Magnetic Intensification of Fenton Processes Using Superconducting Technology for Enhanced Treatment of Printing and Dyeing Wastewater: Mechanisms and Applications

Abstract

The rapid industrial development in recent years has led to severe pollution of aquatic environments. It is necessary to develop green and highly efficient treatment technologies for addressing environmental pollution and realizing carbon peaking and carbon neutrality goals. This study aims to explore the effect of magnetic fields on chemical oxygen demand (COD) degradation by Fenton reaction. The experimental results demonstrated the following: (1) Magnetic fields convert macromolecular organic compounds into low-molecular-weight organic compounds, promoting the attack of radicals on organic pollutants. (2) The magnetic Fenton process achieved COD removal efficiency of 60.0%. (magnetic field intensity: 1.5 T, magnetization duration: 5 min, pH = 5.0, Fe²⁺ = 2.0 mmol/L, H₂O₂ = 2.0 mmol/L, reaction time: 40 min). (3) The magnetic Fenton process consumes less acidic reagent. Notably, it achieves a 33.3% reduction in both catalyst and oxidant usage under the same COD removal efficiency. This study verifies the feasibility of applying the method in real sewage treatment plants, demonstrating promising application prospects.

1. Introduction

The accelerated industrialization in China has precipitated severe aquatic ecosystem degradation, with printing and dyeing wastewater emerging as a critical environmental challenge [1]. As a cornerstone of China’s textile industry, this sector discharges 2.0–2.3 billion tons of wastewater annually, constituting 11% of national industrial effluent [2]. In alignment with the “Carbon peaking and carbon neutrality goals” strategic objectives, the China National Textile and Apparel Council’s 14th Five-Year Plan (2021) mandates urgent development of energy-efficient, low-carbon wastewater treatment technologies. This policy framework underscores the imperative for innovative solutions integrating pollution control with resource recovery.

Printing and dyeing wastewater (PDW) is the wastewater discharged from printing and dyeing factories producing wool, silk, cloth, etc., which is characterized by high concentration of dyes, high alkalinity, difficult biodegradation, a large volume, and large changes in water quality [3]. The main components of printing and dyeing wastewater are dyes, dyeing auxiliaries, surfactants, plasticizers, etc., which ultimately form a mixture of organic pollutants that are harmful and difficult to degrade [4]. Organic pollutants in printing and dyeing wastewater consume dissolved oxygen in the water body, while organic pollutants are anaerobically decomposed to produce hydrogen sulfide, causing the water body to be black and smelly and endangering the aquatic ecological environment. On the other hand, the dyes in the wastewater migrate to the soil and groundwater through the water body, causing greater environmental pollution [5]. Therefore, it is imperative to explore efficient technology for dyeing and printing wastewater to meet the discharge standards.

At present, the deep treatment methods of printing and dyeing wastewater mainly include chemical coagulation and precipitation [6], ion exchange [7], adsorption [8], membrane separation [9], and advanced oxidation [10]. Among them, the advanced oxidation method has become one of the most simple, efficient, and feasible deep treatment technologies due to its simple process, thorough mineralization, and low energy consumption [11].

The Fenton process, a benchmark AOP, generates hydroxyl radicals (·OH, E° = 2.80 V) via Fe²⁺/H₂O₂ reactions at acidic conditions (pH 2.5–3.5), exhibiting pseudo-first-order kinetics (k = 10⁶–10⁹ M⁻¹s⁻¹) [12]. The Fenton reaction mechanism, as illustrated in Equations (1) to (6), operates through a cyclic redox framework. The catalytic process primarily involves the oxidation of Fe²⁺ to Fe³⁺ with concurrent generation of hydroxyl radicals (·OH) and the subsequent reduction of Fe³⁺ back to Fe²⁺, accompanied by hydroperoxyl radical (HO₂·) formation (Equations (1)–(2)). These predominant reactions are accompanied by secondary pathways (Equations (3) to (6)) that influence the overall catalytic efficiency. The reaction scheme explicitly demonstrates that sustained Fe³⁺/Fe²⁺ redox cycling constitutes the fundamental prerequisite for maintaining high process efficiency in Fenton systems.

Fe²⁺ + H₂O₂ → Fe³⁺ + ·OH + OH⁻

(1)

Fe³⁺ + H₂O₂ → Fe²⁺ + HO₂· + H⁺

(2)

Fe²⁺ + ·OH → Fe³⁺ + ·OH

(3)

H₂O₂ + ·OH → HO₂· + H₂O

(4)

·OH + ·OH → H₂O₂

(5)

·OH + HO₂· → H₂O + O₂

(6)

However, inherent limitations, including narrow pH adaptability, iron sludge production (1.5–3.0 kg/m³), and H₂O₂ storage risks, hinder its practical implementation. Additionally, the Fenton process may also lead to heavy iron ions water pollution, causing various negative health consequences [13,14]. Recent advancements focus on process intensification through external energy fields: Photo-Fenton systems enhance Fe³⁺/Fe²⁺ cycling via UV–vis irradiation (λ = 300–500 nm), achieving 91.9 ± 1.5% nitrogen removal in hybrid anammox systems. Nevertheless, photon utilization efficiency remains suboptimal (<15%) [15,16]. Electro-Fenton configurations enable in situ H₂O₂ generation (yield 1.2–3.4 mg/L·min) through oxygen reduction reactions, reducing chemical consumption by 33–45%. Copper-based catalysts demonstrate exceptional stability (>20 cycles) in neutral conditions [17]. Sono-Fenton integration exploits ultrasonic cavitation (20–100 kHz) to enhance mass transfer coefficients by 2–3 orders of magnitude, achieving 98.38% pharmaceutical degradation through synergistic mechano-chemical effects [18]. Emerging investigations reveal that applied magnetic fields induce significant modifications to water’s physicochemical properties. Magnetic treatment (13 min exposure) reduces surface tension by 9% and increases viscosity by 10%, alterations attributed to magnetic dynamic restructuring of hydrogen-bond networks. This phenomenon involves both shortened hydrogen bond distances (enhancing bond strength) and fragmentation of large molecular clusters into smaller, more reactive aggregates [19]. These magnetically induced changes demonstrate potential for enhancing reaction kinetics across various aqueous-phase processes, including dissolution, crystallization, and coagulation phenomena.

Dobranszki et al. [20] discovered that magnetic fields alter the ratio of para- to ortho-isomers in water, accelerating the conversion of para-isomers into ortho-isomers, which exhibit higher physical and chemical reactivity. Wang et al. [21] found that the magnetic dipole moment generated by the field inhibits the transition of triplet states (T states) to singlet states (S states), leading to chemical bonds’ cleavage. Additionally, electron spin resonance induced by the magnetic field promotes bond dissociation, thereby enhancing the oxidation of organic pollutants by hydroxyl radicals (·OH) [22]. Given that Fenton reactions primarily rely on radical mechanisms, and magnetic fields have been shown to significantly influence radical behavior, it can be inferred that magnetic fields enhance Fenton reactions, improving the removal efficiency of organic pollutants.

Xiao et al. [23] integrated magnetic fields with Fenton reactions to degrade methylene blue, using terephthalic acid as a hydroxyl radical scavenger. Fluorescence spectroscopy revealed that under optimal conditions, the magnetic field accelerated the conversion of Fe²⁺ to Fe³⁺, increased the decomposition rate of H₂O₂, and enhanced the formation rate of hydroxyl radicals. As the magnetic field strength increased, the mineralization rate of methylene blue improved by over 10%. Despite these promising results, the optimal magnetic conditions for advanced oxidation processes in printing and dyeing wastewater treatment remain underexplored. Magnetic fields are typically categorized into four types based on strength: weak (<0.001 T), moderate (0.001–1 T), strong (1–5 T), and ultra-strong (>5 T) [24].

Based on the above analysis, magnetic fields may enhance Fenton treatment’s efficiency; however, the underlying mechanisms and practical engineering applications remain unclear. In this study, a strong magnetic field ranging from 1.5 T to 3 T was employed to investigate its impact on advanced oxidation reactions, aiming to optimize the process for industrial applications. The magnetic field promotes Fe²⁺ cycling and ·OH generation, accelerates the decomposition of hydrogen peroxide, and may release H⁺, leading to a decrease in pH value.

2. Materials and Methods

2.1. Wastewater

The wastewater in this study was taken from the biochemical effluent of the sewage plant in the printing and dyeing industrial park. The wastewater is neutral (pH = 7.89), with a chemical oxygen demand (COD) of 90–100 mg/L, suspended solids (SSs) concentration ≤ 33 mg/L, and conductivity ≤ 2.05 ms/cm, indicating a low content of soluble salts. The concentration of nitrogen and phosphorus pollutants is at a relatively low level, with ammonia nitrogen (NH₄⁺-N) ≤ 1 mg/L, nitrate nitrogen (NO₃⁻-N) ≤ 6.92 mg/L, total nitrogen (TN) ≤ 8 mg/L, and total phosphorus (TP) ≤ 0.55 mg/L, which meets the general emission standards. Therefore, the main purpose of this experiment is to degrade COD.

2.2. Magnetizing Equipment

The superconducting magnetic device (Hangzhou Zhongke Kailing Technology Co., Ltd., Hangzhou, China) for the production of magnetization in this study is shown in Figure 1a, with permanent magnets or electromagnets. Superconducting magnets have a higher magnetic field strength and wider magnetic field range. Electromagnets usually have a magnetic field strength of about 1 T and consume large amounts of electricity. Permanent magnets have low field strengths and decay sharply above 5 mm from the magnet surface. When a superconducting material is cooled below its critical temperature, its electrical resistance is zero, so that large currents can be generated and maintained without energy loss. A current is then injected into the superconductor by an external power source to generate a magnetic field inside the superconductor, which is gradually enhanced when the external current is gradually increased.

Since the coil is in a superconducting state, only a small current is required to generate a strong magnetic field. Therefore, the superconducting magnet has the remarkable features of a small size and low energy consumption. In this study, this superconducting magnet is used for the experiments, and the magnetic field strength is adjusted by adjusting the current size, and the magnetic field strength that can be generated by this magnet ranges from 1.5 T to 3 T.

2.3. Experiments

2.3.1. Determination of Surface Tension

The wastewater was magnetized in a 1.5 T magnetic field environment for 5 min and then placed in an insulated box containing an ice bag and sent to the laboratory, where the surface tension was determined by the hanging drop method.

2.3.2. Determination of Molecular Weight of Organic Compounds

The water samples were filtered through a 0.45 μm glass fiber membrane to remove suspended particulate matter. The filtrate was poured into an ultrafiltration cup (Shanghai Limin Industrial Co., Ltd., Shanghai, China) and passed through the ultrafiltration membranes with molecular weight cut-offs of 100 kDa, 30 kDa, 10 kDa, and 3 kDa in parallel under the condition of a high pressure nitrogen cylinder with an air pressure of 0.1 MPa. The first 20 mL of filtrate was discarded, and the DOC (Dissolved Organic Carbon) and UV254 (refers to the absorbance value of ultraviolet light at a wavelength of 254 nm) values of the filtrate were determined. The DOC concentration and UV254 absorbance for each molecular weight fraction were determined by the differential method. Specific ultraviolet absorbance (SUVA) for each fraction was then calculated as the ratio of UV254 to DOC.

The molecular weight distribution characteristics of organic matter before and after magnetization of printing and dyeing wastewater were analyzed by three indicators. The ultrafiltration cup device is shown in Figure 1b.

2.3.3. Magnetic Fenton Experiments

We took 100 mL of printing and dyeing wastewater in a beaker. and adjusted the pH by adding H₂SO₄ (Yonghua Chemical Technology Co., Ltd., Changshu, Jiangsu, China) and NaOH (Yonghua Chemical Technology Co., Ltd., Changshu, Jiangsu, China). We weighed a certain amount of FeSO₄·7H₂O (Shanghai Shenggong Biological Engineering Co., Ltd., Shanghai, China) into the beaker, placed it in the superconducting magnetic equipment, added a certain amount of H₂O₂ (Hangzhou Xiaoshan Chemical Reagent Factory, Hangzhou, China), and placed it in a magnetic stirrer, reacting for 60 min. After that, we adjusted the pH to neutral value, precipitated it for 30 min, and determined the COD by using the supernatant rapid elimination spectrophotometric method. The experiment flowchart is shown in Figure 2. It is imperative that the reactor is placed in the identical location for each experiment to mitigate the influence of magnetic field inhomogeneity on experimental repeatability.

2.4. The Pilot-Scale Experiment

We compared the fluidized bed Fenton process with the magnetic Fenton process to calculate their costs. The pilot-scale experiment applied permanent magnets without electricity consumption. After adding the reagents, the influent wastewater passed through the magnetic field.

2.5. The Calculation of COD Removal Rate (COD RR)

$$COD RR={\displaystyle \frac{{COD}_0-COD}{{COD}_0}}\times 100\%$$

In the equation, COD₀ presents the initial COD; COD presents effluent COD.

2.6. AI Tool

An AI tool helped us to correct grammatical errors and enhance the language fluency.

3. Results and Discussion

3.1. Effect of Magnetic Field on Surface Tension

Surface tension is a key thermodynamic parameter describing the fundamental properties of liquids and interfacial phenomena, and it is related to cohesion, surface energy, and work of adhesion. Therefore, the measurement of surface tension coefficients is scientifically important for understanding the molecular structure and motion of liquids, as well as applying them to specific industrial fields. However, due to the limitations of the experimental conditions, this experiment only tested the change in surface tension of wastewater after magnetization for 5 min at a magnetic field strength of 1.5 T. The results are shown in Figure 3, where Figure 3a shows the surface tension of the liquid before magnetization, and Figure 3b shows the surface tension of the liquid after magnetization.

As a result, the surface tension of the solution decreased from 78.031 mN/m to 71.775 mN/m after 5 min of magnetization, a decrease of 8.02%. This is consistent with the study of Ehsan et al. [25] showing that the surface tension of the solution decreases under the effect of a magnetic field. Cao et al. [26] placed pure water in a magnetic field from 0 to 5 T. The results showed that the surface tension decreased rapidly in the range of magnetic field strength from 0 to 1.0 T. The surface tension increased rapidly in the range of 1.0~1.2 T and began to increase slowly when it was greater than 1.2 T. The applied magnetic field induced a maximum decrease in surface tension by 11%. The decrease in surface tension of water after magnetization is an indication of the decrease in surface free energy and intermolecular cohesion [25]. Ren et al. [27] showed that due to magnetization, the number of hydrogen bonds between water molecules is increased, resulting in a decrease in the surface tension of the liquid. Therefore, the phenomenon can prove that magnetization changes the microstructure of water. Meanwhile, the decrease in cohesion may reduce the formation of molecular clusters in the solution, making it easier for reactant molecules to disperse and come into contact with reactive sites. In homogeneous catalytic reactions, improving the dispersion of reactants may enhance reaction efficiency.

3.2. Effect of Magnetic Field on Molecular Weight Distribution of Organic Matter in Wastewater

The hydrophobic organic matter content in raw water is higher when the SUVA in the water body is >4, and the hydrophilic organic matter content in raw water is higher when the SUVA in the water body is <3. As can be seen from

Table 1. Characteristics of DOC and UV254 organic compounds in different molecular weight intervals.

	Parameter	Raw Water	>100 kDa	30~100 kDa	10~30 kDa	3~10 kDa	<3 kDa
Unmagnetized water	DOC	19.43	2.65	4.92	2.06	2.27	7.53
	Proportion	/	13.6%	25.3%	10.6%	11.7%	38.8%
	UV254	0.851	0.147	0.158	0.146	0.125	0.275
	Proportion	/	17.3%	18.6%	17.2%	14.7%	32.3%
	SUVA	4.38	5.55	3.21	7.09	5.51	3.65
Magnetic water	DOC	18.16	2.82	3.46	1.38	2.21	8.29
	Proportion	/	15.5%	19.1%	7.6%	12.2%	45.6%
	UV254	0.854	0.148	0.194	0.091	0.16	0.261
	Proportion	/	17.3%	22.7%	10.7%	18.7%	30.6%
	SUVA	4.7	5.25	5.61	6.59	7.24	3.15

, the SUVA of the dyeing wastewater was 4.38, which indicates a high content of hydrophobic organic matter. The DOC and UV254 indicators of organic matter in different molecular weight intervals measured after the ultrafiltration experiment are shown in Table 1. The molecular weight distribution characteristics of organic matter in dyeing wastewater are shown in Figure 4.

The data in the table is the average of the three replicates. As shown in Table 1, the organic matter of printing and dyeing wastewater was dominated by a molecular weight < 3 kDa, accounting for 38.8% of the total organic matter. The rest were mainly concentrated in the 30–100 kDa molecular weight range, accounting for 25.3%. Among them, the molecular weights of 3~10 kDa and 10~30 kDa accounted for similar proportions, 11.7% and 10.6%, respectively. UV254 is an important index to characterize the content of unsaturated organic matter in water samples. The content of unsaturated organic matter with a molecular weight < 3 kDa was high, accounting for 32.3%. Figure 4 visualizes that the molecular weight of organic matter in printing and dyeing wastewater is dominated by <3 kDa, and there is no obvious correlation between DOC and UV254 in each molecular weight interval. After the wastewater was magnetized, the organic matter was dominated by a molecular weight < 3 kDa, accounting for 45.6% of the total organic matter. The rest was mainly concentrated in the 30–100 kDa molecular weight interval, accounting for 19.1%. Compared with the unmagnetized wastewater, the proportion of the 30–100 kDa molecular weight range became smaller after magnetization, while the proportion of organic matter with a molecular weight < 3 kDa became larger. This significant shift indicates that magnetic exposure disrupts hydrogen bonding or van der Waals forces within macromolecular organic aggregates, fragmenting them into smaller, more reactive constituents. This magnetically induced redistribution of organic matter toward lower molecular weights profoundly enhances the subsequent Fenton oxidation process, as the smaller fragments exhibit higher diffusivity and are more readily degraded by ·OH.

3.3. Magnetic-Enhanced Fenton Deep Treatment of Wastewater

3.3.1. pH

pH is a critical factor influencing Fenton oxidation. Experiments were conducted for 60 min under the conditions of Fe²⁺ dosage (1 mmol/L) and H₂O₂ dosage (3 mmol/L) to investigate the effect of pH (range 2–6) on COD degradation by conventional Fenton and magnetic Fenton processes. Specific results are presented in Figure 5. The excessively low system pH (high acidity) inhibits the conversion of Fe³⁺ to Fe²⁺, resulting in a lower concentration of catalytic Fe²⁺. Conversely, excessively high system pH suppresses the generation of ·OH radicals. Furthermore, Fe²⁺ and Fe³⁺ ions react with OH- to form precipitates, which results in a decrease in soluble iron concentration. Additionally, at a higher pH, H₂O₂ undergoes self-decomposition to form O₂, which possesses a weaker oxidizing capability, consequently diminishing the system’s oxidative capacity towards organic matter. Corroborating the latest findings of Vasudhevan et al., the presence of organic ligands such as citrate and EDTA in Fe(II)/PS systems effectively enhances the degradation of chlorinated aromatic contaminants across a broad pH spectrum. This is achieved by mitigating iron precipitation and preserving the reactivity of iron ions, even under neutral to alkaline conditions [28].

The optimal COD removal rates for both Fenton reactions occurred at pH 3: 51.2% for conventional Fenton and 57.9% for magnetic Fenton. Across the pH range of 2–6, the applied magnetic field enhanced the Fenton process, increasing the COD removal rate by 6.2% to 10%. This enhancement effect was particularly pronounced under non-optimal pH conditions. For instance, at pH 5, the COD removal rate achieved by magnetic Fenton was 49.7%, representing a 10.0% increase compared to conventional Fenton under identical reaction conditions. These results indicate that the magnetic field broadens the effective pH range for the Fenton reaction to some extent. A recognized limitation of conventional Fenton is its narrow optimal pH range, typically achieving better degradation efficacy only at pH 3–4. In contrast, this study demonstrates that magnetic Fenton maintains a significantly better COD removal performance even at pH 5. This phenomenon may be attributed to the magnetic field inducing an additional magnetic moment in water molecules, generating supplementary magnetic fields and energy. This leads to a decrease in the cohesive forces within the diamagnetic liquid and a reduction in molecular potential barriers. Consequently, the liquid surface tension decreases, the diffusion coefficient increases, solubility improves, and ion exchange processes are accelerated. Therefore, pH 3 (where both systems exhibit their peak performance) and pH 5 (where the magnetic enhancement effect under non-optimal conditions is particularly significant and demonstrably effective for magnetic Fenton) were selected for subsequent experiments.

3.3.2. Fe²⁺ Dosage

The influence of Fe²⁺ dosage (1, 2, 3, 5, and 10 mmol/L) on Fenton and magnetic Fenton processes was examined at pH 3 and 5 with a fixed H₂O₂ concentration of 3 mmol/L. Samples were collected at 10 min intervals during the 60 min reaction for COD analysis, with results presented in Figure 6.

At pH 3, both systems achieved maximum COD removal at 3 mmol/L Fe²⁺: conventional Fenton attained 59.5% removal (effluent COD: 40.47 mg/L), while magnetic Fenton reached 65.2% (effluent COD: 34.84 mg/L). Below this optimal dosage, an increasing Fe²⁺ concentration accelerated H₂O₂ decomposition and ·OH generation, enhancing organic degradation and COD removal. Conversely, dosages exceeding 3 mmol/L resulted in excess Fe²⁺ quenching ·OH radicals (Fe²⁺ + ·OH → Fe³⁺ + OH⁻), promoting radical recombination, and reducing oxidation efficiency [29]. Notably, magnetic Fenton at 1 mmol/L Fe²⁺ achieved 57.9% removal (effluent COD: 42.14 mg/L), comparable to conventional Fenton at 3 mmol/L (59.5%), demonstrating significant catalyst economy. Under identical reagent conditions at pH 3, magnetic Fenton improved COD removal by 5.6–6.7% relative to conventional Fenton.

At pH 5, peak performance occurred at 3 mmol/L Fe²⁺: conventional Fenton yielded 49.0% removal (effluent COD: 51.05 mg/L), versus 59.5% for magnetic Fenton (effluent COD: 40.48 mg/L). Magnetic Fenton at 1 mmol/L Fe²⁺ achieved 49.7% removal (effluent COD: 50.32 mg/L), representing a 10.0% enhancement over conventional Fenton at the same dosage. This performance matched conventional Fenton at 3 mmol/L (49.0%) while reducing catalyst consumption by 66.7%. When operated at identical dosages at pH 5, magnetic Fenton increased COD removal by 5.6–10.0%. At 2 mmol/L Fe²⁺ and pH 5, magnetic Fenton produced effluent COD of 45.13 mg/L. Based on an optimal balance between treatment efficiency and reagent economy, 2 mmol/L Fe²⁺ was selected for subsequent magnetic Fenton experiments. The quenching effect of excess Fe(II) on sulfate radicals is a key factor leading to a decrease in efficiency, a phenomenon that has also been verified in our previous research [28].

3.3.3. H₂O₂ Dosage

Under the conditions of pH 3, Fe²⁺ dosage of 3 mmol/L, and reaction time of 60 min, the effect of different H₂O₂ dosages on COD removal by Fenton was investigated, and the experimental results are shown in Figure 7a. Under the conditions of pH 5, Fe²⁺ dosage of 2 mmol/L, and reaction time of 60 min, the effect of different H₂O₂ dosages on COD removal by magnetic Fenton was investigated. The experimental results are shown in Figure 7b. A H₂O₂ dosage of 1 mmol/L, 2 mmol/L, 3 mmol/L, 4 mmol/L, and 5 mmol/L was selected for the Fenton and magnetic Fenton experiments.

As shown in the figure, when the concentration of H₂O₂ in the system is too low, more ·OH can be generated to oxidize the organic matter as the concentration of H₂O₂ increases. However, after putting in excess H₂O₂, H₂O₂ will continue to react with ·OH to produce HO₂⁻, consuming H₂O₂ and ·OH; in addition, excess H₂O₂ will rapidly oxidize Fe²⁺ in the system to Fe³⁺, which affects the catalytic production of Fe²⁺ to generate ·OH [30]. As can be seen from the figure, at pH 3, Fe²⁺ dosage of 3 mmol/L, and H₂O₂ dosage of 3 mmol/L, the COD removal rate of the Fenton reaction reached the highest value of 62.7%, and the COD effluent was 37.29 mg/L. Therefore, the optimal experimental parameters of Fenton were pH = 3, Fe²⁺ = 3 mmol/L, and H₂O₂ = 3 mmol/L; COD removal was 62.7%, and COD effluent was 37.29 mg/L. As shown in the figure, the magnetic Fenton COD removal was 60.0%, and COD effluent was 40.0 mg/L at pH = 5, Fe^{2 +} = 2 mmol/L when the H₂O₂ dosage was 2 mmol/L.

3.3.4. Time

The Fenton and magnetic Fenton reactions were carried out at pH = 5, Fe²⁺ dosing of 2 mmol/L, and H₂O₂ dosing of 2 mmol/L. Samples were taken at 0, 10, 20, 30, 40, and 60 min of the reaction, and the experiments were repeated three times to determine the COD concentration in the wastewater. Reaction time is one of the important factors affecting the operating cost in practical applications. As shown in Figure 8, as the reaction time grows, the COD removal rate is greater, both the Fenton reaction and the magnetic Fenton reaction complete the oxidation reaction after 40 min, and the COD removal rate tends to remain unchanged as the reaction time is extended. COD removal rate tends to be unchanged. Under the same reaction conditions, the magnetic Fenton removal rate is better than ordinary Fenton. The COD removal rate reached 60.0%; COD effluent was 40.0 mg/L to meet the discharge standards.

3.3.5. Effect of Magnetization Conditions on the Reaction

The reaction was carried out at pH = 5, Fe²⁺ = 2 mmol/L, H₂O₂ = 2 mmol/L for 40 min to investigate the effect of different magnetic field strengths (1.5 T, 2 T, 2.5 T, 3 T) on the Fenton reaction, and the results are shown in Figure 9a. The magnetic effect was best when the magnetic field strength was 1.5 T, and the COD removal rate reached 60.0%. When the magnetic field strength is higher than 1.5 T, and then one increases the magnetic field strength, the magnetic field has no obvious strengthening effect on the Fenton reaction, and the removal rate of COD remains almost the same. Therefore, the optimal reaction parameter of magnetic Fenton was 1.5 T. The reaction was carried out for 40 min under the conditions of pH = 5, Fe²⁺ = 2 mmol/L, H₂O₂ = 2 mmol/, and a magnetic field strength of 1.5 T. The effects of different magnetization times (1 min, 3 min, 5 min, 10 min) on the Fenton were investigated. As shown in Figure 9b, the COD removal rate did not change significantly with the increase in magnetization time, so the optimal magnetization time for magnetic Fenton was 5 min.

3.3.6. Kinetic Model and Mechanism Elucidation

To quantitatively evaluate the enhancement effect of the magnetic field and establish a theoretical foundation for parameter optimization, the degradation kinetics of COD were analyzed. The degradation of organic pollutants by advanced oxidation processes (AOPs), including the Fenton reaction, often follows pseudo-first-order kinetics, as described by the equation

−dt/dC = k_obsC

where C is the COD concentration (mg/L) at time t (min), and k_obs is the observed pseudo-first-order rate constant (min⁻¹). Integration of this equation yields the linear form ln(C₀/C_t) = k_obst.

The experimental data obtained under the respective optimal conditions for each process were fitted to this kinetic model. The calculated rate constants are presented in

Table 2. Pseudo-first-order reaction rate constants for COD degradation under optimal conditions.

Process	Reaction	Kobs (min−1)	R2
Fenton	pH = 3, Fe2+ = 3 mmol/L, H2O2 = 3 mmol/L	0.024	0.987
Magnetic Fenton	pH = 5, Fe2+ = 2 mmol/L, H2O2 = 2 mmol/L, B = 1.5 T	0.023	0.991

.

The high linear correlation coefficients (R² > 0.98) confirm that the pseudo-first-order model accurately describes the COD degradation in both systems. A critical finding is that the magnetic Fenton process achieved a kinetic rate constant (k_{obs MF} = 0.023 min⁻¹) comparable to that of the conventional process (k_{obs F} = 0.024 min⁻¹), despite operating under significantly milder conditions: a higher pH (5 vs. 3) and a 33.3% reduction in the dosage of both catalyst (Fe²⁺) and oxidant (H₂O₂). This kinetic equivalence underscores the efficiency of the magnetic Fenton system, demonstrating that comparable performance can be achieved with reduced chemical consumption and lower acidity requirements.

The enhancement mechanism was further probed using the Langmuir–Hinshelwood (L-H) model, which is often applied to surface-mediated reactions. Although the classic Fenton system is homogeneous, the application of a magnetic field is hypothesized to induce microstructural changes in the aqueous medium and organic pollutants, introducing interfacial interactions analogous to surface effects. The L-H model is expressed as

r = −dt/dC = KCkr/1 + KC

where r is the degradation rate, kr is the intrinsic surface reaction rate constant, and K is the adsorption equilibrium constant of organic pollutants at the reactive sites. Based on our experimental results, we propose that the magnetic field pretreatment effectively increases the apparent K value within this conceptual framework. By reducing aqueous surface tension and disrupting large molecular clusters into smaller, more reactive units (as evidenced in Section 3.1 and Section 3.2), magnetization improves the dispersibility and mass transfer efficiency of organic substrates. This increases their effective concentration or “availability” at the reactive sites, thereby facilitating contact and reaction with ·OH radicals. Consequently, an increased overall reaction rate (r) is observed for given concentrations of Fe²⁺ and H₂O₂. This mechanistic framework aligns with the experimental observations: the magnetic field broadens the operable pH window and lowers reagent demands, as the process becomes less dependent on highly acidic conditions to solubilize iron and more efficient at promoting the critical interactions between radicals and pollutants.

4. Engineering Application

The pilot-scale equipment is shown in Figure 10. The influent and effluent water quality for advanced treatment are summarized in

Table 3. Influent and effluent quality in magnetic Fenton and fluidized bed Fenton.

	COD (mg/L)	BOD5 (mg/L)	SS (mg/L)	NH4+-N (mg/L)	TN (mg/L)
influent	≤150	≤20	≤50	≤5	≤15
effluent	≤50	≤10	≤10	≤5	≤15

. A pilot-scale comparative study was conducted between fluidized bed Fenton and magnetic Fenton processes, with a treatment capacity of 1000 L/h and daily throughput of 6–8 m³/d. The magnetic Fenton system operated at a magnetic field intensity of 1 T. Process parameters, established based on bench-scale testing, were set as follows: fluidized bed Fenton operated at pH 3–4 with 450 mg/L ferrous sulfate and 370 mg/L hydrogen peroxide (27.5% concentration), while magnetic Fenton operated at pH 4–5 with 300 mg/L ferrous sulfate and 250 mg/L hydrogen peroxide (27.5% concentration).

After optimizing chemical dosages to determine optimal operating conditions, both systems achieved stable operation. Results presented in Figure 11 and

Table 4. Experimental results from 5-day operation of magnetic Fenton and fluidized bed Fenton processes.

Days	COD (mg/L)	Magnetic Fenton	COD Removal Rate (%)	Fluidized Bed Fenton	COD Removal Rate (%)
1	influent	68	44.1	73	45
	effluent	38		40
2	influent	78	48.7	80	41.3
	effluent	40		47
3	influent	63	44.4	69	39.1
	effluent	35		42
4	influent	78	39.7	79	39.2
	effluent	47		48
5	influent	74	45.9	72	41.7

demonstrate that under influent COD conditions of 63–78 mg/L, the magnetic Fenton system produced effluent COD of 35–47 mg/L, corresponding to removal rates of 39.7–48.7%. The fluidized bed Fenton system, treating influent COD of 69–80 mg/L, yielded effluent COD of 40–48 mg/L, with removal rates of 39.1–45.0%. Both technologies met the Class 1A standards specified in China’s Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants.

Cost analysis based on pilot operation data (

Table 5. Experimental average drug dosage.

Process	27.5%H2O2 mg/L	90%FeSO4 mg/L	98%H2SO4 mg/L	30%NaOH mg/L	PAM mg/L
Fluidized bed Fenton	364	326	268	176	2
Magnetic Fenton	120	220	190	142	2

,

Table 6. Cost of magnetic Fenton reagent.

No.	Chemical Reagent	Specification	Consumption	Unit Price	Cost
1	FeSO4	90%	220 g/m3	0.5 CNY/kg	0.11 CNY/m3
2	H2SO4	98%	190 g/m3	0.6 CNY/kg	0.114 CNY/m3
3	H2O2	27.5%	120 g/m3	1.0 CNY/kg	0.12 CNY/m3
4	NaOH	30%	142 g/m3	0.8 CNY/kg	0.114 CNY/m3
5	PAM	—	2 g/m3	12 CNY/kg	0.024 CNY/m3
	Total: 0.482 CNY/m3				0.482 CNY/m3

and

Table 7. Cost of fluidized bed Fenton reagent.

No.	Chemical Reagent	Specification	Consumption	Unit Price	Cost
1	FeSO4	90%	326 g/m3	0.5 CNY/kg	0.163 CNY/m3
2	H2SO4	98%	268 g/m3	0.6 CNY/kg	0.161 CNY/m3
3	H2O2	27.5%	364 g/m3	1.0 CNY/kg	0.364 CNY/m3
4	NaOH	30%	176 g/m3	0.8 CNY/kg	0.141 CNY/m3
5	PAM	—	2 g/m3	12 CNY/kg	0.024 CNY/m3
	Total: 0.853 CNY/m3				0.853 CNY/m3

) revealed that magnetic Fenton achieved compliant treatment at a chemical cost of just 0.482 CNY/m³, significantly lower than fluidized bed Fenton. The fluidized bed process requires porous carriers that initially reduce sludge production and improve decolorization through adsorption. However, after carrier saturation occurs, chemical consumption and sludge production increase substantially, while decolorization efficiency declines. This necessitates periodic carrier replacement at approximately 0.12 CNY/m³. Furthermore, the fluidized bed reactors are prone to fouling and clogging during extended operation, demanding frequent maintenance.

Conversely, the magnetic Fenton system eliminates carrier requirements, thereby avoiding fouling and clogging issues while reducing maintenance needs. The process design omits recirculation aeration systems and eliminates degassing requirements in the stabilization tank, resulting in electricity cost savings of 0.05 CNY/m³ compared to fluidized bed Fenton. Additionally, the reduced ferrous iron dosage in magnetic Fenton decreases the production of iron sludge, lowering sludge disposal costs by 0.019 CNY/m³. Collectively, magnetic Fenton demonstrates superior economic viability through reduced power consumption, elimination of carrier costs, decreased sludge production, and lower chemical requirements.

The 12-month long-term performance of the magnetic Fenton process was assessed via monthly average COD measurements (Figure 12). The effluent COD concentration remained stable at an average of 40 mg/L with minimal fluctuation, underscoring the reliability of the process.

5. Conclusions

(1)

Magnetization enhances hydrogen bonding between water molecules, causing the cleavage of large water molecular clusters into smaller clusters. This facilitates the transformation of large-molecular-weight organic matter into small-molecular-weight organic matter, which is conducive to subsequent degradation treatment.

(2)

Magnetic Fenton demonstrates significant advantages over conventional Fenton. Crucially, it achieves comparable COD removal efficiency (60.0%) at pH 5 using reduced reagent dosages (Fe²⁺: 2 mmol/L, H₂O₂: 2 mmol/L), while conventional Fenton requires highly acidic conditions (pH 3) and higher reagent levels (3 mmol/L each) for optimal performance (62.7% removal). The magnetic field (1.5 T, 5 min magnetization) broadens the effective pH range, enabling efficient operation at pH 5, where conventional Fenton performance drops substantially. This translates to reduced acid consumption, 33.3% lower catalyst and oxidant usage, and effluent COD (40.0 mg/L) meeting stringent discharge standards (GB 18918-2002 Class 1A) within 40 min. The magnetic enhancement is attributed to improved reaction kinetics and mass transfer, allowing high efficiency under milder, more economical conditions.

(3)

In engineering applications of the magnetic Fenton process, comparative analysis with fluidized bed Fenton technology demonstrates that when effluent meets the Class 1A standard specified in the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants (GB 18918-2002), the reagent cost for magnetic Fenton (0.482 CNY/m³) is significantly lower than that of fluidized bed Fenton (0.853 CNY/m³). The magnetic Fenton process offers integrated advantages, including elimination of carrier material requirements, reduced electricity consumption, and lower iron sludge production, thereby enhancing its economic viability.

6. Novelty

(1)

Coupling a magnetic field with Fenton, applying physical and chemical methods to water treatment, providing a basis for engineering applications.

(2)

The mechanism of magnetic field-enhanced Fenton reaction was analyzed and verified.