Degradation of Acid Red B by Manganese doped iron oxychloride and Permonosulfate: Performance and Inhomogeneous Activation Mechanism

: manganese doped iron oxychloride (Mn-FeOCl) was synthesized by partial 10 pyrolysis method. The Mn-FeOCl was used as heterogeneous catalyst to activate permonosulfate 11 (PMS) for the degradation of azo dye acid red B(ARB) for the first time. The Mn-FeOCl was 12 characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), 13 transmission electron microscopy (TEM) and X-ray diffraction spectroscopy (XRD). The effects of 14 Mn-FeOCl dosage, PMS concentration, initial pH value, Cl ion concentration and humic acid (HA) 15 dosage on the degradation of ARB by Mn-FeOCl/PMS were investigated. Results showed that the 16 ARB was degraded effectively by Mn-FeOCl/PMS. The mineralization rate of ARB reached 42.5%. 17 As the Mn-FeOCl dosage was 0.1g/L, PMS concentration was 1mmol/L, and ARB concentration 18 was 0.05mmol/L, the degradation rate of ARB reached 99.4% in 30 minutes. With the increase of 19 PMS dosage, Mn-FeOCl dosage, Cl - ion concentration and initial pH value, the decolorization effect 20 of ARB increased. The reaction mechanism was analyzed by free radical quenching experiment and 21 XPS. The main active species were determined as ·OH and SO 4 · - which generated by PMS activation. The SO 4 · -- was the main active species.


Introduction 26
Azo dyes that contain one or more azo bonds ( -N=N -) in their molecular structure 27 (Robinson et al. 2001).It is widely used in various industries, including in textiles, food, leather, 28 cosmetics and pharmaceuticals, according to statistics, the total output of dyes in the world is as 29 high as 10000t, of which more than 15% are lost in the dyeing process (Ghanavatkar et al. 2021). 30 The discharge of azo wastewater decreases the transparency and destroys the ornamental value of 31 natural water. In addition, azo dyes have high chromaticity, high toxicity, complex composition and 32 are difficult to be degraded (Zou et al. 2020) Meanwhile, the discharge of this wastewater also 33 threatens ecosystem and human health because these compounds and their degradation products 34 exhibit ecotoxicity. Thus, the removal and degradation of azo dyes have long received attention.

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Advanced oxidation process (AOPs) is an efficient and inexpensive method for the treatment 36 of refractory organic wastewater. AOPs can produce a large number of active free radicals, mainly 37 hydroxyl radical (·OH) and sulfate radical (SO4· -). The Fenton reaction system based on ·OH has a 38 narrow pH range (pH=2.5-3.5), and produces iron sludge, resulting in secondary pollution.

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Compared with ·OH, SO4· -has a higher REDOX potential (2.5V-3.1V) (Hao et al. 2014). It has high 40 efficiency and oxidation selectivity for the degradation of pollutants containing unsaturated bonds. 41 Longer half-life and wider application range of pH (Tugba et  and transition metal (Malik et al. 2016) . Among them, transition metal activation cost is lower and 47 more efficient. The asymmetric molecular structure of PMS is more easily activated by metals and 48 their oxides than that of PDS (Ding et al. 2020). The commonly used transition metal catalysts 49 include cobalt, copper, manganese, iron and materials containing one or more transition metal 50 catalysts. Cobalt and copper as catalysts release toxic metal ions in the process of reaction, which 51 will also cause environmental pollution and limit their practical application. However, Fe-based 52 catalyst is a better choice because of its non-toxic and large storage capacity.

Preparation of Mn-FeOCl 84
The catalyst of Mn-FeOCl was prepared according to one-pot method. Firstly, a mixture of 1.0 85 g FeCl3·6H2O and 0.2 g MnCl2·4H2O was dissolved in 0.8 mL H2O in a porcelain crucible, and then 86 sonicated for 10 min. The crucible was subsequently placed in an oven at 50 °C for 10 hours to 87 achieve coagulation. Following coagulation, the dried sample was sealed with nitrogen and the 88 temperature was raised to 250°C at a rate of 1°C / min and held for 60 min in a muffle furnace. After 89 being naturally cooled to room temperature, the sample was ground to powder and washed with 90 acetone subsequently by centrifuge. The collected sample was placed in a vacuum drying box at 91 60 ℃ for 8 h to finally obtain Mn-FeOCl (Tan et al. 2021). 92

Degradation experiment 93
Accurately weigh a certain amount of Mn-FeOCl into 100mL of an acid red B(ARB) solution 94 of a specific concentration, and sonicate it for 5minutes to make it uniformly dispersed. Use 95 0.01mol/L H2SO4 and NaOH solutions to adjust pH. Then magnetically stir 30 min to reach 96 adsorption/desorption equilibrium. Adding PMS to start the reaction. Samples were taken at a 97 predetermined time and quenched with 0.20mol/L NaNO2 to stop the reaction. The absorbance of 98 the remaining dyes was measured after the samples were filtered with a 0.45μm filter head. Two 99 parallel reactions were set for each group. 100 The reaction solution was centrifuged, washed with pure water, repeated 3 times, and vacuum 101 dried to obtain use Mn-FeOCl. The above steps were repeated 3 times to check its repeatability and 102 stability. 103

Analytical method 104
The morphology of the materials was characterized by Quanta FEG 250 scanning electron 105 microscope (SEM-EDS). The crystal structure of the catalyst was analyzed by Bruker D8Adwance 106 X-ray diffraction (XRD) in Germany. Thermofisher X-ray photoelectron spectroscopy (XPS) was 107 used to detect the element valence states before and after the use of the catalyst.PHS-3C type pH 108 meter to measure the pH of the solution; The concentration of acid red B was quantified on a UV-109 vis spectrophotometer (Mapada UV 1600(PC)) by monitoring the absorbance at the maximum 110 wavelength of 515nm, Total organic carbon (TOC) measurement was carried out by a Shimazu TOC 111 analyzer (TOC-LCPH, Shimazu, Japan). The electron spin resonance (EPR) spectrometer (JEOL-112 FA200, Japan) was applied to measure SO4· -by using 5, 5-2-methyl-1-pyrroline-N-oxide (DMPO) 113 as the radical spin trapping reagent. The removal efficiency was measured according to (Eq. (1)) 114 Removal efficiency (%)=(C-C0)/C0×100% (1) Where C0 and C represent the initial and final concentrations of pollutants 115

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Characterization 117 Fig. 1 shows the SEM, TEM and EDS images of Mn-FeOCl. As displayed from the SEM 118 results in Fig. 1(a-c), the Mn-FeOCl catalyst has a rectangular parallelepiped configuration, which 119 belongs to a typical orthorhombic crystal system, and its obvious lamellar structure can be seen, 120 with a length of about 1-5μm. This is consistent with the conclusion obtained by Yang (Yang et al. 121 2016). Such a lamella structure effectively expands the specific surface area of the catalyst, which 122 is beneficial to the contact between the catalyst and the pollutants. On the other hand, it increases 123 the number of active sites of the catalyst and increases the output of free radicals. The TEM results 124 Fig. 1(d-f) furtherly showed the same lamella morphology, with tight bonding between layers, neat 125 edges, and a thickness of about 30nm, indicated the surface stripe morphology. To explore the 126 existence of Mn element, the corresponding EDS characterization was analyzed. Results indicated 127 that Mn, Fe, O and Cl were all present catalyst from EDS spectrum Fig. 1(g-j) and the mole ratio of 128 the four elements were determined to be 6.6%, 41.2%, 19.5% and 32.6%, respectively. Furthermore, 129 EDS mapping results Fig which is used as the reference peak for calibration. Fig. 3  shown in Fig.5(b). 188 With the increase of the catalyst dosage, the degradation rate of ARB is increasing. As the 189 catalyst dosage increases from 0.03g/L to 0.2g/L, after 30 minutes of reaction, the degradation rate 190 of ARB increases from 92.5 % to 99.4%. The first-order rate constant is positively correlated with 191 the dosage of the material. This is because the catalyst has an activation effect on PMS. As the 192 dosage increases, Mn-FeOCl provides More active sites are generated, thereby generating more 193 free radicals and improving the degradation efficiency of ARB . 194

Effect of PMS dosage on the degradation of ARB 195
The degradation of ABR in Mn-FeOCl/PMS system with different PMS doses was investigated 196 and shown in Fig. 5(c-d).

Effect of initial pH on the degradation of ARB 205
Generally speaking, the influence of the initial pH value is usually related to the zeta potential the degradation rate of ARB also continues to increase. As the pH rises from 5 to 9, the degradation 216 rate of ARB rises from 77.8% to 99.5%. Although the degradation effect of pH=7 and pH=9 is not 217 much different, the reaction rate is significantly faster as pH=9. This is because under alkaline 218 conditions, PMS will be activated by alkali, forming a metal-OH complex in the Mn-FeOCl material, 219 and at a higher pH can accelerate the production of SO4· -and accelerate the degradation of ARB

Effect of humic acid on the degradation of ARB 239
The actual production wastewater containing natural organic matter (NOM) will also affect the 240 reaction process. The addition of humic acid (HA) to the reaction system simulates the degradation 241 ability of the Mn-FeOCl/PMS system on the ARB in the actual water body. As the Fig.6(c-d) shown 242 that the dosage of HA is 10, 20, 50 mg/L, the degradation rate of ARB within 30 minutes is 95.2%, 243 90.8%, and 68.4%, respectively, which is inhibited. The analysis may be because HA consumes the 244 reaction system. Free radicals, thereby inhibiting the degradation of ARB (Latifoglu et al. 2003). 245 However, the content of NOM in natural waters is generally below 20mg/L, and its impact on the 246 Mn-FeOCl/PMS system is also relatively limited.

Radical identification 252
In order to investigate the reaction mechanism of ARB degradation by Mn-FeOCl/PMS system, 253 a free radical quenching experiment was carried out. According to reports, the transition metal 254 activated PMS mainly produces ·OH and SO4· -; methanol (MeOH) and tert-butanol (TBA) are 255 selected as a quencher to identify the main active species in the system (Buxton et al. 1988) . MeOH 256 has higher reaction rates for ·OH and SO4· -, respectively (1.2~2.8) ×10 9 mol -1 ·s -1 and (1.6~7.7) ×10 7 257 mol -1 ·s -1 , which can be used for Quenching ·OH and SO4· -; while the reaction rate of TBA to SO4· -258 is only (4.0~9.1) ×10 5 mol -1 ·s -1 , which is much lower than that with ·OH (3.8~7.6) ×10 8 mol -1 ·s -1 , 259 so it is often used to quench the identification of ·OH in the reaction. Fig. 7(a) shows the effect of 260 two different quenchers on the degradation of ARB. As the dosage of TBA is 50 and 500mmol/L, 261 the removal efficiency of ARB drops to 84.15% and 75.78% after 30 minutes of reaction. The results 262 show that ·OH has a certain effect on the degradation of ARB. As the dosage of MeOH was 50 and 263 500mmol/L, the removal rate of ARB after 30 minutes of reaction was 66.8% and 54.8%. The 264 inhibitory effect is more obvious, which indicates that both ·OH and SO4· -exist in the Mn-265 FeOCl/PMS system, and DMPO is used as a free radical trapping agent, and the electron spin 266 resonance paramagnetic spectrometer (EPR) is used for the reaction process. As shown in Fig. 7(b), 267 the characteristic signals of DMPO-SO4· -adduct and DMPO-·OH adduct were detected (Wang et 268 al. 2015), where SO4· -is the main free radical and the detection showed that a small amount of ·OH 269 was present in the system at the same time, which was due to the reaction of SO4· -with OHin the 270 solution to generate ·OH (Eq. (10)). EPR shows that SO4· -dominates in the Mn-FeOCl/PMS system.

Possible mechanism 276
In order to understand the changes of surface elements during the reaction, XPS was used to 277 detect the element valences in Mn-FeOCl before and after the reaction. The results are shown in Fig.  278 8 and Fe 3+ are 56.6% and 43.4% respectively. During the reaction, the Fe 3+ content increased by 13.6%, 283 which is consistent with the previous study by previous researchers. (Chen et al. 2013;Qu et al.2020).

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The significant change in the valence state indicates the existence of electron transfer during the 285 reaction. In addition, the spectrum of Mn 2p as shown in Fig.8(b) shows different peak shapes, 286 which may be due to the low load and high background value (Ma et al. 2019). It can be observed 287 that the peak value of Mn 2p is significantly reduced after the reaction, and it can be seen that Mn 288 is also involved in the reaction. On the basis of free radical identification and XPS analysis, a possible catalytic mechanism in 294 the Mn-FeOCl/PMS system is proposed Fig. 9: Fe 2+ /Fe 3+ and Mn 2+ /Mn 3+ and other redox pairs 295 distributed on the catalyst surface are in contact with PMS, and a variety of reactions occur (Eq.

Reusability of Mn-FeOCl 305
In actual application, the reusability of the material is an important indicator to measure the 306 performance of the catalyst. In order to evaluate the reusability of the Mn-FeOCl, the reacted 307 material is centrifuged and cleaned and dried with deionized water and absolute ethanol and then 308 used again. The ratio of reused catalyst after each reaction was presented in Fig. 10

The mineralization ability of Mn-FeOCl/PMS 321
In order to further study the mineralization effect and reaction process of Mn-FeOCl/PMS 322 system on ARB, UV-vis spectrum scanning and TOC test were carried out. Fig.11(a) shows the 323 spectrum change process during the degradation of ARB. It can be seen that ARB has two main 324 characteristic peaks at 310nm and 515nm, corresponding to the naphthalene ring and the even single 325 bond chromophore (Yang et al. 2004, Lan et al. 2015. With the progress of the reaction, the 326 characteristic peak at 515nm continued to decrease, indicating that the Mn-FeOCl/PMS system can 327 oxidize the chromophoric groups in the ARB and has a good decolorization effect. With the 328 continuous extension of the reaction, the characteristic peak of the naphthalene ring at 310nm also 329 continued to decrease, indicating that the reaction can further oxidize the intermediate products 330 produced by the degradation of ARB, and has a certain mineralization ability. 331 Fig. 11(b) shows the ability of the system to mineralize ARB.

Conclusion 344
The Mn-FeOCl material was successfully prepared by partial pyrolysis, which can effectively 345 activate PMS to degrade the azo dye ARB, and has good decolorization effect and mineralization 346 ability. In the Mn-FeOCl/PMS system, the degradation efficiency of ARB increases with the 347 increase of the dosage of Mn-FeOCl, the dosage of PMS, the initial pH and the concentration of Cl -; 348 HA versus Mn-FeOCl/PMS system versus ARB Degradation has an inhibitory effect. The Mn-349 FeOCl/PMS system has a better degradation effect on ARB under neutral and alkaline conditions. 350 Through XPS analysis, the degradation mechanism is inferred. The reaction system produces 351 both ·OH and SO4· -, of which SO4· -is dominant. her constant encouragement and guidance. I am indebted to professor Yan-mao Dong owing to his 355 guidance in this manuscript. And I also thank other authers for their contributions to the manuscript.

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Availability of data and materials The datasets used and/or analyzed during the current study 358 are available from the corresponding author on reasonable request.

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Author contribution All authors contributed to the study conception and design. Material 361 preparation, data collection, and analysis were performed by Rong Chen and Chengrun Cai. The 362 first draft of the manuscript was written by Rong Chen and checked by Yan Yuan and Yanmao Dong. 363 the logic and grammar of the manuscript were examined by Dan Zhao and Zhili Li. All authors 364 commented on previous versions of the manuscript. All authors read and approved the final 365 manuscript.

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Funding This work was supported by Suzhou Regional Water Quality Improvement and Water 367 Ecological Security Technology and Comprehensive Demonstration Project (2017ZX07205) and 368 Suzhou Industrialization Prospect Project (SYG201744). 369

Declarations 370
Ethics approval and consent to participate Not applicable.

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Consent for publication Not applicable.

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Competing interests The authors declare no competing interests. 375