, the generation of H₂O₂ cannot be neglected. Through combination of ·OH and HO₂· species (Eqs. 4 and 5), H₂O₂ could be generated in the VUV/UVC photolysis water process. As shown in Fig. 2a, H₂O₂ concentration increased and leveled off at a quasi-stationary concentration of 15.15 × 10^− 6 M (0.52 mg/L), and the maximum concentration was 38.56 × 10^− 6 M (1.31 mg/L) at 5 min. H₂O₂ concentration raised continuously until to the maximum in a circular photo-reactor also reported on previous study (Moussavi et al. 2018). In this process, the H₂O₂ was formed mostly by recombination of ·OH (Zhang et al. 2020). After that, H₂O₂ was decomposed with other reactions (Eqs. 6 and 7) and UVC photolysis (Eq. 8), then secondary radical generated. Ultimately, Formation and decomposition of H₂O₂ form a dynamic equilibrium, resulting in a steady-state concentrations.

\({2HO}_{2}\text{·}\to {H}_{2}{O}_{2}+{O}_{2}\)

k = 8.4 × 10⁵ M^− 1s^− 1(5)

\({H}_{2}{O}_{2}+\bullet OH\to {HO}_{2}^{\bullet }+{H}_{2}O\)

k = 3.3 × 10⁷ M^− 1s^− 1(6)

\({e}_{aq}^{-}+{H}_{2}{O}_{2}\to \bullet OH+{OH}^{-}\)

k = 1.1 × 10¹⁰ M^− 1s^− 1(7)

\({H}_{2}{O}_{2}+\underleftrightarrow{\lambda \text{=}254nm}2·OH\)

Φ(H₂O₂) = 0.5 (8)

To investigate the individual contribution of UVC photolysis, and UVC/H₂O₂ to MB degradation in VUV/UVC system, the degradation experiments were conducted individually in above three systems. As shown in Fig. 2b, direct photolysis by UVC caused slight degradation of MB, where only 6.96% of MB was removed after 30 min. By contrast, MB was degraded quickly in both VUV/UVC irradiation process and UVC/H₂O₂ process. The remove rate of MB was 100% and 96.82% for VUV/UVC and UVC/H₂O₂ process, respectively.

Sodium bicarbonate (SB) and 1, 4-benzoquinone (BQ) were respectively selected as organic quencher for ·OH and HO₂·. As shown in Fig. S3, MB removal efficiency and pseudo-first-order kinetic constant k reduced leveled off as SB and BQ concentration were 60 and 0.5 mM, respectively, suggesting that ·OH and HO₂· were the ROS in VUV/UVC irradiation process. The degradation of MB in UVC alone, VUV/UVC alone and UVC/H₂O₂ process are shown in Fig. S4. MB degradation by UVC irradiation, VUV/UVC irradiation and UVC/H₂O₂ system well followed a pseudo-first-order kinetics and the degradation rate constant were 0.002, 0.168 and 0.161 min^-1, respectively. As shown in Fig. S4 (a), no obvious effect on MB degradation was found in the UVC irradiation process with addition of quenchers. It was confirmed that quenchers had negligible influence on the degradation of MB in UVC process. As can be seen in Fig. S3 (b and c), degradation of MB has been inhibited in VUV/UVC irradiation and UVC/H₂O₂ process observably when SB or BQ added. It demonstrated that ·OH and HO₂· had been generated in VUV/UVC irradiation and UVC/H₂O₂ process. Additionally, it should be noted that the weak MB degradation was observed though ·OH and HO₂·has been quenched, which indicated the effect of direct photolysis on MB degradation.

The contribution of ·OH, HO₂· and UVC photolysis to degradation of MB was further evaluated with experiments performed with the ROS quenching tests. The contribution ratios of ·OH, HO₂· and UV photolysis were calculated as below equations: (Zhang et al. 2019b):

\({R}_{·\text{O}\text{H}}=\frac{{k}_{MB}-{k}_{SB}}{{k}_{MB}}\) (9)

\({R}_{{\text{H}\text{O}}_{2}·}=\frac{{k}_{SB}-{k}_{BQ}}{{k}_{MB}}\) (10)

Where R_·OH and R_HO2· represent the contribution ratios of ·OH and HO₂· in the overall MB degradation, respectively; k_MB, k_SB, and k_BQ are the degradation rate constants of MB with or without the presence of SB and BQ, respectively. As shown in Fig. 2c, the contribution ratios of ·OH, HO₂·, and UVC photolysis were 90.12%, 3.85%, and 6.03% in VUV/UVC irradiation process and 50.92%, 41.83%, and 5.25% in UV/H₂O₂ process. These results suggested that ·OH played a significant role on MB degradation in VUV/UVC irradiation process. Unlike this result, Li et al. (Li et al. 2016) reported that HO₂· was the predominant ROS leading to MB degradation in VUV/UVC/Fenton process. Huang et al. (Huang et al. 2013) reported that ·OH and energetic photons were the primary ROS on MB degradation in VUV/UVC/TiO₂ process. In the UVC/H₂O₂ process, ·OH can be produced through direct dissociation of H₂O₂ under UVC irradiation. In this process, numerous H₂O₂ reacted with ·OH, and generated a great deal of HO₂·. Although the oxidizing power of HO₂· is lower than ·OH (Haji et al. 2011), its lifetime is several orders longer than that of the ·OH [26, 44], and played remarkable role on MB degradation. Therefore, it concluded that both ·OH and HO₂· were the main active specie for degradation of MB in UVC/H₂O₂ process. In VUV/UVC irradiation process, ·OH formation firstly by VUV photochemical ionization of water, and then HO₂· formatted by ·OH reacting with H₂O₂. However, the concentration of H₂O₂ was lower and resulted in a small quantity of HO₂· generation. Thus, the contribution of ·OH to the degradation of MB is expected to be much higher than that of HO₂·.

UV-Vis spectrum evolution of MB in degradation process in VUV/UVC and UVC/H₂O₂ systems have been presented in Fig. 3. A rapid decrease of the absorbance of MB was observed in VUV/UVC and UVC/H₂O₂ system (Fig. 3a and 3d). The absorbance of MB was slightly decrease as addition of SB and BQ in VUV/UVC irradiation system (Fig. 3b and 3c). In the UVC/H₂O₂, the absorbance of MB decreased significantly as addition of SB and have no evident changes with BQ addition (Fig. 3e and 3f).

The maximum absorption peak blue-shifted in various systems were shown in Fig. S5. The blue-shifted was observed in maximum absorption wavelength at 664 nm in VUV/UVC and UVC/H₂O₂ system, the same phenomenon were found in photocatalytic degradation of MB process (Mir et al. 2017). The blue-shifted intensity was followed the order: VUV/UVC > UVC/H₂O₂ > UVC/H₂O₂ + SB, and other systems could be negligible. In UVC/H₂O₂ + SB process, ·OH was quenched and blue-shifted was significantly suppressed in contrast to UVC/H₂O₂ system. Moreover, the formation of demethylate products during the MB degradation process induced blue-shift of UV-Vis spectra (Rauf et al. 2010, Sandoval et al. 2017). It indicates that ·OH was the main active species for demethylation of MB. In other words, HO₂· degraded MB by destroy the chromophores.

3.3 Products and proposal of MB degradation pathway

Since the mineralization of MB results in the formation of S and N containing intermediates, the concentration of NH₄⁺, NO₂^-, NO₃^-, SO₄^2-, and HCOO^- in the VUV/UVC irradiation system were measured (Fig. 4). NH₄⁺ was produced due to the oxidation of the nitrogen-containing groups in MB molecules, but NO₂^- and NO₃^- were not generated. SO₄^2- was not detected at the initial stage of the reaction until 10 min. In other words, nitrogen-containing groups were oxidized easier than the sulfur-containing groups in VUV/UVC irradiation system. Additionally, HCOO^- generated and remarkably increased during MB degradation, indicating that demethylation was one of the pathways for MB degradation. In addition, TOC removal rate of MB was observed low to be only 15.64% (Fig. S6).

To clarify the degradation pathway of MB by UVU/UVC irradiation, the degradation intermediates were detected by LC-MS analysis. According to the typical total ion chromatogram (TIC) shown in Fig. S7, the abundance of the intermediates during MB degradation variable in time (Fig. 5a-5f). Six major intermedias of MB degradation were identified in VUV/UVC irradiation process ([M + 1]⁺= 272, 290, 304; [M]⁺= 242, 256-1, 256-2). Abundance of all the intermediates first accumulated and then their abundance decreased with prolonged irradiation time.

The degradation pathway of MB under VUV/UVC irradiation is proposed in Fig. 5g. Based on the intermediates, the two possible major photo-degradation pathways of MB are proposed. In the pathway I, the parent of MB was attacked by ·OH and formatted the demethylated intermediate [M]⁺= 256-1 or [M]⁺= 256-2. Intermediates [M]⁺= 242 formed from [M]⁺= 256-1 or [M]⁺= 256-2 by further demethylation, and then intermediates [M]⁺= 272 were produced from [M]⁺= 256-1 by oxidation. In the pathway II, parent MB were oxide and produced [M + 1]⁺= 304, which was further demethylated to [M + 1]⁺= 290.

To further elucidate the degradation pathway, DFT calculation was used and the optimized structure of the MB molecule shows in Fig. 6a. As shown in Fig. 6c, the calculating Fukui functions showed that the most prominent regions of f- are localized in N and S, which indicates that these sites are susceptible for ·OH attacks. According to the MB degradation products, ·OH tended to attack the N15 and N18, agreement with the results of theoretical calculation.

This work investigated the degradation kinetics and degradation mechanism of MB in chemical-free VUV/UVC irradiation system. VUV power and initial concentration of MB significantly affected the MB degradation rate, while there were few influences from pH and temperature. The MB degradation mechanism of VUV/UVC irradiation system was different from UVC/H₂O₂ system. In the VUV/UVC irradiation system, ·OH was the major active species for MB degradation, while ·OH and HO₂· were both the primary active species in UVC/H₂O₂ system. MB was degraded through two pathways including demethylation and oxidation. DFT study was carried out and supported the degradation pathways of ·OH attack. Therefore, VUV/UVC irradiation process is an excellent method to degrade MB without any chemicals addition.

Author Contributions

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Jialiang Liang, Zhijie Liang and Weichun Gao. Yinyan Guan drafted the work or revised it critically for important intellectual content. The first draft of the manuscript was written by Cong Geng and all authors commented on previous versions of the manuscript. Zhiwei Zhao and Jiyan Liang read and approved the final manuscript.

Funding

Project supported by Natural Science Foundation of China (Grant No. 51878090) and LiaoNing Revitalization Talents Program, China (Grant No. XLYC1908034，XLYC2007170).

Data availability

The datasets used and analyzed during the current study are available from the corresponding author upon reasonable request.

Ethics approval and consent to participate Not applicable.

Consent for publication Not applicable.

Competing interests The authors declare no competing interests.

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Kinetics of methylene blue degradation in VUV/UVC irradiation process and the role of hydroxyl radical

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Abstract

Figures

1. Introduction

2. Material And Method

2.1. Chemicals

2.2. Experimental procedures

2.3. Analytical methods

3. Results And Discussion

3.1 Effect of typical factors on the MB degradation

3.1.1 Effect of VUV power

3.1.2 Effect of temperature

3.1.3 Effect of initial pH

3.1.4 Effect of initial MB concentration

3.2 Degradation mechanism of MB in different reaction process

3.3 Products and proposal of MB degradation pathway

4. Conclusions

Declarations

References

Supplementary Files

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