Electro-activated persulfate oxidation of malachite green by boron-doped diamond (BDD) anode: effect of degradation process parameters

Water pollution has been a global problem with the increase of industrialization in various countries. A large amount of wastewater, including printing and dyeing wastewater, chemical wastewater, pharmaceutical wastewater, and tannery wastewater, are continuously produced in many industries (Diaz et al. 2011; Zhou et al. 2011; Katsoni et al. 2014; Mei et al. 2018a; Siedlecka et al. 2018). It is important to find efficient and low-energy demand technologies due to the large refractory sewage emission. Malachite green (MG) is a typical synthetic organic dye and widely used in dyes and fungicides (Srivastava et al. 2004; García-Rodríguez et al. 2016). MG and its intermediate products are seriously toxic to the water environment because of its biotoxicity and low biodegradability (Perez-Estrada et al. 2008; Gopinathan et al. 2015; Qu et al. 2019). As an emerging effective wastewater treatment technology, electrochemical advanced oxidation processes (EAOPs) can produce a highly oxidizing substance ·OH (E⁰ = 2.7 V, Vs. standard hydrogen electrode, SHE) by applying an electric potential to the anode (Guenfoud et al. 2014; Sasidharan Pillai & Gupta 2016; Ansari & Nematollahi 2018), and the generated hydroxyl radical can decompose the organic compounds efficiently thus leads a high degradation performance (Anglada et al. 2011; Chaplin 2014; Flores et al. 2017).

In recent years, different electrode materials, including graphite, boron-doped diamond (BDD) (Zhou et al. 2016), titanium electrodes, and lead oxide have been used to degrade organic sewage with different physical and chemical properties (Fernandes et al. 2014; Xing et al. 2018; Taner Can et al. 2019; Wachter et al. 2019). In particular, BDD has been widely used to treat various types of wastewater because of its large potential window and good chemical corrosion resistance (Deng et al. 2017; Candia-Onfray et al. 2018; Mei et al. 2018b; Zhu et al. 2018; Zheng et al. 2019). The modification of BDD has attracted great attention to improve the specific surface area and change the doping elements to produce hydroxyl radicals effectively (He et al. 2015; Mei et al. 2019). However, the effective mineralization of organic pollutants is also limited due to the short half-life time of ·OH (10⁻⁹s), thus the active radicals can only survive in the surface nearby anode which results in high energy consumption and low degradation efficiency (Haidar et al. 2013).

Persulfate is a strong oxidizing reagent (E⁰ = 2.01 V, SHE) that is close to ozone (E⁰ = 2.07 V, SHE) (Cao et al. 2019; Su et al. 2019). Persulfate is relatively stable at room temperature, but it is easily activated under specific external conditions such as ultraviolet, ultrasonic, heat, electricity, and transition metals to produce ·SO₄⁻ (E = 2.5–3.1 V) (Xiong et al. 2014; Chen et al. 2019a; Gao et al. 2019; Hayat et al. 2019; Lin et al. 2019; Sun et al. 2019). ·SO₄⁻ obtained greater oxidizing ability than ·OH in a neutral and alkaline environment. More importantly, compared with ·OH, ·SO₄⁻ has a longer half-life and it can react with organic compounds much easier in bulk solution (Long et al. 2019). Amongst the various activation method, electro-activated persulfate technology is more efficient, simpler and does not cause secondary pollution (Matzek & Carter 2016). Combining persulfate activation technology with electrochemical oxidation technology can achieve higher degradation efficiency and reduce power consumption (Carter & Farrell 2008). Some works compared the variety of electrode materials for electroactive persulfate (as: SnO₂, Pt, BDD) and considered that BDD can obtain satisfactory persulfate activation and degradation efficiency (Cai et al. 2014; Chen et al. 2019b; Ding et al. 2019).

Herein, we report our work with decomposing malachite green dye wastewater by BDD electro-activated persulfate (BDD-EAP). Firstly, the BDD anode was successfully prepared by the hot filament chemical vapor deposition (HFCVD) method, and the morphology and structure of BDD were characterized. Secondly, the degradation efficiency and energy consumption of BDD-EAP technology were investigated in detail compared with the traditional BDD-EAP system, and then the optimization of the degradation parameters such as persulfate concentration, current density, and initial pH were developed. Lastly, possible mechanisms for BDD-EAP process were proposed.

The BDD electrode used in the experiment was deposited on a 50 mm × 60 mm × 2 mm Si substrate by hot filament chemical vapor deposition (HFCVD) of six straight wires arranged in parallel. The distance between the hot filaments was 10 mm, and the distance from the hot filaments to the substrate was 8 mm. The mixture of gas was in the ratio of B₂H₆ (diluted by 95% of hydrogen): CH₄: H₂ = 0.6 sccm (standard-state cubic centimeter per minute) : 4 sccm: 100 sccm. The temperature of the deposition system was kept at 850 °C and the pressure was maintained at 3 kPa, the deposition time was 2 h. The deposition process was later changed to H₂: B₂H₆: CH₄₌100 sccm: 3 sccm: 0.45 sccm, deposition time was 10 h, and deposition temperature was 850 °C.

The surface morphology and grain size of BDD films were analyzed by scanning electron microscopy (SEM). The boron doping and diamond phase purity of BDD films were detected by Raman spectroscopy. The SEM used in this experiment was a Nova Nano SEM 230 field emission scanning electron microscope. The Raman curve was recorded on a LabRAM HR800 laser microscopic Raman spectrometer.

The initial absorbance or TOC of the dye and the absorbance or TOC at the time of degradation t is expressed as A₀ and A_t, respectively.

The SEM and Raman of the BDD electrode are shown in Figure 1. The diamond have high sp³/sp² ratio due to Raman curve does not show obvious peaks around 1,580 cm⁻¹ (sp²) (Zhang et al. 2020), and the characteristic peaks of boron doping are found around 500 cm⁻¹ and 1,220 cm⁻¹ (Zhang et al. 2019). As illustrated in Figure 1(b), the diamond size distribution is uniform, and the particle size distribution is 3–5 μm.

The MG removal rate by electro-activated persulfate oxidation is summarized in Figure 2. The color removal rate presents a trend that BDD-EAP > PS > BDD-EO (Figure 2(a)). The color removal rate of BDD-EAP was 95.92% after 30 min, which increased by 3.37 times compared to BDD-EO (28.46%) and 1.80 times compared to PS (53.24%). The TOC removal of BDD-EAP increases by 2.2 times for BDD-EO and 1.7 times for PS at 120 min (Figure 2(b)). This might be the lower yield of ·SO₄⁻ produced by persulfate activation leading to weaker degradation efficiency at room temperature.

Besides, further investigation of BDD-EAP removal for MG was analyzed by UV-Vis (Figure S1(a)) (Supplementary Material). The characteristic wavelength absorbance peaks of MG are located at 315 nm, 425 nm and 618 nm. It should be noted that there is a blue-shift of maximum absorption peak from 617 nm to 604 nm due to the N-demethylation reaction caused by sulfate radical (Liang et al. 2017). The damage of the MG conjugate breakage of the whole conjugated aromatic structure can be reflected by the peak drop at 425 nm (Ansari & Nematollahi 2018). The removal rate of the chromophore in the MG can be reflected by the absorbance in the UV-Vis spectrum at 604 nm. It is important to note that there is a new absorption peak at 300 nm, possibly attributed to the benzene ring-opening or the cleavage of central carbon (Liang et al. 2017). As shown in Figure S1(b), the intensity of the mark benzene ring at 300 nm is not lowered after the PS addition until 17 h and 70 h. It indicates that the generated degradation byproducts are recalcitrant to persulfate radicals due to the oxidizing ability of persulfate is lower than sulfate radical. Figure S1(c) shows the removal rate of the PS is close to the intensity of BDD-PS at 604 nm. However, it can be seen from UV-Vis spectra that the intensity at 425 nm reflecting the conjugated structure of MG and the intensity at 300 nm indicating the structure of the benzene ring is still high. The chromophore of MG is destroyed by the attack of persulfate, but the conjugated structure and the benzene ring are still unable to be effectively destroyed. This result showed that the BDD-EAP can effectively improve the efficiency of persulfate degradation, and the performance is significantly better than the BDD-EO degradation system.

The removal of the MG in two electrolytes is shown in Figure 3. The color removal rate for Na₂SO₄ reached 48.90% at 60 min, whereas the color removal for persulfate reached 99.68%. The color removal rate of the persulfate electrolyte was more than 90% at 45 min, which was two times that of sulfate electrolyte. The unit energy consumption is very important for practical application. The unit energy consumption of the persulfate electrolyte is significantly lower than that of the sulfate electrolyte for color removal (Figure 3(b)). As indicated in Figure 3(c), TOC removal increased from 28% (sulfate) to 52% (persulfate). The higher degradation efficiency for persulfate electrolyte than that of sulfate electrolyte due to the half-life of the ·SO₄⁻ with 30–40 μs is usually longer than that of the ·OH (<1 μs), and thus ·SO₄⁻ has better mass transfer performance and contact chance with the target pollutants (Zhi et al. 2020). Besides, persulfate electrolyte can significantly reduce the unit energy consumption for TOC removal (Figure 3(d)). The energy consumption of TOC for persulfate electrolyte is a remarkable decrease compared with the sulfate electrolyte. These experimental phenomena indicate that the BDD-EAP not only improves the removal efficiency but also reduces energy consumption.

As indicated above, the electrochemical oxidation of a single organic pollutant can be described by a first-order kinetic model. In this part, the effect of persulfate concentration on MG degradation efficiency was explored, and the first-order kinetic model was plotted to analyze the effect of persulfate concentration on the MG removal efficiency.

As illustrated in Figure 4(a), the color removal for persulfate (PS) of 0.02 M, 0.06 M and 0.1 M reached 99.68%, 99.93% and 99.75%, respectively. The energy consumption required to remove unit color decreases as the persulfate concentration increases (Figure 4(b)). A control experiment with sulfate as supporting electrolyte to verify the excellent degradation efficiency of BDD-EAP. As shown in Figure S2(a), the color removal rate for Na₂SO₄ of 0.02 M, 0.06 M, and 0.1 M reached 48.90%, 81.89%, and 61.47% at 120 min, respectively. This indicates that the degradation efficiency is significantly improved with the increase of sulfate electrolyte concentration. As a comparison, persulfate electrolyte can achieve high degradation efficiency at low electrolyte concentrations (Figure S2(c)), the rate constants for persulfate concentration of 0.02 M, 0.04 M, 0.06 M, 0.08 M and 0.1 M are 0.091, 0.131, 0.155, 0.167, and 0.179 min⁻¹, respectively. The TOC removal rate gradually increases with the persulfate concentration increase; this is attributed to the production of ·SO₄⁻ increasing with the increase of persulfate electrolyte concentration (Figure 4(c)). The trend of energy consumption to remove the unit TOC is also consistent with the trend of color removal as Figure 4(d).

Figure 4(e) shown as the change of removal rate for three different concentrations of MG by BDD-EAP process. The dye concentration is increased has little effect for MG removal performance, which all can achieve more than 95% within 30 min. This phenomenon is similar to the work of Mei (Mei et al. 2018a), which indicates the BDD-EAP can exert strong ability of degradation within a certain organic concentration range.

In the electrochemical oxidation system, the degradation performance is correlated with current density intimately. As shown in Figure 5(a), the MG removal efficiency is gradually increased with the increases of electrode current density. Besides, the color removal at the initial stage of degradation (10 min) increased with the increase of current density from 1.7 mA/cm² (52.40%) to 11.7 mA/(84.82%). The color removal rate with current densities greater than 3.4 mA/cm² all exceeded 90% within 15 min. As the kinetic model shows, the degradation rate constants increased from 0.133 to 0.223 min⁻¹ with the current density increases, and TOC removal increases from 42.3% to 60.1% (Figure 5(c)). For energy consumption, the unit energy consumption versus TOC removal is rapidly increased with the increase of current density. As seen in Figure 5(a), increasing current density to 5 mA/cm² increases significantly the rate of MG degradation and has lower energy consumption. Therefore, 5 mA/cm² is the optimal current density.

In summary, the increase of current density will lead to the degradation efficiency increasing because it will promote the generation and activation of persulfate. However, the increase of current density will also increase the rate of side reactions, which may lead to the increase in energy consumption and decrease of current efficiency, thereby increasing operating costs.

The actual type of wastewater is complex, thus with a wide pH range. The ·OH has extremely high sensitivity to the pH of the system. In this paper, the degradation efficiency of MG by BDD-EAP technology for different pH range (2–10) was explored. As shown in Figure 6, the degradation efficiency of the electro-activated persulfate will be affected by the initial pH of water. The degradation efficiency is decreased with the increase of pH (Figure 6(a)). The degradation efficiency is not much different in neutral and weakly alkaline conditions. The same rule can be obtained from the TOC removal rate in Figure 6(b). In traditional hydroxyl radical-based EO process, hydroxyl radicals are unable to maintain high oxidizing power under alkaline pH environments (Haidar et al. 2013; Wachter et al. 2019). The BDD-EAP technology in this paper can maintain high degradation efficiency in a wide range of pH. The color removal rate all exceeded 87% in pH 2.3–9.8 within 20 min. It is further explained that the technology can suitable for degradation in Widely pH range.

Besides, we compared the degradation efficiency of BDD-EAP and other reported technologies for the electrochemical treatment of MG in Table 1 (El-Ghenymy et al. 2015; Ansari & Nematollahi 2018; Jiang et al. 2018; Ray et al. 2018; Ergut et al. 2019). BDD electro-activated persulfate technology is more efficient than other technologies, which indicates the proposed BDD-EAP process is more privileged in practical application, and in comparison with other related works of BDD-EAP. Song and colleagues (Song et al. 2017) used Ti/Pt activated persulfate to degrade organic matter, the changed law of degradation efficiency at pH 1–7 was explored. In this paper, degradation efficiency was measured over a wider pH range (2–10). Frontistis and colleagues (Frontistis et al. 2018) reports the law of BDD-EAP degradation of ampicillin by different current densities (5–110 mA/cm²). In order to reduce energy consumption, this paper explores the efficient degradation of BDD-EAP at low current density (1.7–11.7 mA/cm²). Compared with Zhang and colleagues (Zhang et al. 2014), this paper investigates the degradation efficiency within a wider range of persulfate (0.02–0.1 M) concentration.

In this paper, BDD electro-activated persulfate degradation technology is proposed to degrade MG dye organic pollutants. The relationship between persulfate concentration, current density, initial degradation of pH and degradation efficiency are investigated in this paper. The corresponding results indicated this technology possesses the advantages of low energy consumption and high efficiency in a wide pH range, which indicates the proposed BDD-EAP technology is privileged for practical refractory wastewater treatment.

We gratefully acknowledge the National Key Research and Development Program of China (No. 2016YFB0301402, No. 2016YFB0402705), the National Natural Science Foundation of China (No. 51601226, No. 51874370, No. 51302173), the State Key Laboratory of Powder Metallurgy, the Fundamental Research Funds for the Central Universities of Central South University (2018zzts014 and 2017gczd024), Hunan Provincial Innovation Foundation for Postgraduate (CX2018B085) and the Open-End Fund for Valuable and Precision Instruments of Central South University for financial support.

The Supplementary Material for this paper is available online at https://dx.doi.org/10.2166/wst.2020.176.