Activation of Peracetic Acid by Ozone for Recalcitrant Pollutant Degradation: Accelerated Kinetics, Byproduct Mitigation, and Microbial Inactivation

Abstract

Iopamidol (IPM), as a typical recalcitrant emerging pollutant and precursor of iodinated disinfection by-products (I-DBPs), is unsuccessfully removed by conventional wastewater treatment processes. This study comprehensively evaluated the ozone/peracetic acid (O₃/PAA) process for IPM degradation, focusing on degradation kinetics, environmental impacts, transformation products, ecotoxicity, disinfection byproducts (DBPs), and microbial inactivation. The O₃/PAA system synergistically activates PAA via O₃ to generate hydroxyl radicals (^•OH) and organic radicals (CH₃COO^• and CH₃CO(O)O^•), achieving an IPM degradation rate constant of 0.10 min⁻¹, which was significantly higher than individual O₃ or PAA treatments. The degradation efficiency of IPM in the O₃/PAA system exhibited a positive correlation with solution pH, achieving a maximum degradation rate constant of 0.23 min⁻¹ under alkaline conditions (pH 9.0). Furthermore, the process demonstrated strong resistance to interference from coexisting anions, maintaining robust IPM removal efficiency in the presence of common aqueous matrix constituents. Furthermore, quenching experiments revealed ^•OH dominated IPM degradation in O₃/PAA system, while the direct oxidation by O₃ and R-O^• played secondary roles. Additionally, based on transformation products (TPs) identification and ECOSAR predictions, the primary degradation pathways were elucidated and the potential ecotoxicity of TPs was systematically assessed. DBPs analysis after chlorination revealed that the O₃/PAA (2.5:3) system achieved the lowest total DBPs concentration (99.88 μg/L), representing a 71.5% reduction compared to PAA alone. Amongst, dichloroacetamide (DCAM) dominated the DBPs profile, comprising > 60% of total species. Furthermore, the O₃/PAA process achieved rapid 5–6 log reductions of E. coli. and S. aureus within 3 min. These results highlight the dual advantages of O₃/PAA in effective disinfection and byproduct control, supporting its application in sustainable wastewater treatment.

1. Introduction

Water is a vital resource essential for the survival and advancement of human civilization, with its quality being intimately linked to public health. In the context of global pandemics (e.g., COVID-19, influenza A, and the Ebola virus, etc.) the demand for disinfectants has risen sharply due to the growing need for wastewater reuse. A critical challenge in the disinfection process is the formation of disinfection byproducts (DBPs) when disinfectants react with various precursors, including natural organic matter (NOM) and anthropogenic pollutants present in water sources. To date, more than 700 DBPs have been identified in different water systems [1,2]. Toxicological studies consistently demonstrate that most halogenated DBPs exhibit concerning biological effects, including cytotoxicity, neurotoxicity, genotoxicity, carcinogenicity, mutagenicity, and teratogenicity [3]. Their toxicity generally following this order: iodinated disinfection byproducts (I-DBPs) > brominated DBPs ≫ chlorinated DBPs [4]. Moreover, Allen et al. identified I-DBPs as the primary contributors to the overall cytotoxicity and genotoxicity observed in drinking water [5]. Consequently, increasing research attention has focused on developing effective strategies to mitigate I-DBP formation in water treatment processes.

Iopamidol (IPM) is an iodinated contrast agent widely used in medical imaging procedures such as angiography, exhibiting biological inertness in human physiology [6]. However, conventional wastewater treatment systems fail to fully remove IPM, allowing it to persist as a major organic iodine precursor for the I-DBPs formation during subsequent chlorination [7,8,9]. Therefore, it is particularly important to efficiently degrade IPM and minimize the formation of I-DBPs, which is critical to safe and sustainable reuse of water. Advanced oxidation processes (AOPs) and pre-oxidation techniques have demonstrated significant efficacy in eliminating DBPs precursors [10,11,12,13].

Ozone (O₃), due to its high oxidation potential (2.07 eV) and the ability to generate reactive radicals (e.g., hydroxyl radicals (^•OH) and superoxide radicals (O₂^•−)), has been widely employed in urban wastewater treatment [14]. For example, López-Prieto et al. found that the O₃-UV-Cl₂ pretreatment could reduce the formation of DBPs by up to 53% compared to the conventional UV-Cl₂ process [15]. Meanwhile, peracetic acid (PAA), as a potent oxidant, had attracted increasing attention in water treatment due to its strong disinfection efficiency, operational simplicity, and minimal toxic byproducts generation [16,17]. Various activation strategies of PAA can generate both free radicals (^•OH, CH₃CHOO^•, and CH₃CHOOO^•) and non-radical species (¹O₂), making it a promising candidate for peroxide-based AOPs targeting micropollutant degradation [18]. Our prior work revealed that the O₃/PAA process achieved a 2.9-fold higher atrazine degradation rate (0.0839 min⁻¹) compared to O₃ alone (0.0244 min⁻¹), with organic radicals (R-O^•, i.e., CH₃COO^• and CH₃CO(O)O^•) and ^•OH identified as the primary reactive species [19]. Thus, the O₃/PAA process holds great promise for efficiently degrading IPM while simultaneously mitigating the potential formation of I-DBPs.

In this study, the effectiveness of the O₃/PAA process for degrading IPM was investigated, and the effects of various environmental factors on the degradation of IPM in the O₃/PAA process were investigated, which included the amount of oxidant dosing, initial pH, and water matrix composition. Meanwhile, the potential degradation pathways of IPM were predicted and the ecotoxicity of its degradation products was evaluated. Moreover, the potential formation of DBPs during the degradation of IPM in the O₃/PAA process was thoroughly investigated. Finally, the inactivation performance of the O₃/PAA process against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) was investigated. This study would provide valuable insights for a more comprehensive understanding and practical application of the O₃/PAA process in wastewater purification and disinfection.

2. Materials and Methods

2.1. Materials

All reagents used were of the highest available purity. DBP standards such as trichloromethane (TCM), 2,2-dichloroacetamide (DCAM), iodoacetonitrile (IAN), chloroacetic acid (CAA), dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), 1,1,3-trichloropropanone (1,1,3-TCP), iodoform (TIM), dichloroacetonitrile (DCAN), and dichloroacetaldehyde (DCAL) were purchased from Sinopharm Sigma Aldrich (St. Louis, MO, USA). IPM (>98% purity) was obtained from First Standard, Alta Scientific Co., Ltd. (Tianjin, China). The PAA stock solution (disinfectant, assay > 15%) and bovine liver (CAT, ≥3000 units mg⁻¹ protein) were obtained from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). The experimental PAA solution was prepared with addition of catalase from bovine liver to remove H₂O₂ from the PAA stock solution.

2.2. Experimental Procedure

The semi-continuous ozonation experiments were conducted in a 500 mL glass reactor with magnetic stirring and a water bath. Each experiment was carried out in 7.5 mM phosphate buffer solution at 25 °C. After being diluted to the desired concentrations, the PAA and IPM stock solutions were added to the buffer solution. O₃ was continuously supplied into the reactor at a flow rate of 0.5 L min⁻¹ using a laboratory-scale O₃ generator (3S-A3, Tonglin Hi-Tech, Beijing, China) and delivered through a stainless-steel diffuser. At predetermined time intervals, 1.0−1.5 mL samples were obtained from the reactor, immediately quenched with an excess of Na₂S₂O₃ solution, and held at 4 °C for subsequent analysis.

2.3. Analytical Methods

The specific concentration of PAA was determined followed the method provided by GB/T 19104-2021 (China). The IPM sample were separated using a Waters e2695 HPLC (Milford, MA, USA) fitted with a Sunfire-C18 column (150 mm × 4.6 mm, 5 μm), and detected using a Waters 2489 UV detector (Milford, MA, USA) at 242 nm. The IPM samples were measured using 0.1% acetic acid/acetonitrile (90:10, v/v) at a flow rate of 0.8 mL min⁻¹. Total organic carbon (TOC) of humic acid (HA, Sigma-Aldrich, St. Louis, MO, USA) was quantified using an Analytikjena multi N/C 3100 (Jena, Germany). The transformation products (TPs) of IPM were analyzed by an Agilent 1290 UPLC-MS system (6545B Q-TOF MS, Agilent, Santa Clara, CA, USA) and the comprehensive analytical parameters can be found in Text S1.

2.4. DBP Formation Potential

Following pretreatment under various experimental conditions, samples were subjected to DBP formation potential testing. A certain amount of sodium hypochlorite solution was added to the amber glass bottles and stored for 12 h. Each sample of DBPs were analyzed following the modified EPA Method 551.1 for DBPs formation analysis and the comprehensive analytical parameters can be found in Text S2 [20].

2.5. Ecotoxicity Evaluation

The acute toxicity (LC₅₀ or EC₅₀) and chronic toxicity (ChV) of IPM and its intermediates to three aquatic species (green algae, fish, and daphnia) were forecasted and evaluated using the Ecological Structure–Activity Relationship (ECOSAR) software (Version 2.0, Washington, PA, USA) [21].

E. coli (BNCC 269342) and S. aureus (BNCC 336423) were inoculated into 400 mL of Agar Medium and incubated separately at 37 °C with shaking (120 rpm) until the cultures reached the stable exponential growth phase (∼18 h). Bacterial inactivation experiments were conducted in 200 mL Erlenmeyer flasks by centrifuging the broth cultures to retain bacterial cells at an initial concentration of 10⁶–10⁷ CFU/mL. Subsequently, the required volumes of PAA or/and ozone stock solutions were added, and 1.5 mL samples were collected at predetermined time intervals and immediately quenched with an excess of sodium thiosulfate. The quenched reaction mixtures were then serially diluted 10–10⁶-fold and plated onto fresh Luria–Bertani (LB) agar plates, followed by incubation in a shaking incubator at 37 °C for 12–16 h. The viable colony density was determined using the standard plate count method. Each experiment was performed twice.

3. Results and Discussion

3.1. IPM Degradation in Various Systems

The degradation of IPM by direct oxidation of PAA was very limited (less than 3% within 30 min), revealing a very low oxidative capacity of PAA to IPM (Figure 1a). Meanwhile, the O₃/PAA process achieved significantly enhanced IPM removal efficiency (83.4%) and higher degradation rate constant (0.1013 min⁻¹) compared to ozonation alone (65.7% removal and 0.0848 min⁻¹) within 15 min (Figure 1b).

Subsequently, tert-butanol (TBA) and 2,4-hexadiene (2,4-HD) were employed to elucidate the roles of reactive species in the O₃/PAA system. TBA serves as a scavenger for ^•OH, with a reported rate constant of $k_{{}^{•}\mathrm{OH},TBA}$ = (3.8–7.6) × 10⁸ M⁻¹ s⁻¹ [22]. Meanwhile, 2,4-HD reacts rapidly with both acetyl(per)oxyl radicals (CH₃C(O)OO^• and CH₃C(O)O^•) and ^•OH, with a rate constant of $k_{{}^{•}\mathrm{OH},2,4-HD}$ = 9.16 × 10⁹ M⁻¹ s⁻¹ [23]. The degradation of IPM was markedly inhibited in the presence of both TBA and 2,4-HD in the O₃/PAA system, indicating that acetyl(per)oxyl radicals and ^•OH are likely the key reactive oxygen species (ROS) involved in IPM degradation (Figure 2). Notably, similar inhibitory effects were observed with the addition of excess TBA (5 mM) and 2,4-HD (2 mM), suggesting that ^•OH, rather than acetyl(per)oxyl radicals, was the predominant ROS responsible for IPM degradation (Figure 2).

In addition, our previous study confirmed that O₃ could activate PAA to generate both R-O^• and ^•OH, and the presence of PAA promoted the formation of ^•OH from O₃. Among these, R-O^• played a secondary role, contributing 16–35% to ATZ removal [19]. However, unlike the degradation of ATZ, the similar inhibitory effects of TBA and 2,4-HD suggested that R-O^• exhibited low reactivity toward IPM and played a minor role for its degradation. Moreover, the incomplete inhibition of IPM degradation by TBA or 2,4-HD indicated that direct oxidation by O₃ also played an important role in the overall degradation process. Therefore, based on the same reaction conditions and experimental setup, the enhanced degradation of IPM in the O₃/PAA process was likely driven primarily by the accelerated generation of ^•OH from O₃ induced by PAA.

3.2. Effects of Crucial Factors via O₃/PAA System

3.2.1. O₃ and PAA Dose

As the inlet O₃ concentration increased from 3 to 15 mg/L, the pseudo-first-order rate constant (k_obs) for IPM removal increased from 0.0903 to 0.02268 min⁻¹, representing an approximately 1.5-times enhancement (Figure 3). This acceleration can be attributed to improved O₃ mass transfer efficiency at elevated concentrations, which simultaneously promotes both direct molecular O₃ oxidation and PAA activation through radical generation pathways [24]. In addition, as the PAA concentration raised from 0.5 to 3 mg/L, the k_obs of IPM gradually accelerated from 0.0973 to 0.1075 min⁻¹ (Figure 4). As the primary precursor of reactive oxygen species (ROS) in this system, higher PAA concentration could be beneficial for the production of more ROS, thereby accelerating IPM degradation [19]. However, when the concentration of PAA was further increased from 3 to 5 mg/L, the k_obs of IPM slightly declined to 0.1013 min⁻¹ (Figure 4). This counterintuitive trend likely stems from competitive consumption of ROS between excess PAA and IPM, thereby resulting in the inhibition of the degradation efficiency [25].

3.2.2. Solution pH

The solution pH demonstrated significant regulatory effects on IPM degradation in both ozonation and O₃/PAA systems. As illustrated in Figure 5, the increment of pH from 5.0 to 9.0 could obviously promote the k_obs of IPM from 0.0251 to 0.3883 min⁻¹ for ozonation and from 0.0294 to 0.2302 min⁻¹ for the O₃/PAA process, respectively. This pH-dependent acceleration originates from elevated OH⁻ concentrations promoting radical chain propagation reactions, particularly through enhanced ^•OH generation via O₃ decomposition at alkaline conditions [26]. Notably, when the pH increased to 8.0 and 9.0, ozonation outperformed the O₃/PAA process, with k_obs values of 0.24 vs. 0.21 min⁻¹ at pH 8.0 and 0.39 vs. 0.23 min⁻¹ at pH 9.0. This inversion can be mechanistically explained by the pH-dependent speciation of PAA (pKa = 8.2). Below pH 8.0, neutral PAA (PAA⁰) predominates and undergoes efficient O₃-mediated activation to produce ^•OH and R-O^• [27]. While, under alkaline conditions (pH 8.0–9.0), deprotonated PAA⁻ becomes the dominant species and preferentially undergo direct oxidation by O₃ rather than radical generation, resulting in substantial reduction of reactive species, thereby diminishing the synergistic effect in the O₃/PAA system.

3.2.3. Water Matrix

The effects of common inorganic anions (SO₄²⁻, NO₃⁻, Cl⁻, and HCO₃⁻) at different concentrations on IPM degradation in the O₃/PAA system were systematically investigated (Figure S1). Among the investigated anions, SO₄²⁻, NO₃⁻, and Cl⁻ had negligible effects on IPM degradation in O₃/PAA process (Figure S1a,b), suggesting their limited radical scavenging capacity in this oxidation system. Moreover, the presence of 10 mM HCO₃⁻ caused substantial suppression of IPM degradation, significantly reducing the removal efficiency from 91.9% to 70.0% within 20 min (Figure S1c). It is well known that HCO₃⁻ and CO₃²⁻ can react with ^•OH to generate CO₃^•− radicals (Equations (1) and (2)), which exhibit a lower redox potential (E⁰ = 1.59 V vs. NHE) compared to ^•OH (E⁰ = 2.80 V), thereby suppressing the degradation efficiency of IPM [27,28].

$${\mathrm{HCO}}_3^{-}+{}^{·}\mathrm{O}\mathrm{H}\to {\mathrm{CO}}_3^{·-}+{\mathrm{H}}_2\mathrm{O} k=8.5\times {10}^6{ \mathrm{M}}^{-1}{\mathrm{s}}^{-1}$$

(1)

$${\mathrm{CO}}_3^{2-}+{}^{·}\mathrm{O}\mathrm{H}\to {\mathrm{CO}}_3^{·-}+{\mathrm{OH}}^{-} k=4.2\times {10}^8{ \mathrm{M}}^{-1}{\mathrm{s}}^{-1}$$

(2)

3.3. Degradation Pathways and Ecotoxicity Evaluation

About 15 TPs of IPM were detected in the O₃/PAA process by using UPLC-Q-TOF-MS analysis and the detailed information of TPs were depicted in Table S1. As illustrated in Figure 6, the degradation of IPM primarily proceeded through five pathways: deiodination, hydrogen abstraction, amide hydrolysis, amino oxidation, and hydroxyl substitution, with hydrogen abstraction and amide hydrolysis constituting the predominant mechanisms.

Firstly, the deiodination reaction initiated on the benzene ring of IPM, yielding the intermediate P652. Concurrently, the hydrogen abstraction pathway occurred on the side chains of IPM, generating P775 and P773, likely mediated by the oxidation of ^•OH. Meanwhile, the de-iodinated product P652 subsequently underwent further hydrogen abstraction to form P649. The amide hydrolysis reaction played a critical role in degrading both the benzene ring and the side chain of IPM, producing P703 and P705. Additionally, the amide hydrolysis of the side chain in P775 resulted in the formation of P701. Specifically, the intermediate P705 served as a pivotal branching point for multiple degradation routes: Route 1: Sequential amide hydrolysis and hydrogen abstraction converted P705 to P629; Route 2: P705 underwent hydrogen abstraction followed by amino oxidation to produce P731. A parallel pathway involving amino oxidation prior to hydrogen abstraction also converged to P731; Route 3: Hydroxylation of P705 resulted in the formation of P595; Route 4: Amide hydrolysis of P705 produced P735, with subsequent hydrolysis yielding P587.

Subsequently, the ECOSAR software was used to predict the acute and chronic toxicity of IPM and its TPs (Table S2 and Figure 7). The acute toxicity of IPM was categorized as “harmful” to fish (93,600 mg L⁻¹), daphnid (152,000 mg L⁻¹), and “very toxic” to green algae (3320 mg L⁻¹), while the chronic toxicity of IPM was classified as “toxic” to fish (278 mg L⁻¹), “very toxic” to daphnia (7250 mg L⁻¹) and green algae (477 mg L⁻¹) as seen in Table S2. Notably, all intermediates derived from P705 (i.e., P631, P735, P731, P661, P587, and P732) exhibited distinct higher chronic and acute toxicity than the parent compound IPM. In contrast, other intermediates (i.e., P652, P649, P775, and P773) displayed lower acute/chronic toxicity to fish, green algae, and Daphnia compared to IPM (1–2 orders of magnitude). These findings highlight the necessity of prioritizing the monitoring and removal of P705 during IPM degradation processes. Future studies should focus on elucidating the formation kinetics and elimination efficiency of P705 to mitigate its hazardous effects on aquatic ecosystems and improve the safety and efficacy of water treatment technologies.

3.4. DBPs Formation

To simulate practical application, the formation trends of DBPs during chlorination following O₃/PAA pre-oxidation were systematically investigated. Under different initial molar ratios of O₃ and PAA (0:3, 2.5:0, 2.5:1, 2.5:3, and 2.5:5), as well as different pH conditions, total 10 DBPs species (TCM, DCAM, IAN, CAA, DCAA, TCAA, 1,1,3-TCP, TIM, DCAN, DBAN) were detected, with their concentrations at different reaction times presented in Figure 8 (with detailed data provided in Table S3).

Figure 8a illustrates the variation in total DBPs concentrations after 5 and 30 min of pre-oxidation under different O₃/PAA molar ratios. During the single PAA pre-oxidation process (0:3), the total DBPs concentration reached 348.88 μg L⁻¹ after 30 min of chlorination. By contrast, ozonation alone (2.5:0) produced only 75.60 μg L⁻¹ of the total DBPs, less than one-sixth of the PAA-only process. This disparity may be attributed to the presence of acetic acid and the limited degradability of IPM by PAA alone, as well as the potential role of PAA itself in contributing DBP precursors [29,30]. When O₃ and PAA were combined at molar ratios of 2.5:1, 2.5:3, and 2.5:5, the total DBPs concentrations slightly increased to 93.08, 100.21, and 135.39 µg L⁻¹ at 5 min, and to 128.65, 99.88, and 119.66 µg L⁻¹ at 30 min, respectively. Although the combined O₃/PAA process resulted in a slight increase in total DBPs compared to ozonation alone, the overall DBP levels were still far lower than those in the PAA-only process.

Further, the species composition distribution of DBPs after 30 min of pre-oxidation under different O₃/PAA molar ratios was illustrated in Figure 8b. Notably, only two I-DBPs such as IAN and TIM were detected across all treatment processes, with concentrations below 1.6 and 10.05 µg/L, respectively (Table S3). Meanwhile, IPM served as both a precursor for I-DBPs and a nitrogen source contributing to the formation of nitrogenous DBPs (N-DBPs). DCAM, a representative halogenated acetamide, was the dominant DBP species across all treatment conditions, accounting for over 60% of the total DBPs and primarily driving the variations in total DBP levels among different processes. In addition, the concentration of CAA in the PAA-alone process reached 43.97 µg/L—nearly 50 times higher than those in the O₃ and O₃/PAA processes (<1 µg/L). This may be attributed to the acetic acid present in PAA, which could be rapidly chlorinated to form halogenated acetic acids. In contrast, in the O₃ or O₃/PAA systems, ^•OH effectively oxidizes acetic acid to CO₂ and H₂O, thereby preventing further chlorination [19,31].

The influence of pH on DBPs formation was evaluated in the O₃/PAA (2.5:3) system following 30 min of pre-oxidation (Figure 8c, detailed data in Table S3). The total DBPs at pH 7~9 (98.56~99.88 µg/L) were significantly higher than that of pH 6 (90.99 µg/L), likely attributed to the pH-dependent variations in reactive species selectivity toward organic precursors. Our prior study demonstrated that ^•OH and O₃ contributed 52% and 48% to ATZ degradation at pH 6 in O₃/PAA system. While, at pH 7~9 the contributions shifted to 60%~79% for ^•OH and 5~6% for O₃ [19]. Moreover, DBPs formation exhibited stronger dependence on pre-oxidation time than pH. At pH 9, complete IPM degradation after 30 min of pre-oxidation resulted in the formation of 98.56 μg/L DBPs, whereas shortening the duration to 15 min achieved >97% IPM removal with only 1.23 μg/L DBPs formation. These results underscore the critical trade-off between treatment duration and byproduct generation, suggesting that optimizing pre-oxidation time can minimize DBP risks without compromising contaminant removal efficiency.

3.5. Sterilization Performance Evaluation

The microbial sterilization efficacy of O₃, PAA, and the combined O₃/PAA process was evaluated using E. coli and S. aureus as model organisms and the results were represented in Figure 9. Both PAA and O₃ exhibited limited bactericidal activity against E. coli and S. aureus within 3 min, with log reductions of E. coli and S. aureus reaching 2.49, 2.33, 0.89, and 3.32, respectively (Figure 9). PAA and O₃, as highly reactive oxidants, are capable of interacting with various cellular components, such as cell walls and DNA structures, leading to bacterial inactivation [32,33]. Furthermore, the combined O₃/PAA process markedly enhanced disinfection efficiency, achieving log reductions of 6.30 for E. coli and 4.65 for S. aureus within 3 min, significantly surpassing the performance of individual treatments (Figure 9). These results demonstrate the superior efficacy of the O₃/PAA synergy for microbial disinfection in wastewater treatment.

4. Conclusions

The O₃/PAA process significantly enhanced IPM degradation efficiency, increasing the k_obs of IPM degradation from 0.0848 to 0.1013 min⁻¹ compared to ozonation alone. Due to enhanced O₃ decomposition into ^•OH under alkaline conditions, the k_obs of IPM degradation in O₃/PAA system increased from 0.0294 to 0.2302 min⁻¹ as the pH rose from 5 to 9. The system revealed strong resistance to common anions (SO₄²⁻, NO₃⁻, Cl⁻), while the presence of HCO₃⁻ (10 mM) partially inhibited IPM degradation by 21.9% due to ^•OH scavenging. Meanwhile, ^•OH contributed a predominant role in the degradation of IPM, followed by direct oxidation by O₃, while R-O^• played a minor role in O₃/PAA system. Furthermore, the hydrogen abstraction and amide hydrolysis emerged as the dominated degradation pathways of IPM, generating about 15 TPs in O₃/PAA process, among which P705 and its derivatives exhibited elevated ecotoxicity. The O₃/PAA system markedly reduced DBPs formation compared to PAA pre-oxidation alone, achieving a minimum total DBPs concentration of 99.88 µg/L (O₃: PAA = 2.5:3) versus 348.87 µg/L for PAA-only treatment. However, possibly due to the low reactivity between IPM and R-O^• radicals, the formation of chlorinated DBPs after the O₃/PAA pretreatment were higher than those observed with O₃ alone. Given the selective reactivity of R-O^• toward organic contaminants, the broad applicability of the O₃/PAA process requires further evaluation. In addition, IPM served as a dual precursor for I-DBPs and N-DBPs, with DCAM dominating the DBPs profile and accounting for >60% of the total. Furthermore, the O₃/PAA system demonstrated rapid microbial inactivation, achieving 6.30-log and 4.65-log reductions of S. aureus and E. coli, respectively, within 3 min. These findings provide new insights into the O₃/PAA process for efficient contaminant degradation and the minimization of disinfection risks, emphasizing its practical potential for sustainable wastewater treatment.