Study of Fe₃O₄/PS System in Degrading BPA in Aqueous Solution

Abstract

The degradation of bisphenol A (BPA) by Fe₃O₄/persulfate system was investigated in aqueous solution. The influences of the initial concentrations of Fe₃O₄, persulfate (PS) and BPA, pH value, and initial reaction temperature on BPA removal were studied. The radical species was investigated by adding excessive dose of scavenger (methanol (MeOH) and tert-butanol (TBA)) into Fe₃O₄/PS system for the purpose of radical scavenging. The degradation products of BPA were detected by gas chromatography-mass spectrometry (GC-MS). The recyclability of Fe₃O₄ was also evaluated. The BPA removal rate of 80.7% was achieved under the following conditions: [BPA]₀ = 1 mg L^-1, [PS]₀ = 0.2 mM, [Fe₃O₄]₀ = 0.1 g L^-1, T₀ = 20 ± 1 ºC, pH₀ = 6.8 ± 0.2. The results confirmed that the main free radicals in the reaction process were sulfate radicals, followed by hydroxyl radicals. Some intermediate products of BPA degradation, such as phenols, benzoquinones and benzoic acid were identified by GC-MS.

Keywords:
sulfate radicals; Fe₃O₄; bisphenol A; hydroxyl radicals; advanced oxidation processes

Introduction

Bisphenol A (2,2-bis(4-hydroxyphenyl)propane, BPA), a white solid, has been widely used as a ubiquitous intermediate in manufacturing polycarbonate plastics, epoxy resins and polysulfone.¹,² Therefore, BPA is present in many daily supplies, such as toys, bottles, food and beverage packaging, water supply pipes, and the polymers used in dental treatment.³,⁴ BPA is a well-known and studied endocrine disrupting compound (EDC), which can mimic hormones and cause reproductive damage, cancer and other adverse effects on the human body and the ecological environment.⁵,⁶ Previous studies⁷,⁸ have shown that BPA can interfere with the endocrine system of humans and animals even at concentrations below 1 μg m^-3. Due to the discharge of domestic sewage and industrial wastewater, as well as the infiltration of landfill leachate, BPA has frequently been found in surface water and groundwater.⁹,¹⁰ The study showed that the BPA concentration in some water environments can be as high as 100 μg L^-1.¹⁰ At present, the concentration of BPA in water environment and its harm to aquatic organisms have attracted the attention of researchers who are working on the research of refractory organics removal methods.

Since BPA is a refractory pollutant with two benzene rings, conventional biological processes in wastewater treatment plants are not efficient for complete removal of BPA.¹¹ There are several alternative approaches that have been used to deal with the BPA over the years, such as adsorption,¹²,¹³ chlorination with sodium hypochlorite,¹⁴ and the advanced oxidation processes (AOPs). The adsorption is restricted owing to the low hydrophobicity (logK_ow) of BPA that limits the adsorption efficiency,¹⁵ and the chlorinated metabolites formed during chlorination with sodium hypochlorite cause some side effects.¹⁶ In contrast, AOPs are popular due to their ability to decompose refractory organics into biodegradable and benign products by powerful free radicals such as hydroxyl radicals (•OH) or sulfate radicals (SO₄^•-).¹⁷ In AOPs, ozone (O₃),¹⁸ hydrogen peroxide (H₂O₂), peroxymonosulfate (PMS), and persulfate (PS) are commonly used oxidants, which can generate free radicals after being activated by certain ways.¹¹ For instance, in the UV/O₃, UV/H₂O₂, and Fe^II/H₂O₂ processes, O₃ and H₂O₂ can produce •OH due to UV or iron activation; in the UV/PS, Fe⁰/PS, Fe^II/PS, and Fe₃O₄/PS processes, PS can generate SO₄^•- also due to UV or iron activation.¹¹,¹⁸ Although SO₄^•- and •OH have close oxidizing potentials (for SO₄^•-, E₀ = 2.6-3.1 V; for •OH, E₀ = 1.8-2.7 V), the half-life of SO₄^•- is longer than that of •OH, and the oxidant PS is more stable than O₃ and H₂O₂. On the one hand, iron is a common material with benign property, which makes it as an activator to show economic and environmental advantages. On the other hand, from the standpoint of stability, ease of storage, and generation of sulfate radicals, PS is considered to be more suitable as an oxidant in AOPs. Therefore, the use of iron to activate PS to generate free radicals in degradation of refractory organics has become one of the research hotspots in AOPs. The mechanisms by which various forms of iron activate PS are attributed to the redox reactions between Fe^II and PS. Based on the existence form of iron, iron-based activator can be classified into homogeneous (such as Fe^II) and heterogeneous (such as Fe⁰ and Fe₃O₄).¹⁹ As a homogeneous activator, Fe^II is completely dissolved in the solution, which easily causes excessive Fe^II, thereby trapping the generated free radicals and reducing the degradation efficiency of organic matter.²⁰,²¹ On the contrary, as heterogeneous activates, Fe⁰ and Fe₃O₄ can release Fe^II into the water solution at a certain rate to avoid the extinction of free radicals caused by excessive Fe^II. Fe₃O₄ is a preferred activator with mixed valence oxide containing Fe^II and Fe^III, and it is able to constantly produce Fe^II to react with PS and generate free radicals. In addition, Fe₃O₄ is the main component of magnetite in nature, so it is easy to obtain. Compared with Fe^II and Fe⁰, it is easy to recover Fe₃O₄ from wastewater based on its sub-magnetism. Fe₃O₄ can also reduce the cost of large volumes of waste effluent treatment and will not result in secondary pollution.²²

Some researchers used Fe^II and Fe⁰ to activate PS for the treatment of BPA,²²,²³ and some researchers studied sulfamonomethoxine, ciprofloxacin hydrochloride degradation in Fe₃O₄/PS systems.²²,²⁴,²⁵ However, there are few studies towards BPA removal using Fe₃O₄/PS systems, especially the scavenging experiment of free radicals and the intermediate identification of BPA degradation products still need be further studied. In this paper, Fe₃O₄ magnetic particles have been used as activator to activate PS to produce free radicals for the removal of BPA. The performance of the Fe₃O₄/PS system on the degradation of BPA was systematically investigated, the possible effects of environmental factors on BPA removal, and BPA degradation mechanism were discussed in detail. Moreover, the role of active radical species was also explored using scavenging experiments, and the recycling performance of Fe₃O₄ was evaluated based on Fe₃O₄ reuse experiment.

Experimental

Chemicals and equipment

The main chemicals and equipment used in the experiments are present in Tables 1 and 2, respectively. The content of Fe₃O₄ in the iron oxide material is greater than 99.5%, the particle size of iron oxide is 20 nm, and the specific surface area of iron oxide is 51.46 m² g^-1.

Batch experiments

A 1000 mg L^-1 of BPA aqueous stock solution was prepared for dilution into a series of concentrations in the batch experiments. The experiments were conducted in the 500-mL beakers placed on a magnetic heating stirrer with the operating speed of 200 rpm. Firstly, the BPA solution (500 mL) with the designed concentration was added to the beaker, then predetermined amount of PS and Fe₃O₄ was quickly plunge into the beaker to trigger the reaction. The reaction temperature was adjusted by the heater, and the pH was adjusted by adding 0.1 M NaOH or H₂SO₄ to the solution. The entire reaction time was set to 60 min. Aliquots (0.5 mL) were taken out of the beaker and put into the sample bottle. Then, 0.5 mL ethanol was immediately added into the sample bottle to stop the reaction. The samples were taken every 10 min from the beaker, and the residual concentration of BPA of the final samples were detected by high-performance liquid chromatography (HPLC). For the scavenging of free radicals generated in the reaction, the experiments were performed by adding methanol (MeOH) and tert-butanol (TBA) with preset concentrations in beakers.

Table 1
The main experimental chemicals

Chemical	Molecular formula	Purity	Supplier
Bisphenol A	C15H16O2	premium grade	McLean Co., Ltd. (Shanghai, China)
Iron oxide	Fe3O4	analytical grade	McLean Co., Ltd. (Shanghai, China)
Sodium persulfate	Na2S2O8	analytical grade	Sinopharm Co., Ltd. (Shanghai, China)
Sodium hydroxide	NaOH	analytical grade	Tianjin Chemical Reagent Factory (Tianjin, China)
Sulfuric acid	H2SO4	analytical grade	Beijing Chemical Reagent Factory (Beijing, China)
Methanol	CH3OH	chromatographically grade	Norsch Co., Ltd. (Chengdu, China)
tert-Butanol	C4H10O	chromatographically grade	McLean Co., Ltd. (Shanghai, China)
Hexane	C6H14	analytical grade	Beijing Chemical Reagent Factory (Beijing, China)
Dichloromethane	CH2Cl2	analytical grade	Beijing Chemical Reagent Factory (Beijing, China)
Sodium chloride	NaCl	analytical grade	Beijing Chemical Reagent Factory (Beijing, China)
Anhydrous sodium sulfate	Na2SO4	analytical grade	Xilong Science Co., Ltd. (Shantou, China)

Table 2
The main equipment

Equipment	Model	Manufacturer
Ultra pure water machine	Elemental 1810 D	Chongqing Moore Water Treatment Equipment Co., Ltd. (Chongqing, China)
Electronic analytical balance	FA2104A	Shanghai Leigu Instrument Co., Ltd. (Shanghai, China)
Magnetic stirrer	SH-II-4C	Guangdong Foheng Instrument Co., Ltd. (Foshan, China)
pH meter	pHS-3C	Shanghai INESA Scientific Instrument Co., Ltd. (Shanghai, China)
High-phase liquid chromatography (HPLC)	G7121A	Agilent Technologies (China) Co., Ltd. (Shanghai, China)
Nitrogen blowing concentrator	XSF-12	Shanghai Xuansheng Scientific Instrument Co., Ltd. (Shanghai, China)
Gas chromatography mass spectrometry (GC-MS)	7890B-7000C	Agilent Technologies (China) Co., Ltd. (Shanghai, China)
Oven	GHG-9040N	Hangzhou Zhuochi Instrument Co., Ltd. (Hangzhou, China)

Analytical methods

The BPA concentration in samples was examined by a HPLC equipped with a C18 column (4.6 mm × 150 mm, 4 μm) by using a fluorescence detector at 228 nm. The mobile phase was 70% methanol at a flow rate of 0.8 mL min^-1. The excitation and emission wavelengths are 228 and 312 nm, respectively. The temperature of the column compartment was 25 ºC and the injection volume was 20 μL. The retention time of BPA under above conditions was 3.48 min.

The oxidation products of BPA were carried out by a gas chromatography-mass spectrometry (GC-MS). The chromatographic analysis was performed via injection in splitless mode (split ratio of 10:1, injection volume of 1 μL) at 260 ºC using a quartz capillary column (length: 30 m, inner diameter: 0.25 mm, film thickness: 0.25 μm) with a helium flow of 1.0 mL min^-1. The qualitative analysis was performed with electron ionization (EI) at 70 eV using the full scan mode in the m/z range of 45-280. The ion source and quadrupole temperature were maintained at 230 and 150 ºC, respectively.

The removal rate η (%) of BPA is calculated using equation 1, where C₀ is the initial concentration of BPA (mg L^-1), C_t is the BPA concentration after t min treatment (mg L^-1). The fitting of the kinetic equations in the experiment are based on the first-order reaction kinetic equation, as shown in equation 2, where k is the kinetic constant (min^-1), b is the y intercept (no unit).

(1)$$\mathrm{\eta }=\frac{{\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}\times 100$$

(2)$$\ln \left(\frac{{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}\right)=-\mathrm{k}\mathrm{t}+\mathrm{b}$$

Results and Discussion

The effect of the initial Fe₃O₄ dose

As the activator of PS, Fe₃O₄ plays a very important role in the reaction of Fe₃O₄/PS system. We investigated the effects of six Fe₃O₄ doses used in activating PS on removing BPA. The experimental results are presented in Figure 1. Without adding Fe₃O₄, the BPA removal rate can reach 19.68% by PS alone within 60 min. We speculate that this was due to temperature or light energy, which promote PS to produce SO₄^•- (equation 3), thereby removing BPA. A previous research²⁶ confirmed that PS can be catalyzed by ambient temperature to form SO₄^•- to degrade organic matter. As adding Fe₃O₄ to the system, the removal rate of BPA was significantly increased. The Fe^II on the surface of Fe₃O₄ can react with PS to generate SO₄^•- (equation 4), and the BPA was removed. As the dose of Fe₃O₄ increased from 0.1 to 0.2 g L^-1, the BPA removal rate increased from 41.94 to 56.49% within 60 min. When the dose of Fe₃O₄ was increased to 0.3-0.5 g L^-1, the removal rate of BPA did not change much, compared with the BPA removal rate of 0.2 g L^-1 of Fe₃O₄. We speculate that there are three possible reasons for this BPA degradation curve. Firstly, due to the increase of Fe₃O₄ dosage, more Fe^II can be produced, and excessive Fe^II will extinguish SO₄^•- (equation 5),²⁷ which will reduce the BPA removal rate. Secondly, the excessive SO₄^•- formed can react between themselves and reduce the number of SO₄^•- (equation 6).²⁸ Finally, the increase in the amount of Fe₃O₄ (0.3-0.5 g L^-1) causes the consumption of PS to increase. At 40 min, PS is exhausted and the number of free radicals decreases, so the degradation rate of BPA increases slowly.

(3)$${\mathrm{S}}_2{\mathrm{O}}_8{}^{2-}\stackrel{\text{energy}}{⟶}2{\mathrm{S}\mathrm{O}}_4{}^{\bullet -}$$

(4)$${\mathrm{F}\mathrm{e}}^{\mathrm{I}\mathrm{I}}+{\mathrm{S}}_2{\mathrm{O}}_8{}^{2-}\to {\mathrm{F}\mathrm{e}}^{\mathrm{I}\mathrm{I}\mathrm{I}}+{\mathrm{S}\mathrm{O}}_4{}^{\bullet -}+{\mathrm{S}\mathrm{O}}_4{}^{2-}$$

(5)$${\mathrm{F}\mathrm{e}}^{\mathrm{I}\mathrm{I}}+{\mathrm{S}\mathrm{O}}_4{}^{\bullet -}\to {\mathrm{F}\mathrm{e}}^{\mathrm{I}\mathrm{I}\mathrm{I}}+{\mathrm{S}\mathrm{O}}_4{}^{2-}$$

(6)$${\mathrm{S}\mathrm{O}}_4{}^{\bullet -}+{\mathrm{S}\mathrm{O}}_4{}^{\bullet -}\to {\mathrm{S}}_2{\mathrm{O}}_8{}^{2-}$$

Figure 1
The effect of the initial Fe₃O₄ concentration on BPA removal. [BPA]₀ = 5 mg L^-1, [PS]₀ = 0.2 mM, [Fe₃O₄]₀ = 0-0.5 g L^-1, T₀ = 20 ± 1 ºC, pH₀ = 6.8 ± 0.2.

The effect of the initial PS concentration

The initial concentration of PS is an important factor affecting the BPA removal by Fe₃O₄ activation. Figure 2 gives the effect of the PS initial concentration on BPA degradation by Fe₃O₄ activation. In the absence of PS, it was found that 10.3% of BPA was removed within 60 min, indicating that Fe₃O₄ could adsorb BPA as an adsorbent. Sun et al.²⁹ also found that nano-Fe₃O₄ can adsorb 2,4-dichlorophenol, but the efficiency was very low compared with Fe₃O₄/PS catalytic degradation. With the increase in the PS concentration from 0.1 to 0.3 mM, the BPA removal rate was elevated from 34.3 to 65.2% within 60 min, which could be due to the larger amount of SO₄^•- produced by PS/Fe₃O₄ system (equation 4). However, when the initial concentration of PS increased from 0.3 to 0.5 mM, the BPA removal rate increased only by 1.4% within 60 min. We have identified three possible reasons for this result. Firstly, excessive PS (0.4-0.5 mM) slowed down the degradation rate of BPA due to the elimination of SO₄^•- by PS (equation 7). Secondly, more SO₄^•- could also react with each other and lose their oxidation performance (equation 6). Finally, the increase in PS concentration causes the continuous consumption of Fe₃O₄, and the amount of Fe^II released from Fe₃O₄ cannot meet the demand for high concentrations of PS. Therefore, the relatively insufficient amount of catalyst results in a slower reaction rate of catalyzing PS to SO₄^•-, and ultimately affects the degradation efficiency of BPA.

(7)$${\mathrm{S}}_2{\mathrm{O}}_8^{2-}+{\mathrm{S}\mathrm{O}}_4^{\bullet -}\to {\mathrm{S}}_2{\mathrm{O}}_8^{\bullet -}+{\mathrm{S}\mathrm{O}}_4^{2-}$$

Figure 2
The effect of the initial PS concentration on BPA removal. [BPA]₀ = 5 mg L^-1, [PS]₀ = 0-0.5 mM, [Fe₃O₄]₀ = 0.1 g L^-1, T₀ = 20 ± 1 ºC, pH₀ = 6.8 ± 0.2.

The effect of the initial BPA concentration

The concentration of the substrate is also an important factor determining the degradation efficiency. It can be seen from the Figure 3 that when the initial BPA concentration increased from 1 to 5 mg L^-1, after 60 min of reaction, the BPA removal rate dropped from 80.70 to 40.30%. When the initial concentration of BPA is 5 mg L^-1, the kinetic constant for BPA removal is 0.00942 min^-1 (ln(C_t/C₀) = -0.00942t + 0.02804), and the reaction is the slowest. When the initial concentration of BPA was 1 mg L^-1, the kinetic constant for BPA increased to 0.02466 min¹ (ln(C_t/C₀) = -0.02466t + 0.09686), which was 2.6 times faster than the degradation rate of 5 mg L^-1 BPA. As the amount of PS and Fe₃O₄ remained unchanged, the number of SO₄^•- generated did not increase, and the increase of BPA concentration resulted to a relative shortage of SO₄^•-, which reduced the BPA removal rate. Therefore, in order to improve the degradation efficiency of BPA, the amount of oxidant and catalyst needs to be increased accordingly.

The effect of the initial pH

Figure 3
The effect of the initial BPA concentration on BPA removal. [BPA]₀ = 1-5 mg L^-1, [PS]₀ = 0.2 mM, [Fe₃O₄]₀ = 0.1 g L^-1, T₀ = 20 ± 1 ºC, pH₀ = 6.8 ± 0.2.

It is clear that the advanced oxidation reaction in which PS is activated by transition metals is heavily dependent on solution pH. Therefore, we examined the BPA removal in Fe₃O₄/PS system at different pH of 3, 5, 7 and 9 (Figure 4). According to the BPA removal in descending order, the corresponding pH values were 5, 3, 7, 9, respectively. In other words, the pH value at the maximum BPA removal rate was 5, with the removal rate of 59.2%. Under acidic conditions of pH 3-5, the reaction proceed more efficiently due to the solubilization of the Fe^II on the surface of heterogeneous activator Fe₃O₄. Therefore, the homogeneous activation of Fe^II occurs more effectively for generating SO₄^•- towards removing BPA under acidic conditions. However, under neutral or alkaline conditions, Fe^II can hydrolyze to produce the precipitant Fe(OH)₂ which forms a passivation layer on the surface of Fe₃O₄, which hinders the migration of Fe^II into water, and ultimately slows down the catalytic reaction. In addition, SO₄^•- can react with H₂O and OH to produce •OH (equation 8) whose oxidizing power is less than SO₄^•-. These reasons resulted in the reduction of BPA oxidation performance under alkaline conditions. This finding was similar to the study of Liu et al.³⁰ about treatment of landfill leachate biochemical effluent using Fe₃O₄/PS system.

(8)$${\mathrm{O}\mathrm{H}}^{-}+{\mathrm{S}\mathrm{O}}_4^{\bullet -}\to \bullet \mathrm{O}\mathrm{H}+{\mathrm{S}\mathrm{O}}_4^{2-}$$

The effect of the reaction temperature

Temperature is a crucial parameter in the treatment of wastewater as it can affect the rate of chemical reactions. Some scholars have reported experiments that high temperature can activate PS.³¹-³³ We investigated the removal of BPA under four temperature conditions (20, 35, 50 and 70 ºC) in Fe₃O₄/PS and only PS system. The experimental results with or without Fe₃O₄ appearing in PS system at different temperatures are shown in Figure 5. In Figure 5, the curve marked with blank is the BPA removal without Fe₃O₄ (only PS). When only PS was added to the system, the removal rate of BPA increased significantly with the temperature increasing. The highest removal rate occurred at 70 ºC and the lowest removal rate occurred at 20 ºC. When the temperature was 70 ºC, the fastest kinetic constant for BPA removal was 0.00821 min^-1 (ln(C_t/C₀) = -0.00821t + 0.01496), which was 2 times faster than the 20 ºC rate of 0.00414 min^-1 (ln(C_t/C₀) = -0.00414t + 0.00818). Since heat can activate PS to produce SO₄^•- (equation 3), the increase in temperature was conducive to the degradation of BPA. When both PS and Fe₃O₄ were added to the system, the removal rate of BPA was significantly increased compared to the system with only PS. At 20 and 70 ºC, the removal rate of BPA was 40.46 and 87.10% after 60 min, respectively. This showed high temperature and Fe₃O₄ have the ability to synergistically catalyze the degradation of BPA by PS. However, the higher the temperature, the more energy is consumed. The actual situation of sewage treatment should be considered to determine the best temperature from the perspective of energy saving. In addition, as the temperature increased from 20 to 70 ºC, only adding Fe₃O₄ to the system, the BPA removal rate increased from 10.22 to 12.18% (data not shown). In other words, as the temperature increased, Fe₃O₄ alone cannot improve the efficiency of its adsorption of BPA.

Figure 4
The effect of the initial pH concentration on BPA removal. [BPA]₀ = 5 mg L^-1, [PS]₀ = 0.2 mM, [Fe₃O₄]₀ = 0.1 g L^-1, T₀ = 20 ± 1 ºC, pH₀ = 3-9.

Figure 5
The effect of the reaction temperature on BPA removal. [BPA]₀ = 5 mg L^-1, [PS]₀ = 0.2 mM, [Fe₃O₄]₀ = 0 g L^-1, 0.1 g L^-1, T₀ = 20-70 ºC, pH₀ = 6.8 ± 0.2.

The scavenging of the free radicals

MeOH and TBA are commonly used chemical scavengers to distinguish SO₄^•- and •OH as they react with free radicals with different rate constants. MeOH is an effective scavenger for •OH (reaction rate constant, k = 9.7 × 10⁸ M^-1 s^-1) and SO₄^•- (k = 1.1 × 10⁷ M^-1 s^-1), while TBA is an effective scavenger for •OH (k = 6.0 × 10⁸ M^-1 s^-1) and not for SO₄^•- (k = 9.1 × 10⁵ M^-1 s^-1). As observed in Figure 6, without adding the MeOH or TBA, 48.5% of BPA degradation rate was acquired within 60 min. The MeOH and TBA of 80 mM added separately to the system resulted in BPA removal to drop to 25.9 and 45.4%, respectively. As the concentration of MeOH and TBA increased to 800 mM, the degradation rate of BPA further decreased to 12.8 and 39.9%. The addition of MeOH had a more obvious effect on the decrease of BPA degradation rate. Therefore, it was inferred that the SO₄^•- generated in the reaction was the main free radical, followed by the •OH.

Figure 6
The effect of MeOH and TBA on BPA removal. [BPA]₀ = 5 mg L^-1, [PS]₀ = 0.2 mM, [Fe₃O₄]₀ = 0.1 g L^-1, T₀ = 35 ± 1 ºC, pH₀ = 6.8 ± 0.2.

BPA degradation products and pathways

GC-MS was used to qualitatively analyze the intermediate product of BPA degradation by nano-Fe₃O₄ activated PS. The reaction conditions are as follows: [BPA]₀ = 5 mg L^-1, [PS]₀ = 0.2 mM, [Fe₃O₄]₀ = 0.4 g L^-1, T₀ = 35 ± 1 ºC, pH₀ = 5.0. The possible intermediate products of BPA degradation are shown in Table 3. The detected intermediate products were mainly aromatic compounds, including phenol, p-benzoquinone, p-hydroquinone, 4-(2-hydroxypropan-2-yl) phenol, 4-isopropenylphenol, p-hydroxybenzene propanoic acid and 4-hydroxybenzoic acid. The possible degradation pathways of BPA are shown in Figure 7. The free radicals generated from nano-Fe₃O₄/PS system attacked the C-C bond between isopropyl and benzene rings of bisphenol A, forming 4-(2-hydroxypropan-2-yl)phenol and phenol.³⁴ 4-(2-Hydroxypropan-2-yl) phenol was gradually oxidized into 4-isopropenylphenol, p-hydroxyphenylpropionic acid, and 4-hydroxybenzoic acid.³⁵,³⁶ Phenol was gradually oxidized into p-hydroquinone and p-benzoquinone. Then, all the aromatic compounds produced by two pathways were oxidized into small molecular compounds,¹⁷,³⁷ some of which will be mineralized into CO₂ and H₂O under the conditions set in Fe₃O₄/PS system.

Figure 7
The possible degradation pathways of BPA.

Reusability of Fe₃O₄

In order to investigate the recyclability of Fe₃O₄, a magnet was used to separate Fe₃O₄ from the solution. After separation, the Fe₃O₄ was rinsed with deionized water, and dried indoors with natural ventilation for use in the next experiment. It can be seen from Figure 8 that the removal rate of BPA dropped from 68.0 to 26.9% after five cycles of Fe₃O₄. In the absence of Fe₃O₄, PS alone could remove about 20% of BPA, so it is believed that after five cycles of Fe₃O₄, the catalytic activity of Fe₃O₄ is already very low. It is speculated that the decrease in the activity of Fe₃O₄ is due to the fact that most of the Fe^II becomes Fe^III that cannot effectively activate PS.³⁸

Table 3
The possible intermediates of BPA degradation

Serial number	Chemical name	Molecular formula	Molecular weight / (g mol-1)	Molecular structure
1	bisphenol A	C15H16O2	228
2	phenol	C6H6O	94
3	p-hydroquinone	C6H6O2	110
4	p-benzoquinone	C6H4O2	108
5	4-(2-hydroxypropan-2-yl) phenol	C9H12O2	152
6	4-isopropenylphenol	C9H10O	134
7	p-hydroxybenzene propanoic acid	C9H10O3	166
8	4-hydroxybenzoic acid	C7H6O3	138

Figure 8
Reuse of Fe₃O₄ and BPA removal rate. [BPA]₀ = 5 mg L^-1, [PS]₀ = 0.2 mM, [Fe₃O₄]₀ = 0.2 g L^-1, T₀ = 35 ± 1 ºC, pH₀ = 5 ± 0.2.

Conclusions

Fe₃O₄/PS performed well as a sulfate radical-based AOP process in degradation of endocrine disruptor BPA. The removal rate of BPA was related to Fe₃O₄ dose, PS concentration, BPA concentration, pH and temperature. The best BPA removal rate of 80.7% could be acquired under the selected conditions of [BPA]₀ = 1 mg L^-1, [PS]₀ = 0.2 mM, [Fe₃O₄]₀ = 0.1 g L^-1, T₀ = 20 ± 1 ºC, pH₀ = 6.8 ± 0.2. The oxidation of BPA by Fe₃O₄/PS mainly relies on sulfate radicals generated during the reaction, and hydroxyl radicals play a minor role. The possible intermediates of BPA degradation were determined as phenol, p-benzoquinone, p-hydroquinone, 4-(2-hydroxypropan-2-yl) phenol, 4-isopropenylphenol, p-hydroxybenzene propanoic acid and 4-hydroxybenzoic acid. The performance of Fe₃O₄ was significantly reduced after five times of reuse.

Acknowledgments

The authors wish to acknowledge the financial support from the National Natural Science Foundation of China (grants No. 42077160) and Jilin Province Education Department of China (grants No. JJKH20200288KJ).

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Publication Dates

• Publication in this collection
  26 Nov 2021
• Date of issue
  Dec 2021

History

• Received
  05 May 2021
• Accepted
  19 Aug 2021