3.1 Structure, morphology, and optical properties of synthesized photoanode
The SEM image of BiVO4 photoanode was shown in Fig.S1(a). The nanorods of BiVO4 with a thickness of about 200–500 nm were successfully grown on FTO. XRD patterns showed that the crystal structure of BiVO4 photoanode as presented in Fig.S1(b). The diffraction peaks of pure BiVO4 correspond to those of monoclinic scheelite BiVO4 (JCPDS no.14–0688). The main peaks centered at 18.6°, 18.9°, 28.9°, 30.5°, 35.2°, 37.8°, 47.2° that belong to the typical (110), (011), (121), (040), (002), (220), (042) crystal plane of monoclinic BiVO4, respectively(Sun et al. 2020). All characterizations indicated that BiVO4 had been successfully synthesized. The UV-Vis absorption spectra of the prepared electrode are illustrated in Fig. S3. For the pristine BiVO4 appearing at wavelength about 500 nm due to its intrinsic band gap absorption. The band gap of bare BiVO4 can be estimated from Tauc plots of (αhv)2~hv and the value is about 2.38 eV shown in Fig. S2, which is closely consistent with the previous report(Li et al. 2021c).
3.2 Enhanced degradation of SMX by PS addition
BiVO4 photoanode was used to evaluate the influence of SMX removal efficiency with or without the presence of PS. The photoelectrocatalytic activity of BiVO4 for SMX degradation was also systematically evaluated in different systems, PEC (at a potential of 0.5 V with visible light illumination without the addition of PS), PC/PS (just only illumination with 2 mM PS), EC/PS (at potential of 0.5 V with 2 mM PS) and PEC/PS (at a potential of 0.5 V vs.Ag/AgCl with visible light illumination in the presence of 2 mM PS) shown in Fig. 1 (a). The SMX degradation efficiency was 50.4%, 57.1%, 2.5% and 68.4% in the PEC, PC/PS, EC/PS and PEC/PS, respectively. The PEC/PS system showed the highest removal efficiency, which confirmed the potential of PEC as a strategy for PS activation. It was hypothesized that PS impeded charge recombination and generated SO4·− under visible light irradiation, and underwent reductive conversion via CB electrons(Son et al. 2021). Furthermore, the degradation kinetics and pseudo rate constant were determined by the first decay model (–ln (C/C0) = kt) and linearized regression in Fig. 1(b), where C is the final concentration after the reaction, C0 represents the initial SMX concentration, “t” and “k” are constants. The removal rate constant indicated that the first-order degradation kinetics could better fit the degradation rate of SMX. Notably, the rate constant listed in Table S2 of PEC/PS system was 0.00948 min− 1, which was 1.37 times higher than photocatalytic (0.0069 min− 1) and 56.4 times higher than electrocatalytic (0.0012 min− 1). This indicated that photocatalysis plays a major role in the degradation of SMX in PEC system, while electrocatalytic is an adjective role. The main reason for excellent activity of PEC system due to its better charge-separation, which prevents the recombination of photogenerated electron-hole pairs and enhanced catalytic performance(Bacha et al. 2020).
In order to further study the interaction between photocatalysis and electrocatalysis, and the enhancement factor R, according to Eq. was used to evaluate the synergistic effect of SMX removal in the PEC system. Generally, R > 1 shows a synergistic effect and if R < 1 it represents antagonism. According to the results in Fig. 1(a), the obtained R value was 1.15, which is higher than 1, indicating a significant synergistic effect in the PEC system. The synergistic effect is attributed to benefits of combining PC and EC, resulting in the additional generation of active species by PS decomposition.
3.3 Effects of initial PS and SMX concentration on its degradation
As is shown in the Fig. 1(c). In the absence of PS, the PEC system at a potential of 0.5 V with visible light irradiation just removed 43.8% of SMX after 100 min, while the degradation efficiency was increased up to 60.7% with the addition of 2 mM PS. With the increase of PS dosing concentration, the removal efficiency of SMX also obviously increased. In the presence of PS with 3 mM, 5 mM and 7 mM, the SMX removal efficiency was enhanced to 77.0%, 97.3% and 97.6%, respectively. As is illustrated in Fig. 1(d). the degradation rate constant was 0.0092 min− 1, 0.0147 min− 1, 0.0388 min− 1 and 0.0412 min− 1 with the addition of 2 mM, 3 mM, 5 mM and 7 mM, respectively, which was higher about 1.67, 2.67, 7.05 and 7.49 times than no addition of PS (0.0055 min− 1). This represents that PS plays an important role in SMX degradation. The degradation rate was meaningfully improved as the PS concentration increased, but with a concentration greater than 5 mM, the degradation efficiency can only increase limitedly. Considering the economic benefits, 5 mM of PS is the optimal dosage.
Moreover, the effect of different initial concentrations was further investigated. As exhibited in Fig. 1(e), the degradation efficiency of SMX was obviously decreased with the increased of initial concentrations of SMX. Although the degradation efficiency ranged from 97.3–73.1% after 100 min with the concentration increased from 10 mg/L to 30 mg/L, it should be noticed that the absolute amounts of the degradation of SMX are increased. Pseudo-first-order kinetics was also used for fitting the degradation of SMX under different initial SMX concentrations. As displayed in Fig. 1(f), the kinetic constants decreased from 0.0388 min− 1 to 0.0128 min− 1 as the concentration increased. The reason for decreasing removal efficiency may be due to the competition of reactive species with a high concentration of SMX, so it is not enough for them to degrade extra pollutants.
3.4 Effects of initial pH on SMX degradation
The solution pH is the main parameter in pollutant treatment as it directly influences PEC activity, generation of reactive species, the formation of catalyst species, and their availability for reaction with the oxidant and degradation rate. To investigate pH effect on SMX degradation in the PEC/PS system, experiments were performed under identical conditions with different solutions of pH 4.0 to pH 10.0(± 0.05), and the obtained results are shown in Fig.S3. The results inferred that the heterogeneous system could remove SMX at a broad range of pH from 4.0 to 10.0, which covered the pH range of most real wastewater. The high degradation efficiency of SMX under acidic conditions because it is easier to produce SO4−• Eqs. (1), (2) with the higher reaction of 4*108 M− 1·s− 1 and 6.1*105 M− 1·s− 1(Fayyaz et al. 2021), pollutants can be efficiently removed due to the strong oxidation and non-selectivity of sulfate radicals. While in an alkaline environment, SO4−• will be consumed and produce ·OH Eq. (3) which has a lower redox potential of 1.8 ~ 2.7 V, t. As is shown in FigS4, the pH was decreased into 3.80–5.11 in the degradation process with initial pH changed from 3.99 to 9.97, which may due to the decomposition of PS and the formation of residual intermediates(Duan et al. 2016).
S2O82− + H+ → HS2O8− (1)
HS2O8− + e− → SO4−• + SO42− +H+ (2)
SO4−• + OH− → ·OH + SO42− (3)
In addition, SMX has different forms including protonated, non-protonated and deprotonated at different pH values, which would be protonated at its amine group at pH 1.6(pKa1) and transformed into an anion at pH5.7(pKa2) by deprotonating the sulfonamide NH group(Anne L. Boreen 2004, Ge et al. 2019). At pH = 2.0 and 4.0, SMX was in formation of non-protonated, with the pH increasing to 6.0, SMX with the deprotonated sulfonamide NH group became the predominant form of SMX in water, which displayed higher sensitivity towards the radicals than another two forms of SMX. Hence, the PEC/PS system showed the optimized removal efficiency at pH 6.0.
3.5 Probable mechanism of PS activated by BiVO4 photoanode on SMX degradation
To identify the role of PS as photogenerated electron acceptor, the OCP was measured in different electrolytes. As shown in Fig. 2(a), the OCP of BiVO4 photoanode was about − 76 mV under visible light irradiation, but it shifted negatively to − 418 mV with the presence of 5 mM of Na2SO3. The higher OCP output promoted by injecting electrons into the conduction band because Na2SO3 captured the photogenerated holes scavenge (Zhu et al. 2017). In comparison, the OCP of BiVO4 photoanode positively shifts to − 13 mV when PS was introduced into the system. This can be considered that the decrease of electrons injected into the conduction band is attributed to the PS reduction by consume photoelectrons (Zhu et al. 2018). As an electron acceptor, PS is supposed to promote photogenerated charge separation by consuming the photoelectrons.
EIS was measured at 0.6 V vs.Ag/AgCl to verify the kinetics of photogenerated charges transfer. Equivalent circuit diagram fitted by Z-View shown in Fig. 2(b). While Rs represents the resistance of the outer metal wire circuits, Rct represents the charge transfer resistance across the electrode/electrolyte interface and CPE is the corresponding Helmholtz Capacitance. Generally, the smaller radius of the fitted charge transfer of resistance represents the faster charge transfer rate. It can be seen from Table S3 that the charge transfer resistance was obviously decreased than blank BiVO4 with the presence of PS, which proved that PS can promote a rapid charge transfer and achieve the separation of photogenerated electron-hole pairs. Moreover, the injection of SMX into solution further decreased the radius of photoanode, which revealed a more efficient transfer of photogenerated charge. This indicated that more photogenerated holes can participate in the oxidative degradation of SMX. In addition, PS will also be reduced by electrons to produce sulfate radicals to effectively remove SMX.
Electrochemical characterization was tested to explain the activation mechanism of PS by BiVO4. Different solution conditions in 0.1 M Na2SO4 electrolyte with 5 mM PS only, with SMX 10 mg/L only, with both 5 mM PS and 10 mg/L SMX was labeled as BVO/PS,BVO/SMX and BVO/PS/SMX, respectively. The LSV curves were tested to explore the interfacial generation and separation dynamics of photogenerated charges with the assistance of PS. As is exhibited in Fig. 2(c), the current significantly increased in the presence of PS and SMX compared to blank BiVO4 in visible light illumination, suggesting the PS and SMX can promote the separation of photogenerated charges.The enhanced photocurrent is speculated that PS can attract and accept photogenerated electrons from BiVO4 photoanode as an electron acceptor leading a more effective separation of photogenerated charges and increasing the numbers of photogenerated holes. While the photocurrent peak with SMX injection increased by direct oxidation of SMX on BiVO4 photoanode. Therefore, the maximum photocurrent was exhibited in the coexistence of PS and SMX. The photocurrent responses of BiVO4 photoanode in the SMX degradation process had been measured in Fig. 2(d). Higher photocurrent responses in PEC/PS system suggested that PS can improve the separation of photogenerated holes and electrons, causing the high degradation efficiency of SMX.
To determine roles of reactive oxidation species, free radical capture experiment was examined. Tert-butyl alcohol (TBA) is reported to quickly react with ·OH at a constant of 3.8–7.6 × 108 M− 1s− 1, whereas methanol (MeOH) displays strong reactivity for both SO4·− and ·OH with the constants of k·OH= 9.7 × 108 M− 1s− 1 and kSO4·− = 2.5 × 107 M− 1s− 1, respectively(Yin et al. 2018). Disodium ethylenediaminetetraacetic acid (EDTA-2Na) and p-benzoquinone(p-BQ) can usually be used for quenching h+ and ·O2−, respectively. As is displayed in Fig. 3.(a), the removal efficiency of SMX had seriously inhibited after adding EDTA-2Na and p-BQ indicating that h+ and ·O2− were the main radicals in the degradation of SMX. As depicted in Eq. (8), Moreover, the degradation rate decreased to 73.3% and 77.2% after 100 min with the addition of MeOH and TBA to the SMX degradation process, which indicated that ·OH was another reactive oxidation species for slightly inhibiting the SMX removal(Zhong et al. 2019), while there was only a small amount of sulfate radical were generated on the PEC process. The SMX degradation under different scavengers followspseudo-first-order kinetics with a declined constant rate of 0.0135 min− 1, 0.0151 min− 1, 0.0066 min− 1 and 0.0056 min− 1 when MeOH, TBA, EDTA-2Na and p-BQ were induced into the PEC system, respectively (Fig. 3(b)). All the results indicated that h+ and ·O2− were dominant reactive oxidation species by directly oxidating pollutant, while the indirect S2O82− transfer pathway with producing •OH played a minor role.
To further verify the main radicals in the PEC/PS system, EPR experiment had been carried out with the capture of DMPO agent. As is exhibited in Fig. 3.(c) and Fig. 3(d), no signals of DMPO-·O2−, DMPO-·OH and DMPO-SO4·− had been discovered in dark, indicating that PS would not be activated by BiVO4 photoanode in the dark. But strong intensity peaks of DMPO-·O2−and DMPO-·OH had been detected, which illustrated that ·O2− and ·OH played a positive role in the degradation of SMX. Slight signals of SO4·− indicated that PS had been activated by BiVO4 photoanode under visible light irradiation and participated in the degradation of SMX.
Based on above results and previous work, a plausible mechanism of SMX degradation was proposed(Liu et al. 2017, Sun et al. 2021):
BiVO4 + hv → hVB+ + eCB− (4)
eCB− + O2 → ·O2− (5)
eCB− + S2O82− →[S2O82−]− (6)
[S2O82−]− + O2 → S2O82− + ·O2− (7)
eCB− + S2O82− →SO4·− + SO42− (8)
SO4·− + H2O → ·OH + SO42− +H+ (9)
h+/·O2−/·OH/·SO4− + pollutants → degradation products (10)
Under the irradiation of visible light, BiVO4 absorb solar energy and generate holes and electrons pairs (Eq. (4)).. It is sufficient for photogenerated electrons to reduce O2 into ·O2− (Eq. (5)). Meanwhile, PS as an electron acceptor can capture the photogenerated electrons and induce O2 to produce ·O2− (Eq. (6)-(7)). Moreover, PS can also be activated by photogenerated electrons to generate ·SO4− Eq. (8)), and then SO4·− will react with H2O to produce ·OH (Eq. (9)). The h+ generated from the valence band (VB) of BiVO4 photoanode under visible light illumination can directly participate the oxidation of organic pollutants. As shown in Fig. 3(e), organic pollutants will degrade under the combined action of h+, ·O2−, ·OH and SO4·− (Eq. (10)).
3.6 Degradation of mixed pollutants in DI water and natural water
To further study the photoelectrocatalytic performance of PS activated by BiVO4 photoanode for removal mixed pollutants, CIP and TC (10 mg/L) were added to above SMX solution with pH adjusted to 6 under visible light irradiation at potential of 0.5 V vs.Ag/AgCl. As is displayed in Fig. 4(a), the degradation efficiency of SMX, TC and CIP was 64.4%, 79.6% and 99.4%, respectively. It indicated that TC and CIP could also be better removed in mixed pollutants and CIP is easiest to be degraded, while the removal efficiency of SMX was inhibited to a lesser extent compared to a single pollutant in PEC/PS system (97.3%). It may be due to the addition of CIP and TC occupying more active sites, forming a competitive relationship with SMX. Since the degradation rate of CIP was quik in the first 10 minutes, the first-order kinetics fitting was performed in 10–60 minutes. The kinetic rate constants of SMX, TC and CIP was 0.0079 min− 1, 0.0143 min− 1 and 0.0343 min− 1, respectively (Fig. 4.(b)). This was consistent with the results of removal efficiency, which indicated that BiVO4 photoanode synergistic catalytic activation of PS has good application prospects in the degradation of mixed pollutants. In order to further explore the degradation of mixed pollutants by PEC/PS system in natural water, SMX, TC and CIP (10 mg/L each) were added into the natural water taken from the central lake outside the gate of South China University of Technology. Surprisingly, as is shown in Fig. 4(c), CIP was almost completely degraded after 20 min. And the removal efficiency of TC reached 95.9% within 20 min, which is a huge improvement compared to DI water. As is depicted in Fig.S5, the rate constant of TC in natural water (0.159 min− 1) was 8.83 times higher than that of DI water (0.018 min− 1). While there was a slight increase of SMX degradation in natural water. To explain the reason for this performance increase, the photocurrent responses had been monitored. As depicted in Fig. 4(d), the photocurrent response in natural water was higher than in DI water, which suggested the improved separation of photogenerated holes and electrons. This indicates that the separation of light-generated holes and electrons has been improved, which is consistent with the improvement of degradation effect, and there is still SO4·- in the combined system, so it can be inferred that the degradation performance of antibiotics in the actual water is improved mainly due to the following two reasons: 1) In a complex ionic environment, some anions such as Cl− can act as receptors for transferring h+, and then are oxidized to produce active chlorine free radicals. The specific reaction formula is shown in (Eq. (11)), which enhances the degradation effect of antibiotics. 2) The SO4·− and ·OH produced in the system can also further oxidize Cl− to produce chlorine free radicals, which are shown in Eq. (12)-(14) to promote the degradation of antibiotics.
Cl− + h+ → Cl· (11)
Cl− + ·OH → ClOH·− (12)
ClOH·− + H+ → Cl· + H2O (13)
Cl− + SO4·− → SO42− + Cl· (14)
Cl· + Cl− → Cl2· (15)
3.7 Transformation products identification and pathway inference
In order to further study the chemical structure of transformation products (TPs) of each pollutant in the mixed system, the intermediate products of SMX, TC and CIP were identified based on HPLC/MS spectra. The molecular formulae, molecular masses, and proposed structures of the TPs are listed in Table S4-S6. Based on the observed products.
3.7.1 The probable pathway of SMX
As is displayed in Fig, a total of 7 TPs had been detected on the SMX degradation progress. As reported in previous work, the main degradation pathway of SMX included desulfonation (TP1 m/z = 189),cleavage of sulfonamide bond (TP2, m/z = 99), oxidation (TP4, m/z = 283)༌deamination, hydroxylation (TP5, m/z = 299), (Kim et al. 2017, Li et al. 2021b). Pathway 1(P1) is a kind of desulfonation process with losing -SO2, and then degrade into smaller molecules by the attack of free radicals in the PEC/PS system. Pathway 2(P2) is formed by the directly cleavage of sulfonamide bond and transformed into TP3(m/z = 209) by the coupling reaction of -NH2 with oxidation occurring in the isoxazole ring(Xu et al. 2019). The pathway3 (P3) with two possible TPs (m/z = 284) are formed through the oxidation of the –CH3 on the isoxazole ring yielding a carboxylic acid group (TP4-1) and the oxidation of the –NH2 transforming to the nitro-SMX derivative (TP4-2) (Chen and Wang 2021). The formed TP4-2 can then convert to TP2, andTP5 through the cleavage of N-S bond and hydroxylation. The hydroxylation process happened through the generated •OH and other active species attacking the benzene ring in the PEC/PS system, which becomes more active to decompose. The formed TP5 can further transform into TP6(m/z = 219) and TP2 through sulfonamide bond hydrolysis, resulting in the cleavage of the S-N bond. Furthermore the TP7 (m/z = 504) is a kind of self-coupling product formed in the coupling of amino groups of two SMX molecules(Pu et al. 2020). After through a series of radical reactions, some organic products would convert to small molecules or CO2 and H2O.
3.7.2 The probable pathway of CIP
The major CIP transformation intermediates and the possible pathways were further investigated to better understand the photoelectrocatalytic performance on CIP degradation. As depicted in Fig. 6 The pathway 1(P1) is a kind of hydroxylation and defluorination process. CIP is firstly converted to TP1(m/z = 347) by electrophilic reaction in the addition of hydroxyl at the C = C bond(Fang et al. 2021).Next, hydroxylation reaction, an important role in the defluorination process, takes place on the quinolone ring and further hydroxylated to produce TP2(m/z = 345)(You et al. 2021). TP3(m/z = 317) is one of defluorinated products formed from TP2 through the breaking of C = C bond with the loss of an ethylene group. The TP3 is further converted to TP4(m/z = 301) through dehydroxylated and can be eventually mineralized into CO2 and H2O or other small molecule products(Chen et al. 2021).As reported in other literature, N (3) and N (6) sites of the piperazine ring were vulnerable to attack by h+(Wang et al. 2021). Thus, pathway 2 (P2) is a process of breaking of the piperazine ring under PEC/PS system for CIP degradation. The piperazine ring is attacked in the first place and produce TP5 (m/z 362), then-CO is eliminated to transform to TP6(m/z = 333) with two probable structures labeled as TP6-1 or TP6-2, and further loss of -CO achieved TP7(m/z = 305). The compound with the m/z values of 289(TP8) and 262 (TP9) are formed through dehydroxylated and further destruction of piperazine ring, respectively. TP9 is further converted to TP10(m/z = 280) through the breaking of C-C bonds with the loss of an ethylene group. At last, TP10 was further transformed into TP11(m/z = 207) with losing -COOH and -CO. Under the effect of active species, the above transformation products would continue to be mineralized to CO2 and H2O or other small molecule products through a series of reactions.
3.7.3 The probable pathway of TC
A total of 19 TPs had been detected in the degradation process of TC at PEC/PS system. The probable degradation pathway has been depicted in Fig. 7, which shows that there are 4 major pathways of TC. According to the structure of TC, 3 types of functional groups, including double bonds, phenolic groups and amine groups are easily attacked by radicals. The double bond is more active than the other two functional, which shows a higher electron density in the position of C11a-C12. The P1 is a hydroxylation process, TP1(m/z = 461) is produced by hydroxyl addition reaction at C11a-C12 position and further converted to TP2(m/z = 474) and TP3(m/z = 477) under the attack of ·OH which attacked in the one methyl on amine group and double bonds in the position of C2-C3, respectively. Addition, TP3 can transfer to TP5(m/z = 459) by losing a H2O molecule(Li and Hu 2016). Another way of TP1 is a dealkylation process on amino groups in the formation of TP5(m/z = 432) under the attack of h+. The P2 is a kind of dehydration process. The compound of TP6(m/z = 427) further transform to TP7(m/z = 394) by removing amide bond. P3 occurs under the attacking by a series of reactive species, which is mainly a process of dealkylation. After losing a -CH3, the TP8(m/z = 431) can convert toTP9(m/z = 413) and TP10(m/z = 416) through losing a H2O molecule and amino group. The former intermediate products undergo a ring-opening reaction under the attack of free radicals, and the opening ring compounds including TP11(m/z = 363) and TP12(m/z = 376) can further occur ring-open reaction until it is oxidized into water and CO2. P4 is a successive fragmentation induced by reactive species. The generation of TP15(m/z = 403) is formed through dealkylation and deamidation, and further convert to TP16(m/z = 360) by losing a methylamino group. Then the TP16 can fragment to TP17(m/z = 317) through opening ring、dealkylation and dehydroxylation. Besides, the formation of TP18(m/z = 279) and TP19(m/z = 273) is attributed to the loss of acetyl group and hydroxyl and oxidation reaction(Yang et al. 2018). Finally, these small molecules can be mineralized into H2O and CO2 under the attack of a variety of active radicals in the PEC/PS system.
3.8 Stability analysis
Generally, the stability of the photoanode was considered as an important concern for the photoelectrocatalytic activity. Therefore, in order to evaluate the stability of BiVO4 photoanode materials, the repetitive experiments of BiVO4 activated PS to degrade SMX were carried out and the respective results were shown in Fig.S6. The degradation performance of SMX in PEC/PS system was almost no significant decline with a high efficiency of 96.6% after four cycles of experiments which showed good recyclability with better stabilities and stable catalytic efficiency of the BiVO4 photoanode. To further prove the high stability of BiVO4 photoanode, the XRD patterns of the fresh and used catalyst were displayed in Fig.S7. It can be seen that the strong characteristic peaks for sample are clearly observed even after four cycles, which means that the phase and the structure was unchanged. The results were clearly shown that the photoanode has high stability for PS activation for SMX removal under visible light irradiation. Therefore, the high stability of BiVO4 photoanode for PS activation can provide a theoretical foundation for practical photoelectrocatalytic applications.