Oxidative degradation of p-chlorophenol by the persulfate-doped Fe–Mn bimetallic hydroxide, the parametrical significance, and systematical optimization

Aqueous Fe(II)-catalyzed activation commonly deteriorates the oxidation performance of persulfate (PS) to the treatment of organic contaminants. In this study, a PS-doped layered bimetallic hydroxide (Fe–Mn hydroxide) was synthesized to construct a heterogeneously catalytic system to solve the issue brought by homogeneity. The molar ratio of Fe(II) to Mn(II) and the mass ratio of PS to Fe–Mn hydroxide both had a significant impact on the catalytic degradation of p-CP. Reaction temperatures engaged in the most essential role in influencing the degradation and removal of p-chlorophenol (p-CP). The optimal combination of factors for the preparation of PS-hydroxide and the treatment of p-CP was finally determined by significance analysis. The degradation process was appropriately fitted by the pseudo-first-order kinetic model. The benzene ring in p-CP was broken by PS-hydroxide during the adsorption. The surface modification of PS-hydroxide caused by the valence transition of Mn was beneficial to the adsorption and catalytic degradation of p-CP.


Introduction
Chlorinated phenols (Cl-phenols) are extensively used in some chemical and pharmaceutical-related industries, which are commonly considered persistent organic pollutants (POPs) in the treatment of wastewater (Mikkonen et al. 2018;Tubic et al. 2019). As a vital member of Cl-phenols, p-chlorophenol (p-CP) can cause severe ecological harm to the surrounding environment and humans, such as carcinogenicity, acute toxicity, and recalcitrance (Pirzada et al. 2019;Sharma and Khare 2018). Therefore, it is essential to explore some effective technologies for the rapid removal of p-CP to avoid further contamination. Until now, some techniques have been proposed and applied on the remediation of wastewater containing p-CP, such as extraction, adsorption, membrane separation, electrochemical oxidation, zero-valent iron (ZVI) reduction, and biological degradation (Bao et al. 2019;Kuntail et al. 2019;Wei et al. 2019). However, some shortcomings such as the large dosage of chemicals, low removal efficiency, a long-lasting disposal cycle, and the secondary pollution drawn by the secondary organic species have perplexed and dwarfed the wide application of these traditional technologies (Ababneh et al. 2019;Kim et al. 2019).
In recent years, the advanced-oxidation methods (AOP) have attracted more attention particularly in treating nonbiodegradable POPs in the wastewater due to their operational convenience and effectiveness (El Fakir et al. 2019;Lv et al. 2020;Xiao et al. 2020a). Among the oxidants, the utilization of hydroxyl radical (•OH) for the oxidation degradation of some organic compounds is most mentioned in various treatment processes (Laysandra et al. 2019;Shokoohi et al. 2019;Xiao et al. 2020b). •OH has a higher potential compared with other common oxidants. Furthermore, •OH can be easily produced by several technologies such as Fenton, ozonation, photocatalysis, electrocatalysis, and photo-electrocatalysis (Das et al. 2019;Li et al. 2020;Wang et al. 2020;Zhou et al. 2019). Nevertheless, the redox potential of •OH is changeable in different pH environments (e.g., +2.7 V in acidity and +1.8 V in neutrality), which explicitly weakened its performance to some extent. Comparably, due to the distinctive solubility and operativity in the wide pH range, the sulfate radical (SO 4 •− , E 0 = 2.60 V)-based heterogeneous oxidation has attracted more favor in degrading POPs in different pH environments (Guerra-Rodriguez et al. 2018;Huang and Zhang 2019;Liu et al. 2018).
SO 4 •− can be generated by activating persulfate (PS) under ambient or heating conditions (Crincoli et al. 2020;Xu et al. 2020). The activation of PS is commonly conducted by some methods, such as heating, microwave, transition-metal catalysis, UV radiation, and electrolysis. Generally, among these pathways, the transition-metal catalysis with the participation of aqueous Fe 2+ is preferably adopted for the activation of PS in the application due to its lower temperature request and running cost  To solve the abovementioned issues, the layered bimetallic Fe-Mn hydroxide was synthesized as the heterogeneous activator of PS in this study. PS was doped to the bimetallic hydroxide using a mechanochemical method to increase the activation activities. To directly characterize the influence of the preparation parameters and the aqueous conditions on the oxidation performance of the PS-doped bimetallic hydroxide (PS-hydroxide), two orthogonal designs with seven factors in total were arranged and quantitatively analyzed based on the removal amounts of p-CP. The synthesis and treatment processes were optimized by the significance analysis and the marginal results. To specify the degradation mechanisms of p-CP by PS-hydroxide, FTIR, SEM, and EDS (i.e., energy dispersive X-ray spectroscopy) were employed in combination for the characterization analysis of the solid and aqueous samples obtained from different conditions. This study solves the performance issue of PS drawn by the aqueous Fe(II)activated drawback for the degradation of p-CP and clarifies the role of Mn(II) in the iron-dominated catalysis process.

Preparation of the PS-doped Fe(II)-Mn(II) hydroxide
All the glass containers and gadgets have been rinsed three times using DI water and dried before any use. The whole preparation process was conducted in a vacuum glove box to avoid the oxidation of Fe(II) and Mn(II) by O 2 . The Fe-Mn bimetallic hydroxide was synthesized by the coprecipitation of the dissolved Fe(II) and Mn(II) species before the synthesis of the PS-doped Fe-Mn oxidative inducer. FeSO 4 ·7H 2 O and MnCl 2 ·4H 2 O were weighted at the molar ratios of Fe(II) to Mn(II) of 1, 3, and 5, respectively, and both dissolved in DI water. NaOH was weighted at some molar ratios of OH − and Fe(II) and dissolved to obtain an alkaline solution. The dark-green flocculated precipitation was immediately generated once the alkaline solution was poured into the water containing Fe(II) and Mn(II) cations. Afterward, the mixture was mechanically stirred to make full precipitation, sequentially centrifuged using a centrifuge (KH19A, KAIDA, China) at 8000 rpm, and dried at 75°C by a vacuum drying oven (ZK-50A, Biao Cheng Share Laboratory Co. Ltd, China). Finally, the PS-doped Fe-Mn hydroxide (PShydroxide) was prepared by mixing Fe-Mn hydroxide with PS at some mass ratios (i.e., the mass ratios of PS to Fe-Mn hydroxide of 1-5) and mechanically milling at 120 rpm over 2 h. The PS-hydroxide samples were stored in the glovebox for further employment.

Experimental procedure
A certain amount of oxidative reducer (the PS-hydroxide) was weighted and added to the p-CP stock solution in the Erlenmeyer flask (250 mL). A water-bath shaker (COS-100B, Shanghai BILON INSTRUMENTS Co. LTD, China) was employed to control the shaking time (6 h) and the temperature of the solution. The influence of the preparation parameters (including the molar ratio of Fe(II) to Mn(II) (Fe/Mn ratio), the mass ratio of PS to Fe-Mn hydroxide (PS/Fe-Mn ratio), and the molar ratio of OH − to Fe(II) (OH − /Fe ratio)) and the aqueous conditions (including the initial pH, concentration, PS-hydroxide, and temperature) on the degradation and removal of p-CP were comprehensively analyzed in batch experiments. Two orthogonal designs with different factors and levels were constructed, respectively, to optimize the synthesis of the PS-hydroxide and effectively arrange the degradation experiments affected by the changes in the preparation parameters and the aqueous conditions. Variance analysis (ANOVA) was used to obtain marginal means and quantitively evaluate the significance of each factor. The factors and levels with specific labels are correspondingly listed in Table S1 and Table S2 in the Supporting information (SI). The orthogonal designs with the p-CP removal results are shown in Table 1 and Table 2, respectively. To testify the removal kinetics of p-CP using PS-hydroxide, a pseudofirst-order kinetic model was adopted to describe the removal process following Eq. (1), where c 0 and c t are the concentrations of p-CP in solutions at the initial and t times, respectively, and k is the constant reaction rate bounded to the physicochemical characteristics of PS-hydroxide (h −1 ).
Calculations and analysis Liquid samples taken off at a specific time were centrifuged at 5000 rpm and filtered through a membrane filter (0.22 μm) before being subjected to high-performance liquid chromatography (HPLC, ULTIMATE-3000, USA, equipped with a UV-vis detector at 280 nm and a C18 column) for the determination of p-CP concentration. The measurement process was conducted in duplication to minimize the manual deviations. The samples of the P-CP solution and the PS-hydroxide Table 1 Orthogonal design with the removal amount of p-CP for the preparation parameters (Experimental conditions: pH of 7, the p-CP concentration of 6 mM, the PS-hydroxide dosage of 1.5 g/L, and the temperature of 50°C) No. Three factors with three levels Removal amount (mM)  before and after the degradation process were characterized by a Fourier transform infrared spectrometer (FTIR, Thermo Electron Co.-380 FTIR, USA) over a range of 400-4000 cm −1 at every step of 4 cm −1 and a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS, Carl Zeiss AG, Germany). The KBr particles were meticulously grounded to power, molded, and pressured to a small circular transparent sheet. The P-CP solution was spread evenly on the sheet, dried under a high-power incandescent lamp, and installed at a hollow bracket before the FTIR measurement.

Results and discussion
Characterization of Fe-Mn hydroxide and PShydroxide FTIR spectra of Fe-Mn hydroxide affected by Fe/Mn ratio and the PS-hydroxide influenced by both Fe/Mn ratio and PS/Fe-Mn ratio are shown in Fig. 1. The morphologies and EDS distribution of samples are correspondingly displayed in Fig. 2 and Fig. S1 in the SI, respectively. The peaks at the wavenumbers (cm −1 ) of 3420, 1626, 1384, 1121, 983, 612, and 465 were matched, respectively, at the Fe/Mn ratio of 1 for the Fe-Mn hydroxide (Fig.  1a). However, the peak at 983 cm −1 disappeared in terms of the spectra at the Fe/Mn ratio of 3 and 5. Moreover, the peak at 465 cm −1 was shifted to 494 cm −1 for the Fe/Mn ratio of 3 and a brand peak at 571 cm −1 was newly formed for the Fe/Mn ratio of 5. The partial oxidation of Fe(II) and the substitution of Fe into the Mn-related hydroxide potentially analyzed the changes in the FTIR spectra of Fe-Mn hydroxide with the increase of Fe/Mn ratios Lin et al. 2019). Comparably, more peaks were observed in the spectra of PS-hydroxide although the changes in the Fe/Mn ratios had an unobvious effect on the change of the spectral pattern (Fig. 1b). The results confirmed that the PS had been mechanochemically doped into the Fe-Mn hydroxide, which covered the change of spectra caused by Fe/ Mn ratios (Cao et al. 2017). Furthermore, the peaks within the bands of 592-690 cm −1 and 1060-1274 cm −1 gradually disappeared with the increase of the PS/Fe-Mn ratio (Fig. 1c), which reconfirmed the PS loading on the Fe-Mn hydroxide. The layered precipitation was observed in the SEM image of Fe-Mn hydroxide (Fig. 2a). The morphologies of the Fe-Mn hydroxide confirmed that the increase of Fe in the hydroxide facilitated the formation of the layered bimetallic hydroxide (Fig. 2a). The Fe-Mn hydroxide was tightly wrapped by the amorphous (also flocculated) PS in the PS-hydroxide (Fig. 2b). The flocculated morphology of PS was different from its originally smooth surface, which directly indicated the partial catalysis of PS to sulfate radicals (Tan et al. 2021 (Fig. S1, SI). Generally, the mass contents of Fe, Mn, and S semiquantitatively reflected the reliability of Fe/Mn ratios and PS/Fe-Mn ratios.

Effect of preparation parameters and aqueous conditions
The main effect of the preparation parameters on p-CP removal (default α = 0.05) is shown in Table S3 in the SI. The quantitative results analyzed by ANONA were acceptable due to the R 2 value (R: regression coefficient) which was close to 1. The significance coefficients (Sig.) were 0.040, 0.038, and 0.318 for factor A (i.e., Fe/Mn ratio), B (i.e., PS/Fe-Mn ratio), and C (i.e., OH − /Fe ratio), respectively. The preparation parameters including A and B factors both had a significant effect on the removal of p-CP using PS-hydroxide. Contrarily, the influence of the OH − /Fe ratio on the removal of p-CP was unobvious. The significance order of the preparation parameters followed the descending pattern as Factor B > Factor A > Factor C. The parameter of PS/Fe-Mn ratio played a most important role among the factors in strengthening the performance of PS-hydroxide toward p-CP removal. The different morphologies of PS-hydroxide ( Fig. 2a and b) caused by the Fe/Mn ratio and PS/Fe-Mn ratio explicitly impacted the reactions between PS-hydroxide and p-CP to some extent. The main effect of the aqueous conditions on p-CP removal (default α = 0.05) is shown in Table S4 in the SI. The R 2 value of ANONA analysis was 0.822, indicating that the quantitative results listed in Table S4 were moderately acceptable. The Sig. values were 0.457, 0.364, 0.799, and 0.012 for the factor D (i.e., Initial pH), E (i.e., P-CP concentration), F (i.e., PShydroxide dosage), and G (i.e., Temperature), respectively. Only Sig. value of factor F among the aqueous conditions was lower than the default α of 0.05, meaning only the temperature of the solution significantly affected the removal of p-CP achieved by PS-hydroxide. The impact of the other three factors on the removal results was insignificant while nonnegligible. Similarly, the significant order of the aqueous conditions followed the descending rank as Factor G > Factor E > Factor D > Factor F. Obviously, the variable of temperature engaged in the most essential role in alternating the degradation process of p-CP (Chao et al. 2018).

Optimization of p-CP removal
Marginal means of the preparation parameters and aqueous conditions in the orthogonal design (default α = 0.05) are shown in Fig. 3. Although, the maximum removal amounts (mM) of 5.37 and 8.12 were obtained in the parametrical combination of A 2 B 1 C 3 (i.e., Fe/Mn ratio of 3, PS/Fe-Mn ratio of 1, and OH − /Fe ratio of 6) and D 5 E 5 F 3 G 5 (i.e., Initial pH of 11, the p-CP concentration of 10 mM, Dosage of 1.5 g/L, and Temperature of 70°C) in the two orthogonal designs, respectively (Table 1 and Table 2). The results conflicted with the significance-analyzed ones (Table S3 and Table S4, SI), which cannot be used to represent the data in the full experiments with 3 3 plus 4 5 runs. The marginal means of factors combining with the significance analysis should be employed for the determination of systematical optimization. As seen in Fig. 3, for the factor of Fe/Mn ratio, the level (i.e., 1, 3, and 5) of 3 (A 2 ) was most conducive to the removal peak of p-CP. Similarly, the factors with the specific levels including B 1 , C 1 , D 5 , E 4 , F 3 , and G 5 were assured as the most preferred values for the optimal removal means. Furthermore, the effect significances of all factors were ranked in the whole following G > B > A > C > E > D > F under the significance results (Table S3 and Table S4, SI). It was clear that Dosage, Initial pH, and p-CP concentration were the three least significant factors. Therefore, the medium levels were preferred to maintain cost-effectiveness to some extent. The high temperature in the solution (e.g., 60 and 70°C), the low PS/Fe-Mn ratios (e.g., 1), and the medium Fe/Mn ratio (e.g., 3) were selected in favor of the significant importance of the three factors. As a result of the analysis, the optimal combination of factors for the preparation of PS-hydroxide and the treatment of p-CP was finalized for A 2 B 1 C 2 D 3 E 3 F 3 G 5 .

Kinetics description for p-CP removal
Removal amounts of p-CP affected by the preparation parameters and the aqueous conditions over time (360 min) are shown in Fig. 4. The other six factors were maintained as the same as those in the optimal combination of A 2 B 1 C 2 D 3 E 3 F 3 G 5 when any one single factor was changed over levels and time to evaluate its influence on the removal amounts of p-CP by PS-hydroxide. For the factor of Fe/Mn ratio, an increasing curve with a slower rate before a fast one over the whole running time was observed at all three levels.
The PS/Fe-Mn ratio performed a similar trend in comparison with the Fe/Mn ratio. However, the OH − /Fe ratio influenced the removal curves differently with a nearly constant rate obtained in the middle and later stages of experiments. However, the removal process of p-CP was intensely changed by the change of the initial pH in the middle-later phase (i.e., from 210 until to 360 min). A high p-CP concentration (e.g., 6, 8, and 10 mM) enhanced the removal rate of p-CP for the PS-hydroxide generally despite its inconspicuous significance analysis (Table S4, SI). The increase in the dosage of PShydroxide based on the medium value (i.e., 2.0 and 2.5 g/L) had not supported the continuing elevation of p-CP removal although the reaction rate was strengthened in the initial and middle stages of the experiment (i.e., before 240 min). Notably, the temperature factor generated the largest removal difference in the level range among the seven factors. The significance of temperature was visually confirmed by the different Fig. 6 FTIR of p-CP before and after the degradation process (a) and the PS-hydroxide after the degradation process (b) removal curves over time. The removal data from the each-factorchanged experiments were adopted to conduct the kinetic analysis. The linear fit of the pseudo-first-order kinetic model affected by the preparation parameters and aqueous conditions is shown in Fig. 5. The parametrical statistics for the kinetics fitting are correspondingly listed in Table S5 in the SI. As seen, most of the adjusted R 2 values (i.e., Adj. R 2 ) were larger than the default value of 0.95. Overall, the change of ln(c t /c 0 ) vs time can be appropriately fitted by the pseudo-first-order kinetic model. The suitableness of the model directly clarified that the removal of p-CP mostly endured a heterogeneous mechanism containing both physical and chemical pathways (Bao et al. 2019;Pirzada et al. 2019;Wan and Li 2017). Most p-CP was adsorbed onto the surface of hydroxide before the subsequent immediate oxidation by SO 4 •− with only a minor moiety was degraded through a homogeneous process (Brienza and Katsoyiannis 2017).

Mechanism exploration for p-CP degradation
FTIR spectra of p-CP before and after the degradation process and the PS-hydroxide after the degradation process are shown in Fig. 6. As seen, fewer peaks were matched in the spectra of the degraded p-CP compared with the raw sample (Fig. 6a). The increase in the concentration of p-CP would increase the number of peaks. To analyze the changes in the molecular structure of p-CP before and after experiments, the FTIR spectrum of benzene was analyzed and shown in Fig. S2 in the SI. The standard peaks of benzene were observed at 3447,2429,1694,1384,1328,1296,936, and 709 cm −1 , respectively. Except for the peaks at 3447 and 1384 cm −1 , the spectra of the degraded p-CP were different from benzene. The benzene ring in p-CP had been broken by PS-hydroxide during the test. The peak at 3447 cm −1 is commonly associated with the O-H band in the water molecular. The peaks at 1634, 1429, and 1384 cm −1 represent C=O, C-C, and C=C bonds, respectively (He et al. 2015;Huang et al. 2017;Taherian et al. 2013). The results indicated that p-CP had experienced an incomplete mineralization process potentially with some by-products being produced. Similarly, fewer peaks were matched in the spectra of PS-hydroxide after the degradation (Fig. 6b) in comparison with the raw counterpart (Fig. 1b, c). The peaks at 1297, 1060, 690, 592, and 559 cm −1 had disappeared after the degradation, which reflected the activating decomposition of PS and the changes of bimetallic hydroxide. Contrarily, two peaks at 1430 and 1481 cm −1 were newly formed, which was attributed to the remaining bending vibration of C-H. The disappearance and contrary formation of peaks in the spectra of PS-hydroxide after the degradation embodied the decomposition of PS and the oxidative influence of PS-hydroxide to the p-CP. The morphology of the PS-hydroxide after the degradation process is shown in Fig. 7. Some different morphological characteristics were observed for the used PShydroxide sample. A complicated physicochemical mechanism foreseeably participated in the degradation process. PS is activated into SO •− 4 radical mainly by heat, UV (also lightdriven) radiation, transition metal ions, and electrolysis (Crincoli et al. 2020;Forouzesh et al. 2019). Generally, the reaction mechanism can be substantially surmised based on the above analysis and results. PS doped into the Fe-Mn hydroxide was dissolved into the solid-liquid interface once PShydroxide was added to the solution (Fig. 7), which thereupon was activated by ferrous ions to SO •− 4 (Eq. 2) that dominantly Fig. 7 Morphological characteristics of the PS-hydroxide after the degradation process participated in the de-chlorination, loop disintegration, carbon-chain breaking, and mineralization processes (Table S3, Fig. 4, and Fig. 6) (Nguyen and Juang 2015;Taherian et al. 2013). More pores, cracks, and flocculation on PS-hydroxide (Fig. 7) compared to the hydroxide potentially reflected the oxidation of divalent Mn ) and the activation of PS involving Fe(II) ions (Eq. 4, Fig. 5 and Table S5, SI) . Considering the removal of p-CP by PS-hydroxide mainly was a heterogeneous process (Fig. 5 and Table S5), the surface modification of the PS-hydroxide was caused by the valence transition of Mn was beneficial to the adsorption of p-CP and subsequent removal or degradation. However, too high or too low Fe/Mn ratios (i.e., A 1 and A 3 ) were both unacceptable during the preparation of PS-hydroxide (Table S1 and Table 1). A high Fe/Mn ratio inhibited the modification drawn by Mn while a low Fe/Mn ratio reduced the generation of SO •− 4 (Eq. 2). Heating or increasing the temperature of solution clearly enhanced the production of SO •− 4 and further increased the degradation of p-CP (Eq. 3, Table S4, and Fig. 4), which supported the setting of both PS/Fe-Mn ratio and aqueous temperature (Table 1 and Table 2).
Conclusion PS was mechanochemically doped into the layered Fe-Mn hydroxide to solve the homogenous issue for the degradation of p-CP, which caused the amorphous and flocculation in the PS-hydroxide. The impact of the Fe/Mn ratio, the PS/Fe-Mn ratio, and the temperature on the removal of p-CP was significant among all factors. The quantitative influence of factors on the removal of p-CP ranked as G > B > A > C > E > D > F. The maximum removal amounts (mM) of 5.37 and 8.12 were obtained in the parametrical combination of A 2 B 1 C 3 and D 5 E 5 F 3 G 5 in the two orthogonal designs, respectively. The optimal combination of A 2 B 1 C 2 D 3 E 3 F 3 G 5 was determined for the preparation of PS-hydroxide and the treatment of p-CP based on the significance analysis and the marginal means of the experimental results. The linear fit of the pseudo-firstorder kinetic model was confirmed for the preparation parameters and aqueous conditions, which demonstrated that most p-CP was adsorbed onto the surface of hydroxide and then experienced the immediate oxidation by SO 4 •− . The spectra of the degraded p-CP were different from the standard peaks of benzene. The benzene ring in p-CP had been broken by PShydroxide during the removal process. Suitable surface modification for the PS-hydroxide caused by the valence transition of Mn was beneficial to the adsorption of p-CP and the subsequent degradation. Data availability There are no datasets generated and/or analyzed during the current study.

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Competing interests
The authors declare no competing interests.