Chemical Treatment Technology for Dye Wastewater

1 Electrochemical Method

Electrochemical methods have previously suffered from drawbacks such as high energy consumption, high cost, and side reactions like oxygen and hydrogen evolution at the electrodes. In recent years, researchers have developed many new electrode materials, and novel electrodes with high oxygen evolution overpotential and high hydrogen evolution overpotential have emerged in electrooxidation and electroreduction, which have improved treatment efficiency and provided another reasonable option for dye wastewater treatment processes. Electrochemical methods can be divided into electrochemical reduction, electrochemical oxidation, and electrocoagulation-electroflotation in principle.wastewatertreatment process provides another reasonable option. Electrochemical methods can be divided into electrochemical reduction, electrochemical oxidation, electrocoagulation-electroflotation, etc. in principle.

1.1 Electrochemical Reduction and Electrochemical Oxidation

Electrochemical reduction is the process of removing environmental pollutants through cathodic reduction, which can be divided into direct electroreduction and indirect electroreduction. Electrochemical oxidation is the process of removing pollutants through the anode or strong oxidizing substances generated by the anode [superoxide radical (·O2), H2O2, hydroxyl radical (·OH), etc.], which can be divided into direct electrochemical oxidation and indirect electrochemical oxidation. During the electrooxidation process, the main side reaction is anode oxygen evolution from water discharge and decomposition. During the electroreduction process, the main side reaction is hydrogen evolution from water discharge and decomposition. For the entire electrochemical system, electrode material is the core of electrochemical water treatment technology and has been a hot topic in research applied to dye wastewater treatment processes in recent years.

Ali et al. used carbon sponge (CS) as the cathode material to treat alkaline blue 3 wastewater. Compared with the traditional carbon felt (CF) cathode, the CS cathode produced three times the amount of hydrogen peroxide. The study also investigated the effects of applied current, electrolyte type, oxygen flow rate, pH, and temperature on hydrogen peroxide production. The results showed that the maximum hydrogen peroxide production was achieved when the applied current was 100mA (5.6mA·cm-2). Among these factors, applied current, oxygen flow rate, pH, and temperature had a significant impact on hydrogen peroxide production. After 8 hours of treatment, the TOC removal rate of alkaline blue 3 was 91.6% (CS cathode) and 50.8% (CF cathode). The mineralization current efficiency of the CS cathode was four times that of the CF electrode.

Zhou et al. conducted a comparative experimental study on the degradation of azo dye methyl orange using mixed metal oxide and boron-doped electrodes. The experiment investigated the effects of current density, electrolyte type, pH, and initial pollutant concentration on the removal of color, COD, and TOC in dye wastewater. The experimental results showed differences in the degradation behavior of pollutants on the two types of electrodes. Boron-doped electrodes have a wider range of process applicability for dye wastewater compared to mixed metal oxide electrodes. From an economic perspective, boron-doped electrodes are a better choice for dye mineralization.

Yao et al. used a TiO2 oblique film photoelectrocatalytic (PEC) reactor to treat dye wastewater. The experiment investigated the treatment of Rhodamine B wastewater by a TiO2 nanotube electrode and conducted a comparative study of the efficiency of the PEC reactor and a traditional photocatalytic reactor. Simultaneously, the treatment effects of anodized TiO2 nanotube electrodes and TiO2 sol-gel electrodes on Rhodamine B were compared. The results showed that after treating 20mg·L-1 Rhodamine B for 180 minutes, the treatment effect of anodized TiO2 nanotubes was 30% higher than that of sol-gel TiO2 electrodes. Further research indicated that the oblique film PEC reactor, due to improved mass transfer efficiency, achieved better treatment results for dye wastewater than TiO2 sol-gel electrodes.

Xu et al. also developed a TiO2/Ti rotating disk photoelectrocatalytic (PEC) reactor for treating Rhodamine B wastewater. In the high-efficiency thin-film PEC reactor, the upper part of the disk is a coated photoanode, and the wastewater forms a thin film on the electrode surface, exposed to the air. Ultraviolet light is used to irradiate the wastewater, while the remaining part is immersed in the water. The disk electrode rotates at a constant speed, continuously renewing the liquid film on the photoanode surface, thereby improving mass transfer efficiency and degradation of pollutants in both the upper part and the water body. For Rhodamine B concentrations of 20-150 mg·L-1, within 1 hour, color removal ranged from 27% to 84%, and TOC removal was 7% to 48%. The rotating disk photoelectrocatalytic reactor offers a new process option for dye wastewater treatment.

1.2 Electrocoagulation-Dissolved Air Flotation

Under an external voltage, a large number of cations are generated using soluble anodes (iron or aluminum) to coagulate colloidal wastewater. Simultaneously, a large number of hydrogen microbubbles are precipitated on the cathode, which adhere to the flocs and float up. This method is called electrocoagulation-dissolved air flotation. In the water treatment process, the bubbles adhere well to suspended particles, thereby improving the treatment efficiency for dye wastewater. In addition, under the action of current, some dyes in the wastewater may be directly oxidized to CO2 and H2O. Partially oxidized organic matter can also be adsorbed and coagulated by Fe(OH)3 or Al(OH)3 along with suspended solid particles and separated by flotation driven by hydrogen and oxygen. Electrocoagulation-dissolved air flotation is a synergistic effect of multiple processes.

Balla et al. applied the electrocoagulation-dissolved air flotation process (aluminum/iron electrodes) to treat synthetic dyes and actual textile wastewater. Three disperse dyes, three reactive dyes, and mixtures of these two dye classes were selected as target pollutants. The research results showed that for disperse dyes, the aluminum electrode had a better treatment effect than the iron electrode; while the iron electrode was more suitable for treating reductive dyes and mixed synthetic dyes. 20 minutes was the optimal electrolysis time, and the optimal current density was 40mA·cm-2. For reactive dye and mixed dye wastewater, the optimal treatment pH was 7.5, and for disperse dyes, it was 6.2. The decolorization rate of the electrocoagulation-dissolved air flotation method for all three types of model pollutants was above 90%. At the same time, Balla et al. also conducted an energy consumption analysis of the process: the energy consumption for treating reductive dyes, disperse dyes, and mixed synthetic dyes were 170, 120, and 50 kW·h·(kg dye)-1, respectively.

2 Advanced Oxidation Processes

Advanced oxidation technology is an emerging water treatment technology in recent years. Because this technology can generate highly oxidizing hydroxyl radicals (·HO) during the treatment process, it can convert many stable or even difficult-to-biodegrade organic molecules into non-toxic, harmless, and biodegradable low-molecular-weight substances. The final products of the reaction are mostly carbon dioxide, water, and inorganic ions, and no residual sludge or concentrate is produced. Therefore, this technology has become a research hotspot for treating dye wastewater in recent years.

2.1 Photocatalytic Oxidation

Photocatalytic oxidation technology is developed on the basis of photochemical oxidation. Compared with the photochemical method, it has stronger oxidation ability and can degrade organic pollutants more thoroughly. In recent years, photocatalytic oxidation technology with TiO2 as a catalyst has become a research hotspot. Commonly used catalysts for photocatalytic oxidation technology include TiO2, ZnO, WO3, CdS, ZnS, SnO2, and Fe3O4.

Sun et al. synthesized microcrystalline ZnO by hydrothermal method as a photocatalytic oxidant to degrade three dye wastewaters: crystal violet, methyl violet, and methylene blue. After 75 minutes, the decolorization rates reached 68.0%, 99.0%, and 98.5%, respectively. The TOC removal rates were 43.2%, 59.4%, and 70.6%, which were 16% to 22% higher than the catalytic effect of commercial ZnO. Aber et al. used UV-induced zinc sulfide nanocrystals (UV-ZnS) catalytic oxidation method to degrade Acid Blue 9 wastewater. The experiment investigated the effects of UV intensity, S2O82-, and IO4- on the photocatalytic oxidation process. The experimental results showed that the treatment effect of the UV-ZnS system on Acid Blue 9 increased with the increase of UV intensity, S2O82-, and IO4- concentrations. Joshi et al. synthesized nanocrystalline WO3 by sol-gel method and performed decolorization experiments on methyl orange wastewater under visible light induction. Methyl orange wastewater was completely decolorized after 4 hours.

Sema et al. prepared titanium dioxide by hydrothermal method. Under the induction of visible light, the degradation of Congo red wastewater was studied. 20 mg·L-1 Congo red wastewater can be easily degraded in a system with 0.25% (mass) nano-titanium dioxide after 30 minutes of illumination.

Muhammad et al. prepared Cr-TiO2 catalyst containing Cr3+ by sol-gel method. With UV induction, methyl blue wastewater was treated. The experimental results showed that at pH=7, 70% of methyl blue could be degraded, and the reaction followed a pseudo-second-order kinetic equation.

2.2 Fenton and Fenton-like Oxidation Methods

The Fenton method uses iron salts (Fe2+ or Fe3+) as a catalyst, which, in the presence of H2O2, generates highly oxidizing ·HO radicals to oxidize molecules in dye wastewater. The reaction can be carried out at room temperature and atmospheric pressure.

Sun et al. studied the effect of hydrogen peroxide concentration and its ratio with Fe2+, reaction temperature, solution pH, chloride ion concentration, and dye concentration on the treatment of Orange G by the Fenton system. The experimental results showed that the treatment effect was best at an initial pH of 4.0, H2O2 concentration of 1.0×10-2mol·L-1, and a hydrogen peroxide:Fe2+ ratio of 286:1. The decolorization rate of Orange G reached 94.6% within 60 minutes. The decolorization process followed a second-order kinetic equation.

The Fenton method for treating wastewater has problems such as long reaction time, large reagent consumption, and secondary pollution caused by increased COD in wastewater due to excess Fe2+. Researchers have introduced ultraviolet and visible light into the Fenton system and used other transition metals to replace Fe2+. These methods can enhance the oxidative degradation of organic matter and reduce reagent consumption, thereby lowering treatment costs, and are collectively referred to as Fenton-like reactions.

Hsieh et al. studied the effects of the ratio of hydrogen peroxide/Fe2+ in the Fenton system, the amount of nano-iron added, reaction time, and initial pH on COD removal rate and decolorization efficiency. At a water-soluble azo dye concentration of 500mg·L-1, a reaction time of 60 minutes, a nano-iron addition of 1g·L-1, and a dye:hydrogen peroxide:Fe2+ ratio of 1:3.6:2.4, the decolorization rate and COD removal rate were 99.91% and 63.36%, respectively.

Kasiri et al. used Fe-ZSM5 zeolite as a catalyst to degrade the indigo dye Acid Blue 74 in a system of ultraviolet light and hydrogen peroxide. In this photo-Fenton system, under reaction conditions of 120 minutes, hydrogen peroxide concentration of 21.4mmol·L-1, catalyst addition of 0.5g·L-1, and pH=5, the TOC removal rate for Acid Blue 74 wastewater was 57%.

2.3 Ozone Oxidation Method and Ultrasound-Ozone Combined Method

O3, due to its strong oxidizing ability (redox potential up to 2.07V in acidic solution), has become the preferred oxidant for many refractory industrial wastewater treatment processes. Khadhraoui et al. found in their study of using ozone to treat Congo Red that in the initial stage of oxidation, ozone itself could completely oxidize and decolorize Congo Red, and the experimental results followed a pseudo-first-order kinetic model. However, simply adding ozone could not completely mineralize Congo Red, and the COD removal rate was only 54%. The sole use of O3 for treating dye wastewater exhibits drawbacks such as selective oxidation and incomplete treatment.

To improve the O3 oxidation method, researchers have introduced ultrasonic technology to enhance the decolorization and degradation of dyes by O3 oxidation. The high temperature and high pressure generated by the instantaneous cavitation of ultrasonic waves can decolorize or completely degrade dyes, making it a feasible process option.

He et al. used ozone, ultrasound, and combined ultrasound-ozone technology to degrade anthraquinone dye Vat Blue 19. The experimental results showed that the combined ultrasound-ozone technology had a better decolorization effect on wastewater than the other two methods.

Zhang et al. used a combination of 20kHz ultrasound and ozone to study the degradation of Acid Orange 7. The experiment investigated the effects of power density, airflow velocity, initial pH, radical scavengers, and dye concentration on the decolorization rate. The experimental results showed that the decolorization kinetics of Acid Orange 7 conforms to a pseudo-0.5-order kinetic equation. The thermal radiation effect of ultrasound significantly promoted the degradation of Acid Orange 7.