Electro-Fenton mineralization of real textile wastewater by micron-sized ZVI powder anode

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 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.

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.

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.

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.

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).

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.

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.