Zero-Valent Manganese Promoted Fe²⁺/H₂O₂ Oxidation for Acid Orange 7 Degradation: Performance, Mechanism and Water Matrix Effects

Abstract

Fenton oxidation is effective for degrading refractory organic contaminants, but slow Fe³⁺-to-Fe²⁺ regeneration under acidic conditions limits continuous H₂O₂ activation. Here, Acid Orange 7 (AO7) was used as a model pollutant to evaluate a coupled Mn⁰/Fe/H₂O₂ system designed to promote iron redox cycling and enhance H₂O₂ activation. AO7 degradation was examined under different operating conditions, and reactive species and mechanisms were identified using electron spin resonance spectroscopy, chemical probes, and quenching experiments. Under optimized conditions (pH 3.0, 10 μM AO7, 2 mM H₂O₂, 9 μM Fe²⁺, and 0.05 g/L Mn⁰), 88% of AO7 was degraded within 30 min. Hydroxyl radicals (•OH) and FeO²⁺ were identified as the main reactive species, accounting for 88.6% and 11.4% of AO7 degradation, respectively. Mn⁰ promoted AO7 degradation by directly activating H₂O₂ to form •OH and by reducing Fe³⁺ to Fe²⁺, thereby sustaining Fe²⁺/Fe³⁺ cycling and facilitating •OH and FeO²⁺ generation. Sulfate and carbonate had negligible effects, whereas fulvic acid, nitrate, and chloride inhibited degradation. AO7 degradation decreased to 40% in lake water and 60% in river water. These results demonstrate that Mn⁰ addition can enhance Fenton-like oxidation by accelerating iron redox cycling, suggesting its potential in treating dye-contaminated wastewater under controlled operational conditions.

1. Introduction

Dyes are extensively used in a wide range of industrial sectors, including textile, leather, paper, printing, and food processing industries. Due to incomplete fixation during manufacturing and application processes, substantial amounts of dyes are discharged into industrial effluents, resulting in wastewater with high organic load, intense color, complex composition, and elevated suspended solids [1]. In addition to causing serious aesthetic deterioration of water bodies, many dyes and their degradation intermediates exhibit persistence, toxicity, and potential ecological risks [2]. Therefore, the efficient treatment of dye-containing wastewater has become an important issue in water pollution control.

Hydrogen peroxide (H₂O₂)-based advanced oxidation processes (AOPs) have attracted extensive attention because of their simplicity, strong oxidation capacity, and effectiveness in degrading refractory organic pollutants [3,4,5]. Among them, the Fenton process is one of the most widely studied systems, in which Fe²⁺ activates H₂O₂ to generate hydroxyl radicals (•OH) for rapid oxidation of organic compounds (Equation (1)) [6]. However, the practical efficiency of the Fenton process is often limited by the slow regeneration of Fe²⁺ from Fe³⁺, which suppresses the Fe³⁺/Fe²⁺ redox cycle and lowers H₂O₂ utilization efficiency. Therefore, accelerating Fe²⁺ regeneration is crucial for improving the performance of Fenton oxidation systems.

Fe²⁺ + H₂O₂ → •OH + Fe³⁺ + OH⁻

(1)

Various approaches have been developed to enhance the Fe³⁺/Fe²⁺ cycle in the Fenton process. For example, several reducing agents, such as thiosulfate [7], gallic acid [8], hydroxylamine [9] and ascorbic acid [10], have been introduced to promote the reduction of Fe³⁺ to Fe²⁺, and their effectiveness in enhancing contaminant degradation has been demonstrated. However, the addition of soluble organic or inorganic reductants may introduce secondary contaminants, consume reactive oxygen species, or compete with target pollutants for •OH, thereby lowering the overall oxidation efficiency [11]. Zero-valent iron (Fe⁰) has also been employed to strengthen Fenton systems because it can continuously provide Fe²⁺ through corrosion under acidic conditions [12]. Nevertheless, the corrosion and electron-transfer processes of Fe⁰ are strongly affected by the formation and thickening of surface oxide layers, which increase electron-transfer resistance and hinder the reduction of Fe³⁺ by Fe⁰. In addition, external energy-assisted methods, such as ultraviolet irradiation [13] and electric assistance [14], have been reported to effectively promote Fe³⁺ reduction and enhance H₂O₂ activation. However, these methods generally require additional energy input and may show limited resistance to water matrix interference, which constrains their practical application in complex wastewater treatment. Accordingly, it is highly necessary to explore novel, feasible modification strategies to optimize and upgrade traditional Fenton technology.

Compared with the direct addition of soluble reductants, zero-valent metals are attractive because they can continuously donate electrons and facilitate Fe³⁺ reduction without introducing large amounts of dissolved organic additives. Among them, zero-valent copper (Cu⁰) has been reported to effectively accelerate the Fe³⁺/Fe²⁺ cycle, thereby improving H₂O₂ activation and pollutant degradation efficiency [15]. However, Cu⁺ generated during the corrosion of Cu⁰ can be readily oxidized by dissolved oxygen to Cu²⁺, leading to inefficient electron utilization and potentially limiting the long-term performance of the system [16]. In this context, zero-valent manganese (Mn⁰) may represent a promising alternative. Owing to the lower redox potential of the Mn²⁺/Mn⁰ couple relative to that of Cu²⁺/Cu⁰, Mn⁰ is expected to exhibit a stronger reducing ability and thus greater potential to promote Fe³⁺ reduction in Fenton systems [17]. Moreover, manganese species are generally considered less toxic than copper species, which may make Mn⁰ more favorable for practical water treatment applications [18]. Although Mn-based materials have been investigated in some oxidation systems, the performance and mechanism of the Mn⁰/Fe²⁺/H₂O₂ process for pollutant degradation have not yet been systematically clarified. In particular, the role of Mn⁰ in promoting H₂O₂ activation, the dominant reactive oxygen species involved, and the applicability of this process in complex water matrices remain insufficiently understood.

In this study, dye degradation by the Mn⁰/Fe²⁺/H₂O₂ system was systematically investigated. Among dyes, Acid Orange 7 (AO7), a typical azo dye with high chromaticity and recalcitrance, was chosen as a target pollutant. The objectives of this work were: (i) to evaluate the enhancement effect of Mn⁰ on AO7 degradation in the Fe²⁺/H₂O₂ system; (ii) to elucidate the dominant reactive oxygen species and the associated H₂O₂ activation mechanism; and (iii) to assess the applicability of the Mn⁰/Fe²⁺/H₂O₂ system in real water matrices and its resistance to interference from common coexisting substances. This work is expected to provide mechanistic insight into Mn⁰-assisted Fenton oxidation and to support the development of efficient and practical advanced oxidation technologies for dye-containing wastewater treatment.

2. Results and Discussion

2.1. AO7 Degradation in Different Reaction Systems

Figure 1a shows the AO7 degradation performance of five reaction systems, including H₂O₂, Mn⁰, Fe²⁺/H₂O₂, Mn⁰/H₂O₂, and Mn⁰/Fe²⁺/H₂O₂ systems. Negligible AO7 degradation was observed in the H₂O₂ system and Mn⁰ systems within 30 min, indicating that neither H₂O₂ nor Mn⁰ alone was effective for AO7 degradation under the tested conditions. In the Fe²⁺/H₂O₂ system, approximately 25% of AO7 was rapidly degraded in the initial 1 min, after which the pollutant concentration remained nearly unchanged. This trend is consistent with previous studies, in which Fe²⁺ rapidly activated H₂O₂ at the initial stage, whereas the slow reduction of Fe³⁺ to Fe²⁺ limited the sustained generation of reactive oxidizing species [9]. By contrast, the Mn⁰/H₂O₂ system achieved about 30% AO7 degradation within 30 min, suggesting that Mn⁰ was able to activate H₂O₂ to some extent. More importantly, the Mn⁰/Fe²⁺/H₂O₂ system exhibited the best performance, achieving 88% AO7 degradation within 30 min. This efficiency was substantially higher than those of the Fe²⁺/H₂O₂ and Mn⁰/H₂O₂ systems, suggesting a synergistic effect between Mn⁰ and Fe²⁺ in promoting H₂O₂ activation. Furthermore, a systematic comparison of AO7 degradation performance among various Fenton-like processes is summarized in Table S1. Compared to mineral-catalyzed or energy-assisted Fenton processes, the Mn⁰/Fe²⁺/H₂O₂ system exhibits highly competitive degradation efficiency rapidly (within 30 min) at remarkably lower chemical dosages and without external energy input. Its primary advantage lies in the rapid and sustained regeneration of Fe²⁺ driven by Mn⁰.

To quantitatively compare the reaction kinetics, a pseudo-first-order kinetic model was applied (Figure 1b). However, this model was not suitable for the Fe²⁺/H₂O₂ system. In that system, the reaction exhibited a rapid initial burst but virtually stalled within the first minute due to the swift depletion of Fe²⁺ and the slow regeneration from Fe³⁺, leading to a premature plateau. Consequently, kinetic fitting for the Fe²⁺/H₂O₂ system was omitted to avoid statistically invalid comparisons. For the applicable systems, the rate constants (k₁) were calculated over the 30 min fitting range. The Mn⁰/Fe²⁺/H₂O₂ system exhibited a k₁ of 0.194 ± 0.071 min⁻¹ (R² = 0.97), which was notably higher than that of the Mn⁰/H₂O₂ system (k₁ = 0.059 ± 0.023 min⁻¹, R² = 0.92). These kinetic results quantitatively support that the introduction of Mn⁰ helps sustain the catalytic oxidation and mitigates the rapid stagnation typically observed in the Fenton process.

2.2. Identification of Reactive Oxygen Species in the Mn⁰/Fe²⁺/H₂O₂ System

The identification of reactive oxygen species (ROS) is critical for elucidating the oxidation mechanism of advanced oxidation processes. In Fe²⁺-activated H₂O₂ systems, •OH has traditionally been considered the predominant oxidizing species [14,15]. However, increasing evidence suggests that ferryl Fe(IV), such as FeO²⁺, may also be formed during Fenton-like reactions and contribute to pollutant oxidation [19,20]. Accordingly, both •OH and FeO²⁺ were considered potential reactive species for the degradation of target pollutants in the Mn⁰/Fe²⁺/H₂O₂ system.

To clarify the ROS involved in AO7 degradation, ESR analysis and scavenging experiments were performed. As shown in Figure 2a, a characteristic four-line ESR signal with an intensity ratio of 1:2:2:1 was observed in the presence of DMPO, which is attributable to the DMPO-•OH adduct. This result confirms the generation of •OH in the Mn⁰/Fe²⁺/H₂O₂ system.

To further evaluate the contributions of different oxidizing species, nitrobenzene and ethanol were employed as chemical scavengers. Nitrobenzene is known to react rapidly with •OH but negligibly with FeO²⁺ [21]. As shown in Figure 2b, the addition of 1 mM nitrobenzene markedly suppressed AO7 degradation, and the degradation efficiency decreased to approximately 10%, indicating that •OH was the dominant oxidizing species in the system. In contrast, ethanol was capable of simultaneously scavenging both •OH and FeO²⁺ [22]. When 2 M ethanol was introduced, AO7 degradation was almost completely inhibited (Figure 2b). The stronger inhibition observed with ethanol than with nitrobenzene suggests that FeO²⁺ also participated in AO7 oxidation, although its contribution was secondary to that of •OH.

In addition, PMSO was adopted as a specific probe for FeO²⁺. It has been documented that FeO²⁺ can selectively oxidize PMSO to produce PMSO₂, whereas •OH tends to yield other byproducts such as hydroxylated derivatives and biphenyl substances [23]. As shown in Figure S1, nearly 5 μM PMSO₂ was detected after 1 mM PMSO was added to the Mn⁰/Fe²⁺/H₂O₂ system, providing additional evidence for the formation of FeO²⁺. The generation of FeO²⁺ may be described by Equation (2) [24]:

Fe²⁺ + H₂O₂ → FeO²⁺ + H₂O

(2)

Although classical Fenton chemistry predominantly generates •OH at bulk pH < 3.0 (consistent with the 88.6% contribution of •OH in this study), the detection of FeO²⁺ (11.4% contribution) indicates the co-existence of the two-electron transfer pathway. This could be attributed to the corrosive consumption of protons by Mn⁰ at the solid-liquid interface, which increases the pH near the Mn⁰ surface, creating a microenvironment that favors the heterolytic cleavage of H₂O₂ to generate FeO²⁺.

Collectively, all of the above results indicate that •OH and FeO²⁺ were involved in AO7 degradation in the Mn⁰/Fe²⁺/H₂O₂ system, with •OH serving as the predominant reactive species. Based on the inhibition extents observed in the scavenging experiments, the relative contributions of •OH and FeO²⁺ to AO7 degradation were estimated to be 88.6% and 11.4%, respectively.

2.3. Enhancement Mechanism of Mn⁰/Fe²⁺/H₂O₂ System

The superior AO7 degradation observed in the Mn⁰/Fe²⁺/H₂O₂ system suggests a synergistic interaction between Mn⁰ and Fe²⁺ during H₂O₂ activation. As shown in Figure S2, negligible AO7 degradation occurred in the H₂O₂, Mn⁰, and Mn²⁺/H₂O₂ systems, indicating that dissolved Mn²⁺ contributed little to H₂O₂ activation under the tested conditions. During the reaction, Mn⁰ was gradually oxidized and released predominantly as Mn²⁺. Therefore, the reactivity observed in the Mn⁰/H₂O₂ system can be reasonably ascribed to direct H₂O₂ activation at the Mn⁰ surface. Specifically, the solid-liquid interfacial electron transfer from Mn⁰ to H₂O₂ is likely to promote the cleavage of the O-O bond, generating •OH as proposed in Equation (3). Previous studies have likewise shown that the coupling of Fe²⁺ and Mn⁰ can enhance H₂O₂ activation [17,25], thereby greatly improving the degradation efficiency of AO7.

2H₂O₂ + Mn⁰ → Mn²⁺ + 2•OH + 2OH⁻

(3)

To further elucidate the enhancement mechanism, the variations in Fe²⁺ and Mn⁰ concentrations were monitored. As shown in Figure 3, Fe²⁺ in the Fe²⁺/H₂O₂ system decreased from 9 μM to 0 μM, whereas 0.5–1 μM Fe²⁺ remained in the Mn⁰/Fe²⁺/H₂O₂ system throughout the reaction. The persistence of Fe²⁺ suggests that Mn⁰ facilitated the regeneration of Fe²⁺ from Fe³⁺. To experimentally verify this, we monitored the generation of Fe²⁺ in a control Mn⁰/Fe³⁺ system ([Mn⁰]₀ = 0.05 g/L, [Fe³⁺]₀ = 10 μM, pH₀ = 3.0 ± 0.1 and 25 ± 2 °C). The results revealed that 9.50 ± 0.21 μM of Fe²⁺ was rapidly produced within 5 min of reaction, providing direct evidence that Mn⁰ efficiently reduces Fe³⁺ to maintain the iron redox cycle (as shown in Equation (4)).

2Fe³⁺ + Mn⁰ → Mn²⁺ + 2Fe²⁺

(4)

In addition, Mn²⁺ release increased slightly from 246 to 259 μM after Fe²⁺ was introduced into the Mn⁰/H₂O₂ system (Figure 3), which is consistent with Mn⁰ oxidation coupled with Fe³⁺ reduction. Given the initial Mn⁰ dosage of 0.05 g/L (approximately 0.91 mM), the released Mn²⁺ accounts for a sacrificial consumption of 28.5% of the initial Mn⁰, with the remaining 71.5% remaining in the solid phase. Meanwhile, the homogeneous iron species remained dissolved and 100% conserved in the liquid phase under the acidic conditions. These results indicate that Mn⁰ promoted Fe³⁺/Fe²⁺ redox cycling in the coupled system.

It should be noted that while this sacrificial dissolution of Mn⁰ effectively drives the iron cycle, it leads to a final Mn²⁺ accumulation of 259 μM (14.2 mg/L). This value exceeds the Mn discharge limit of 2.0 mg/L specified in the Integrated Wastewater Discharge Standard (GB 8978-1996) [26]. Therefore, to prevent secondary metal contamination in practical applications, a subsequent alkaline precipitation step (e.g., adjusting effluent pH to ≥9.0) is necessary as a post-treatment to efficiently recover the dissolved Mn as solid precipitates.

The accelerated iron redox cycle also enhanced H₂O₂ utilization. As shown in Figure S3, H₂O₂ decomposition in the Mn⁰/Fe²⁺/H₂O₂ system was significantly higher than that in either the Mn⁰/H₂O₂ or Fe²⁺/H₂O₂ system. At the end of the reaction, the H₂O₂ consumed in the ternary system reached 0.36 mM (corresponding to an utilization efficiency of 2.4% for AO7 degradation), whereas the Fe²⁺/H₂O₂ system only consumed 0.03 mM (with a nominal efficiency of 8.3%). This result suggests that Mn⁰ not only participated in H₂O₂ activation directly, but also sustained the availability of Fe²⁺ through Fe³⁺ reduction, thereby promoting continuous H₂O₂ decomposition and ROS generation.

Overall, the enhanced performance of the Mn⁰/Fe²⁺/H₂O₂ system can be attributed to a highly efficient heterogeneous–homogeneous redox coupling mechanism involving two synergistic pathways: (i) direct activation of H₂O₂ by Mn⁰ and (ii) Mn⁰-mediated regeneration of Fe²⁺ from Fe³⁺, which accelerated the Fe³⁺/Fe²⁺ redox cycle and improved H₂O₂ activation efficiency. In this process, Fe²⁺ activated H₂O₂ to generate •OH and FeO²⁺ via Equations (1) and (2), while Mn⁰ continuously replenished Fe²⁺ through Equation (4), thereby sustaining ROS production and enhancing AO7 degradation. A schematic illustration of the proposed mechanism is presented in Figure 4.

2.4. Effects of Reaction Factors on AO7 Degradation in the Mn⁰/Fe²⁺/H₂O₂ System

Based on the results discussed above, Fe²⁺ played an important role in enhancing AO7 degradation capacity in the Mn⁰/H₂O₂ system. Thus, investigating the impact of Fe²⁺ concentration on AO7 degradation in the Mn⁰/Fe²⁺/H₂O₂ system was of great significance. As shown in Figure 5a, AO7 degradation increased continuously as the Fe²⁺ concentration increased from 0 to 15 μM. Specifically, the degradation efficiency after 30 min increased from 30% to 96%. This enhancement can be attributed to the greater availability of Fe²⁺ for H₂O₂ activation, which facilitated Fe³⁺/Fe²⁺ redox cycling and promoted the formation of •OH and FeO²⁺ for AO7 degradation.

The dosage of Mn⁰ directly dictates the availability of reactive sites for H₂O₂ activation. As shown in Figure 5b, increasing the Mn⁰ dosage from 0 to 0.05 g/L significantly enhanced AO7 degradation from 26% to 88% within 30 min. This profound improvement indicates that an adequate Mn⁰ supply provides sufficient reactive sites to trigger H₂O₂ decomposition, ensuring continuous •OH production and rapid Fe³⁺/Fe²⁺ interconversion. However, a further increase in Mn⁰ dosage to 0.1 g/L resulted in a slight decline in degradation efficiency to 79%. This inhibitory effect is likely due to the parasitic scavenging of •OH and FeO²⁺ by excess Mn⁰ particles.

The oxidant dosage is another critical parameter governing reactive species yield. The effect of H₂O₂ dosage is shown in Figure 5c. Within the investigated range of 0–2.0 mM, AO7 degradation increased with increasing H₂O₂ concentration. After 30 min, the degradation efficiency increased from 2% to 88%. The improved performance can be ascribed to the increased availability of H₂O₂ for activation by Fe²⁺ and Mn⁰, which enhanced the production of •OH and FeO²⁺ responsible for AO7 degradation.

pH fundamentally governs iron speciation and redox potential, heavily influencing the performance of advanced oxidation processes [27]. As illustrated in Figure 5d, the AO7 degradation efficiency exhibited a strong pH dependence, declining steadily from 88% to 46% as the initial pH increased from 3.0 to 7.0 in the Mn⁰/Fe²⁺/H₂O₂ system. As discussed in Section 2.3, efficient Fe³⁺/Fe²⁺ redox cycling is essential for sustained H₂O₂ activation in this system. Ferric speciation is strongly pH-dependent, and the dominant Fe³⁺ species shift from soluble Fe³⁺ and FeOH²⁺ to Fe(OH)₂⁺ and insoluble Fe(OH)₃ as pH increases from 3 to 7 (Figure S4). Compared with soluble ferric species under acidic conditions, these hydrolyzed forms are less reactive and more prone to precipitation as iron hydroxides. Consequently, the regeneration of Fe²⁺ from Fe³⁺ becomes less efficient at higher pH due to the surface passivation by precipitates and the decreased redox potential of iron couples [28,29], leading to suppressed H₂O₂ activation and lower AO7 degradation.

Overall, these results demonstrate that AO7 degradation in the Mn⁰/Fe²⁺/H₂O₂ system was strongly dependent on Fe²⁺ concentration, Mn⁰ dosage, H₂O₂ dosage, and initial pH. Efficient degradation was achieved at higher Fe²⁺ and H₂O₂ concentrations, an appropriate Mn⁰ dosage, and acidic conditions.

2.5. Effects of Water Matrix Constituents and Performance in Real Water Samples

Coexisting inorganic anions and natural organic matter are ubiquitous in natural waters and may affect contaminant degradation during advanced oxidation processes [30]. A unified concentration of 1 mM for inorganic anions (and 1 mg/L for HA) was selected. This value was adopted as a standardized condition, commonly utilized in fundamental AOP studies, to clearly observe and compare the distinct inhibitory impacts and fundamental radical-scavenging mechanisms of different water matrix constituents on AO7 degradation. As shown in Figure 6a, the addition of 1 mM carbonate species or SO₄²⁻ caused little change in AO7 degradation in the Mn⁰/Fe²⁺/H₂O₂ system under the tested conditions. In contrast, the presence of Cl⁻ slightly decreased the AO7 degradation efficiency from 88% to 83%. This inhibition was mainly attributed to the reaction of Cl⁻ with •OH, which can convert •OH into less reactive chlorine-based radicals/species and thereby reduce the overall oxidation efficiency [31,32]. Among the tested anions, NO₃⁻ exhibited a more pronounced inhibitory effect, decreasing the AO7 degradation efficiency from 88% to 73%. This result may be related to the direct reaction between NO₃⁻ and Mn⁰ (Equation (5)), which consumes Mn⁰ and reduces its availability for H₂O₂ activation and Fe³⁺ reduction [33]. Humic acid showed the strongest suppression effect, and AO7 degradation decreased sharply to 35%. This substantial inhibition can be ascribed to the competitive scavenging of reactive oxidizing species, including •OH and FeO²⁺, by humic substances [34].

3Mn⁰ + 2NO₃⁻ + 8H⁺ → 2NO + 4H₂O + 3Mn²⁺

(5)

To further evaluate the practical applicability of the Mn⁰/Fe²⁺/H₂O₂ process, AO7 degradation was examined in authentic water samples, including lake water and river water. As shown in Figure 6b, both real water matrices substantially suppressed AO7 degradation, with final degradation efficiencies of 40% in lake water and 60% in river water, respectively. The stronger inhibition in lake water is likely due to its typically higher accumulation of natural organic matter and salinity compared to flowing river water. Moreover, the background impurities in natural water, particularly natural organic matter (NOM) and inorganic ions (e.g., Cl⁻ and NO₃⁻), competed with AO7 for reactive oxygen species such as •OH and FeO²⁺. This competitive effect was the key reason for the weakened pollutant degradation capacity in real water environments. Consequently, overcoming this scavenging effect in complex matrices would inevitably increase chemical demands, such as higher H₂O₂ consumption.

Furthermore, scaling up this system for real wastewater treatment presents several operational limitations. The optimal performance strictly depends on an acidic environment (pH 3.0), which necessitates costly acidification prior to treatment and subsequent neutralization. Additionally, the sacrificial dissolution of Mn⁰ introduces metal ions into the effluent, requiring post-treatment alkaline precipitation to meet discharge standards, which inevitably generates metal hydroxide sludge. These results and operational constraints indicate that although the Mn⁰/Fe²⁺/H₂O₂ system exhibits high degradation efficiency in model solutions, its practical application potential requires further evaluation.

3. Materials and Methods

3.1. Chemicals

Acid Orange 7 (AO7, C₁₆H₁₁N₂NaO₄S, >85%), zero-valent manganese (Mn⁰, >99.9%), ferrous sulfate heptahydrate (FeSO₄·7H₂O, >99%), hydrogen peroxide (H₂O₂, 30 wt%), sodium hydroxide (NaOH, >96%), sulfuric acid (H₂SO₄, 95–98%), and 1,10-phenanthroline monohydrate (C₁₂H₈N₂·H₂O, >98%), sodium carbonate (Na₂CO₃, >99%), sodium chloride (NaCl, >99.8%), sodium sulfate (Na₂SO₄, >99%), sodium nitrate (NaNO₃, >99%), humic acid (≥90%), nitrobenzene (C₆H₅NO₂, >99.5%), 5,5-dimethyl-1-pyrroline N-oxide (C₆H₁₁NO, DMPO, >97%), sodium thiosulfate (Na₂S₂O₃, >99%), methyl phenyl sulfoxide (PMSO, >95%), methyl phenyl sulfone (PMSO₂, >97%) and tert-butyl alcohol (C₄H₁₀O, >98%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Deionized water was used to prepare all solutions. In addition, lake water from Taiping Lake and river water from the Xin’an River were used as water matrices for AO7 solution preparation.

3.2. Experimental Procedures

Batch degradation experiments were carried out in 250 mL conical flasks with continuous magnetic stirring (500 rpm) at room temperature (25 ± 2 °C). Before each experiment, the initial pH of the AO7 solution was adjusted to the desired value using 0.01 M NaOH solutions and 0.01 M H₂SO₄. In a typical experiment with the Mn⁰/Fe²⁺/H₂O₂ system, 100 mL of AO7 solution (initial concentration of 10 μM) was transferred into the flask, followed by the addition of a predetermined volume of H₂O₂ and FeSO₄ solution. Mn⁰ was then added within a few seconds to initiate the reaction. At predetermined time intervals, 1 mL samples were collected and immediately quenched with 1 mL of 0.5 M sodium thiosulfate solution, followed by filtration through a 0.45 μm membrane before analyzing. For comparison, control experiments using Fe²⁺/H₂O₂, Mn⁰/H₂O₂, Fe²⁺ alone, and Mn⁰ alone were also conducted. For the active species quenching experiments, the quencher at a specified concentration was mixed with the AO7 solution prior to the addition of other substances. The relative contributions of •OH and FeO²⁺ were estimated from the differences in AO7 degradation efficiencies in the presence of selective scavengers. Each experiment was performed in triplicate, and the results are reported as mean values with standard deviations.

3.3. Analytical Methods

The residual AO7 concentration was determined from its absorbance at 484 nm using a UV–visible spectrophotometer (U-3900, Hitachi Limited, Tokyo, Japan) based on a standard calibration curve. The degradation efficiency of AO7 was calculated using the following Equation (6), and the degradation kinetics were fitted to the pseudo-first-order model described by Equation (7):

Degradation Efficiency (%) = (C₀ − C_t)/C₀ × 100%

(6)

In(C_t/C₀) = −k₁t

(7)

where C₀ and C_t are the initial concentration and the concentration of AO7 at reaction time t (μM), respectively; k₁ represents the pseudo-first-order rate constant (min⁻¹), and t is the reaction time (min).

Solution pH was monitored with a PH2-25 pH meter (Shanghai INESA Scientific Instrument Co., Ltd., Shanghai, China). The content of Fe²⁺ was quantified at 510 nm using the 1,10-phenanthroline monohydrate colorimetric method [35], while H₂O₂ concentration was determined by the ABTS method at 415 nm [36]. The concentration of M²⁺ was determined using a Flame Atomic Absorption Spectrophotometer (FAAS, TAS-990, Beijing Purkinje General Instrument Co., Ltd., Beijing, China). The analysis was conducted using an air-acetylene flame. The analytical wavelength for Mn was set at 279.5 nm with a slit width of 0.3 nm. Standard calibration curves were established prior to the measurements, exhibiting excellent linearity (R² > 0.999). To ensure the reliability of the results, all samples were filtered through a 0.22 μm membrane and appropriately diluted before measurement, and the analyses were performed in triplicate. An electron spin resonance (ESR) spectrometer (Bruker EMXplus, Bruker Corporation, Karlsruhe, Germany) was employed to detect reactive oxygen species using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the spin-trapping agent.

4. Conclusions

In this study, we developed an efficient Mn⁰/Fe²⁺/H₂O₂ system for rapid AO7 degradation, achieving 88% removal within 30 min under optimized conditions. It was demonstrated that the synergistic coupling of zero-valent manganese (Mn⁰) and Fe²⁺ significantly enhances H₂O₂ activation. Specifically, Mn⁰ plays a crucial dual role: it facilitates direct H₂O₂ cleavage and continuously regenerates Fe²⁺ from Fe³⁺, thereby sustaining the iron redox cycle. Both hydroxyl radicals (•OH) and high-valent iron species (FeO²⁺) cooperatively drive the oxidation process. While the system exhibits exceptional reactivity in model solutions, its efficiency is attenuated in authentic water matrices due to competitive scavenging by natural constituents.

Overall, this work deepens the understanding of reactive species generation and iron cycling in Mn⁰-assisted systems, providing a feasible strategy for strengthening Fenton-like oxidation via metal-mediated redox coupling. Nevertheless, for future applications in complex wastewater treatment, operational limitations, including the need for pH adjustment, H₂O₂ consumption, and the management of metal–hydroxide sludge, must be carefully addressed. Additionally, comprehensive evaluations of mineralization efficiency (e.g., TOC/COD removal), transformation by-products, and effluent toxicity will be imperative to ensure the environmental safety of the treated water. Addressing these engineering and environmental aspects will ultimately improve the overall techno-economic viability of this advanced oxidation technology.