Degradation of Bisphenols by Air Micro-Nano Bubbles Activated Persulfate

Abstract

Micro-nano bubbles (MNBs) have been widely used in water treatment due to their large specific surface area, long retention time, and high zeta potential. This study investigated the degradation of bisphenols by activating persulfate (PDS, an oxidizing agent) with air MNBs (MNBs/PDS). The removal rate of bisphenol A (BPA) in the MNBs/PDS process was 98.3% within 25 min, while there was almost no degradation observed by PDS or MNBs alone. This enhancement was attributed to the huge amount of energy released during the collapse of MNBs, sufficient to break the O–H bonds of water molecules or the O–O bond of PDS to induce the formation of reactive oxygen species (ROS, such as HO^• and SO₄^•−). To qualitatively analyze ROS generated in this system, electron paramagnetic resonance and quenching experiments were conducted, and the HO^• and SO₄^•− were detected in MNBs/PDS. Furthermore, the degradation percentages of bisphenols after 25 min of MNBs/PDS treatment followed the order of bisphenol B (100%) > BPA (98.3%) > bisphenol E (87.9%) > bisphenol F (86.5%) > bisphenol AF (84.9%) > bisphenol S (51%). Higher PDS dosage, higher gas flow rate, and lower pH values were preferred for the degradation. Moreover, the MNBs/PDS treatment reduced the TOC of secondary effluent containing BPA by 45.8% in one hour, indicating the application potential of MNBs/PDS in the advanced treatment of wastewater.

1. Introduction

In recent years, the types and concentrations of endocrine disruptors in the water environment have increased significantly. As a typical representative of endocrine disruptors, bisphenol A (BPA) has attracted considerable attention worldwide. As a result, more than 20 BPA analogs have been synthesized to replace BPA in consumer products, including BPAF, BPB, BPS, BPE, and BPF. These bisphenols have been found to be detected in both terrestrial and aqueous environments, which ultimately cause significant health problems such as cancer, reproductive diseases, and developmental abnormalities by interfering with hormonal regulation [1]. Various technologies have been applied to remedy their presence in water and soil, including biological (e.g., bacteria, peroxidase, and laccase) [2,3], chemical (e.g., chlorine, ozonation, and photocatalytic) [4,5,6], and physical (e.g., adsorption, membrane separation, and electron-coagulation) [7,8,9,10] approaches. These traditional technologies have their own limits and often fail to adequately degrade organic pollutants. For instance, chemical methods are prone to causing secondary pollution, and biological methods often take a long time. As a result, the advanced oxidation process (AOP) that generates HO^• (E₀ = 1.9–2.8 V) and other reactive oxygen species (ROS) has been proven effective in the degradation of BPA and its analogs [11]. Compared to HO^•, SO₄^•− (E₀ = 2.6–3.1 V) possesses the advantages of strong oxidizing capacity, a wide applicable pH range, easy control, and has a broader range of application prospects in the field of advanced treatment of water [12,13]. SO₄^•− can be produced by activating sodium persulfate (PDS, S₂O₈²⁻) [14,15,16]. Catalysts such as carbon-based materials, transition metals, and metal oxides usually activate PDS through the heterolytic cleavage of O–O bonds with single electron transfer (SET). External energy input, such as ultraviolet irradiation, visible light, heating, and ultrasonic waves, can all cause the O–O bonds to break homolytically, thereby generating SO₄^•− [17,18,19]. Chen et al. [20] used magnetite (Fe₃O₄)/persulfate (PS) system to degrade BPA, under which BPA removal efficiency reached 59.54% ([magnetite] = 0.3 g/L, [PS] = 0.26 mM, and pH = 4.9), and Li et al. [21] explored that the FeMo@CN/PDS system could degrade BPA completely (k_obs = 0.12 min⁻¹) within 1 h. However, the above-mentioned methods may bring increased costs or only modest improvements to degradation efficiency. Therefore, air micro-nano bubbles (MNBs) technology, as a chemical-free strategy, offers a promising alternative means to enhance organic pollutant degradation efficiency by supplying gaseous oxygen to solutions.

Numerous technologies have been applied in PDS activation, but they have some limitations. For example, in photocatalysis, high-speed electron–hole recombination may cause unsatisfactory oxidation efficiency; thermal activation depends on continuous heating to supply energy to break O–O bond; and metal ions may have a leaching problem resulting in secondary pollution and catalyst deactivation. Different from those activation methods, MNBs technology, initially utilized for enhancing aeration in aquaculture, has emerged as a promising method for wastewater treatment, groundwater remediation, and drinking water disinfection over the past few decades. Bubbles with a particle size smaller than 100 μm can be classified as MNBs. Compared with large bubbles or common bubbles (2–5 mm), MNBs have a larger specific surface area, higher dissolved oxygen (DO), and longer solution residence time. More importantly, bursting of MNBs can result in localized heating, thereby generating HO^• with strong oxidizing properties [22]. Dong et al. [23] found that the efficiency of methyl orange degradation by ozone MNBs was 96.04% within 120 min. Xia et al. [24] used ozone MNBs for groundwater remediation at a trichloroethylene contaminated site in Japan, showing a removal rate of approximately 100% in 6 days (9 h per day). Hu et al. [25] investigated the treatment of secondary effluent from wastewater treatment plants with ozone MNBs. The removal rates of COD, chroma, and UV₂₅₄ were 67.79%, 90.47%, and 58.53%, respectively, and the biodegradability of the wastewater was significantly enhanced. In addition, Tang et al. [26] found that ozone MNBs can be used as an efficient pretreatment technology for wastewater from oxytetracycline production. Fan et al. [27] combined photocatalysis with air MNBs to effectively inactivate bacterial spores in water, and H₂O₂ and HO^• were the main ROS causing the inactivation. Although air is the most environmentally friendly gas source, current studies on pollutant removal using air MNBs are very limited due to the low ROS generation rate [28]. Therefore, innovative strategies should be investigated to enhance the ROS formation by air MNBs. One possible approach is by activating PDS using air MNBs to produce SO₄^•− and HO^•, considering their high reaction rate constants with a wide variety of electron-rich compounds, such as BPA (k _HO^•−_BPA = 8.77 × 10⁹ M⁻¹s⁻¹, k _SO4^•−_BPA = 1.37 × 10⁹ M⁻¹s⁻¹) [29,30]. SO₄^•− possesses long half-life (30–40 μs), high selectivity, and strong oxidation ability. MNBs form large amounts of HO^• either spontaneously or under dynamic stimulation, providing high-temperature sites that may enhance PDS activation when the gas–liquid interface pressure normalizes upon collapse.

This study aims to investigate the feasibility of the PDS assisted with air MNBs for enhancing organic pollutant removal. It systematically reveals (1) the performance of MNBs/PDS on the degradation of various bisphenols; (2) ROS generation and mechanism of BPA degradation in MNBs/PDS system; (3) the effect of air intake, MNBs generator pressure, pH, PDS dosages, inorganic ions, and NOM on BPA degradation; and (4) applications in real water matrices. The results not only provide experimental evidence for the treatment of BPA and its analogs, but also evaluate the application potential of AOP induced by air MNBs in the advanced treatment of wastewater. Moreover, it clarifies the mechanism of MNBs/PDS in the degradation of BPA, which has not been mentioned in previous studies.

2. Results and Discussion

2.1. Degradation of BPA and Its Analogs by MNBs/PDS

As shown in Figure 1, MNBs/PDS degraded BPA by 98.3% in 25 min, whereas both PDS and MNBs alone were ineffective for BPA degradation, achieving only 6.7% and 9.8% removal, respectively, indicating that MNBs and PDS had good synergistic effects in the degradation of BPA. It should be noted that the MNB generator released heat continuously during operation, even though an ice-water bath was used. Within 25 min, the temperature increased from room temperature to about 46 °C. Therefore, the effect of thermally activated PDS on the degradation of BPA was investigated by simulating this heating process (Figure 1). In the first 15 min of the MNBs/PDS reaction, the temperature was below 40 °C, indicating that the thermal activation effect was relatively weak, and the activation of PDS by MNBs was dominant. However, after 15 min, the temperature continued to rise, and thermal activation of PDS dominated, while MNBs played a weaker role. In addition, an extra experiment was conducted on BPA degradation using heat/PDS technology at 25 °C, 35 °C, 45 °C, and 55 °C, respectively. The results revealed that high temperature (≥45 °C) was more conducive to enhancing BPA degradation, which better explained the dominant role of thermal activation after 15 min of reaction. The higher the temperature, the more energy was obtained in the solution, which caused the cleavage of O—O bonds in PDS and produced more SO₄^•−. Overall, the process of MNBs/PDS system degrading BPA was the result of the combined effect of multiple factors, including the role of MNBs (MNBs rupture generating ROS), direct oxidation of PDS, and thermal activation of PDS.

To investigate the degradation of BPA and its analogs, six endocrine-disrupting bisphenols were treated by MNBs/PDS and heat/PDS (Figure 2a). The concentrations of BPA, BPS, BPE, BPB, BPAF, and BPF were reduced by 98.3%, 51.0%, 87.9%, 100.0%, 84.9%, and 86.5%, respectively, within 25 min (degradation rate: BPB > BPA > BPE > BPF > BPAF > BPS). The lowest degradation rates of BPS were attributed to the higher electron withdrawal of the SO₂ group of BPS, which led to its lower reactivity with SO₄^•− (k_SO4^•−_,BPS = 1.09 × 10⁸ M⁻¹s⁻¹) and HO^• (k_•OH,BPS = 5.17 × 10⁸ M⁻¹s⁻¹) compared to other bisphenols (Table S1). For BPAF, the fluorine atom is an electron-absorbing group, which is inactive against oxidants, and the two CF₃ groups of BPAF are attached to the central carbon atom of BPAF, which makes its structure highly stable and not easily damaged.

The temperature changes in the thermal activation of the PDS experiment simulated the actual temperature changes in the MNB generator to explore the effect of temperature on bisphenol degradation (Figure 2b). After 25 min of treatment with heat-activated PDS, the target pollutants were eliminated to differing extents, as follows: BPS (7.0%), BPE (27.9%), BPB (29.5%), BPAF (6.0%), BPF (42.5%), and BPA (78.3%). According to the data from MNBs/PDS and heat/PDS experiments, a bar graph exhibiting the proportion of thermal effects in the MNBs/PDS system on the degradation of pollutants could be obtained (Figure 2c,d). At 25 min, the contribution of the thermal effect on the degradation of BPA was the highest among the six bisphenols (79.6%), while BPAF had the lowest contribution (8.3%). Different from the degradation of BPA, its analogs in MNBs/PDS system did not show a clear trend of “MNBs dominating PDS activation at low temperature (time = 15 min); while thermal effect dominating PDS activation at high temperature (time = 25 min)”. Under either high or low temperature conditions, the thermal effect was relatively weak for BPA analogs.

2.2. Reaction Mechanisms in MNBs/PDS System

To explore the reaction mechanism, quenching experiments and electron paramagnetic resonance (EPR) experiments were conducted to qualitatively determine the free radicals present in the MNBs/PDS system. The second-order rate constants of the reaction between reagents and free radicals are shown in Table S2.

2.2.1. Quenching Experiment

To clarify the types of free radicals involved in BPA degradation and their contributions, radical quenching experiments were conducted. tert-butyl alcohol (TBA), methanol(MeOH), L-histidine, and superoxide dismutase (SOD) (Table S3) were used as quenching agents for HO^•, SO₄^•−, ¹O₂, and O₂^•−, respectively. It is worth noting that when some quenching agents are used excessively, they may generate other oxidizing or reducing substances, which can affect the determination of ROS. Moreover, if the dosage of the quenching agent is inadequate, the corresponding ROS cannot be completely quenched. Therefore, it is necessary to test different concentrations of quenching agents to achieve the appropriate quenching effect. As shown in Figure 3a, 50, 100, and 200 mmol/L TBA were selected to inhibit HO^•, and 50 mmol/L TBA had a more obvious inhibitory effect on the reaction, reducing the BPA degradation rate from 98.3% to 42% within 25 min, indicating the presence of HO^• in the system. As a quenching agent for HO^• and SO₄^•−, 200 mmol/L MeOH significantly inhibited the reaction, reducing the degradation rate to 22.5% within 25 min, indicating the presence of SO₄^•− in the system (Figure 3b). L-histidine had relatively high second-order rates with HO^•, SO₄^•−, ¹O₂, and O₂^•−, and the degradation rate decreased to 9.94% within 25 min after addition of 75 mmol/L L-histidine (Figure 3c). SOD, as a quencher of O₂^•−, inhibited the degradation rate of BPA by nearly 57.8% after its addition (Figure 3d), owing to the fact that O₂^•− was a precursor of ROS, and quenching O₂^•− also simultaneously prevented the generation of other ROS [31]. Through Equation (1), it could be obtained that the addition of TBA, MeOH, L-histidine, and SOD inhibited the reaction by 57.3%, 77.1%, 89.9%, and 58.8%, respectively.

$$\mathrm{Inhibitory}\mathrm{rate}={\displaystyle \frac{{\mathrm{D}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}_{\left(\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}\right)}-{\mathrm{D}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}_{\left(\mathrm{a}\mathrm{d}\mathrm{d}\mathrm{s}\mathrm{c}\mathrm{a}\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{e}\mathrm{r}\right)}}{{\mathrm{D}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}_{\left(\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}\right)}}}$$

(1)

In addition, considering the temperature rise in MNBs/PDS, the effect of adding quenching agents on thermally activated PDS was investigated. This experiment was a continuous heating process. The temperature rose from room temperature to about 46 °C within 25 min, simulating the heating process during MNBs/PDS. As shown in Figure S1, 50 mmol/L TBA, 200 mmol/L MeOH, 75 mmol/L L-histidine, and 2 mg/L SOD were added to the heat/PDS system. The degradation efficiencies of BPA were 2.5%, 13%, 1.3%, and 54.3%, respectively. The degradation efficiency of BPA without quenching agents was 52.6%. SOD only quenches O₂^•−, and its oxidation capacity is inferior to that of HO^• and SO₄^•−. Moreover, previous studies showed that temperature had a significant impact on the activity of SOD enzymes. Huma Naz et al. reported that the SOD activity reached the maximum value at 30 °C [32]. Therefore, when the temperature rises above 30 °C, the activity of the SOD enzyme decreases, resulting in a reduced quenching effect. The degradation rate was inhibited when other quenching agents were added, which indicated that HO^• and SO₄^•− were present in the heat/PDS system. It is worth noting that the prerequisite for the establishment of the ROS quenching experiment is that the added quenching agent has no effect on other components in the reaction system, but in most cases, this prerequisite is extremely difficult to guarantee. Therefore, the conclusions drawn in this section need to be corroborated by other free radical identification experiments, such as the EPR experiment.

2.2.2. EPR Experiment

To further and more accurately identify the free radicals present in the system, spin trapping reagents were added during the reaction process to capture the free radicals and detect them using an EPR instrument. As a free radical trapping agent, 5,5-dimethyl-1-pyrroline N-oxide (DMPO) can capture short-lived free radicals SO₄^•− and HO^• and convert them into stable or metastable DMPO radical addition products; while 2,2,6,6-tetramethylpiperidine (TEMP) can capture ¹O₂. Different types of free radicals can be accurately identified based on the hyperfine coupling constants AN and AH. As shown in Figure 4a, after the addition of 50 mmol/L DMPO, the MNBs/PDS system detected a DMPO-HO^• signal with a characteristic peak of 1:2:2:1 (αN = αH = 14.9 G) within 25 min. Meanwhile, relatively low-intensity DMPO-SO₄^•− signals with characteristic peaks of 1:1:1:1:1:1 (αN = 13.2 G, αH = 9.6 G, αH = 1.48, αH = 0.78) were identified. In addition, the triple characteristic peak of ¹O₂ (1:1:1, αN = 16.9 G) was also detected using 60 mmol/L TEMP, as shown in Figure 4b. In summary, the EPR experiments indicated that MNBs effectively activated PDS to produce SO₄^•−, HO^•, and ¹O₂ in the MNBs/PDS system.

2.2.3. Reaction Mechanisms of MNBs/PDS

By summarizing the results of ROS formation, the reaction mechanism of MNBs/PDS was proposed (Figure 5). On the one hand, during the rupture of MNBs, extremely high local temperatures and pressure conditions are generated, which can instantly release a huge amount of energy. This is sufficient to break the O–H bonds of water molecules and induce the formation of ROS, such as HO^•. ROS reacts with PDS to produce SO₄^•−. On the other hand, the energy generated by the rupture of MNBs can also trigger the O–O bond cleavage of PDS, thereby generating SO₄^•− and other ROS [33]. Therefore, the synergistic effect of MNBs and PDS can generate more active species, thereby enhancing the degradation efficiency of pollutants.

2.3. Influencing Factors of BPA Degradation by MNBs/PDS

2.3.1. PDS Dosages

The degradation of BPA by MNBs/PDS at PDS doses varying from 0 to 20 mmol/L was investigated at pH = 7. All concentrations were sampled at 5 min intervals for 25 min, except for 20 mmol/L PDS, which was sampled every 2 min for a total of 10 min. As shown in Figure 6a, within 25 min, PDS at concentrations of 0, 0.1, 1, 2, and 5 mmol/L removed 9.8%, 89.4%, 99.2%, 98.3%, and 98.6% of BPA, respectively. The degradation efficiency in 10 min with 20 mmol/L PDS was 91.9%. It could be seen that a high concentration of PDS favored the degradation of BPA, which was similar to Cerrón-calle et al. [34]. Their study confirmed the enhancement of BPA abatement when PDS dosages rose from 1 mmol/L to 4 mmol/L, with an increasing degradation efficiency varying from 64% to 99% in 60 min. The above phenomenon can be explained by that of the higher production rate of radicals derived from high PDS dosage can degrade BPA. Figure 6b reflects the degradation rate of the thermally activated PDS system for degrading BPA. The degradation rates of BPA in 25 min under 0.1, 1, 2, 5 mmol/L, and 20 mmol/L PDS were 17.0%, 52.6%, 78.3%, 100%, and 100%, respectively. In the case of 20 mmol/L PDS, it had the highest degradation rate in both systems, and this well illustrated the indispensable role of PDS in BPA degradation.

Figure S2 reflects the ratio of the contribution of thermal effects in MNBs to BPA degradation to the overall contribution at 15 min. For 5 mmol/L PDS and 20 mmol/L PDS, the contribution of thermal effects to BPA degradation was greater than that of MNBs itself, while the activation of PDS by MNBs itself dominated at lower PDS concentrations. This further reinforced the fact that, in the first 15 min, when the temperature of the system had not risen to a high value, the activation of PDS was mainly by MNBs; while after 15 min, thermal activation dominated the degradation of BPA in the MNBs/PDS system. Moreover, as the PDS concentration increased, the contribution of MNBs thermal effects (e.g., bubble rupture to produce ROS) on BPA degradation increased.

2.3.2. Air Intake and MNBs Generator Pressure

The gas flow rate of 100–400 mL/min was selected to evaluate the effect of air intake. As shown in Figure S3, the optimal gas inlet was 400 mL/min. Gas flow rate at the lower level resulted in an inhomogeneous gas–liquid ratio, which in turn affected the concentration of MNBs in solution. When the gas flow rate was 400 mL/min, the reaction vessel contained a large number of stable and continuous MNBs, which formed airborne MNBs. The rupture of these air MNBs generated more free radicals, which promoted the degradation of BPA [35]. However, excessively high gas flow rate was not conducive to the removal of BPA. As the bubble size increased, the specific interfacial area and internal pressure of the bubbles decreased, thereby reducing the concentration of ROS and resulting in a decrease in the BPA removal efficiency [36].

Pressure control is important for MNBs generators because the pressure directly affects the physical properties of the MNBs, such as size and number. Therefore, the effect of pressure of the system on BPA degradation was explored (Figure S4). According to the guidelines of the MNB generator, the system pressures of 0.2 MPa to 0.4 MPa were tested. The highest efficiency of BPA degradation was achieved at a pressure of 0.4 MPa. Higher pressure might cause MNBs to become larger, affecting their solubility. Lower pressure might lead to a decrease in MNBs concentration, thereby affecting the BPA degradation. Therefore, in all experiments, the intake volume and pressure of MNBs were adjusted to 400 mL/min and 0.4 MPa, respectively.

2.3.3. pH Value

The initial pH might influence the chemical properties of the solution and the formation of active components, because the oxidation potential of ROS and the stability of MNBs differ at different pH values [37,38]. Therefore, the effect of pH on the degradation efficiency of BPA was investigated in the range of pH from 5 to 11. An acidic condition was conducive to the degradation of BPA, while an alkaline condition significantly reduced the removal rate of BPA (Figure 7a). This result was similar to many previous research findings. For example, a study on the removal of BPA by a PS-based hybrid system showed that the acidic conditions were more favorable for BPA degradation than the case under neutral and alkaline conditions [35]. For thermal activation, there was no significant pattern of BPA degradation by pH, and the degradation rates were all lower than those of BPA when introducing MNBs (Figure 7b). The reasons were as follows: (1) rapid dissociation of PDS to SO₄^•− at extreme acid pH values [1]; (2) lower HO^• redox potential under alkaline conditions, thus decreasing the degradation of organic compounds; and (3) degradation of BPA into carbon dioxide (CO₂) introducing the bicarbonate ions (HCO₃⁻) and carbonate ions (CO₃²⁻) into the MNBs/PDS system at alkaline conditions, and then inhibiting any further degradation process of BPA due to scavenging effects of HCO₃⁻ and CO₃²⁻ on SO₄^•− and HO^• [2].

2.3.4. Inorganic Ions and Natural Organic Matter (NOM)

Natural water is a complex matrix where a wide variety of inorganic ions are usually present. The effect of ions on ROS production and MNBs stability had been reported [36]. Therefore, this study investigated the role of chloride (Cl⁻), nitrate (NO₃⁻), carbonate (CO₃²⁻), and humic acid (HA), as shown in Figure 8.

As shown in Figure 8a, when the Cl⁻ concentration was higher than 5 mmol/L, it had a slight promoting effect on degradation. When the concentration was less than 0.5 mmol/L, Cl⁻ showed an inhibiting trend. Adding 0.1 mmol/L of Cl⁻ inhibited the reaction by nearly 51.3%. This result was consistent with the degradation results of BPA in the UV/PDS [37] systems. Cl⁻ in water may react with HO^• and SO₄^•− to form reactive chlorine species (RCS, such as Cl^•, Cl₂^•− and ClOH^•−) with relatively weak oxidizing capacity. RCS readily reacts with pollutants containing electron-donating groups, such as BPA. When the concentration of Cl⁻ was less than 0.5 mmol/L, the scavenging effect of RCS was dominant, resulting in a decrease in the degradation rate of BPA. When the concentration of Cl⁻ was higher than 5 mmol/L, a large amount of RCS was formed, accelerating the degradation of BPA.

After adding 0.1, 0.5, 3, and 10 mmol/L of CO₃²⁻, pH values were 8.4, 10.2, 10.9, and 11.2, respectively, before the reactions. The pH values after the reactions were 7.4, 9.7, 10.5, and 10.9, respectively (Table S5). As shown in Figure 8b, based on the pH changes before and after the reaction, CO₃²⁻ concentrations from 0.1 to 10 mmol/L were compared with pH = 7.5 (with BPA degradation rate of 100%), pH = 10 (with BPA degradation rate of 71%), and pH = 11 (with BPA degradation rate of 42.7%). Except for 0.1 mmol/L CO₃²⁻, which had an insignificant effect, CO₃²⁻ promoted the degradation of BPA at all other concentrations. Literature also reported promotion of CO₃²⁻ (0.4, 2.2, and 4.0 mmol/L) on BPA degradation [38]. It captured HO^• and SO₄^•− to generate CO₃^•−, which was more selective for electron-rich groups [33]. The second-order reaction rate of CO₃^•− with BPA is approximately 2.2 × 10⁸ M⁻¹s⁻¹ [39], thus demonstrating the promotion of BPA degradation.

The presence of NO₃⁻ slightly increased the degradation rate of BPA, and the promoting effect was more obvious at higher concentrations (Figure 8c). This result was different from Abdual et al. [40], who found that BPA degradation slightly decreased in the presence of NO₃⁻ (1, 10 mmol/L) due to the scavenging effect on HO^• and SO₄^•−. The enhanced degradation might be due to the formation of HO^• [41], assuming that NO₃⁻ could be activated by MNBs to produce more HO^• and reactive nitrogen species (RNS, such as NO₂^•). This will be further investigated in future work.

Humic acid (HA) is one of the representative substances of NOM, containing various functional groups such as carboxyl, carbonyl, and phenolic groups. As shown in Figure 8d, 0.2 to 1 mg C/L of HA exhibited an inhibitory effect on the degradation of BPA, and the corresponding degradation rate of BPA was 93.5% within 25 min. This inhibitory effect was observed in other studies. For example, Wang et al. [42] found a significant inhibition trend for the degradation of BPA in the heat/PDS system with NOM concentrations of 2, 4, 6, and 10 mg/L. The inhibition was ascribed to the competition of HA with BPA for HO^•, because HA contains various unsaturated groups such as –SH and C=C, to endow it with a certain degree of reducing property.

2.4. Advanced Treatment of Wastewater by MNBs/PDS

To further verify the practical feasibility of MNBs technology-based AOP, degradation experiments of BPA were conducted in the secondary effluent of a wastewater treatment plant in Shanghai. The secondary effluent was filtered through 0.45 µm filter membranes. Table S7 summarizes the water quality parameters after the filtration. The treatment result of BPA in wastewater by MNBs/PDS was far inferior to that in pure water. The degradation efficiency of the former was nearly 79.1% lower than that of the latter. Figure 9a shows the degradation rates of BPA in wastewater by different oxidation systems: MNBs, MNBs/PDS, and heat/PDS. The degradation effect of MNBs and heat/PDS on BPA was almost zero, while the combination of MNBs and PDS had a certain effect on the degradation of BPA.

In addition, comparison of TOC removal rates by MNBs/PDS and heat/PDS in wastewater containing BPA was investigated in the presence of 2 and 20 mmol/L PDS. As shown in Figure 9b, regardless of the activation system, increasing the dosage of PDS could significantly improve the removal rate of TOC. However, at the same PDS concentration, the thermal activation efficiency was significantly lower than that of MNBs activation. When the initial concentration of PDS was 2 mmol/L, the TOC removal rates of MNBs/PDS and heat/PDS within one hour were 23.4% and 6.7%, respectively. When the initial concentration of PDS increased to 20 mmol/L, the TOC removal rates of MNBs/PDS and thermal/PDS within one hour were 45.8% and 13.2%, respectively. The above-mentioned influencing factor experiments indicated that the MNBs/PDS system was less disturbed by inorganic ions and NOM. Therefore, the MNBs/PDS system has the application potential to treat wastewater and is worthy of in-depth and systematic research in the future. However, to achieve more satisfactory results, dynamic addition of PDS can be considered in case of the fast consumption of PDS.

3. Materials and Methods

3.1. Chemicals

Detailed information on the chemicals and reagents in this study is presented in Table S6. Stock BPA solutions (100 µmol/L) were prepared in ultrapure water. In each experiment, 4 L reaction solutions were treated with a BPA concentration of 1 µmol/L in a 5 L round quartz beaker. Real water samples were taken from a local wastewater treatment plant. Water samples were all filtered using a glass fiber membrane to remove suspended matter, particulate matter, etc.

3.2. Experimental Set-Up and Procedures

An aqueous suspension of MNBs was produced by an MNB generator (NANO-MF5000, Xingheng, Shanghai, China) based on the principle of “high-pressure dissolution and low-pressure release” (Figure S5). Air was introduced into the generator at 400 mL/min. To investigate the kinetics and mechanism of BPA degradation, experiments were conducted in a 4 L reaction solution in a 5 L glass tank within an ice-water bath containing 1 μmol/L BPA, 2 mmol/L PDS, and 1 mmol/L phosphate buffer (PBS). Samples were collected at regular intervals (every 5 min) and quenched with 10 mmol/L Na₂S₂O₃ to terminate the reaction. In addition, to qualitatively explore ROS, TBA, MeOH, L-histidine, and SOD were used as quenching agents for HO^•, SO₄^•−, ¹O₂, and O₂^•−, respectively. The second-order reaction rate constants of ROS with common probe compounds are listed in Table S2.

Degradation efficiency of contaminants can be calculated with Equation (2):

removal rate% = [(C₀−C_t)/C₀] × 100%

(2)

where C₀ is the initial concentration of BPA or its analogs, C_t is the concentration of BPA or its analogs at time = t min.

3.3. Analytical Methods

BPA was detected at 227 nm by HPLC (Utimate3000, Thermo Fisher, Waltham, MA, USA), with the mobile phase consisting of 65% acetonitrile (Merck, Rahway, NJ, USA, 99.9%) and 35% Milli-Q water. The analytical methods for BPA analogs are listed in Table S4. The PDS concentration was measured by UV spectrophotometry (DR3900, HACH, Loveland, CO, USA) using iodometry [43]. The mineralization rate of BPA was analyzed by a TOC analyzer (multi N/C 3100, Jena, Germany). EPR experiments were performed using a Bruker (EMXplus, Bruker, Germany). DMPO and TEMP were used as radical spin-trapping agents. In this study, all the experiments were repeated at least 3 times.

4. Conclusions

This work studied the performance and mechanisms of the BPA degradation during the MNBs/PDS treatments, showing a more favorable effect on BPA removal than MNBs alone or PDS alone. The conclusions can be obtained as follows: (1) The bisphenols were degraded efficiently by ROS generated through the integration of air MNBs with PDS, especially BPA, BPB, and BPE. (2) Through quenching and EPR experiments, HO^• and SO₄^•− were produced in the MNBs/PDS system. (3) The SO₄^•− formation mechanism of MNBs/PDS was proposed to be either through O–O bond cleavage of PDS by the rupture of MNBs, or via the reaction of HO^• with PDS. (4) Higher PDS dosage, higher gas flow rate, and lower pH values were preferred for BPA degradation, while the influences of inorganic ions and NOM were not significant. (5) In the secondary effluent containing BPA by MNBs/PDS for one hour, the TOC removal rate achieved 45.8%, which further indicated the application potential of MNBs/PDS in the advanced treatment of wastewater. This study provides fundamentals for further utilization of air MNBs in advanced oxidation technologies to decompose organic pollutants in various aqueous matrices.