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Article

Enhanced Degradation of 4-Nitrophenol via a Two-Stage Co-Catalytic Fenton Packed-Bed Reactor with External Circulation

by
Yan Liu
1,
Jingyu Liu
1,
Yongyou Hu
1,*,
Yueyue Shi
2,
Chaoyang Tang
1,
Jianhua Cheng
1,
Xiaoqiang Zhu
1,3,
Guobin Wang
3 and
Jieyun Xie
3
1
Ministry of Education Key Laboratory of Pollution Control and Ecological Remediation for Industrial Agglomeration Area, College of Environment and Energy, South China University of Technology, Guangzhou 510006, China
2
School of Civil and Environmental Engineering, Zhengzhou University of Aeronautics, Zhengzhou 450046, China
3
Guangzhou Pengkai Environment Technology Co., Ltd., Guangzhou 511493, China
*
Author to whom correspondence should be addressed.
Environments 2025, 12(8), 280; https://doi.org/10.3390/environments12080280
Submission received: 12 July 2025 / Revised: 7 August 2025 / Accepted: 9 August 2025 / Published: 14 August 2025
(This article belongs to the Special Issue Advances in Heavy Metal Remediation Technologies)
Browse Figures
Versions Notes

Abstract

To mitigate the consumption of active sites on co-catalysts by H2O2 and to enhance the efficiency and stability of co-catalytic Fenton reactions, an external circulation two-stage packed-bed reactor (ECTPBR) was developed using DPW (diatomite plate@polydopamine@WC) as a co-catalyst to degrade 4-nitrophenol (4-NP). Under suitable conditions, the ECTPBR could achieve over 91.97% 4-NP degradation, with low iron sludge production (11.97 mg/L) and minimal tungsten leaching (3.6363 mg/L). The two-stage strategy enabled spatial separation of Fe3+ reduction and Fenton reactions, minimizing the loss of active sites on DPW, ensuring long-term system stability, and reducing the toxicity of 4-NPdegradation products. In addition, external circulation enhanced mass transfer and improved resistance to shock loads. These advantages suggest that the ECTPBR may serve as an effective strategy for applying co-catalytic Fenton reactions in the treatment of toxic and refractory organic wastewater.

1. Introduction

The essence of the co-catalytic Fenton reaction lies in the exposed M4+ sites (M = Mo, W, Co, etc.) on the co-catalyst, which facilitate the reduction of Fe3+ back to Fe2+ during the Fenton reaction [1,2,3,4]. This process accelerates Fe3+/Fe2+ cycle, reduces iron sludge formation [5,6], and demonstrates great potential for degrading toxic and refractory organic pollutants in water [7]. In order to promote the practical implementation of co-catalytic Fenton, it is crucial to develop effective co-catalyst application strategies and design suitable reactors. In this regard, various supported co-catalysts have been investigated, such as MoS2-loaded sponges [8,9], MoS2-loaded hydrogels [10], CoS2-loaded graphene aerogels [3], and MoS2-loaded silica particles [11], all of which enable efficient co-catalyst recovery. In our previous study, a high-mechanical-strength 3D diatomite plate@polydopamine@WC (DPW) co-catalyst was developed using a two-step impregnation method [12]. DPW exhibited excellent co-catalytic activity, adaptability, and stability, making it highly promising for practical applications.
Continuous-flow packed-bed reactors were widely used as co-catalytic Fenton reactors due to their simple structure and high mass transfer efficiency [11,13,14]. However, in these reactors, both the Fenton reaction and Fe2+ regeneration occurred in the same zone. As a result, the co-catalysts not only reduced Fe3+ to Fe2+ but also directly activated H2O2 to generate reactive oxygen species (ROS), consuming some of its reduction sites [15,16]. Previous studies have demonstrated that the compartmentalized reaction mode and intermittent addition of H2O2 could significantly enhance the catalytic activity and sustainability of the co-catalytic Fenton process [17,18,19,20].
4-Nitrophenol (4-NP) is an important chemical raw material and intermediate widely used in the manufacture of pesticides, pharmaceuticals, textiles, and dyes [21,22]. Large-scale production and extensive application have inevitably led to the discharge of 4-NP into the environment [23]. The strong electron-withdrawing nitro group renders 4-NP highly persistent and bio-refractory, leading to its accumulation and ecological risks in aquatic environments [24]. Due to high toxicity and persistence [25,26], 4-NP has been listed as a priority pollutant by the U.S. Environmental Protection Agency (EPA) [27]. However, conventional treatment methods such as adsorption, photocatalysis, and biological treatment, have shown limited efficiency in degrading 4-NP [28,29]. Therefore, it is highly important to develop an efficient method to remove 4-NP from wastewater.
To this end, this study employed DPW as a co-catalyst and constructed an external circulation two-stage packed-bed reactor (ECTPBR) system for the degradation of 4-NP. An internal circulation compartmentalized packed-bed reactor (ICCPBR) and a conventional packed-bed reactor (PBR) were employed as reference reactors to elucidate the degradation performance and mechanism of the ECTPBR. The effects of H2O2 addition strategy, DPW filling ratio, pH, Fe2+ and H2O2 dosage, reaction time, 4-NP concentration, and circulation flow rate on the degradation of 4-NP in the ECTPBR were systematically investigated. Kinetic fitting analysis was conducted to identify the rate-limiting factors of 4-NP degradation in the ECTPBR, based on which regulation strategies are proposed. A 20-cycle comparative experiment was carried out to verify the advantages of the two-stage reactor in enhancing co-catalytic reaction efficiency, improving 4-NP degradation performance, facilitating mass transfer, controlling iron sludge formation, and ensuring system stability. X-ray photoelectron spectroscopy (XPS) was used to analyze changes in the valence states and elemental composition of DPW before and after the reaction. The operational mechanism of the ECTPBR was elucidated, and a regulation strategy was proposed. This study aims to provide theoretical foundations and technical support for the development of two-stage co-catalytic Fenton reaction strategies and their application in the degradation of toxic and refractory organic pollutants in wastewater.

2. Materials and Methods

2.1. Material and DPW

All reagents are listed in Text S1 of Supplementary Material. DPW was prepared according to the previous research of our group [12], with detailed synthesis procedures provided in Text S2. Each DPW block (2 cm × 2 cm × 2 cm) was uniformly loaded with approximately 12 mg of WC. The surface elements of the co-catalysts were characterized by X-ray photoelectron spectroscopy (XPS) using a Thermo Scientific Nexsa (Thermo Fisher Scientific, Hillsboro, OR, USA). The binding energies were calibrated using the surface-contaminated C 1s peak at 284.6 eV as a reference. The radiation source of X-ray photoelectron spectroscopy was an Al target, the energy was 1486.8 eV, the current was 10 mA, and the scan step was 0.05 eV.

2.2. Design and Operation of Reactors

2.2.1. External Circulation Two-Stage Packed-Bed Reactor (ECTPBR)

The schematic diagram of the ECTPBR is shown in Figure 1a. This system is composed of a packed-bed reactor connected in series with a mixing reactor. The packed-bed reactor was made of PVC, with an inner diameter of 12.00 cm, a height of 58.00 cm, and an effective volume of 7.13 L. It was filled with 10% DPW co-catalyst, where the co-catalytic Fe3+ reduction takes place. The mixing reactor, a square chamber measuring 21.00 × 21.00 × 20.00 cm with an effective volume of 8.82 L, served as the zone for the Fenton reaction. Water was pumped from the bottom of the packed-bed reactor, flowed upward through the reactor, and then entered the mixing reactor, forming a closed-loop circulation. To monitor changes in iron ion and pollutant concentrations, sampling ports were positioned at both the inlet and outlet of the packed-bed reactor.

2.2.2. Internal Circulation Compartmentalized Packed-Bed Reactor (ICCPBR)

The ICCPBR featured a concentric inner-outer cylinder structure made of PVC (Figure 1b). The inner cylinder had a diameter of 10.00 cm, a height of 60.00 cm, and an effective volume of 4.71 L, while the outer cylinder had a diameter of 14.00 cm, a height of 73.00 cm, and a total effective volume of 11.24 L. The inner cylinder was packed with DPW, where the co-catalytic Fe3+ reduction reaction occurred, while the outer cylinder served as the Fenton reaction zone. An outlet was positioned 7.00 cm above the bottom of the outer cylinder and connected to a pump. Water entered the bottom of the reactor and flowed upward through the packed bed in the inner cylinder, driven by a pipeline pump. It then overflowed from the top due to gravity and flowed downward through the gap between the inner and outer cylinders before reaching the reactor bottom again. The circulation flow can be regulated by adjusting the flow rate. To monitor the variations in iron-ion and pollutant concentrations at different locations within the reactor, sampling ports were installed at both the inlet and the outlet.

2.2.3. Packed-Bed Reactor (PBR)

The PBR (Figure 1c) was a cylindrical reactor made of PVC with an inner diameter of 14.00 cm, a height of 63 cm, and an effective volume of 11.24 L. Inside the reactor, DPW was used as the packing material. In this setup, both the Fenton reaction and the Fe3+ reduction reaction occurred in the same reaction zone. Water entered the bottom of the reactor, flowed upward to the top outlet, and was then directed to a pipeline pump for pressurization. The circulation flow was regulated by adjusting the flow rate. To monitor variations in iron-ion and pollutant concentrations at different positions within the reactor, sampling ports were installed at both the inlet and the outlet.

2.2.4. Operation Process of Reactors

A water sample containing 30 μM FeSO4·7H2O and 10 mg/L 4-NP was prepared using laboratory tap water, with the pH adjusted to 3 using 1 M HCl. A total of 100 DPW blocks ([WC] = 1200 mg, [DPW] = 10 blocks/L) were added to the reactor, achieving a filling ratio of 10%. The reaction solution was then introduced, and a submersible pump was activated at a flow rate of 100 L/h to ensure continuous liquid circulation within the reactor. To initiate the reaction, the 0.1 M H2O2 solution was continuously added using a gravity-fed infusion set at a rate of 1.33 mL/min. In the ECTPBR, the solution was introduced into the mixing reactor; in the ICCPBR, it was added to the outer cylinder; and in the PBR, it was directly introduced into the reactor. At predetermined time intervals, 6 mL water samples were collected from the sampling ports, filtered through a 0.22 μm syringe filter, and analyzed for 4-NP concentration and iron-ion content. Each reaction cycle involved the treatment of 10 L of water. At the end of each cycle, the reactor was drained via the bottom discharge pipe.

2.3. Analytical Methods

The determination methods for Fe2+, Fe3+, total iron concentration, and iron sludge generation are provided in Texts S3 and S4. Details on the measurement of 4-NP concentration and its intermediate products, toxicological evaluation, and other analytical methods can be found in Texts S5 and S6. The data processing methods are described in detail in Text S7.

3. Results and Discussion

3.1. 4-NP Degradation Performance of ECTPBR

3.1.1. Effect of H2O2 Addition Method on 4-NP Degradation Performance of ECTPBR

H2O2 is typically added in a single dose in existing studies on co-catalytic Fenton reactions [7,30,31]. However, if the Fe2+ concentration in the co-catalytic Fenton system is insufficient to fully react with the single H2O2 addition, the unreacted H2O2 might enter the Fe3+ co-catalytic reduction zone, undermining the intended compartmentalization [17]. Therefore, a sequential H2O2 addition strategy was necessary to achieve spatial separation of Fe3+ reduction and Fenton reaction. The method of H2O2 addition significantly influenced 4-NP degradation in the ECTPBR (Figure 2a). After 60 min, 84.12% of 4-NP was degraded with H2O2 single addition, whereas H2O2 sequential addition led to a much higher degradation efficiency of 92.30%. In both cases, the degradation of 4-NP followed pseudo-first-order kinetics, with a reaction rate constant (k) of 0.04151 min−1 under H2O2 sequential addition (Figure 2b), an improvement of 51.96% compared to the single addition. This demonstrated that H2O2 sequential addition enhanced 4-NP degradation more effectively. As shown in Figure 2c, sequential H2O2 addition markedly improved TOC removal in the ECTPBR, increasing from 17.28% to 34.29%, which reflects enhanced mineralization performance. The excessive local concentration of H2O2 caused by single addition could have led to ·OH scavenging by H2O2 (Equations (1) and (2)) [32], thereby reducing both 4-NP degradation and TOC removal efficiency.
The concentrations of Fe2+ and Fe3+ ions after reactions with different H2O2 addition strategies are shown in Figure 2d. The concentration ratio of Fe2+ to Fe3+ in the sequential addition and single addition systems was 2.67 and 1.34, respectively, indicating superior Fe3+/Fe2+ cycling efficiency in the sequential addition system. This could be attributed to the large amount of Fe3+ generated during the single addition system, while the limited reduction sites on the DPW were unable to rapidly convert Fe3+ back to Fe2+. Meanwhile, DPW also reacted with the excess H2O2 and was partially consumed, further hindering the regeneration of Fe2+. In the H2O2 sequential addition system, Fe2+ could adequately react with a suitable amount of H2O2, while the generated Fe3+ was effectively reduced back to Fe2+ by DPW co-catalyst. Overall, sequential addition of H2O2 maintained an optimal Fe2+/H2O2 stoichiometric ratio in Fenton reaction zone, enhanced the conversion efficiency of H2O2 to ·OH [33,34,35], and facilitated the iron cycling process in the DPW co-catalytic Fenton system.
H2O2 + ·OH → H2O + ·O2H
·O2H + ·OH → H2O + O2

3.1.2. Effect of Main Factors on 4-NP Degradation by Reactors

The effect of DPW filling ratio on 4-NP degradation in the ECTPBR is shown in Figure 3a. As the DPW filling ratio increased from 5% to 20%, the 4-NP degradation efficiency significantly improved. When the DPW filling ratio reached 10%, the 4-NP degradation efficiency peaked at 92.06%. When the DPW filling ratio exceeded 10%, further increases in DPW loading did not result in a significant improvement in either degradation efficiency or reaction rate. This could be attributed to the fact that a 10% filling ratio of DPW was sufficient to activate the Fenton system and produce enough ROS for 4-NP degradation. Taking cost-effectiveness into account, a DPW filling ratio of 10% was adopted.
The effect of initial pH on 4-NP degradation in the ECTPBR is presented in Figure 3b. As pH increased from 3.0 to 3.5, the 4-NP degradation efficiency remained above 87.93%. The highest degradation efficiency (92.06%) and reaction rate constant (0.0415 min−1, Figure S1b) were achieved at pH 3.0. However, when pH exceeded 4.0, both degradation efficiency and k-value decreased significantly. This decline can be attributed to the hydrolysis and polymerization of Fe3+ at higher pH, leading to the formation of ferric hydroxide gels, which hinder the reaction. Conversely, at pH 2.0, the degradation efficiency sharply dropped to 36.43%, likely due to the protonation of H2O2 under highly acidic conditions, resulting in the formation of the stable H3O2+ species (Equation (3)), which substantially reduces its reactivity with Fe2+ [36].
H2O2 + H+ → H2O2+
The effect of Fe2+ concentration on 4-NP degradation in the ECTPBR is shown in Figure 3c. As the Fe2+ concentration increased from 10 μM to 40 μM, the 4-NP degradation efficiency improved from 75.92% (10 μM) to 97.19% (40 μM), and the corresponding k-value increased from 0.02344 min−1 to 0.05699 min−1 (Figure S1c). However, when the Fe2+ concentration exceeded 40 μM, the degradation efficiency of 4-NP no longer increased significantly.
The effect of H2O2 dosage on 4-NP degradation in the ECTPBR was investigated over a range of 2–8 mmol (Figure 3d). As H2O2 dosage increased from 2 to 4 mmol, the degradation efficiency of 4-NP improved from 82.77% to 92.25%. The maximum degradation rate of 4-NP was observed when the H2O2 dosage reached 6 mmol. However, further increasing the H2O2 dosage slightly reduced both the degradation efficiency and the reaction rate constant (Figure S1d), likely due to the scavenging of ·OH radicals by excess H2O2 [18,37,38]. Therefore, the optimal H2O2 dosage was determined to be 4 mmol.
Figure 3e illustrates the effect of reaction time on the degradation of 4-NP by ECTPBR. As the reaction time increased from 15 to 90 min, the degradation efficiency of 4-NP gradually improved. After 30 min, the degradation rate reached 91.97%, and the k-value peaked at 0.0792 min−1. Extending the reaction time further did not result in a significant increase in the degradation rate, and the reaction rate constant decreased (Figure S1e). Therefore, the optimal reaction time for the reactor was determined to be no less than 30 min.
Figure 3f illustrates the effect of initial 4-NP concentration on the degradation of 4-NP by the ECTPBR. As the initial 4-NP concentration increased from 5 to 10 mg/L, the degradation efficiency by the ECTPBR remained above 91.97%. However, when the 4-NP concentration exceeded 10 mg/L, both the degradation efficiency and the reaction rate constant (Figure S1f) declined. Therefore, the maximum tolerated concentration of 4-NP for the reactor was determined to be 10 mg/L.

3.1.3. Circulating and Reaction Equilibrium of ECTPBR

The effect of circulation flow rate(Q) on the degradation of 4-NP by the ECTPBR is shown in Figure 4a. As the circulation flow rate increased from 50 to 100 L/h, the degradation efficiency of 4-NP by the ECTPBR gradually improved, reaching a maximum at 100 L/h. However, further increases in flow rate led to a decline in degradation efficiency. Under fixed reaction time and total H2O2 dosage, the circulation flow rate determined the Fe2+ generation rate in the catalytic reaction zone, and the sequential added H2O2 had to match this Fe2+ generation rate accordingly. In this study, the Fe2+ generation rate was defined as the difference between the Fe2+ concentration in the reduction reaction and the Fe2+ concentration in the Fenton reaction, divided by the reduction reaction time. Figure 4b shows the Fe2+ generation rate under different circulation flow rates. As the flow rate increased, both the Fe2+ generation rate and 4-NP degradation followed the same trend: both increased from 50 to 100 L/h and then decreased from 100 to 200 L/h. When the circulation flow rate was either too high or too low, the Fe2+ generation rate failed to match the continuously added H2O2, resulting in unstable 4-NP degradation efficiency.
Based on the experimental results, the optimal conditions for 4-NP degradation by the ECTPBR were as follows: a DPW filling rate of 10–20%, pH 3.0, [Fe2+] = 30–40 μM, an H2O2 dosage of 4–8 mmol, a reaction time of 30–90 min, a maximum tolerated 4-NP concentration of 10 mg/L, and a circulation flow rate of 100 L/h. Under these conditions, the degradation efficiency of 4-NP by the ECTPBR was at least 91.97%.

3.2. Process Mechanism of ECTPBR

The optimal operating parameters for the ECTPBR have been determined in the previous section. Based on these parameters, a long-term comparative experiment was further conducted among the ECTPBR, ICCPBR, and PBR. Fe2+ was added only before the first reaction cycle, and no additional iron source was replenished in subsequent cycles. In each operation cycle, 100 mg of 4-NP was introduced into each reactor, and 40 mL of 0.1 M H2O2 solution was continuously added. The degradation efficiency of 4-NP, iron-ion concentrations in different regions, changes in iron-ion concentrations, and the amount of iron sludge produced were compared to assess the effectiveness of the partition strategy and to reveal the operational advantages and mechanisms of the ECTPBR.

3.2.1. Verification of the Reliability of Partition Reaction Strategy

The differences in 4-NP degradation efficiency among the three reactors over 20 operation cycles were shown in Figure 5a. As illustrated, during the first three cycles, the degradation efficiencies of 4-NP in the ECTPBR, ICCPBR, and PBR all exceeded 90.00%. At this stage, sufficient W4+ and W0 were present on DPW in all reactors. However, W4+ and W0 were progressively consumed, resulting in a gradual decline in 4-NP degradation efficiency, following the order PBR > ICCPBR > ECTPBR. In PBR, the co-catalytic and Fenton reactions occurred simultaneously within the same zone, leading to the consumption of W4+ and W0 by H2O2 and thereby restricting the co-catalytic activity. The degradation efficiencies of 4-NP in the ECTPBR and the ICCPBR remained similar during the first seven cycles. However, after 20 cycles, ECTPBR still maintained a degradation efficiency of 68.90%, which was significantly higher than the 39.47% observed in the ICCPBR. A one-way ANOVA analysis showed that the final degradation efficiency of ECTPBR was significantly higher than that of the ICCPBR and the PBR (p < 0.05), demonstrating its superior long-term performance. The calculated circulation flow velocities in the ECTPBR and the ICCPBR were 14.74 cm/min and 20.62 cm/min, respectively, reflecting the operational differences between compartmentalized and two-stage reaction strategies. The higher circulation velocity may have led to hydraulic flushing of DPW, causing the loss of active components and gradual deactivation [39]. To verify the effectiveness of the two-stage reaction in the ECTPBR and the compartmentalized reaction in the ICCPBR, Fe2+ concentrations in Fenton and reduction zones were measured during reactor operation (Figure 5b). In both reactors, the concentration of Fe2+ in the reduction zone consistently exceeded that in the Fenton zone, ensuring the efficient regeneration of Fe2+. This further validated the effectiveness of spatially separating Fenton oxidation and co-catalytic reduction processes in both systems. At the initial stage, the ratio of Fe2+ concentration in the reduction zone to that in the reaction zone was approximately 1.3 in both reactors. As the reaction progressed, this ratio stabilized at 1.22–1.28 in the the ECTPBR, which was higher than the 1.16–1.23 observed in the ICCPBR. By the 20th cycle, the ratio had decreased to 1.16 in the ECTPBR and 1.08 in the ICCPBR. Moreover, the concentration of Fe2+ in the ECTPBR remained consistently higher than those in the ICCPBR and the PBR throughout 20 reaction cycles (Figure 6), indicating that the two-stage reaction strategy employed in the ECTPBR was more effective than the compartmentalized reaction strategy in the ICCPBR in maintaining the long-term stability of DPW. Following the method described in Xiao et al. [19], the mass transfer coefficients of the three reactors were calculated as 1.03, 0.76, and 0.57 cm/s, respectively (detailed in Text S8). The corresponding mass transfer efficiency followed the order ECTPBR > ICCPBR > PBR, which was consistent with the ranking of 4-NP degradation efficiencies among the three reactors.

3.2.2. Fe2+/Fe3+ Circulation and Iron Sludge

The temporal variations of Fe2+ and Fe3+ concentrations in the three reactors are shown in Figure 6. At the initial stage, all three reactors contained sufficient W0 and W4+ on DPW, resulting in negligible differences in total iron concentration. However, as the reaction progressed, the total iron concentrations in the ECTPBR, ICCPBR, and PBR gradually decreased, reaching 26.25 μM, 23.12 μM, and 18.75 μM, respectively, after 20 reaction cycles. These differences can be attributed to whether H2O2 directly reacted with DPW and the varying rates of active component loss from DPW. Additionally, the concentration ratio of Fe2+ to Fe3+ in all reactors declined over time. Initially, the ratios were 3.03, 2.92, and 2.82 for the ECTPBR, ICCPBR, and PBR, respectively. By the 16th cycle, the concentration ratio of Fe2+ to Fe3+ in the PBR dropped below 1, while in the ECTPBR and ICCPBR, it declined to 1.35 and 0.89, respectively, by the 20th cycle. After 20 cycles, the reaction solutions in all reactors were neutralized to pH 7, and the amount of iron sludge produced was measured (Figure S2). The iron sludge concentrations in the ECTPBR, ICCPBR, and PBR were 11.97, 26.45, and 34.65 mg/L, respectively. Compared to PBR, the iron sludge formation in ECTPBR and ICCPBR was reduced by 65.45% and 23.60%, respectively. This finding highlighted the improved iron-ion recycling efficiency in the two-stage and compartmentalized reaction strategies, with ECTPBR demonstrating the most effective iron cycling performance. Although the ECTPBR produced less sludge, the accumulated solid waste still contained iron and tungsten, which may pose environmental concerns if not properly managed [40,41]. Given the presence of valuable metals, converting ECTPBR sludge into reusable catalysts via alkaline treatment and thermal calcination [42] could be a promising future direction.

3.2.3. Stability of DPW and ECTPBR

Figure 7a presents photographs of DPW from the three reactors before and after 20 reaction cycles. The diatomite-based support exhibited excellent mechanical strength, maintaining its structural integrity in aqueous environments without any deformation or breakage. As a result, DPW retained its original morphology and remained structurally intact in all three reactors throughout the entire reaction process, exhibiting no visible physical damage. However, a noticeable yellowing of DPW was observed, likely attributable to partial detachment of WC and the accumulation of iron sludge. Figure 7b,c display the XPS survey spectra and high-resolution W 4f spectra before and after 20 reaction cycles. The elemental composition on DPW surface remained unchanged after the reactions. The peak at 30.96 eV corresponds to W0, while peaks at 33.11 eV and 34.50 eV are attributed to W4+ 4f7/2 and W4+ 4f5/2, respectively. Additionally, the peaks at 36.49 eV and 37.63 eV correspond to W6+ 4f7/2 and W6+ 4f5/2, respectively [12,43]. Initially, the prepared DPW contained 30.77% W0, 61.05% W4+, and 8.18% W6+. After 20 reaction cycles, the W0 content in the ECTPBR, ICCPBR, and PBR decreased to 25.97%, 25.43%, and 17.04%, respectively, while the W4+ content declined to 55.54%, 54.13%, and 44.63%. In contrast, the W6+ content increased to 18.49%, 20.44%, and 38.33%, respectively. The reductions in W0 and W4+ contents were comparable in the ECTPBR and ICCPBR, whereas in the PBR, the depletion of W0 was 2.53 and 2.30 times as much as that in the ECTPBR and ICCPBR, respectively, and the loss of W4+ was 2.78 and 2.37 times as much as that in the ECTPBR and ICCPBR, respectively. These findings indicated that direct contact between DPW and H2O2 leads to rapid depletion of the active sites W0 and W4+, ultimately contributing to DPW deactivation.
The changes in the elemental composition of DPW before and after the reactions in the three reactors are summarized in Table 1. Initially, DPW contained 1.83% W atoms and 57.92% C atoms. After 20 reaction cycles, the W atomic content in the ECTPBR, ICCPBR, and PBR decreased to 1.76%, 0.64%, and 1.74%, respectively, while the C atomic content dropped to 54.24%, 33.72%, and 44.97%. The reduction in W atomic content was 0.07%, 1.19%, and 0.09% for the ECTPBR, ICCPBR, and PBR, respectively, whereas the C atomic content decreased by 3.68%, 24.20%, and 12.95%. These results indicated that the ICCPBR exhibited the highest loss of both W and C atoms, suggesting significant leaching of active components during the reaction. This is consistent with the high recirculation flow rate in the ICCPBR, where intense hydraulic flushing leads to substantial loss of active species. Additionally, Fe atoms were detected on DPW surfaces in all three reactors, with concentrations of 0.73%, 0.86%, and 1.30% for the ECTPBR, ICCPBR, and PBR, respectively. The iron accumulation is likely due to the deposition of Fe(OH)3, formed when Fe3+ could not be effectively reduced due to weakened co-catalytic reactions. The observed trend in Fe accumulation suggests that the rate of co-catalytic activity decay follows the order ECTPBR < ICCPBR < PBR.
The time-dependent leaching of metal ions from DPW in the three reactors over 20 reaction cycles is summarized in Table 2. As the reaction progressed, the concentration of leached W ions gradually increased in all reactors, following the trend PBR > ICCPBR > ECTPBR. At the end of the reaction cycles, the cumulative concentrations of W ions in the ECTPBR, ICCPBR, and PBR were 3.6363, 6.1206, and 6.2678 mg/L, respectively, indicating varying degrees of active site loss from DPW. The leaching was significantly mitigated in the segmented reaction of the ECTPBR. The consumption of DPW caused by the activation of H2O2 is higher than that caused by hydraulic flushing. Consequently, the two-stage and compartmentalized reaction strategies effectively prolonged the lifespan of DPW, improved reactor stability, and minimized secondary pollution. To further assess reactor stability, a batch experiment was conducted after draining the water at the end of 20 cycles. The results showed that the ECTPBR still achieved an 81.39% degradation rate of 4-NP, while the ICCPBR and PBR achieved 62.84% and 59.27%, respectively—comparable to the Fenton system (55.43%). These findings suggested that after 20 reaction cycles, the DPW in the ECTPBR retained strong co-catalytic activity, demonstrating high stability and sustained pollutant degradation efficiency. Moreover, according to our previous work [12], DPW could be regenerated by immersion in a WC-containing solution, effectively restoring its catalytic performance.

3.2.4. Mechanism of Degradation of 4-NP

Identification of ROS
To identify the ROS involved in the degradation of 4-NP in the ECTPBR, quenching experiments were conducted using TBA, BQ, and L-HD as scavengers for ·OH, ·O2−, and 1O2, respectively [44,45]. Figure 8a shows that the degradation efficiency of 4-NP decreased by 55.22%, 62.51%, and 67.01% after the addition of 1 mM, 5 mM, and 10 mM TBA, respectively, indicating that ·OH played an important role in this process. Figure 8b shows that increasing the BQ concentration from 0 to 5 mM reduced 4-NP degradation efficiency from 91.97% to 47.14%, with the corresponding k decreasing from 0.0792 to 0.0208 min−1. Similarly, as shown in Figure 8c, increasing L-HD concentration (0–5 mM) also led to a gradual decline in 4-NP degradation efficiency. However, the inhibitory effect of L-HD was less pronounced compared to those of TBA and BQ, suggesting that while 1O2 was generated in the system, it was not the dominant ROS. Furthermore, the EPR experiments were performed using TEMP and DMPO as trapping agents to detect the ROS generated in the ECTPBR (Figure 8d). The characteristic signals of DMPO-·OH, TEMP-1O2, and DMPO-·O2− adducts were distinctly detected, confirming the presence of ·OH, ·O2−, and 1O2 in the ECTPBR. These findings indicated that the degradation of 4-NP in the ECTPBR involved a synergistic effect of ·OH, ·O2−, and 1O2, with ·OH playing a dominant role, which was a typical characteristic of the co-catalytic Fenton reaction.
Degradation Products and Toxicity Evaluation
An LC-MS/MS analysis identified the intermediate products generated during the degradation of 4-NP in the ECTPBR, as summarized in Table S1, with the proposed degradation pathways illustrated in Figure 9. Typically, the reaction between ·OH and aromatic groups occurs via electrophilic addition [46]. In Pathway I,·OH preferentially attacked the ortho-position of the -OH group on the benzene ring, leading to the formation of 4-nitrocatechol (P1). Further ·OH attack on P1 yielded 1,2,4-trihydroxybenzene (P2). The formation of 3,4,5-trihydroxy-nitrobenzene (P3) was attributed to ·OH addition at the ortho-position of 4-nitrocatechol [47]. Due to the strong electrophilic effect of ·OH, the C-C bonds between adjacent hydroxyl groups became unstable, leading to the ring opening of 1,2,4-trihydroxybenzene and subsequent formation of 2-hydroxy-2,4-hexadienedioic acid (P4) [47]. Pathway II involved the elimination of the -NO2 group from 4-NP, followed by ·OH-mediated transformation to hydroquinone. Additionally, due to the reductive properties of WC, some 4-NP molecules in the reaction system might have gained electrons on the WC surface, being reduced to p-nitrosophenol (P5), which was subsequently reduced to p-aminophenol (P6) [48,49]. Moreover, ROS can oxidize 4-NP to hydroquinone (P7), which can then be further oxidized to p-benzoquinone (P8) [50,51,52]. Continued oxidation of these intermediates led to aromatic ring cleavage and carbon chain shortening, ultimately producing low-molecular-weight compounds such as fumaric acid (P9), oxalic acid (P10), and pentanoic acid (P11), which were eventually mineralized to CO2 and H2O.
Additionally, the toxicity of 4-NP and degradation intermediates was evaluated by QSAR using the Toxicity Estimation Software Tool (T.E.S.T). The toxicity prediction values of 4-NP degradation products toward bioconcentration factor, fathead minnow, and Daphnia magna are shown in Figure 9b–d. As for the bioconcentration factor (Figure 9b), most of the intermediates were lower than 4-NP, except for p7. For the fathead minnow LC50, all intermediates had significantly higher values than 4-NP (Figure 9c). Similarly, for Daphnia magna, 4-NP had an LC50 of 7.51 mg/L, while most intermediates showed higher LC50 values, except for P1, P3, and P6 (Figure 9d), indicating a significant reduction in acute toxicity. These results collectively demonstrated that the ECTPBR effectively reduced the adverse effects of 4-NP and its degradation products on aquatic organisms and the environment.

3.2.5. Mechanism of Degradation of 4-NP by ECTPBR

Based on the findings above, a schematic diagram illustrating the proposed mechanism of 4-NP degradation in the ECTPBR is presented in Figure 10. The ECTPBR consisted of a packed-bed reactor and a mixing reactor. In the mixing reactor, H2O2 was continuously added at a controlled rate based on stoichiometric calculations. Fe2+ rapidly reacted with H2O2 to generate ·OH, while being oxidized to Fe3+ (Equation (4)). Additionally, Fe2+ can react with O2 to produce ·O2− (Equation (5)). The interconversion between ·OH and ·O2− also generated 1O2 (Equations (6) and (7)). Under continuous attack by ROS (Equation (8)), 4-NP was gradually transformed into less toxic intermediates and ultimately mineralized into CO2 and H2O. Fe3+ was then transported via a pump to the packed-bed reactor, which was packed with DPW. The exposed W0 and W4+ sites on the DPW surface acted as electron donors, effectively reducing Fe3+ to Fe2+while being oxidized to W6+ (Equations (9) and (10)). This process played a key role in sustaining the Fe3+/Fe2+ redox cycle critical for continuous ·OH generation. In addition, W6+ could be partially reduced back to W4+ by H2O2 (Equation (11)), contributing to the regeneration of active sites. The regenerated Fe2+ flowed out from the top of the reactor, returning to the mixing reactor to activate H2O2 and sustain pollutant degradation. The combination of the packed-bed and mixing reactors effectively separated the Fe3+ reduction and Fenton reaction, minimizing side reactions associated with DPW-mediated H2O2 activation [16], thereby enabling the long-term operation of the co-catalytic Fenton system. The external circulation and two-stage packed-bed design enhanced the contact between the reaction solution and DPW, facilitating mass transfer between Fe3+ and DPW and increasing reaction efficiency. Additionally, the continuous dilution effect of circulation improved the reactor’s resistance to shock loads. A flow control valve at the reactor’s base allowed precise regulation of the influent flow rate. In the ICCPBR, an inner-outer tube design was adopted to separate the Fenton reaction from Fe3+ reduction. Unlike the ECTPBR, ICCPBR operated at a higher internal flow velocity, leading to significant hydraulic flushing of DPW during the reaction. This high-flow condition accelerated the loss of active sites on the DPW surface, thereby compromising reaction stability and long-term performance.
In the mixing reactor:
Fe2+ + H2O2 → Fe3+ + ·OH + OH
Fe2+ + O2 → ·O2− + Fe3+
4·OH → 1O2 + 2H2O
·O2− + ·OH → 1O2 + OH
·OH/·O2−/·OH + 4-NP → products + CO2 + H2O
In the packed-bed reactor:
Fe3+ + W0 → Fe2+ + W4+/W6+
Fe3+ + W4+ → Fe2+ + W6+
W6+ + H2O2 → W4+ + H2O+ O2

3.2.6. Regulation Strategies of ECTPBR

In the ECTPBR, the controllable parameters mainly included DPW filling rate, pH, Fe2+ concentration, H2O2 dosage, reaction time, and circulating flow rate. For the treatment of 10 mg/L of 4-NP wastewater, we proposed the following control strategies: maintaining the DPW filling rate at 10–20%, keeping the pH at 3, setting Fe2+ concentration at 30–40 μM, controlling the H2O2 dosage at 4–8 mmol, ensuring a reaction time of no less than 30 min, and maintaining a circulating flow rate of approximately 100 L/h. Under the above conditions, the ECTPBR can achieve efficient degradation and maintain stable performance during long-term continuous operation. To further reduce the consumption of catalytic active sites by H2O2 and to prevent direct contact between DPW and H2O2, the system employed continuous H2O2 addition and adjusted the circulating flow rate to balance the generation rate of Fe2+ with the constant H2O2 dosage.

3.3. Tungsten Release and Potential Post-Treatment Strategies

The segmented and low-flushing design of the ECTPBR effectively suppressed the loss of active sites and metal leaching compared to the other systems. However, the potential long-term environmental risks associated with residual tungsten should not be overlooked. After 20 continuous reaction cycles, the cumulative concentration of tungsten in the ECTPBR effluent reached 3.6363 mg/L. By adjusting the effluent to alkaline conditions through the addition of NaOH, this value was reduced to 2.5453 mg/L, indicating that chemical precipitation may offer a promising approach for tungsten removal. Considering the persistence and bioaccumulative nature of tungsten, further investigation into secondary purification methods, such as adsorption [53,54], coagulation [55], and microbial remediation [56] is highly warranted. This will be an important step in our future efforts to enhance the environmental compatibility of the ECTPBR.

4. Conclusions

In this study, a novel external circulation two-stage packed-bed reactor was developed, effectively reducing the consumption of active sites on the co-catalyst by H2O2 and enhancing the efficiency and stability of the co-catalytic Fenton reaction. Under optimal conditions, the ECTPBR achieved a 4-NP degradation efficiency of no less than 91.97% at an initial concentration of 10 mg/L. Results from 20 reaction cycles demonstrated that, compared to the ICCPBR and PBR, the ECTPBR showed significant advantages in degradation efficiency, mass transfer, iron sludge control, and long-term stability. The two-stage and compartmentalized configuration spatially separated the Fenton and Fe3+ reduction reactions, reducing the contact between DPW and H2O2, and thus minimizing active site loss while accelerating the Fe2+/Fe3+ cycle. Compared with the ICCPBR, the lower flow-rate requirement of the ECTPBR resulted in less hydraulic shear on DPW, decreasing the leaching of active components. Additionally, 4-NP was effectively degraded, with a significant reduction in toxicity. The proposed two-stage strategy mitigates the consumption of active sites on the co-catalyst caused by H2O2 activation, enhancing co-catalyst stability and promoting the practical application of the co-catalytic Fenton system.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/environments12080280/s1, Text S1. Chemicals, Text S2. Synthesis and characterization of DPW, Text S3. Detection of iron ions concentration [57], Text S4. Detection of iron sludge, Text S5. Determination and toxicity assessment of 4-NP concentration and its intermediates, Text S6. Analytical methods, Text S7. Data processing method, Text S8. Detection of overall mass transfer co-efficient; Table S1. The identified intermediates from catalytic reactions removal of 4-NP in ECTSPBR by LC–MS/MS, Table S2. XPS binding energy, Table S3 One-way ANOVA results for the degradation efficiency of 4-NP among different reactors; Figure S1. Kinetic calculation of various parameters, including different DPW filling ratio (a), initial pH (b), Fe2+ concentration (c), H2O2 dosage (d), reaction time (e) and 4-NP concentration (f), Figure S2. The amount of iron sludge in ECTPBR, ICCPBR and PBR.

Author Contributions

Y.L.: investigation, methodology, software, formal analysis, writing—original draft. J.L.: writing—review and editing, software, formal analysis. Y.H.: conceptualization, funding acquisition, writing—review and editing, supervision. Y.S.: validation. C.T.: software. J.C.: funding acquisition. X.Z.: project administration. G.W.: resources, project administration. J.X.: project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (21976060) and Guangzhou Pengkai Environment Technology Co., Ltd.

Data Availability Statement

The data presented in this study are not publicly available due to privacy issues.

Conflicts of Interest

The authors employed by Guangzhou Pengkai Environment Technology Co., Ltd. are Xiaoqiang Zhu, Guobin Wang, and Jieyun Xie. The company only provided funding for the research and had no role in study design, data collection, analysis, interpretation, manuscript writing, or the decision to submit for publication. The authors declare no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
WCTungsten carbide
ECTPBRExternal circulation two-stage packed-bed reactor
ICCPBRInternal circulation compartmentalized packed-bed reactor
PBRPacked-bed reactor
DPWdiatomite plate@polydopamine@WC
4-NP4-nitrophenol

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Figure 1. Schematic diagram of the ECTPBR (a), ICCPBR (b), and PBR (c).
Figure 1. Schematic diagram of the ECTPBR (a), ICCPBR (b), and PBR (c).
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Environments 12 00280 g002
Figure 2. Effect of H2O2 addition method on 4-NP degradation (a), the corresponding pseudo-first-order kinetic fitting results (b). TOC removal efficiency (c). The concentrations of Fe2+ and Fe3+ (d). Experimental conditions: DPW filling ratio = 10%, pH = 3, [Fe2+] = 30μM, [H2O2] = 4 mmol, reaction time = 60min, [4-NP] = 10 mg/L, and Q = 100 L/h.
Figure 2. Effect of H2O2 addition method on 4-NP degradation (a), the corresponding pseudo-first-order kinetic fitting results (b). TOC removal efficiency (c). The concentrations of Fe2+ and Fe3+ (d). Experimental conditions: DPW filling ratio = 10%, pH = 3, [Fe2+] = 30μM, [H2O2] = 4 mmol, reaction time = 60min, [4-NP] = 10 mg/L, and Q = 100 L/h.
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Figure 3. Effects of DPW filling ratio (a), initial pH (b), Fe2+ concentration (c), H2O2 dosage (d), reaction time (e), and 4-NP concentration (f) on 4-NP degradation efficiency in the ECTPBR. Experimental conditions: DPW filling ratio = 10%, pH = 3, [Fe2+] = 30μM, [H2O2] =4 mmol, reaction time = 60 min, [4-NP] = 10 mg/L, and Q = 100 L/h.
Figure 3. Effects of DPW filling ratio (a), initial pH (b), Fe2+ concentration (c), H2O2 dosage (d), reaction time (e), and 4-NP concentration (f) on 4-NP degradation efficiency in the ECTPBR. Experimental conditions: DPW filling ratio = 10%, pH = 3, [Fe2+] = 30μM, [H2O2] =4 mmol, reaction time = 60 min, [4-NP] = 10 mg/L, and Q = 100 L/h.
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Figure 4. Effects of circulating flow rate on 4-NP degradation (a). The rate of Fe2+ generation in ECTPBR under different circulating flow rates (b). Experimental conditions: DPW filling ratio = 10%, pH = 3, [Fe2+] = 30 μM, [H2O2] = 4 mmol, reaction time = 30 min, [4-NP] = 10 mg/L, and Q = 100 L/h.
Figure 4. Effects of circulating flow rate on 4-NP degradation (a). The rate of Fe2+ generation in ECTPBR under different circulating flow rates (b). Experimental conditions: DPW filling ratio = 10%, pH = 3, [Fe2+] = 30 μM, [H2O2] = 4 mmol, reaction time = 30 min, [4-NP] = 10 mg/L, and Q = 100 L/h.
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Figure 5. Degradation efficiency of 4-NP in ECTPBR, ICCPBR, and PBR (a). The concentrations of Fe2+ and Fe3+ of different regions in the ECTPBR and ICCPBR (b). Experimental conditions: DPW filling ratio = 10%, pH = 3, [Fe2+] = 30 μM, [H2O2] = 4 mmol, reaction time = 30 min, [4-NP] = 10 mg/L, and Q = 100 L/h.
Figure 5. Degradation efficiency of 4-NP in ECTPBR, ICCPBR, and PBR (a). The concentrations of Fe2+ and Fe3+ of different regions in the ECTPBR and ICCPBR (b). Experimental conditions: DPW filling ratio = 10%, pH = 3, [Fe2+] = 30 μM, [H2O2] = 4 mmol, reaction time = 30 min, [4-NP] = 10 mg/L, and Q = 100 L/h.
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Figure 6. The concentrations of Fe2+ and Fe3+ in ECTPBR, ICCPBR, and PBR. Experimental conditions: DPW filling ratio = 10%, pH = 3, [Fe2+] = 30 μM, [H2O2] = 4 mmol, reaction time = 30 min, [4-NP] = 10 mg/L, and Q = 100 L/h.
Figure 6. The concentrations of Fe2+ and Fe3+ in ECTPBR, ICCPBR, and PBR. Experimental conditions: DPW filling ratio = 10%, pH = 3, [Fe2+] = 30 μM, [H2O2] = 4 mmol, reaction time = 30 min, [4-NP] = 10 mg/L, and Q = 100 L/h.
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Figure 7. Changes in DPW before and after the reaction in the ECTPBR, ICCPBR, and PBR: photograph (a), XPS survey spectrum (b), high-resolution W 4f spectra (c).
Figure 7. Changes in DPW before and after the reaction in the ECTPBR, ICCPBR, and PBR: photograph (a), XPS survey spectrum (b), high-resolution W 4f spectra (c).
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Figure 8. Effects of different scavenger concentrations on 4-NP degradation in the ECTPBR: TBA (a), BQ (b), TEMP (c). EPR spectra of DMPO-OH, DMPO-O2−, and TEMP-1O2 (d).
Figure 8. Effects of different scavenger concentrations on 4-NP degradation in the ECTPBR: TBA (a), BQ (b), TEMP (c). EPR spectra of DMPO-OH, DMPO-O2−, and TEMP-1O2 (d).
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Figure 9. Possible pathways of 4-NP degradation in the ECTPBR (a). Bioconcentration factor (b), fathead minnow LC50 (96h) (c), and Daphnia magna LC50 (48 h) (d) of 4-NP and its degradation intermediates.
Figure 9. Possible pathways of 4-NP degradation in the ECTPBR (a). Bioconcentration factor (b), fathead minnow LC50 (96h) (c), and Daphnia magna LC50 (48 h) (d) of 4-NP and its degradation intermediates.
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Figure 10. Proposed process mechanism in the ECTPBR.
Figure 10. Proposed process mechanism in the ECTPBR.
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Table 1. The change in elemental content in DPW before and after the reaction in three reactors.
Table 1. The change in elemental content in DPW before and after the reaction in three reactors.
ElementOriginal DPWECTPBRICCPBR
C (Atomic %)57.8254.2433.72
O (Atomic %)40.3543.2664.78
W (Atomic %)1.831.760.64
Fe (Atomic %)N.A.0.730.86
Table 2. Changes in the concentration of leached W ions during 20 cycles in the ECTPBR, ICCPBR, and PBR.
Table 2. Changes in the concentration of leached W ions during 20 cycles in the ECTPBR, ICCPBR, and PBR.
CycleECTPBR
(mg/L)		ICCPBR
(mg/L)		PBR
(mg/L)
10.14510.4140 0.4751
50.98271.93751.9573
101.65802.70812.8532
152.54534.48104.7892
203.63636.12066.2678
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Liu, Y.; Liu, J.; Hu, Y.; Shi, Y.; Tang, C.; Cheng, J.; Zhu, X.; Wang, G.; Xie, J. Enhanced Degradation of 4-Nitrophenol via a Two-Stage Co-Catalytic Fenton Packed-Bed Reactor with External Circulation. Environments 2025, 12, 280. https://doi.org/10.3390/environments12080280

AMA Style

Liu Y, Liu J, Hu Y, Shi Y, Tang C, Cheng J, Zhu X, Wang G, Xie J. Enhanced Degradation of 4-Nitrophenol via a Two-Stage Co-Catalytic Fenton Packed-Bed Reactor with External Circulation. Environments. 2025; 12(8):280. https://doi.org/10.3390/environments12080280

Chicago/Turabian Style

Liu, Yan, Jingyu Liu, Yongyou Hu, Yueyue Shi, Chaoyang Tang, Jianhua Cheng, Xiaoqiang Zhu, Guobin Wang, and Jieyun Xie. 2025. "Enhanced Degradation of 4-Nitrophenol via a Two-Stage Co-Catalytic Fenton Packed-Bed Reactor with External Circulation" Environments 12, no. 8: 280. https://doi.org/10.3390/environments12080280

APA Style

Liu, Y., Liu, J., Hu, Y., Shi, Y., Tang, C., Cheng, J., Zhu, X., Wang, G., & Xie, J. (2025). Enhanced Degradation of 4-Nitrophenol via a Two-Stage Co-Catalytic Fenton Packed-Bed Reactor with External Circulation. Environments, 12(8), 280. https://doi.org/10.3390/environments12080280

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