Degradation of bisphenol A by persulfate activation of MoS₂ composite iron tailings

The production of steel has been on the rise globally due to industrial needs, especially in China. The increase in iron and steel production will lead to the increase in iron tailings. China produces a large amount of iron-bearing tailings every year, and there is a growing trend (Zhou & Yang 2016). The accumulation of tailings has various dangers. For example, the accumulation of a large number of tailings will occupy land resources and cause scarcity of land resources. Due to the small sand particles in the tailing pond, the tailings will cause weathering after long-term stacking, resulting in the generation of dust. In addition, the tailings also contain a variety of heavy metal ions, which will lead to the leaching of heavy metals after being washed by rainwater (Geng et al. 2019), and the heavy metal ions released will have an important impact on the local soil and water environment. Therefore, we need to treat tailings in a non-hazardous way. At present, the harmless ways of tailings are mainly to mine backfill raw materials, raw materials of building materials (such as making cement with tailings, firing building bricks, making ceramics, glass-ceramics, and so on), extraction of valuable elements, synthesis of mesoporous molecular sieve using silicon sources, preparation of soil amendments and production of color coatings. However, the mine backfill waste resources, the production of building, ceramics, and glass–ceramic temperature above 1,000 °C, and large energy consumption, as building material durability is poor, are not suitable for use in the occasion of greater pressure, low economic efficiency, and valuable metal extraction of tailing composition requirements. Therefore, many scholars are seeking the application of tailings in environmental remediation, such as the preparation of catalysts (Lu et al. 2022). The high content of iron in many tailings has the potential to prepare heterogeneous iron-based catalysts, which can be used in advanced oxidation technology of persulfate.

Bisphenol A (BPA) is mainly used to synthesize some chemical materials, such as polycarbonate (PC), epoxy resin, and so on. Their existence can be detected in many plastic products. BPA has been found in humans, where it enters the body through consumption in ordinary life. It is an endocrine-disrupting chemical that disrupts the body's hormonal balance in the presence of low doses (Mohammed et al. 2020). The presence of BPA may be inextricably linked with precocious puberty in adolescents, nervous system development in infants and adolescents, adult fertility and other diseases (Vitku et al. 2016). Therefore, there is an urgent need for the treatment of BPA and other similar contaminants.

Advanced oxidation technology refers to the generation of free radicals with strong oxidation ability to degrade organic pollutants under different reaction conditions. According to the different conditions for generating free radicals, it can be divided into advanced oxidation technologies such as Fenton oxidation, electrochemical anodizing, photocatalytic oxidation, catalytic ozonization and ultrasonic chemistry, where the product free radicals are mainly ·OH, and advanced oxidation technologies where the product free radicals are mainly ⁠. In the traditional Fenton reaction (Li et al. 2012), when the pH is close to 3, Fe²⁺ reacts with hydrogen peroxide to produce ·OH, which is similar to the mechanism of produced by iron-based catalyst and persulfate. The redox potential of sulfuric acid is higher than that of hydroxyl radical generated in traditional Fenton reactions and its life span is longer (He et al. 2022). It has a wider range of pH and does not produce iron sludge like the traditional Fenton reaction. Therefore, the S-AOP technology based on sulfate radicals has more advantages than the traditional Fenton technology in the treatment of organic pollutants (Ghanbari & Moradi 2017). At present, more and more scholars have paid attention to the method of activating persulfate as shown in Table 1.

Table 1

Persulfate activation techniques

Activation mode .	Activation mechanism .	Advantage .	Disadvantage .	Reference .
Thermal activation		Room temperature activation	High energy consumption, low efficiency	Potakis et al. (2016)
Light activation		Ultraviolet radiation, sunlight is enough	Low efficiency	Macías-Vargas et al. (2022)
Alkali activation		pH neutrality produces ·OH and SO4•−	pH affects the form of contaminants and soil	Bolade et al. (2021)
Transition metal activation		Activation under normal temperature and pressure.	It is difficult to recover metal ions	Ren et al. (2021)
Zero-valent iron		It can provide Fe2+ activation and may act as adsorbent	Zero-valent iron is unstable	Lei et al. (2023)
Ironoxide/hydroxide		Environmentally friendly and relatively non-toxic	The efficiency is low, the response is not sustained	Li et al. (2019)
Iron-based polymetal		Accelerates the Fe (II)/Fe (III) cycle	Copper is toxic and cobalt is expensive	Zhang et al. (2022a)
MoS2/Iron tailings		Reaction cycle, metal iron and molybdenum are harmless to human body

Activation mode .	Activation mechanism .	Advantage .	Disadvantage .	Reference .
Thermal activation		Room temperature activation	High energy consumption, low efficiency	Potakis et al. (2016)
Light activation		Ultraviolet radiation, sunlight is enough	Low efficiency	Macías-Vargas et al. (2022)
Alkali activation		pH neutrality produces ·OH and SO4•−	pH affects the form of contaminants and soil	Bolade et al. (2021)
Transition metal activation		Activation under normal temperature and pressure.	It is difficult to recover metal ions	Ren et al. (2021)
Zero-valent iron		It can provide Fe2+ activation and may act as adsorbent	Zero-valent iron is unstable	Lei et al. (2023)
Ironoxide/hydroxide		Environmentally friendly and relatively non-toxic	The efficiency is low, the response is not sustained	Li et al. (2019)
Iron-based polymetal		Accelerates the Fe (II)/Fe (III) cycle	Copper is toxic and cobalt is expensive	Zhang et al. (2022a)
MoS2/Iron tailings		Reaction cycle, metal iron and molybdenum are harmless to human body

View Large

This work aims to prepare an iron-based catalyst that can effectively activate persulfate by using iron-containing tailings as an iron source and introduce MoS₂ to facilitate the redox loop of Fe(II)/Fe(III), which improve the activation efficiency of persulfate. Therefore, within this work, an array of MoS₂/iron-containing tailing catalysts has been manufactured by a single-step hydrothermal methodology using iron-bearing tailings as the iron source. All catalyst materials were used in the study of activating PMS to generate active radicals to degrade BPA. The experimental results show that the composite catalyst prepared after the introduction of MoS₂ exhibits higher catalytic efficiency than the pure tailing sample during the activation of the PMS. It has been found that MoS₂ has a synergistic effect as well as the iron activation sites over the catalyst surface to hasten the redox cycling process of Fe(II)/Fe(III), thus greatly improving the degradation effectiveness of BPA. In general, this study prepared a high-efficiency iron-based catalyst for activating persulfate by using tailings as raw materials, providing a new idea for tailing resource utilization, and achieving the goal of ‘wastewater treatment with waste’.

Chemical reagents utilized in this design are available in Table S1. The iron-bearing tailings used in this work are vanadium–titanium–magnetite tailings (CT) from Luanping County. During the collection process, we use the multi-point sampling method to collect the tailings. Impurities in the tailing sample are removed and 5 kg of sample is taken for the experiment. The material-to-ball ratio in the ball mill was 1:3, the rotational speed of the ball mill was 1,500 rpm min⁻¹ and the grinding time was 2 h. Finally, the resulting products adopted a 200–300 mesh nylon sieve. The compositions of the tailing sample are given in Table S2.

MoS₂/CT catalyst was prepared by hydrothermal methodology. First, 1 mmol (NH₄)₆Mo₇O₂₄·4H₂O and 30 mmol H₂NCSNH₂ were weighed into a beaker, then 70 mL of deionized water was dumped into the beaker and stirred it continuously for 30 min. Then, a quantitative volume of CT was placed into the solution, and ultrasonic irradiation was then performed for 15 min. The pre-prepared solution was transferred to a 100 mL reactor and left to heat at different temperatures (180, 200, 220°C) for 24 h. The product was washed several times with deionized water, filtered and collected. The product was dried at 80°C for 5 h and passed through 200 mesh sieve and used as catalyst. The specific preparation conditions and short names of different MoS₂/iron tailing composite catalysts are shown in Table S3. CTM stands for MoS₂/iron tailing catalyst. The suffix of the catalyst abbreviation represents the proportion of iron tailings added and hydrothermal temperature during preparation, such as CTM 1–200 represents the preparation of MoS₂/iron tailing composite catalyst. Adding iron tailings: MoS₂ = 1:1, hydrothermal temperature is 200 °C.

The phase structures of all catalysts are analyzed by X-ray powder diffractometer (XRD, BRUKER D8 FOCUS) using Cu K_α radiation on the condition of 40 KW and 100 mA. N₂ adsorption–desorption isotherms were determined on a physicochemical adsorption instrument (Autosorb-IQ2-C-TPX). Material-specific surface area was measured by a specific surface area analyzer (3H-2000PM). Scanning electron microscope (SEM, ZEISS Sigma 300) was applied to observe the morphology of the sample and energy dispersive spectrometer (EDS) was applied to document the element distribution over the specimen surface. X-ray photoelectron spectroscopy (XPS) has been applied to analyze the chemical content of the surface. The OPTIMA 8300 inductively coupled plasma emission spectrometer was used for the quantitative analysis of elements in tailings. Transmission electron microscope (TEM) patterns of the catalyst were tested by a Tecnai F20 field emission high-resolution electron transmission microscope produced by Philips-FEI, and the lattice fringes of the catalyst were characterized in the high-resolution mode. The electron paramagnetic resonance (EPR) spectra are to be measured on Chinainstru&Quantumtech (Hefei) EPR200-Plus. Methylene blue (MB) and tetracycline hydrochloride (TC) were tested and analyzed using a UV2800 Ultraviolet and visible (UV-Vis) spectrophotometer produced by Shanghai Sunny Hengping Scientific Instrument Co, Ltd Ultra-pure water was used as blank control, in which the absorption wavelength of MB was set to 665 nm, while the absorption wavelength of tetracycline hydrochloride was set to 358 nm. BPA, phenol and p-nitrophenol were detected by Agilent G7129A high-performance liquid chromatography (HPLC). The HPLC test criteria are shown in Table S4. The mass spectra (MS) were acquired in both negative electrospray (ESI−) and positive electrospray (ESI+) modes in the m/z range of 50–1,000.

The degradation of pollutants is uniformly carried out at room temperature (25 °C). First, adding 100 mL of BPA solution (0.1 mM) into a 200 mL beaker, the pH value required for phenol degradation is regulated with 0.1 M sulfuric acid and sodium hydroxide. Subsequently, various masses of catalyst were thrown into the contaminant solution and stirred for 30 min (300 rpm min⁻¹), and then a pre-determined concentration of PMS will be added for the reaction (catalyst and PMS added to different reaction systems is different, the specific content is described in Section 3.2.2). During the reaction, 1 mL sample at regular intervals was removed with a 2 mL syringe (1, 3, 5, 7, 10, 15, 20, 30 min), and the product was then injected into a liquid-phase vial by filtering the sample through a 0.22 μm polyethersulfone filter with a syringe filter and injected into a liquid-phase vial (pre-filled with 100 μL of methanol), which then awaited to be tested by HPLC. Different quenching agents were added before the reaction, and the remaining steps were basically unchanged. The types and concentrations of quenchers and the active species targeted during the experiment are shown in Table S5.

N₂ adsorption–desorption tests were performed to better explore the effects of different CT additions and hydrothermal reaction temperatures on the surface structure of MoS₂/CT. Figure S3a has type IV adsorption–desorption of type h3 hysteresis loop, indicating that its pore structure may be a slit hole formed by a sheet structure (Cui et al. 2022). Figure S3b shows the aperture distribution of samples added in different CT proportions, and the aperture is mostly distributed between 3 and 20 nm. In combination with Table S6, it can be seen that when the MoS₂ generation quantity increases, the sample has a larger specific surface area and a larger average pore diameter, which can provide a larger contact area and more active sites for MoS₂/CT catalyst to activate PMS. Compared with pure CT, when CT:MoS₂ = 1:1, the specific surface area is more suitable, and combined with the degradation effect data, it can be seen that adding too much CT will not be conducive to the activation of PMS.

In addition, through the analysis of (Figure S4), it is found that the curves are similar to those in the previous section. Hydrothermal temperature also affects the specific surface area, but does not change the pore structure of the material (Table S7). When the hydrothermal temperature of the catalyst is increased, the specific surface area of the catalyst will decrease, which is not conducive to the activation of PMS molecules. Compared with CTM 1–200, CTM 1–220 has a degradation efficiency of only 70% for BPA within 30 min (Figure S5b). The specific surface area of CTM 1–180 is larger than that of CTM 1–200, but the degradation efficiency is low, indicating that it is not a major factor in determining the activity of the material (Figure S4). Based on the above analysis, 200 °C is selected as the best hydrothermal temperature for the preparation of CTM 1 catalyst, while the mechanism of material activation needs to be further studied.

To investigate the photocatalytic properties of the materials, we also explored the degradation of BPA by MoS₂/PMS or CTM 1–200/PMS systems in the dark (Figure 4(b)) (Zhou et al. 2018). The experiment showed that the degradation efficiency of BPA was affected to some extent during 30 min. Under dark conditions, MoS₂ significantly reduced the degradation efficiency of BPA, while CM 1–200 did not. However, the degradation rate of BPA could still be accelerated by light at the early stage of the reaction, which indicates that CTM 1–200 photocatalytic participation in the activation process of PMS in the process of reaction. This further proves the presence of Fe doping in MoS₂.

As shown in Figure S5a, the removal rate of the pure MoS₂ system is very low, and the removal rate of BPA begins to rise after the addition of CT. When CT:MoS₂ = 2:1, the removal rate of BPA was 43%. With the decrease of CT addition, the removal rate of BPA becomes greater and greater. When CT:MoS₂ = 1:1, BPA can be completely removed within 30 min. When CT:MoS₂ = 0.5:1, the degradation effect decreased slightly. As shown in Figure S5c, when CT:MoS₂ = 1:1, k_obs = 0.213 is the largest. The additional amount of CT has an important effect on the removal of BPA. When the amount of CT is too much, the MoS₂ wafer on the surface will not grow evenly, and when the amount of CT is too little, the surface layer will be stacked. Therefore, the optimal ratio of MoS₂ to CT is confirmed by experiments as 1:1. As shown in Figure S5b, when the hydrothermal temperature is 200 °C, the removal rate of BPA reaches 100% in 30 min. When the hydrothermal temperature was 180 and 220 °C, the removal rate of BPA was only 28% at 30 min. As shown in Figure S5d, when the hydrothermal temperature is 200 °C, the maximum k_obs = 0.218. This shows that hydrothermal temperature is a very important factor in the preparation of catalysts. Too high a temperature will cause the degree of crystallization of MoS₂ to become stronger, reduce the defects in MoS₂, and hinder the flow of electrons. When the temperature is too low, the peaks of MoS₂ are not obvious, and MoS₂ cannot grow well on CT, resulting in a decrease in the activation efficiency of PMS.

Figure 5(c) shows that the CTM 1–200/PMS is an efficient BPA removal system over a wide pH range. The CTM 1–200/PMS system can achieve a removal efficiency of more than 97% in the range of pH = 3–10. At pH = 11, the removal efficiency of BPA dropped to 15%, which indicated that the catalyst had no activation effect on PMS at this time. Interestingly, the removal rate of BPA also decreases at low pH, which might result from the formation of (Fe (H₂O)₆)²⁺ and (Fe (H₂O)₆)³⁺ in extreme acidic conditions (Peng et al. 2022). Fig. S6c shows that the k_obs reached its maximum when pH = 7, so neutral conditions were selected as the initial environment for subsequent research. Fig. S6d shows the Zeta potential value on the catalyst surface. The data show that the change in pH has little effect on the Zeta potential value on the catalyst surface, which indicate that the change of potential on the catalyst surface is not the main reason for the degradation. In addition, we selected tetracycline hydrochloride (TC), p-nitrophenol (PNP), phenol (phenol) and MB as model pollutants to evaluate the removal of various pollutants by the CTM 1–200/PMS system. As shown in Figure 5(d), the total removal of BPA and phenol in less than 30 min. MB could be totally eliminated within 40 min. The rate of removal of TC and PNP also exceeded 85% within 40 min, which indicated that the CTM 1–200/PMS system is effective in removing a wide range of organic contaminants and can be used in a variety of complex environments.

The role of metal sites in MoS₂ has been established, and the role of sulfur species on its surface needs to be studied. The change of sulfur species on the surface is detected by XPS. Figure 7(d) shows the fine spectrum of S 2p of CTM 1–200 catalyst before and after the reaction. Before the reaction, 162.6 and 163.7 eV were attributed to S 2p_3/2 and S 2p_1/2, and after 169.6 eV was attributed to produced by the reaction. According to literature reports (Kuang et al. 2020), Mo(IV) has also been found to promote the conversion of Fe³⁺ to Fe²⁺ in MoO₂/Fe³⁺/PS system, but its catalytic efficiency is lower than that of MoS₂/Fe³⁺/PS system, indicating that in addition to the role of reducing metal sites, the presence of unsaturated S also plays an important role. According to Huang et al. (2021), surface defects of molybdenum disulfide can promote the conversion of Fe³⁺ to Fe²⁺. It is reported that the presence of S defect in MoS₂ can promote the formation of electron-deficient centers, increase the electron density around Mo and reduce the valence state of Mo on the surface of MoS₂, so as to better play the role of reducing metal sites. Before the reaction, the proportion of and was 7.39 and 14.54%, respectively. After the reaction, the proportion of was basically unchanged, while the proportion of increased significantly. Other than the metal site, reducing sulfur is the only electron donor that produces Fe(III) (Kuang et al. 2020). did not change much before and after the reaction, indicating that the metal reaction site provided by Mo played a major role in loading MoS₂ on the CT surface, while the influence of S species was small.

Therefore, there may be two factors affecting the increase of Fe(II), one is the effect of reducing metal sites, and the other is the effect of unsaturated S.

Electron paramagnetic spin resonance (ESR) (Zhou et al. 2020) was used to further identify reactive radicals and other reactive species produced during the reaction. In Figure 8(b), feature peaks of and appeared. However, the characteristic peak of DMPO-•OH is relatively inconspicuous, which is also in line with the quenching experiment, indicating that it is not •OH but that acts as the main player in the reaction process. As shown in Figure 8(c) and 8(d), the existence of and was also found during the reaction. From the ESR spectra, we found that and also existed in the CTM 1–200/PMS system apart from the conventional and •OH.

From the above description, the mechanism of the CTM 1–200/PMS/BPA system was summarized. As shown in Fig. S7, the Fe(II) active sites on the catalyst surface react with PMS to generate and •OH, and plays a major role. At the same time, electron transfer occurs on the catalyst surface, and Fe(III) is reduced to Fe(II) by Mo(IV) on the surface of MoS₂, and which process is dynamic. The generated Mo(V) may be converted into Mo(VI). is generated by the reaction of in solution with Mo(VI), resulting in the active species acting in solution as ⁠, and •OH. The resulting active substance reacts with BPA to generate certain intermediate products which are finally completely mineralized into CO₂ and H₂O.

For the purpose of further determining the intermediate process of conversion of BPA to H₂O and CO₂ in the CTM 1–200 system, the transformation of BPA into inorganic products using ESI-MS analysis, and three possible reaction routes of BPA in the CTM 1–200/PMS system were proposed (Fig. S8) (Darsinou et al. 2015). In part I, the oxidation of reactive substances at C1 and C5 positions on the benzene ring resulted in the production of product-01 (bisphenol A catechol) and product-02. Nucleophilic attack occurs at C3 and C10 in part II, resulting in beta cleavage of the isopropyl group between the two phenyl groups; thus, product-06, product-05, product-04 or product-03 was generated. In part III, due to the cleavage of the benzene ring at C10 and C12, product-07 or product-08 is produced. After the above three pathways, oxidation and ring-opening reactions will further occur in the generated products to generate low-molecular-weight ring-opening products and finally mineralize into CO₂ and H₂O.

In order to explore the influence of the contact between iron tailings and water when using a catalyst, the effective amount of heavy metal leaching experiment was carried out on the prepared CTM 1–200.

5 g of sifted MoS₂ composite iron tailings were weighed and placed in a 500 mL beaker, and 250 mL of ultra-pure water was added as the extraction agent according to the liquid–solid ratio of 50:1. Place the beaker on the magnetic stirrer and run it at 500 rpm for 3 h. During this period, the pH of the leaching solution was adjusted with 1 mol/L nitric acid solution to stabilize the pH between 7 ± 0.05. The leached solution was filtered with a 0.45 μm filter membrane and stored. Place the filter membrane and its trapped sample into the original 250 mL beaker and add ultra-pure water so that its total weight is the same as in the first step. Place on the magnetic stirrer and run at the same speed for 3 h. During this period, the pH of the leaching solution was adjusted with 1 mol/L nitric acid solution to stabilize the pH between 4 ± 0.05. The remaining steps are identical to the previous one. The leachate was assessed by uniformly mixing it with the leachate held in the initial stage. The experimental results of effective leaching of heavy metals are shown in Table S8.

The main raw material of the composite catalyst produced by the technology in this study is iron tailings, which are mostly concentrated in a tailing reservoir, and the recycling process is convenient and saves a lot of transportation resources. Iron tailings are used as industrial solid waste and reused as the main raw material of catalyst, which saves the storage and management cost of tailings and the treatment and restoration cost of polluted environment. The recycling of iron tailings, in response to national policies, is subject to national technical subsidies, and the comprehensive cost is reduced, which can achieve better economic effects and create conditions based on the market.

This work prepared a series of composite catalysts using iron-containing tailings as the iron source and used BPA as the main objective pollutants to explore the degradation capabilities of varied reaction systems on BPA and the optimal preparation conditions of the catalysts. Hydrothermal loading of MoS₂ on CT surface can significantly improve the ability of activating PMS to degrade BPA. In the CTM 1–200/PMS system, the rise in the ratio of Fe(II) sites is related to the existence of reducing metal sites Mo(VI), electrons on the surface of the CTM 1–200 catalyst transfer from the reduced metal sites at the junction to Fe(III), which accelerates the Fe(II)/Fe(III) redox cycle and greatly improves the degradation efficiency to model pollutants. In general, this work prepared a highly efficient iron-based catalyst for persulfate activation using tailings as raw materials, which provides a new idea for the resource utilization of tailings.

In this paper, the resource utilization of iron-bearing tailings is applied to the treatment of wastewater, and MoS₂ is introduced to improve the degradation efficiency of bisphenol A wastewater, which realizes our purpose of ‘treating wastewater with waste water’. However, there are still several areas that need further exploration:

• 1.

  There is no strong research on the treatment and recovery of the subsequent catalyst, and an economical and simple method should be explored for the subsequent treatment of the catalyst after use.

• 2.

  Only one kind of waste containing iron tailings is reused in this topic, and the resource utilization of other iron-containing waste is not considered. Pickling sludge, iron slag and waste electrodes, on the basis of introducing MoS₂, may be used as effective catalysts for activating persulfate.

H. Z. rendered support in formal analysis, investigated the data, supported in data curation, wrote the original draft, wrote the review and edited the article, and visualized the project. X. W. arranged the resources, investigated the data, and visualized the article. S. Y. investigated the data and arranged the resources. G. X. conceptualized the whole article, developed the methodology, validated the data, supervised the work, and administered the project work. C. G. supervised the work and administered the project work. L. W. supervised the work and conceptualized the whole article. X. D. supervised the work and conceptualized the whole article. Y. W. supervised the work and conceptualized the whole article. G. T. supervised the work and conceptualized the whole article. S. Z. supervised the work and conceptualized the whole article.

This work was supported by the National Natural Science Foundation of China (Grant No.U20A20132) and the National Natural Science Foundation of China (No. 42277369) to carry out this research.

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

The authors declare there is no conflict.