Degradation of bisphenol A in an oxidation system constructed from Mo2C MXene and peroxymonosulfate

The etched Mo2C MXene with a layered structure was characterized as an environmentally friendly catalyst in the bisphenol A (BPA) removal by advanced oxidation. 99.75% of BPA was degraded in the oxidation system constructed using Mo2C MXene and peroxymonosulfate (PMS). The Mo2C MXene was recyclable, with a high removal percentage (89.29%) of BPA after even four cycles. The catalysis of Mo2C MXene on PMS was due to the Mo-deficit vacancy defects. The electron paramagnetic resonance technique and density functional theory (DFT)-based density of states calculations verified defect signals. In addition, the defective Mo2C MXene and PMS have strong binding and electron transfer capabilities. The reactive oxygen species (ROS, including O2•−, 1O2, SO4•−, and •OH) produced by Mo2C MXene activates PMS, leading to BPA degradation. The condensed Fukui function predicted the active sites of the BPA molecule and found that the O1, O2, C3, C4, C6, C12, C15, and C16 sites have higher electrophilic reactivity. The C, C–C bonds, or C4/C16 sites in the isopropyl group connecting the two phenolic rings were attacked first, then further transformed BPA into non-toxic or low toxic small molecule degradation products through a series of reactions such as bond-breaking, addition, hydroxylation, and ring-opening. Moreover, the Mo2C MXene/PMS system has strong applicability in actual water bodies. The study provides valuable insights into PMS activation by two-dimensional MXenes to remove toxic organic pollutants in an aqueous matrix.


INTRODUCTION
Bisphenol A (BPA) is an important organic chemical raw material used in the preparation of polymer materials such as epoxy resin and polycarbonate, which impact everyday life in that people are in close contact 1,2 . The US Environmental Protection Agency stipulated that the BPA flowing into the environment through various channels exceeds one million pounds annually 3 . BPA has a wide range of uses but is persistent in environmental matrices; hence it can contaminate food chains, groundwater, and oceans. The concentration of BPA in water, wastewater, food, biological liquids, consumer, and personal care products, and semi-solids were 251.00 μg L −1 , 384.80 μg L −1 , 937.49 μg kg −1 , 208.55 μg L −1 , 3590.00 μg g −1 , and 154820.00 μg g −1 , respectively 4 . In surface water and groundwater of Europe and America, the concentrations range from 0.001 to 100 mg m −3 5 . In Ondo State, southwestern Nigeria, BPA concentrations were 0. 41-5.19 μg L −1 in river water, 0.64-10.60 μg kg −1 in river sediments, and 0.63-0.68 μg L −1 in well water 6 . Also, BPA has endocrinedisrupting effects on aquatic wildlife and humans, with its acute toxicity in mammals valued at 6.50 g kg −1 7,8 . BPA was detected in the urine of approximately 95% of adults in the USA and Asia 4 . And the maximum daily intake in adults was as high as 11.00 μg kg −1 , which was much higher than the average (30.76 ng kg −1 ). The effects of BPA in humans include cardiovascular problems, reproductive effects, breast development problems, obesity, hormone-related cancers such as breast or prostate cancer, etc. 9 . Attention has focused on removing BPA in water, with numerous proposed technologies, such as physical adsorption, chemical adsorption or/and degradation, and biotransformation 10 . Among them, the chemical oxidation method was widely used because of its fast effect and high rate, such as TiO 2 , MnFe 2 O 4 modified g-C 3 N 4 /diatomite photocatalytic activation of peroxymonosulfate (PMS) [11][12][13] . However, the current oxidation methods also have some shortcomings, for example, requiring additional light sources; the cost of preparing composite catalysts is high and the process is complicated; the active components are easily changed and are not conducive to recycling. It is an urgent task to find a catalytic oxidation system that has a low cost of catalyst preparation and maintains a highefficiency catalytic effect.
The MXene materials are shaped like a potato chip and have a two-dimensional (2D) nanostructure. Those can be produced by etching the A layer in MAX (M: transition metal, A: IIIA/IVA group element, X: C/N) 23,24 . Mo 2 C MXene material is a transition metal carbide. MXene materials have a layered or sheet structure and unique advantages, such as a large specific surface area, surface functional group design, and considerably coordinated unsaturated external atoms [25][26][27] . These characteristics make it possible to use MXenes alone or in combination with other materials, such as MOFs, to prepare more advantageous composites in some fields 28 . Like the reported MOFs, MXene materials can also enhance their related application mechanisms through their own rational design, including the introduction of functional ligands, as well as structural and energy band regulation 29 . In addition, MXene materials generally have single-atom vacancy defects, which would provide a new mechanism for its application. It shows many unique characteristics and promising applications in the field of electronics. Some MXene-based materials can remove organic and inorganic contaminants to purify water and remediate polluted environments 30 . However, the research of Mo 2 C MXene in catalysis is mainly the hydrogen evolution reaction 31,32 . The study of Mo 2 C MXene as a peroxides activator has not been reported yet. In this work, we try to use the Mo 2 C MXene material as the activator of PMS to degrade toxic organic pollutants oxidatively.
In this study, we utilized Mo 2 C MXene and PMS to build an advanced catalytic oxidation system generating reactive oxygen species (ROS). BPA was used as a typical contaminant to examine the performance of the system. This work has the following objectives: (a) Morphology, phase, and mass transfer resistance were analyzed to characterize Mo 2 C MXene; (b) the batch experiments were investigated to explore the activation efficiency of Mo 2 C MXene on PMS and the degradation effect of Mo 2 C MXene/PMS system on BPA and evaluate the stability and recyclability of Mo 2 C MXene; (c) the ROS, the defect sites, and calculation of the binding capacity between defective Mo 2 C/nondefective Mo 2 C and PMS were analyzed to elucidate the catalytic mechanism; (d) the transformation products were measured, combined with condensed Fukui function calculation, to clarify the transformation pathways of BPA.

RESULTS AND DISCUSSION Characteristics of Mo 2 C MXene
The Mo 2 Ga 2 C sample was first etched with 40% hydrofluoric acid solution for 36 h at 50°C to remove the Ga element as much as possible and form defects. As shown in Fig. 1a, the phase of Mo 2 Ga 2 C by XRD changed obviously after etching, and some diffraction peaks disappeared, especially the diffraction peak at 8.94°shifted to the left, and the peak intensity was noticeably enhanced. The peaks at 8.94°, 28.86°, 34.57°, 37.07°, 42.10°, 61.82°, Fig. 1 Phase and morphology analysis. a Morphological analysis of Mo 2 Ga 2 C and Mo 2 C MXene by X-ray diffraction (XRD). b Scanning electron microscopy (SEM) image of Mo 2 Ga 2 C. c SEM images of Mo 2 C MXene. d SEM-EDS of Mo 2 C MXene. e Transmission electron microscopy (TEM) images of Mo 2 C MXene. f Cyclic voltammograms of Mo 2 Ga 2 C and Mo 2 C MXene. g Nyquist plots of Mo 2 Ga 2 C and Mo 2 C MXene. and 65.04°of 2θ are the diffraction peaks of the Mo 2 C MXene, which indicated that the MXene material was successfully prepared. In Fig. 1b, c, SEM images showed that Mo 2 Ga 2 C was a solid block, and it is an apparent layered Mo 2 C MXene structure after etching. The elemental content and morphology of Mo 2 C MXene were further analyzed by SEM-EDS and TEM in Fig. 1d, e. Furthermore, single vacancy is the only point defect observed in MXene in experiments 33 . Also, according to previous studies, Mo 2 C MXene is more prone to form Mo vacancy defects due to its relatively low single-vacancy formation energy (0.96 eV) 34 . Regarding this characteristic, its mechanism shall be discussed in detail.
To verify the electrochemically active surface area of the Mo 2 C MXene, the CV of raw material Mo 2 Ga 2 C and Mo 2 C MXene was measured in Fig. 1f. It was found that the Mo 2 C MXene has a larger surface area, which implies that there were more active centers on its surface 35 . About the Nyquist plots of EIS (Fig. 1g), the radius of the arc shows the interface resistance of the electrode surface, and the smaller the radius, the lower the charge transfer resistance 36 . Among them, the Mo 2 C MXene has a shorter radius than Mo 2 Ga 2 C, indicating that the Mo 2 C MXene obtained after Mo 2 Ga 2 C etching has reduced charge transfer resistance, which not only accelerated the electron transfer in the Mo 2 C MXene but also promoted the catalytic activity 37 . Those characteristics suggest that Mo 2 C MXene is a possible material for activating PMS.
The performance of Mo 2 C MXene for PMS activation to degrade BPA As exhibited in Fig. 2a, single-use of Mo 2 C MXene or PMS resulted in no BPA degradation, indicating that Mo 2 C MXene and PMS have almost no BPA degradation effect. When the raw material Mo 2 Ga 2 C and PMS coexist, the removal rate of BPA was minimal (only 16.30%), which may be caused by the adsorption of BPA by Mo 2 Ga 2 C or the weak activation of PMS by the exposed transition metal atoms on Mo 2 Ga 2 C surface. This phenomenon implies that Mo 2 Ga 2 C has no effective activation effect on PMS. But when 0.5 g L −1 Mo 2 C MXene and 2.0 mmol L −1 PMS exist simultaneously, the degradation percentage of BPA reached 99.75% within 240 min. And this reaction fitted the pseudo-first-order kinetic equation, with the rate constant being 0.0227 min −1 . This result indicates that Mo 2 C MXene has excellent activation performance on PMS compared to other catalysts, and the production of stronger oxidizing substances led to the removal of BPA. For example, 0.5 g L −1 70.00% copper oxide/halloysite nanotubes activated PMS to degrade 50.0 mg L −1 BPA, and the degradation percentage of BPA was less than 80.00% within 240 min 38 . Also, the Me(Ba, Ca, and Mg)SrCoO were used to catalytically generate • OH and 1 O 2 for the degradation of dye orange II; the reaction was slow and the degradation percentage was about 90.00% within 8 h 39 . The Mo 2 C MXene used from two to four cycles resulted in BPA degradation percentages of 100%, 90.20%, and 89.29%, respectively (Fig. 2b). This slight inactivation may be due to the active sites being occupied by partial decomposition of organic residues, as some defective sites on the Mo 2 C MXene were oxidized 15,40 . Overall, Mo 2 C MXenes have good recyclability compared to some catalysts in AOPs. It has been reported that the degradation activity of the traditional iron oxide Fenton catalyst decreased from 70.00% to 10.00% in only 4 cycles 39 .
The influences of Mo 2 C MXene and PMS dosage, the initial solution pH, and widespread inorganic anions for BPA removal were investigated in Fig. 2c. Additionally, the reaction rate constants (Fig. 2d) were obtained by fitting the pseudo-firstorder degradation kinetic curves (Supplementary Fig. 1) to get. For the concentration of Mo 2 C MXene and PMS at 0.1-1.0 g L −1 and 0.5-5.0 mmol L −1 , as the dosage increased, the degradation rate of BPA increased first and then decreased. Still, the degradation percentage was close to 100% at high doses. When Mo 2 C MXene and PMS were 0.5 g L −1 and 2.0 mmol L −1 , respectively, there was a great increase in k value, indicating that the two perfectly cooperate to continuously generate a large amount of ROS at this dose. Mo 2 C MXene can provide more active sites and surface area, and higher PMS can offer greater contact with Mo 2 C MXene 41 . However, when Mo 2 C MXene was excessive, its contact with PMS was limited. And when PMS was excessive, the large amount of ROS generated instantaneously would be self-quenched, resulting in a decrease of k of BPA degradation 42,43 .
BPA degradation first decreased from pH 3 to pH 7 before increasing to pH 10. BPA can be entirely degraded under acidic conditions, 92.66% under basic conditions, and 73.61% under neutral conditions. This shows that Mo 2 C MXene has a wider range of applications than some Fenton-like catalysts. The biochar/iron oxide composite degraded 10 mg L −1 BPA at pH 3 through a Fenton-like reaction, and the degradation percentage was only 69.80% within 1000 min. And with the increase of solution pH, the degradation percentage of BPA tended to decrease 44 . The zero charge point (pH pzc ) of Mo 2 C MXene is lower than 3 in Supplementary Fig. 2, indicating that its surface is negatively charged between pH 3-10, which is not conducive to binding with PMS molecules to produce ROS; therefore, the degradation rate of BPA gradually decreases at pH 3-7. However, there may be a process of base activation at higher pH (pH 10), which led to BPA degradation, owing to the negatively charged OH − being easier to donate electrons than H 2 O molecules 43 . This result implies that Mo 2 C MXene/PMS system is effective for acidic and base conditions. Moreover, the initial unadjusted pH value of our experiment should be between 3 and 5.
Increasing the reaction temperature had a positive effect on the degradation of BPA ( Supplementary Fig. 3a, b), which increased the degradation percentage and improved the reaction rate. The degradation percentage of BPA increased from 89.11% to 99.75% in 240 min from 15°C to 25°C, and further increased to 35°C, BPA was completely degraded within 100 min. This was mainly because temperature accelerated the movement of molecules, which was favorable for ROS production and the interaction between ROS and pollutants, and high temperature may also contribute to the activation of PMS 45 . In addition, we calculated the activation energy using the Arrhenius equation (Supplementary Methods; Supplementary Fig. 3c), which was 524.12 J mol −1 .
Other types of contamination were used to evaluate the Mo 2 C MXene's application potential, including the activation efficiency of Mo 2 C MXene for different kinds of oxidants and the degradation effects of oxidation systems for different types of pollutants ( Supplementary Fig. 4). Both PMS and PS are peroxides, and the degradation percentage of BPA in the Mo 2 C MXene/PMS system was 99.75%. However, when Mo 2 C MXene activated PS, BPA can be completely degraded within 80 min. This suggests that Mo 2 C MXene has good applicability to persulfate-based peroxides. In addition, the degradation efficiency of Mo 2 C MXene-activated PMS to BPA, bisphenol S (BPS), benzyl butyl phthalate (BBP), and naphthalene (NAP) was also explored. These pollutants were classified as endocrine disruptors, plasticizers, and PAHs, respectively. The degradation percentages of BPA, BPS, BBP, and NAP in the Mo 2 C MXene/PMS system were 99.75%, 100%, 77.42%, and 100%, respectively. This indicates that the Mo 2 C MXene/PMS system may have broad-spectrum removal efficiency for environmental contaminants.
Since there are generally a large number of inorganic anions remaining in the actual wastewater, it is very meaningful to study the influence of inorganic anions on the degradation of organic matter. As shown in Fig. 2c, d, NO 3 − has a weak improving efficiency for BPA removal in the Mo 2 C MXene/PMS system, and Cl − greatly promotes the degradation rate of BPA. This was mainly due to the reaction of Cl − with HSO 5 − , SO 4 •− , and • OH to generate active chlorine species, such as Cl • (E 0 = 2.4 V), Cl 2 •− , Cl 2 , etc., which might synergistically degrade BPA. Also, Cl • also can catalyze the transport reaction to increase the enhancement of SO 4 •− , which was conducive to the removal of BPA 46 . Moreover, Cl • has a higher reaction rate than • OH for electron-rich organic pollutants 47 . This has a similar effect to Cl − in the degradation of BPA, PAHs, and arsanilic acid by pyrrolic N-rich biochars/ peroxydisulfate, polystyrene-coated persulfate polyacrylonitrile beads, and CuFe 2 O 4 /PMS, respectively [48][49][50] . According to Eqs. 1-4, Cl − can be converted into active chlorine or hypochlorous substances 7,47,51,52 .
The active chloride (Cl • and Cl 2 •− ) may attack the unsaturated bonds of pollutants to produce adsorbable organic halides that were conducive to the conversion of BPA on the surface of the Mo 2 C MXene. Similar research findings have also been reported in some studies; for example, CuFe 2 O 4 /kaolinite activated PMS to degrade BPA, nZVI/carbon fiber activated PMS to degrade levofloxacin, and the degradation of Acid Orange 7 by SO 4 •− -AOPs 7,53,54 . PO 4 3− also promotes BPA removal efficiency; this result was consistent with Khajeh et al., whereby PO 4 3− ions can catch the • OH to generate PO 4 2• according to Eq. 5 55 .
HCO 3 − has an inhibitory effect on BPA removal in the Mo 2 C MXene/PMS system, which was different from previous studies. Dong et al. believed that HCO 3 − promoted BPA removal in the system constructed by 40%-CF/K composite and PMS because it led to more active oxygen production 7 . And our research is consistent with Hammad et al., that HCO 3 − usually acts as a free radical quencher in AOPs 56 . In addition, SO 4 2− also greatly inhibits the BPA degradation percentage because SO 4 2− is the product of SO 4 •− reaction, so it would affect its conversion. In order to explore the environmental applicability of the Mo 2 C MXene/PMS system in real water, we applied it in five natural water bodies, namely Pond 1, Pond 2, Huai River, Guo River, and Qinhuai River. The sampling points of these five water bodies are shown in Fig. 2e, and the basic situation of water quality was also measured. The total organic carbon (TOC) contents were 11.10, 20.60, 10.10, 24.80, and 5.50 mg L −1 , respectively, for Pond 1, Pond 2, Huai River, Guo River, and Qinhuai River. UV 254 value is the absorbance of some organic substances in the water under the ultraviolet light of 254 nm wavelength, reflecting the amount of humic macromolecular organic substances naturally present in the water. The UV 254 value of Pond 1, Pond 2, Huai River, Guo River, and Qinhuai River were 0.05, 0.05, 0.11, 0.10, and 0.05 AU cm −1 , respectively. The degradation of BPA by the Mo 2 C MXene/PMS system in Pond 1, Pond 2, Huai River, Guo River, and Qinhuai River is shown in Fig. 2f; the degradation percentages were 73.24%, 78.46%, 77.16%, 80.00%, and 72.14%, respectively. The result indicates that BPA still remains at a high degradation percentage in the real environment, and the Mo 2 C MXene/PMS system has good environmental applicability.

Identification and analysis of ROS
To explore the catalytic mechanism of Mo 2 C MXene on PMS, the EPR technique and quenching experiments were performed 57 . The concentration of PMS from 2.0 mmol L −1 decreased to 0.5 mmol L −1 after the reaction, which indicates that the PMS was decomposed (Fig. 3a). In Fig. 3b •− adduct was weaker, which may be due to the rapid transformation of SO 4 •− to • OH by nucleophilic substitution 60 .
In addition, we have also detected the existence of DMPO-O 2 To distinguish the contribution of ROS, we conducted a quenching experiment. Among them, TBA acts on • OH, EtOH quenches SO 4 •− and • OH, p-BQ is a quencher for O 2 •− , and L-Histidine for 1 O 2 . Figure 3c shows that after adding the quencher TBA, EtOH, p-BQ, and L-Histidine, the degradation percentage of BPA decreased to 53.21%, 36.43%, 5.67%, and 13.31%, respectively. The pseudo-firstorder degradation kinetics of each ROS was fitted in Supplementary Fig. 5, with the reaction rate constant and the individual contribution being shown in Fig. 3d. The k value was significantly decreased compared with in the absence of quencher, and the corresponding ROS contribution was O 2 , and 1 O 2 were identified as ROS that degraded BPA to non-toxic or low-toxic small molecule degradation products or caused it to mineralize (Eq. 11).
Catalytic mechanism of Mo 2 C MXene To further explore the electron transfer mechanism between Mo 2 C MXene and PMS, the Mo 2 C MXene samples before and after the reaction were explored by XPS. As displayed in Fig. 4a   The functional groups from the surface of Mo 2 C MXene were identified using FTIR with wavenumbers from 400 nm to 4000 nm in Fig. 4c. It was found that there were no apparent bands related to catalysis on the fresh Mo 2 C MXene. Therefore, the possibility of surface functional group catalysis was excluded in the Mo 2 C MXene/PMS system. Figure 4c also showed several obvious absorption peaks of bands on the used Mo 2 C MXene. The two newly formed bands at 2953.9 cm and 2919.1 cm were due to C-H stretching vibration, another two bands at 1454.5 cm and 1381.1 cm were attributable to the C=O bond formed by surface oxidation, and the band at 3300 cm was O-H bond stretching vibration attached on the surface of Mo 2 C MXene 66,67 . This suggests that some groups would be adsorbed on the surface after Mo 2 C MXene catalyzed the PMS.
It has been reported that MXene samples (i.e., 312 phase Ti 3 C 2 ) etched would form corresponding Ti-deficit vacancy defects, which have a strong reducing ability [68][69][70] . We speculated that Mo 2 C MXene activation of PMS was also achieved through Modeficit vacancy defects on its surface. PMS first combined on the vacancy defects of Mo 2 C MXene, then received electrons from unpaired spintronics on the defective site to generate ROS, while the vacancy defects transformed holes due to the loss of electrons. We measured the defect signal and analyzed the combination between defective Mo 2 C/undefective Mo 2 C MXene and PMS.
The density of states (DOS) of Mo 2 C MXene is displayed in Fig. 4d. The energy 0 point is the Fermi level, and the d orbital electrons of Mo mainly contribute to the electron DOS. The electron DOS with upward and downward spin was symmetrically distributed. It further proved that Mo element sites are easier to form defects and may retain electrons. Moreover, there was a pronounced signal peak of Mo 2 C MXene based on EPR analysis (Fig. 4e), and the g value of 2.003 is characteristic of delocalized/free electrons caused by defects in the surface of catalyst 71 .
The binding energies between undefective Mo 2 C MXene, defective Mo 2 C MXene (there was a Mo-deficit vacancy defect in the structure), and PMS molecule (HSO 5 − ) were analyzed by computational chemistry based on DFT. First, the undefective Mo 2 C MXene, defective Mo 2 C MXene, and HSO 5 − structural models were constructed. The energies of the three structures optimized by DFT were −465.89, −453.89, and −41.92 eV, respectively (Supplementary Fig. 6 and Fig. 5a). Then, the binding energies between undefective Mo 2 C MXene/defective Mo 2 C MXene and HSO 5 − were calculated using the Vienna ab-initio simulation package (VASP) with the frozen-core all-electron projector-augment-wave (PAW) method (Fig. 5b, c). It was found that the combination of HSO 5 − and undefective Mo 2 C MXene does not cause the breakage of the O-O bond, but binds to the surface of the Mo 2 C MXene in the form of Mo-OH-SO 3 -O-Mo, and the binding energy was 4.78 eV. But after HSO 5 − was combined with the defective Mo 2 C MXene, the O-O bond would be directly broken to generate OH and SO 4 groups, which were connected to the Mo atoms around the defects, and the binding energy was 6.38 eV. Eventually, the SO 4 would be converted into SO 4 •− by electron transfer. Those results indicate that the existence of defects was more favorable for the interaction between Mo 2 C MXene and PMS to generate ROS for BPA degradation (Eqs. [6][7][8][9][10][11]. In addition, according to the XPS and FTIR results, the hydroxyl group content increased after Mo 2 C MXene participation in the reaction. The interaction between Mo 2 C MXene and the hydroxyl group was analyzed. Supplementary Fig. 7 shows the DOS of Mo 2 C MXene with an adsorbed hydroxyl group. It was found that the Mo 2 C MXene has strong adsorption of the hydroxyl group, with an adsorption energy of −5.31 eV. The electron DOS of spin up and down were distributed asymmetrically after the hydroxyl group was adsorbed. At the interface, the O charge of the hydroxyl group accumulates, showing a charge-rich state (yellow area). The combined Mo 2 C MXene loses charge around the Mo atoms (blue area). It shows that when the hydroxyl group was adsorbed on Mo 2 C MXene, it obtained electrons from Mo 2 C MXene. The electron-rich hydroxyl group on the surface of the material can not only promote electron transfer between the catalyst and PMS to accelerate ROS generation, but also promote the diffusion of the catalyst in the water phase matrix. The advantage of surface hydroxyl groups in activating PMS for the degradation of toxic organic pollutants has also been explored in other studies 46 . Those properties further increase catalytic activity and enhance the mass transfer of model pollutants 7 . Those were the reason for the better repeatability of the Mo 2 C MXene.
During the Mo 2 C MXene/PMS cycles, the content of Mo ions in the system was also determined, with an average value of 9.7 mg L −1 (Supplementary Fig. 8a). It shows that Mo ions would be leached from Mo 2 C MXene in the catalytic oxidation process. To further verify the contribution of leached Mo ions in BPA degradation, 10.0 mg L −1 Mo ions were utilized to activate PMS. The removal percentage of BPA was 20.00% within 240 min ( Supplementary Fig. 8b), which indicated that the homogeneous catalysis also weakly affects the removal of BPA in the Mo 2 C MXene/PMS degradation system.

Condensed Fukui function and proposed transformation pathways of BPA
To deeply analyze the transformation mechanism of BPA in the Mo 2 C MXene/PMS system, we calculated the condensed Fukui function of BPA (Fig. 6a). It can predict the reaction site. The reddest atom in the figure, the larger the condensed Fukui function value f 0 , the easier it is for electrophilic reactions to occur. For BPA, O1, O2, C3, C4, C6, C12, C15, and C16 are all sites with very high reactivity with f 0 > 0.04. The mass spectra of the possible transformation products from BPA degradation in the Mo 2 C MXene/PMS system were shown in Supplementary Figs. 9-30. Figure 6b shows that BPA was first attacked by O 2 •− and • OH, which resulted in the C-C bond cleavage and hydroxylation, forming P1, then P2 generated by ring-opening 72 . Second, BPA can be oxidized by ROS to phenol (P3 and P4), then dehydrogenated and oxygenated to P6 before being hydroxylated and dehydroxylated to P7 and P9 72,73 . After BPA decomposition, C3 or/ and C15 were attacked and dehydroxylated to form P5, and P6 was converted to P8 through ring-opening 3,5 . Third, BPA was oxidatively demethylated by free radicals to become P10 and P12, and P10 can also be dehydrogenated to P11. P12 transforms the hydroxylated product P13 and P14 through the electrophilic addition of • OH to C4, C16, and C14, which was finally oxidized by SO 4 •− and • OH to cause ring-opening and form P15. This process has also been found in other AOPs 66 . Fourth, BPA can degrade to P19 by dehydroxylation and oxidation at the C16 site and then become small molecule acids P20, P21, and P22 through C-C cleavage, • OH addition, and elimination reactions 3 . Also, similar to the above route, BPA can convert through dehydroxylation (P17) and oxidation (P18) at C4 and C16 sites 7 . In addition, through ROS oxidation, the two methyl functional groups in the isopropyl group connecting the two phenolic rings can be transformed into alcohol and carboxyl groups (P16) 66 . Most of these intermediates were unstable and would soon be further mineralized into H 2 O and CO 2 . Notably, the transformation products of BPA were found to be non-toxic or low-toxic (listed in Supplementary Table 2). The acute toxicity of BPA to Mouse (intraperitoneal) and Cavia porcellus (oral) was 150.0 and 4.0 mg kg −1 , respectively. And the However, only the products P3, P4, P5, P6, P7, P9, P12, P20, and P22 have toxic records. Most of its toxicity values (expressed as LD 50 ) in rats (oral, inhalation, dermal) and rabbit (dermal) were as high as several hundred, or even tens of thousands of mg kg −1 . This implies that the transformation products of BPA after oxidation treatment by the Mo 2 C MXene/PMS system were environmentally friendly.
In this work, the Mo 2 C MXene with Mo-deficit vacancy defects has a good catalytic effect on PMS, producing a high reaction rate that effectively degrades BPA. It is important that the defective Mo 2 C MXene has strong PMS combining and electron transfer ability. The O 2 •− , 1 O 2 , SO 4 •− , and • OH generated by Mo 2 C MXene activating PMS degraded BPA, and each had different contributions. The condensed Fukui function calculation was applied to predict the highly active sites in BPA and explored the possible removal pathway of BPA in the Mo 2 C MXene/PMS system, according to HPLC-MS/MS analysis. This mechanism of defective Mo 2 C MXene activation of PMS in BPA degradation is summarized in Fig. 7. The oxidation system constructed by Mo 2 C MXene and PMS has many advantages. First, Mo 2 C MXene has high adaptability for persulfate-based oxidants (including PMS and PS). Second, the Mo 2 C MXene/PMS system could oxidatively degrade various pollutants, including bisphenol endocrine disruptors, polycyclic aromatic hydrocarbon persistent pollutants, and plasticizers. Third, compared with some catalysts, the applicable pH range of Mo 2 C MXene/PMS system was wider, and the degradation percentage of BPA was above 74% under all pH conditions. Fourth, some hydroxyl groups would be loaded on the surface when Mo 2 C MXene participated in the activation of PMS, which improved the repeatability of Mo 2 C MXene. Furthermore, the transformation product of BPA was environmentally friendly and would not harm the ecological environment. Therefore, the Mo 2 C MXene/PMS system was suitable for BPA degradation in actual water, and the degradation percentage remained high.

Catalytic activity measurement
The chemical experiments reacted in 20 mL glass bottles. First, 9.5 mL of the prepared 2 mg L −1 BPA was put into the reaction flask, then 10 mg Mo 2 C MXene was weighed into the system, before finally adding 0.5 mL of 40 mmol L −1 PMS to trigger the Fig. 7 The degradation mechanism. Proposed mechanism of Mo 2 C MXene activating peroxymonosulfate (PMS) for the degradation of bisphenol A (BPA). experiment. The dosage of Mo 2 C MXene was 0.5 g L −1 , and PMS was 2 mmol L −1 in the system. And using 1 mol L −1 H 2 SO 4 and NaOH to adjust the solution pH (3)(4)(5)(6)(7)(8)(9)(10). The batch experiment was under a constant temperature oscillator at 25°C and protected from light. At the specific time point, 0.5 mL of methanol was used to quench an equal amount of the reaction solution, then filter the mixture of methanol and reaction solution using an organic filter (0.22 µm). And the HPLC (Shimadzu DGU-20A 5 , Japan) with an inertsil C18 column was used to analyze the residual BPA content. The experiments were done in triplicates for comparison, and the correlation is accepted at p < 0.05.

Characterization of Mo 2 C MXene
The phase analysis of Mo 2 Ga 2 C and Mo 2 C MXene was done using X-ray diffraction (XRD, X'TRA, Thermo Fisher Scientific, USA) at a Cu-Ka X-ray source with 40 kV pipe pressure and 40 mA pipe flow. The surface morphology was analyzed using a field emission scanning electron microscope (SEM; Zeiss HD, Germany) and a transmission electron microscope (TEM; JEOL JEM 2100F, Japan). And the surface element state determination was done using an X-ray photoelectron spectrometer (XPS, Axis Ultra DLD Kratos AXIS SUPRA, Japan) equipped at an Al-Ka source, and calibrated by C-C (284.8), while the surface functional group analysis was a Fourier transform infrared spectroscopy (FTIR, Bruker Vertex 70, Germany) on KBr tablets and the surface vacancy defects were measured by electron paramagnetic resonance (EPR, E500-9.5/12, Bruker, Germany). The total concentration of leached Mo was analyzed by inductively coupled plasma-optical emission spectroscopy (Optima 5300DV, Perkin Elmer, USA). The degradation efficiency calculation equation and pseudo-first-order kinetics model were shown in Supplementary Methods.

Cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS) tests
The electrochemical workstation was utilized to investigate the cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS). The experiments include glassy carbon, Ag/AgCl, and platinum electrode. And KOH with 0.1 mol L −1 was chosen as the electrolyte. Moreover, the sweep speed was 50 mV s −1 , and the CV sweep range was −1.0 to 0.5 V. The EIS was tested under open circuit potential (frequency: 1 MHz-0.1 Hz, amplitude: 5 mV AC).
Electron paramagnetic resonance test EPR was used to determine ROS: SO 4 •− , • OH, and 1 O 2 were measured in the water phase, while O 2 •− was measured in the DMSO phase. SO 4 •− , • OH, and O 2 •− were captured by 100 mmol L −1 of DMPO, while 1 O 2 was captured by 30 mmol L −1 of TEMP. The degradation products of BPA were analyzed by a high-performance liquid chromatography-mass spectrometer (HPLC-MS), and the MS was electrospray ionization mass spectrometry (Thermo Fisher, USA). The full scan spectrum was measured in the positive ion mode in the m/z 50-1000 scan range, using high-purity nitrogen as the atomizer and drying gas.

Computational chemical analysis
The binding energies are calculated based on density functional theory (DFT), which utilizes the VASP 74,75 with the frozen-core allelectron PAW 76,77 . The Perdew-Burke-Ernzerhof 78 of generalized gradient approximation was used to explain the exchange and correlation potential. A monolayer 4 × 4 defective Mo 2 C/undefective Mo 2 C (001) supercell was used. The HSO 5 − molecule was placed on the top-site of Mo in Mo 2 C MXene, and a 20 Å vacuum area was added to decrease the interaction. Perform geometric optimization until the force on each ion drops below 0.01 eV/Å. The Monkhorst-Pack k-point sampling was set to 2 × 2 × 1 79 . The binding energy (E bind ) is calculated using the Eq. 12.
where E surface is the energy of defective Mo 2 C/undefective Mo 2 C (001) supercell, E molecule represents the energy of HSO 5 − , E molecule + surface represents the total energy.
The calculation method of the DOS is shown in Supplementary Methods. The condensed Fukui function was calculated through computational chemistry optimization. Firstly, Chemoffice 19 software was used to draw the molecular structure of BPA, and then Gaussian 09 W software was utilized to optimize the molecule by the DFT at the ωB97XD/6-31G basis set. Finally, Multiwfn 3.8 was used in the calculations before displaying using VMD 80 .