Enhanced Rhodamine B degradation by GAC/Mn–Sn particles electrodes Open Access

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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

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.