Enhanced Oxidation of Carbamazepine Using Mn(II)-Activated Peracetic Acid: A Novel Advanced Oxidation Process Involving the Significant Role of Ligand Effects

Abstract

In recent years, extensive attention has been paid to advanced oxidation processes (AOPs) with peracetic acid (PAA), a widely used disinfectant, using transition metal ions for the degradation of organic contaminants within water environments. Mn(II) has been widely used as an effective homogeneous transition metal catalyst for oxidant activation, but it has shown poor performances with PAA. Since the stability of manganese species can be enhanced through the addition of ligands, this study systematically investigated a novel AOP for the oxidation of carbamazepine (CBZ) using an Mn(II)/PAA system with several different ligands added. The reactive species were explored through UV-vis spectrometry, scavengers, and probe compounds. The results suggest that Mn(III)–ligand complexes and other high-valent Mn species (Mn(V)) were generated and contributed obviously toward efficient CBZ oxidation, while radicals like CH₃CO₂^• and CH₃CO₃^• were minor contributors. The oxidation efficiency of Mn(II)/PAA/ligands depended highly on ligand species, as ethylene diamine tetraacetic acid (EDTA) and oxalate (SO) could promote the oxidation of CBZ, while pyrophosphate (PPP) showed modest enhancement. The results obtained here might contribute to the removal of residue pharmaceuticals under manganese-rich waters and also shed light on PAA-based AOPs that could help broaden our present knowledge of manganese chemistry for decontamination in water treatment.

1. Introduction

With the development of urbanization and industrialization, attention to environmental issues concerning aquatic ecosystems has increased attention due to the growing number of emerging contaminants, i.e., pharmaceuticals, personal care products, endocrine disruptors, artificial sweeteners, disinfection by-products, and pesticides, which are continuously introduced into waterbodies worldwide [1]. Thus, advanced oxidation processes (AOPs) are promoted for the elimination of these recalcitrant contaminants in water and have attracted extensive attention due to their special advantages, including higher mineralization efficiency and mild operating conditions. The main principle of AOPs is the generation of reactive oxidation radicals (ROS) or reactive species with high redox potential from oxidants through activators like catalysts, UV, and heat. Specifically, peracetic acid (PAA) is an organic peroxyacid originally used in, for example, disinfectants, sanitizers, bleach, food industry, oxidizers, and polymerization catalysts, and unlike common oxidants like H₂O₂, peroxymonosulfate (PMS), and persulfate (PS), it has been confirmed to be effective in generating ROS with high reaction activity and thus able to degrade organic contaminants like diclofenac (DFC) and sulfamethexazole (SMX) through PAA-based AOPs [2,3,4,5].

Compared with some conventional water disinfectants and microbial inactivation, such as through chlorination, ozonation, and ultraviolet (UV), PAA is advantageous due to its high sterilization ability, lower pH dependence, easier technical implementation, and reduced formation of toxic by-products in the treated effluent [1,6,7]. In addition to its use as a disinfectant, it has also been reported that PAA has the potential for degrading aqueous organic contaminants, such as pharmaceuticals, due to its high oxidation potential, which ranges between 1.06 and 1.96 based on the working pH [8,9]. However, the application of PAA in AOPs for the removal of organic contaminants from water has received much less attention to date.

Generally, PAA can be activated using, for example, external energy like radiation (i.e., solar radiation, UV radiation, and microwave) and catalysts to produce reactive species for the oxidation of organic pollutants like fluoroquinolone, sulfadiazine, and bisphenol-A [10,11,12,13,14]. Based on previous studies, PAA can be decomposed into reactive species including peroxyl radicals (CH₃CO₂^• and CH₃CO₃^•) from PAA molecules and ^•OH from either PAA or coexisting H₂O₂ [15,16].

In addition to some common transition metal ions such as Fe(II/III), Cu(II), Mn(II), and their divertive composites are widely used in catalysis fields due to their special characterizations like natural abundant and valence variation. Manganese (Mn) exists naturally on earth and in space in several oxidation states, including Mn(II), Mn(III), Mn(IV), and Mn(V). However, in water, only Mn(II) is stable as a free ion. Soluble forms of higher oxidation states—such as Mn(III), Mn(V), and Mn(VI)—are generally unstable in solution. In other words, these Mn species in higher oxidation states do not typically exist as free ions in water. Instead, they are either short-lived intermediates that can be stabilized through complexation with ligands, or they quickly convert into other Mn species. They tend to undergo disproportionation, meaning they break down into more stable forms like Mn(II), Mn(IV) (which often forms insoluble oxides), or Mn(VII) (permanganate) [17,18]. Among them, aqueous Mn(II) ion is one of the dominant species in natural water environments; it is often regarded as an efficient trigger or acceleration for the activation of several oxidants such as persulfates and H₂O₂ [19,20,21]. However, the homogeneous activation of PAA using soluble Mn(II) has seldom been investigated.

It has been reported that the addition of some complexing ligands into the AOP reaction system with transitional metal ions might pose potential influence on the oxidation process with accelerated organic contamination degradation. As reported by previous works, ligands like ethylene diamine tetraacetic acid (EDTA), nitrilotriacetic acid (NTA), and sodium tripolyphosphate (TPP) could help boost the working efficiency of some Fe(II)-based AOPs, i.e., dissolved Fe(II), pyrite, magnetite, and Fe(II)-bearing clay minerals, by increasing the yield of ^•OH production. This is because ligands might help utilize the electronic properties of Fe(II), thereby improving its ability in AOPs for the degradation of organic contaminants. For example, Xie et al. reported that the induction of specific ligands with high electron donation ability could obviously promote the production of ^•OH through the use of Fe(II) since the decreasing electron donation ability of critical ligands could coordinate with Fe²⁺. In other words, regulating Fe(II) species using ligands with slight electron donation ability through ion exchange could enhance electron utilization efficiency during sediment oxygenation [22]. Zeng et al. reported that the presence of ligands could promote the yield and rate of ^•OH production with an accelerated electron transfer rate since ligands can dissolve a part of solid Fe(II) into liquid Fe(III) [23]. Furthermore, some studies have also reported that in spite of the enhanced reactive species generation, the production of some other high-valent metal species after the addition of electron-donating ligands might also promote the oxidation and degradation of organic pollutants through the production of reactive intermediates. These high-valent metal species can activate oxidants like PMS and H₂O₂ through non-radical means, possibly due to the unique redox properties of the centered metal atom, and also play a major role as an alternative to reactive species in the removal of organic pollutants [24,25]. For example, Wang et al. reported that the presence of electron-donating ligands, the 4-aminopyridine species, accelerated the degradation of Reactive Red 195 using activated H₂O₂ due to the formation of IDA/Cu(II)/4-ampy complex. The generated high-valent copper–oxo species, [^•O-Cu^III(IDA)(4-ampy)₂], was proposed as the possible main working active species since ^•OH was not detected during the reaction [26]. Li et al. reported that the complex of Fe^III-TAML was not only capable of PMS activation but also acted as the key active species responsible for the elimination of p-chlorophenol within a wide pH range [27].

The introduction of specific complexing ligands has been confirmed to promote the working efficiency within several Fenton-like AOPs activated by transitional metal species, i.e., PMS and H₂O₂ activated by Fe or Mn species, for the degradation of several organic pollutants such as dyes and pharmaceuticals. However, whether similar effects would occur within an AOP using PAA has rarely been investigated. Thus, there still lies a knowledge gap regarding the effects of ligands within Mn(II)/PAA, the reaction mechanism behind it, the kinds of main working reactive species, and the possible influence of additive ligands on the oxidation of certain organic contaminants.

The influences from additive ligands were explored using representatives such as sodium oxalate (SO), ethylene diamine tetraacetic acid (EDTA), and pyrophosphate (PPP). EDTA is known as an important chemical raw material widely used in the industrial production of textiles and chemicals. It is also a suitable chelating agent for a Fenton-type process, since with an iron–EDTA complex, the effective reduction of Fe(III) to Fe(II) using hydrogen peroxide drastically improves the catalytic nature of iron at near-neutral pH [28]. PPP belongs to the group of inorganic phosphates commonly used in meat industries [29]. It can not only improve the degradation of trichloroethylene (TCE) at neutral pH as an iron-stabilizing agent but also works as a strong complexing ligand to sequester Fe(V) oxidation during chemical oxidation [30,31]. The acceleration performances of SO within both a heterogeneous Fenton-like system and ozonation using activated Ce(III/IV) for efficient water treatment have also been significant [32,33]. Compared to with Fe species, the impacts from ligands toward oxidations using Mn species have been less investigated.

Thus, in this work, we systematically aimed to investigate (1) the activation performance of PAA using Mn(II) for the oxidation of a specific pharmaceutical contaminant, (2) the effects and reaction mechanisms of the additive ligands, namely EDTA, SO, and PPP, on the oxidation efficiency of the Mn(II)/PAA/ligand system, and (3) the identification of the major reactive species’ generated and their contributions within the oxidation. Herein, carbamazepine (CBZ) with antiepileptic and psychotropic activities was chosen as a model contaminant due to its low sorption properties and high resistance to biodegradation. In addition, not only was CBZ detected within municipal sewage treatment plants and domestic wastewater, but it was also one of the most frequently detected pharmaceuticals in drinking water systems in Europe and the USA [34].

2. Results and Discussion

This section is divided into subsections providing a concise and precise description of the experimental results, their interpretation, and the experimental conclusions that can be drawn.

2.1. Effect of Complexing Ligands in the Activation of PAA Using Mn(II)

The oxidation of CBZ with PAA using Mn(II) was investigated, and the results are provided in Figure 1. The results show that with no Mn(II) or PAA added, the concentration of CBZ remained almost unchanged during the whole reaction. The simultaneous presence of Mn(II) and PAA could barely help to change this situation, as residue CBZ decreased slightly after the reaction, with only about 12% removed. These situations reveal that PAA might hardly be activated by Mn²⁺ without further assistance.

(General working conditions: [Mn(II)] = 50 μM, [PAA] = 200 μM, [SO] = [EDTA] = [PPP] = 100 μM, pH = 5.5)

Thus, another series of experiments with three different ligands, namely EDTA, SO, and PPP, added into the Mn(II)/PAA system were performed. In order to determine the optimal working conditions for CBZ oxidation with ligands, the working parameters, including pH and the concentrations of Mn(II), ligands, and PAA, were investigated separately.

The effects of three ligands, namely EDTA, PPP, and SO, on the oxidation of CBZ are shown in Figure 1. The results imply that the three ligands posed significantly different impacts on the oxidation of CBZ, as EDTA and SO could promote CBZ oxidation, while PPP showed modest effects on the reaction. More than 80% and 50% of CBZ were removed within 30 min with the addition of SO or EDTA, separately. Furthermore, the oxidation trend of CBZ using SO and EDTA showed slight differences, as the one using SO fitted the pseudo-first-order model roughly, while the one using EDTA could be roughly divided into two stages: an initial rapid stage followed by a lagged second one.

As revealed in Figure 2, while increasing the Mn(II) concentration and keeping the ligands unchanged, the reduction of CBZ was promoted gradually, in general, with EDTA or SO added. This suggests a possible enhanced activation of PAA or the generation of some other oxidants with high redox potential using Mn(II) with co-existed EDTA or SO. The impacts caused by the change in the PAA concentration on CBZ oxidation are observed in Figure 3, showing a similar phenomenon but not as obvious as the previous ones. The above situations indicated the possible integration between PAA, Mn(II), and EDTA or SO, and PAA might not act as the only oxidant during the process. The roles of EDTA and SO might not be limited to the assistance of PAA activation. However, it should be noted that little promotion of CBZ oxidation (less than 5%) could be detected, while the ratios between Mn(II) and SO or EDTA increased beyond 1:2, except for one at 1:4 (about 17% of CBZ remained after oxidation in the 1:1 group, about 20% in the 3:4 group, and about 22% in the 1:2 group). This might be caused by the limited amount of ligands or Mn(II) added. Based on the above situations, it could be inferred that the combination of Mn(II) and SO or EDTA played an important role upon CBZ oxidation, and the decomposition of PAA was less significant in comparison.

(Working conditions: [Mn(II)] = 25, 50, 75, 100 μM, [PAA] = 200 μM, [SO] = [EDTA] = [PPP] = 100 μΜ, pH = 5.5)

(Working conditions: [Mn(II)] = 50 μM, [PAA] = 50, 100, 200, 300 μM, [SO] = [EDTA] = [PPP] = 100 μΜ, pH = 5.5)

The characteristics of the ligands might also interfere with the oxidation of PAA due to their interactions with Mn(II). As a complex molecule containing multiple coordinating atoms, EDTA with six coordination sites (four O-donors from the carboxyl groups and two N-donors from amino groups) could form stable five-membered chelate rings with Mn(II), which accelerated effective PAA activation. In addition, the tetra-negative anion originated from the N-donors and O-donors of EDTA might create a tetra-negative anion that attracts Mn(II) [35]. The retarded oxidation of CBZ with PPP, a tetradentate ligand, might be caused by its fewer coordinating sites, with only four. Thus, more PPP is needed to chelate Mn(II). In addition, the more rigid and bulky structure of PPP also made it less effective at forming tight complexes with Mn(II) [36,37]. SO was the simplest dicarboxylic acid with lower molecular weight, significant acidity, and chelating ability. Relevant studies have also reported that oxalate radicals might react with dissolved oxygen within water to form radicals like HO₂^•and O₂^•−, which could further disproportionate to form H₂O₂ [38,39]. In addition, it has been reported that the reaction rate between oxalate or its dissociated species and ^•OH is relatively slow. The slight ^•OH scavenging rate of oxalates also implies it has little competition with the target contaminant.

The influence of working pH was also detected within the range of 3.0–10.0 to further investigate the optimal working condition for CBZ oxidation using Mn(II)/PAA/ligands, as indicated in Figure 4. Generally, more than 88% of CBZ oxidized could be found at pH 5.5 using SO, while little CBZ was removed in extreme acidic or alkaline environments. However, it was clear that the oxidation of CBZ with either SO or EDTA added was faster in the first 10 min at pH = 5.5 and was retarded obviously afterward. These phenomena might be caused by the speciation of PAA, as the pKa of PAA was ~8.2, indicating the predominant role of neutral species within the range of pH = 3–7. At the same time, at pH over 8.2, PAA mainly existed as disassociated species, PAA⁻, with lower oxidation ability than the protonated PAA near-neutral pH [40,41]. This specific situation helps explain the significantly inhibited CBZ oxidation in basic environments. Thus, a near-neutral working pH was more advantageous for the Mn(II)/PAA/ligand system.

(General working conditions: [Mn(II)] = 50 μM, [PAA] = 200 μM, [SO] = [EDTA] = [PPP] = 100 μΜ)

In order to further investigate the important role PAA played during CBZ oxidation, a series of control experiments were performed using the raw materials for PAA synthesis, in other words, H₂O₂ and acetic acid (AA, CH₃COOH). In addition, H₂O₂ and CH₃COOH were also directly mixed without the addition of H₂SO₄, and the results are presented in Figure 5. It is obvious that none of H₂O₂, CH₃COOH, or CH₃COOH + H₂O₂ led to the oxidation of CBZ with Mn(II) added for homogeneous catalyis since the concentration of CBZ remained almost unchanged after the reaction. It has been widely reported that H₂O₂ could be effectively activated using transition metal ions or their oxides with the generation of ^•OH, a common reactive species with high redox potential at around 2.8V [42]. However, the situation of H₂O₂ catalyzed with Mn(II) seemed no better than the others, which is consistent with previous studies in which H₂O₂ could hardly be activated using only Mn(II) [43]. Therefore, it is reasonable to conclude that PAA, instead of other reagents, was the main oxidant that could be activated with Mn(II) for the oxidation of CBZ.

(General working conditions: [Mn(II)] = 50 μM, [PAA] = 200 μM, [AA] = [H₂O₂] = 200 μM [SO] = [EDTA] = [PPP] = 100 μΜ, pH = 5.5)

2.2. Generation and Involvement of Mn(III) Complex

The above results indicate that the addition of specific ligands posed obvious interference on CBZ oxidation due to their impacts on PAA activation.

During the reaction between PAA and Mn(II), the disproportionation of Mn(III) was feasible to produce and stabilize with ligands or under acidic environments in situ, which helped prevent its dismutation into Mn(II) and Mn(IV) [36,44]. A similar situation was also detected within AOPs using oxidants like H₂O₂ and PMS, where the produced Mn(III) might turn into more stabilized Mn(III) species (e.g., Mn(III)-EDTA, Mn(III)-EDDS, and Mn(III)-ligand complexes with humic acid), demonstrating an effective reactive intermediate that contributes to the oxidation of organic contaminants like p-nitrophenol through carboxyl oxygen atoms, chlorophene, and atrazine [36,45,46,47].

In order to further clarify the changes in Mn(II)/Mn(III) potential in the presence of the three different chelating agents, a cyclic voltammetry test was performed (Figure 6). The cyclic voltammetry (CV) curve of bare Mn²⁺ solution showed two oxidation peaks located at around 0.68 and 0.99 V, which could be attributed to the transformation of Mn(II) to Mn(III) and of Mn(III) to Mn(IV) on the electrode surface, respectively [48,49]. The addition of SO significantly promoted the conversion of Mn(II) to Mn(III) by lowering the oxidation potential of Mn(II)/Mn(III) (ΔE ≈ −0.22 V) and enhanced the peak current. These findings reveal the coordination of the accelerated electron transfer process, where the strong chelating ability of SO contributes to the stabilization of the Mn(III) species and thus reduces the activation energy barrier [50,51,52].

A similar phenomenon was also found with the addition of EDTA, but with only a slight shift in the oxidation potential (ΔE ≈ −0.15 V). These effects might be caused by its stronger chelation strength (logK = 13.6) compared to that of SO (logK = 5.2), leading to the inhibitory behavior caused by the over-coordination observed in stronger ligand systems [51,53]. Herein, it could be concluded that the addition of SO showed higher tendency to stabilize Mn(III) species [52]. However, the peak current with the addition of PPP reduced significantly, which might result from the suppression of electron transfer. In addition, the oxidation potential shifted toward higher potential, which indicated a stronger activation energy barrier [54]. The ligand environment in Mn-based catalysts might critically regulate intermediate stability during hydrogenation reactions. It has been reported that PPP is likely to coordinate with Mn(II) to form more stable complexes, thereby reducing its propensity to oxidize into highly reactive Mn(III/IV) species [55]. In addition, PPP might also induce the formation of Mn(II)-P₂O₇ precipitates at near-neutral pH, thereby further reducing the concentration of active manganese species [56].

(General working conditions: [Mn(II)] = 50 μM, [PAA] = 200 μM, [SO] = [EDTA] = [PPP] = 100 μΜ, pH = 5.5)

The presence of Mn(III) species within the PAA activation system could be determined using UV-vis spectra within the range of 200–500 nm since Mn(III) species have a characteristic adsorption peak at around 270 nm [45,57,58]. Figure 7 implies that the signals attributed to Mn(III) species were detected in both EDTA and SO systems minutes after the addition of PAA but with different strengths and formation rates. Generally, the strength of Mn(III) species’ signal increased dramatically in the first 4 min and slowed down gradually afterward. The electron transfer from both the outer sphere and intramolecular electron transfer from EDTA to Mn(III) helped make it more stable [59]. Compared to EDTA, Mn(III) appeared earlier with SO addition, indicating the possible acceleration of Mn(III) formation caused by SO. In contrast, there was almost no Mn(III) signal detected in the PPP system, revealing that Mn(III) could hardly be stabilized with PPP. The above findings imply the feasibility of oxidizing Mn(II) into Mn(III) species within PAA environments and that the complexing ligands play critical roles during this process. The different impacts on the oxidation of CBZ could be explained by the specific characterizations within the ligands. The Mn(III) species could hardly be formed through PPP, a non-redox active compound, which might be caused due to ligand hydrolysis and disproportionation [59,60]. Therefore, it is reasonable to infer that stable Mn(III) species might be more difficult to generate within a Mn(II)/PAA/PPP system than the one using EDTA.

As for SO, it could coordinate with Mn(II) through carboxyl oxygen atoms. The oxalate tended to bind to Mn(II) in a bidentate mode via both deprotonated oxygen atoms (O⁻) of the carboxylate groups, forming a stable five-membered chelate ring [61,62]. In this reaction system, such coordination helped to stabilize Mn(III), thus facilitating further oxidations [63,64]. However, the formation of Mn(III) might undergo disproportionation with the production of Mn(II) and MnO₂, which showed less reactivity, as indicated by Equation (1). Herein, the presence of SO could help to stabilize Mn(III) species through the generation of Mn(III)–ligand complexes [65].

2Mn(III) + 2H₂O → Mn(II) + MnO₂ + 4H⁺

(1)

In addition, the simpler structure of SO showed lower steric hindrance than EDTA and PPP [66,67]. This might be conducive to the oxidation process with a more stable complex generated. Therefore, it is reasonable to infer that the generation and participation of Mn(III) species during the oxidation of CBZ using Mn(II)/PAA/ligands play critical roles.

2.3. Identification of ROS

The above results reveal that the oxidation of CBZ under Mn(II)/PAA/ligands was possibly due to the generation of reactive species from both PAA and Mn(III) species. Thus, a series of scavenging tests were performed using scavengers, including MeOH (methanol) and TBA (tertiary butanol). TBA is a highly reactive quencher toward ^•OH (k_•OH/TBA = 3.8−7.6 × 10⁸ M⁻¹·s⁻¹), while MeOH can quench ^•OH and ROH^• spontaneously (k_•OH/MeOH = 9.7 × 10⁸ M⁻¹·s⁻¹) [68,69].

(General working conditions: [Mn(II)] = 50 μM, [PAA] = 200 μM, [SO] = [EDTA] = [PPP] = 100 μΜ, pH = 5.5, [TBA] = [MeOH] = 50 mM, [PMSO] = 5 mM)

Figure 8 reveals that although the addition of excessive TBA retarded the CBZ oxidation to a certain extent, the inhibition was slight, as residual CBZ only increased within 10% compared to the normal ones. The reaction was never terminated in the ones using SO and EDTA, indicating that ^•OH was not the only effective reactive species involved in the oxidation of CBZ. The situation for the PPP group remained unchanged, with almost no CBZ oxidized.

The results in Figure 8 show far more obvious inhibition in CBZ oxidation with 50 mM MeOH addition compared to that with TBA, implying that the roles of ROH^• (i.e., CH₃CO₂^•, CH₃CO₃^•, CH₃^•, and CH₃O₂^•) played a more critical role in the reaction. In addition, the recombination of CH₃^• and PAA with dissolved O₂ (DO) within the reaction system accelerated the formation of another peroxy radical, CH₃O₂^•, which might also promote oxidation [70]. The anaerobic experiments under N₂ plugging in Figure 8 showed that the oxidation of CBZ was partly interfered with compared to the normal ones, with a modest reduction in CBZ oxidation. This indicates that CH₃O₂^• might only contribute to the oxidation process slightly.

Furthermore, since some high-valent Mn species (i.e., Mn(IV) and Mn(V) species) with high reactivity could be formed within the Mn(II)/PAA/ligand system using ligands like PICA, HA, and EDDS, some other investigations needed to be performed to distinguish their contributions from the others [36,44,71]. Herein, PMSO (methyl phenyl sulfoxide) was adopted as a mechanic probe to confirm the existence and contribution of high-valent Mn species like Mn(V). It has been reported that PMSO can hardly react with PAA and its characteristic reactive species, including CH₃CO₂^• and CH₃CO₃^•, but can be converted to PMSO₂ by high-valent metal species through oxygen atom transfer [72,73]. The products oxidized by free radicals like SO₄^•− and ^•OH were generally hydroxylated and/or polymeric products. Thus, the existence and contribution of high-valent Mn species could be determined using the generation rate and yield of PMSO₂ (i.e., the molar ratio of produced PMSO₂ to converted PMSO, η (PMSO₂)). The results in Figure 9 reveal that the conversion of PMSO into PMSO₂ using Mn(II)/PAA/ligands varied significantly over time at pH = 5.5. Generally, the addition of SO accelerated the conversion of PMSO into PMSO₂ significantly with an obvious conversion yield, about 100%, in less than 5 min at most. Compared to SO, the conversion of PMSO with EDTA was slightly slower, with less than 85% converted into PMSO₂ in about 10 min. As for PPP, almost no PMSO₂ was generated. These situations indicate that the Mn(III) formed initially due to the reaction of Mn(II)-SO and Mn(II)-EDTA, with PAA showing a higher conversion rate in high-valent Mn species (Mn(V)) later than with PPP.

(General working conditions: [Mn(II)] = 50 μM, [PAA] = 200 μM, [SO] = [EDTA] = [PPP] = 100 μΜ, [PMSO] = 20 μM, pH = 5.5)

Based on the above results, it is clear that several reactive species and Mn species with different valences can be generated with the Mn(II)/PAA/ligand system for the oxidation of CBZ. However, MeOH could also quench the Mn(III) species or any high-valent Mn species generated in addition to CH₃CO₂^•, CH₃CO₃^•, and ^•OH. Since traditional quenching experiments could hardly distinguish the relative contributions of CH₃CO₂^•, CH₃CO₃^•, and high-valent Mn species (Mn(V)) within a Mn(II)/PAA/ligand system, further investigations were performed. Herein, excessive PMSO was added into the reaction system as a scavenger, as indicated in Figure 9. The results show that the addition of PMSO inhibited CBZ oxidation using SO and EDTA seriously, where almost no CBZ could be removed throughout the reaction. This situation reveals the importance of high-valent Mn species (Mn(V)) in the oxidation of CBZ, while the contribution of CH₃CO₂^• and CH₃CO₃^• were slight.

Generally, both the traditional quenching experiments and PMSO oxidations helped in the determination of the main working reactive species during the Mn(II)/PAA/ligand process in the oxidation of CBZ. Except for PPP, the experiments using SO and EDTA showed that in addition to major ROS, including CH₃CO₂^• and CH₃CO₃^•, Mn(III) species and high-valent Mn(V) also exhibited unignorably impacts. The limited influence from TBA and obvious inhibition from PMSO indicated the participation of Mn(III) and high-valent Mn(V) species within CBZ oxidation when SO and EDTA were added. However, the high conversion rate of PMSO into PMSO₂ and the retardation from excessive PMSO on CBZ oxidation indicated the more dominant role of high-valent Mn(V) species compared to the others. Therefore, it could be concluded that Mn(III) species like Mn(III)-SO and Mn(III)-EDTA could be formed during the activation of PAA using Mn(II) with corresponding ligands added, while PPP would hardly facilitate the oxidation of Mn(II) into Mn(III) species. Furthermore, high-valent Mn(V) species could also be further formed with appropriate ligands like SO and EDTA within the Mn(II)/PAA/ligand system, which contributed largely to the oxidation of CBZ.

Thus, the possible reaction mechanism of Mn(II)/PAA/SO and Mn(II)/PAA/EDTA could be expressed using Equations (2)–(9) below. The coordination between Mn(II) and the ligands (SO or EDTA) led to the generation of Mn(II)-SO or Mn(II)-EDTA complexes. Then, the electron transfer between the Mn(II)-SO or Mn(II)-EDTA complex and PAA resulted in the formation of reactive species like CH₃C(O)O^• and Mn(III) species. Afterward, the Mn(III) species would further react with PAA to produce high-valent Mn(V) species and/or CH₃C(O)O^•. These generated reactive species, including organic radicals (CH₃C(O)O^• and CH₃C(O)OO^•) and Mn(V), contributed to the oxidation of CBZ [65,71].

For the Mn(II)/PAA/SO system:

Mn(II) + SO → Mn(II) − SO

(2)

Mn(II) − SO + CH₃C(O)OOH → Mn(III) − SO + CH₃C(O)O^• + OH⁻

(3)

Mn(III) − SO + CH₃C(O)OOH → Mn(V) − SO + CH₃C(O)O^• + OH⁻

(4)

Mn(III) − SO + CH₃C(O)OOH → Mn(II) − SO + CH₃C(O)OO^• + H⁺

(5)

For the Mn(II)/PAA/EDTA system:

Mn(II) + EDTA → Mn(II) − EDTA

(6)

Mn(II) − EDTA + CH₃C(O)OOH → Mn(III) − EDTA + CH₃C(O)O^• + OH⁻

(7)

Mn(III) − EDTA + CH3C(O)OOH → Mn(V) − EDTA + CH₃C(O)O^• + OH⁻

(8)

Mn(III) − EDTA + CH3C(O)OOH → Mn(II) − EDTA + CH₃C(O)OO^• + H⁺

(9)

2.4. Proposed Oxidation Pathway of CBZ

Generally, three oxidation pathways could be identified while the electrophilic attack on the olefinic double bond by reactive species initiated the oxidation (Figure 10). Subsequently, the following intermediates could be formed through subsequent reactions such as deacetylation, ring contraction, dehydrogenation, oxygen transfer, and dehydration, during which their structure was gradually simplified and their molar weight decreased. Among them, the electron-rich groups like C=C and C-N tended to be attacked by ^•OH and high-valent Mn species, while the hydroxylation products might have resulted from the attack of ^•OH with compounds containing electron-donating groups (such as –NH₂) [63,74,75].

(General working conditions: [Mn(II)] = 50 μM, [PAA] = 200 μM, [SO] = 100 μΜ, pH = 5.5)

3. Materials and Methods

3.1. Chemical Reagents

Except for PAA, the other chemical reagents in this work were all of analytical purity or over and were used directly as obtained without further purification. The chemical reagents, including acetic acid (AA), 30% hydrogen peroxide (H₂O₂), MnSO₄·H₂O (Mn(II)), sodium oxalate (SO), sodium pyrophosphate (PPP), and sodium ethylene diamine tetraacetic acid (EDTA), were purchased from Macklin. Methyl phenyl sulfoxide (PMSO), methyl phenyl sulfone (PMSO₂), tert-butyl-alcohol (TBA), and carbamazepine (CBZ) were obtained from Sigma-Aldrich. Acetonitrile (ACN) and methanol (MeOH) were purchased from Merck. All solutions were prepared using Milliq water (>18.2 MΩ·cm) from a Millipore system (Millipore, Billerica, MA, USA).

3.2. Synthesis of PAA

The PAA solution used in this work was prepared with acetic acid (AA) and 30% hydrogen peroxide (H₂O₂) under the catalysis of 98% H₂SO₄ in liquid phase at room temperature (25 °C), as stated by Equation (10).

$${\mathrm{CH}}_3\mathrm{COOH}+{\mathrm{H}}_2{\mathrm{O}}_2\stackrel{{\mathrm{H}}_2{\mathrm{SO}}_4}{\to }{\mathrm{CH}}_3\mathrm{COOOH}+{\mathrm{H}}_2\mathrm{O}$$

(10)

Briefly, 60 mL AA and 40 mL H₂O₂ were mixed in a 125 mL amber glass reagent bottle with a frosted mouth and cover, and 1 mL H₂SO₄ was added as the catalyst. Then, this airtight bottle was then placed under continuous magnetic stirring for 20 h at a speed of 1200 r/min to make the reaction fully processed [76,77]. All syntheses were performed in a fuming hood since AA, H₂O₂, and PAA were all volatile. The concentration of the obtained PAA after 20 h synthesis was determined via iodometric titration [7]. The PAA concentration of the synthesized solution was about 2.3 mol/L fresh and decreased by about 5% after 7 days according to our testing. Therefore, PAA was synthesized every 3 days to keep the freshness and a relatively accurate concentration.

3.3. Batch Experiments

CBZ was oxidized in a 50 mL amber iodine measuring flask with a frosted mouth and cover under continuous magnetic stirring in a fuming hood. Batch experiments were initiated by adding a predetermined amount of PAA into the mixed solution, which already contained the target compound, Mn(II), and ligands. The Mn(II) for the reaction came from the MnSO₄·H₂O stock solution. The dosages of PAA, Mn(II), and ligands varied according to the experiments and are stated in detail in the figure captions. The initial pH of the reactions was adjusted by H₂SO₄ (0.1 mol/L) and NaOH (0.1 mol/L). At given time intervals, 0.5 mL of the working solutions was withdrawn using a syringe and quickly quenched with excessive Na₂S₂O₃ (10 mM). In addition to the traditional control experiments, control experiments were performed using the raw materials of PAA synthesis, in other words, using H₂O₂, AA, and H₂O₂ + AA as oxidants.

Traditional quenching experiments were performed to determine the generation and participation of reactive species like ^•OH, peroxyl radicals (CH₃CO₂^• and CH₃CO₃^•), and high-valent Mn species. Herein, scavengers including tert-butyl alcohol (TBA), methanol (MeOH), and methyl phenyl sulfoxide (PMSO) were adopted.

All oxidation experiments were conducted thrice with relative standard deviations less than 5%.

3.4. Analytical Methods

The concentrations of CBZ, PMSO, and PMSO₂ were analyzed using a High-Performance Liquid Chromatography (HPLC, Waters, MA, USA) system equipped with a DAD detector and a reverse-phase C18 column (Restek, 5 μm, 250 × 4.6 mm) at a flow rate of 1 mL/min under room temperature. The HPLC analyses for CBZ, PMSO, and PMSO₂ were all performed using isocratic elution. The mobile phase for CBZ consisted of 40% of MilliQ water and 60% of ACN, with the detection wavelength set at 286 nm. The mobile phase for PMSO consisted of 50% of 0.1% (v/v) formic acid in water and 50% of ACN, with the detection wavelength set at 230 nm. The mobile phase for PMSO₂ consisted of 50% of 0.1% (v/v) formic acid in water and 50% of ACN, with the detection wavelength set at 215 nm [78]. The oxidation intermediates of CBZ were identified using Liquid Chromatography coupled with an Orbitrap Mass Spectrometer (LC-MS, Thermo Scientific, MA, USA) with positive-mode electrospray ionization (ESI⁺). Chromatographic separation was carried out on a C18 HPLC capillary column (5 μm, 100 mm × 2.1 mm). The eluents consisted of deionized water (eluent A, containing 0.1% formic acid) and ACN (eluent B) with a flow rate of 0.25 mL min⁻¹. The gradient program was started at 95% A for 12 min, followed by a decrement to 5% A in 5 min and then went back to 95% A in 5 min. The column temperature and injection volume were set at 30 °C and 10 μL. The cyclic voltammetry (CV) was performed with a Chenhua electrochemical workstation in a three-electrode electrochemical cell comprising a Pt wire counter electrode, a saturated Ag/AgCl reference electrode, and a glassy carbon electrode as the working electrode at room temperature. The voltammetry results were collected at a potential scan rate of 10 mV/s. Finally, the UV-vis spectra were scanned using a Shimadzu UV2600i spectrometer (Shimadzu, Japan).

4. Conclusions

This study mainly focused on the efficient oxidation of CBZ through the activation of PAA, a sufficient disinfectant traditionally used in water treatment, in the textile industry, and in the sterilization of equipment in healthcare settings. As PAA is difficult to activate using solely Mn(II), this work focused on the enhanced oxidation efficiency of a Mn(II)/PAA system through the addition of chelated ligands. The optimal working parameters, such as the working pH, dosages of PAA, and ratio between Mn(II) and ligands, were investigated separately. The results show that SO and EDTA could significantly promote oxidation efficiency at near-neutral pH, while PPP only slightly influenced the reactions. Mn(III) species could be stabilized effectively using appropriate ligands like SO and EDTA, and later investigations showed that these stabilized Mn (III) species could even be further generated into high-valent Mn species (Mn(V)), which also contributed obviously to the oxidation of CBZ in addition to CH₃CO₂^• and CH₃CO₃^•. This study sheds light on a new advanced oxidation process, which helps broaden present knowledge of PAA activation as well as manganese chemistry for decontamination in water treatment.