Role of weak magnetic field for enhanced oxidation of orange G by magnetic Fenton

The role of weak magnetic field (WMF) on the degradation of a common textile azo-dye, orange G (OG), by magnetic Fenton system was investigated in detail. The results showed that the presence of WMF can provide better performance of the Fe3O4/H2O2 system for OG degradation. The optimized reaction conditions were contained at 1 mM Fe3O4 as Fe, 20 mT of magnetic field intensity, 20 mM H2O2, and initial pH of 3.0. The removal efficiency of OG by Fe3O4/H2O2 coupling with WMF increased largely from 56.3 to 82.3% compared with Fe3O4/H2O2 process. Both the electron paramagnetic resonance (EPR) analysis and the quenching effect of tert-butyl alcohol (TBA) confirmed that hydroxyl radical (•OH) was the primary reactive oxygen species in WMF-Fe3O4/H2O2 system. The improving effect of WMF was explained by the magnetoconvection theory. The presence of WMF could accelerate the corrosion rate of Fe3O4 and thus promoted the release of Fe(II), which led to the increased production of •OH and enhanced the degradation of OG. Moreover, it was surprising to observe that the WMF induced improvement in OG degradation by heterogeneous Fenton involving the iron sludge, namely FeOOH and Fe2O3, as catalysts. These results indicated that WMF could be utilized as an efficient and cost-effective strategy to improve the removal of organic pollutants by iron oxide–based Fenton process.


Introduction
Advanced oxidation processes (AOPs) have been intensively studied for the removal of refractory organic pollutants in water and wastewater treatment. Among various AOPs, the Fenton reaction which uses ferrous ions and hydrogen peroxide (H 2 O 2 ) for the formation of nonselective hydroxyl radicals (•OH) is an especially powerful method due to its ease of implementation and high catalytic efficiency. However, the large-scale application of homogeneous Fenton system is typically restricted by the low operation pH range (pH=2.5-3.5) and the generation of undesirable iron sludge (Brillas et al. 2009). To minimize negative effects of homogeneous Fenton process, various heterogeneous Fenton catalysts have been developed (Nidheesh 2015;Thomas et al. 2020).
Iron oxides, such as goethite (α-FeOOH), magnetite (Fe 3 O 4 ), and hematite (α-Fe 2 O 3 ), are often treated as heterogeneous Fenton catalysts because of their abundance in earth's crust, low cost, negligible toxicity, and environmentally benign (Thomas et al. 2020). Compared with hematite and goethite, magnetite (Fe 3 O 4 ) has gain much more attention with its outstanding properties. Fe 3 O 4 is usually represented by the formula (Fe(III)) tet [Fe(II)Fe(III)] oct O 4 where Fe(II) ions occupy octahedral sites and Fe(III) ions are equally in both octahedral and tetrahedral sites (Avetta et al. 2015). Due to its redox properties, Fe 3 O 4 can provide high catalytic activity in the oxidation processes. It has been applied as a heterogeneous Fenton catalyst for abatement of various organic pollutants, such as p-nitrophenol (Sun and Lemley 2011), aniline (Zhang et al. 2009), phenol (Hou et al. 2014), and polycyclic aromatic hydrocarbons (Usman et al. 2012). It is noteworthy that the catalytic efficiency of Fe 3 O 4 is still unsatisfactory due to the insufficient Fe(III)/Fe(II) cycle which needs further improvement. To date, researchers have developed numerous Responsible Editor: Ricardo Torres-Palma countermeasures, including introducing external energy (e.g., ultraviolet irradiation (Minella et al. 2014), ultrasound (Hou et al. 2016), microwave (Vieira et al. 2020), and electricity (Choe et al. 2021)), adding chelating or reducing agents (such as nitrilotriacetic acid , citrate (Xue et al. 2009b), and ascorbic acid (Sun et al. 2020)), and doping other metals (e.g., Cu (Jin et al. 2017), Mn (Zhong et al. 2014), and Ce (Xu and Wang 2012b)) into the magnetite structure, to promote the Fe(III)/Fe(II) cycle of iron oxide in heterogeneous Fenton reactions. Nonetheless, these methods always suffer from operational complexity, being costly, and ecological toxicity. Therefore, it is imperative to explore an efficient, lowcost, and environmentally friendly process to improve the removal efficiency of organic compounds by Fe 3 O 4 /H 2 O 2 .
Recently, researchers have reported that the irradiation of weak magnetic field (WMF) is an effective and economic strategy to enhance removal efficiency of contaminants by Fe 0 activated H 2 O 2 or persulfate (Xiong et al. 2014(Xiong et al. , 2015. In the combination of WMF with Fe 0 , the WMF gradient forces cause paramagnetic Fe(II) ions to move in highgradient field regions, leading to accelerate the dissolution rate of Fe 0 and thus accompany the leaching of Fe(II) from Fe 0 particles (Xiong et al. 2015). As a result, the generation of radicals in the solution is increased, which is conducive to enhance the abatement of contaminants. Because of the presence of Fe(II) in Fe 3 O 4 , the superimposed WMF on Fe 3 O 4 might also highly activate H 2 O 2 for treating the contaminants. To our best knowledge, no studies have been done so far to explore the influence of the WMF on Fe 3 O 4 /H 2 O 2 process.
Thus, the aim of this work was to investigate the feasibility and mechanism of heterogeneous Fenton process coupling with weak magnetic field (WMF-Fe 3 O 4 /H 2 O 2 ), which can also be named as magnetic Fenton. Orange G (OG), which is a common textile azo-dye, was chosen as a model organic compound. Then, influencing parameters, such as pH, H 2 O 2 and Fe 3 O 4 dosage, and magnetic field intensity, were studied, and reactive species was identified by electron paramagnetic resonance (EPR) spectroscopy and radical scavenger experiments. Additionally, various characterization technologies (e.g., XRD, BET, TEM, and XPS) and Fe leaching experiment were employed to further demonstrate the mechanism of WMF-Fe 3 O 4 /H 2 O 2 system. Finally, the suitability and effectiveness of heterogeneous Fenton process based on another two iron sources (FeOOH and Fe 2 O 3 ) as a catalyst was also evaluated.

Materials
Fe 3 O 4 was supplied by Nanjing Emperor Nano Material Co., Ltd. OG, sulfuric acid, sodium hydroxide, Fe 2 O 3 (average diameter 300 nm, 98%), tert-butyl alcohol (TBA), and H 2 O 2 (30%) were obtained from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China). These chemicals were of analytical grade and used without further purification. The goethite (FeOOH) catalyst was prepared with the method used by Lin et al. (2012). Methanol of HPLC grade was provided by Merck (Darmstadt, Germany). All solutions were prepared with Milli-Q water (18.2 MΩ cm).

Characterization
The phase identification of Fe 3 O 4 was performed by X-ray diffraction (XRD, ARL, X'TRA ) with a Bruker D8-Advance using Cu Kα radiation. The morphology and size distribution of Fe 3 O 4 were obtained from a transmission electron microscope (TEM) of H-7500 (Hitachi) operating at 80 kV accelerated voltage. The Brunauer-Emmett-Teller (BET) specific surface area was determined by nitrogen adsorption-desorption measurement on an ASAP 2020 instrument (Micromeritics) with analysis bath temperature 77 K. The metal oxidation states on Fe 3 O 4 surface before and after the Fenton reaction were recorded by the X-ray photoelectron spectroscopy (XPS, Thermal scientific, ESCALAB 250Xi) with monochromatic Al Kα X-ray radiation at 1486.71 eV, and the XPSPEAK 4.1 software was used for data analysis. The binding energy at 284.6 eV of C 1s peak was used to calibrate binding energy of all the spectra.

Experimental procedures
All experiments were conducted in a series of borosilicate glass jars continuously mixed by mechanical stirring and maintained at 25 ± 1°C by a thermostatic water bath. As shown in Fig. S1, two thin cylindrical neodymium-iron-boron permanent magnets were assembled under the reactor to provide magnetic field. The magnetic field intensity was measured with a Teslameter (HT201, Shanghai Hengtong Magnetic & Electric Technology Co., Ltd) at the bottom of the reactor. The tests were initiated by simultaneously adding Fe 3 O 4 and H 2 O 2 into 500-mL unbuffered reaction solution containing OG. The solution of initial pH was adjusted with sulfuric acid and sodium hydroxide. During treatment, the solution was mixing by a mechanical stirrer at 400 rpm to prevent the aggregation of Fe 3 O 4 particles. At the given intervals, water samples were withdrawn and quenched by methanol immediately, and the mixture was filtered through 0.22 = μm membranes to remove the suspended solids before OG analysis. All batch experiments were conducted in duplicates, and the average data were obtained as the mean of the two replicates.

Analytical methods
OG concentration was examined with an automatic scanning UV-Vis spectrophotometers (TU-1902, Purkinje) at 478 nm. Dissolved Fe ion concentration was analyzed by inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 5900). Electron paramagnetic resonance (EPR) experiments were explored on a Bruker A200 system with 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO) as a spin trapping agent. The mixture of DMPO and sample was mixed for 30s and then transferred to a glass tube, which was inserted into the cavity of EPR. The EPR instrument was operated in the following parameters: center field 353.5 mT, sweep width 7 mT, microwave frequency 9.85 GHz, microwave power 6.1 mW, a sweep time 81.92 s, modulation frequency 100 kHz, and modulation amplitude 0.05 mT.

Performance of WMF-Fe 3 O 4 /H 2 O 2 process
The degradation efficiency of OG along time under different experimental conditions was assessed as demonstrated in Figs. 1 and S2. It was observed that only 5.0% of OG was removed by only 20 mM H 2 O 2 within 360 min of reaction (Fig. S2a), attributed to the weak oxidation potential of H 2 O 2 . Less than 13% of OG removal was achieved in the control reactions with 1 mM Fe 3 O 4 (as Fe) alone, which was mainly expected to surface adsorption (Xue et al. 2009a). As seen in Fig.  1, the addition of Fe 3 O 4 to H 2 O 2 at pH 3.0 could remove 36.3% OG after 6 h of reaction, suggesting the high catalytic ability of Fe 3 O 4 to H 2 O 2 . Surprisingly, the introduction of WMF greatly enhanced the removal efficiency of OG to 82.3% under the same condition. As either the WMF-H 2 O 2 or the WMF-Fe 3 O 4 systems had negligible influence on OG degradation (Fig. S2b), the presence of WMF might promote the degradation of OG in the WMF-Fe 3 O 4 /H 2 O 2 system by enhancing the Fenton reaction. Besides, it is still easy to separate the Fe 3 O 4 by magnet after reaction. In order to optimize the WMF-Fe 3 O 4 /H 2 O 2 process, the influence of initial solution pH, magnetic field intensity, Fe 3 O 4 dosage, and H 2 O 2 concentration were examined systematically in the following sections.

Effect of initial solution pH
The pH value is one of the most important factors because it determines the route of the Fenton processes. In this study, the decomposition of OG was carried out at pH range from 2.0 to 3.5 in both Fe 3 O 4 /H 2 O 2 and WMF-Fe 3 O 4 /H 2 O 2 systems (Fig.  2a). The degradation efficiency of OG by WMF-Fe 3 O 4 /H 2 O 2 at pH 2.5, 3.0, and 3.5 was increased by 5.6%, 46%, and 7.8%, respectively. The enhancement was likely related to the fact that the amount of leaching of Fe ions from the Fe 3 O 4 particle under WMF was higher than that without WMF, which would be shown in the next section. Thus, the more •OH generated in homogeneous reaction can be ascribed to the enhancement of OG removal during the WMF-Fe 3 O 4 /H 2 O 2 process. It was also observed that the rates of OG removal drastically decreased with the pH increasing from 2.5 to 3.5 in both systems, suggesting OG degradation was closely pH-dependent. This phenomenon might be ascribed to the concentration of iron ions in the bulk which is relatively low as pH increases (Li et al. 2016). Moreover, when the initial pH decreased from 2.5 to 2.0, the presence of WMF had negligible effect on the degradation of OG by Fe 3 O 4 /H 2 O 2 process, and the performance of both systems was much less effective. It may be attributed to the generation of complex species [Fe(H 2 O) 6 ] 2+ at lower acid environment, which reacts more slowly with (Li et al. 2016). Additionally, the strong acidity could have a scavenging effect on both H 2 O 2 and •OH. Due to the remarkable enhancement of OG by the WMF-Fe 3 O 4 /H 2 O 2 system, the solution pH of 3.0 was chosen in the following studies.

Effect of the magnetic field intensity
As shown in Fig. 2b intensity increasing from 0 to 20 mT. According to the previous studies (Fan et al. 2019;Sun et al. 2017), two forces, including the magnetic field gradient force (F B ) and the Lorentz force (F L ), have been proposed to be responsible for the WMF effect. However, it was recently verified that the magnetic field gradient force is the major driving force for the enhancing effect of WMF ). The magnetic field gradient force (F B ) could drive paramagnetic ions (Fe (II)) along the magnetic field from far away to close to the Fe 3 O 4 surfaces. Then, electromagnetic forces and galvanic coupling could promote paramagnetic ion movement and the Fe 3 O 4 surface corrosion. Additionally, it was also reported that the additional convection induced by the F L in the presence of WMF could increase the mass transport of H + toward the Fe 3 O 4 particle surface, leading to the lower pH at the Fe 3 O 4 particle surface than that without WMF (Xiong et al. 2015). These physical processes were conducive to increase the dissolution of Fe ions from particles, responsible for enhancement of OG decomposition in WMF-Fe 3 O 4 /H 2 O 2 process. Besides, with increasing the WMF intensity, the influence of paramagnetic ions transport induced by the F B would be further strengthened. The dissolving rate of Fe(II) from Fe 3 O 4 surface can be accelerated, resulting in the more yield of •OH. However, further increasing the WMF intensity to 30 mT leads to a dropped OG removal, which should be resulted from the Fe 3 O 4 aggregation with MF intensity greater than 20 mT. The phenomenon about the inhibition of OG removal in the presence of excessive MF intensity was consistent with previous studies (Xiong et al. 2014(Xiong et al. , 2015. The aggregation might result in the decrease of active sites of Fe 3 O 4 , which could deteriorate the performance of catalyst for OG removal. Thus, 20 mT is the best for the intensity of WMF on the removal efficiency of OG in the Fe 3 O 4 /H 2 O 2 process. It was evident that the whole reaction of OG degradation by Fe 3 O 4 /H 2 O 2 process with or without WMF can be divided into three stages. The first stage lasts for 2 min, during which the surface adsorption of a little fraction of OG (7%) on Fe 3 O 4 is the dominant procedure. Subsequently, a lag period (second stage) and a followed rapid degradation stage (third stage) which displayed apparent first-order kinetic were observed (Fig. S3), which was also demonstrated by previous studies (Xu and Wang 2012a). As shown in Fig. S3, the kinetic constant (k, min -1 ) of the third stage was obviously higher than that of the second stage. The k of second stage slightly increased from 3 × 10 -4 to 7 × 10 -4 min -1 with the increase of MF intensity from 0 to 20 mT, while the k of third stage at MF intensity of 20 mT was 5.8 times higher than that without WMF. It was well known that in the Fe 3 O 4 /H 2 O 2 system, the homogeneous reaction dominated under acidic conditions, and the heterogeneous reaction only played the minor role (Sun et al. 2013). The significantly increased oxidation efficiency at the third stage may be attributed to the more dissolved fraction of Fe ions generated under WMF. The leaching of Fe ion during the Fe 3 O 4 /H 2 O 2 reaction superimposed WMF was specifically discussed in Section "Mechanism of WMF-Fe3O4/ H2O2 process."

Identification of reactive radicals
The formation of reactive oxygen species in the WMF-Fe 3 O 4 / H 2 O 2 system was firstly investigated with the addition of radical scavenger. TBA is a common •OH scavenger because of its high-rate constant reaction with •OH (k= 6.0×10 8 M -1 s -1 ) (Shi et al. 2019). As shown in Fig. 3a (Fig. 4), which was mainly attributed to the influence of the external magnetic field. Figure S4 shows that the XRD patterns of virgin  Fig. 5, peaks at the binding energies of 284, 530, and 711 eV in the wide-scan XPS spectrum of Fe 3 O 4 indexed to C 1s, O 1s, and Fe 2p, respectively. Peaks at 711 and 725 eV in the Fe 2p spectrum can be ascribed to Fe 2p 3/2 and Fe 2p 1/2 , respectively. Then, the Fe 2p 3/2 spectrum was deconvoluted into two peaks at 710.7 and 712.4 eV to further quantify the surface Fe(II) and Fe(III) fractions in the sample (Jia et al. 2018). For the Fe 3 O 4 sample treated by WMF-Fe 3 O 4 /H 2 O 2 system, the surface Fe(II)/Fe(III) ratio of the Fe 2p 3/2 peak was 0.22 (Table S1), which was much lower than that from virgin Fe 3 O 4 (1.00) and Fe 3 O 4 (0.76) used in Fe 3 O 4 /H 2 O 2 process. The reduced ratio of surface Fe(II)/ Fe(III) in Fe 3 O 4 sample after the Fenton reaction was mainly associated with the following reasons: (1) the release of Fe(II) ion from catalyst at acidic conditions and (2) the H 2 O 2 oxidation of surface Fe(II) to Fe(III), according to the classical Haber-Weiss mechanism (Eqs. (3)-(4)) (Hu et al. 2011). Previous studies reported that the introduction of WMF had negligible influence on Fe(III)/Fe(II) cycle either in the solution or on the surface during the Fenton reaction, but significantly accelerated the leaching of Fe(II) (Xiong et al. 2015). This could lead to the lower Fe(II) content in the catalyst after the WMF-Fe 3 O 4 /H 2 O 2 process.
We assumed that the significant enhancement in WMF-Fe 3 O 4 /H 2 O 2 system might be caused by the more dissolution of Fe ions into the solution. To clarify this issue, the total Fe ion concentration in solution was determined by ICP-OES during the degradation process (Fig. 6). As expected, the concentration of dissolved Fe ion in both Fe 3 O 4 /H 2 O 2 and WMF-Fe 3 O 4 /H 2 O 2 systems increased steadily. It should be noted that no Fe (II) was detected during Fe 3 O 4 /H 2 O 2 reaction with and without WMF, implying that Fe (II) can be promptly oxidized by H 2 O 2 once it was leached from Fe 3 O 4 . By comparison, the leaching of iron during OG degradation by WMF-Fe 3 O 4 /H 2 O 2 system was much faster than by Fe 3 O 4 /H 2 O 2 system. Thus, the superimposed WMF facilitated the dissolution of Fe 3 O 4 , resulting in the enhanced Fenton reaction, which performed an important function for OG removal. The concentration of dissolved iron in Fe 3 O 4 /H 2 O 2 and WMF-Fe 3 O 4 /H 2 O 2 systems after 660 min was detected at 3.0 μM and 4.2 μM, respectively, equivalently to only 0.30% and 0.42% of total iron of catalyst used. To evaluate the contribution of Fe ions to the decomposition of OG, the homogeneous Fenton experiments were taken with 4.2 μM and 3 μM Fe(II), respectively, based on the maximal amount of iron leached from the catalyst during the above experiments. As can be seen in Fig. 6, the whole removal of OG in homogeneous Fenton was higher than that in Fe 3 O 4 /H 2 O 2 and WMF/Fe 3 O 4 /H 2 O 2 processes, which indicated the catalytic contribution from the dissolved Fe(II) ions is significant. This result further supported that the homogeneous Fenton reaction in the WMF-Fe 3 O 4 /H 2 O 2 system is expected predominantly in OG degradation.
Based on the above experimental data, a possible mechanism of Fe 3 O 4 /H 2 O 2 coupling with WMF was proposed. The degradation of OG by WMF-Fe 3 O 4 /H 2 O 2 system may be mainly ascribed to the homogenous Fenton reaction in aqueous solution. Under acidic conditions, the dissolved Fe ion, including Fe(II) and Fe(III), from Fe 3 O 4 surface can activate H 2 O 2 to generate reactive oxygen species (•OH) in bulk solution. The generated •OH was primarily responsible for the degradation of OG due to their high oxidizing potential. The theory of magnetoconvection could well explain the enhancing effect of WMF in our study (Waskaas and Kharkats 1999). In the presence of WMF, the paramagnetic ions (Fe(II)) pulled by magnetic gradient force could move along magnetic lines to the place with higher MF flux intensity, leading to the uneven distribution of Fe(II) and eventually a gradient of Fe(II) concentration. Moreover, with the introduction of WMF, the additional driving force existing as the same direction of the gradient of the paramagnetic ions could act on the reaction mixture and then induce a redistribution of velocities in the diffusion layer . Consequently, WMF caused an additional convective transfer of all constituents of the mixture, that is, acceleration of releasing iron. Thus, the enhancing effect of WMF can be related to cause the more dissolved fraction of iron species in solution. Hence, more concentration of •OH is generated for oxidizing OG during the homogenous Fenton reaction.  Figure S6 depicts that the recycled Fe 3 O 4 still exhibited high activity for the removal of OG after five consecutive runs. About 64% OG was decomposed in the WMF-Fe 3 O 4 /H 2 O 2 process, and the enhanced effect of WMF was still obvious. The results demonstrate that the Fe 3 O 4 was durable in the WMF-Fe 3 O 4 /H 2 O 2 system and could be reused several times, which was of economic significance.

Wide suitability and environmental implication of magnetic Fenton process
It was surprising to observe that the WMF-induced enhancement in OG abatement by heterogeneous Fenton process was also adaptable for another two iron species as a catalyst, including FeOOH and Fe 2 O 3 , as demonstrated in Fig. 7. In comparison with the absence of WMF, the degradation efficiency of OG by WMF-Fe 2 O 3 /H 2 O 2 and WMF-FeOOH/ H 2 O 2 systems obviously increased by 16.5% and 26.3%, respectively, at the end of reactions. It has been reported that surface Fe(II) could be produced on the surface of iron oxide based on the surface complexation mechanism in the heterogeneous Fenton system (Hou et al. 2017), as described by Eqs.
(5)-(7). With the introduction of WMF, the dissolution rate of newly generated Fe(II) might be accelerated from Fe 2 O 3 or FeOOH surface to the bulk solution, resulting in the more yield of •OH. Both FeOOH and Fe 2 O 3 can be seen as the main components of industrial iron sludge. Previous studies have often reused the iron sludge as an iron source for Fenton process. The most widely used way is the addition of regenerated Fe (III) via acidification dissolution of iron sludge to the classic Fenton reactor. In our study, the superimposed WMF to promote removal of organic pollutants by heterogeneous Fenton process could be conducive to the recycling and reuse of iron sludge. This magnetic Fenton process can avoid the utilization of expensive corrosive substances which can increase risk to the environment and humans. Therefore, employing WMF to improve the reactivity of iron oxidebased Fenton process can provide a promising alternative for the abatement of contaminants in water and wastewater treatment. WMF impacted the mass transfer and thus promoted the releasing of Fe(II), which was conducive to produce more •OH in bulk solution. Moreover, the introduction of WMF was also suitable for enhancing catalytic reactivity of other two iron sources, such as FeOOH and Fe 2 O 3 , as a catalyst, which shed a light on the recycling and reuse of iron sludge in heterogeneous Fenton process.
Author contribution Material preparation and background analysis were performed by ZYS, RJZ, and JZ. The manuscript specific experimental design and operation and experimental data processing were performed by ZYS and RJZ. Data analysis was performed by ZYS, RJZ, and JZ. ZYS and JZ performed writing, review, and editing; JZ supervised the research.
Funding Data availability All data generated or analyzed during this study are included in this published article.

Declarations
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Competing interests The authors declare no competing interests.