Enhanced removal of ibuprofen by heterogeneous photo-Fenton-like process over sludge-based Fe₃O₄-MnO₂ catalysts

With the development of urbanization and industrialization, the problem of water pollution has become increasingly severe. Pharmaceuticals and personal care products (PPCPs), as a classic type of persistent organic pollutants, have been frequently detected in water bodies in recent years, which has caused concern and worries from all walks of life (Ebele et al. 2017). PPCPs have complex components and diverse types (Su et al. 2020). They can exist stably in the environment for a long time, and it is easy to produce bioaccumulation through the food chain, which causes irreversible damage to human health and the ecological environment (Lin et al. 2016; Liu et al. 2020). Ibuprofen (IBP), as a common non-steroidal anti-inflammatory analgesic, is one of the most widely used PPCPs (Rainsford 2013). It enters the water environment through a variety of ways during production and use, and is a persistent organic pollutant ubiquitous in the water environment (Davarnejad et al. 2018). The source and migration process of ibuprofen are shown in Figure 1. Although the detection concentration of ibuprofen is not high overall (Tran et al. 2018; Khazri et al. 2019; Ngubane et al. 2019), the migration and transformation of ibuprofen in the environment still poses potential threats to human health and ecosystems. Therefore, it is urgent to explore the technology that can efficiently and completely remove ibuprofen in wastewater.

At present, using traditional physical, chemical and biological methods, it is difficult to effectively remove ibuprofen from wastewater (Mezzelani et al. 2018; Prud'homme et al. 2018). Advanced Oxidation Processes (AOPs) can generate hydroxyl radicals (·OH), superoxide radicals (·O²⁻) and other free radicals with strong oxidizing ability. Under specific reaction conditions, the macromolecular organic matter is oxidized into low-toxic or non-toxic small-molecule substances (Gu et al. 2019; Jia et al. 2020). The removal of ibuprofen has been studied under different AOP systems (Guo et al. 2018; Kwon et al. 2018; Minella et al. 2019). Kwon et al. (2018) reported that ibuprofen was effectively removed by the formation of hydroxyl and active chlorine substances under the UV/chlorine system. Guo et al. (2018) degraded ibuprofen in aqueous solution through ozonation process and transition metal homogeneous catalysis process, respectively, and found that the degradation effect of ibuprofen in the ozonation process is poor (40–60%), while the addition of different catalysts can significantly improve the degradation efficiency (60–75%). At present, catalysts generally have the problems of chemical corrosion, small specific surface area, and high production cost (Prince-Pike et al. 2015). Therefore, researchers are pushing ahead to develop low-cost, excellent catalytic performance and structurally stable catalysts.

Sludge is an inevitable by-product of sewage treatment plants. It has the characteristics of being perishable, having high water content and strong odor, and also contains heavy metals, organic pollutants and pathogenic microorganisms (Cheng et al. 2014; Tomasi et al. 2018). More and more attention has been paid to the rational utilization and treatment of sludge. Sludge contains a large amount of organic matter, which can be converted into porous activated carbon after chemical treatment or other treatment conditions (Zhang & Wang 2016; Lin et al. 2018; Almahbashi et al. 2021). The combination of carbon and metal oxides can accelerate the charge transfer at the interface and significantly improve the catalytic performance (Zhang et al. 2021). Compared with other metal oxides, Fe₃O₄ contains Fe(II) and Fe(III), which can provide electron transfer space (Zhen et al. 2018). With its unique magnetic characteristics, it is easy to achieve solid-liquid separation under the action of an external magnetic field. In the Fe₃O₄/Fenton system, Fe(II) reacts with hydrogen peroxide to generate hydroxyl radicals and initiates oxidation reaction; Fe(III) reacts with hydrogen peroxide to generate Fe(II) (Yang et al. 2021), which enables hydroxyl radicals to degrade organic pollutants non-selectively. In addition, in order to accelerate the Fe(II)/Fe(III) cycle and the decomposition of hydrogen peroxide, transition metal elements are often added to the catalyst (Rusevova et al. 2012). Manganese (Mn) is considered to be the most promising Fenton catalyst due to its multiple valence states. In the process of catalyzing hydrogen peroxide by manganese oxide, the unpaired free electrons of Mn(II) and Mn(III) can promote the transfer of effective electrons to hydrogen peroxide to generate hydroxyl radicals (Debnath et al. 2016), which improves the efficiency of degrading organic pollutants.

In this study, two heterogeneous photo-Fenton-like catalysts called the iron-manganese modified sludge-based catalyst (Sludge/Fe₃O₄-MnO₂) and the iron-manganese modified sludge activated carbon (SAC/Fe₃O₄-MnO₂), respectively, were prepared by high temperature calcination-coprecipitation method. The main experimental contents include: (1) The optimal preparation conditions of the catalyst were obtained by orthogonal experiments; (2) Prepared materials were characterized by analysis of the specific surface area, surface state, crystal structure, functional group and magnetic analysis; (3) In order to obtain the performance of the two catalysts, the degradation experiments of ibuprofen were carried out respectively, and the catalytic mechanism was further explored; (4) The degradation mechanism of ibuprofen catalyzed by Sludge/Fe₃O₄-MnO₂was discussed based on the degradation products analysis results of mass spectrometry.

Ibuprofen (≥ 98%) was purchased from Shanghai source biotechnology Co. Ltd (Shanghai, China), and other reagents were obtained from KaiTong chemical reagent Co., Ltd (Tianjin, China). All reagents were of analytical reagent grade and used without further purification. Deionized water was used for the preparation of samples and solutions unless otherwise stated. Sewage sludge was collected from the First Municipal Wastewater Treatment Plant of Tai'an, Shandong Province, China. The characteristics of sewage sludge are shown in Table 1.

An orthogonal experimental design was employed to optimize the multivariate analysis of the prepared composites, which were selected based on preliminary results. Five common influencing factors were selected as indicators in this experiment: calcination time, calcination temperature, activator (ZnCl₂) concentration, solid-liquid ratio, and mass ratio of sludge to nano Fe₃O₄-MnO₂. Taking the degradation rate of IBP as the inspection standard, the orthogonal experiment of L₁₆(4⁵) was designed with SPSS software (SPSS Inc., Chicago, IL, USA). The designed orthogonal procedure is shown in Table 2 and Table S1.

The mixed nano Fe₃O₄-MnO₂ solution was placed in a thermostatic water bath at 90 °C and allowed to stand for 4 h. When the temperature was cooled to room temperature, the solution was filtered and rinsed with deionized water until pH was neutral. The nano Fe₃O₄-MnO₂ was placed in a beaker and dried to constant weight in a vacuum drying oven at 105 °C. Grind through an 80-mesh sieve and seal for later use.

The sludge obtained from the sewage treatment plant was placed in an oven and dried to its constant weight at 60 °C. Subsequently, the dried sludge was ground through an 80-mesh sieve with a diameter of 178 μm, and the obtained dry sludge powders were stored in drying vessels to avoid humidity for later use.

The concentrations of sludge, nano Fe₃O₄-MnO₂ and ZnCl₂ were prepared according to the results of orthogonal experiments to prepare sludge-based catalysts. The specific operation are as follows: Under the action of the activator, the sludge base was placed in an oven at 60 °C, activated for 8 h to fully activate the sludge and nanomaterials. It was placed in an oven at 105 °C for drying treatment, until the weight no longer changed. The dry sludge base was ground through an 80-mesh sieve, and placed in the porcelain ark (not exceeding 2/3 of the volume). The porcelain ark was calcined at a high temperature in the muffle furnace continuously filled with nitrogen. The highest temperature was 500 °C, and the duration of the highest temperature was 60 minutes. The calcined sludge-based catalyst was washed with a 1 M hydrochloric acid solution. When the color of the filtrate changed from yellow to colorless, the catalyst was washed with deionized water until the pH was neutral. The catalyst was placed in a beaker and dried to constant weight in a vacuum drying oven at 105 °C. Grind through an 80-mesh sieve and seal for later use; the product was Sludge/Fe₃O₄-MnO₂.

SAC/Fe₃O₄-MnO₂ was prepared by the method of calcining activated carbon first and then hydrothermal load. Activated sludge as in the previous subsection was first calcined in muffle furnace at high temperature. The obtained products were washed and sieved, which were recorded as SAC. Then the nano Fe₃O₄-MnO₂ and SAC were fully mixed according to a certain mass ratio, and 4 h water bath was carried out. The obtained product was washed with deionized water until the pH was neutral, and dried to constant weight in a vacuum drying oven at 105 °C. The obtained product was SAC/Fe₃O₄-MnO₂.

The Brunauer-Emmett-Teller (BET) surface area measurements of the samples were collected in nitrogen by a Autosorb IQ analyzer (Quantachrome, USA). The surface morphology was characterized by scanning electron microscopy (SEM, Sigma 300, Carl Zeiss AG, Germany). X-ray diffraction (XRD, X'Pert PRO, PANalytical B.V., Netherlands) patterns of in situ prepared sludge-based catalysts were acquired using Cu Kα photons from a diffractometer (X'Pert PRO) operated at 40 kV * 30 mA with 2θ ranging from 5° to 90°. The surface functional groups were analyzed by using Fourier transform infrared (FTIR, RAffinity-1, Shimadzu, Japan). The FTIR measurement analysis was performed with KBr as the background, the resolution was 4.0, and the wavelength was in the range of 400–4,000 cm⁻¹. A vibrating sample magnetometer (VSM, MPMSSQUID, Quantum Design, USA) was applied to determine the magnetic properties of the resultant samples.

At each time, a specific mass of catalyst and 50 mL 0.04 g·L⁻¹ ibuprofen solution (including a specific concentration of hydrogen peroxide) were added to a 100 mL photolysis tube in triplicate. The reaction system was shaken under constant stirring in a photochemical reactor (XPA, Xujiang electrochemical plant, China) for given time with a 500 W xenon lamp. The solution was immediately filtered with 0.22 μm filter into a high performance liquid chromatography (HPLC, Shimadzu, Japan) vial for analysis. For the degradation performance experimental parameters, refer to Text S1.

The concentration of IBP was determined by using HPLC (C18 reverse-phase column) equipped with a UV detector at λ = 220 nm and C18 reversed phase column (250*3.0 mm). The mobile phase was a mixture of acetate buffer and acetonitrile (30/70, v/v) at a flow rate of 1.0 mL min⁻¹. The degradation products of ibuprofen degraded by Sludge/Fe₃O₄-MnO₂ were analyzed by using a mass spectrometer (1200HPLC/MSD, Agilent Technologies, USA). The equation used in the analysis refers to Text S2.

Orthogonal experiment results are shown in Table S2. In the results obtained, the K value refers to the average value of each factor at the same level, which is used to determine the best preparation conditions for the catalyst. The R value is the third type of square sum, which reflects the degree of influence of the changes in the level of various factors on the experimental indicators (Yu et al. 2021). It can be seen that the order of the various influencing factors in the catalytic ibuprofen effect was: Activator concentration > Sludge and Fe₃O₄-MnO₂ mass ratio > Calcination time > Solid-liquid ratio > Calcination temperature. Orthogonal experimental design is proved reasonable, and its significance was 0.015 < 0.05. Subsequent catalyst preparation conditions were A2B2C2D2E2, which specifically showed that the ZnCl₂ concentration was 2.0 M, the solid-liquid ratio was 1:2.5, the calcination temperature was 500 °C, the calcination time was 60 min, and the sludge and Fe₃O₄-MnO₂ mass ratio was 25:2.

The BET surface area was measured by using N₂ adsorption analysis. The specific BET surface areas of the samples derived from the calculation of the adsorption–desorption isotherms are listed in Table 3. The BET areas of Sludge/Fe₃O₄-MnO₂, SAC/Fe₃O₄-MnO₂ and SAC were 434.51, 362.68 and 398.74 m²·g⁻¹, respectively. The BJH cumulative desorbing volumes of pores in Sludge/Fe₃O₄-MnO₂, SAC/Fe₃O₄-MnO₂ and SAC were 0.355, 0.302 and 0.312 cm²·g⁻¹, respectively. Because nanoparticles block the pores on the activated carbon surface, they cannot penetrate deep into the pores of activated carbon (Lu et al. 2019). The surface structure of activated carbon collapsed during the calcination process, resulting in the specific surface area of SAC/Fe₃O₄-MnO₂ being smaller than that of SAC. The pore diameters of the three are similar, indicating that ZnCl₂ does not change with external conditions during the pore-making process, and its performance is relatively stable (Wang et al. 2021).

The SEM of the sludge-based catalyst is shown in Figure 2. It can be clearly seen that the surface of the Sludge/Fe₃O₄-MnO₂ was rough, the surface was collapsed and etched more severely, and the pores distributed on the surface were large. Nanoparticles were embedded in the pores of activated carbon, where Fe-Mn nanoparticles of smaller size (61–77 nm) were observed. The surface of SAC/Fe₃O₄-MnO₂ was blocked by nanoparticles, and a small part goes deep into the pores. Compared with Sludge/Fe₃O₄-MnO₂, the particle size (102–113 nm) was larger. By comparison, it was found that the surface pores of SAC are more and larger than those of SAC/Fe₃O₄-MnO₂. The conclusion obtained by SEM analysis is consistent with the BET measurement result. Sludge/Fe₃O₄-MnO₂ has a larger pore structure and surface area.

The XRD of Sludge/Fe₃O₄-MnO₂ and SAC/Fe₃O₄-MnO₂ is shown in Figures S1 and S2. The phase components of the two catalysts were basically the same. The carbon structure of the crystal plane was observed at 2θ of 20° and 28° (Sajjadi et al. 2019), the crystal rows were not damaged, and the carbon structure was good. Obvious Fe₃O₄ diffraction peaks appeared at 2θ of 30° and 35° (Yuan et al. 2019), indicating that after activation and calcination, nanoparticles were loaded on activated carbon in a stable form. No visible crystalline peak was detected for MnO₂ in all lines, which demonstrated that the MnO₂ in the nanocomposites is amorphous. At 2θ of 40–90°, there are some dense peaks, corresponding to crystalline carbon structure and amorphous carbon structure. This phenomenon was caused by the blockage of nanoparticles in the pores or voids of the activated carbon. In addition, the specific surface area was reduced, which was consistent with the results of BET and SEM analysis.

The FTIR of the two catalysts is shown in Figure 3. The characteristic absorption peak types of Sludge/Fe₃O₄-MnO₂ and SAC/Fe₃O₄-MnO₂ were almost the same, indicating that the main functional groups that constitute the catalyst do not change with the preparation process. In the high-frequency spectrum, there are absorption peaks of different intensity (3,435 cm⁻¹), which are typical hydroxyl absorption peaks in this material (Thakur et al. 2019). The peak intensity of Sludge/Fe₃O₄-MnO₂ and SAC/Fe₃O₄-MnO₂ were weak and the widths of the peaks are large, due to the gradual decrease of element H during the pyrolysis process and the oxidation of some organic groups to generate a small amount of carboxyl groups. When the absorption peak was 1,500 cm⁻¹, it is mainly the N-H/C-N group of the protein in the sludge (Li et al. 2019; Li et al. 2021). The absorption peaks at 1,150 cm⁻¹ were C = O and C = C bonds, which may be caused by the decomposition and bonding of C-O-C and C = O. It may also be C-O, -C-C (aldehydes) tensile vibration, showing the presence of ether, ester, and alcohol compounds (Yao & Ma 2018; Li et al. 2020). The peak value of metal oxides appeared in the range of 600–800 cm⁻¹ (Xiong et al. 2017), indicating that the preparation methods of these two catalysts are feasible. The peak value of SAC/Fe₃O₄-MnO₂ was large, while the peak value of Sludge/Fe₃O₄-MnO₂ was small. The reason for this phenomenon was that the sludge and nanoparticles were fully activated in the process of preparing the catalyst, and the organic matter and inorganic matter were fully and effectively decomposed. Based on the above analysis, it was concluded that the surface of the sludge-based catalyst is mainly composed of these functional groups: carboxyl groups, hydroxyl groups, lactone groups, and metal oxides. The existence of these functional groups shows that the sludge-based catalyst has adsorption efficiency.

As shown in Figure 4, Sludge/Fe₃O₄-MnO₂ had higher magnetic properties than SAC/Fe₃O₄-MnO₂, and its maximum saturation magnetization was roughly 0.015 emu·mg⁻¹. In the test range, as the magnetic field increases, the magnetic performance becomes more obvious. Both of them cross the zero point, indicating that they do not have remanence and coercivity, and the two sludge-based catalysts have superparamagnetic properties (Wang et al. 2020). Under the action of a magnetic field, they can be easily separated from the aqueous solution by magnetization (Wang et al. 2021).

Degradation of IBP by H₂O₂ in different systems was evaluated. The variation of ibuprofen degradation rate with time is shown in Figure 5. From the results, it can be seen that only relying on hydrogen peroxide, the self-degradation of ibuprofen was very slow, and the degradation rate was about 20%. In other systems, the process of catalytic hydrogen peroxide degradation of ibuprofen is divided into a fast reaction stage (0–0.5 h) and a slow reaction stage (0.5–4.5 h). The fast reaction stage is mainly due to the adsorption of SAC and a part of the hydroxyl groups that are quickly produced by the contact between Sludge/Fe₃O₄-MnO₂ and hydrogen peroxide. The slow reaction stage is divided into two categories. One is the reducibility of nanomaterials, which oxidize to produce a part of hydroxyl radicals. In the other, hydrogen peroxide penetrates into the interior of the material and combines with iron to form a part of the hydroxyl group. The results showed that the degradation rate of ibuprofen in the blank control group (only hydrogen peroxide) was about 20%. Without the addition of catalyst, the self-degradation of ibuprofen was very slow, and its degradation rate was low. Both SAC and SAC/Fe₃O₄-MnO₂ had a relatively high degradation rate for the photocatalytic degradation of ibuprofen, and the degradation rate exceeded 80% after 2.5 h of light. This indicates that the abundant functional groups on the surface of activated carbon contribute to the photocatalytic degradation of ibuprofen. Sludge/Fe₃O₄-MnO₂ had excellent degradation effects on ibuprofen, and the degradation rate exceeded 90% after 2.5 h of light. Thus, it can be concluded that the degradation effect of Sludge/Fe₃O₄-MnO₂ was the best, and all experiments used it as a catalyst.

When the initial concentration of ibuprofen was 40 mg·L⁻¹, the dosage range of Sludge/Fe₃O₄-MnO₂ was 0.1–0.5 g·L⁻¹. After 2.5 h of light, the photolysis balance was determined and the photolysis effect was investigated. Figure 6 shows the effect of catalyst dosage on the degradation rate of ibuprofen. When the dosage of Sludge/Fe₃O₄-MnO₂ was less than 0.4 g·L⁻¹, the degradation rate of ibuprofen increased with the increase of the dosage of Sludge/Fe₃O₄-MnO₂. When the dosage of sludge-based catalyst reached 0.5 g·L⁻¹, the degradation rate of ibuprofen did not change significantly. Although the increase in dosage can provide more active sites for the degradation of ibuprofen, when the dosage reaches a certain level, it will hinder the transfer rate of hydrogen peroxide in the solution, resulting in a decrease in the reaction rate. Since the sludge-based catalyst catalyzed the production of hydroxyl radicals by hydrogen peroxide, the hydroxyl radicals generated by the increase in dosage will also increase. The hydroxyl radicals inevitably collided with each other and may be quenched, resulting in a decrease in the degradation rate of ibuprofen. It can be concluded that the optimal dosage of Sludge/Fe₃O₄-MnO₂ was 0.4 g·L⁻¹.

When the solution is alkaline or the pH gradually increases, it will hinder the generation of hydroxyl radicals, resulting in a decrease in the degradation rate (Cho et al. 2011). Therefore, in the study of the catalytic degradation of ibuprofen performance of Sludge/Fe₃O₄-MnO₂, pH 3.3 should be selected for the experiment.

When the hydrogen peroxide is too much, the excess hydroxyl radicals produced will react with the hydrogen peroxide itself, resulting in the reduction of the hydroxyl groups that play an important role in degradation. The generated HO₂· reactivity to pollutants is far less than the activity of hydroxyl radicals on pollutants (Chen et al. 2017). In conclusion, the optimal experimental concentration of hydrogen peroxide was 3.0 mL·L⁻¹.

The effect of different times on degradation is shown in Figure 5. As the contact time between Sludge/Fe₃O₄-MnO₂ and ibuprofen increased, the catalytic oxidation reaction gradually reached equilibrium. The degradation rate increased significantly within 2.5 h and reached 94.17% rapidly. The degradation rate of ibuprofen did not change significantly within 2.5–4.0 h, and the degradation rate was 93.46–94.40%. In summary, the optimal catalysis reaction time was 2.5 h.

The experimental data were fitted with a pseudo-first-order kinetic model and a pseudo-second-order kinetic model, as shown in Figure 9 and Table 4. The equilibrium reaction capacity of the experimental data was compared with the pseudo-first-order and pseudo-second-order adsorption kinetics models, and the pseudo-second-order kinetics model was closer to the actual value. The correlation coefficient (R² = 0.9938) was higher than that of the pseudo-first-order kinetic model (R² = 0.9236). The results showed that the pseudo-second-order kinetics model could better express the actual degradation process of ibuprofen, mainly chemical degradation (Li et al. 2021). The pollutants are adsorbed at the center of the sludge-based catalyst by using the adsorbability of the sludge-based catalyst itself, and the catalysis takes a dominant position over time. The pseudo-second-order kinetic model includes all the degradation processes, such as surface adsorption, external liquid film diffusion and particle internal diffusion (Mashayekh-Salehi & Moussavi 2016), which can better reflect the degradation mechanism of sludge-based catalysts for ibuprofen. In order to further determine that the degradation of ibuprofen was due to catalysis, the degradation products were analyzed.

The catalytic degradation process of Sludge/Fe₃O₄-MnO₂ mainly included the oxidation of hydrogen peroxide, the adsorption of sludge-based activated carbon, and the process of catalyzing hydrogen peroxide to produce hydroxyl radicals by Sludge/Fe₃O₄-MnO₂. The process of generating hydroxyl radicals included two parts: One part is the process in which Sludge/Fe₃O₄-MnO₂ catalyzes the decomposition of hydrogen peroxide to produce OH·, as shown in Figure 10 (2) and (3); The other part is due to the gain and loss of electrons in nanomaterials, and the combination of electrons and dissolved oxygen generates superoxide free radicals (·O²⁻). Superoxide radicals combine with hydrogen ions to generate hydrogen peroxide, which in turn generates hydroxyl radicals (Fang et al. 2013), as shown in Figure 10 (1) and (3).

In order to further explore the degradation reaction mechanism, the degraded products were identified by liquid chromatography mass spectrometry (LC-MS). The mass spectrum of the degradation product is shown in Figure S3. Ion peaks appeared at 100 and 142.1 m·z⁻¹, inferred from the mass spectrometry characteristics and combined with the mass spectrum library: the ion peak of ibuprofen at 206 m·z⁻¹, and the ion peak at 100, 142.1 m·z⁻¹ represented C₆H₁₀O and C₆H₁₀O₄, respectively (Li et al. 2021). From this, we can conclude that ibuprofen is decomposed by Sludge/Fe₃O₄-MnO₂ under light conditions, not just adsorbed on the sludge activated carbon by the sludge-based catalyst. According to the structure diagram of ibuprofen, we can infer the general path of the catalyst to degrade ibuprofen, as shown in Figure 11. Under the adsorption of sludge-based activated carbon, the ibuprofen molecules in the solution are adsorbed to the surface of Sludge/Fe₃O₄-MnO₂. The oxidation of nanomaterials in the catalysts produces hydroxyl radicals and the oxidation of a part of hydrogen peroxide. Decarboxylation occurs in the side chain of the benzene ring to form C₁₂H₁₈ (Chen et al. 2017). As the reaction occurs, the catalysts catalyze hydrogen peroxide to generate a large number of hydroxyl radicals. The hydroxyl radical acts on the benzene ring of C₁₂H₁₈, which will break the benzene ring to generate C₆H₁₀O (100 m·z⁻¹). Meanwhile, C₁₂H₁₈ may also generate C₆H₁₀O₄ (142 m·z⁻¹) under the action of hydroxyl radicals (Ding et al. 2017; Li et al. 2017). In summary, the degradation process of Sludge/Fe₃O₄-MnO₂ for ibuprofen was not simply an adsorption transfer. It was a process in which the benzene ring of ibuprofen was opened by hydroxyl radicals, which degraded macromolecular organic substances into small molecular substances, thereby achieving the purpose of removing ibuprofen.

In summary, magnetic sludge-based catalysts with low cost and little hazard were synthesized, and then used as catalysts in the heterogeneous photo-Fenton-like process to remove ibuprofen. The experimental conditions were 0.4 g·L⁻¹ catalyst dosage, 3.0 mL·L⁻¹ hydrogen peroxide concentration, pH 3.3, contact time 2.5 h, and the degradation rate could exceed 95%. The degradation process of ibuprofen conforms to the pseudo-second-order kinetic model. Hydroxyl radicals were considered to be the free radical species in the catalytic degradation system. The catalytic mechanism was proposed according to the FTIR analysis and mass spectrometry product analysis; it was mainly attributed to the interaction between hydroxyl groups and benzene rings. These results revealed that Sludge/Fe₃O₄-MnO₂ was an effective, environmentally friendly and low-cost catalyst, which can effectively degrade organic pollutants by relying on the heterogeneous photo-Fenton process. Sludge-based catalysts had proven their potential for photodegradation of drugs and might stimulate more attempts with sludge-based catalysts.

All relevant data are included in the paper or its Supplementary Information.