Efficiency and Mechanism of Naproxen Degradation in the Mo/Fe³⁺/H₂O₂ System

Abstract

Naproxen (NPX) is a widely occurring, refractory organic contaminant that cannot be removed by conventional water treatment processes. In response to the growing environmental pollution caused by NPX, an innovative and highly efficient green degradation method has been developed, designed on the principles of sustainability to promote long-term ecosystem health and advance a circular economy. In this study, using zero-valent molybdenum as a catalyst in combination with trivalent iron (Fe³⁺) and hydrogen peroxide (H₂O₂), we constructed a Mo/Fe³⁺/H₂O₂ system to treat NPX-contaminated water. The effects of solution pH, H₂O₂ concentration, Fe³⁺ concentration, Mo dosage, and co-existing water-matrix constituents (Cl⁻, HCO₃⁻, PO₄³⁻, NO₃⁻, and humic acid (HA)) on NPX removal were investigated; reactive species were identified; and the reusability of Mo as well as its performance under the continuous-flow condition were evaluated. The results showed that the optimal pH was 3 and the appropriate Fe³⁺ dosage is 100 µM. With 500 µM H₂O₂, 87.9% of NPX was removed within 7 min, and a moderate increase in Fe³⁺ concentration, together with a suitable H₂O₂ level, enhanced the removal efficiency. HCO₃⁻, Cl⁻, and HA exerted slight inhibition, whereas PO₄³⁻ markedly suppressed NPX degradation. Recycling tests and the 6 h continuous-flow treatment demonstrated excellent reusability and stability of Mo. Quenching experiments revealed that HO^• and Fe(IV) were the dominant reactive species responsible for NPX degradation.

1. Introduction

Non-steroidal anti-inflammatory drugs (NSAIDs), belonging to the ubiquitous pharmaceuticals and personal care products (PPCPs), are widely used for treating disorders such as headaches and arthritis [1,2,3]. Naproxen (NPX), a representative NSAID, is poorly removed in conventional water treatment processes [4] and has been a frequently detected component in the effluents of Chinese wastewater treatment plants [5]. Its recalcitrant biodegradability leads to its widespread environmental occurrence [6], where it undergoes continuous migration and accumulation, posing potential threats to aquatic ecosystems and human health [7,8]. Fenton and Fenton-like processes, which primarily use H₂O₂ as the oxidant, demonstrate good efficiency for removing organic pollutants that are difficult to degrade by traditionally activated sludge processes [9,10,11,12,13]. Iron-based catalysts can effectively activate H₂O₂ to generate highly reactive free radicals for degrading organic pollutants [14] and are widely applied in Fenton and Fenton-like systems. However, the conventional activated sludge process exhibits poor efficacy in the degradation of NPX, which possesses a stable aromatic structure. In contrast, the Fenton reaction can achieve relatively favorable degradation performance. Nevertheless, it is associated with several limitations, including a relatively narrow applicable pH range (typically below 3.0) and slow regeneration of iron ions, which renders ferric iron in the system more susceptible to precipitation and consequently leads to the accumulation of iron sludge [15]. To address these limitations, reducing agents can be introduced into the Fenton system to promote the conversion of Fe³⁺ to Fe²⁺, thereby enabling continuous activation of H₂O₂. Molybdenum (Mo) plays a synergistic catalytic role in the system. The reactive sites it possesses can accelerate the regeneration of iron ions, thereby reducing iron sludge and speeding up the reaction process. Mo and its compounds have attracted widespread attention due to their excellent physical and chemical properties [16,17,18,19]. Studies have shown that molybdenum disulfide (MoS₂) can effectively activate persulfate to degrade sulfamethoxazole [20,21,22]. It is reported that Mo sites with varied oxidation states and oxygen vacancies favor peroxymonosulfate (PMS) activation and the formation of critical singlet oxygen (¹O₂). Furthermore, H₂O₂ is known to decompose into reactive oxygen species (ROS) on MoS₂ surfaces. Given these findings, it is reasonable to speculate that elemental Mo may also exhibit excellent catalytic potential in H₂O₂-based systems. The elemental form of Mo possesses more reactive sites and holds great potential for enhancing the efficiency of Fenton treatment [23,24,25].

This study employs elemental molybdenum powder (Mo) as a co-catalyst to construct a Mo/Fe³⁺/H₂O₂ system, using NPX as the target pollutant. This system treats NPX-contaminated wastewater, directly advancing the sustainability of water cycles and contributing to a closed-loop water management model. The effects of solution pH, H₂O₂ concentration, Fe³⁺ concentration, naproxen concentration, and water quality parameters (Cl⁻, HCO₃⁻, PO₄³⁻, humic acid) on the degradation efficiency of naproxen in this system were investigated. The reusability of Mo and its stability in continuous flow experiments were tested. Additionally, quenching experiments were conducted to explore the reactive species present in the reaction system and their contributions to NPX degradation. The findings present a promising alternative for the oxidative degradation of organic pollutants in Fenton and Fenton-like systems, thereby advancing the development of advanced oxidation technologies. While much attention has been focused on fabricating composite materials to enhance synergistic effects, investigating the heterogeneous reactions within the Mo/Fe³⁺/H₂O₂ system can provide novel insights and foundational data for the design of such composites.

2. Materials and Methods

2.1. Materials

All reagents used in the experiments were of analytical grade or higher. Naproxen (NPX), anhydrous ferric sulfate (Fe₂(SO₄)₃), humic acid (HA), methyl phenyl sulfoxide (PMSO), and phenyl methyl sulfone (PMSO₂) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China) Tert-butanol (TBA), 30% hydrogen peroxide (H₂O₂), glacial acetic acid, sodium sulfate (Na₂SO₄), sodium dihydrogen phosphate (NaH₂PO₄), disodium hydrogen phosphate (Na₂HPO₄), and sodium thiosulfate (Na₂S₂O₃) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) Methanol (MeOH) and acetonitrile were purchased from Tedia. The ultrapure water (18 MΩ-cm) used in all chemical solutions was produced by a water purifier (Micro Pure UV, Thermo Fisher Scientific, Niederelbert, Germany).

2.2. Method

All experiments were conducted at room temperature (25 ± 1 °C) in 100 mL conical flasks placed in a constant-temperature water bath. First, the NPX stock solution was added to the reactor, followed by the Mo powder and H₂O₂. The initial pH was quickly adjusted to 3 using H₂SO₄ and NaOH. Finally, the Fe³⁺ solution was added to the conical flask, and the magnetic stirrer was set to 600 r/min. At specified time intervals (At 30 s, 1 min, 2 min, 3 min, 5 min, 7 min), 1 mL of the reaction mixture was collected and mixed with excess Na₂S₂O₃ as a quenching agent. In the quenching experiments with MeOH and TBA, NPX was first added, followed by the oxidant H₂O₂. The reaction was then initiated by the simultaneous addition of Fe³⁺ and Mo powder, with samples taken at 30 s, 1, 2, 3, 5, and 7 min.

For the experiment designed to trap high-valent iron species, PMSO was introduced first, followed by H₂O₂. The reaction was immediately (within 2 s) started by concurrently adding Fe³⁺ and Mo powder, with sampling conducted at the same time intervals. Each experimental group was performed with at least two replicates.

After the reaction, the Mo material was filtered and separated, then dried. This material was subsequently used for NPX degradation cycling experiments. In a pure-water background, a 180 mm × 300 mm spherical condenser tube was filled with sponge material loaded with Mo powder, and a continuous-flow experiment was carried out at a flow rate of 0.5 L/h with a hydraulic retention time of 10 min.

2.3. Analytical Method

NPX, PMSO, and PMSO₂ were measured using a high-performance liquid chromatograph (Shimadzu LC-20, Shimadzu Corporation, Nishinokyo Kuwabara-cho, Nakagyo-ku, Kyoto 604-8511, Japan) equipped with a Shim-pack GIST C18-AQ column (5 µm, 250 mm × 4.6 mm). The HPLC conditions for NPX analysis were as follows: mobile phase consisting of 0.1% acetic acid and acetonitrile at a ratio of 55:45, flow rate set at 0.2 mL/min, injection volume of 5.0 µL, and detection wavelength of 231 nm. For PMSO and PMSO₂, the conditions were: mobile phase of 0.1% acetic acid and acetonitrile (55:45), flow rate of 0.2 mL/min, injection volume of 5.0 µL, with simultaneous detection of PMSO and PMSO₂ absorption intensities at wavelengths of 215 nm and 230 nm, respectively.

3. Results and Discussion

3.1. Degradation Effect of NPX Under Different Systems

As shown in Figure 1, the degradation rates of NPX after 7 min of mixing with Fe³⁺, H₂O₂, or Fe³⁺/H₂O₂ were 1%, 0%, and 0%, respectively. Only a marginal 3% increase in the degradation rate was observed upon adding Mo powder, either alone or to Fe³⁺ or H₂O₂, which was attributed to the adsorption by Mo powder. This suggested that Mo adsorption was minimal and can be considered negligible. When Mo powder was introduced into the Fe³⁺/H₂O₂ system, the degradation rate of NPX reached as high as 87.9% within 7 min. This indicates that the Mo/Fe³⁺/H₂O₂ system can effectively degrade NPX, which is because Mo promotes the reduction in Fe³⁺ to Fe²⁺. The increased Fe concentration in turn accelerates the activation of H₂O₂, thereby generating more free radicals that rapidly degrade NPX through oxidation [26]. The degradation process of NPX by the Mo/Fe³⁺/H₂O₂ system was fitted, and the reaction conformed to first-order kinetics. The calculated apparent rate constant K_obs was 0.4504 min⁻¹ (R² = 0.98) (Figure S1). The kinetic equation is shown as Equation (1) [27].

$$\mathrm{l}\mathrm{n}\left({\displaystyle \frac{\mathbf{C}}{{\mathbf{C}}_{\mathbf{0}}}}\right)={\mathrm{K}}_{\mathrm{o}\mathrm{b}\mathrm{s}}\mathrm{t}$$

(1)

where

• c is the concentration of NPX in the solution at any given time, in μM;
• ${\mathrm{c}}_0$ is the initial concentration of NPX, in μM;
• K_obs is the apparent rate constant, in min⁻¹;
• t is the reaction time, in min.

Figure 1. The Effect of Different Reaction Systems on the Removal of NPX. Conditions: [Mo]₀ = 0.1 g/L, [H₂O₂]₀ = 500 μM, [Fe³⁺]₀ = 100 μM, pH = 3.0 ± 0.1, temperature 25 ± 1 °C.

Figure 1. The Effect of Different Reaction Systems on the Removal of NPX. Conditions: [Mo]₀ = 0.1 g/L, [H₂O₂]₀ = 500 μM, [Fe³⁺]₀ = 100 μM, pH = 3.0 ± 0.1, temperature 25 ± 1 °C.

3.2. The Removal Efficiency of NPX with Changes in Single Reaction Conditions

When the Fe³⁺ dosage was gradually increased from 50 μM to 1000 μM, the reaction progressively accelerated. As shown in Figure 2a, the degradation rate within 7 min gradually increased, and K_obs rose from 0.1950 min⁻¹ to 0.9554 min⁻¹ (Figure S2a). High concentrations of Fe³⁺ significantly enhanced the reaction rate, which is attributed to the increased reaction rate with Mo powder, thereby accelerating the generation of Fe²⁺ and improving the catalytic efficiency of the reaction. However, excessive Fe³⁺ poses potential risks: an excessively high generation rate of HO^• may trigger mutual quenching of free radicals, which could reduce the utilization efficiency of H₂O₂. To minimize Fe³⁺ pollution and improve H₂O₂ utilization, lower concentrations of Fe³⁺ should be selected. For subsequent experiments, 100 μM Fe³⁺ was chosen for the following study.

The dosage of Mo powder directly affects the available surface area of the reaction system. When the Mo dosage ranged from 0 to 0.1 g/L, the removal rate of NPX within 7 min increased with the higher Mo dosage, and K_obs rose from 0.1034 min⁻¹ to 0.4504 min⁻¹ (Figure S2b), indicating a significant enhancement in degradation efficiency (Figure 2b). This improvement is attributed to the increased available surface area between the Mo powder and the reaction system as the dosage increases, which accelerates the conversion of ferric iron (Fe³⁺) to ferrous iron (Fe²⁺). This, in turn, promotes the activation of hydrogen peroxide (H₂O₂), generating more reactive species. However, when the Mo dosage was further increased to 0.15 g/L, K_obs remained around 0.67 min⁻¹ (Figure S2b), and further increases did not lead to additional improvement in degradation efficiency. This may be because, at Mo dosages above 0.15 g/L, the rate of Fe²⁺ generation already exceeds the rate at which Fe²⁺ activates H₂O₂. Thus, the overall reaction rate becomes limited by the Fe²⁺-activated H₂O₂ process, and further increasing the Mo dosage does not accelerate the reaction. To minimize Mo powder consumption, a dosage of 0.1 g/L was selected for subsequent experiments.

H₂O₂ is activated by Fe²⁺ and serves as the direct source of reactive species in the system. Its dosage directly affects the degradation rate of NPX. To some extent, an adequate amount of H₂O₂ can enhance the thoroughness of pollutant degradation. Therefore, exploring the appropriate H₂O₂ concentration is of great significance for achieving optimal degradation effects and cost control. In this context, based on the baseline reaction conditions, the effects of different H₂O₂ concentrations were investigated. As shown in Figure 2c, when the H₂O₂ dosage increased from 100 μM to 500 μM, the NPX removal rate gradually improved from 55% to 85%, and K_obs increased from 0.4074 min⁻¹ to 0.4504 min⁻¹ (Figure S2c). However, further increases in H₂O₂ dosage provided limited additional improvement in the degradation efficiency. Excessive H₂O₂ can lead to the self-disproportionation pathway in the Fenton reaction, which quenches hydroxyl radicals, specifically by reacting with HO^• to generate HO₂^• with a lower redox potential, as shown in Equation (2) [27,28]. High concentrations of H₂O₂ also offer limited economic benefits. Considering both cost-effectiveness and treatment efficiency, 500 μM H₂O₂ was selected for subsequent experiments [29].

An appropriate pH is crucial for enhancing the treatment efficiency of the Fenton-like system. Excessively high pH can lead to the hydrolysis of Fe³⁺, reducing the concentration of iron ions in the system and slowing down the reaction rate. The results indicated that the system achieved optimal treatment efficiency at pH = 3 (Figure 2d). Under neutral or alkaline conditions, Fe³⁺ underwent significant hydrolysis, leading to an excessively low concentration of iron ions in the system and inhibiting the reaction. On the other hand, a higher concentration of H⁺ can interact with Mo powder to generate more active sites, thereby promoting the conversion of Fe³⁺ to Fe²⁺. Figure 2d demonstrates that the performance at pH 3 is significantly stronger than that at higher pH levels. Although the system does not achieve a breakthrough across a broader pH range, it exhibits superior reaction kinetics and removal efficiency. Therefore, pH 3 was selected as the benchmark condition for the reaction to achieve optimal removal performance.

The concentration of NPX directly influences the consumption of reactive species. As shown in Figure S4, the degradation efficiency decreases with increasing NPX concentration.

$${\mathrm{H}\mathrm{O}}^{•}+{\mathrm{H}}_2{\mathrm{O}}_2={\mathrm{H}}_2\mathrm{O}+{\mathrm{H}\mathrm{O}\mathrm{O}}^{•}$$

(2)

3.3. Identification of Active Species

Quenching experiments were conducted to identify the reactive species generated in the system. Based on the references, I anticipated the possible presence of hydroxyl radicals and high-valent iron in the system [30]. When excessive amounts of TBA and MeOH were added separately to quench the reactive species generated in the system. As shown in Figure 3a, NPX degradation was almost completely inhibited after adding MeOH, indicating that all reactive species in the system were completely quenched. TBA can scavenge hydroxyl radicals (HO^•) [31]. When 10 mM TBA was added, the degradation efficiency was 52%. Increasing the TBA concentration to 50 mM resulted in a degradation efficiency of 40%. Since a five-fold increase in concentration only inhibited degradation by 12%, it can be concluded that hydroxyl radicals in the system were effectively scavenged at 50 mM TBA. Under this condition, a degradation efficiency of 40% was still achieved, indicating the presence of active species other than hydroxyl radicals in the system. PMSO can be oxidized by high-valent iron to form PMSO₂ [32]. To this end, 100 μM PMSO was added to the system, and the concentrations of PMSO and PMSO₂ were monitored over time (Figure 3b). The results showed that the concentration of PMSO gradually decreased to 60 μM, while the concentration of PMSO₂ synchronously increased to 38 μM. The total concentration of PMSO and PMSO₂ was maintained at approximately 100 μM, indicating the presence of high-valent iron in the system. These findings demonstrate that both HO^• and Fe(IV) exist in the reaction system and play significant roles [33]. The possible reactions involved are as follows; [26,28,29,34,35,36]:

$$6{\mathrm{F}\mathrm{e}}^{3+}+\mathrm{M}\mathrm{o}\to 6{\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{M}\mathrm{o}}^{6+}$$

(3)

$${{\mathrm{H}}^{+}+\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{F}\mathrm{e}}^{2+}\to {\mathrm{H}\mathrm{O}}^{•}+{\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{H}}_2\mathrm{O}$$

(4)

$${\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{F}\mathrm{e}\mathrm{O}}^{2+}+{\mathrm{H}}_2\mathrm{O}$$

(5)

$${\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}\mathrm{O}}_2^{•}+{\mathrm{H}}^{+}$$

(6)

$${\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{H}\mathrm{O}}_2^{•}\to {\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{O}}_2+{\mathrm{H}}^{+}$$

(7)

3.4. Impact of Water Quality Parameters

Natural water contains various ions and organic substances, which can influence the activation of H₂O₂ in Fenton-like reactions. Under the baseline conditions—including a reaction temperature of 25 ± 1 °C, NPX concentration of 10 μM, Mo dosage of 0.1 g/L, H₂O₂ concentration of 500 μM, Fe³⁺ concentration of 10 μM, and pH₀ of 3.0 ± 0.1—the effects of different water quality parameters (Cl⁻, HCO₃⁻, PO₄³⁻, NO₃⁻ and HA) on the oxidation performance of the system were investigated.

As shown in Figure 4a, Cl⁻ slightly inhibited NPX degradation. The possible reactions of Cl⁻ in water are as follows (Equations (8)–(12) [37,38]). When the dosage reaches 0.5 mmol/L, the degradation efficiency decreases from 85% to 71% (Figure 4a). Cl⁻ can scavenge HO^•, generating less reactive species such as Cl^• and Cl₂^•−, thereby reducing the degradation rate when Cl⁻ is introduced.

In contrast, HCO₃⁻ significantly promotes the reaction. At a dosage of 2 mmol/L, the degradation efficiency increases from 87.9% to 99.5% (Figure 4b), and the K_obs value rises from 0.4504 min⁻¹ to 0.9868 min⁻¹ within 3 min (Figure S3a). In water, HCO₃⁻ may react with HO^• (Equations (13) and (14) [39,40]), producing HCO₃^• and CO₃^•−, which likely contribute to enhancing NPX degradation, HCO₃⁻ is considered to promote the Fenton reaction through its complexation potential [41].

PO₄³⁻ markedly inhibits NPX degradation. As the PO₄³⁻ dosage increases, the degradation efficiency continuously declines. At a dosage of 2 mmol/L, the degradation efficiency drops to 20% (Figure 4c) with K_obs decreasing to 0.0577 min⁻¹ (Figure S3d). The possible reaction (Equation (15)) suggests that PO₄³⁻ increases the concentration of H₂PO₄⁻, which reacts with Fe³⁺ to form precipitates, reducing the concentration of iron ions in the system and leading to decreased NPX degradation efficiency.

Humic acid (HA) in water can scavenge free radicals, affecting the NPX degradation. HA at a concentration of 2 mg/L reduced the degradation efficiency by 4% (Figure 4d) with no significant change in K_obs during the first three minutes of the reaction. Even with increasing HA dosage, the NPX degradation efficiency can still reach 75%, indicating that HA has a relatively minor impact on the system.

The toxicity was assessed using the luminescent bacteria method after the reaction was conducted in ultrapure water. As shown in Figure S6, the toxicity generated was low, only slightly reducing the number of luminescent bacteria.

$$\mathrm{C}{\mathrm{l}}^{-}+\mathrm{H}{\mathrm{O}}^{•}\leftrightarrow \mathrm{C}\mathrm{l}\mathrm{H}{\mathrm{O}}^{•-}$$

(8)

$$\mathrm{C}\mathrm{l}\mathrm{H}{\mathrm{O}}^{•-}+{\mathrm{H}}^{+}\to \mathrm{C}{\mathrm{l}}^{•}+{\mathrm{H}}_2\mathrm{O}$$

(9)

$$\mathrm{C}{\mathrm{l}}^{•}+\mathrm{C}{\mathrm{l}}^{-}\to \mathrm{C}{\mathrm{l}}_2^{•-}$$

(10)

$$\mathrm{C}{\mathrm{l}}_2^{•-}+{\mathrm{C}\mathrm{l}}_2\to 2\mathrm{C}{\mathrm{l}}^{-}$$

(11)

$$\mathrm{C}{\mathrm{l}}_{2\left(\mathrm{a}\mathrm{q}\right)}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{H}\mathrm{O}\mathrm{C}\mathrm{l}+{\mathrm{H}}^{+}+\mathrm{C}{\mathrm{l}}^{-}$$

(12)

$$\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}+\mathrm{H}{\mathrm{O}}^{•}\to \mathrm{H}\mathrm{C}{\mathrm{O}}_3^{•}+\mathrm{O}{\mathrm{H}}^{-}$$

(13)

$$\mathrm{H}\mathrm{C}{\mathrm{r}}_3^{•}\to \mathrm{C}{\mathrm{O}}_3^{•-}+{\mathrm{H}}^{\mp }$$

(14)

$${\mathrm{F}\mathrm{e}}^{3+}+{{\mathrm{H}}_2\mathrm{P}\mathrm{O}}_4^{-}\to \mathrm{F}{\mathrm{e}\mathrm{P}\mathrm{O}}_4^{-}\downarrow +2{\mathrm{H}}^{+}$$

(15)

3.5. Continuous Flow Experiment

To investigate the catalytic stability of the Mo material, a continuous-flow experiment was designed. The Mo material was loaded onto the surface of a sponge, packed into a condenser tube, and a solution containing 30 μM NPX, 300 μM Fe³⁺, and 1500 μM H₂O₂ was introduced. Samples were collected at the outlet every 30 min. The flow rate was set at 0.5 L/h with a hydraulic retention time of 10 min. As shown in Figure 5a, the degradation efficiency stabilized at 97% within 30 min of operation and remained consistent between 97% and 98% over the subsequent 6 h. The results indicate that the Mo/Fe³⁺/H₂O₂ system exhibits excellent performance in degrading NPX under continuous-flow conditions, with Mo powder demonstrating good catalytic stability.

To investigate the reusability of the Mo material, four-cycle experiments were designed. Under the conditions of a Mo dosage of 0.1 g/L, Fe³⁺ dosage of 100 μM, initial H₂O₂ concentration of 500 μM, initial NPX concentration of 10 μM, initial pH of 3.0, and reaction temperature of 25 °C, the reaction was carried out for 7 min. After the reaction, the solution was filtered under vacuum, and the filter paper was rinsed with pure water to obtain the Mo powder suspension. The steps of vacuum filtration and rinsing with pure water were repeated several times, and the material was then placed in a vacuum drying oven for 12 h. The dried Mo powder obtained after the reaction was used for the next cycle of experiments. As shown in Figure 5b, the results indicate that after four cycles, the degradation efficiency can still reach 90%, demonstrating that Mo powder exhibits excellent reusability.

To evaluate the practical applicability of the reaction, secondary effluent from the Tangxun Lake Wastewater Treatment Plant in Wuhan, Hubei Province, was treated, and the total organic carbon (TOC) was measured afterward. As shown in Figures S5 and S7, the system also achieved favorable performance in real water, with a relatively low TOC level remaining after the reaction.

4. Conclusions

Under optimal conditions, the Mo/Fe(III)/H₂O₂ system achieved an NPX removal efficiency of 87.9%. The degradation performance was enhanced with increasing initial concentrations of Fe(III) and Mo, whereas an excessive dosage of H₂O₂ somewhat reduced the degradation efficiency. The reaction followed pseudo-first-order kinetics, with an apparent rate constant of 0.4074 min⁻¹ within the first 3 min. Mechanistic investigations revealed that the main reactive species generated in the system were high-valent iron (Fe(IV)) and hydroxyl radicals (HO^•), both contributing to the oxidative degradation of NPX. Assessment of environmental factors indicated that Cl⁻ and humic acid (HA) consumed a portion of the radicals, leading to a slight decrease in NPX degradation; in the presence of 2 mM Cl⁻ or HA, the degradation efficiency decreased to 71% and 75%, respectively. In contrast, HCO₃⁻ improved the degradation, reaching 99.5% at a dosage of 2 mM. Phosphate (PO₄³⁻) markedly inhibited NPX degradation by reacting with Fe(III), which reduced the available iron concentration in the system, resulting in a degradation efficiency of only 20% at 2 mM PO₄³⁻. Furthermore, molybdenum powder exhibited excellent reusability, maintaining high catalytic activity during prolonged operation (6 h) and over multiple consecutive cycles (4 cycles).