The diverse compositions and complex nature of the textile wastewater make it imperative to find an economical and suitable degradation pathway. The degradation of real textile wastewater on a novel heterogeneous electro-Fenton system was carried out with a composite anode of magnetically fixed micron ZVI coupling with a Ti/RuO2-IrO2 sheet. The influences of different variables such as mZVI dosage, H2O2 amount, applied voltage and pH value on both total organic carbon and chemical oxygen demand removal efficiencies and energy consumption were investigated. The optimized parameters were simultaneously verified by using electrochemical workstation Tafel curves and Nyquist plots. The optimal operating conditions for evaluating the wastewater treatment were H2O2 dosage of 0.10 mol·L−1, applied voltage of 5.0 V, mZVI amount of 1.0 g·L−1 and initial pH value of 3.0. The high TOC and COD removal efficiencies of 92.44 and 82.84% could be achieved simultaneously in 60 min, respectively. XRD, XPS and SEM-EDS were used to investigate the interaction between the pollutant and the mZVI. GC-MS analysis was performed on untreated and treated wastewater to determine the degradation of pollutants in dyeing wastewater during the electro-Fenton process and to effectively propose a suitable degradation mechanism for this system.

  • A heterogeneous electro-Fenton process was performed on mZVI anode.

  • This study is performed on the real textile wastewater.

  • High COD and TOC removal efficiencies were achieved by the heterogeneous E-Fenton process.

  • The performance of electro-Fenton for contaminant removal was evaluated at different parameters.

  • The mechanisms were proposed based on the physiochemical and electrochemical properties of the anode.

Graphical Abstract

Graphical Abstract
Graphical Abstract

In recent years, considerable amounts of wastewater with significant organic carbon levels have aroused a widespread concern of the whole society. Among them, the discharge from the textile factory is a significant pollution source. According to statistics, the amount of wastewater from textile enterprises in China is as high as 4 million tons per day (Bilińska et al. 2017). However, the wastewater contains various organic pollutants with high concentrations and some heavy metals (Yamjala et al. 2016) due to the numerous chemicals used in the textile dyeing process. In addition to the complex chemical structures, other different characteristics of dyestuffs such as photo-resistance, biodegradation resistance, variable pH, carcinogenicity and mutagenicity make textile wastewater even more difficult to properly be treated (Nidheesh & Gandhimathi 2012; Naje et al. 2017). Therefore, it is imperative to explore better, more energy-saving and inexpensive technology for the degradation of textile effluent.

The conventional treatment options mainly include biological and physico-chemical processes (Khlifi et al. 2010; Samuchiwal et al. 2021). Although biological treatment is quite economical and eco-friendly, the long treatment cycle is an important shortcoming. Moreover, dyes have complex aromatic structures, which can produce carcinogenic, toxic and mutagenic aromatic amines if the dyes are decomposed by anaerobic microorganisms (Sarikaya et al. 2012; Yamjala et al. 2016; Paz et al. 2017). Additionally, textile wastewater includes non-biodegradable and refractory dyes indicating a low biodegradation index, which further leads to typical biological treatment inefficiencies (Somensi et al. 2010; Nakhate et al. 2019). Chemical removal (Bahadur & Bhargava 2019), nanofiltration adsorption (Chen et al. 2015) and other physico-chemical approaches also have their own drawbacks. They often encounter problems such as high-cost, failure to meet discharge limits and membrane fouling. Recently, advanced oxidation processes (AOPs) have attracted the great attention of many researchers because of their high effectiveness and mineralization efficiency (Dolatabadi et al. 2021; Li et al. 2021). Hydroxyl radical has a significant impact on the AOPs. Its redox potential (2.80 V) is much larger than those of typical oxidants such as potassium permanganate or hydrogen peroxide, and also more efficient than radical-based oxidants such as (2.60 V) (Ileri & Dogu 2022). Fenton (Sayin et al. 2022), O3 processes (Bilinska et al. 2020), peroxides (Chow & Leung 2019), methods based on ultraviolet light or ultrasonic decomposition (Cai et al. 2016), photocatalytic oxidation (Ju et al. 2022) and processes in which multiple methods are synergistic with each other are all AOPs (Huang et al. 2020; Pan & Qian 2022). Among many AOPs, electro-Fenton has extremely attractive advantages in treating hard-to-degrade wastewater due to its high oxidation capability, low installation cost, and high efficiency as shown in the following equations.
formula
(1)
formula
(2)
formula
(3)
formula
(4)

The in situ generation of hydrogen peroxide and ferrous iron in the electro-Fenton system results in high wastewater treatment efficiency compared to the routine Fenton approach. However, most of the current electro-Fenton studies are based on simulated wastewater with one or several dyes (Kenova et al. 2018). Therefore, extensive research is still required for the electro-Fenton application to actual industrial wastewater.

It is also well known that the choice of electrode material is closely related to the electrochemical performance of the pollutant degradation process. An application of boron-doped diamond (BDD) electrode has shown excellent electrochemical stability in wastewater treatment. However, BDD electrodes are high in cost (Kaur et al. 2018). Dimensionally stable anode (DSA), consisting of titanium-based metals covered with a thin conductive layer of metal oxides, have become widely used in wastewater treatment (Feng et al. 2016; Baddouh et al. 2019a; Santos et al. 2020). DSA-type electrodes (IrO2 and RuO2-IrO2) are very effective in the degradation of pollutants (Baddouh et al. 2019b; Kishor et al. 2021; Baddouh et al. 2022). The degradation of 1,4-benzoquinone was carried out on Ti/IrO2 anodes and the main mineralization step of benzene ring breakage readily occurred, although the byproduct of carboxylic acids slightly exits and ultimately transforms to non-toxic degradants (Pulgarin et al. 1994). Meanwhile, according to Equations (5)–(7, RuO2-Ti electrodes have also been reported to facilitate the production of reactive chlorine species (HClO, Cl2 and ClO) (Paździor et al. 2019). It is important to note that active chlorines (RCS, such as Cl• and ClO•) are theoretically effective oxidants for pollutants because they react with the electron-rich part through single-electron oxidation. For textile wastewater, high chlorine content is also one of its important characteristics. Therefore, RuO2-IrO2-Ti anodes have great potential for decolorization and degradation of dyestuffs.
formula
(5)
formula
(6)
formula
(7)

Zero-valent metals have a high surface activity and are one of the methods used to degrade pollutants in water sources (Fu et al. 2014). Among them, zero-valent iron (ZVI) with a redox potential (E0(Fe2+/Fe0)) of −0.44 V found wide application as a strong reducing agent for refractory organic pollutants. Electro-Fenton with zero-valent plate may lead to severe passivation inhibiting the release of Fe2+ from the anodic process due to the generation of a tiny iron oxide layer on the electrode. Apparently, ZVI powder is assumed to have undergone a more convenient electrochemical corrosion into ferrous ions because of the large surface area and high reactivity in the electro-Fenton reaction.

In this study, several composite anodes were constructed by magnetically immobilized mZVI particles on RuO2-IrO2-Ti (RuO2-IrO2/mZVI-Ti) and graphite was utilized as the cathode in this electro-Fenton process. They were then applied to the treatment of actual textile wastewater to investigate the comprehensive performance of the electro-Fenton system. The influences of several variables, such as pH, applied voltage, dosage of mZVI and concentration of H2O2 on its degradation capability of textile wastewater were systematically estimated from the aspect of COD and TOC removal rates. The physico-chemical characteristics of the reactant precipitates and mZVI electrochemical behaviors were studied to strengthen mechanism research by electrochemical impedance spectroscopy (EIS), Tafel curves, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), scanning electron microscopy-energy dispersive spectrometry (SEM-EDS), respectively. The possible organic dyes present in the raw wastewater along with the final intermediates were evaluated by gas chromatography-mass spectrometry (GC-MS), and finally, the degradation mechanism was proposed.

Wastewater sample and chemicals

The textile wastewater used in the study was collected from a textile company in Shaoxing, Zhejiang Province. The wastewater solution was preserved at 4°C before further analysis. Due to the good electrical conductivity of the wastewater sample, no additional electrolyte was required for electrochemical treatment. The characteristics of the textile wastewater are listed in Table S1.

All reagents used in the experiments were analytically pure. Micron ZVI powder (>99.9%) was purchased from Qinghe Chuanjia Welding Material Corporation (Xingtai, China). Ti/RuO2-IrO2 plates and graphite flakes were purchased from Schultech Industrial Technology (Suzhou, China) and Baofeng Graphite Co (Qingdao, China). The mass ratio of IrO2 to RuO2 coated on the titanium plate is approximately 3:8. The initial pH of wastewater solution was regulated by H2SO4 (1 M) and NaOH (3 M) during this experiment.

Experimental setup and operation

Figure S1 shows a schematic view of the experimental system. This study was carried out in a glass reaction tank (50 mm × 50 mm × 100 mm). RuO2-IrO2/mZVI-Ti and graphite sheet (30 mm × 100 mm × 1 mm) were utilized for the anode and cathode, respectively. mZVI powder covers an area of ∼ 800 mm2 (20 mm × 40 mm) on the surface of RuO2-IrO2-Ti electrode with a magnetic source outside the reaction tank. The total area of the electrode immersed in the wastewater was 2,100 mm2. Both anode and cathode were immersed in the wastewater and connected by a direct current power supply (MCH, K303D-II, China) with a constant distance 20 mm. The wastewater with 200 mL in volume was adjusted to various pH levels (2.0, 3.0, 5.0 and 7.0) using 1 M H2SO4 solution or 3 M NaOH solution under different constant voltages (8.0, 5.0 and 3.0 V) in all batch experiments. During the reaction, water samples were extracted at the specified intervals and routinely filtered before analysis via a polytetrafluoroethylene syringe membrane filter with a pore size of 0.45-μm. All data points on the curves are the average of three tests with error bars.

Analysis and characterization

A total organic carbon analyzer (Shimadzu) was employed to determine TOC value and its removal rate was calculated using Equation (8). An appropriate amount of manganese dioxide was added to the sample at 60 °C for 30 min to remove the interference of excessive H2O2 on COD measurement. After filtration, COD values were determined using a rapid COD analyzer (5B-3F, Lianhua Technology Co., Ltd, China). TN and TP were analyzed with a continuous flow analyzer (AutoAnalyzer3, Germany). The content of heavy metals Sb and Cr was readily determined by graphite furnace atomic absorption spectrophotometry (AA-7000, Shimadzu). The conductivity probe (Shanghai Rex) and pH electrode (FiveEasy) were used to record the pH and conductivity of the wastewater solutions, respectively. Ferrous iron ion concentrations (DFe2+) and total iron ion (TDFe) were analyzed using a O-phenanthroline spectrophotometer method (UV2600, Shimadzu, Japan). The morphology and structure of the precipitate flocs produced in the electro-Fenton system were observed by field-emission SEM (Sigma300, Zeiss, Germany) and EDS (Smart). In addition, the physical properties of crystalline flocs powders were monitored by XRD (D8 Advance diffractometer, Bruker). XPS (K-Alpha, Thermo Fisher Co., US) was performed to record the chemical valences of iron, carbon, and oxygen of the precipitate. Anodic oxidation analysis was performed by a Chi 660A electrochemical workstation (China) using EIS and Tafel analysis. Potential contaminants in the wastewater before and after the reaction were detected by GC-MS (Agilent 5977A, America) with a pressure of 100 kPa for the nitrogen carrier gas and the injector and detector temperatures of 220 and 280 °C, respectively.
formula
(8)
where η is the TOC removal efficiency; C0 and Ct are the initial TOC concentration and final TOC concentration (mg·L−1), respectively.
The EEO was used as a figure of merit to determine the electrical energy required to reduce pollutant concentrations by an order of magnitude (Bolton et al. 2001).
formula
(9)
where P is the rated power for electro-Fenton reaction (kW); t is the reaction time (min); V is the volume of wastewater (L); CODi and CODt are initial and final COD concentrations (mg·L−1);
The COD removal of textile effluent was investigated using the pseudo-first-order kinetic model, as shown in the following equation.
formula
(10)
where k is the pseudo-first-order rate constant for the decay of the effluent COD concentration (min−1).

Influencing factors of the mZVI powder anode electro-Fenton system

mZVI dosage

The catalytic ferrous ions amount has a crucial influence on the degradation effect of organic pollutants in electro-Fenton reaction. The effect of mZVI loading amount on the removal rate of COD and TOC of dyeing wastewater was studied because the ferrous ion concentration is directly determined by the dosage of mZVI under other parameters unchanged. Figure 1(a) and 1(b) show the removal rates of COD and TOC increasing with the mZVI amount from 0.5 to 1.0 g·L−1 and a simultaneous increase in H2O2 production. The COD and TOC removal rates achieve 82.84 and 92.44% at 1.0 g·L−1, respectively. However, as the mZVI amount was further increased to 1.5 and 2.0 g·L−1, the removal rates of both COD and TOC decreased to different degrees. On the one hand, it could be caused by the lower anodic dissolution of agglomerated mZVI at the anode due to the excessive addition amount. On the other side, excessive Fe2+ consumes more OH· and competes with OH· for the mineralization of pollutants, resulting in a decrease in the utilization of hydroxyl radicals. Additionally, excessive iron ions form more iron sludges during the reaction process, thus causing secondary pollution and making the effluent unable to meet the discharge requirements. Therefore, the optimal mZVI loading was set to 1.0 g·L−1 in this study.
Figure 1

Influence of mZVI dosage on (a) COD; (b) TOC removal; influence of various H2O2 concentrations on (c) COD; (d) TOC removal.

Figure 1

Influence of mZVI dosage on (a) COD; (b) TOC removal; influence of various H2O2 concentrations on (c) COD; (d) TOC removal.

Close modal

Hydrogen peroxide concentration

In the electro-Fenton reaction, oxygen transfers two electrons at the cathode to generate hydrogen peroxide, which reacts with divalent iron ions from mZVI electrodissolution at the anode to produce hydroxyl radicals for pollutant degradation. As shown in Figure 1(c) and 1(d), the COD and TOC removal rates were 50.98 and 56.76% without the addition of H2O2, respectively. The COD and TOC removal efficiencies initially enhanced as the increase of H2O2 concentrations because of the additional formation of OH· (Ghalebizade & Ayati 2019). The removal rates of COD and TOC reached 82.84 and 92.42% when the H2O2 concentration was 0.1 mol·L−1, respectively, indicating that dyeing pollutants were of high mineralization efficiencies. However, the removal rate decreased as further increase in H2O2 concentration to 0.15 mol·L−1 and obvious flocculent foams were observed on the surface of the wastewater. This proves that the excess of hydrogen peroxide leads to its severe self-consumption (Equation (11)) and concomitant oxygen carries the iron species-based flocs to the surface. Herein, the H2O2 concentration was chosen to 0.1 mol·L−1 for better utilization of hydroxyl radicals in this electro-Fenton reaction.
formula
(11)

Initial pH value

Similarly, the wastewater pH has a significant impact on the electro-Fenton system, which influences the wastewater conductivity, the present form of iron species and the utilization efficiency of H2O2 (Naje et al. 2017). In this experiment, the variation of COD and TOC of textile wastewater was studied at different initial pH values (2.0, 3.0, 5.0 and 7.0). Figure 2(a) and 2(b) shows that the COD and TOC removal rates are of the same variation trends. The removal efficiencies of the pollutants at pH of 2.0, 3.0, 5.0 were much higher than that at pH = 7.0, and a maximum removal efficiency value was observed at pH of 3.0 for this electro-Fenton reaction.
Figure 2

Influence of different solution acidities on (a) COD and (b) TOC removal efficiencies; effect of different applied voltages on (c) COD and (d) TOC removal efficiencies.

Figure 2

Influence of different solution acidities on (a) COD and (b) TOC removal efficiencies; effect of different applied voltages on (c) COD and (d) TOC removal efficiencies.

Close modal

It is reported that Fe(OH)2 present at pH = 3.0 has higher Fenton reaction activity than Fe2+ (Pignatello et al. 2006). However, Fe2+ could easily be converted to Fe(OH)3 precipitates from pH value equal to 3.7 (Liu & Wang 2007). Hence, as the pH value was further increased, Fe2+ ions were largely converted into Fe3+ and further formed Fe(OH)3 precipitates, thus weakening the reaction activity. An initial pH value (3.0) of wastewater was set for this study. Figure S3 reveals the pH value variation of the wastewater during the reaction. The values were kept rising as a result of the interaction between Fe2+ and hydrogen peroxide in 60 min; however, the overall increase degree was not significant, which could be impeded by the accumulation of carboxylic acid and CO2 from efficient degradation of dyeing pollutants (Bakheet et al. 2013).

Applied voltage

The loaded electric field is the main dynamic force of electro-Fenton system. Normally, the higher the applied voltage, the more intense the reaction and the higher the pollutant removal efficiency. This is confirmed by results shown in Figure 2(c) and 2(d), where the COD and TOC removal efficiencies increased by 11.85 and 9.59% with the increasing voltage (3.0–5.0 V), respectively. This can be attributed to the increase in hydrogen peroxide production on the cathode surface due to the increase in current intensity (Martínez-Huitle & Brillas 2009; Khataee et al. 2011). However, as the applied voltage was further increased to 8.0 V, there was a slight decrease in the removal rate. The principal reason for this decrease is due to enhanced electrode polarization and the occurrence of severe cathodic and anodic side reactions (Equation (12)) at higher applied voltage. The higher current causes a more violent side reaction of water electrolysis with byproduct O2, which results in a decrease of ·OH (Zheng et al. 2018). As listed in Table S2, the energy consumption of the reaction at different voltages by Equation (9) also proves that a large amount of electrical energy will be consumed by the side reaction at a higher voltage. Here, 5.0 V was chosen as the appropriate voltage for this study. Under optimal conditions, the actual energy consumption required is 7.79 kWhm−3, which is advantageous when compared with other methods for degrading actual wastewater (Table S4). In addition, the raw materials required for the experiment and the costs are briefly calculated in the supporting information (Table S3).
formula
(12)

Kinetics of COD removal by electro-Fenton process

The COD removal under various voltages was in accordance with the model of pseudo-first-order kinetic (Figure S2 and Table S5). The whole electro-Fenton process involved two different phases of rapid reaction and slow reaction, and the removal efficiency within the first 10 min was much greater than that in the latter stage within 1 h. In the fast reaction stage, a high concentration of H2O2 including the electrocatalytically generated H2O2 reacted with Fe2+ to produce more ·OH to rapidly degrade the dyeing pollutants. The lower efficiency in the later stage was caused by the fact that Fe3+ cannot be rapidly reduced to Fe2+ and the efficiency of ·OH generation decreases due to H2O2 consumption.

Electrochemical analysis

Tafel and Nyquist diagrams at different H2O2 concentrations were conducted to further investigate the electro-Fenton performance for the mineralization of textile wastewater (Figure 3). Potential window ranging from −1.2 to 1.2 V was set for Tafel curves. The corrosion potentials were measured as 0.21, −0.05 and 0.04 V for hydrogen peroxide concentrations of 0.15, 0.10 and 0.05, respectively. Obviously, the Ecorr is much smaller than the values under other conditions at a hydrogen peroxide concentration of 0.10 mol·L−1 indicating the highest electrode activity for this electro-Fenton reaction. As the concentration is further increased, the critical value for the occurrence of the reaction increases and the Ecorr moves in the positive direction, which is consistent with the previous analysis that the larger H2O2 concentration causes severe side reactions. The corrosion currents were 4.44, 1.63 and 3.76 mA at different H2O2 concentrations of 0.15, 0.10 and 0.05 mol·L−1, respectively. Lower corrosion potential and corrosion current at 0.10 mol·L−1 of H2O2 concentration imply that easier anodic oxidation and slower anodic corrosion rate occurred, thus reducing the generation of iron sludge and making the Fe3+/Fe2+ conversion more adequate. Nyquist plots showed that the interfacial transfer resistance (Rct) at different H2O2 concentrations (0.05, 0.10, 0.15 mol·L−1) were 10.00, 7.32 and 9.07 Ω, respectively, with the smallest Rct value at 0.10 mol·L−1, which is consistent with above observation that mZVI is more susceptible to anodic oxidation at 0.10 mol·L−1 of H2O2 concentration.
Figure 3

(a) Tafel plots (b) Nyquist plots, of mZVI anode at various H2O2 concentrations. Reaction conditions: Pollutants = 200 mL; pH = 3.0; [Fe]0 = 1.0 g·L−1; Applied voltage = 5.0 V.

Figure 3

(a) Tafel plots (b) Nyquist plots, of mZVI anode at various H2O2 concentrations. Reaction conditions: Pollutants = 200 mL; pH = 3.0; [Fe]0 = 1.0 g·L−1; Applied voltage = 5.0 V.

Close modal

Reusability of anode material

Ti/RuO2-IrO2 flakes and mZVI powder are two important components of the composite anode. Herein, the utilization of mZVI powder in the electro-Fenton reaction at various initial pH values (Figure 4(a)) and the loss of Ti/RuO2-IrO2 flakes under multiple repetitions of the test were examined, respectively. First, the concentration of Fe2+ in the wastewater under neutral conditions was much smaller than that under acidic conditions. The Fe2+ concentration reached a maximum value at the initial pH = 3 after 60 min of reaction. In addition, the value of [DFe2+]/[TDFe] at pH = 2.0 (8.4%) was significantly lower than that pH = 3.0 (38.9%). Obviously, the Fe2+ utilization at pH = 2.0 is low indicating that a large amount of hydrated ferric hydroxide would be produced after the reaction. Figure 4(b) presents the average TOC removal efficiency maintaining a value as high as 91.5% after five replicate experiments, indicating that the Ti/RuO2-IrO2 electrode was not effected during the experiments after simple washing with deionized water for each experiment. Therefore, the composite anode in this experiment is of low cost and good stability.
Figure 4

(a) Utilization of mZVI at different initial pH; (b) Reusability of the anode material after five cycles of experiment. Reaction conditions: Pollutants = 200 mL; [Fe]0 = 1.0 g·L−1; H2O2 = 2 mol·L−1; Applied voltage = 5.0 V.

Figure 4

(a) Utilization of mZVI at different initial pH; (b) Reusability of the anode material after five cycles of experiment. Reaction conditions: Pollutants = 200 mL; [Fe]0 = 1.0 g·L−1; H2O2 = 2 mol·L−1; Applied voltage = 5.0 V.

Close modal

Characterization of precipitation

Iron-based precipitate flocs were formed in the textile wastewater after 60 min treatment via electro-Fenton process. The morphologic structure and chemical elements of the flocs were analyzed via SEM-EDS (Figure 5(a) and 5(b)). SEM shows that the precipitates are irregular spherical aggregates with a rough surface and spherical particles with a particle size of about 100 nm. Elemental mapping illustrated that the precipitates were composed of C (16.15 wt%), N (4.3 wt%), O (39.20 wt%), Fe (37.36 wt%) and S (2.99 wt%). The results indicate that the primary components of formed flocs precipitate are ferric oxides and ferric hydroxides (e.g. Fe2O3, Fe3O4, Fe(OH)2, Fe(OH)3 and FeO(OH)). A small amount of sulfur in the precipitate illustrates that the dyeing pollutants are partially removed by the flocculation. XRD analysis was performed on mZVI before the reaction and the precipitate after the reaction to further investigate the composition of the precipitate (Figure 5(c)). However, the XRD patterns of the precipitated samples showed no characteristic peak of Fe(OH)3 because the generated Fe(OH)3 flocs were non-crystalline and therefore could not be readily measured by XRD analysis (Yoon et al. 2016). After calcination of the precipitate sample at 400 °C, eight major characteristic peaks of the XRD pattern were observed and well fitted with the standard card (PDF#73-2234) of iron oxide, which laterally proved that the precipitate were flocs of iron hydroxide.
Figure 5

Scanning electron microscopy images (a), element mappings (b) and XRD spectra (c,d) of mZVI and precipitate before and after calcination.

Figure 5

Scanning electron microscopy images (a), element mappings (b) and XRD spectra (c,d) of mZVI and precipitate before and after calcination.

Close modal
Figure 6 displays the XPS spectra of the iron-based flocs precipitate. XPS survey spectrum shows the existence of carbon, oxygen, sulfur, nitrogen and iron elements in composition (Figure 6(a)). High-resolution core-level scans of the O 1s, C 1s and Fe 2p were also performed. Figure 6(b) demonstrates that the C 1s spectrum contains three main peaks with binding energies, corresponding to carbon–carbon single bond (C-C, 284.5 eV) and carbon–carbon double bond (C = C, 286.5 eV) bonds and carbon–oxygen double bond (C = O, 288.4 eV), respectively, which indicates that the dye pollutants removal process is based on synergistic effects involving dominant degradation and partial flocs adsorption (Mattevi et al. 2009). Three intense peaks are observed at 529.8, 531.3 and 532.2 eV in the O 1s spectrum (Figure 6(c)). The O1s peak located at 529.7–530.1 eV corresponds to oxygen (O2−) on the basis of the report by Piumetti et al. The peak with a binding energy of 531.3 eV belongs to the surface adsorbed oxygen (OH) and the binding energy at 532.2 eV can be attributed to the physical or chemical adsorption of water (H2O) (Sun et al. 2020). Furthermore, the peak position of Fe 2p3/2 is between 710.6 and 711.2 eV, and Fe 2p1/2 is located in 723.2–724.8 eV (Yamashita & Hayes 2008; Do et al. 2013). In this study, the Fe 2p spectrum includes characteristic peaks at 710.8 and 724.5 eV, corresponding to the Fe 2p3/2 and Fe 2p1/2 states, respectively (Figure 6(d)). The peak at 710.5 eV implies the presence of the ferric state in Fe2O3 (Kumar et al. 2015). The peak located at 712.2 eV is ferrous iron of Fe3O4. In summary, the ferrous compounds in the flocs precipitate can be classified into iron hydroxide (i. e. Fe(OH)2, Fe(OH)3 and FeO(OH)) and iron oxide (i. e. Fe2O3 and Fe3O4). The existence of low intensity of N and S implies that N, S-containing organics were degraded and marginally adsorbed on the flocs, which is supported by the evidence of little increase of the sulfate and nitrate concentration in wastewater during electro-Fenton reaction.
Figure 6

XPS survey spectra of the flocs precipitate from Electro-Fenton process, (a) survey spectrum, (b) high-resolution C 1 s core level, (c) O 1 s core level and (d) Fe 2p core level.

Figure 6

XPS survey spectra of the flocs precipitate from Electro-Fenton process, (a) survey spectrum, (b) high-resolution C 1 s core level, (c) O 1 s core level and (d) Fe 2p core level.

Close modal
Figure 7

Rational mechanism of electro-Fenton reaction under optimum conditions.

Figure 7

Rational mechanism of electro-Fenton reaction under optimum conditions.

Close modal

Degradation mechanism

GC-MS analysis on treated and untreated real textile wastewater was performed to investigate the degradation mechanism. The results are presented in Table S6 and the corresponding mass spectra are listed in the supporting information (Figure S4). These include amide compounds, ester auxiliaries, polysiloxanes and halogen-containing flame retardants. Among them, the amide compounds are effective in improving the surface dyeing depth of the dyed and finished fabrics and improving their deficiencies in color light shading, resulting in a much higher surface depth and color fixation rate of the fabrics. Polysiloxane has excellent flexibility, thermal stability and chemical stability. In the textile process, it is normally used in the co-condensation of polysiloxane into the co-polyester molecular chain, reducing the dyeing temperature of blended fibers to avoid high temperature and high voltage dyeing technology on the blended fiber damage. Evidently, the GC-MS analysis proved that the large molecules containing benzene ring and other hard-to-degrade pollutants were opened to form aliphatic hydrocarbons and then further degraded to small molecules with a high mineralization rate during the electro-Fenton degradation process. However, slight ester pollutants were still detected at the end of the degradation reaction, which may be caused by the complexity of the actual wastewater and the acidic environment even after the reaction unfavorable for esters degradation. Therefore, the high efficiency of this electro-Fenton can be considered a pre-treatment process to achieve a cost-effective treatment of dyeing wastewater.

The plausible degradation mechanism of the electro-Fenton reaction is illustrated in Figure 7. The combination of Fe2+ from the anode dissolution and H2O2 generated on the cathode produces ·OH to degrade pollutants, which are mineralized or partially mineralized into CO2 and H2O and other inorganic ions. At the same time, Fe2+/Fe3+ reduction occurs at the cathode under the electric field, allowing the Fenton reaction to circulate. In addition, iron ions are electro-dissoluted into the wastewater to produce iron hydroxides as coagulants, which adsorb colloidal or soluble pollutants for further pollutant removal.

In this study, we investigated the mineralization of real textile wastewater by novel heterogeneous E-Fention with a composite anode composed of magnetically fixed mZVI and RuO2-IrO2-Ti sheets. The specific degradation efficiencies were studied under different parameters on the basis of COD and TOC removal. It was demonstrated that the COD and TOC removal efficiencies of 82.84 and 92.44%, respectively, could be achieved at an mZVI loading of 1.0 g·L−1, an H2O2 concentration of 0.10 mol·L−1, a load voltage of 5.0 V and an initial pH of 3.0 within 60 min. This high mineralization efficiency was supported by the GC-MS investigation according to the types of pollutants. Several main pollutants in raw wastewater (amides, ester additives, polysiloxane and halogenated flame retardants, etc.) are degraded into small molecules and completely removed. The removal efficiencies of COD follow a pseudo primary kinetic model. The whole electro-Fenton removal process was divided into two stages: fast reaction and slow reaction. In addition, the physico-chemical analysis of precipitate by EDS and XPS proved that the precipitate was iron oxide/iron hydroxide, which could be used as a coagulant to further remove the pollutants. The mZVI-based electro-Fenton system has a high [DFe2+]/[TDFe] value with an excellent electrochemical performance. In summary, this study showed that RuO2-IrO2/mZVI-Ti anode can effectively be employed in the electro-Fenton reaction to treat real textile wastewater.

We acknowledge the financial support for this work provided by Open Foundation of State Environmental Protection Key Laboratory of Mineral Metallurgical Resources Utilization and Pollution Control (HB201909). Authors have full power to use these grants.

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

The authors declare there is no conflict.

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