Rhodamine B (RhB) wastewater could be degraded by a three-dimensional electrolytic reactor with surface-modified titanium anodes, and a variety of materials had been tried to prepare for particles electrodes to enhance its removal effects, among them, granular activated carbon (GAC) with large specific surface areas and stable chemical properties was selected as particles materials and coated by manganese oxidation (Mn) as the main active ingredient. The experimental results showed that 98.3% of RhB and 60.7% of chemical oxygen demand were removed respectively, and the RhB wastewater's biodegradability was improved either. On the superficial sites of GAC/Mn–Sn particles, hydroxyl radicals were generated, and some absorbed RhB molecular was initially decolored by hypochlorite removing the two ethyl groups on both sides of the molecular, then oxidized by hydroxyl, and continually decomposed by these strong oxidants into a variety of intermediates.

  • The removal rate of Rhodamine B (RhB) was increased obviously by the three-dimensional electrode reactor, packed with surface-modified GAC/Mn–Sn particles electrodes.

  • Hydroxyl radicals were probably generated by the surficial active sites of GAC/Mn–Sn particles electrodes that metastable solid Mn–Sn oxides were catalytic and conductive to the electrochemical process.

  • The biodegradability of wastewater was increased.

Graphical Abstract

Graphical Abstract

It is about 80–100 m3 water for one ton of printing and dyeing textiles to consume, and about 80–90% of consumed water will become wastewater (Wang et al. 2018), as a whole, about 2 billion tons of printing and dyeing wastewater per year was produced, ranked as the fifth place in the total industrial wastewater discharge in China (Oller et al. 2011; Asghar et al. 2015). Dyes are the main pollutants in printing and dyeing wastewater, which are chemically stable and have carcinogenic, teratogenic and mutagenic effects on organisms (Turhan et al. 2012; Aquino et al. 2014). It will affect the water's light transmittance and gas solubility, and endanger the health of aquatic organisms and humans (Alves de Lima et al. 2007; Markandeya et al. 2015; Wong et al. 2019). Rhodamine B (RhB) dyes were among the earliest and most broadly applied synthetic dyes, and were utilized in a variety of industrial applications (Amalin et al. 2022).

At present, adsorption (Deng et al. 2019), membrane separation (Puspasari & Peinemann 2016), biochemical (Paz et al. 2017), electro-flocculation (Nippatla & Philip 2019), advanced oxidation (Baldisserri et al. 2018) and other methods have been widely used to treat printing and dyeing wastewater, among them, advanced oxidation process (AOP), such as photocatalytic degradation, is considered to be the most effective treatment methods because it can decompose refractory organic matter by generating strong oxidizing substances, such as hydroxyl radical (•OH) with a redox potential of 2.80 eV (Wang et al. 2014; Asghar et al. 2015; Xu & Ma 2021). The three-dimensional electrode reactor (TDER), packed with conductive particles such as granular activated carbon (GAC), ceramics and γ-Al2O3 between anode electrodes and cathode electrodes as third particles electrodes, was applied to the wastewater treatment (Sun et al. 2017; Pedersen et al. 2019; Zhang et al. 2019). These conductive particles could be polarized to form a large number of charged microelectrodes, which significantly increase the specific surface area of the electrodes and improve the electrolysis efficiency (Xiong et al. 2003; Zhang et al. 2013).

GAC, with a large specific surface area and stable chemical properties, had the potential to be used as particles electrode, and the surficial characteristic of GAC could be modified and improved by coating active materials (Li et al. 2013, 2014). Manganese oxide, with good conductivity and oxygen evolution reaction, could be used as an active catalytic material coated on the surfaces of electrode particles (Chen et al. 2019), for example, the synthetic δ-MnO2/MWCNT nanocomposite catalyst for the oxidative degradation of Anionic Reactive Blue 19 (RB19) dye solution was investigated in detail (Fathy et al. 2013). Montmorillonite coated with manganese oxide was also used to adsorb Methylene blue (He et al. 2018). Other than manganese oxide, SnO2, with high oxygen evolution potential, excellent conductivity and chemical stability, is also used as an excellent doping material (Li et al. 2016a). SnO2–Sb-doped TiO2 was ever coated on the surface of GAC and used to increase the hydroxyl radical generation rate in the electrochemical oxidation of synthetic dyeing wastewater (Li et al. 2016b).

However, as particles electrodes, few studies were about the use of SnO2-doped manganese oxide on GAC to degrade dye wastewater. In this paper, as particles electrodes, surface-modified GAC/Mn–Sn was prepared and used to enhance the RhB removal efficiency. The particles electrodes were characterized and analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffractometry (XRD). The electrochemical oxidation mechanism was explained by the analytical data of UPLC-MS.

In recent years, our research group had focused on the electrochemical degradation of target pollutants, such as nitrate nitrogen of industrial wastewaters, many types of metal materials had been tried to fabricate anodes and cathodes, and furthermore, polymer plastic was also experimented to use as third particles electrode besides GAC particles.

Preparation of GAC/Mn–Sn particles electrodes

GAC was purchased from McLean Reagent. Co., Ltd (Shanghai, China). All the other chemicals were from Sinopharm Group Chemical Reagent Co., Ltd (Beijing, China) and were of analytical grade (AR). Deionized water was manufactured using the Millipore apparatus.

In this work, MnO2 and SnO2 were selected by added doping reagents, for its catalysis for chemical organic reactions and excellent conductivity characteristics.

The GAC/Mn–Sn particles electrodes were prepared by the chemical precipitation-roasting method that was suggested by Ma et al. with minor modifications (Ma et al. 2009). 25 g of washed and dried GAC was immersed in solutions of MnCl2 (0.04 mol/L, 250 mL) with vigorously magnetic stirring at room temperature for 2 h. 250 mL of 0.1 mol/L KMnO4 solution was subsequently added into the suspension to make the solution precipitate, which indicated the formation of MnO2 according to the following equation:
(1)

After that, 100 mL of 0.02 mol/L of SnCl2 solution was poured into the solution that had generated MnO2, and stirring was continued for 2 h. The suspension was filtered, and the solid obtained was washed several times with deionized water and dried at 105 °C for 12 h in an oven. Then, the pre-prepared GAC/Mn–Sn samples were roasted at 450 °C for 2 h to obtain GAC/Mn–Sn particles electrodes.

Experimental setup and main analysis

The TDER was a rectangular tank made of polypropylene with an effective volume of 100 mL (50 mm × 20 mm × 100 mm). The main electrodes including a catalytic plate anode (45 mm × 100 mm × 1 mm) and a titanium plate cathode with the same dimension, were positioned vertically and parallel to each other with an inner gap of 2 cm. The granular GAC/Mn–Sn particles electrode was packed between the anode and cathode, and the air was blown into the wastewater through a microporous aeration stone at the bottom of the tank and the aeration rate was adjusted by a flowmeter. The electric field was supplied using digital DC power (WYK-605D, Shanda Electronics Co., Ltd, Guangdong Province, China). NaCl was used as an electrolyte. The anodes–cathodes and particles electrodes used in the experiments are shown in Figure 1(a) for a glance.
Figure 1

Materials used in the experiments (a) and cyclic volt-ampere (b) and Tafel curves (c) of three different anode plates.

Figure 1

Materials used in the experiments (a) and cyclic volt-ampere (b) and Tafel curves (c) of three different anode plates.

Close modal

The electrolysis reaction was carried out at atmospheric pressure and ambient temperature, the reaction time was controlled to 30 min and the RhB water sample was taken out and measured every 5 min.

The catalytic plate, which surface was modified by Ru and Sn oxides, was used as anodes, its characteristic of CV and Tafel curves was tested in detail compared with the titanium plate and stainless plate, seen in Figure 1(b), from −0.4 to 0.7 V, the conductivity of catalytic plate was bigger than that of other plates, it indicated that the active RuO2, IrO2 and SnO2 were contributed to the generation of radicals and degradation of dyes. Figure 1(c) shows that the corrosion current density of the catalytic plate was higher than other plates, which had some resistance to corrosion in use.

The surface morphology and energy-dispersive spectroscopy of the GAC-Mn/Sn were obtained using SEM and EDS (SEM, JSM-6700F, JEOL, Japan), respectively. The crystal structures of the particles electrodes were investigated with an X-ray diffractometer (XRD, D/MAX-2500/PC, Rigaku, Japan). The remaining RhB concentration was analyzed at 554 nm with a UV–Vis spectrophotometer (UV-2600, Shimadzu, Japan), and the adsorption spectra of the liquid samples were measured on it either.

The possible intermediates of RhB degradation were identified by a UPLC-MS (WATERS UPLC-Q/T of Xevo G2-XS) mass spectrometer (Ji et al. 2018). The process was as follows: the pH of the samples was adjusted to 2 with sulfuric acid, and 1 mL of pH adjusted solution was extracted using 1 mL CH2Cl2, then the dichloromethane extract was dehydrated with anhydrous sodium sulfate. The final sample was automatically injected into a UPLC-MS system equipped with a DB-5 column (30 m × 0.25 μm × 0.25 mm) with a heating mode followed by an initial temperature of 50 °C held for 5 min, and then ramped to 60 °C at 5 °C/min, ramped to 290 °C at 10 °C/min and held at 290 °C for 15 min (Diao et al. 2017). The decomposition products were analyzed using NIST08.LTB mass spectrometry library data.

The RhB removal rate (%), the chemical oxygen demand (COD) removal rate (%) and the energy consumption (Es, kWh kg−1 COD) were estimated through the following equations (Shen et al. 2017):
(2)
(3)
(4)
where C0 and Ct are the initial and final concentrations of RhB (mg/L), COD0 and CODt are the initial and final concentrations of the wastewater calculated as COD (mg/L), U is the voltage (V), I is the average current (A), t is the treatment time (h) and V is the volume of the simulated wastewater (L).

Comparison of eight different types of conductive particles electrodes

It is important to select an appropriate material to be used as conductive particles electrodes, and its characteristics must be as follows: good mass transfer efficiency, electric conductivity and large specific surface areas.

For testing, both GAC and γ-Al2O3 (activated alumina) were selected as basic materials to manufacture conductive particles electrodes, while oxides of Mn–Sn–Sb were tried as the main active material, thus eight types of conductive particles were made for TDER experiments, they were named as GAC, GAC-Mn, GAC/Mn–Sb, GAC/Mn–Sn, γ-Al2O3, γ-Al2O3–Mn, γ-Al2O3/Mn–Sb and γ-Al2O3/Mn–Sn conductive particles, respectively.

GAC and other particles’ conductive characteristic were critical as kinds of Third particles electrodes, while they all have certain adsorption properties themselves on RhB, thus, the saturation adsorption tests were required for the GAC and GAC/Mn–Sn particles before its usage as conductive particles electrodes, the process was as follows: they were immersed in the RhB solution, and the RhB concentrations of solutions were periodically measured and changed until these particles reached the adsorption saturation of RhB to avoid its influence of adsorption on electrolysis experiments (Pedersen et al. 2019).

Four types of GAC particles coated with different active ingredients, GAC, GAC-Mn, GAC/Mn–Sb and GAC/Mn–Sn, were tested under the same conditions. Figure 2(a) shows that the degradation efficiency of RhB by GAC/Mn–Sn particles was 93.5% in 30 min, bigger than that by GAC, GAC/Mn–Sb and GAC/Mn particles.
Figure 2

Effect of different types of particles electrodes on the degradation of RhB. (a) Different GAC particles and (b) different γ-Al2O3 particles.

Figure 2

Effect of different types of particles electrodes on the degradation of RhB. (a) Different GAC particles and (b) different γ-Al2O3 particles.

Close modal

In the same way, four types of γ-Al2O3 particles coated with different active ingredients, γ-Al2O3, γ-Al2O3/Mn, γ-Al2O3/Mn–Sn and γ-Al2O3/Mn–Sb, were also tested in detail. Figure 2(b) shows that the degradation efficiency of RhB by γ-Al2O3/Mn–Sn particles was 89.1% in 30 min, also bigger than that by γ-Al2O3, γ-Al2O3/Mn and γ-Al2O3/Mn–Sb particles. The results were similar to that of surface-modified GAC particles.

Based on the results mentioned above, it seemed that the degradation efficiencies of RhB could be improved obviously by particles electrodes, and especially by GAC/Mn–Sn particles, which could be actually selected as particles electrode to fill in TDER, the results could be explained that some H2O was absorbed on the active MnO2–SnO2 sites, and may be decomposed to hydroxy radicals that would attack RhB dyes lately.

Enhanced removal effects of RhB by GAC/Mn–Sn particles electrodes

Three control experiments were carried out on the removal of RhB (300 mg/L) using two-dimensional electrolytic reactor (2DER), TDER-GAC (with GAC particles electrodes) and TDER-GAC/Mn–Sn (with GAC/Mn–Sn particles electrodes), respectively.

As seen from Figure 3(a) and 3(b), the RhB removal rates by 2DER, TDER-GAC and TDER-GAC/Mn–Sn after 60 min electrolysis were 65.8, 80.7, and 94.3%, respectively. The removal rates of TDER showed significantly higher than that of 2DER. The added particles electrodes increased the reacting areas of the reactor, facilitated the mass transfer and improved electrolysis efficiency (Shen et al. 2017). Compared with GAC particles electrodes, the GAC/Mn–Sn particles electrodes had a better electro-catalytic performance for RhB removal efficiency. This indicated that it was the manganese oxide coated on the surface of GAC particles to improve the performance (Li et al. 2015).
Figure 3

Pretreatment of RhB by different types of electrochemical reactors: (a) control experiments of adsorption saturation and (b) effects of different types of electrochemical reactors (8 V of cell voltage and 0.6 g/L of electrolyte concentration).

Figure 3

Pretreatment of RhB by different types of electrochemical reactors: (a) control experiments of adsorption saturation and (b) effects of different types of electrochemical reactors (8 V of cell voltage and 0.6 g/L of electrolyte concentration).

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A series of single-factor experiments were carried out to find out the optimal variables of TDER, including the electrolysis voltage, the electrolyte concentration, the dosage of GAC/Mn–Sn particles and the initial concentration of RhB.

The electrolysis voltage provides electrical power for the electrochemical reaction. Figure 4(a) shows that the degradation rate of RhB was increased with the increasing voltage, but the larger electrolysis voltage means greater energy consumption (Jung et al. 2015), therefore, the 8 V electrolysis voltage was chosen as the applied electrolysis voltage.
Figure 4

Effect of the main factor on RhB removal rate: (a) electrolysis voltages; (b) electrolyte concentration; (c) dosage of GAC/Mn–Sn particles and (d) initial concentration of RhB.

Figure 4

Effect of the main factor on RhB removal rate: (a) electrolysis voltages; (b) electrolyte concentration; (c) dosage of GAC/Mn–Sn particles and (d) initial concentration of RhB.

Close modal

Sodium chloride is used as the electrolyte, Cl of the solution will generate Cl2 and ClO during the electrolysis process, and increase the oxidative degradation ability of organic pollutants (Zambrano & Min 2019). Figure 4(b) shows that the RhB removal rate was increased linearly with the sodium chloride concentrations, when the concentration of sodium chloride was increased from 0.4 to 0.8 g/L, the removal rate of RhB could also increased from 82.1 to 95.5%. The effect of Cl on the degradation efficiency was not inhibitory as that in the photocatalytic process (Isari et al. 2018).

The GAC/Mn–Sn particles electrodes prepared by the precipitation-roasting method have no adsorption properties, which was prone to be polarized to form large amounts of small electric micro-electrolytic cells between the anodes and cathode plates (Feng et al. 2017), The conductive particles coated by the catalytic ingredients MnO2 and SnO2 were equivalent to significantly increasing the active specific surface areas of the electrodes. Figure 4(c) shows that the degradation rate of RhB was increased from 82.4 to 92.5% while increasing the particles dosage. Then, 40 g/L was selected as the dosage of GAC-Mn/Sn particles in TDER.

The effects of different initial concentrations on the degradation of RhB were also investigated in detail, as shown in Figure 4(d), while the initial concentration of RhB was increased from 300 to 700 mg/L, and that of RhB was decreased from 93.5 to 63.6% in 30 min. This was probably because the hydroxyl radicals and hypochlorous acid generated by the electrodes were also certain, and not enough to decompose more RhB. Then, the initial RhB concentration was determined as 300 mg/L.

Characteristic changes of RhB before and after treatment

The degradation effect of RhB by TDER can also be shown directly from the UV–Vis spectrum, a UV–Visible spectrophotometer (Shimadzu UV-2600) was used to scan the RhB wastewater samples in the range of 200–1,000 nm every 5 min. As Figure 5 shows, RhB has distinct characteristic peaks at 250, 350 and 554 nm, of which 554 nm corresponds to an ethyl chromophore and 255 nm corresponds to a benzene ring structure (Chen et al. 2016; Radoń et al. 2019). With the increase of the electrolysis time, the height of the peaks at three positions continued to decrease, and finally became a smooth straight line, indicating that the electrolysis destroyed the structure of the RhB molecule. A complete degradation performance of RhB could be achieved within 30 min by TDER packed with GAC/Mn–Sn particles electrodes.
Figure 5

UV–Vis spectrum of RhB wastewater samples (9.45 V of voltage, 0.79 g/L of electrolyte, 32.37 g/L of GAC/Mn–Sn dosage and 300 mg/L of initial RhB concentration).

Figure 5

UV–Vis spectrum of RhB wastewater samples (9.45 V of voltage, 0.79 g/L of electrolyte, 32.37 g/L of GAC/Mn–Sn dosage and 300 mg/L of initial RhB concentration).

Close modal
When RhB was electrochemically fully decomposed, a mineralization process would occur simultaneously, Figure 6 shows, under the optimal conditions, the COD removal rate of RhB simulated wastewater can reach 89.2% in the TDER, and its energy consumption was 20.1 kWh kg−1 COD, compared with other TDERs, seen from Table 1, the energy consumption of this study is significantly lower, thus it is significant to decompose RhB wastewater by the electrochemical process.
Table 1

Comparison of COD removal rate and energy consumption

ContaminantsCOD removal rate (%)Ec (kWh kg−1 COD)Ref
RhB textile dyehouse effluents 60.7 20.1 This work 
80 135 Tsantaki et al. (2012)  
RhB industrial wastewater 60–70 400 Li et al. (2016b)  
50 180 Can et al. (2014)  
ContaminantsCOD removal rate (%)Ec (kWh kg−1 COD)Ref
RhB textile dyehouse effluents 60.7 20.1 This work 
80 135 Tsantaki et al. (2012)  
RhB industrial wastewater 60–70 400 Li et al. (2016b)  
50 180 Can et al. (2014)  
Figure 6

COD removal efficiency in the electrochemical process.

Figure 6

COD removal efficiency in the electrochemical process.

Close modal
In general, electrolysis was merely used as a kind of AOP way to pretreat dyes wastewater, because the treated water's COD was uncertain up to the local environmental standards yet, however, it is significant to pretreat dyes wastewater and increase its B/C values (ratio of biochemical oxygen demand to chemical oxygen demand) or its biodegradability, thus it could continuously be treated by the successive biological process that was regarded as low operational cost. Sludge oxygen utilization rate (SOUR) was used to represent the biodegradation of RhB wastewater samples treated at different times, here, a DO meter was used to measure the dissolved oxygen content of the wastewater samples at different electrolysis times added with activated sludge, and the oxygen consumption of the sludges was calculated. Figure 7 shows the SOUR values of RhB wastewater samples treated at different times, which unit is mgDO/[gMLSS.h]. All the SOUR values are significantly different. Among them, the activated sludge had the largest oxygen consumption in RhB wastewater samples treated at 30 min. The SOUR of differently activated sludges were distilled water (0.2404), raw water (0.2987), 10 min (0.5056), 20 min (0.8618) and 30 min (0.9224). The results indicated that catalytic-electrolysis of RhB wastewater samples by TDER is conducive to the wastewater's improvement of biodegradability.
Figure 7

Oxygen consumption of RhB wastewater samples at different times.

Figure 7

Oxygen consumption of RhB wastewater samples at different times.

Close modal
Other than RhB, three dyes including Methylene blue, Methyl orange and Methyl violet, were also tried to treat by the same TDER packed with GAC/Mn–Sn particles electrodes, and the results are shown in Figure 8(d). The three different dyes were decomposed excellently, the degradation rate of Methyl violet was the fastest within 60 min of electrolysis time, which had already reached more than 90% in 30 min, followed by Methyl orange and Methylene blue.
Figure 8

Degradation of three kinds of dyes. UV–Visible full-band scan (a) Methylene blue; (b) Methyl orange; (c) Methyl violet and (d) removal rate of dyes by TDER.

Figure 8

Degradation of three kinds of dyes. UV–Visible full-band scan (a) Methylene blue; (b) Methyl orange; (c) Methyl violet and (d) removal rate of dyes by TDER.

Close modal

Seen from the UV–Visible full-band scans in Figure 8(a)–8(c), the height of the peaks of the three dye molecules gradually decreases until they become smooth curves, the results indicated that the electrolytic system destroyed the molecular structures of the three different dyes through direct and indirect reactions, and achieved the degradation of the different structural dye molecules, thus, dyes wastewater could be degraded very well by TDER packed with GAC/Mn–Sn particles electrodes. It was decolorized and mineralized obviously while pre-treated electrochemically, and was much more easily to be degraded biochemically by the subsequent conventional activated sludge process.

Characteristics of GAC/Mn–Sn particles electrodes

The SEM images were taken to observe the differences in surface morphology of GAC and GAC/Mn–Sn particles. Figure 9(a) shows that the surface of pure GAC was rough and had fine cracks and micro-pores, while that of GAC/Mn–Sn in Figure 9(b) was relatively uniform, and its particle size was smaller. This was because the micro-pores of GAC surface were coated and filled by manganese-tin oxides during the modification process, which was well dispersed and distributed on its surfaces to provide an abundance of reaction sites (Li et al. 2016b; You et al. 2017). The elements of GAC/Mn–Sn particles electrodes were analyzed, and the result is shown in Figure 9(c). The result showed that C, Mn, Sn and O are the main elements on the surface of GAC/Mn–Sn particles electrodes, and Mn and Sn are actually detected in the form of oxide.
Figure 9

SEM images and EDS spectrum of GAC/Mn–Sn particles electrodes. (a) GAC, (b) GAC/Mn–Sn and (c) EDS spectrum of GAC/Mn–Sn.

Figure 9

SEM images and EDS spectrum of GAC/Mn–Sn particles electrodes. (a) GAC, (b) GAC/Mn–Sn and (c) EDS spectrum of GAC/Mn–Sn.

Close modal
The surface characteristics of GAC/Mn–Sn are also analyzed at a scanning angle of 5–90° by XRD, and the result is shown in Figure 10. After being modified by manganese, all diffraction peaks of GAC were weakened, but obvious diffraction peaks appeared at 2θ = 28.7° and 42.9 ° were attributed to the (110) and (111) planes of MnO2 (PDF#65-2821), when added with Sn, the crystal structure of the modified GAC is also changed. Another XRD pattern of GAC-Mn/Sn showed that several obvious diffraction peaks match the reflection of SnO2 (PDF#99-0024), and the diffraction peaks of GAC were enhanced, but no diffraction peak of Mn metallic oxides was clearly found. However, the EDS results confirmed the presence of Mn, indicating that a catalytic metastable solid of SnO2–MnOx was formed, while both SnO2 and MnOx have excellent catalysis and conductivity, helpful to generate hydroxyl radicals and accelerate the electrochemical decomposition of dyes.
Figure 10

XRD spectrums. (a) GAC, (b) GAC/Mn and (c) GAC/Mn–Sn.

Figure 10

XRD spectrums. (a) GAC, (b) GAC/Mn and (c) GAC/Mn–Sn.

Close modal

Reusability of GAC/Mn–Sn particles electrodes

To GAC particles electrodes, it is significant to maintain stably electro-catalytic performance in application, thus the recovery and reuse efficiency of the GAC/Mn–Sn particles electrodes was measured. The same batch of GAC/Mn–Sn particles electrodes were reused seven times, which were ultrasonically cleaned prior to use to remove organic matter attached during the last application. As shown in Figure 11, after seven cycles of consecutive operations, the removal efficiency of RhB decreased slightly, but it could still be maintained above 85%, the results indicated that the GAC/Mn–Sn particles electrodes still had a relatively stable catalytic effect. It could be explained that the manganese oxide coated on the surface of the GAC/Mn–Sn particles electrode will fall off to some extent due to the long-term electric field and the organic matter adheres to the particles’ surface (Liu et al. 2018; Zhang et al. 2019), but it still has the same good performance on reuse in TDER. Many carbon-based materials have absorbent properties and could function independently as particles electrodes toward the degradation of RhB, however, it was usually regarded as dangerous solid waste when amended (Ajiboye et al. 2021; Rono et al. 2021).
Figure 11

Effects of reused numbers of GAC/Mn–Sn particles.

Figure 11

Effects of reused numbers of GAC/Mn–Sn particles.

Close modal
Figure 12

Inhibition of hydroxyl radicals by methanol (MA 10 mM), tert-butanol (TBA 10 mM) and phenol (phenol 10 mM) on RhB removal.

Figure 12

Inhibition of hydroxyl radicals by methanol (MA 10 mM), tert-butanol (TBA 10 mM) and phenol (phenol 10 mM) on RhB removal.

Close modal
Figure 13

Schematic diagram of catalytic electrochemical reactions in TDER.

Figure 13

Schematic diagram of catalytic electrochemical reactions in TDER.

Close modal

Degradation mechanism

In order to verify that •OH is the main active radical for TBER to degrade organic matter, three kinds of radical scavengers were selected to add to TBER to affect the degradation process of RhB. Both methanol and tert-butanol are commonly used free radical scavengers that react rapidly with •OH (1.2 × 109–2.8 × 109 M−1 S−1 for methanol and 3.8 × 108–7.8 × 108 M−1 S−1 for tert-butanol) (Chen et al. 2016). In addition, phenol is also used as a radical scavenger because it reacts fast with •OH (6.6 × 109 M−1 S−1) (Lindsey & Tarr 2000). However, phenol is more hydrophobic than methanol and tert-butanol, and thus is more easily adsorbed on the electrodes surfaces. The use of phenol can be speculated whether the degradation of RhB occurs on the electrodes surface or in the aqueous solution (Zhao et al. 2017). It can be seen from Figure 12 that the addition of methanol (10 mM) and tert-butanol (10 mM) reduced the RhB removal rate, which indicates that the •OH was the main active substance for degrading RhB. Moreover, with the addition of phenol (10 mM), the removal rate of RhB was significantly lower than that of methanol and tert-butanol. The results showed that RhB mainly was decomposed by •OH on these electrodes’ surfaces during the degradation process.

The possible ways to degrade RhB wastewater mainly include direct and indirect catalytic-oxidation reactions. As shown in Figure 13, macromolecular organic matter is adsorbed to the surfaces of the catalytic plates and conductive particles electrodes under the action of the electric field, and is decomposed into small molecule organic matter and continuously converted into non-toxic and harmless carbon dioxide and water molecules, by losing electrons, or attacked by hydroxy radicals.

Hydroxy radicals could be generated on the surfaces of anodes and particles electrodes, coated by some active components (Ru, Ir, Pd, Sn and Mn), the active ingredients can directly cause water molecules or OH adsorbed on the electrodes’ surfaces to undergo an electro-catalytic reaction to produce •OH, the reaction as follows (Flores et al. 2017):
(5)
(6)
(7)

As a strong oxidant, •OH can degrade most organic substances without selectivity, in addition.

NaCl as an electrolyte also plays a certain role in the process of degrading organic matter. Cl is oxidized to Cl2 at the anode, and Cl2 easily reacts with water, to produce HClO, ClO (Equations (8)–(10)), which are strong oxidants to accelerate the degradation of pollutants (Scialdone 2009).
(8)
(9)
(10)
In order to explain how the RhB was degraded by TDER with GAC/Mn–Sn particles electrodes, its intermediate byproducts in the electrochemical reaction process were identified by UPLC-MS (WATERS UPLC-Q/Tof Xevo G2-XS). The chromatograms before and after the degradation of RhB are shown in Figure 14. It can be seen from Figure 15(a) that the peak with a retention time of 1.70 min is a peak of RhB. In the chromatogram after degradation (Figure 15(b)), no obvious peak appears when the retention time was 1.7 min, but at 0.25, 1.29, 2.06, 2.7, 3.45, 5.81 and 5.89 min appears different levels of peaks, respectively, indicating that RhB had been degraded into other substances. Figure 15 shows mass spectrums of several major intermediates of TDER degradation of RhB, and these intermediates are listed in Table 2.
Table 2

Intermediate byproducts identified during the degradation of RhB

No.CompoundMolecular formulaStructure
RhB C28H32N2O3  
2-(3,6-Biscchlorooxy)-3H-xanthen-9-yl)benzoic acid C20H12Cl2O5  
2-(2,4-Dihydroxybenzyl)benzoic acid C14H12O4  
Cyclohexa-2,5-dienol C6H8 
Resorcinol C6H6O2  
Phthalic acid C8H6O4  
(2Z,4E)-Hexa-2,4-dienoic acid C6H8O2  
(2E,4E)-Hexa-2,4-dienedioic acid C6H6O4  
No.CompoundMolecular formulaStructure
RhB C28H32N2O3  
2-(3,6-Biscchlorooxy)-3H-xanthen-9-yl)benzoic acid C20H12Cl2O5  
2-(2,4-Dihydroxybenzyl)benzoic acid C14H12O4  
Cyclohexa-2,5-dienol C6H8 
Resorcinol C6H6O2  
Phthalic acid C8H6O4  
(2Z,4E)-Hexa-2,4-dienoic acid C6H8O2  
(2E,4E)-Hexa-2,4-dienedioic acid C6H6O4  
Figure 14

UPLC chromatogram of RhB wastewater. (a) Untreated and (b) after treatment.

Figure 14

UPLC chromatogram of RhB wastewater. (a) Untreated and (b) after treatment.

Close modal
Figure 15

Mass spectrum of intermediate products. (a) RhB, (b) 2-(2,4-dihydroxybenzyl) benzoic acid, (c) cyclohexa-2,5-dienol, (d) phthalic acid, (e) (2Z,4E)-hexa-2,4-dienoic acid.

Figure 15

Mass spectrum of intermediate products. (a) RhB, (b) 2-(2,4-dihydroxybenzyl) benzoic acid, (c) cyclohexa-2,5-dienol, (d) phthalic acid, (e) (2Z,4E)-hexa-2,4-dienoic acid.

Close modal
Based on the analysis of the detected intermediates and combined the previous studies on the degradation pathway of RhB (Nidheesh et al. 2014; Diao et al. 2017; Ji et al. 2018), a possible degradation pathway is proposed in Figure 16. RhB molecule is first decolorized by hypochlorite to generate 2-[(3,6-biscchlorooxy)-3H-xanthen-9-yl]benzoic acid, then under the action of •OH oxidation and substitution, its conjugate structure is broken down and decomposed into 2-(2,4-dihydroxybenzyl) benzoic acid and cyclohexa-2,5-dienol, 2-(2,4-dihydroxybenzyl) benzoic acid is also oxidized and decomposed into resorcinol and phthalic acid. Cyclohexa-2,5-dienol continues to be oxidized to form (2Z,4E)-hexa-2,4-dienoic acid, and both the benzene ring structures of resorcinol and phthalic acid are hydroxylated to open and form (2E,4E)-hexa-2,4-dienedioic acid. Finally, the latter and (2Z,4E)-hexa-2,4-dienoic acid all are mineralized to produce CO2 and H2O.
Figure 16

Proposed degradation pathway of RhB.

Figure 16

Proposed degradation pathway of RhB.

Close modal

The RhB degradation was carried out by TDERs packed with GAC/Mn–Sn particles electrodes, and the main conclusions are as follows: (1) the optimal operating factors were 9.45 V of voltage, 0.79 g/L of electrolyte and 32.37 g/L of particles electrodes dosage, respectively, and the removal rates of 98.3% of RhB, 60.7% of COD and 20.1 kWh kg−1 COD of energy consumption was obtained. (2) GAC and Mn–Sn oxides could be used as based materials to prepare for particles electrodes, and the Mn–Sn oxidizes coated on its surfaces was favorable to generate hydroxyl and accelerate the dyes’ degradation process. (3) RhB molecule is first decolorized by hypochlorite, then oxidized by hydroxyl, and continually decomposed by these strong oxidants into a variety of intermediates that was finally mineralized into water and carbon dioxides, the biodegradability of wastewater was increased obviously after treated by the electrochemical process.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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