Degradation of Reactive Brilliant Red X-3B by Photo-Fenton-like Process: Effects of Water Chemistry Factors and Degradation Mechanism

: Azo dye wastewater belongs to the highly concentrated organic wastewater, which is difﬁcult to be treated by traditional biological processes. The oxidation efﬁciency of a single physicochemical method is not considerable. Recent research indicated that the advanced oxidation processes (AOPs) based on the highly reactive hydroxyl radical ( · OH) became one of the preferred methods in dealing with such dye wastewater. In this paper, the typical azo dye, reactive brilliant red X-3B, was employed as the target pollutant, and the transition metal Mn and hydrogen peroxide as the catalysts. A photo-Fenton-like process, UV/Mn 2+ -H 2 O 2 system, was established, which enables a combination of various technologies to improve azo dye degradation efﬁciency while reducing disposal costs. The results indicated that the UV/Mn 2+ -H 2 O 2 system had the synergism of Mn 2+ /H 2 O 2 and UV/H 2 O 2 , which was 2.6 times greater than the sum of the two individual effects. And the degradation of X-3B reached the optimum under the conditions of 0.59 mmol/L of the Mn 2+ , 10 mmol/L of the H 2 O 2 , pH = 6 and a high level of DO. The · OH, generated from chem-catalytic and photocatalytic decomposition of H 2 O 2 , played the predominant role in the decolorization of X-3B and mineralization of its intermediates. The · OH tended to attack and break the chromophore group, resulting in the rapid decolorization of X-3B. The azo bond in X-3B was easy to be decomposed in the form of N 2 , while the triazinyl group was recalcitrant for ring opening. The degradation process of the UV/Mn 2+ -H 2 O 2 system preferred to be conducted at an acidic condition and appropriate concentrations of Mn 2+ and H 2 O 2 . The alkaline condition would decrease the utilization of H 2 O 2 , and excessive H 2 O 2 would also quench the · OH.


Introduction
Azo dyes, as the most used type of commercial dyes, are widely applied in various fields, including textile, printing, leather, paint, plastics, and food processing [1,2]. Reactive brilliant red X-3B is an indispensable azo dye in the roll dyeing, dip dyeing and tiedyeing of cotton and viscose fibers, as well as the dyeing of polyester-cotton and polyestercotton blended fabrics, and accounts for more than 60% of total dyes [3,4], which is very representative. The discharge of large amounts of dye pollutants into water bodies will threaten the aquatic organisms and human health [5]. The presence of dyes in the natural aquatic ecosystem will prevent light penetration and oxygen transfer into water, resulting in the reduction of the dissolved oxygen (DO) levels, and increases of the chemical oxygen 2 of 15 demand (COD) and biochemical oxygen demand (BOD) levels in water sources [5,6]. Furthermore, the metabolic cleavage of azo linkage during degradation produces toxic byproducts, which are carcinogenic to both human beings and aquatic life [4,7]. Therefore, effective treatment of textile effluents is crucial prior to their ultimate discharge to the environment [5].
With the continuous progress and widespread use of dye synthesis technology, the treatment of printing and dyeing wastewater has become more difficult. Printing and dyeing wastewater discharge standards are becoming more and more stringent, and the existing treatment facilities have difficulties to meet the requirements of printing and dyeing wastewater discharge standards [8]. Seeking an efficient and economical method for treating dye wastewater and minimizing the discharge of wastewater and pollutants has become an urgent problem to be solved in the dye manufacturing industry and the printing and dyeing industry, which has attracted the attention of a wide range of scholars [9,10]. There are many techniques that have been developed to deal with the dye wastewater including physical, chemical and biological methods, such as ultrasound irradiation [11], photocatalytic processes [7], coagulation-flocculation [12], adsorption [13], electrochemical oxidation [14], and biodegradation [15]. At present, the treatment methods for various dye wastewater at home and abroad are mainly biochemical methods, supplemented by chemical methods or physicochemical methods. Nevertheless, due to the low removal rates of COD and color by biochemical methods, the post-treatment of physical methods is relatively low and prone to produce the secondary pollution; and the chemical method has the disadvantage of high cost as an effective way to improve the efficiency of sewage treatment. As a powerful and environmentally friendly approach, advanced oxidation processes (AOPs), have received attention for treating recalcitrant dye wastewater, by generating highly reactive transient species (i.e., ·OH) in situ to achieve high removal efficiency and total mineralization of dye molecules [10,16].
Among various AOPs for the treatment of printing and dyeing wastewater, H 2 O 2 as a commonly used oxidant has a higher oxidation potential, which can react with metal ions to produce highly active ·OH. The ·OH involved in AOPs is a kind of nonspecific oxidant that can oxidize and mineralize almost all the organic molecules owing to its high oxidation potential (E • = +2.8 eV) [17,18]. The H 2 O 2 is often used in combination with Fe 2+ to comprise of the Fenton process, which proves to be an efficient, inexpensive, and green technology for the treatment of wastewaters containing a variety of toxic substances [19,20]. In addition, the light irradiation can promote the generation of ·OH and degradation of organic matter in the Fenton system, and therefore, some studies have attempted the application of the UV/Fenton (photo-Fenton) system to the treatment of printing and dyeing wastewater [21,22].
Nevertheless, Fe 2+ (Fe 3+ ), as a catalyst in the Fenton and photo-Fenton processes in homogeneous media, must be used in an acidic environment (pH < 3), which requires high corrosion resistance of the equipment and greatly reduces the promotion of applications. A large amount of sludge containing Fe ions is produced after the wastewater treatment. Moreover, the utilization rate of H 2 O 2 is not ideal. Many problems need to be solved for the application of the Fenton process in wastewater treatment [23]. Fortunately, in recent years, many studies have found that the oxidizability of Fenton-like (Fe 3+ /H 2 O 2 ) oxidation system can be effectively improved under the action of Ce 3+ and Mn 2+ [24,25]. The Ce 3+ and Mn 2+ enhanced Fenton-like oxidation system has a stronger oxidation ability and higher reagent utilization rate. The catalytic oxidation method using transition metals as catalysts has a high removal rate of COD and color, and can even oxidize most of the soluble dyes [24]. Compared with the original Fenton-like oxidation system, it has shown good prospects for development [25]. In short, although there are many methods for the treatment of dye wastewater, each has its own limitations, which makes it difficult to achieve the unity of the removal effect, economic cost and ecological protection. The application of the multi-stage combined degradation method is more advantageous, and it is expected to effectively reduce the chroma and ensure the effluent without increasing costs to meet emission standards.
Currently, little is known about the synergistic effects produced by the combination of UV/Fenton and Fenton-like systems. In this context, the reactive brilliant red X-3B was used to simulate dye wastewater, and the removal efficiency and degradation effects of different degradation systems were compared including Mn 2+ , H 2 O 2 , UV/Mn 2+ , Mn 2+ /H 2 O 2 , UV/H 2 O 2 , and UV/Mn 2+ -H 2 O 2 . The synergistic and degradation mechanisms of the UV/Mn 2+ -H 2 O 2 system were preliminarily investigated. The effects of Mn 2+ and H 2 O 2 concentration, initial solution pH, and X-3B initial dosage on the degradation of X-3B by the UV/Mn 2+ -H 2 O 2 process were comprehensively investigated. The research on this combined process is expected to effectively improve the degradation efficiency of azo dyes and reduce the treatment cost, and provide important information for the research on the operating conditions and mechanism of the process, which can provide some references for its application and promotion.

Preparation and Testing of Dye Wastewater
The commercial azo dye X-3B was used to prepare 1 L of 150 mg/L simulated wastewater. The chemical structure and UV-vis absorption spectrum of X-3B are given in Figure 1. X-3B had five absorption peaks in the 200-800 nm wavelength range, i.e., 236 nm, 285 nm, 329 nm, 512 nm, and 538 nm. According to the theory of organic spectroscopy [26,27] and the chemical structure of X-3B, it could be seen that: 236 nm and 285 nm were the characteristic absorption peaks of the benzene ring; 329 nm was the absorption peak of the naphthalene ring. The azo bond (−N=N−) is an atomic group containing π electrons, which forms a conjugated color system of 8-naphthol-3 with the benzene ring and a 6-disulfonic acid with the naphthalene ring, and the characteristic absorption peaks appeared at 512 nm and 538 nm, respectively [28]. In this study, 538 nm was selected as the characteristic absorption peak of X-3B in the UV-vis region, which was represented by A 538 . advantageous, and it is expected to effectively reduce the chroma and ensure the effluent without increasing costs to meet emission standards. Currently, little is known about the synergistic effects produced by the combination of UV/Fenton and Fenton-like systems. In this context, the reactive brilliant red X-3B was used to simulate dye wastewater, and the removal efficiency and degradation effects of different degradation systems were compared including Mn 2+ , H2O2, UV/Mn 2+ , Mn 2+ /H2O2, UV/H2O2, and UV/Mn 2+ -H2O2. The synergistic and degradation mechanisms of the UV/Mn 2+ -H2O2 system were preliminarily investigated. The effects of Mn 2+ and H2O2 concentration, initial solution pH, and X-3B initial dosage on the degradation of X-3B by the UV/Mn 2+ -H2O2 process were comprehensively investigated. The research on this combined process is expected to effectively improve the degradation efficiency of azo dyes and reduce the treatment cost, and provide important information for the research on the operating conditions and mechanism of the process, which can provide some references for its application and promotion.

Preparation and Testing of Dye Wastewater
The commercial azo dye X-3B was used to prepare 1 L of 150 mg/L simulated wastewater. The chemical structure and UV-vis absorption spectrum of X-3B are given in Figure 1. X-3B had five absorption peaks in the 200-800 nm wavelength range, i.e., 236 nm, 285 nm, 329 nm, 512 nm, and 538 nm. According to the theory of organic spectroscopy [26,27] and the chemical structure of X-3B, it could be seen that: 236 nm and 285 nm were the characteristic absorption peaks of the benzene ring; 329 nm was the absorption peak of the naphthalene ring. The azo bond (−N=N−) is an atomic group containing π electrons, which forms a conjugated color system of 8-naphthol-3 with the benzene ring and a 6-disulfonic acid with the naphthalene ring, and the characteristic absorption peaks appeared at 512 nm and 538 nm, respectively [28]. In this study, 538 nm was selected as the characteristic absorption peak of X-3B in the UV-vis region, which was represented by A538.

Experimental Apparatus and Procedures
The schematic diagram of the experimental apparatus is shown in Figure 2. For a typical degradation experiment, a certain concentration of MnSO4 and H2O2 were added into X-3B solution in the reactor, and the initial pH value was adjusted with 20% H2SO4 and 10% NaOH. After sequentially turning on the aerating apparatus and UV lamp, the degradation reaction was started. At a certain interval, solution samples were taken for chemical analyses.

Experimental Apparatus and Procedures
The schematic diagram of the experimental apparatus is shown in Figure 2. For a typical degradation experiment, a certain concentration of MnSO 4 and H 2 O 2 were added into X-3B solution in the reactor, and the initial pH value was adjusted with 20% H 2 SO 4 and 10% NaOH. After sequentially turning on the aerating apparatus and UV lamp, the degradation reaction was started. At a certain interval, solution samples were taken for chemical analyses. For the choice of ideal catalyst, a certain volume of X-3B solution of 50 mg/L was prepared. The candidate catalysts (i.e., MnSO4, MnO2, CeO2, CuSO4 or ZnSO4) of 0.2 g/L and 1 mL H2O2 were added into X-3B solution. The degradation experiments were conducted at room temperature, and the degradation of X-3B was monitored at 0, 30, 60, 90 and 120 min, respectively. The comparative results indicated the most ideal catalyst was MnSO4 (see the results section below).
To investigate the degradation pathway, the changes of UV-vis spectra of X-3B and anions (Cl − , NO3 − and SO4 2− ) concentrations were recorded during the degradation process of the UV/Mn 2+ -H2O2 system.

Analytical Methods
For the measurements of UV-vis absorption spectra and the concentration of X-3B, an UV-1201 UV-vis spectrophotometer (Beijing Ruili Analytical Instrument Co., Ltd., Beijing, China) was used with scanning wavelength range of 200-800 nm. The pathlength of quartz cuvette was 1 cm, and distilled water was used as a reference. A538 was employed for the quantitative analysis of X-3B. The TOC and TN were determined by a For the study of synergistic mechanisms, different doses of methanol (0, 0.24, 0.29 and 0.32 mmol/L) were added into X-3B solution to conduct the degradation experiments with the UV/Mn 2+ -H 2 O 2 system. After that, the degradation experiments were conducted in the following three scenarios: (1) 1 L of 150 mg/L X-3B, 10 mmol/L of H 2 O 2 , 0.59 mmol/L of Mn 2+ , and UV irradiation; (2) 1 L of 150 mg/L X-3B, 10 mmol/L of H 2 O 2 , 0 mmol/L of Mn 2+ , and UV irradiation; and (3) 1 L of 150 mg/L X-3B, 10 mmol/L of H 2 O 2 , 0.59 mmol/L of Mn 2+ , and without UV irradiation. Solution samples were taken at 0, 30, 60, 90 and 120 min, respectively, and the residual H 2 O 2 , total organic carbon (TOC) and total nitrogen (TN) were measured.
To investigate the degradation pathway, the changes of UV-vis spectra of X-3B and anions (Cl − , NO 3 − and SO 4 2− ) concentrations were recorded during the degradation process of the UV/Mn 2+ -H 2 O 2 system.

Analytical Methods
For the measurements of UV-vis absorption spectra and the concentration of X-3B, an UV-1201 UV-vis spectrophotometer (Beijing Ruili Analytical Instrument Co., Ltd., Beijing, China) was used with scanning wavelength range of 200-800 nm. The pathlength of quartz cuvette was 1 cm, and distilled water was used as a reference. A 538 was employed for the quantitative analysis of X-3B. The TOC and TN were determined by a Multi N/C TOC/TN analyzer (Jena Analytical Instruments Co., Ltd., Thuringia, Germany). For the measure-Water 2022, 14, 380 5 of 15 ment of residual H 2 O 2 , ammonium molybdate spectrophotometry was employed using 2.4 mmol/L ammonium molybdate and absorbance at 350 nm. The ion chromatographic analyzer ICS-900 (Dionex, Sunnyvale, CA, USA), equipped with Ion Pac AS23 ion separation column, DS5 conductivity detector and MMS300 suppressor, was used to analyze the concentration of Cl − , NO 3 − , and SO 4 2− during the degradation reactions. The sample injection volume was 10 µL, and KOH solution was employed as the eluent with a flow rate of 1.0 mL/min. Before the measurement, the sample was diluted, decolorized using On Guard P type pretreatment column, and filtered through a 0.45 µm filter membrane.

Decolorization of X-3B by H 2 O 2 with Different Catalysts
The decolorization rates of X-3B by H 2 O 2 with different catalysts showed great difference ( Figure 3). In the catalytic degradation experiments, the order of the ability of these five catalysts to decompose H 2 O 2 is: CuSO 4 > MnO 2 > MnSO 4 > CeO 2 > ZnSO 4 , respectively. Among these five catalysts, Cu, Mn, and Zn belong to transition metal elements, while Ce belongs to rare earth elements. The experimental results showed that the catalytic performance of CeO 2 was not very obvious. CuSO 4 has the highest catalytic activity, which is consistent with the results of other studies [29]. The catalytic activities of MnSO 4 and MnO 2 are similar, affording~25% of decolorization rate of X-3B after 120 min reaction. Previous studies have shown that Mn 2+ has good catalytic performance in homogeneous catalytic ozonation reactions, and MnO 2 also has strong heterogeneous catalytic performance [30,31]. However, red-brown flocs appeared in the solution with CuSO 4 , and the powdery MnO 2 was insoluble in water, which would interfere with the measurements of the water chemistry parameters. The catalytic activity of ZnSO 4 was the worst, probably because Zn 2+ has no variable valence state, so it can be considered that ZnSO 4 has no catalytic activity for the catalytic decomposition of H 2 O 2 . Therefore, MnSO 4 was finally selected as the ideal catalyst for further investigations. Multi N/C TOC/TN analyzer (Jena Analytical Instruments Co., Ltd., Thuringia, Germany). For the measurement of residual H2O2, ammonium molybdate spectrophotometry was employed using 2.4 mmol/L ammonium molybdate and absorbance at 350 nm. The ion chromatographic analyzer ICS-900 (Dionex, Sunnyvale, CA, USA), equipped with Ion Pac AS23 ion separation column, DS5 conductivity detector and MMS300 suppressor, was used to analyze the concentration of Cl − , NO3 − , and SO4 2− during the degradation reactions. The sample injection volume was 10 μL, and KOH solution was employed as the eluent with a flow rate of 1.0 mL/min. Before the measurement, the sample was diluted, decolorized using On Guard P type pretreatment column, and filtered through a 0.45 μm filter membrane.

Decolorization of X-3B by H2O2 with Different Catalysts
The decolorization rates of X-3B by H2O2 with different catalysts showed great difference ( Figure 3). In the catalytic degradation experiments, the order of the ability of these five catalysts to decompose H2O2 is: CuSO4 > MnO2 > MnSO4 > CeO2 > ZnSO4, respectively. Among these five catalysts, Cu, Mn, and Zn belong to transition metal elements, while Ce belongs to rare earth elements. The experimental results showed that the catalytic performance of CeO2 was not very obvious. CuSO4 has the highest catalytic activity, which is consistent with the results of other studies [29]. The catalytic activities of MnSO4 and MnO2 are similar, affording ~25% of decolorization rate of X-3B after 120 min reaction. Previous studies have shown that Mn 2+ has good catalytic performance in homogeneous catalytic ozonation reactions, and MnO2 also has strong heterogeneous catalytic performance [30,31]. However, red-brown flocs appeared in the solution with CuSO4, and the powdery MnO2 was insoluble in water, which would interfere with the measurements of the water chemistry parameters. The catalytic activity of ZnSO4 was the worst, probably because Zn 2+ has no variable valence state, so it can be considered that ZnSO4 has no catalytic activity for the catalytic decomposition of H2O2. Therefore, MnSO4 was finally selected as the ideal catalyst for further investigations.

Degradation of X-3B in Different Systems
The removal efficiencies of X-3B by Mn 2+  degrade X-3B, and the catalytic oxidation effect was not obvious even under the action of UV photolysis. It can be assumed that X-3B cannot be directly degraded by these single processes. However, when Mn 2+ and H 2 O 2 coexist, the degradation effect of X-3B appears, which indicates that the catalysis of Mn 2+ is crucial for the oxidation of hydrogen peroxide.

Degradation of X-3B in Different Systems
The removal efficiencies of X-3B by Mn 2+ , H2O2, UV/Mn 2+ , Mn 2+ /H2O2, UV/H2O2, and UV/Mn 2+ -H2O2 are presented in Figure 4. According to the experimental results, the degradation rates of X-3B in these systems can be summarized as: UV/Mn 2+ -H2O2 > UV/H2O2 > Mn 2+ /H2O2 > H2O2 > UV > Mn 2+ . It can be seen that X-3B was stable under the catalysis of Mn 2+ alone or H2O2 alone, indicating that Mn 2+ cannot directly degrade X-3B, and the catalytic oxidation effect was not obvious even under the action of UV photolysis. It can be assumed that X-3B cannot be directly degraded by these single processes. However, when Mn 2+ and H2O2 coexist, the degradation effect of X-3B appears, which indicates that the catalysis of Mn 2+ is crucial for the oxidation of hydrogen peroxide. For the combined systems, i.e., Mn 2+ /H2O2, UV/H2O2 and UV/Mn 2+ -H2O2, the degradation kinetics of X-3B was fitted by the first-order kinetic equation [32]. The calculated apparent reaction rate constant (kapp) for UV/Mn 2+ -H2O2, UV/H2O2, and Mn 2+ /H2O2 was 0.0951, 0.0368 and 0.0030 min −1 , respectively. It can be seen that X-3B was degraded the fastest in UV/Mn 2+ -H2O2 system, and the kapp is 2.6 times the sum of the other two systems. It is feasible to draw the conclusion that the synergistic effect of photocatalysis and Mn 2+ catalysis is crucial to improve the degradation ability of the UV/Mn 2+ -H2O2 system.

Effects of Mn 2+ and H2O2 Concentrations
Mn 2+ , as a catalyst for the UV/Mn 2+ -H2O2 reaction system, is a necessary factor for generating •OH. As shown in Figure 5a, increasing the concentration of Mn 2+ from 0.3 to 0.59 mmol/L accelerated the decomposition of H2O2. However, continuously increasing the Mn 2+ concentration resulted in an inhibition on the degradation of X-3B. Mn 2+ participates in the Fenton-like reaction [24]. An obvious acceleration in the decomposition rate of H2O2 was observed by adding Mn 2+ , and a large amount of •OH was produced. When the dosage of Mn 2+ was too high, •OH would be produced in a very short time and many of them would be consumed by side reactions, which is not conducive to the effective use of •OH. Therefore, the optimal concentration of Mn 2+ was determined to be 0.59 mmol/L. For the combined systems, i.e., Mn 2+ /H 2 O 2 , UV/H 2 O 2 and UV/Mn 2+ -H 2 O 2 , the degradation kinetics of X-3B was fitted by the first-order kinetic equation [32]. The calculated apparent reaction rate constant (k app ) for UV/Mn 2+ -H 2 O 2 , UV/H 2 O 2 , and Mn 2+ /H 2 O 2 was 0.0951, 0.0368 and 0.0030 min −1 , respectively. It can be seen that X-3B was degraded the fastest in UV/Mn 2+ -H 2 O 2 system, and the k app is 2.6 times the sum of the other two systems. It is feasible to draw the conclusion that the synergistic effect of photocatalysis and Mn 2+ catalysis is crucial to improve the degradation ability of the UV/Mn 2+ -H 2 O 2 system.

Effects of Mn 2+ and H 2 O 2 Concentrations
Mn 2+ , as a catalyst for the UV/Mn 2+ -H 2 O 2 reaction system, is a necessary factor for generating ·OH. As shown in Figure 5a, increasing the concentration of Mn 2+ from 0.3 to 0.59 mmol/L accelerated the decomposition of H 2 O 2 . However, continuously increasing the Mn 2+ concentration resulted in an inhibition on the degradation of X-3B. Mn 2+ participates in the Fenton-like reaction [24]. An obvious acceleration in the decomposition rate of H 2 O 2 was observed by adding Mn 2+ , and a large amount of ·OH was produced. When the dosage of Mn 2+ was too high, ·OH would be produced in a very short time and many of them would be consumed by side reactions, which is not conducive to the effective use of ·OH. Therefore, the optimal concentration of Mn 2+ was determined to be 0.59 mmol/L. As an oxidant, the amount of H2O2 had a great influence on the UV/Mn 2+ -H2O2 degradation system. It can be seen from Figure 5b that the final decolorization rate gradually increased, as the H2O2 concentration was increased from 5 to 20 mmol/L. However, within the initial 10 min of the degradation reaction, the degradation rate with 10 mmol/L H2O2 was significantly better than that with 20 mmol/L. The H2O2 can not only produce •OH, but also acts as a radical scavenger in the system [11]. The excessive H2O2 would consume •OH, leading to the reduction of available •OH for the oxidation of X-3B. Considering that excessive H2O2 will increase the COD in the effluent and increase the treatment cost, the ideal concentration of H2O2 was determined to be 10 mmol/L. Figure 6 shows the effects of the initial solution pH on the X-3B decolorization rate by the UV/Mn 2+ -H2O2 system. When the initial condition of the experiment was increased from pH = 2 to pH = 8, the degradation rate of X-3B was gradually slowed down, but all could be completely degraded after 60 min. An increase in initial pH (OH − concentration) will decrease the utilization of H2O2, and reduce •OH concentration. Under alkaline conditions, H2O2 would like to decompose into HO2 − and H2O [33]. On the contrary, H2O2 is relatively stable under acidic conditions, but can be decomposed by UV light and soluble Mn 2+ to generate •OH [34]. As the reaction progressed, the organic carbon in the solution was continuously converted into inorganic carbon. Under alkaline conditions, inorganic carbon existed in the form of HCO3 − or CO3 2− , which could react with •OH, and then consume •OH to reduce the catalytic activity [35]. Furthermore, in order to ensure good dyeing properties of azo dyes, they should be used under acidic or neutral conditions, due to its anionic properties. On the other hand, under alkaline conditions, Mn 2+ could hydrolyze and be transformed into precipitation, which will inhibit the reactions and reduce the generation of •OH [25]. For the comprehensive consideration of the above aspects, the pH value of 6 was selected in the degradation experiments. As an oxidant, the amount of H 2 O 2 had a great influence on the UV/Mn 2+ -H 2 O 2 degradation system. It can be seen from Figure 5b that the final decolorization rate gradually increased, as the H 2 O 2 concentration was increased from 5 to 20 mmol/L. However, within the initial 10 min of the degradation reaction, the degradation rate with 10 mmol/L H 2 O 2 was significantly better than that with 20 mmol/L. The H 2 O 2 can not only produce ·OH, but also acts as a radical scavenger in the system [11]. The excessive H 2 O 2 would consume ·OH, leading to the reduction of available ·OH for the oxidation of X-3B. Considering that excessive H 2 O 2 will increase the COD in the effluent and increase the treatment cost, the ideal concentration of H 2 O 2 was determined to be 10 mmol/L. Figure 6 shows the effects of the initial solution pH on the X-3B decolorization rate by the UV/Mn 2+ -H 2 O 2 system. When the initial condition of the experiment was increased from pH = 2 to pH = 8, the degradation rate of X-3B was gradually slowed down, but all could be completely degraded after 60 min. An increase in initial pH (OH − concentration) will decrease the utilization of H 2 O 2 , and reduce ·OH concentration. Under alkaline conditions, H 2 O 2 would like to decompose into HO 2 − and H 2 O [33]. On the contrary, H 2 O 2 is relatively stable under acidic conditions, but can be decomposed by UV light and soluble Mn 2+ to generate ·OH [34]. As the reaction progressed, the organic carbon in the solution was continuously converted into inorganic carbon. Under alkaline conditions, inorganic carbon existed in the form of HCO 3 − or CO 3 2− , which could react with ·OH, and then consume ·OH to reduce the catalytic activity [35]. Furthermore, in order to ensure good dyeing properties of azo dyes, they should be used under acidic or neutral conditions, due to its anionic properties. On the other hand, under alkaline conditions, Mn 2+ could hydrolyze and be transformed into precipitation, which will inhibit the reactions and reduce the generation of ·OH [25]. For the comprehensive consideration of the above aspects, the pH value of 6 was selected in the degradation experiments. Water 2022, 14, x FOR PEER REVIEW 8 of 16 Figure 6. Effect of initial solution pH value on the decolorization of X-3B by the UV/Mn 2+ -H2O2 system.

Effect of Initial X-3B Concentration
For different processes, X-3B concentration in effluent printing and dyeing plants will significantly fluctuate. Therefore, the effect of initial X-3B concentration on X-3B removal efficiency is worth studying. As shown in Figure 7, as the initial concentration increased, X-3B removal efficiency decreased. The reason is that the dosage of the oxidizing agent was certain, and the amount of •OH produced was limited [36]. The amount of •OH used to attack the chromogenic group of X-3B was not enough to completely break the chromogenic groups, resulting in the decrease of decolorization rate. The light shielding effects caused by excess X-3B may also provide a negative impact for the photo-Fenton-like process [37]. In addition, the increased amount of X-3B molecules would also produce more intermediates that competed with X-3B parent molecules for •OH [38].

Effect of Initial X-3B Concentration
For different processes, X-3B concentration in effluent printing and dyeing plants will significantly fluctuate. Therefore, the effect of initial X-3B concentration on X-3B removal efficiency is worth studying. As shown in Figure 7, as the initial concentration increased, X-3B removal efficiency decreased. The reason is that the dosage of the oxidizing agent was certain, and the amount of ·OH produced was limited [36]. The amount of ·OH used to attack the chromogenic group of X-3B was not enough to completely break the chromogenic groups, resulting in the decrease of decolorization rate. The light shielding effects caused by excess X-3B may also provide a negative impact for the photo-Fenton-like process [37]. In addition, the increased amount of X-3B molecules would also produce more intermediates that competed with X-3B parent molecules for ·OH [38].

Effect of Initial X-3B Concentration
For different processes, X-3B concentration in effluent printing and dyeing plants will significantly fluctuate. Therefore, the effect of initial X-3B concentration on X-3B removal efficiency is worth studying. As shown in Figure 7, as the initial concentration increased, X-3B removal efficiency decreased. The reason is that the dosage of the oxidizing agent was certain, and the amount of •OH produced was limited [36]. The amount of •OH used to attack the chromogenic group of X-3B was not enough to completely break the chromogenic groups, resulting in the decrease of decolorization rate. The light shielding effects caused by excess X-3B may also provide a negative impact for the photo-Fenton-like process [37]. In addition, the increased amount of X-3B molecules would also produce more intermediates that competed with X-3B parent molecules for •OH [38].

Role of ·OH in the Degradation System of UV/Mn 2+ -H 2 O 2
The ·OH was considered to be the main factor in the degradation of organic pollutants in AOPs [39]. Adding different concentrations of ·OH scavengers could indirectly analyze the change of ·OH in the reaction system, and qualitatively explain the existence and contribution of ·OH to the degradation of organic pollutants [40,41]. There were many kinds of ·OH scavengers, and alcohols are the most widely used [42,43]. Here, methanol was selected as the ·OH scavenger. As shown in Figure 8, with the addition of methanol, the degradation rate of X-3B gradually decreased. With the addition of 0.32 mol/L methanol, the degradation of X-3B was inhibited by 72%. The experimental results indicated that the degradation of X-3B by the UV/Mn 2+ -H 2 O 2 system mainly resulted from the oxidation of ·OH [44].
Water 2022, 14, x FOR PEER REVIEW 9 of 16

Role of •OH in the Degradation System of UV/Mn 2+ -H2O2
The •OH was considered to be the main factor in the degradation of organic pollutants in AOPs [39]. Adding different concentrations of •OH scavengers could indirectly analyze the change of •OH in the reaction system, and qualitatively explain the existence and contribution of •OH to the degradation of organic pollutants [40,41]. There were many kinds of •OH scavengers, and alcohols are the most widely used [42,43]. Here, methanol was selected as the •OH scavenger. As shown in Figure 8, with the addition of methanol, the degradation rate of X-3B gradually decreased. With the addition of 0.32 mol/L methanol, the degradation of X-3B was inhibited by 72%. The experimental results indicated that the degradation of X-3B by the UV/Mn 2+ -H2O2 system mainly resulted from the oxidation of •OH [44].  Figure 9 shows the changes of the residual H2O2 in the three degradation systems of UV/H2O2, Mn 2+ /H2O2 and UV/Mn 2+ -H2O2 over time. When the reaction time was 60 min, the decomposition of H2O2 in the three systems was not complete, the residual concentration of H2O2 was 7.07, 8.82 and 6.18 mmol/L, respectively. When the reaction progressed to 120 min, the residual H2O2 (8.47 mmol/L) in the Mn 2+ /H2O2 system did not change significantly, indicating the catalytic effect of single Mn 2+ was not remarkable, and the amount of •OH produced was not enough to oxidize X-3B. H2O2 was still excited to form a certain amount of •OH after 60 min in the UV/H2O2 system, and the residual H2O2 continuously decreased to 3.7 mmol/L at 120 min. In the UV/Mn 2+ -H2O2 system, the residual H2O2 was the lowest and further decreased to 1.52 mmol/L at 120 min. These results indicated in the UV/Mn 2+ -H2O2 system, in addition to UV irradiation, the Fenton-like effect of Mn 2+ and H2O2 was also very important [45]. Under the dual catalysis, H2O2 decomposed rapidly and produced a large amount of •OH for the degradation of X-3B. The reaction mechanism is as follows: 23 Figure 9 shows the changes of the residual H 2 O 2 in the three degradation systems of UV/H 2 O 2 , Mn 2+ /H 2 O 2 and UV/Mn 2+ -H 2 O 2 over time. When the reaction time was 60 min, the decomposition of H 2 O 2 in the three systems was not complete, the residual concentration of H 2 O 2 was 7.07, 8.82 and 6.18 mmol/L, respectively. When the reaction progressed to 120 min, the residual H 2 O 2 (8.47 mmol/L) in the Mn 2+ /H 2 O 2 system did not change significantly, indicating the catalytic effect of single Mn 2+ was not remarkable, and the amount of ·OH produced was not enough to oxidize X-3B. H 2 O 2 was still excited to form a certain amount of ·OH after 60 min in the UV/H 2 O 2 system, and the residual H 2 O 2 continuously decreased to 3.7 mmol/L at 120 min. In the UV/Mn 2+ -H 2 O 2 system, the residual H 2 O 2 was the lowest and further decreased to 1.52 mmol/L at 120 min. These results indicated in the UV/Mn 2+ -H 2 O 2 system, in addition to UV irradiation, the Fentonlike effect of Mn 2+ and H 2 O 2 was also very important [45]. Under the dual catalysis, H 2 O 2 decomposed rapidly and produced a large amount of ·OH for the degradation of X-3B. The reaction mechanism is as follows: After X-3B was degraded, small molecular nitrides (such as NO3 − , NO2 − , NH4 + etc.) would be produced, which could affect the water discharge quality, and TN reflects the total nitrogen content in the solution [46]. As shown in Figure 10a, the removal rate of TN in the two systems of UV/H2O2 and Mn 2+ /H2O2 was not ideal, and even the TN content in the solution increased at the beginning of the reaction. The UV/Mn 2+ -H2O2 system showed a considerable effect for the TN removal, and obtained a TN removal rate of 20.65% at 120 min. After X-3B was degraded, small molecular nitrides (such as NO 3 − , NO 2 − , NH 4 + etc.) would be produced, which could affect the water discharge quality, and TN reflects the total nitrogen content in the solution [46]. As shown in Figure 10a, the removal rate of TN in the two systems of UV/H 2 O 2 and Mn 2+ /H 2 O 2 was not ideal, and even the TN content in the solution increased at the beginning of the reaction. The UV/Mn 2+ -H 2 O 2 system showed a considerable effect for the TN removal, and obtained a TN removal rate of 20.65% at 120 min. After X-3B was degraded, small molecular nitrides (such as NO3 − , NO2 − , NH4 + etc.) would be produced, which could affect the water discharge quality, and TN reflects the total nitrogen content in the solution [46]. As shown in Figure 10a, the removal rate of TN in the two systems of UV/H2O2 and Mn 2+ /H2O2 was not ideal, and even the TN content in the solution increased at the beginning of the reaction. The UV/Mn 2+ -H2O2 system showed a considerable effect for the TN removal, and obtained a TN removal rate of 20.65% at 120 min. The TOC removal rate represents the degree of mineralization of the X-3B solution [47]. It can be seen from Figure 10b that the Mn 2+ /H2O2 system had almost no effect on the degradation of TOC in the X-3B solution, and the removal rate of TOC at 120 min was only 3.48%, indicating that Mn 2+ /H2O2 system was difficult to mineralize X-3B. The UV/Mn 2+ -H2O2 system showed the best effect on TOC removal, and obtained a TOC removal rate of 56.45% at 120 min. It could be observed that the removal of TOC in the three systems were relatively slow at the beginning. Decolorization was completed in the early stage of the reaction, because •OH will first attack the chromophore group of X-3B The TOC removal rate represents the degree of mineralization of the X-3B solution [47]. It can be seen from Figure 10b that the Mn 2+ /H 2 O 2 system had almost no effect on the degradation of TOC in the X-3B solution, and the removal rate of TOC at 120 min was only 3.48%, indicating that Mn 2+ /H 2 O 2 system was difficult to mineralize X-3B. The UV/Mn 2+ -H 2 O 2 system showed the best effect on TOC removal, and obtained a TOC removal rate of 56.45% at 120 min. It could be observed that the removal of TOC in the three systems were relatively slow at the beginning. Decolorization was completed in the early stage of the reaction, because ·OH will first attack the chromophore group of X-3B and cause it to be cleaved, and after the decolorization was completed, the organic intermediate molecules in the solution began to be oxidized and mineralized into H 2 O and CO 2 [28,48], leading to the quick decline of TOC.

Degradation Mechanism of UV/Mn 2+ -H 2 O 2 System
The azo bond in X-3B is unable to maintain a steady state when exposed to light, heat, and in acidic or alkaline media, and it is prone to react, release N 2 , and decolorize [49]. In the UV-vis absorption spectrum of X-3B, the chromophore group is the azo bond (the conjugated π system) corresponding to the main absorption bands at 512 and 538 nm, and the peaks at 236, 329 and 285 nm in the ultraviolet region are ascribed to the aromatic rings. In the reaction process, the intensity of characteristic absorption peaks at 512 and 538 nm obviously reduced until they disappeared completely after 60 min (Figure 11), indicating that chromogenic group of X-3B had been completely cleaved. The peaks of the benzene ring in the UV range of 200-350 nm were also diminished to a certain extent, which was well in accord with TOC removal rate [50]. After 120 min degradation, the solution still had a strong terminal absorption at about 200 nm, which may be the strong absorption of various intermediates. and cause it to be cleaved, and after the decolorization was completed, the organic intermediate molecules in the solution began to be oxidized and mineralized into H2O and CO2 [28,48], leading to the quick decline of TOC. The azo bond in X-3B is unable to maintain a steady state when exposed to light, heat, and in acidic or alkaline media, and it is prone to react, release N2, and decolorize [49]. In the UV-vis absorption spectrum of X-3B, the chromophore group is the azo bond (the conjugated π system) corresponding to the main absorption bands at 512 and 538 nm, and the peaks at 236, 329 and 285 nm in the ultraviolet region are ascribed to the aromatic rings. In the reaction process, the intensity of characteristic absorption peaks at 512 and 538 nm obviously reduced until they disappeared completely after 60 min (Figure 11), indicating that chromogenic group of X-3B had been completely cleaved. The peaks of the benzene ring in the UV range of 200-350 nm were also diminished to a certain extent, which was well in accord with TOC removal rate [50]. After 120 min degradation, the solution still had a strong terminal absorption at about 200 nm, which may be the strong absorption of various intermediates. Figure 11. Change of the UV-Vis absorption spectra during the degradation of X-3B by the UV/Mn 2+ -H2O2 system.
In the comparison of ion chromatograms, the peaks of Cl − , SO4 2− and NO3 − at 120 min were conspicuously higher than that at 30 min ( Figure 12). The concentration of X-3B in the experiment was 150 mg/L, and its molarity was about 0.25 mmol/L. According to the molecular formula of X-3B, theoretically, the molar concentrations of Cl, N and S elements in the reaction solution were 0.5 mmol/L, 1.5 mmol/L and 0.5 mmol/L, respectively. In addition, the initial concentration of SO4 2− in the catalyst used in the experiment was 0.6 mmol/L. With the progress of the reaction, the concentration of Cl − , NO3 − and SO4 2− in the solution continued to increase, indicating that the degree of oxidation was further deepened. After 60 min, the concentration of Cl − and SO4 2− in the solution was relatively stable and close to the theoretical value, while NO3 − concentration was small. When the reaction proceeded to 120 min, Cl − , NO3 − and SO4 2− concentrations were 0.4 mmol/L, 0.039 mmol/L, 0.877 mmol/L ( Figure 13). The NO3 − in the solution may theoretically come from the azo bond, amino group and triazinyl. Nevertheless, the azo bond decomposed in the form of N2, and the triazinyl group was stable and difficult to open ring, so the NO3 − in the solution was mainly provided by the amino group [51,52]. In the comparison of ion chromatograms, the peaks of Cl − , SO 4 2− and NO 3 − at 120 min were conspicuously higher than that at 30 min ( Figure 12). The concentration of X-3B in the experiment was 150 mg/L, and its molarity was about 0.25 mmol/L. According to the molecular formula of X-3B, theoretically, the molar concentrations of Cl, N and S elements in the reaction solution were 0.5 mmol/L, 1.5 mmol/L and 0.5 mmol/L, respectively. In addition, the initial concentration of SO 4 2− in the catalyst used in the experiment was 0.6 mmol/L. With the progress of the reaction, the concentration of Cl − , NO 3 − and SO 4 2− in the solution continued to increase, indicating that the degree of oxidation was further deepened. After 60 min, the concentration of Cl − and SO 4 2− in the solution was relatively stable and close to the theoretical value, while NO 3 − concentration was small. When the reaction proceeded to 120 min, Cl − , NO 3 − and SO 4 2− concentrations were 0.4 mmol/L, 0.039 mmol/L, 0.877 mmol/L ( Figure 13). The NO 3 − in the solution may theoretically come from the azo bond, amino group and triazinyl. Nevertheless, the azo bond decomposed in the form of N 2 , and the triazinyl group was stable and difficult to open ring, so the NO 3 − in the solution was mainly provided by the amino group [51,52].  According to the above experimental results, combined with the structure of X-3B and the previous research [52], a probable degradation pathway of X-3B in the UV/Mn 2+ -H2O2 system was proposed (Figure 14). The •OH generated during the photo-Fenton-like process attacked the X-3B molecules, increased the electron cloud density of the adjacent and para positions, and the electron cloud density of the N in −NH− also enlarged due to the lone pair electrons. Therefore, it is speculated that •OH would attack the ortho position of the −OH and N atom in −NH− to break the chromophore group, which is consistent with the result of the rapid decolorization of X-3B in the UV/Mn 2+ -H2O2 system. The o-hydroxybenzenediazonium salt released N2, and then was further oxidized to small molecular acids or aldehydes. At the same time, the sulfonic acid group on the naphthyl ring was broken to form SO4 2− , and the Cl atom on the triazine ring was replaced by −OH and entered the solution to form Cl − .  According to the above experimental results, combined with the structure of X-3B and the previous research [52], a probable degradation pathway of X-3B in the UV/Mn 2+ -H2O2 system was proposed (Figure 14). The •OH generated during the photo-Fenton-like process attacked the X-3B molecules, increased the electron cloud density of the adjacent and para positions, and the electron cloud density of the N in −NH− also enlarged due to the lone pair electrons. Therefore, it is speculated that •OH would attack the ortho position of the −OH and N atom in −NH− to break the chromophore group, which is consistent with the result of the rapid decolorization of X-3B in the UV/Mn 2+ -H2O2 system. The o-hydroxybenzenediazonium salt released N2, and then was further oxidized to small molecular acids or aldehydes. At the same time, the sulfonic acid group on the naphthyl ring was broken to form SO4 2− , and the Cl atom on the triazine ring was replaced by −OH and entered the solution to form Cl − . According to the above experimental results, combined with the structure of X-3B and the previous research [52], a probable degradation pathway of X-3B in the UV/Mn 2+ -H 2 O 2 system was proposed (Figure 14). The ·OH generated during the photo-Fenton-like process attacked the X-3B molecules, increased the electron cloud density of the adjacent and para positions, and the electron cloud density of the N in −NH− also enlarged due to the lone pair electrons. Therefore, it is speculated that ·OH would attack the ortho position of the −OH and N atom in −NH− to break the chromophore group, which is consistent with the result of the rapid decolorization of X-3B in the UV/Mn 2+ -H 2 O 2 system. The o-hydroxybenzenediazonium salt released N 2 , and then was further oxidized to small molecular acids or aldehydes. At the same time, the sulfonic acid group on the naphthyl ring was broken to form SO 4 2− , and the Cl atom on the triazine ring was replaced by −OH and entered the solution to form Cl − .  Figure 14. Probable degradation pathway of X-3B in the UV/Mn 2+ -H2O2 system.

Conclusions
The combination of the photocatalysis with the Mn 2+ -H2O2 Fenton-like process was proven to have a synergetic effect on enhancing the degradation of X-3B, which is 2.6 times that of the combined effect of Mn 2+ /H2O2 and UV/H2O2 alone. The Mn 2+ had no oxidizing ability, but acted as an active catalyst in the UV/Mn 2+ -H2O2 system, which enhanced the production of •OH from chem-catalytic and photocatalytic decomposition of H2O2, and it was also an indispensable part of the whole system. The •OH was the main contributor to decolorization of X-3B and mineralization of intermediate products to inorganic substances, whose function was to attack and destroy the chromophore of X-3B to rapidly decolorize. The •OH tended to attack and break the chromophore group, and therefore, the decolorization of X-3B was a relatively rapid process. In addition, various experimental conditions, such as Mn 2+ and H2O2 concentrations, the initial solution of pH and the X-3B initial dosage had significant influences on the degradation efficiency. The degradation of X-3B reached the optimum under the conditions of 0.59 mmol/L of the Mn 2+ , 10 mmol/L of the H2O2, pH = 6 and a high level of DO. The study opens up new ideas for the combined use of various technologies in the treatment of printing and dye wastewater, caters to the future development trend, and provides an important reference for the application conditions of this technology.

Conclusions
The combination of the photocatalysis with the Mn 2+ -H 2 O 2 Fenton-like process was proven to have a synergetic effect on enhancing the degradation of X-3B, which is 2.6 times that of the combined effect of Mn 2+ /H 2 O 2 and UV/H 2 O 2 alone. The Mn 2+ had no oxidizing ability, but acted as an active catalyst in the UV/Mn 2+ -H 2 O 2 system, which enhanced the production of ·OH from chem-catalytic and photocatalytic decomposition of H 2 O 2 , and it was also an indispensable part of the whole system. The ·OH was the main contributor to decolorization of X-3B and mineralization of intermediate products to inorganic substances, whose function was to attack and destroy the chromophore of X-3B to rapidly decolorize. The ·OH tended to attack and break the chromophore group, and therefore, the decolorization of X-3B was a relatively rapid process. In addition, various experimental conditions, such as Mn 2+ and H 2 O 2 concentrations, the initial solution of pH and the X-3B initial dosage had significant influences on the degradation efficiency. The degradation of X-3B reached the optimum under the conditions of 0.59 mmol/L of the Mn 2+ , 10 mmol/L of the H 2 O 2 , pH = 6 and a high level of DO. The study opens up new ideas for the combined use of various technologies in the treatment of printing and dye wastewater, caters to the future development trend, and provides an important reference for the application conditions of this technology.