Picolinic Acid-Mediated Catalysis of Mn(II) for Peracetic Acid Oxidation Processes: Formation of High-Valent Mn Species

Metal-based advanced oxidation processes (AOPs) with peracetic acid (PAA) have been extensively studied to degrade micropollutants (MPs) in wastewater. Mn(II) is a commonly used homogeneous metal catalyst for oxidant activation, but it performs poorly with PAA. This study identifies that the biodegradable chelating ligand picolinic acid (PICA) can significantly mediate Mn(II) activation of PAA for accelerated MP degradation. Results show that, while Mn(II) alone has minimal reactivity toward PAA, the presence of PICA accelerates PAA loss by Mn(II). The PAA-Mn(II)-PICA system removes various MPs (methylene blue, bisphenol A, naproxen, sulfamethoxazole, carbamazepine, and trimethoprim) rapidly at neutral pH, achieving >60% removal within 10 min in clean and wastewater matrices. Coexistent H2O2 and acetic acid in PAA play a negligible role in rapid MP degradation. In-depth evaluation with scavengers and probe compounds (tert-butyl alcohol, methanol, methyl phenyl sulfoxide, and methyl phenyl sulfone) suggested that high-valent Mn species (Mn(V)) is a likely main reactive species leading to rapid MP degradation, whereas soluble Mn(III)-PICA and radicals (CH3C(O)O• and CH3C(O)OO•) are minor reactive species. This study broadens the mechanistic understanding of metal-based AOPs using PAA in combination with chelating agents and indicates the PAA-Mn(II)-PICA system as a novel AOP for wastewater treatment.


■ INTRODUCTION
−16 Recent reviews have reported that PAAtreated wastewater effluents have low ecotoxicological impacts, and PAA-based pulp bleaching generates little persistent toxic or mutagenic residuals or byproducts. 7,17−29 While PAA has relatively low reactivity toward MPs with the exception of sulfur moieties, 30 PAA can be activated to produce highly reactive species that can more efficiently degrade MPs. The • OH is a well-known strong oxidant to degrade a wide range of MPs.CH 3 C(O)OO • is also a powerful oxidant that rapidly reacts with MPs via electron transfer. 23,31CH 3 C(O)O • has high oxidation power but is subject to rapid self-dissociation to less reactive • CH 3 and CO 2 (k = 2.3 × 10 5 s −1 ). 324][25][26]33 Only small amounts of those metals (10− 200 μM) can efficiently activate PAA (100−200 μM) and degrade a wide range of MPs, including pharmaceuticals, antibiotics, and dyes. It ispostulated that high-valent metal species are generated when metal catalysts are applied to activate PAA.High-valent metal species are known to be strong oxidants but with greater selectivity in reactivity than free radicals; 34−36 thus, they may persist longer in environmental water matrices that contain various reactive speciesscavenging constituents.
Manganese is a common transition metal found in natural environments and has a wide range of oxidation states from +2 to +7 in aqueous solutions.Water-soluble Mn(II) has been extensively studied as a homogeneous activator for many oxidants, 37−40 but there is little information on PAA activation by Mn(II).To the best of our knowledge, only one study 41 evaluated the abatement of MP by Mn(II)-PAA in aqueous samples.They reported that Orange II was degraded by Mn(II)-PAA at pH 9.4; however, a high dosage of PAA ([PAA] = 5−20 mM, [Mn(II)] = 100 μM) was required.Moreover, at acidic to neutral pHs (pH 3−7), the degradation of Orange II was limited in similar conditions.They proposed the formation of Mn(III) and Mn(IV), which was supported by UV/visible spectral changes and electron paramagnetic resonance (EPR) spectroscopy.It was also postulated that Mn(II) complexes with PAA to form the peroxo complex, which undergoes heterolytic cleavage to form Mn(IV)-oxo species and/or homolytic bond cleavage to yield Mn(III) and organic radical.
Catalytic activity of metals can be improved by in situ addition of some chelating agents.In our recent study, we found that picolinic acid (2-pyridinecarboxylic acid; PICA) is a highly efficient chelating agent that can accelerate the reaction of PAA with Fe(III), thereby facilitating the oxidation of MPs by the Fe(III)-PAA process at neutral pH conditions. 42PICA has been widely used as a chelating agent in chemical and pharmaceutical applications. 43,44Recently, it has been applied to water treatment, owing to its biodegradability and lower toxicity compared to other types of chelating agents. 42,45−48 The σ-donor (and weak π-acceptor) features of the aromatic nitrogen of PICA boost the nucleophilicity of the metal center and the catalytic activity of the metal complex. 47uilding upon the previous work, this study investigated whether PICA could improve the catalytic efficiency of Mn(II) for PAA oxidation processes in a wide pH range (pH 3.0−9.0).The study objectives included: (i) demonstrating the capability of the PAA-Mn(II)-PICA system to degrade a model MP, methylene blue (MB), under a wide range of reaction conditions (i.e., molar ratios of metal to ligand (Mn(II) to PICA), molar ratios of metal to oxidant (Mn(II) to PAA), solution pHs, and presence of other anions), (ii) identifying the generation of major reactive species and their contributions to MP degradation in the PAA-Mn(II)-PICA system by using scavengers and probe compounds, and (iii) investigating the
■ EXPERIMENTAL SECTION Chemicals.Sources of chemicals and reagents are provided in Text S1.
In the scavenging experiments to evaluate the influence of Analytical Methods.The DPD method was used to determine the PAA concentration. 19,49The additional information on other analytical methods is available in Text S1.

■ RESULTS AND DISCUSSION
Reaction of PAA with Mn(II)-PICA.First, the decay of PAA by Mn(II) in the absence and presence of PICA was monitored (conditions: [PAA] 0 = 200 μM, [Mn(II)] 0 = 10− 40 μM, [PICA] 0 = 0 or 50−200 μM where [Mn(II)]:[PICA] ratio = 1:5, initial pH = 5.0).As shown in Figure 1A, the loss of PAA by Mn(II) without PICA was negligible.In contrast, the addition of PICA noticeably increased the loss of PAA, resulting in approximately 51.8−90.4% of PAA consumption within 10 min at increased [Mn(II)]/[PICA] concentrations, strongly suggesting that PICA enhances the reactivity of Mn(II) toward PAA.Importantly, the loss of PAA continued without adding additional Mn(II)-PICA (Figure S1), suggesting that reactive species (possibly intermediate Mn species) formed through the reaction of PAA with Mn(II)-PICA could further consume PAA rapidly.
Based on previous research on the reaction of PAA with metals such as Fe(II), 20 Co(II), 24,29 Ru(III), 26 and Fe(III)-PICA, 42 we proposed the following reactions in the PAA-Mn(II)-PICA system.In the presence of Mn(II) and PICA, the reaction of PAA with Mn(II)-PICA could form reactive species, such as Mn(III), Mn(IV/V), and/or CH 3 C(O)O • , according to eqs 1−4: Other possible reactions, generating • OH and CH 3 C(O)OO • , may also be considered (eqs 5−8): (5) The possible decay of PAA by Mn(III)-PICA was then investigated separately (conditions: [PAA] 0 = 200 μM, [Mn(III)] 0 = 20 μM, [PICA] 0 = 100 μM, initial pH = 5.0).As shown in Figure 1B, the rate of PAA loss was much faster when the reaction was initiated by Mn(III)-PICA.This result supports the above hypothesis that Mn(III)-PICA is likely an Environmental Science & Technology intermediate which has greater reactivity toward PAA.The fast reaction of Mn(III)-PICA with PAA could lead to the formation of higher valent Mn species, such as Mn(IV/V).
Since the PAA solution contained about 32% PAA, 6% H 2 O 2 , and 40% acetic acid, 83.8 μM H 2 O 2 and 316.7 μM acetic acid were present in the 200 μM PAA solution.Reactions related to coexistent H 2 O 2 might form reactive species (eqs 9 and 10): (9) (10) However, we found that H 2 O 2 did not decay by either Mn(II)-PICA or Mn(III)-PICA, whether or not acetic acid was present  S2).These results confirmed that coexistent H 2 O 2 and acetic acid in the PAA solution played a negligible role in the overall reaction.Furthermore, the addition of TBA, a highly reactive quencher of 50 had a negligible effect on PAA decomposition by Mn(II)-PICA or Mn(III)-PICA (Figure 1D).As PAA is known to have high reactivity to • OH, 19,51 the little impact of TBA indicated that the generation of • OH was likely negligible in the reaction of PAA with Mn(II)-PICA and Mn(III)-PICA.Degradation of MPs by the PAA-Mn(II)-PICA System.First, the PAA-Mn(II) system without PICA showed almost no removal of MPs (<0.7%) (Figure 2A).The degradation % of MPs ([MP] removal,% ) and the initial first-order rate constant (k initial in min −1 ) for the MP degradation were used to compare the efficiency of MP abatement.[MP] removal,% was obtained for a reaction time of 10 min.k initial in min −1 was calculated by the slope of ln(C t /C 0 ) versus time at the initial stage of the reaction, where the reaction could be considered to follow pseudo-first-order kinetics (Figure S3).
Effect of Mn(II) to PICA Molar Ratio.The effect of PICA on the degradation of a model compound, MB, by the PAA-Mn(II) system was investigated.The addition of PICA significantly enhanced MB degradation by PAA-Mn(II) (Figure 2A and Table S2).Different molar ratios of [Mn(II)]:[PICA] ranging from 1:1 to 1:10 were investigated
In this study, the [Mn(II)]:[PICA] ratio of 1:5 was selected to further assess the impacts of different reaction conditions (i.e., PAA and Mn(II) dosages, solution pH, coexistent H 2 O 2 and acetic acid, water matrix constituents).
Effects of PAA and Mn(II) Dosages.The effects of PAA and Mn(II) dosages on MB degradation by the PAA-Mn(II)-PICA system were investigated.First, the effect of PAA dosage was investigated at the initial pH of 5.0 under two reaction conditions (molar ratio [PAA]   S4).

Effects of Coexistent
− had minimal impact on the MB degradation by PAA-Mn(II)-PICA (Figure S6B).In contrast, previous research reported minimal to moderate impacts of HCO 3 − on PAA-Ru(III) and PAA-Fe(III)-PICA, 26,42 while having a significant inhibitory effect on PAA-Co(II). 24,29ompared to Cl − and HCO 3 − , H 2 PO 4 − /HPO 4 2− moderately retarded MB degradation by the PAA-Mn(II)-PICA system (k initial decreasing from (2.78 ± 0.24) × 10 −1 to (2.14 ± 0.35) × 10 −1 min −1 ) and reduced the overall abatement from 80.5% to 68.9% (Figure S6C).The complex formation . 57When the anion concentration is 10 mM, Mn(II) forms only weak complexes with Cl − (f MnCl = 0.01) but forms strong complexes with HCO 3 ) and , MB degradation was only slightly to moderately impeded, indicating that PICA could still competitively interact with Mn(II) (and intermediate Mn species) and mediate the reaction.The above results suggest that environmental waters containing high concentrations of H 2 PO 4 − /HPO 4 2− will exert some inhibitory effect on MB degradation by the PAA-Mn(II)-PICA system through complexation competition.
Major Reactive Species in the PAA-Mn(II)-PICA System.Impacts of Mn(III).Mn(III) formed by the reaction of Mn(II) with PAA could be an oxidant to oxidize MPs.Mn(III) is unstable and rapidly disproportionates to Mn(II) and Mn(IV). 58−61 Similarly, in this study, the presence of PICA stabilized Mn(III), thus generating minimal Mn(IV) (colloidal MnO 2 ), which was confirmed by no peak observed at 350−420 nm in Mn(III)-PICA (Figure S7B).Separately, we investigated the impact of Mn(III)-PICA on the degradation of MPs (MB, BPA, CBZ, NPX, and SMX).Degradation of CBZ, NPX, and SMX was negligible during 10 min, but MB and BPA were degraded to some extent with [MP] removal,% of 8.6% and 54.4%, respectively (Figure 4; conditions: [MP] 0 = 15 μM, [Mn(III)] 0 = 20 μM, [PICA] 0 = 100 μM, initial pH = 7.0).The reactivities of soluble Mn(III) toward MPs are scarcely reported, but several studies reported the selectivity of Mn(III) in degrading MPs.Jiang et al. 62 reported that Mn(III) did not show reactivity toward CBZ, while Mn(III) was an intermediate enhancing the degradation of BPA during Mn(VII) oxidation in another study. 63Sun et al. 64 reported enhanced degradation of MB by Mn(III) formed by the activation of MnO 2 by S(IV).Our results generally corroborate with the previous studies.

Environmental Science & Technology
system was faster than that in the PAA-Mn(II)-PICA system, which was likely due to (i) Mn(III)-PICA reacting with PAA faster than Mn(II)-PICA owing to higher reactivity and/or (ii) the reaction of Mn(II)-PICA with PAA initially forming Mn(III) which was further converted to high-valent Mn species (Mn(V)).Further oxidized species (Mn(VII) could be ruled out, because it was confirmed that Mn(VII) was not formed during the reaction (no peak at 525 nm, Figure S7C,D).Additionally, previous studies have reported that Mn(VI/VII) degrades PMSO much more slowly than Mn(V). 67,68ontribution of Reactive Species to MP Degradation.The contribution of reactive species to MB degradation was evaluated by adding several scavengers (Figure 5C, conditions: [MB] 0 = μM, [PAA] 0 = 200 μM, [Mn(II)] 0 = 20 μM, [PICA] 0 = 100 μM, initial pH = 5.0).Addition of 50 mM TBA minimally influenced the degradation of MB, indicating that • OH was not important.MeOH is also a well-known scavenger for free radicals including • OH (k •OH/TBA = 6.0 × 10 8 M −1 • s −1 ). 69Meanwhile, MeOH is a probable scavenger for CH 3 C(O)O • /CH 3 C(O)OO • , as Wang et al. 23 reported that MeOH suppressed the degradation of MP in the PAA-Co(II) system where little plays a major role in MP degradation.Note that Mn(IV) is inert to MeOH, but Mn(V) may have some reactivity with MeOH. 70Adding 50 mM MeOH slightly retarded MB degradation (k initial was decreased from (2.82 ± 0.19) to (2.13 ± 0.20) × 10 0 min −1 , Table S8), suggesting that CH 3 C(O)O • /CH 3 C(O)OO • and/or Mn(V) could contribute to MB degradation.
Then, the relative contributions of CH 3 C(O)O • /CH 3 C-(O)OO • versus Mn(V) to MB degradation by PAA-Mn(II)-PAA was assessed by using PMSO.Addition of 5.0 mM PMSO completely inhibited the degradation of MB, indicating the importance of high-valent Mn species (Mn(V)) in MB degradation in the PAA-Mn(II)-PICA system, and the contribution of CH 3 C(O)O • /CH 3 C(O)OO • was minimal.The impact of O 2 was also assessed by investigating the degradation in a purged reaction solution, and MB degradation by the PAA-Mn(II)-PICA system was minimally influenced by the presence or absence of O 2 .Overall, with limited impact from TBA but significant inhibition of MB degradation by PMSO as well as a high conversion yield of PMSO to PMSO 2 , high-valent Mn species (Mn(V)), rather than radicals, were likely the predominant reactive species leading to MPs' degradation.
Other Minor Reactive Species.First, as mentioned above, we confirmed that MPs' degradation in H 2 O 2 -Mn(II)-PICA was limited (Figure 3B), indicating that the reactive species generated from the reaction of H 2 O 2 with Mn (including highvalent species)-PICA played a minor role in MPs' degradation.Minimal impact of TBA ruled out the contribution of • OH to MPs' degradation (Figure 5C).HO 2 • /O 2 •− has low reactivity with MPs (k HOd 2
The feasibility of the PAA-Mn(II)-PICA system to degrade MPs (MB, NPX, SMX, and BPA) in real water matrices was

Environmental Science & Technology
assessed in a tertiary wastewater effluent.Results showed minimal to mild impact of real water matrices, and >66% of MPs' degradation was achieved (Figure 6B).These results demonstrate the effectiveness of the PAA-Mn(II)-PICA system for degrading MPs in wastewater containing complex matrix components such as organic matter and divalent metals.
Comparison to the PAA-Fe(III)-PICA System.The chelating agent PICA significantly enhances the capability of both Mn(II) and Fe(III) to activate PAA for MP degradation. 42Both PAA-Mn(II)-PICA and PAA-Fe(III)-PICA systems showed much less degradation of MPs at acidic pH than at higher pH, due to protonation of PICA hindering metal complexation.Under comparable reaction conditions (i.e., 1:5 metal-to-PICA molar ratio and PAA dose), both systems could achieve a high percentage of MP degradation at the initial pH of 7 within 10 min (>56% for Fe(III): [PAA] 0 = 500 μM, [Fe(III)] 0 = 50 μM, [PICA] 0 = 125 μM; >50% for Mn(II): [PAA] 0 = 200 μM, [Mn(II)] 0 = 20 μM, [PICA] 0 = 100 μM).The presence of Cl − has little impact on MP degradation in both systems.In the PAA-Fe(III)-PICA system, HCO 3 − moderately reduces MP abatement from 90% to 77%, while H 2 PO 4 − /HPO 4 2− completely inhibits MP degradation.Comparatively, the inhibitory effects of HCO 3 − and H 2 PO 4 − / HPO 4 2− are weaker in the PAA-Mn(II)-PICA system.Different effects of anions are likely due to their different ability to compete with PICA for complexing Fe(III) versus Mn(II).High-valent metal species, rather than radicals, are major reactive species contributing to MP degradation in both systems.PAA shows little reactivity for several transition metals, such as Fe(III), Mn(II), Mn(III), Cu(II), and Ni(II). 27t would be worthwhile to expand research further into the application of PICA to such transition metals in PAA systems.
Environmental Significance and Implications.The PAA-Mn(II) AOP requires high dosages of PAA and Mn(II) to achieve sufficient efficiency for MP removal.This study showed that the chelating agent, PICA, can dramatically enhance the efficiency of PAA-Mn(II) AOP to degrade a range of MPs in the pH range of 3.0−9.0.With low dosages of PAA and Mn(II), significant abatement of MPs was achieved, and the impacts of real water matrices were minimal, demonstrating the PAA-Mn(II)-PICA system to be a promising AOP.The robust evaluation suggested that high-valent Mn species (Mn(V)) were likely the major reactive species contributing to MP degradation by the PAA-Mn(II)-PICA system.The minimal impacts of water matrices could be attributed to the formation of selective Mn(V).A variety of chelating agents, such as EDTA and NTA, have been applied to metal ion-based AOPs to improve the efficiency of catalytic activity for oxidants.PICA is less toxic and more biodegradable than other chelating agents, which is beneficial in minimizing negative environmental risks. 45,78Furthermore, among metals, Mn has low toxicity and high abundance in natural environments, while PAA exhibits several advantages over other conventional oxidants.Thus, the PAA-Mn(II)-PICA process could be a promising AOP that is suitable for a wide pH range and complex water matrices.Further research is needed to evaluate in detail the potential impacts of degradation products of MPs as well as PICA and its products.