Heterogeneous photo-Fenton-like catalysts with low cost, little hazard, high effectiveness and facile separation from aqueous solution were highly desirable. In this study, sludge-based catalysts combining nano Fe3O4-MnO2 and sludge activated carbon were successfully synthesized by high-temperature calcination method and then characterized. These synthetic materials were applied to remove ibuprofen in the heterogeneous photo-Fenton process. The preparation conditions of sludge-based catalysts optimized by orthogonal experiments were 2.0 M of ZnCl2, a temperature of 500 °C, a pyrolysis time of 60 min, and a sludge ratio: Fe3O4-MnO2 of 25:2. In batch experiments, the optimal experimental conditions were determined as catalyst dosage of 0.4 g·L−1, hydrogen peroxide concentration of 3.0 mL·L−1, pH value of 3.3, and contact time of 2.5 h. The degradation rate sludge/Fe3O4-MnO2 catalyst to ibuprofen is up to 95%. The removal process of ibuprofen fitted the pseudo-second-order kinetic model, and the photocatalytic degradation process was the main factor controlling the reaction rate. The catalytic mechanism was proposed according to the Fourier transform infrared analysis and mass spectrometry product analysis; it was mainly attributed to the interaction between hydroxyl groups and benzene rings.

  • A novel approach to reuse waste sludge as a catalyst is suggested.

  • Fe/Mn oxide nanoparticles were embedded in the sludge activated carbon.

  • The catalyst was easy to achieve solid-liquid separation under the action of an external magnetic field.

  • Hydroxyl radicals mainly lead to the degradation of ibuprofen.

Graphical Abstract

Graphical Abstract
Graphical Abstract

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.

Figure 1

Source and migration of ibuprofen in the human environment.

Figure 1

Source and migration of ibuprofen in the human environment.

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.

Materials

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.

Table 1

Characteristics of sewage sludge

PropertiesSludge moisture content (%)Volatile solid content (%)Ash content (%)
Content 78.60 10.08 4.16 
PropertiesSludge moisture content (%)Volatile solid content (%)Ash content (%)
Content 78.60 10.08 4.16 

Preparation and optimization of catalysts

Orthogonal experiment design

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.

Table 2

Levels of factors in the experiment

LevelActivator concentration (M)Solid-liquid ratioCalcination temperature (°C)Calcination time (min)m(Sludge): m(Fe3O4-MnO2)
0.5 1:1 400 45 50:7 
1:2.5 500 60 25:2 
1:3 600 90 50:1 
4.5 1:4.5 750 120 250:1 
LevelActivator concentration (M)Solid-liquid ratioCalcination temperature (°C)Calcination time (min)m(Sludge): m(Fe3O4-MnO2)
0.5 1:1 400 45 50:7 
1:2.5 500 60 25:2 
1:3 600 90 50:1 
4.5 1:4.5 750 120 250:1 

Synthesis of nano Fe3O4-MnO2

3.1281 g FeSO4·7H2O was dissolved in 150 mL distilled water, followed by a thermostatic water bath at 90 °C, and a certain amount of 5 M sodium hydroxide solution was added into the solution. Then the mixture was stirred evenly and a green precipitate appeared. 25 mL of 0.1 M KMnO4 solution was added to the mixture after 2 to 4 minutes under stirring until the potassium permanganate was completely dissolved. During the entire preparation process, the chemical reaction mechanism is shown in Equations (1) and (2):
formula
(1)
formula
(2)

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.

Sludge pretreatment

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.

Synthesis of sludge/Fe3O4-MnO2

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.

Synthesis of SAC/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.

Catalysts characterization

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.

Batch degradation assay

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.

Analysis

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

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.

Characterization of catalysts

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).

Table 3

Summary of sludge base catalyst specific surface area and pore size

Product CharacteristicsSludge/Fe3O4-MnO2SAC/Fe3O4-MnO2SAC
Surface area and porosity SBET (m2 g−1434.512 362.677 398.741 
Pore volume (cm3 g−10.355 0.302 0.312 
Pore diameter (nm) 0.887 0.888 0.887 
Product CharacteristicsSludge/Fe3O4-MnO2SAC/Fe3O4-MnO2SAC
Surface area and porosity SBET (m2 g−1434.512 362.677 398.741 
Pore volume (cm3 g−10.355 0.302 0.312 
Pore diameter (nm) 0.887 0.888 0.887 

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.

Figure 2

SEM micrographs of three catalysts.

Figure 2

SEM micrographs of three catalysts.

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.

Figure 3

FTIR of sludge-based catalysts.

Figure 3

FTIR of sludge-based catalysts.

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).

Figure 4

VSM of sludge-based catalysts.

Figure 4

VSM of sludge-based catalysts.

Degradation performance

Catalytic oxidation of ibuprofen over sludge-based catalysts

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.

Figure 5

Degradation of ibuprofen by the sludge-based catalysts catalyzed the hydrogen peroxide. Reaction conditions: Initial ibuprofen concentration = 40 mg·L−1, Initial H2O2 concentration = 3.0 mL·L−1, pH value = 3.3, catalysts dose = 0.4 g·L−1.

Figure 5

Degradation of ibuprofen by the sludge-based catalysts catalyzed the hydrogen peroxide. Reaction conditions: Initial ibuprofen concentration = 40 mg·L−1, Initial H2O2 concentration = 3.0 mL·L−1, pH value = 3.3, catalysts dose = 0.4 g·L−1.

The effect of catalyst dosage on degradation

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.

Figure 6

The effect of catalyst dosage on the degradation of ibuprofen. Reaction conditions: Initial ibuprofen concentration = 40 mg·L−1, Initial H2O2 concentration = 3.0 mL·L−1, pH value = 3.3; contact time = 4.5 h.

Figure 6

The effect of catalyst dosage on the degradation of ibuprofen. Reaction conditions: Initial ibuprofen concentration = 40 mg·L−1, Initial H2O2 concentration = 3.0 mL·L−1, pH value = 3.3; contact time = 4.5 h.

The effect of pH value on degradation

In the catalytic hydrogen peroxide system, pH not only affects the rate of catalytic hydrogen peroxide, but also has a certain effect on the surface properties of the catalyst. In this experiment, the pH range was selected as 3.3–9.3. The ibuprofen solution was acidic and its pH was 4.3. As shown in Figure 7, with the increase of pH value, the degradation rate of ibuprofen by Sludge/Fe3O4-MnO2 also decreased correspondingly, indicating that acidic conditions are more conducive to the photocatalytic degradation of ibuprofen by Sludge/Fe3O4-MnO2 (Baccar et al. 2012). When the pH value of the ibuprofen solution was 3.3, the degradation reached the best effect, and the degradation rate was about 90%. The ibuprofen solution and hydrogen peroxide diffused to the center of the sludge catalyst, and it was adsorbed on the surface of the sludge catalyst. Then, hydrogen peroxide generates hydroxyl radicals under the catalysis of metal ions. The reaction equation is shown in Equation (3) (Fang et al. 2013):
formula
(3)
Figure 7

The effect of pH on the degradation of ibuprofen. Reaction conditions: Initial ibuprofen concentration = 40 mg·L−1, Initial H2O2 concentration = 3.0 mL·L−1, catalysts dose = 0.4 g·L−1, contact time = 4.5 h.

Figure 7

The effect of pH on the degradation of ibuprofen. Reaction conditions: Initial ibuprofen concentration = 40 mg·L−1, Initial H2O2 concentration = 3.0 mL·L−1, catalysts dose = 0.4 g·L−1, contact time = 4.5 h.

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.

The effect of hydrogen peroxide concentration on degradation

In this experiment, hydrogen peroxide was chosen to be added. The addition of hydrogen peroxide not only won't cause environmental pollution, but also can provide hydroxyl radicals to play a certain role in the process of ibuprofen degradation. The concentration of hydrogen peroxide was set in the range of 0.4–4.0 mL·L−1. As can be seen from Figure 8, when the concentration of hydrogen peroxide ranged from 0.4 to 3.0 mL·L−1, it gradually increased with the increase of the dosage of hydrogen peroxide; when the dosage was 4.0 mL·L−1, the degradation rate decreased. This means that the higher the concentration of hydrogen peroxide, the higher the degradation rate of ibuprofen by Sludge/Fe3O4-MnO2. This phenomenon can be explained by Equation (4) (Fang et al. 2013):
formula
(4)
Figure 8

The effect of hydrogen peroxide concentration on the degradation of ibuprofen. Reaction conditions: Initial ibuprofen concentration = 40 mg·L−1, pH value = 3.3; catalysts dose = 0.4 g·L−1, contact time = 4.5 h.

Figure 8

The effect of hydrogen peroxide concentration on the degradation of ibuprofen. Reaction conditions: Initial ibuprofen concentration = 40 mg·L−1, pH value = 3.3; catalysts dose = 0.4 g·L−1, contact time = 4.5 h.

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.

Reaction kinetics

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.

Table 4

Pseudo-first-order and pseudo-second-order kinetic model parameters

Qe(exp.) (mg/g)Pseudo-first-order kinetic
Pseudo-second-order kinetic
k1 (h−1)Qe(cal.) (mg.g−1)R2k2 (mg−1*h−1)Qe(cal.) (mg.g−1)R2
95.34 5.7092 94.73 0.9236 6.0878 95.26 0.9938 
Qe(exp.) (mg/g)Pseudo-first-order kinetic
Pseudo-second-order kinetic
k1 (h−1)Qe(cal.) (mg.g−1)R2k2 (mg−1*h−1)Qe(cal.) (mg.g−1)R2
95.34 5.7092 94.73 0.9236 6.0878 95.26 0.9938 
Figure 9

The reaction kinetic curve of Sludge/Fe3O4-MnO2 degradation of ibuprofen.

Figure 9

The reaction kinetic curve of Sludge/Fe3O4-MnO2 degradation of ibuprofen.

Degradation mechanism

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).

Figure 10

Hydroxyl radical production process.

Figure 10

Hydroxyl radical production process.

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.

Figure 11

Ibuprofen degradation products analysis.

Figure 11

Ibuprofen degradation products analysis.

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.

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Supplementary data