Advanced Degradation of Aniline in Secondary Effluent from a Chemical Industry Park by Cobalt Ferrite/Peracetic Acid System

Abstract

The residual emerging pollutants in secondary effluent from a chemical industry park contain potential risks for natural waters. Herein, the cobalt ferrite/peracetic acid system was employed to destroy aniline, a typical emerging pollutant, with a reaction rate of 0.0147 min⁻¹ at pH 7.0. Singlet oxygen (¹O₂) served as the predominant reactive species for aniline degradation, with superoxide radicals (O₂⁻) and organic radicals (R-O) acting in secondary roles. The valence transition between Co(II) and Co(III) on the CoFe₂O₄ surface played a determining role in the reaction progression. The presence of anions and humic acids with low concentrations had minimal impact on aniline removal. Additionally, the CoFe₂O₄ catalyst demonstrated excellent recyclability, maintaining a pollutant removal rate above 93% over five consecutive cycles. Lastly, the CoFe₂O₄/PAA system demonstrates effective treatment of typical pollutants, including phenolic compounds, pesticides, antibiotics, and dyes, achieving removal rates of 77.48% to 99.99%. Furthermore, it significantly enhances water quality in the treatment of actual secondary effluent, offering a novel theoretical foundation and practical insights for applying this catalytic system in wastewater treatment.

1. Introduction

Aniline, an essential organic raw material and intermediate, is extensively used in petroleum processing, dyestuff production, plastics manufacturing, rubber synthesis, paint production, tanning, textiles, and other industrial sectors [1,2,3,4,5]. Despite its relatively low concentration in the environment, aniline is susceptible to methemoglobinemia and liver, kidney, and skin damage, leading to acute or severe poisoning and significant toxicity to individual organisms and ecosystems. Pollution caused by the release of aniline into the environment is persistent in nature, difficult to degrade naturally, and bioaccumulates through food chains, leading to a variety of diseases [1,3,4,6]. As a result, the U.S. Environmental Protection Agency has listed aniline as one of the 129 priority environmental pollutants, and it is also included in China’s ‘Black List of Environmental Priority Pollutants’ [4,6]. Hence, aniline must be eliminated before being released into the environment [7]. Moreover, due to its resistance to conventional water treatment, aniline cannot be effectively removed through a biological process [2,8]. Consequently, multiple advanced oxidation processes (AOPs), such as photocatalytic-coupled catalytic ozonation, have been employed to degrade aniline, though their operating costs are high [9]. Hence, there is an immediate need to develop a versatile, eco-friendly, and cost-efficient AOPs method for aniline removal [7,9,10].

Peracetic acid (PAA) offers strong sterilization, low pH dependence, and lower toxicity of disinfection by-products [11,12]. Additionally, PAA has high redox potential (1.96 V) and allows for the degradation of pollutants [13]. PAA can be activated via UV radiation, heating, or transition metals, producing reactive oxygen species (ROS) like hydroxyl radicals (·OH) and organic radicals (CH₃C(O)O·, CH₃C(O)OO·), owing to its easily activatable O–O bond [13,14,15,16]. PAA-based AOPs offer advantages over traditional AOPs, including lower pH dependence, strong resistance to interference, and fewer by-products from disinfection [11,13,15,17]. Moreover, transition metals are regarded as optimal activators due to their universal applicability, energy efficiency, and high catalytic performance [18,19,20].

Among transition metals (Fe(II), Mn(II), Cu(II)), Co(II) exhibits the highest PAA activation efficiency [17,18,21]. However, the main challenges faced by homogeneous activation methods are the inability to recycle and reuse, secondary pollution, and the toxicity of metals, and the free metal ions remain in water, increasing the cost of catalyst addition and potentially causing problems for human health [21,22,23]. Additionally, they are excellent catalytic materials for PAA. Recently, cobalt ferrite (CoFe₂O₄) has demonstrated excellent potential as a PAA activator because of its high structural stability, low metal ion leaching, and bimetallic composition [24,25]. Although the CoFe₂O₄/PAA system has been explored for pollutant removal, most research focuses on individual contaminants, like sulfamethoxazole [13], with limited investigation in real wastewater applications. Limited information exists on which activators are present in the system and how CoFe₂O₄ activates PAA to degrade specific contaminants.

This study systematically investigates the catalytic activity and underlying mechanism of the CoFe₂O₄/PAA system for aniline degradation in real wastewater. First, experiments were conducted to optimize key parameters, including CoFe₂O₄ dosage, PAA concentration, and initial pH, to improve the efficiency of aniline degradation. Subsequently, the influence of water matrix composition on pollutant degradation was further analyzed, and the primary active species involved in aniline removal were identified. Additionally, this study also investigated the structural stability and recyclability of CoFe₂O₄ during PAA activation. Lastly, the catalytic performance of CoFe₂O₄ in PAA activation was thoroughly evaluated for typical pollutants, including phenolic compounds, pesticides, antibiotics, and dyes. Furthermore, the potential of this catalytic system for treating actual secondary effluent was assessed, proposing an efficient strategy for advanced wastewater treatment with broad application prospects.

2. Results and Discussion

2.1. Characterizations of the Catalysts

According to the previous research of our group, scanning electron microscopy (SEM) analysis revealed that CoFe₂O₄ is an irregular solid with uneven particle sizes around 100 nm [26]. Additionally, energy-dispersive spectroscopy (EDS) elemental mapping confirms homogeneous spatial distribution of Co, Fe, and O across nanoparticle surfaces [26].

The XRD spectrum of CoFe₂O₄ displays seven prominent peaks, corresponding to the standard CoFe₂O₄ diffraction pattern (PDF#22-1086) (Figure S1) [27]. The characteristic peaks at 2θ = 18.57°, 30.39°, 35.78°, 43.51°, 53.83°, 57.40°, and 62.79° are indexed to the (111), (220), (311), (400), (422), (511), and (440) Bragg planes, indicating the crystalline structure and high crystallinity of CoFe₂O₄. The FT-IR spectra of CoFe₂O₄ (Figure S2) show absorption peaks near 3416 cm⁻¹ and 1398 cm⁻¹, attributed to OH stretching of surface-adsorbed H₂O [27]. Furthermore, the characteristic peak at 620 cm⁻¹ is assigned to symmetric stretching of metal–oxygen (M–O) bonds, providing additional evidence for the formation Co/Fe-O [25,27].

2.2. Performance of CoFe₂O₄-Activated PAA Oxidation for the Degradation of Aniline

2.2.1. Aniline Degradation by CoFe₂O₄/PAA System Degradation

Figure 1a compares the degradation efficiency of aniline by different reaction systems. Results showed that aniline was not significantly degraded in the PAA-only system, indicating PAA alone was ineffective for its oxidative degradation. In the CoFe₂O₄ system alone, the aniline removal was limited, with a final rate of only 8.2%. This suggests CoFe₂O₄ had poor physical adsorption capacity for aniline. The aniline removal rate in the CoFe₂O₄/H₂O₂ system was only 29.3%, indicating CoFe₂O₄ activation of H₂O₂ was ineffective in promoting aniline degradation. The CoFe₂O₄/PAA system exhibits excellent degradation performance, with a removal rate of 93.4% for aniline, which was attributed to the activation of the decomposition of PAA after chemisorption on the surface of CoFe₂O₄ to generate highly reactive free radicals, such as hydroxyl radicals (OH·) and organic radicals (CH₃C(O)O·, CH₃C(O)OO·), which facilitate the degradation of aniline [13].

2.2.2. Effects of Reaction Factors

The effect of PAA concentration in the range of 0.1–5 mM on aniline degradation is shown in Figure 1b. As shown in the figure, aniline degradation increased with higher PAA concentrations; however, the rate of increase slowed. Increasing PAA concentration from 0.1 mM to 2 mM significantly enhanced aniline removal, rising from 26.9% to 98.1%. However, when PAA concentration increased to 5 mM, the aniline removal rate dropped to 89.6%. Increasing PAA concentration in the system enhances the utilization of active sites on CoFe₂O₄, generating more ROS. However, high PAA concentrations saturated the CoFe₂O₄ surface, reducing ROS generation and limiting the aniline degradation rate. Furthermore, an excess of PAA would undergo reaction with ROS within the system, thereby competing with the pollutants and resulting in a reduction in available ROS for pollutant degradation [28].

Figure 1c shows the effect of different CoFe₂O₄ concentrations on aniline degradation. Aniline degradation increased with CoFe₂O₄ dosage between 0 and 0.10 g/L. Additionally, no further improvement in aniline degradation was observed when CoFe₂O₄ dosage increased to 0.20 g/L. This occurs because an increase in CoFe₂O₄ dosage proportionally increases the number of active sites on its surface. This, in turn, accelerates the decomposition of PAA, thereby facilitating the production of free radicals, which are instrumental in the pollutant degradation process. An excess of CoFe₂O₄ dosage may have two adverse effects. Firstly, CoFe₂O₄ may scavenge free radicals, which could hinder pollutant degradation [29]. Secondly, agglomeration and precipitation of CoFe₂O₄ may reduce the surface active sites, thereby affecting the further decomposition of oxidants into free radicals [30].

As shown in Figure 1d, the degradation of aniline in the CoFe₂O₄/PAA system varies with pH, highlighting the effect of different initial pH levels on its removal efficiency. Aniline degradation in the CoFe₂O₄/PAA system was significantly better under neutral conditions compared to acidic or alkaline conditions. The pH value influences the morphological distribution of PAA in solution. The pK_a of PAA is 8.2 (Figure S3) [31], which suggests that PAA mainly remains in its neutral form (PAA⁰) at pH below 8.2, while it transitions to the deprotonated form (PAA⁻) at pH above 8.2. Additionally, PAA⁰ is more oxidative than PAA⁻, and thus more readily activated. In the meantime, pH exerts an influence on the charge borne by the surface of CoFe₂O₄. The isoelectric point of CoFe₂O₄ is 6.3 (Figure S4), and when the pH of the solution is less than 6.3, the surface of CoFe₂O₄ undergoes a protonation reaction, thereby acquiring a positive charge. Conversely, when the pH of the solution is greater than 6.3, CoFe₂O₄ undergoes a deprotonation reaction, resulting with a negative charge. Thus, at pH > 8.2, most PAA exists in its deprotonated form (PAA⁻), creating a negative surface charge on CoFe₂O₄ that leads to isoelectric repulsion. This reduces the contact opportunity between PAA and CoFe₂O₄, affecting the decomposition of PAA and the generation of free radicals. This, in turn, affects the degradation of aniline. Similarly, under acidic conditions, CoFe₂O₄ becomes protonated, and the surface gains a positive charge. During this process, the O–O bond of PAA is susceptible to forming a hydrogen bond with H⁺, which results in the formation of a positively charged PAA. This induces electrostatic repulsion between PAA and CoFe₂O₄ surfaces, which in turn affects the degradation of aniline [32].

Based on the research of Chen et al. [33], a molar ratio of 50:1 between persulfate and aniline was used to remove aniline. In this experiment, by optimizing the amount of oxidant added, excellent aniline degradation efficiency was still achieved while significantly reducing the amount of oxidant used. Compared with the sequencing batch reactor process adopted by Jiang et al. [34], although this system can achieve the degradation of aniline, it has limitations such as long reaction cycle and low kinetic rate. The catalytic system developed in this study achieved higher pollutant removal rates while significantly reducing reaction time, demonstrating superior degradation kinetics performance.

2.3. Influence of Water Matrix

Common aqueous constituents such as anions and dissolved organic matter influence aniline degradation through two primary mechanisms: ROS scavenging and metal ion complexation. As demonstrated in Figure 2, the existence of Cl⁻, SO₄²⁻, and NO₃⁻ has an insignificant impact on the degradation of aniline. SO₄²⁻ does not undergo a reaction with the free radicals (OH·, R-O·) present in the reaction system; consequently, SO₄²⁻ at high concentrations shows negligible impact on aniline degradation efficiency. The impact of chloride ions can be observed in two key areas: on the one hand, chloride can directly interact with PAA to produce HClO as “a secondary oxidant”. (Equation (1)) [32]. On the other hand, chloride can additionally participate in free radicals (OH·, R-O·) in the system, generating chlorine-containing active species (Cl·, Cl₂⁻·, and HClO⁻·, etc.) (Equations (2)–(5)). These species exhibit varying sensitivities to different kind of pollutants [35].

Cl⁻ + CH₃C(O)OOH → HClO + CH₃C(O)O⁻

(1)

Cl⁻ + OH· → HClO·⁻

(2)

Cl⁻ + CH₃C(O)OO· + H⁺ → Cl· + CH₃C(O)OOH

(3)

Cl· +H₂O ↔ HClO·⁻ + H⁺

(4)

Cl· + Cl⁻ → Cl₂·⁻

(5)

Our results demonstrate that low concentrations of NO₃⁻ exhibit negligible effects on aniline degradation efficiency in the CoFe₂O₄/PAA system, whereas a slight inhibition of the degradation of aniline was observed with an increase in NO₃⁻ concentration [36]. This behavior likely stems from NO₃⁻ radical scavenging capacity, as NO₃⁻ efficiently reacts with system-generated free radicals, resulting in the production of NO₃. This subsequently competes with the contaminants for ROS, thereby inhibiting the degradation of aniline.

The presence of H₂PO₄⁻ significantly inhibited the degradation of aniline. The decrease in the removal rate was associated with two distinct mechanisms: firstly, the free radical scavenging and the production of phosphate radicals [37], and secondly, the formation of precipitates in neutral environments when H₂PO₄⁻ was complexed with CoFe₂O₄ [37,38]. These precipitates exhibited low activity when reacting with PAA, thereby inhibiting the degradation of aniline.

It is evident that HCO₃⁻ demonstrates significant inhibition of aniline degradation. Furthermore, the inhibitory effect is found to be stronger with an increase in HCO₃⁻ concentration. HCO₃⁻ is a common ·OH scavenger with its negligible reactivity with R-O·, therefore, aniline degradation inhibition is not due to competition with reactive radicals [17]. Additionally, CoFe₂O₄ forms a Co–HCO₃ complex with HCO₃⁻, affecting PAA activation by CoFe₂O₄, and thus reducing free radical generation and inhibiting aniline degradation [17,39]. Furthermore, HCO₃⁻ functions as a buffer, influencing the pH of the reaction system [40]. The weak base condition is unfavorable for aniline degradation in the CoFe₂O₄/PAA system.

Incorporation of HA to the CoFe₂O₄/PAA system significantly inhibited the degradation of aniline, and this effect intensified as HA concentration increased. This inhibition primarily stems from HA radical scavenging capacity, which will compete with the target pollutants for free radicals and accelerate the depletion of free radicals, thus competitively reduces radical availability for aniline degradation [35]. Additionally, HA typically possesses -OH and -COOH functional groups, which are readily adsorbed onto the surface of CoFe₂O₄, blocking PAA activation [24,28].

2.4. Identification and Analysis of Reactive Species

The ROS present in the reaction system were identified through the use of free radical inhibition experiments. The PAA-based advanced oxidation system generates multiple reactive species, including ·OH, ·R-O, ·O₂⁻, ¹O₂ and high-valent metal substances (Co (IV), Fe (IV)) [17,41]. TBA was found to scavenge ·OH(k = 6.0 × 10⁸ M⁻¹s⁻¹) effectively [16], but not the aforementioned other radicals. Thus, ·OH to the overall degradation process can be quantitatively evaluated through controlled quenching experiments using excess TBA. As illustrated in Figure 3a, with 200 mM TBA, the degradation rate of aniline has hardly decreased, indicating that ·OH is not the primary ROS in the system. The addition of TBA expedites the forward decomposition of PAA, resulting in the consumption of a portion of the PAA (Figure 3b).

The reaction rate of CHCl₃ with ·O₂⁻ (3.0 × 10¹⁰ M⁻¹s⁻¹) was much higher than that of other ROS, making it suitable for evaluating the influence of ·O₂⁻ in the degradation process [38]. Degradation of aniline was inhibited by excess CHCl₃ (Figure 3a), while CHCl₃ also accelerated PAA decomposition and consumed part of it (Figure 3b). It has been reported that ·O₂⁻ acts as an intermediate in advanced oxidation systems, including the generation of ¹O₂ by ·O₂⁻.

MeOH is commonly used to scavenge OH· (k = 9.7 × 10⁸ M⁻¹s⁻¹) and R-O·, allowing the roles of OH· and R-O· to be distinguished using TBA and MeOH [16]. Clearly, an excess of MeOH reduces the removal efficiency of aniline (Figure 3a). Therefore, to confirm that the inhibitory effect of MeOH was due to the scavenging of R-O· rather than ·OH, the contributions of these radicals to aniline degradation were analyzed using specific probes: pCBA (with a rate constant of 5.0 × 10⁹ M⁻¹s⁻¹ for ·OH) and NAP (with a rate constant of 9.0 × 10⁹ M⁻¹s⁻¹ for R-O) [42]. According to Figure S5, NAP underwent complete degradation in just 10 min, while only 10% of pCBA reacted after 60 min. This stark contrast demonstrates that R-O· radicals, rather than ·OH, are the primary reactive species generated in the CoFe₂O₄/PAA system.

In addition, CH₃C(O)O· and CH₃C(O)OO· are the R-O· in PAA-AOPs that play a key role in pollutant degradation. Typically, CH₃C(O)O· is inherently unstable and rapidly self-decomposes into CH₃· (k = 2.3 × 10⁵ M⁻¹s⁻¹). Additionally, it can react with PAA to generate CH₃C(O)OO· [14,43]. In addition, CH₃C(O)O· is less reactive towards most organic compounds, whereas CH₃C(O)OO· has a stronger oxidizing ability. To provide additional evidence for the involvement of CH₃C(O)OO· in aniline degradation, Mn²⁺ quenching experiments were systematically performed. Mn²⁺ could react quickly with CH₃C(O)OO· via electron transfer (k = 10⁵–10⁶ M⁻¹s⁻¹) but has no effect on OH· and CH₃C(O)O· [14]. As evidenced by Figure S6, the introduction of Mn²⁺ led to a pronounced suppression of aniline degradation, and the removal of aniline decreased from 96.54% to 70.69% within 60 min when 1.5 mM Mn²⁺ was added to the CoFe₂O₄/PAA system. Although Mn²⁺ could slightly promote the decomposition of PAA, Mn²⁺ addition showed minimal impact on aniline degradation (Figure S7). Thus, CH₃C(O)OO· is important for the degradation of aniline.

FFA (k = 1.2 × 10⁸ M⁻¹s⁻¹ with¹O₂) was employed as a selective scavenger to assess the contribution of singlet oxygen to pollutant degradation [42]. The addition of ¹O₂ significantly inhibited aniline degradation compared to other quenchers (Figure 3a), indicating that ¹O₂ is critical in the degradation process. Consequently, adding FFA accelerated PAA decomposition, leading to PAA depletion (Figure 3b).

PMSO was employed as a selective probe to detect the generation of high valence metal species (Co(IV) and Fe(IV)) in the catalytic system. It has been reported that PMSO is converted to PMSO₂ with high valence metallic substances through the oxygen atom transfer pathway, while the free radicals form hydroxylation products [44]. The results confirmed the presence of PMSO₂ in the CoFe₂O₄/PAA system, but PMSO conversion to PMSO₂ was only 2.84% (Figure S8). In addition, the removal of aniline decreased by only 1.65% when an excess of PMSO was introduced (Figure S9). The results show that high valence metals (Co(IV), Fe(IV)) play little role in aniline degradation.

The formation of reactive oxygen species (OH·, R-O·, ¹O₂, and O₂⁻·) in the CoFe₂O₄/PAA system was further verified by EPR experiments [45,46]. DMPO served as the spin trap for detecting OH· and O₂⁻·, while DIPPMPO and TEMP were used for R-O· and ¹O₂, respectively. As shown in Figure 3d–f, DIPPMPO-R-O·, TEMP-¹O₂ (1:1:1), and DMPO- O₂⁻· adducts were detected in the CoFe₂O₄/PAA system. No DMPO-OH· signal was observed (Figure 3c), and EPR signal intensity at 5 min was stronger than at 2 min, indicating that R-O· and ¹O₂ were produced.

In conclusion, quenching experiments, chemical probe analyses, and EPR tests revealed that ¹O₂ is the primary ROS in the CoFe₂O₄/PAA system, O₂⁻· plays a secondary role, and CH₃C(O)OO· contributes to aniline degradation.

2.5. Activation Mechanisms

To better understand the activation mechanism of PAA by CoFe₂O₄, XPS analysis was conducted on the catalyst before and after the reaction. The full-spectrum XPS scans (Figure 4a) confirmed the presence of Co, Fe, and O in CoFe₂O₄. As shown in Figure 4b, the Fe 2p spectra displayed characteristic peaks at 710.24 eV (Fe²⁺) and 712.82 eV (Fe³⁺). After the reaction, the Fe(II) content decreased from 56.19% to 51.43%, while Fe(III) increased from 43.81% to 48.57%, indicating the involvement of a redox cycle between ≡Fe(III) and ≡Fe(II) on the catalyst surface [24].

The Co 2p XPS spectra before and after reaction (Figure 4c) revealed characteristic peaks at 779.57 eV (Co 2p3/2) and 795.1 eV (Co 2p1/2) for Co(III), and 781.08 eV (Co 2p3/2) and 796.8 eV (Co 2p1/2) for Co(II). Post-reaction analysis showed a decrease in Co(III) content from 49.74% to 46.21% with a corresponding increase in Co(II) from 50.26% to 53.79%, confirming the ≡Co(II)/≡Co(III) redox cycle [24]. Notably, while the ≡Fe(III)/≡Fe(II) cycle contributed minimally to PAA activation compared to the cobalt cycle, iron presence facilitated Co(III)→Co(II) conversion (Equations (6)) [47], enhancing the catalyst electron transfer capacity. The O 1s spectra (Figure 4d) displayed three components: lattice oxygen (529.57 eV), surface hydroxyl oxygen (531.05 eV), and adsorbed oxygen (532.49 eV). Reaction-induced changes included lattice oxygen decrease (68.92%→64.59%), with increases in surface hydroxyl (19.58%→22.58%) and adsorbed oxygen (11.5%→12.83%). These shifts reflect Co(III) reduction and surface hydroxyl group formation (Co-OH/Fe-OH, Equations (7)–(9)) [25,47].

Fe(II) + Co(III)→ Co(II) + Fe(III)

(6)

≡Fe³⁺ + H₂O→ ≡FeOH²⁺ + H⁺

(7)

≡Co²⁺ + H₂O→ CoOH⁺ + H⁺

(8)

≡Co²⁺ + ≡FeOH²⁺→ CoOH⁺ + Fe³⁺

(9)

XPS analysis confirms cobalt’s pivotal role in PAA activation within the CoFe₂O₄ system. The observed aniline degradation primarily stems from ROS generation (¹O₂ ·O₂⁻ and CH₃C(O)OO·), suggesting the following mechanism: (1) Surface ≡Co(II) transfers an electron to PAA, producing CH₃C(O)O· while oxidizing to ≡Co(III) (Equation (10)) [25,47]; (2) ≡Co(III) is subsequently reduced back to ≡Co(II) by accepting electrons from PAA, forming CH₃C(O)OO· (Equation (11)). This ≡Co(II)/≡Co(III) redox cycle sustains R-O· production for pollutant degradation. Concurrently, H₂O₂ reacts with R-O· species (CH₃C(O)O·, CH₃C(O)OO·) to yield HO₂· (Equations (12) and (13)) [25,47], which undergoes deprotonation to form ·O₂⁻ (Equation (14)) and subsequently dimerizes to ¹O₂ (Equation (15)) [24,47]. Although iron does not directly activate PAA, its presence enhances interfacial electron transfer between cobalt species, thereby boosting catalytic efficiency.

≡Co(II) + CH₃C(O)OOH→ ≡Co(III) + CH₃C(O)O· + OH⁻

(10)

≡Co(III) + CH₃C(O)OOH→ ≡Co(II) + CH₃C(O)OO· + H⁺

(11)

H₂O₂ + CH₃C(O)O·→ HO₂·+ CH₃C(O)OH

(12)

H₂O₂ + CH₃C(O)OO·→ HO₂·+ CH₃C(O)OOH

(13)

HO₂⁺→ ·O₂⁻ + H⁺

(14)

2O₂⁻ + H₂O→ ¹O₂ + H₂O₂ + OH⁻

(15)

2.6. Reusability and Stability of the CoFe₂O₄

The stability of the catalytic performance of CoFe₂O₄ was evaluated through the implementation of cyclic experiments, and the subsequent catalytic degradation experiments carried out the alkaline desorption of the reacted CoFe₂O₄. Figure 5a illustrates that the CoFe₂O₄/PAA system demonstrated consistent degradation of aniline, with the removal rate of aniline by CoFe₂O₄/PAA remaining above 95% after five consecutive cycles. Additionally, the highest cobalt ion leaching was around 0.0547 mg/L over five cycles (Figure 5b), while no iron ions were detected. The cobalt concentration is significantly below the reference standard limit of 1.0 mg/L set by the Environmental Quality Standard for Surface Water. The aforementioned results demonstrate that CoFe₂O₄ is a highly recoverable and stable PAA activator.

XRD and FT-IR analyses of CoFe₂O₄, both before and after the reaction, were compared to assess its structural stability. The positions of the XRD characteristic diffraction peaks before and after the reaction of CoFe₂O₄ are consistent, and their intensities do not change significantly (Figure 5c), indicating that CoFe₂O₄ exhibits good structural stability during the reaction. As illustrated in Figure 5d, the position of the FT-IR absorption peak before and after the reaction of CoFe₂O₄ showed no significant change. However, the intensity of the absorption peak associated with the bending vibration of ·OH in CoFe₂O₄ increased significantly after the reaction, likely due to the formation of Co–OH and Fe–OH complexes on its surface during the reaction. The increase in the surface hydroxyl oxygen content of O 1s observed in the XPS comparative analysis before and after the reaction of CoFe₂O₄ further confirms this observation. The excellent catalytic properties and structural stability of CoFe₂O₄ create favorable conditions for its practical application in CoFe₂O₄/PAA systems for water treatment.

2.7. Practical Application

2.7.1. Degradation of Different Organic Pollutants by CoFe₂O₄/PAA

Given the complex composition of biochemical tail water containing diverse organic matter, and in order to more effectively evaluate the ability of CoFe₂O₄ to activate PAA, four organic pollutants, ATZ, SMX, phenol, and acid red 14, were degraded under identical reaction conditions ([PAA] = 1 mM, [CoFe₂O₄] = 100 mg/L, [pollutant] = 100 μM, pH = 7.0, T = 25◦ C, reaction time = 60 min) (as pesticides, antibiotics, phenolic pollutants, and dyes). As shown in Figure 6a–d, the degradation efficiencies of ATZ, SMX, phenol, and acid red 14 were 77.48% (k_obs = 0.0129 min⁻¹), 87.02% (k_obs = 0.0122 min⁻¹), 91.74% (k_obs = 0.0364 min⁻¹), and 99.99% (k_obs = 0.0874 min⁻¹). Experimental evidence confirmed that ROS-mediated degradation dominated the removal of all four pollutants, with adsorption and direct PAA oxidation playing negligible roles. These findings demonstrate that CoFe₂O₄ effectively activates PAA, making it a highly efficient peroxyacid oxidant for organic pollutant degradation.

2.7.2. Degradation of Aniline in Secondary Effluent from the Chemical Industry Park

To evaluate practical applicability, we tested aniline removal in real wastewater (secondary effluent from a chemical industry park; characteristics in Table S3). As shown in Figure 7a, the system achieved 79.6% aniline removal at 1 mM PAA ([CoFe₂O₄] = 100 mg/L, [aniline] = 100 μM, pH = 7.0, T = 25 °C). Increasing PAA to 2.0 mM enhanced removal to over 93% (k_obs = 0.0134 min⁻¹). The presence of competing organic compounds in the actual wastewater likely consumed available PAA through parallel oxidation pathways, making oxidant dosage the rate-limiting factor in aniline degradation. This competitive effect explains the observed enhancement in removal efficiency with increased PAA concentration. Post-treatment analysis revealed increases in both TOC and COD values. This increase is attributed to the presence of acetic acid in the PAA solution and the production of fine carbon sources during the activation process. Notably, the C/N ratio rose from 8.73 to 52.44 (Figure 7b), indicating a significant improvement in wastewater biodegradability after treatment with the CoFe₂O₄/PAA system ([CoFe₂O₄] = 100 mg/L, [aniline] = 100 μM, pH = 7.0 ± 0.05, T = 25 °C).

We monitored organic matter removal by measuring UV₂₅₄ absorbance and analyzing fluorescence profiles via three-dimensional excitation-emission matrix (3D-EEM) spectroscopy. According to the UV–Vis absorption spectrum at 254 nm wavelength (Figure 7b), the UV₂₅₄ concentration in raw water was 0.263 cm⁻¹, which decreased to 0.128 cm⁻¹ by the end of the reaction, indicating a removal rate of 51.33% and effective pollutant elimination. As shown in Figure 7c, aniline showed three fluorescence peaks in the biochemical tail water, peak B and peak C fluorescence mainly from the biochemical tail water, belonging to the humic acid-like substances, which is an important part of the natural organic matter, and peak A is the characteristic peak of aniline, which belongs to the aromatic amines, in which the fluorescence intensity of characteristic peaks A, B, and C exhibited significant attenuation, transitioning from high-intensity regions (red/yellow) to low-intensity regions (green) in the 3D-EEM spectra (Figure 7c–f), demonstrating effective removal of the corresponding organic fractions [48]. These analyses demonstrate that the CoFe₂O₄/PAA system effectively removes pollutants and improves wastewater quality, showing great potential for real-world wastewater treatment applications.

3. Materials and Methods

3.1. Chemicals and Reagents

All chemicals were used as received without further purification. Aniline, phenol, atrazine (ATZ), sulfamethoxazole (SMX), furfuryl alcohol (FFA), N,N-diethyl-p-phenylenediamine (DPD), humic acid (HA), p-chlorobenzoic acid (pCBA), and naproxen (NAP) were obtained from Aladdin Corporation Ltd. (Shanghai, China). Tert-butanol (TBA), methanol (MeOH), trichloromethane (CHCl₃), acid red 14, methyl phenyl sulfoxide (PMSO), and methyl phenyl sulfone (PMSO₂) were purchased from McLean Biochemical Company (Shanghai, China). HPLC-grade acetonitrile (CH₃CN) and formic acid (HCOOH) were acquired from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The CoFe₂O₄ nanoparticles (~100 nm) and PAA precursor reagents were supplied by Aladdin Co., Ltd. and Kemiou Chemical Reagent (Tianjin, China), respectively. The PAA working solution was prepared by reacting H₂O₂ with acetic acid (molar ratio 0.7:1) for 24 h. Complete chemical information including CAS numbers is provided in Table S1.

3.2. Degradation Experiments

All degradation experiments were conducted in 250 mL glass beakers containing 100 mL of contaminant solution at 25 °C, maintained using a water bath. The reactions were performed with constant stirring (150 rpm) using an SYC-2A shaker (Shanghai Bonding Instrument, Shanghai, China). After adding CoFe₂O₄ and adjusting the pH (0.5 M H₂SO₄/NaOH), PAA was introduced to initiate the reaction (Figure 8). Aliquots (1.5 mL) were collected at predetermined time intervals (0–60 min), immediately filtered through 0.22 μm PTFE membranes, and quenched with 0.10 mM Na₂S₂O₃ prior to HPLC analysis.

ROS were identified using free radical trapping agents (TBA, MeOH, CHCl₃, FFA) and verified with probe compounds (p-CBA, NAP). To assess CoFe₂O₄ stability, the catalyst was treated with alkali to remove adsorbed substances, rinsed with deionized water, followed by drying at 60 °C in air oven. The effects of co-existing inorganic anions and different water types were tested by adding anions (Cl⁻, SO₄²⁻, NO₃⁻, HCO₃⁻, H₂PO₄⁻), humic acid (HA), and replacing deionized water with actual biochemical tailwater to study their impact on aniline degradation. All experiments were performed in two sets, with error bars representing standard deviations.

Aniline stock solution (10 mM) was prepared in distilled water with continuous stirring and stored at 4 °C. The experimental solution was prepared by diluting the stock solution. The secondary effluent was collected from Yangzhou Zhonghua Huayu Environmental Protection Co. in Yangzhou chemical industry park (Yangzhou, China). The kinetic analysis is detailed in Text S1.

3.3. Analytical Methods

Structural characterization was performed through X-ray diffraction (Thermo Scientific K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA) using Cu Kα radiation (1.542 Å) with a 5–85° 2θ range and 5°/min scan speed. Surface functional groups were characterized by FT-IR spectroscopy (Thermo Scientific Nicolet iS20, Thermo Fisher Scientific, Waltham, MA, USA) employing KBr pellet method, with spectra recorded from 4000 to 400 cm⁻¹. Surface chemical states were investigated by X-ray photoelectron spectroscopy on the K-Alpha platform. The zeta potential of CoFe₂O₄ was measured with a zeta potential meter (Nano ZS90, Malvern Panalytical, Ltd., Malvern, UK).

The PAA stock solution was titrated weekly for calibration, with peroxide concentration determined by iodimetry and hydrogen peroxide concentration by potassium permanganate titration, to calculate the PAA concentration [35]. Residual PAA concentration was measured via DPD spectrophotometry [31]. The concentrations of aniline, pCBA, NAP, PMSO, PMSO₂, SMX, ATZ, and phenol used HPLC (Agilent 1260, Agilent, Santa Clara, CA, USA) equipped with an EC-C18 column (detailed parameters in Table S2). The absorbance of acid red 14 was measured spectrophotometrically at the wavelength of 515 nm. CoFe₂O₄ is a non-uniform solid with uneven particle size. Cobalt and iron ion concentrations were measured using ICP-AES for analysis. The pH measurements were conducted using a Mettler Toledo FE28 pH meter (Mettler-Toledo Instruments, Shanghai, China). Reactive oxygen species were identified through electron paramagnetic resonance spectroscopy employing DMPO, DIPPMPO, and TEMP as spin probes. Fluorescence characterization was performed with a three-dimensional excitation-emission matrix spectrometer (Lengguang F98, Shanghai Lengguang Technology Co., Ltd., Shanghai, China).

4. Conclusions

This study demonstrated that CoFe₂O₄ effectively activates PAA for aniline degradation, with the effects of CoFe₂O₄ dosage, PAA concentration, and pH thoroughly investigated. The optimal aniline degradation was achieved with 0.1 g/L CoFe₂O₄ dosage, 1 mM PAA concentration, and pH 7, with a removal rate of 96.1%. Additionally, the effects of low concentrations of anions and HA on the degradation of aniline by the CoFe₂O₄/PAA system were almost negligible. Mechanism exploration revealed that ¹O₂ is the main ROS in the CoFe₂O₄/PAA system, with O₂⁻· playing a secondary role and R-O· also contributing to aniline degradation. The ≡Co(II)/≡Co(III) redox cycle occurring on the surface of CoFe₂O₄ during the reaction promotes the decomposition of PAA to generate ROS. Fe in CoFe₂O₄ is not directly involved in the activation of PAA, but the interaction between Fe and Co accelerates the rate of electron transfer at the catalytic interface, which facilitates the activation of PAA to generate ROS. Furthermore, as the reaction progressed, the CoFe₂O₄ structure remained stable, and its aniline degradation efficiency still reached 93.9% after five uses. Lastly, besides aniline, the CoFe₂O₄/PAA process effectively removed other organic pollutants (ATZ, SMX, phenol, and acid red 14). The system had a good ability to remove the actual secondary effluent. However, high concentrations of HCO₃⁻, H₂PO₄⁻, and HA inhibited aniline degradation. Therefore, further research is needed to minimize the impact of the water matrix on pollutant removal and to investigate its effects on the activity of ROS-like ¹O₂. This study contributes to the practical application of heterogeneous catalysts in PAA-based AOPs for wastewater treatment.