J. Cent. South Univ. (2018) 25: 1825-1835

DOI: https://doi.org/10.1007/s11771-018-3872-y

Electrochemical oxidation of reactive brilliant orange X-GN dye on boron-doped diamond anode

MA Li(马莉)¹, ZHANG Ming-quan(张明全)¹, ZHU Cheng-wu(朱成武)¹, MEI Rui-qiong(梅瑞琼)¹,

WEI Qiu-ping(魏秋平)¹, ZHOU Bo(周博)², YU Zhi-ming(余志明)¹

1. State Key Laboratory of Powder Metallurgy, School of Materials Science and Engineering,Central South University, Changsha 410083, China;

2. School of Engineering and Materials Science, Queen Mary, University of London, United Kingdom

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract: In this study, the electrochemical oxidation of reactive brilliant orange X-GN dye with a boron-doped diamond (BDD) anode was investigated. The BDD electrodes were deposited on the niobium (Nb) substrates by the hot filament chemical vapor deposition method. The effects of processing parameters, such as film thickness, current density, supporting electrolyte concentration, initial solution pH, solution temperature, and initial dye concentration, were evaluated following the variation in the degradation efficiency. The microstructure and the electrochemical property of BDD were characterized by scanning electron microscopy, Raman spectroscopy, and electrochemical workstation; and the degradation of X-GN was estimated using UV-Vis spectrophotometry. Further, the results indicated that the film thickness of BDD had a significant impact on the electrolysis of X-GN. After 3 h of treatment, 100% color and 63.2% total organic carbon removal was achieved under optimized experimental conditions: current density of 100 mA/cm², supporting electrolyte concentration of 0.05 mol/L, initial solution pH 3.08, and solution temperature of 60 °C.

Key words: reactive brilliant orange X-GN; boron-doped diamond; film thickness; electrochemical oxidation

Cite this article as: MA Li, ZHANG Ming-quan, ZHU Cheng-wu, MEI Rui-qiong, WEI Qiu-ping, ZHOU Bo, YU Zhi-ming. Electrochemical oxidation of reactive brilliant orange X-GN dye on boron-doped diamond anode [J]. Journal of Central South University, 2018, 25(8): 1825–1835. DOI: https://doi.org/10.1007/s11771-018-3872-y.

1 Introduction

Synthetic dyes are widely used in textile, leather, paper, food, and other industries [1]. However, many dye effluents are released in the industrial production process, which have become a refractory pollutant because of their high concentration, high chroma, and good chemical stability. Azo dyes are characterized by the presence of one or more —N=N— chromophoric groups [2]. Azo dyes account for over 70% of commercial dyestuff [3]; thus, they are rated as one of the most important class of synthetic dyes.

The reactive brilliant orange X-GN dye (hereafter referred to as X-GN) is a typical reactive azo dye that is widely used in the dyeing and printing industries [4] and needs to be treated urgently. However, X-GN is difficult to degrade using traditional physical, chemical, and biological methods [5] for various reasons. For example, secondary pollution arises because of the additives used during chemical treatment [6]; ozone and hypochlorite oxidation are not cost-effective due to the high cost of equipment and operation [7]; and the potential toxicity of reactive dyes show high resistance to microbial degradation [8]. Recently, advanced oxidation processes (AOPs) have attracted great attention for the removal of organic pollutants by generating strong oxidants in the effluents [3]. According to the previous reports, the research works on X-GN treatment concentrated on one of the AOPs, called the Fenton method. CHEN et al [9] investigated photocatalytic degradation of X-GN using Fe-Mt/H₂O₂ as heterogeneous photo- Fenton catalysis. WU et al [10] accomplished the treatment of X-GN by a heterogeneous Fenton system using activated carbon-FeOOH catalyst and H₂O₂. LI et al [11] studied Fenton degradation of X-GN using activated carbon-supported zero-valent iron catalyst. However, in this homogeneous Fenton process, iron ions acting as a catalyst are dissolved in water, which results in sludge disposal problems [12].

Electrochemical oxidation, as one of the AOPs, has been widely studied and applied because effluents can be effectively degraded without additional chemicals [13]. There are two methods of conducting the electrochemical oxidation process. One is direct oxidation, in which the organic matter is oxidized directly on the anode’s surface, and the other is indirect oxidation, in which the oxidation of organic matter is mediated by electrogenerated oxidants, such as hydroxyl radicals (·OH), active chlorine, and peroxide [14]. An anodic material is the key element in the degradation effect of organic effluent treatments; thus, various anodes, such as graphite [15], noble metal [16], and metal oxide [17, 18], have been studied. However, these anodes have the corresponding disadvantages of activity, cost, and service life [19–21]. The boron-doped diamond (BDD) anode has attracted great interest due to its excellent properties, such as high overpotential of oxygen evolution, low adsorption, and good stability [22–24]. Further, it has been extensively studied for dye effluent treatments. YAVUZ et al [25] studied anodic oxidation of the reactive black 5 dye on a BDD electrode in a bipolar trickle tower reactor; 97% color, 51% chemical oxygen demand (COD), and 29.3% total organic carbon (TOC) removals were achieved. SOLANO et al [26] studied decontamination of real textile effluents on a BDD electrode; color and COD removal reached the Brazilian legal requirements after 12 h of treatment.

According to the previous reports, very few studies are conducted on electrochemical oxidation of X-GN using the BDD anode, and few studies reported the effect of BDD properties on dye effluent treatments. Electric energy consumption is the primary focus of electrochemical oxidation processes [27]; thus, the best operating parameters and performance of BDD electrodes need to be investigated for attaining a highest degradation efficiency. In this study, BDD electrodes were prepared with three different deposition time by hot filament chemical vapor deposition (HFCVD). The effects of varying operating parameters, such as film thickness, current density, supporting electrolyte concentration, initial solution pH, temperature, and initial dye concentration were investigated. Further, the UV-Vis spectrophotometer, TOC, and energy consumption were used for the characterization of the treatment effect.

2 Experiment

2.1 Preparation and characterization of BDD/Nb

A batch of BDD films deposited on Nb substrates (30 mm×20 mm×1 mm) was prepared by HFCVD. The HFCVD process and the pretreatment of substrates have been discussed in previous literatures [28, 29]. The mixture of 1.0 mL/min CH₄, 99 mL/min H₂, and 0.2 mL/min B₂H₆ (diluted to 5% with hydrogen) fed for boron addition was used as a reaction gas system. The substrate temperature measured by a thermocouple was set at around 800 °C, and the total pressure of BDD deposition was maintained at 3 kPa. The BDD films were deposited with deposition durations of 3, 6 and 12 h (named as 3 h BDD, 6 h BDD, and 12 h BDD) to obtain films with the thicknesses of 899 nm, 2.8 μm and 3.6 μm, respectively.

The morphology and grain size of the BDD films were observed using a FEI Sirion200 field emission SEM system. The boron doping level and phase purity were evaluated by Raman spectroscopy with LabRAM ARAMIS (484 nm and 10 mW). Cyclic voltammetric measurements were performed using the CHI 660E (Shanghai CH Instruments, China) to characterize the overpotential of BDD oxygen evolution.

2.2 Preparation of X-GN solution

X-GN (Figure 1) with the molecular formula C₁₉H₉Na₃Cl₂N₆O₁₀S₃, was purchased from Shanghai Eighth Dyestuff Chemical Plant and used without further purification. The structural formula is shown as follows: The dye solution used in all trials was prepared using deionized water. NaSO₄ was added to the dye solution as a supporting electrolyte due to the poor conductivity of the dye solution. H₂SO₄ and NaOH were used for pH adjustment. The pH value of the dye solution was tested with the pH meter (ST20, OHAUS Company, American). All reagents were bought from Sinopharm Chemical Reagent Co., Ltd, Shanghai.

2.3 Electrochemical oxidation

The bulks of the electrochemical oxidation experiments were conducted in a glass electrolytic cell containing 500 mL X-GN solution by direct- current power supply (RD-3020, Suzhou Wanruida Electric Company, China). The electrolytic cell was placed on a magnetic stirrer (WH220-HT, WIGGENS Company, Germany), which adjusts the stirring rate and temperature of the solution. The BDD/Nb samples were encapsulated as anode and the stainless steel plates were used as cathode. The distance between anode and cathode was maintained at 1 cm. Every experiment in this study was repeated three times to avoid errors, and the average of the three experiments was considered as the final result.

2.4 Analyses

A batch of 10 mL dye solution samples was taken from the electrolytic cell, at regular intervals of 30 min during the experiments, for analyses. A UV-Vis spectrophotometer (UV-2600, Shimadzu, Japan) was used to measure the concentration of X-GN because of the variations in X-GN concentration determined by absorbance at the maximum absorption wavelength of 479 nm. The degradation efficiency of the X-GN dye solution was calculated by the following equation [30]:

Color removal=(A₀-A_t)/A₀×100%            (1)

where A₀ and A_t denote the absorbances before electrochemical oxidation and at time t, respectively.

The mineralization degree of the X-GN dye solution was evaluated by the variation in TOC concentration using a TOC analyzer (TOC-L, Shimadzu, Japan). The specific energy consumption per volume of the treated effluent was estimated in kW·h/m³ by the following equation [31]:

E_c=UIt/(1000V)                          (2)

where U denotes the mean applied voltage (V); I is the current (A); t is the duration of treatment (h); and V is the volume of the treated effluent (m³).

3 Results and discussion

3.1 Effect of electrode with different deposition time

Figure 2 show the SEM images of 3 h BDD,6 h BDD, 12 h BDD, respectively. The continuous grains and clear grain boundaries without cracks or voids can be observed clearly in these images. Meanwhile, the grain size increases and tends to be more homogeneous as the deposition time increases. This can be explained by the fact that crystals are composed of irregular grains in the initial nucleation stage of growth and the grain size increases through columnar growth mode with time [32].

Figure 1 Structural formula of X-GN

Figure 2 SEM images of BDD films with different deposition time:

Figure 3 displays the Raman spectra of the BDD films. All the samples show a sharp diamond peak at 1331 cm^–1, and any obvious peak can be hardly observed around 1580 cm^-1 related to the non-diamond carbon phase, indicating that the samples are of high quality [33]. In addition, the two bands assigned to the boron atoms doping appear at approximately 500 cm^–1 and 1220 cm^–1 [34]. At 1331 cm^–1, the peak intensity gradually decreases with an increase in the deposition time, while the peak intensities of 500 cm^–1 and 1220 cm^–1 increase with an increase in the deposition time. This could be attributed to the Fano effect of boron doping, indicating an increase in boron atoms doped in the BDD film with an increase in thickness. Further, this result is consistent with the previous reports [35, 36], and will affect the electrocatalytic properties of the BDD electrode. The ratios of boron in 3 h BDD, 6 h BDD, and 12 h BDD films were calculated using Eq. (3) [37], and they are 5.27×10¹⁹/cm³, 1.71×10²⁰/cm³ and 1.97×10²⁰/cm³, respectively.

[B]=8.44×10³⁰exp(–0.048W)                (3)

where W is the wave number of the Lorentzian component of the 500 cm^–1 broad peak, and it can be obtained from the Raman spectroscopy results.

Figure 3 Raman spectra of BDD films with different deposition time

The cyclic voltammogram of the 3 h BDD, 6 h BDD, and 12 h BDD films in 0.1 mol/L H₂SO₄ solution is shown in Figure 4. The higher overpotential of oxygen evolution than that of other anode materials guarantees the high current efficiency during the degradation process. The oxygen evolution potentials of 3 h BDD, 6 h BDD and 12 h BDD are 2.6, 3.5 and 3.6 V, respectively. The overpotential increases with increasing deposition time; however, the overpotentials of oxygen evolution of 6 h BDD and 12 h BDD are close.

Figure 5 shows the changes in the UV-Vis spectra and color of the X-GN solution during the degradation process using the 6 h BDD electrode. X-GN can be characterized by the absorption peak at 479 nm attributed to the N=N chromophoric group in the visible region, and by the absorption peak at 298 nm attributed to the ultraviolet region [12]. The peak at 479 nm gradually became weak with time and almost reached zero absorbance values after 1.5 h of treatment, indicating that the N=N chromophoric groups were completely destroyed. The peak at 298 nm also declined with time and nearly disappeared after 2.5 h, indicating that the naphthalene ring also can be destroyed effectively by electrochemical oxidation. Moreover, the absorbance between 200–250 nm is related to the intermediate products, such as phenol, fumaric acid, and oxalic acid [2]. Their values decreased slowly with time, indicating that the dye molecule was breakdown first, and then the intermediate products were mineralized [12].

Figure 4 Cyclic voltammograms of BDD films with different deposition time at scan rate of 100 mV/s in 0.1 mol/L H₂SO₄ solution

Figure 5 UV-Vis spectra of X-GN during degradation process (a) and color variations during degradation process (b) (The experimental conditions: 100 mg/L X-GN, J=100 mA/cm², 0.05 mol/L of NaSO₄, pH 6.47(nature value), and room temperature)

The degradation effects of X-GN using the 3 h BDD, 6 h BDD, and 12 h BDD films were investigated. The degradation efficiency is affected by the performance parameters of the BDD electrodes, such as film thickness, boron concentration, and grain size. For example, FENG et al [21] and BOGDANOWICZ et al [38] observed that high boron concentration can improve film conductivity, catalytic activity, and degradation efficiency of effluent treatments. KROTOVA et al [39] reported that the catalytic activity can be improved with an increase in the crystallite size. As shown in Figure 6, the degradation efficiency of 3 h BDD is lower than that of the others and the degradation efficiency of 6 h BDD and 12 h BDD are close.

Figure 6 Variation of degradation efficiency with film thickness (The experimental conditions: 100 mg/L X-GN, J=100 mA/cm², 0.05 mol/L of NaSO₄, pH nature, and room temperature)

In conclusion, the electrocatalytic properties of the BDD electrodes are enhanced due to the growth of grain, the increase of boron dopant, and overpotential oxygen evolution when the deposition time of the BDD films increases from 3 h to 12 h, while the properties and the degradation efficiency of 12 h BDD were very closed to those of 6 h BDD. It can be speculated that the electrocatalytic properties are enhanced with the film thickness of BDD increasing; however, the degradation efficiency of X-GN is almost unchanged when the film thickness reaches a certain value. Thus, 6 h BDD is chosen to conduct the following experiments considering cost.

3.2 Effect of current density

The influence of current density on the degradation efficiency was investigated using applied current densities at 50, 100 and 150 mA/cm², and the results are depicted in Figure 7. It can be observed that the degradation efficiency was fairly high at the initial stage of degradation; however, it went down gradually as degradation continues. For instance, 80.4% of color removal can be reached after the first half hour, while the color removal increased from 80.4% to 97% during degradation for the next half hour at current density of 150 mA/cm². In the initial period of electrolysis, the electrolysis is under current-limited control due to the high dye concentration, and nearly all the applied current is used to attack the dye molecules. However, the electrolysis is then gradually controlled by mass transport as the dye concentration reduces, resulting in the decrease of degradation efficiency, which can be attributed to the appearance of side reactions, such as oxygen evolution (reaction (4)) [40].

Figure 7 Variation of degradation efficiency with current density (The experimental conditions: 100 mg/L X-GN, 0.05 mol/L of NaSO₄, pH nature, and room temperature)

Furthermore, the increase of degradation efficiency with current density can be obviously observed when other conditions were identical. This can be explained that the rate of electron and mass transfer and production of ·OH increased significantly in higher current density, which is beneficial to indirect oxidation of effluents (reactions (5) and (6), R denoted organic effluent). Further, the formation of mediated oxidants, such as S₂O₈^2–, H₂O_2, and O₃, can be considerably enhanced by increasing the current density using reactions (7)–(9) [41]. To be specific, 58.5%, 88.5% and 97% of color removal were obtained under current densities of 50, 100 and 150 mA/cm², respectively. However, there is no great improvement of degradation efficiency by increasing the current density from 100 to 150 mA/cm². This case can be attributed to the mass transport limitation [40]. Additionally, under the current density of 100 and 150 mA/cm², the color removal reached 99% after 2 h of degradation and the energy consumption values were 22.9 and 47.8 kW·h/m³, respectively. Considering the above analyses, the current density of 100 mA/cm² was considered as the best parameter.

BDD(·OH)→BDD+1/2O₂+H⁺+e       (4)

BDD+H₂O→BDD(·OH)+H⁺+e                    (5)

BDD(·OH)+R→BDD+mCO₂+nH⁺                 (6)

2SO₄^2–→S₂O₈^2–+2e                       (7)

2H₂O→H₂O₂+2e+2H⁺                     (8)

3H₂O→O₃+6e+6H⁺                       (9)

3.3 Effect of supporting electrolyte concentration

Electrolysis experiments of 0.025, 0.05 and 0.100 mol/L NaSO₄ concentration were conducted to investigate the effect of supporting electrolyte concentration on degradation efficiency as shown in Figure 8. It was evidently found that increasing supporting electrolyte concentration lead to the raise of degradation efficiency. The conductivity of solution is enhanced by adding supporting electrolyte, thus the electron transfer and the electrolysis reaction are accelerated [42]. Moreover, the strong oxidant S₂O₈^2– is generated more as reaction (5), thus, stimulating the oxidation reaction further and improving the degradation efficiency.

Figure 8 Variation of degradation efficiency with current density (The experimental conditions: 100 mg/L X-GN, J=100 mA/cm², pH nature, and room temperature)

As shown in Figure 8, after 1.5 h of electrolysis, 88.21% of color removal occurred under 0.025 mol/L electrolyte concentration, while the color removal reached to 97.2% and 98.9%, under 0.05 mol/L and 0.1 mol/L electrolyte concentration, respectively. We can assume that the influence of electrolysis concentration weakens gradually or even disappears when the electrolysis concentration reaches a certain level. Therefore, 0.05 mol/L was chosen as the optimum electrolyte concentration to reduce the cost of chemical without sacrificing the degradation efficiency.

3.4 Effect of initial solution pH

A set of experiments were conducted to evaluate the effect of initial solution pH between 3.05 and 10.94, and the results are shown in Figure 9. The degradation efficiency is favored at acidic initial pH values and inhibited at alkali conditions. Specifically, 94.8%, 92.8%, 88.5%, 85% and 83.1% of color removal were obtained at initial pH of 3.05, 5.08, 6.47 (nature value, without adjustment), 8.95, and 10.94, respectively, after 1 h of treatment. This is because the formation of ·OH and S₂O₈^2– can be promoted at acidic conditions, while they might be inhibited at an alkali condition [29]. The initial solution pH of 3.05 was the optimal pH value of this experiment.

Figure 9 Variation of degradation efficiency with initial pH value (The experimental conditions: 100 mg/L X-GN, J=100 mA/cm², 0.05 mol/L of NaSO₄, and room temperature)

3.5 Effect of solution temperature

Figure 10 illustrates the influence of solution temperature on the degradation efficiency by varying the temperature of solution between 30 and 60 °C. As shown in Figure 10, a higher solution temperature led to higher degradation efficiency. For instance, the color removal reached 69.9%, 73.9%, 76.8% and 82.9% at 30, 40, 50 and 60 °C, respectively, after half an hour of electrolysis. This is because the production rate of ·OH is improved at elevated temperature and this causes an acceleration of the electrolysis reaction [43]. In addition, the applied voltage declined with the increasing rate of mass transfer increased at higher temperature; thus, the energy consumption was reduced. Consequently, the ideal temperature of this experiment is 60 °C.

Figure 10 Variation of degradation efficiency with solution temperature (The experimental conditions:100 mg/L X-GN, J=100 mA/cm², 0.05 mo/L of NaSO₄, and pH nature)

3.6 Effect of X-GN concentration

Various studies were conducted by varying the X-GN concentration between the 50–150 mg/L range to elucidate the effect of X-GN concentration on the degradation efficiency, as depicted in Figure 11. It can be observed that higher degradation efficiency can be achieved with a lower X-GN solution. For example, 99.45% of color removal occurred for 50 mg/L X-GN after 1 h of electrolysis, compared to 88.5% and 79% of color removal for 100 mg/L and 150 mg/L, respectively. This could be ascribed to the fact that the same amount of oxidants is required in the same experiment conditions; thus, it takes longer time to reach the same color removal for X-GN of higher concentration [25,44].

Figure 11 Variation of degradation efficiency with X-GN concentration (The experimental conditions: 100 mg/L X-GN, J=100 mA/cm², 0.05 mol/L of NaSO₄, and 3.05 pH)

It is also worth noting that the mass of X-GN reduced to 24.86, 44.26 and 59.24 mg in the X-GN concentration of 50, 100 and 150 mg/L, respectively after 1 h of treatment. It can be concluded that more X-GN molecules are destroyed at the same time for high X-GN concentration, which can be attributed to the lower limitation of mass transport in higher X-GN concentration [45].

3.7 Energy consumption and TOC removal

The optimal experimental conditions have been determined through a series of experiments. The electrochemical oxidation of 100 mg/L X-GN was conducted under these conditions, and the energy consumption and TOC concentration were estimated. The results demonstrate that 99% of color removal is obtained with 10.6 kW·h/m³ of energy consumption after 1 h of treatment and 63.2% of TOC removal is reached with 31.2 kW·h/m³ of energy consumption after 3 h of electrolysis. The curve of TOC concentration as a function of time is displayed in Figure 12.

4 Conclusions

1) The 3 h BDD, 6 h BDD, and 12 h BDD films were prepared on Nb substrates by the HFCVD method. The performance of the BDD electrodes was characterized, and the electrolysis of X-GN using three types of BDD electrodes was conducted. The results show that the electrocatalytic properties are enhanced with the film thickness of BDD increasing; however, the degradation efficiency of X-GN is almost unchanged when the film thickness reaches a certain value.

Figure 12 Variation of TOC concentration with time (The experimental conditions: 100 mg/L X-GN, I=ted to the ultraviolet region 100 mA/cm², 0.05 mol/L of NaSO₄,pH nature, and 60 °C)

2) The influence of the varying operating parameters on the degradation efficiency was investigated based on color removal using 6 h BDD. Finally, the degradation results degradation of the 100 mg/L X-GN solution indicated that the optimum experimental conditions were as follows: current density 100 mA/cm², 0.05 mol/L supporting electrolyte concentration, initial solution pH 3.08, and 60 °C of solution temperature. Under these conditions, 100% of color removal and 63.2% of TOC removal were achieved with 31.2 kW·h/m³ after 3 h of electrolysis. These results prove that the X-GN solution can be degraded effectively on the BDD anode. Considering the energy consumption and high efficiency in the initial stage, this technique can be used as a pretreatment in other processes.

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(Edited by YANG Hua)

中文导读

硼掺杂金刚石阳极电化学氧化活性橙X-GN偶氮染料废水

摘要：本文研究了硼掺杂金刚石阳极（BDD）对活性艳橙X-GN染料的电化学氧化的影响。BDD电极是采用采用热丝化学气相沉积法（HFCVD）在铌（Nb）衬底表面制备的。研究了工艺参数（如膜的厚度、电流密度、电解液浓度、溶液pH值、溶液温度、染料初始浓度）对降解效率的影响。采用扫描电镜（SEM）、拉曼光谱和电化学工作站表征了BDD的微观结构和电化学性能，采用紫外–可见分光光度计测量了X-GN降解性能。结果表明，BDD薄膜厚度对电化学降解X-GN有显著影响。在优化的实验条件下（电流密度为100 mA/cm²、电解质浓度为0.05 mol/L、溶液初始pH值为3.08和溶液温度为60 °C）处理废水3 h后可得到100%的色度移除率和63.2%的总有机碳量（TOC）去除率。

关键词：活性艳橙X-GN；硼掺杂金刚石，薄膜厚度；电化学氧化

Foundation item: Project(2016YEB0301402) supported by the National Key Research and Development Program of China; Project(51601226) supported by the National Natural Science Foundation of China; Project supported by the Open-End Fund for the Valuable and Precision Instruments of Central South University, China; Project supported by State Key Laboratory of Powder Metallurgy, China

Received date: 2017-02-20; Accepted date: 2017-04-13

Corresponding author: WEI Qiu-ping, PhD, Associate Professor; Tel: +86–731–88876692; E-mail: qiupwei@csu.edu.cn; ORCID: 0000- 0002-8109-5982