Efficiency of H₂O₂-Modified Ferrite Process for High-Concentration PVA Removal and Magnetic Nanoparticle Formation

Abstract

High-concentration polyvinyl alcohol (PVA) wastewater from 3D printing applications presents significant treatment challenges due to PVA’s water solubility, chemical stability, and resistance to biodegradation. In this study, we investigated the enhanced removal of high-concentration PVA (3–7 g/L) using a modified ferrite process with H₂O₂ pre-oxidation, while simultaneously exploring the formation and properties of magnetic precipitates. The effects of PVA concentration, reaction conditions, and thermal treatment (100 °C and 650 °C) on the magnetic precipitates were studied through XRD, TEM, FTIR, and magnetic measurements. Results showed that H₂O₂ pre-oxidation effectively maintained the system pH and improved PVA removal efficiency, achieving a COD reduction of 83% after two-stage treatment for 7 g/L PVA solution. XRD and TEM analyses revealed that precipitates formed at 100 °C consisted of dispersed Fe₃O₄ nanoparticles within PVA fibrous networks, while calcination at 650 °C led to the formation of rod-like structures and agglomerated particles. The magnetic properties varied significantly with treatment conditions, exhibiting the highest saturation magnetization of 10.30 emu/g for sample calcinated at 100 °C. This study demonstrates the potential of the modified ferrite process for treating high-concentration PVA wastewater while producing recoverable magnetic nanoparticles, providing a dual-function approach to address environmental challenges posed by the 3D printing industry.

1. Introduction

Polyvinyl alcohol (PVA) plays a crucial role in the rapidly expanding 3D printing industry, particularly as a water-soluble support material for complex geometric structures [1]. The unique properties of PVA, including excellent water solubility, biodegradability, and mechanical strength, make it ideal for supporting overhanging features during the printing process of complex objects [2]. Once printing is complete, the PVA supports can be dissolved in water, leaving the main structure intact. With the widespread adoption of 3D printing technology in medical devices, construction, and industrial applications, the demand for PVA has increased significantly. Recent applications include patient-specific anatomical models in healthcare, lightweight structural components in automotive manufacturing, and intricate architectural prototypes in construction [3,4,5]. Unlike traditional textile and paper industries where PVA wastewater concentrations range from 0.1 to 1 g/L, 3D printing generates wastewater containing PVA concentrations as high as 20–35 g/L, presenting a new environmental issue [6,7]. This higher concentration results from the complete dissolution of solid PVA supports in relatively small volumes of water during the post-processing phase of 3D printing.

The treatment of such high-concentration PVA wastewater is particularly challenging due to PVA’s unique molecular structure. Its carbon backbone with hydroxyl side groups creates excellent water solubility while imparting significant chemical stability. Conventional biological treatment methods are ineffective, as most microorganisms lack the enzymes necessary to break down the carbon–carbon bonds in PVA’s main chain [8]. Additionally, PVA’s high molecular weight and viscosity properties cause severe fouling in membrane filtration systems, reducing treatment efficiency by up to 85% [6]. The environmental impacts of untreated PVA wastewater are multifaceted and persistent. When discharged into aquatic environments, PVA requires extended periods for natural degradation, with studies showing half-lives exceeding 100 days in natural waters [7]. Its accumulation in water bodies increases viscosity, generates surface foam, and interferes with oxygen transfer, inhibiting aerobic microbial activity essential for ecosystem health. Furthermore, PVA has been shown to facilitate the mobilization and transport of heavy metals in sediments by forming complexes that increase metal solubility, potentially exacerbating environmental pollution. The final destination of PVA in the environment is particularly concerning, as it can eventually degrade into microplastics that may enter the food chain, posing risks to aquatic organisms and potentially humans [9].

Previous studies have revealed multiple solutions in treating high-concentration PVA wastewater. While biological treatment is environmentally friendly, even specialized bacterial strains achieve only 65% degradation efficiency for 5 g/L PVA, requiring treatment periods exceeding 15 days [10]. Advanced oxidation processes, such as UV/H₂O₂ systems, require 4.8 kWh/m³ energy consumption to treat 1 g/L PVA [11], making them cost-prohibitive. Membrane filtration technologies experience up to 85% flux decline when treating high-concentration PVA and face membrane fouling and high operational costs [12]. These technologies become even less efficient and more costly when dealing with high-concentration PVA wastewater from 3D printing. To address these issues, researchers have proposed various treatment methods, including photochemical degradation [13,14,15], ultrasonic technology [16], potassium permanganate oxidation [17], radiation degradation [18], and various adsorption materials [19]. Among these, photoelectrochemical treatment has gained attention due to its environmental friendliness and operational flexibility [20]. Electrocoagulation, as an emerging electrochemical treatment technology, has demonstrated effectiveness in treating various wastewaters through the generation of metal ions from sacrificial anodes to form hydroxides for pollutant removal [21].

In the field of advanced oxidation processes, the persulfate-based system has emerged as a promising oxidant owing to its strong oxidation potential, chemical stability, and versatile activation mechanisms through thermal-, Fe²⁺-, and zero-valent iron (Fe⁽⁰⁾)-mediated pathways [22]. Studies have shown that thermally activated persulfate can completely degrade PVA within 30 min at 60 °C, while at room temperature, Fe²⁺- or Fe⁽⁰⁾-activated persulfate systems achieve complete PVA degradation within 2 h [23]. The Fe⁽⁰⁾-activated system demonstrates superior treatment efficiency, primarily attributed to its sustained release of Fe²⁺, maintaining persulfate activation efficiency. Gas chromatography–mass spectrometry analysis reveals that the main intermediate product of persulfate oxidation of PVA is vinyl acetic acid, which is more biodegradable than PVA due to its lower molecular weight and unsaturated carboxylic acid structure. This indicates that persulfate oxidation significantly enhances the biodegradability of PVA wastewater, creating favorable conditions for subsequent biological treatment [24]. Several limitations exist in applying persulfate oxidation to treat high-concentration PVA wastewater from 3D printing. The thermal activation process exhibits high efficiency but remains energy-intensive and impractical for industrial applications. The understanding of Fe²⁺ and Fe⁽⁰⁾ activation efficiency and mechanisms under high PVA concentrations is still insufficient [25]. The process is further complicated by the potential self-scavenging reactions of excess Fe²⁺, which can reduce overall oxidation efficiency.

The ferrite process, initially developed for heavy metal removal [26,27,28], has shown promise in organic pollutant treatment due to its ability to form easily separable magnetic precipitates. This process involves the co-precipitation of Fe²⁺ and Fe³⁺ ions under alkaline conditions to form ferrite (Fe₃O₄). Spinel ferrite nanoparticles (SFNPs), with their unique superparamagnetic properties, high adsorption capacity, and surface-area-to-volume ratio, demonstrate significant potential in water treatment [29,30,31]. The ferrite structure, with the general formula MFe₂O₄ (M = Mn, Fe, Co, Ni, Cu, Zn, etc.), not only exhibits excellent magnetic properties but can also be surface-modified to enhance selectivity and adsorption performance for pollutants. Studies have shown that SFNPs effectively remove both organic and inorganic pollutants. For instance, Fe₃O₄–chitosan composites achieve adsorption capacities of 1215 mg/g for azo dye acid orange 7, significantly higher than traditional adsorbents like activated carbon (391 mg/g) and graphene oxide (144.92 mg/g), while achieving over 95% removal efficiency for heavy metals such as Pb²⁺ ions [32,33,34].

The ferrite process combines the advantages of precipitation separation and magnetic recovery, demonstrating unique potential in treating complex wastewaters. In situ-generated ferrite materials exhibit smaller particle sizes (50–300 μm) and larger specific surface areas (>200 m²/g) compared to traditional prefabricated ferrites, with abundant active functional groups on their surfaces, leading to superior pollutant removal performance [29]. For instance, the in situ ferrite process achieves over 95% removal efficiency for methylene blue dye within 5 min, with an adsorption capacity of 347.82 mg/g, significantly surpassing traditional adsorbents such as activated carbon (164.9 mg/g), graphene oxide (144.92 mg/g), and bio-adsorbents like cellulose citrate [30]. Additionally, the magnetic properties of ferrite precipitates enable rapid magnetic separation and potential reuse as catalysts, demonstrating significant engineering advantages [31]. However, the application of the ferrite process in treating high-concentration PVA, particularly from 3D printing wastewater, remains under-explored. The interactions between PVA and iron oxides during the ferrite process present both opportunities and challenges [32,33,34]. While PVA may interfere with the formation of crystalline ferrite structures, it might also promote the formation of novel composite materials with enhanced removal capabilities. Understanding these interactions and optimizing process parameters is crucial for developing effective treatment strategies for high-concentration PVA wastewater.

This study proposes a multi-step ferrite process specifically designed for treating high-concentration PVA wastewater from 3D printing applications. The primary objectives of this research are to evaluate the efficiency of an H₂O₂-modified ferrite process for removing high-concentration PVA, to investigate the formation mechanisms and properties of magnetic precipitates generated during the treatment process, and to explore the potential reuse of these precipitates as functional materials. We hypothesize that the introduction of H₂O₂ pre-oxidation will enhance the conventional ferrite process through improved pH regulation and partial oxidative degradation of PVA molecules, thereby facilitating more effective subsequent ferrite formation and PVA removal. Additionally, we posit that PVA concentration will significantly influence the crystalline structure and magnetic properties of the formed precipitates, potentially leading to tailorable magnetic materials. The key contributions of this study to scientific knowledge include the development of an effective treatment approach for high-concentration PVA wastewater that addresses a growing environmental challenge posed by the 3D printing industry, elucidation of the interaction mechanisms between PVA and iron oxides during the modified ferrite process, and demonstration of a resource recovery strategy that transforms waste constituents into potentially valuable magnetic materials. This integrated approach represents an important advancement in sustainable wastewater treatment, aligning with circular economy principles by converting treatment byproducts into resources.

2. Materials and Methods

2.1. Materials and Reagents

High-concentration PVA wastewater was collected from an actual 3D printing process line. Cleaning solution was purchased from Waterworks Company (New York, NY, USA). Deionized water was used throughout all experiments.

2.2. Experimental Methods

The modified ferrite treatment experiments were conducted at room temperature. PVA wastewater was initially prepared at various concentrations (3–7 g/L) by mixing commercial PVA (Mw 89,000–98,000, 99+% hydrolyzed, Sigma Aldrich, St. Louis, MO, USA) with cleaning solution at specific ratios. In a 15 mL reaction system, PVA wastewater addition ranged from 1.20 mL to 2.30 mL, with corresponding adjustments in cleaning solution volume (13.8–12.2 mL). Pre-prepared ferrite solution was then introduced to the reaction system under continuous stirring [20]. The pH was monitored throughout the reaction process, showing a decrease from 9.9 to 7.1 with increasing PVA concentration. After reaction completion, the mixture was settled for 10 h followed by filtration to separate the precipitates and filtrate. The collected precipitates were characterized by XRD analysis, while the filtrate was preserved for subsequent analysis. The morphology of PVA–Fe₃O₄ was characterized using a transmission electron microscope (TEM, Thermo Fisher Scientific, Waltham, MA, USA, Talos F200X G2) operated at 200 kV. Samples for TEM were prepared by drop-casting 10 μL of the suspension onto carbon-coated copper grids and allowing them to dry under ambient conditions for 2 h. XRD patterns were recorded on a Bruker D8 Discover Plus TXS diffractometer (Bruker, Billerica, MA, USA) using Cu Kα radiation (λ = 1.5406 Å) in the 2θ range of 20–80° with a step size of 0.02° and a scan rate of 1°/min. The Fourier-transform infrared (FTIR) spectroscopy analysis was performed using a Shimadzu IRTracer-100 FTIR spectrometer (Shimadzu Corporation, Kyoto, Japan) equipped with an HATR (horizontal attenuated total reflectance) accessory with a ZnSe crystal. Liquid samples were directly applied onto the ATR crystal surface without any pretreatment. For each measurement, approximately 50–100 μL of the solution was placed on the crystal and analyzed. The spectra were recorded in the mid-infrared range of 4000–700 cm⁻¹ with a spectral resolution of 4 cm⁻¹, averaging 64 scans per sample to improve the signal-to-noise ratio. A background spectrum of deionized water was collected before each sample measurement to eliminate solvent interference. The ATR crystal was thoroughly cleaned with ethanol and deionized water between measurements and dried with nitrogen gas to prevent cross-contamination. All measurements were performed at room temperature. Magnetic properties were measured using a Quantum Design MPMS-XL SQUID magnetometer (San Diego, CA, USA) at room temperature. The hysteresis loops were recorded with applied fields ranging from −6000 to 6000 Oe. Samples were prepared by placing approximately 50 mg of dried powder in a gelatin capsule and fixing it in a plastic straw. Chemical oxygen demand (COD) was measured using a HACH DR3900 spectrophotometer (Loveland, CO, USA) with digestion vials (HACH method 8000) at 150 °C for 2 h, followed by colorimetric measurement at 620 nm wavelength.

2.3. Preparation and Recovery of Magnetic Precipitates

An experimental design was developed to investigate the potential reuse of magnetic precipitates generated from the ferrite treatment of PVA wastewater. To enhance experimental reproducibility and controllability, NaOH was employed as the pH regulator instead of commercial cleaning solution. A 2 × 2 factorial design was implemented to study the effects of reaction system scale and pH on magnetic precipitate formation. Specifically, high-concentration PVA solution (37.8 g/L) was used to establish two reaction system scales: 300 mL and 350 mL. In the smaller system (300 mL), 55.5 mL PVA solution was precisely measured and mixed with 244.493 mL NaOH solution for initial pH adjustment. For the 350 mL system, 64.8 mL PVA solution and 285 mL NaOH solution were used accordingly. The pH values were adjusted to different target levels through H₂O₂ pre-oxidation: Series A experiments were adjusted to 11.1, while Series B experiments ranged from 10.9 to 12.3. Under continuous stirring, Fe solution was added sequentially: for the 300 mL system, initial addition of 14.4 mL followed by 8.2 mL; for the 350 mL system, 16.8 mL followed by 9.5 mL. This sequential addition strategy was designed to optimize magnetic precipitate formation. The final PVA concentration was maintained at 7 g/L across all treatment groups to ensure comparability. This precise experimental design facilitated understanding of how reaction system scale and pH influence magnetic precipitate formation while providing essential experimental foundation for developing precipitate recovery and reuse strategies. Through rigorous control of reaction parameters, particularly staged pH adjustment and sequential Fe ion addition, the process aimed to produce stable precipitates with desirable magnetic properties.

2.4. Additional Information Regarding the Sample Preparation

For the ferrite treatment experiments, we used a precisely controlled procedure: Each experiment was conducted in triplicate at room temperature (25 ± 2 °C) in 15 mL glass reactors with magnetic stirring at 300 rpm. PVA wastewater samples were prepared by diluting high-concentration PVA wastewater (37.8 g/L) with deionized water to achieve the target concentrations (3–7 g/L). The pH was continuously monitored using a calibrated Orion pH meter (Thermo Scientific, accuracy ±0.01). For H₂O₂ pre-oxidation, 30% H₂O₂ solution was added at a molar ratio of 1:10 (H₂O₂:Fe) to the PVA solution and mixed for 10 min. Ferrite solution (containing Fe²⁺/Fe³⁺ ions at a molar ratio of 1:2) was then added dropwise (1 mL/min) during continuous stirring. For sequential ferrite treatment, two separate additions of ferrite solution were made with a 30 min interval between additions. For magnetic precipitate recovery, the solution was allowed to settle for 10 h, followed by vacuum filtration using 0.45 μm membrane filters. The precipitates were washed three times with deionized water and then divided into two portions for thermal treatment: one portion was dried at 100 °C for 24 h in a convection oven, while the other was calcinated at 650 °C for 2 h in a muffle furnace with a heating rate of 5 °C/min. All experiments were conducted in triplicate, and results are reported as mean ± standard deviation. The coefficient of variation for treatment efficiency measurements was maintained below 5% across all experimental conditions.

3. Results and Discussion

3.1. Visual Observation of PVA Treatment Effects by Ferrite Process

To evaluate the treatment effectiveness of the modified ferrite process on PVA wastewater at various concentrations, visual observation studies were first conducted. Figure 1 illustrates the appearance changes of PVA solutions before and after treatment. In the initial state after surfactant addition (Figure 1a), the solutions gradually transformed from colorless and transparent to light brown as PVA concentration increased (0–17 g/L), indicating preliminary interactions between PVA and the surfactant. After ferrite treatment (Figure 1b), significant color changes were observed. At a PVA concentration of 4.25 g/L, the solution exhibited a uniform orange-brown color, suggesting effective interactions between Fe²⁺/Fe³⁺ ions and PVA molecules [35]. As PVA concentration increased to 8.5 g/L, the solution color deepened while maintaining uniformity, demonstrating that the ferrite treatment remained effective within this concentration range. Even at higher PVA concentrations (17 g/L), although slight stratification was observed, the solution still displayed distinct treatment reaction characteristics. These phenomena preliminarily indicate that the modified ferrite process shows treatment potential for PVA wastewater across different concentrations. Notably, all PVA-containing samples exhibited significant color changes after treatment, contrasting sharply with the blank control (No PVA), which further confirms the effective action of ferrite treatment on PVA. These remarkable visual changes provide important insights for subsequent in-depth studies on PVA removal mechanisms while demonstrating the feasibility of this process for practical applications.

3.2. Effects of Modified Ferrite Treatment on PVA Molecular Structure

To investigate the treatment effectiveness of the modified ferrite process on PVA wastewater at different concentrations, FTIR analysis was performed on the filtrates before and after treatment. Figure 2 illustrates the FTIR spectral changes of solutions before and after treatment, with initial PVA concentrations ranging from 3 to 7 g/L. In the untreated samples, several characteristic peaks were observed: a strong absorption peak around 1000 cm⁻¹ corresponding to C–O stretching vibration, absorption peak clusters in the 1250–1500 cm⁻¹ range attributed to C–H bending vibrations, and a characteristic absorption at 1650 cm⁻¹ originating from C–C skeletal vibrations [35,36]. The broad peak at 2750–3000 cm⁻¹ represents O–H stretching vibrations. After modified ferrite treatment, samples at all concentrations exhibited similar spectral change patterns. Notably, at lower concentrations of 3 g/L and 4 g/L, the intensity of the C–O stretching vibration peak (1000 cm⁻¹) significantly decreased, indicating partial destruction of alcohol hydroxyl groups in PVA molecules. Simultaneously, the absorption peaks of C–H bending vibrations (1250–1500 cm⁻¹) also weakened, suggesting degradation of PVA molecular chains. However, as initial concentrations increased to 5–7 g/L, the degree of these characteristic peak changes gradually diminished. Particularly at 7 g/L, the spectra before and after treatment almost overlapped, indicating limited impact of the modified ferrite treatment on PVA molecular structure at higher concentrations.

Furthermore, the O–H stretching vibration peak near 3000 cm⁻¹ showed varying degrees of change across all concentration conditions, possibly related to the coordination between Fe ions and hydroxyl groups in PVA molecules during ferrite treatment [37]. This peak shape change was more pronounced at lower concentrations (3–4 g/L), further confirming the formation of Fe–O–C coordination bonds. This coordination interaction might be one of the key mechanisms leading to PVA removal. The FTIR analysis results demonstrate that the modified ferrite treatment can effectively alter PVA molecular structure, though its effectiveness is closely related to initial concentration. The treatment shows optimal results at lower concentrations (3–4 g/L), possibly due to more effective interactions between Fe ions and PVA molecules within this concentration range. As concentration increases, treatment effectiveness gradually decreases, suggesting that treating high-concentration PVA wastewater may require adjusted treatment conditions or multiple treatment cycles.

3.3. Crystal Structure Analysis of Filter Residues at Different PVA Concentrations

Figure 3 shows the XRD pattern analysis, revealing significant structural differences in precipitates formed under varying PVA concentrations. Within the PVA concentration range of 3–5 g/L, the diffraction patterns exhibit typical amorphous characteristics, primarily manifested as a broadened diffraction peak around 35°. This broadening feature indicates that the formed iron oxides possess either low crystallinity or nanoscale grain sizes, possibly due to the interfering effects of PVA molecules during the crystallization process [37,38]. When PVA concentration increases to 6 g/L, besides the broadened background, a sharp diffraction peak begins to emerge near 31.72°, potentially corresponding to the (220) crystal plane of Fe₃O₄. This change suggests that higher PVA concentrations may promote preferential growth of certain crystal planes. Notably, at the high concentration of 7 g/L, the diffraction pattern displays the most distinct crystalline characteristics, with prominent diffraction peaks at 31.7° and 45.4°, corresponding to the (220) and (440) crystal planes of Fe₃O₄, respectively. This concentration-dependent structural evolution reveals the dual role of PVA in iron oxide formation: at low concentrations, it primarily inhibits crystallization, while at higher concentrations, it may promote the growth of specific crystal planes through molecular arrangements. However, XRD data alone are insufficient to definitively determine the specific iron oxide phases, necessitating comprehensive assessment through magnetic testing and other characterization methods. Furthermore, the influence of PVA concentration on crystallinity provides potential pathways for controlling the structure and properties of the final products. The concentration-dependent structural evolution observed in this study reveals the dual role of PVA in iron oxide formation, which has not been previously reported. While Mohammed et al. [34] observed that PVA could inhibit Fe₃O₄ crystallization, they did not investigate the concentration-dependent effects. Our findings demonstrate that at low concentrations (3–5 g/L), PVA primarily inhibits crystallization, whereas at higher concentrations (6–7 g/L), it promotes the growth of specific crystal planes through molecular templating. This differs from the results of Gupta et al. [38], who studied Fe₃O₄/PVA composites but did not observe the concentration threshold effect we identified. This novel finding provides insights into controlling the crystallinity and morphology of magnetic materials through PVA concentration adjustment.

3.4. Visual Observation of H₂O₂ Pre-Oxidation Effects on PVA Removal

Figure 4 compares the reaction phenomena after ferrite treatment at different PVA concentrations (3–7 g/L) with and without H₂O₂ pretreatment. In systems without H₂O₂ pretreatment, increasing PVA concentrations led to substantial non-uniform iron oxide deposits at the reaction vessel walls and solution interfaces, particularly pronounced at higher concentrations of 6–7 g/L. This non-uniform deposition phenomenon suggests interference with ferrite formation in high-concentration PVA environments, potentially reducing treatment effectiveness. In contrast, systems with H₂O₂ pretreatment demonstrated significantly improved reaction characteristics. First, wall deposition was notably suppressed, with deposits remaining relatively uniform even under high PVA concentrations. Second, the solution phase exhibited more distinct stratification, with darker and more compact precipitates at the bottom, indicating that H₂O₂ pretreatment promoted the formation of more complete ferrite structures. This improvement likely stems from reactive species generated during H₂O₂ pre-oxidation, which not only regulated system pH but potentially facilitated partial PVA molecular degradation, enabling more effective subsequent reactions with iron ions. Notably, samples in the low-to-medium concentration range (3–5 g/L) with H₂O₂ pretreatment showed optimal reaction performance, characterized by clear phase separation and uniform precipitate formation. This indicates that within an appropriate concentration range, H₂O₂ pre-oxidation can effectively enhance the stability and efficiency of the ferrite treatment process. This finding provides important experimental evidence for optimizing high-concentration PVA wastewater treatment processes.

3.5. FTIR Analysis of H₂O₂ Pre-Oxidation Effects and Treatment Efficiency

The introduction of H₂O₂ significantly improved the treatment process through pH regulation and enhanced oxidation. Without H₂O₂, high-concentration PVA solutions (5–7 g/L) showed reduced pH values (pH 7–8), potentially affecting ferrite formation and PVA removal efficiency. After H₂O₂ addition, pH levels remained around 12 across all concentrations, providing more favorable conditions for ferrite formation. FTIR spectral analysis, as shown in Figure 5, revealed significant changes in the C–O stretching vibration peak at 1100 cm⁻¹ across all concentrations. At lower concentrations (3–4 g/L), this characteristic peak intensity notably decreased, indicating H₂O₂-promoted C–O bond cleavage in PVA molecules. Absorption peak changes in the 700–900 cm⁻¹ and 1200–1500 cm⁻¹ regions became more pronounced, suggesting that H₂O₂ affected both PVA main chain structure and side chain oxidative degradation. Although spectral changes were relatively minor at higher concentrations (6–7 g/L), treatment efficiency still improved compared to experiments without H₂O₂.

The mechanism of H₂O₂ action under alkaline conditions involves H₂O₂ dissociation at pH 12 generating HO²⁻, which reacts with Fe²⁺ to produce hydroxyl (·OH) and superoxide (O²⁻) radicals. These active radicals attack C–O bonds in PVA molecules, promoting bond cleavage. Subsequently, degraded PVA fragments more readily coordinate with iron ions, facilitating ferrite formation. Quantitative evaluation at the highest PVA concentration (7 g/L) demonstrated that single ferrite treatment achieved 37% COD removal. Implementation of a two-stage treatment strategy significantly improved COD removal to 83%. This enhancement likely results from initial H₂O₂ pre-oxidation and ferrite treatment breaking down PVA macromolecular structure, producing more easily degradable intermediates that readily coordinate with iron ions during the second treatment phase.

The H₂O₂ pre-oxidation approach in our study offers distinct advantages over previously reported PVA treatment methods. While previous research [22,23] focused on complete PVA degradation using persulfate-based oxidation, our approach combines partial oxidation with ferrite formation, achieving both treatment efficiency and valuable material recovery. Our two-stage treatment strategy achieved 83% COD removal for 7 g/L PVA, which is comparable to more energy-intensive methods like UV/H₂O₂ [11], but with the added benefit of producing recoverable magnetic materials. This represents a significant advancement over conventional electrocoagulation methods [21], which achieve PVA removal but do not facilitate resource recovery.

3.6. Characteristics of Filter Residues

3.6.1. Magnetic Response Analysis of Filter Residues in Solution and After Heat Treatment

To investigate the influence of different preparation conditions on the magnetic characteristics of filter residues, this study systematically compared magnetic property changes under four distinct treatment conditions (as shown in

Table 1. Magnetic characteristics of precipitates under different treatment conditions and temperatures.

Sample Code	Treatment Method	Magnetic Properties in Solution	100 °C Thermal Treatment	650 °C Thermal Treatment
A1	Single Ferrite Addition	No magnetic response	No magnetic response	Complete magnetic attraction
B1	H2O2 Pretreatment/ Ferrite Addition	Slight magnetic response	No magnetic response	Nearly complete magnetic attraction
A2	H2O2 Pretreatment/ Sequential Ferrite Addition	Partial precipitate magnetic separation	Nearly complete magnetic attraction	Complete magnetic attraction
B2	Sequential Ferrite Addition	Complete magnetic control of precipitates	Complete magnetic attraction	No magnetic response

Note: Magnetic properties were assessed by external magnetic field response; “Complete magnetic attraction” indicates that the sample can be fully attracted by a magnet; “No magnetic response” indicates no observable magnetic interaction.

). Single ferrite treatment samples (A1) showed no magnetic properties in both solution state and after 100 °C drying, only exhibiting complete magnetic attraction after calcination at 650 °C. This indicates that simple ferrite treatment alone cannot form magnetic iron oxide structures. With the introduction of H₂O₂ pretreatment (B1), slight magnetic response emerged in solution, suggesting that H₂O₂ addition promoted initial formation of magnetic species. However, this magnetic characteristic disappeared after drying at 100 °C, possibly due to the reorganization of nascent magnetic phases under mild heating. Interestingly, the sample regained near-complete magnetic attraction capability after calcination at 650 °C, indicating that high-temperature calcination favors the formation of stable magnetic phases. Samples treated with H₂O₂ pretreatment combined with stepwise ferrite addition (A2) demonstrated the most desirable magnetic evolution process. Partial magnetic separation of precipitates was observed in solution state, with magnetism further enhanced after 100 °C drying, and complete magnetic attraction achieved after 650 °C calcination. This progressive enhancement of magnetic properties suggests that the synergistic effect of H₂O₂ pretreatment and stepwise ferrite treatment facilitates the formation and growth of stable magnetic phases. Samples with pure stepwise ferrite addition strategy (B2) exhibited the strongest magnetic characteristics in both solution and dried states, achieving complete magnetic control of precipitates. However, unexpectedly, these samples completely lost their magnetic properties after calcination at 650 °C. This phenomenon suggests that without H₂O₂ pretreatment, magnetic phases formed through stepwise ferrite treatment may undergo unfavorable phase transitions at high temperatures. Supplementary videos demonstrate the magnetic response of filter residues prepared under different conditions in solution state and after heat treatment.

3.6.2. Visual Appearance Analysis

Figure 6 demonstrates the appearance changes of samples in their original state, after drying at 100 °C, and after calcination at 650 °C. In the original solution state, the four sample groups exhibited distinct color characteristics: A1 appeared light yellowish-brown, B1 showed brown, A2 displayed dark brown, and B2 presented a distinctive black color. These color variations likely reflect differences in oxidation states of ferrite treatment and the degree of interaction with PVA molecules under different processing conditions. Samples with H₂O₂ pretreatment (B1 and B2) showed notably darker colors, suggesting that pre-oxidation might promote ferrite oxidation and complex formation. After drying at 100 °C, all samples transformed into powdery substances while maintaining similar color characteristics, indicating that low-temperature drying primarily removed water content without significantly altering their chemical composition. Notably, at this stage, samples A1 and B1 exhibited weak magnetic properties, while A2 and B2 showed stronger magnetic responses, potentially due to the influence of different ferrite treatment strategies on magnetic oxide formation. Upon calcination at 650 °C, significant color changes occurred. Samples A1, B1, and A2 transformed to deep brownish-red, characteristic of the α-Fe₂O₃ phase, while maintaining good magnetic properties. However, sample B2 not only became lighter in color but also completely lost its magnetic properties after high-temperature calcination. This phenomenon suggests different phase transformation processes of iron oxides under high temperatures depending on the treatment conditions. These observations reveal the critical influence of H₂O₂ pretreatment and stepwise ferrite addition strategies on the final product properties.

3.6.3. Magnetic Properties Analysis of Filter Residues

A detailed magnetic characterization was conducted to understand the influence of PVA on the magnetic evolution of ferrite precipitates. Figure 7 shows magnetic hysteresis loops of samples under different thermal treatment temperatures. Ferrite samples prepared without PVA exhibited relatively weak magnetism, with saturation magnetization of only 0.123 emu/g after drying at 100 °C (Figure 7a), slightly increasing to 0.189 emu/g after calcination at 650 °C (Figure 7b), indicating the difficulty in forming strong magnetic phases under mild conditions through simple ferrite formation. With PVA introduction, sample B1 showed saturation magnetization of 0.152 emu/g after drying at 100 °C (Figure 7c), comparable to PVA-free samples. However, its magnetic properties were significantly enhanced after calcination at 650 °C, with saturation magnetization increasing to 2.43 emu/g (Figure 7d). This substantial magnetic enhancement suggests PVA’s influence on magnetic phase formation at high temperatures, with hysteresis loop shape changes indicating magnetic structure reorganization. Sample B2 demonstrated the most distinctive magnetic evolution characteristics. After drying at 100 °C, its saturation magnetization reached a remarkable 10.30 emu/g (Figure 7e), substantially higher than other samples. However, after calcination at 650 °C, its magnetism dramatically decreased to 0.679 emu/g (Figure 7f). This anomalous magnetic deterioration likely results from phase transitions at high temperatures, suggesting the formation of a unique metastable magnetic phase in the initial B2 sample [39]. The magnetic properties achieved in our study significantly surpass those reported in similar research. Ghanbari et al. [37] reported a maximum saturation magnetization of 5.2 emu/g for PVA-coated Fe₃O₄ nanoparticles, whereas our B2 sample exhibited 10.30 emu/g at 100 °C. This enhanced magnetic performance likely results from our H₂O₂ pre-oxidation method, which alters the interaction between PVA and iron ions. Similarly, Saleem et al. [29] employed different coating strategies for ferrite nanoparticles but achieved lower magnetic performance. The unique metastable magnetic phase formed in our B2 sample demonstrates the potential for tailoring magnetic properties through controlled processing conditions.

3.6.4. Correlation Between XRD Analysis and Magnetic Properties of Filter Residues

Figure 8 presents XRD patterns of samples under different treatment conditions after thermal processing at 100 °C and 650 °C. All samples exhibited characteristic diffraction peaks of Fe₃O_4, including (220), (311), (440), and (422) crystal planes. However, significant differences in crystallinity and phase composition were observed under different treatment conditions. After 100 °C thermal treatment (Figure 8a), sample B2 demonstrated the most complete Fe₃O₄ characteristic peaks, with clearly visible and intense diffraction peaks at 2θ = 30.1° (220), 35.5° (311), and 62.6° (440), corresponding to its excellent magnetic performance (10.30 emu/g). In contrast, samples A1 and B1 showed relatively weak peak intensities despite displaying Fe₃O₄ characteristic peaks, explaining their lower magnetization values (0.123 and 0.152 emu/g, respectively). Upon heating to 650 °C (Figure 8b), significant structural changes occurred. Fe₃O₄ characteristic peaks became more distinct in samples A1 and B1, with B1 showing notably increased magnetization to 2.43 emu/g, consistent with its enhanced crystallinity. However, although sample B2 maintained good crystallinity after high-temperature treatment, its magnetization dramatically decreased to 0.679 emu/g. This anomalous phenomenon might result from partial transformation of Fe₃O₄ to α-Fe₂O₃ at high temperatures, evidenced by the emergence of a new diffraction peak at 33.2°. These findings reveal the complex influence of preparation conditions and thermal treatment temperatures on the final product’s structure and magnetic properties.

3.6.5. Microscopic Morphology Analysis of Filter Residues

TEM analysis revealed the microstructural evolution of sample B1 under different thermal treatment temperatures. After drying at 100 °C (Figure 9a,b), two typical morphological features were observed: dispersed nanoparticles and fibrous network structures. This unique morphology likely results from the interaction between PVA molecular chains and iron oxides, where PVA molecules serve as templates guiding iron oxide growth into oriented structures. High-magnification images (Figure 9b) clearly show the distribution of nanoparticles within the PVA fibrous network, maintaining a high specific surface area and explaining the relatively weak magnetic properties (0.152 emu/g) at 100 °C. After heating to 650 °C (Figure 9c,d), significant morphological changes occurred. Rod-like structures emerged (Figure 9c), indicating temperature-promoted directional crystal growth. High-magnification imaging (Figure 9d) revealed notable particle aggregation forming larger clusters. These morphological changes corroborate XRD analysis and magnetic testing results: high-temperature calcination enhanced crystallinity while cluster formation contributed to magnetic property enhancement, explaining the significant increase in magnetization (2.43 emu/g). This microstructural evolution demonstrates PVA’s crucial role in material formation, providing growth templates at low temperatures and facilitating particle rearrangement during high-temperature decomposition.

3.6.6. Proposed Interaction Mechanism Between PVA and Iron Oxides

The interaction between PVA and iron oxides can be described by a multistage coordination–templating mechanism, as shown in Figure 10, that explains both the observed FTIR spectral changes and the XRD crystallinity patterns. In the initial coordination stage, PVA hydroxyl groups (–OH) act as electron donors that coordinate with Fe²⁺ and Fe³⁺ ions through their oxygen atoms [40,41,42]. This coordination is evidenced by the observed changes in the O–H stretching vibration peak (2750–3000 cm⁻¹) in our FTIR results, which became more pronounced at lower PVA concentrations (3–4 g/L). At lower PVA concentrations (3–5 g/L), a nucleation control mechanism dominates, where polymer chains adsorb onto the surface of forming iron oxide nuclei. The hydroxyl groups of PVA form hydrogen bonds with the surface hydroxyl groups of iron oxide nuclei, creating a steric barrier that inhibits crystal growth and leads to the amorphous characteristics observed in XRD patterns. This explains the broadened diffraction peak around 35° seen in our samples. Conversely, at higher PVA concentrations (6–7 g/L), a template-directed growth process occurs. PVA molecules form ordered structures due to intermolecular hydrogen bonding, creating templates that guide the growth of specific crystal planes. This explains the emergence of preferential growth along certain crystallographic directions, as evidenced by the sharp diffraction peak at 31.72° (corresponding to the (220) crystal plane of Fe₃O₄) in our XRD analysis. The thermal transformation pathway further supports this mechanism. Upon heating, the PVA template undergoes thermal decomposition, which explains the morphological evolution observed in TEM from dispersed nanoparticles within fibrous networks at 100 °C to rod-like structures and aggregated particles at 650 °C. This transformation follows the sequence: PVA–Fe complex → PVA-stabilized Fe₃O₄ nanoparticles → thermally reorganized Fe₃O₄/α-Fe₂O₃ structures. This proposed mechanism aligns with our experimental observations while providing a theoretical framework that connects the molecular interactions to the observed macroscopic properties of the magnetic precipitates.

4. Conclusions

This study developed a modified ferrite treatment process incorporating H₂O₂ pre-oxidation for treating high-concentration PVA wastewater. Through optimized H₂O₂ pre-oxidation and iron ion addition strategies, the process effectively maintained pH stability of the reaction system and achieved efficient removal of high-concentration PVA. A two-stage treatment achieved 83% COD removal for 7 g/L PVA solution. Microscopic morphology and structural analyses revealed PVA’s crucial role in material formation. Under 100 °C drying conditions, PVA molecular chains served as templates, guiding the formation of dispersed Fe₃O₄ nanoparticles and fibrous network structures. When heated to 650 °C, the material evolved into rod-like structures and aggregates due to PVA decomposition and crystal reorganization. This structural evolution directly influenced the material’s magnetic properties, with sample B2 exhibiting the highest magnetization (10.30 emu/g) at 100 °C, demonstrating excellent magnetic characteristics. The PVA concentration exhibited a threshold effect, transitioning from inhibiting crystallization at lower concentrations to promoting specific crystal plane growth at higher concentrations. The findings from this study provide a new technical approach for treating high-concentration organic wastewater while opening pathways for resource utilization in wastewater treatment through the recyclability of magnetic precipitates. This strategy, combining wastewater treatment with functional material preparation, holds significant scientific and practical value for developing environmentally friendly wastewater treatment technologies.