3.1. Characterization
TEM was used to observe the morphology of CMK-3 and OCMK-3, with the results shown in Fig. 1. The well-ordered stripe-like structures could be clearly observed from Fig. 1(a) and (b), indicating that oxidation did not destroy the ordered mesoporous structure of CMK-3. The isotherm curve of both catalysts (inset of Fig. 2) exhibited type IV feature, indicating the typical mesoporous structures. Based on the pore size distributions curves (Fig. 2), the most probable pore sizes of the two materials centered around 5 nm, indicating the uniform mesopores of them, in agreement with the TEM images(Wang and Man et al., 2018). Meanwhile, the specific pore structure parameters of both catalysts are listed in Table S3.
The changes of CMK-3 and OCMK-3 in the composition of functional groups before and after reaction were analyzed using XPS. As shown in the wide scan XPS spectra (Fig. 3(a, b)), The oxygen content of OCMK-3 was higher than that of CMK-3 before and after reaction, indicating that more oxygen-containg groups were introduced after the oxidation treatment, and resulted in the improvement of the catalytic effect. Meanwhile, both CMK-3 and OCMK-3 spectra showed the increased oxygen level after reaction owing to intermediates adsorption and the surface oxidation, which was consistent with the previous studies(Duan and Su et al., 2016). Fig. 3(c-f) shows the de-convolution XPS C1s and O1s spectra before and after the reaction. Before reaction, high solution C1s XPS spectra of CMK-3 could be divided into three sections corresponding to t C graphite/C-C/C-H (283.85 eV), C-OH/C-O-C (284.77 eV) and π-π* (287.94 eV) shake up, respectively. After reaction, a broad C1s peak of CMK-3 was fitted into four component peaks at 284.34, 285.92, 287.12 and 288.42 eV, indicating C graphite/C-C/C-H, C=O, COOH and π-π* shake up. Meanwhile, before reaction, C1s XPS spectra represented three C species on original OCMK-3 in the range of 282–292 eV: C graphite/C-C/C-H, C-OH/C-O-C and π-π* shake up generated at 283.58, 284.77 and 287.77 eV, respectively. After reaction, a broad C1s peak was fitted into four C species on OCMK-3 in the range of 282–292 eV: C graphite/C-C/C-H, C=O, COOH and π-π* shake up generated at 284.34, 285.92 and 287.12 eV, 288.42eV, respectively. After reaction, the C-OH/C-O-C on the surface of both CMK-3 and OCMK-3 decreased, and C=O and COOH increased. Therefore, it could be conferred that in the process of PS activation by the two materials, the hydroxyl groups on the surface were converted into carbonyl and carboxyl groups. Thus hydroxyl on the surface could be regarded as an important functional group for catalysts activation. As shown in Fig. 3(d, f), high solution O1s XPS spectra could be divided into three types of O species: O1(530.98–531.9 eV), O2(532.6–533.6 eV), and O3 (533.5–535.84 eV) indicating C=O, C-OH/C-O-C and chemisorbed oxygen and adsorbed water, respectively. For the speciଁc oxygen groups, both C1s and O1s scans displayed the increase of C-OH/C-O-C and the decrease of C=O after reaction, which might be ascribed to the transformation between the two groups. Therefore, the changes of the speciଁc groups might impose effects on the catalytic performances of the catalysts(Indrawirawan and Sun et al., 2015; Tang and Liu et al., 2018). The influence of trace amounts of nitrogen on the surface of the catalyst was also investigated. The de-convolution XPS N1s spectra of CMK-3 and OCMK-3 before reaction were shown in Fig. S1(a-b). Obviously, after oxidation of CMK-3 using nitric acid, nitrogen-containing functional groups were introduced on the surface. The N1s peak of OCMK-3 was fitted into two sections at 399 and 405 eV, indicating pyridine and −NO2. It was also conferred in other studies that under a high temperature environment, oxidation using HNO3 could effectively generate nitrogen-containing species on the surface(Yang and Li et al., 2016).
FT-IR spectroscopy (Fig. 4) was also used for further studies of the surface chemistry of CMK-3 and OCMK-3. The absorption of stretching vibration of C–O bond was denoted in the absorption band at 1100 cm−1. Peak around 1640 cm−1 corresponded to C=O and -OH stretch vibrations and the characteristic peaks around 2922 cm−1and 3440 cm−1 indicated C-H/O-H and -OH stretch vibrations, respectively. The relative intensity of 1100 cm−1, 1640 cm−1 and 2922 cm−1 bands in OCMK-3 were higher than those of the CMK-3 sample, indicating that more C–O,C=O, –OH and C-H/O-H functional groups were generated after the oxidation(Zolfaghari and Esmaili-Sari et al., 2011). Therefore, FT-IR spectra further demonstrated the presence of more oxygen-containing functional groups onto the OCMK-3 surface, which were favorable for catalytic activation of PS (Liu and Xiong et al., 2019).
3.2. Catalytic performances of OCMK-3
It has been reported that high concentration of acid treatment might lead to collapse of the pore structure of the carbonaceous material, and excessive oxygen-containing functional groups could block the micropores and thus reduce the specific surface area and pore capacity, which might inhibit the oxidative degradation activity of the catalysts(Bazu A and Lu et al., 2008). To obtain the optimum conditions for OMCK-3 synthesis for CIP removal, CIP removal experiments were performed using OCMK-3 prepared under various conditions, with the results shown in Tables S1-S3. Compared with the pristine CMK-3, both the adsorption and removal of ciprofloxacin of OCMK-3 could be enhanced only after treatment with 4 mol/L of nitric acid solution. As shown in Table S2, neither the adsorption capacity nor the catalytic efficiency of OCMK-3 could get enhanced when the synthetic temperature was 70℃ or 90℃. And Table S3 indicated that both of the adsorption and catalytic performance experienced a rise when the modification time increased from 15 min to 45 min, but started to decrease when the time increased to 60min. Therefore, compared with the pristine CMK-3, the adsorption and catalytic performance of OCMK-3 could be enhanced under certain synthetic conditions.
In practical application, the catalytic performance of the OCMK-3/PS system is often affected by environmental parameters. Therefore, the removal efficiency of CIP and the corresponding kinetic performance were studied by changing reaction parameters (e. g. the dosage of OCMK-3 and persulfate, solution pH and CIP initial concentration, temperature and coexistent ions). Firstly, the effect of the dosage of OCMK-3 was investigated, with the results shown in Fig. 5(a). PS alone could hardly oxidize CIP without activation, with the removal rate only about 8%. Herein, with the increase of dosage from 0.1g/L to 0.4g/L, the removal efficiency of CIP was enhanced, which could be attributed to more activators and active sites provided by the increased amounts of OCMK-3. However, kobs did not increase when the dose of OCMK-3 was above 0.3g/L, which might be limited by the dose of PS. The influence of the PS dosage was also investigated. First, in order to eliminate the influence of adsorption, the adsorption experiment was carried out for 3h without the addition of PS. As shown in Fig. 5(b), the adsorption process reached equilibrium within 1h and about 40% of CIP was removed. After adsorption reached equilibrium at 1h, PS was added. It could be observed from Fig. 5(b) that the concentration of CIP decreased significantly. It means that in addition to adsorption, chemical reactions were also involved in the system, which was affected by the PS dosage. The Langmuir-Hinshelwood (L-H) kinetic model was used to fit the removal curve of CIP. Formula (1) and (2) are (L-H) variable forms.
$$-\frac{dC}{dt}={K}_{obs}C$$
1
$$In\left(\frac{{C}_{t}}{{C}_{o}}\right)=-{K}_{obs}t$$
2
where C0 and Ct are the concentrations of CIP, Kobs is the pseudo-first-order reaction rate constant (min−1), and it is the reaction time (min).
When the PS dosage rose from 2mM to 8mM, Kobs increased from 0.00189 / min to 0.00364 / min. An increase in Kobs at the beginning might be resulted from more reactive free radicals through increasing PS dosage. However, Kobs decreased when PS dosage continued to increase, which could be due to the self-quenching effect between excessive free radicals(Guo and Zeng et al., 2017).
The effect of pH on the removal of CIP was studied over the pH range of 3.0 and 5.0, as such concentration of CIP was insoluble under neutral and alkaline conditions. As shown in Fig. 5 (d), the maximum removal of CIP occurred when pH was 4. To further understand the effects of pH, zeta potential for OCMK-3 under various pH values was analyzed. As shown in Fig. 5 (c), the isoelectric point of OCMK-3 is 2.1. Thus when the solution pH is more than 2.1, the surface of OCMK-3 is negatively charged(Saleh and Isik et al., 2021). When the solution pH was 3 or 4, CIP existed as cationic form CIP+ as a result of the protonation of the secondary amine on the piperazine group, while OCMK-3 surface was oppositely charged. However, OCMK-3 was more negatively charged when pH was 4, resulting in the enhanced adsorption amounts. When pH reached 5, CIP could exist as zwitterionic (CIP±). In this stage, both CIP molecules and OCMK-3 surfaces were negatively charged, resulting in depressed adsorption process(Peng and Hu et al., 2015).
As demonstrated in Fig. 6 (a), CIP removal efficiency decreased from 89–57% with the increase of CIP concentration from 100mg/L to 300mg/L. As the concentration of pollutants rose, the increased competition of CIP molecules might be the reason for the reduced removal rate(Ye and Yan et al., 2019; Ahammad and Zulkifli et al., 2021).
To compare CIP removal by the prepared CMK-3 and OCMK-3, comparative experiments were carried out, with the results described in Fig. 6 (b). About 42% and 74% of CIP were removed in 3 h by CMK-3 and OCMK-3, respectively. The removal efficiency of OCMK-3 was about 1.76 times that of CMK-3, which further verified that oxidation treatment could increase the activity of the catalyst. To evaluate the oxidative degradation performance, the mineralization degree of organic contaminants was investigated. It could be clearly observed from Fig. 6 (c) that TOC removal by the OCMK-3/PS system was about 11% higher than that by the CMK-3/PS system. This suggested that the OCMK-3/PS system could not only enhance the removal of CIP but also exhibited higher mineralization efficiency.
The effects of temperature on CIP removal were further investigated, with the results shown in Fig.S2(a). At 25℃, 74% of CIP could be removed. When the temperature increased to 45℃, about 81% of CIP could be removed. Thus it could be inferred that elevated temperatures could lead to enhanced PS activation. There might be two reasons to interprete this phenomenon. Firstly, higher temperature could facilitate the decomposition of PS and therefore accelerated the removal of CIP. Secondly, the elevated temperature could also promote the adsorption process, which was favorable for the electron transfer between OCMK-3 and PS (Tang and Liu et al., 2018).
The trace amounts of inorganic anions often existed in real wastewaters, which might affect catalytic behaviors in very complex ways. Fig.S2(b, c) showed the effects of the different concentrations of HCO3− and Cl− on CIP removal. As shown in Fig.S2(b), when the HCO3− concentration rose from 0 to 10 mM, the removal of CIP was also increased. The reason for the increase might lie in that the addition of HCO3− might increase the solution pH, thus reduced the solubility of CIP molecules(Wu and Li et al., 2013). However, when the HCO3− concentration rose to 20 mM, CIP removal efficiency slightly decreased. As HCO3− could act as an electron donor, it might compete with CIP for reacting with SO4·− thus hindered CIP removal (Eqs. (3)-(4))(Wen and Qiang et al., 2018; Yong and Jie et al., 2018).
$${SO}_{4}{\bullet }^{-}+{HCO}_{3}^{-}\to {HCO}_{3}{\bullet }^{-}+{SO}_{4}^{2-}$$
3
$$\bullet OH+{HCO}_{3}^{-}\to {OH}^{-}+{HCO}_{3}{\bullet }^{-}$$
4
As demonstrated in Fig.S2 (c.), the presence of 5mM and 10mM chlorides slightly decreased the remove rate, as Cl− could act as radical scavengers and combine with SO4·− and ·OH to generate Cl· and Cl2·− (Eqs. (5)-(8))(Guo and Yan et al., 2021). Whereas, an obvious increase of kobs could be observed with the chloride concentration continued to increase to 20 mM. Herein, besides scavenging SO4·− and ·OH, super abundant Cl− could also take part in the removal of CIP, which might act as an electron donor to afford electrons to PS, thus the active chlorine species and sulfate radicals were generated under the catalysis of OCMK-3(Anipsitakis and Dionysiou et al., 2006; Oyekunle and Cai et al., 2021).
$${SO}_{4}{\bullet }^{-}+{Cl}^{-}\to Cl\bullet +{SO}_{4}^{2-}$$
5
$$\bullet OH+{Cl}^{-}\to {ClOH\bullet }^{-}$$
6
$$ClOH{\bullet }^{-}+{H}^{+}\to Cl\bullet +{H}_{2}O$$
7
$$Cl\bullet +{Cl}^{-}\to {Cl}_{2}{\bullet }^{-}$$
8
3.3. Identification of radicals
Herein, to investigate the effects of radicals, radical quenching reaction was analyzed. SO4·−and ·OH have been regarded as two typical reactive species involved in PS activation. Studies had demonstrated the strong quenching ability of Methanol for ·OH (k ·OH=9.7×108M−1s−1, kSO4·− =3.2×106M−1s−1), while EtOH served as an effective quencher for SO4·− (k ·OH = (1.6-7.8) ×107M−1s−1, kSO4·− = (1.2-2.8) ×109M−1s−1)(Buxton and Greenstock et al., 1988). Thus Methanol and EtOH were utilized as quenchers, with the results shown in Fig. 7 (a-b). The CIP removal was restrained from the existence of either MeOH or EtOH, with a decrease of kobs from 0.0044 min−1 to 0.00131 min−1(n[MeOH/PS] = 1000) and 0.00134 min−1(n[EtOH/PS] = 1000), respectively. It could be referred that both SO4·−and ·OH were generated and involved in the reaction. In addition, 1,4-benzoquinone (BQ) was introduced as a quencher to identify the existence of superoxide anion radicals (O2·−), with the results shown in Fig. 7 (c). Slight inhibition in the reaction by BQ could be observed, with a decrease of kobs from 0.0044 min−1 to 0.0034 min−1, indicating that O2·− was also involved in CIP removal.
Electron paramagnetic resonance (EPR) spectroscopy was employed to further conଁrm the existence of radicals. Fig. 7 (d) shows the characteristic peaks of DMPO·OH (hyperfine splitting constants of aN = aH = 14.9 G) and DMPO·SO4(aN = 13.2 G, aH = 9.6 G, aH = 1.48 G and aH = 0.78 G) (Wang and Sun et al., 2015), in agreement with the quenching experiment, and it could be further inferred that ·OH and SO4·− were involved in the reaction(Jiang and Li et al., 2020). Meanwhile, six characteristic peaks of DMPO- O2•− could be observed in Fig. 7. (e), further confirming the occurrence of O2·−(Wang and Lin et al., 2017; Cdab and Zhuo et al., 2021).
3.4. Degradation products and pathways
In this study, 9 transformation intermediates of CIP were identified by the LC-MS technique (Table S2 and Fig.S3). Through interpreting the molecular ion masses, analyzing ESI(+) MS spectra and comparing with previous studies (Guo and Li, 2013; Guo and Gao et al., 2016), three major reaction pathways for CIP degradation were proposed and were illustrated in Fig. 8.
Pathway Ι was initiated by cleaving piperazine ring, and then ·OH, SO4·−, and O2·− attack caused stepwise oxidation This route in agreement with the study that the piperazine ring was determined to be the most active site for the free radical attack(Jiang and Ji et al., 2016). Oxidation of piperazine led to ring opening, resulting in the dialdehyde derivative C1. C1 converted into C2 through removing one formaldehyde from the open piperazine ring, and then transformed into C3 through further loss of second formaldehyde in the open piperazine ring. The compound C3 could be further oxidized into C4 by losses of secondary amine nitrogen(Mahdi-Ahmed and Chiron, 2014).
In Pathway Ⅱ, CIP was firstly converted to C5 by hydroxylation, and then transformed into C6 through OH/F substitution and further hydroxylation. C7 was formed by substituting -COOH with -OH in C6. This pathway was also proposed in other studies(Zhu and Wang et al., 2019).
In Pathway Ⅲ, CIP might convert into C8 by defluorination. On the one side, product C9 was produced through piperazine epoxidation from C8. On the other side, the C8 might also form C5, through hydroxylation, and then with the similar route to pathway II, through further hydroxylation and decarboxylation process were similar to pathway II, the same intermediate C6 and C7 was generated. This pathway in agreement with that proposed by Yja, B., et al.(Yja and Cf et al., 2014).
3.5. Electron transfer pathways
Based on the above analysis, the mechanism of CIP removal using the OCMK-3/PS system was proposed and shown in Scheme 1. The unique electronic circumstance on the surface of OCMK-3, resulted in the defective edge sites and free-flowing π electrons of the sp2-hybridized carbon network, which might impose effects on the electron configuration of PS and weaken the O-O bond to generate metastable complexes(Duan and Sun et al., 2015). In addition, the large specific surface area and porous structure of OCMK-3 could facilitate the adsorption of PS and CIP molecules onto its surface, and thus promoted their contact and subsequent reactions. Accordingly, two possible pathways were proposed in the following evolution.
The radical pathway: PS was easily hydrolyzed by the metastable complex (Eq. (9)), and then the oxidation between HO2− and S2O82− occurred via one-electron transfer process, leading to the generation of SO4·− and O2·−(Eq. (10)). SO4·− could also convert to ·OH via Eq. (11). Here, OCMK-3 played a vital role as the electron transfer mediator(Jya and Xh et al., 2020). The generated SO4·−, ·OH and O2·− could then participated in the degradation of CIP(Duan and Sun et al., 2018).
The nonradical pathway: In PS nonradical activation systems, when the electron-rich CIP molecule approached the activated PS molecule, a ternary system of electron donor (CIP)-mediator (OCMK-3)-acceptor (PS) was established. PS was used up in CIP oxidation by getting two electrons from organics (Eq. (12)(Yao and Zhang et al., 2019) ), and the radical reactions did not occur due to the steady state of PS (Chen and Ma et al., 2019; Zhang and Tang et al., 2020).
The radical and nonradical oxidation processes both took part in the reaction in this system. OCMK-3 acted as an electron transfer mediator, which could facilitate the transfer of electrons from CIP or H2O to PS. The electron transfer pathways were conformed to the CMK-3/PS system studied previously(Tang and Liu et al., 2018; Liu and Lai et al., 2019), indicating that OCMK-3 did not change the degradation pathway.
$${S}_{2}{O}_{8}^{2-}+2{H}_{2}O\to {HO}_{2}^{-}+2{SO}_{4}^{2-}+3{H}^{+}$$
9
$${S}_{2}{O}_{8}^{2-}+{HO}_{2}^{-}\to {SO}_{4}{\bullet }^{-}+{SO}_{4}^{2-}+{O}_{2}{\bullet }^{-}+{H}^{+}$$
10
$${SO}_{4}{\bullet }^{-}+{OH}^{-}\to {SO}_{4}^{2-}+\bullet OH$$
11
$${S}_{2}{O}_{8}^{2-}+2{e}^{-}\to 2{SO}_{4}^{2-}$$
12
3.6. Stability of OCMK-3 and Real wastewater application
The stability of the catalyst OCMK-3 for continuous PS activation was evaluated for further application. The experiment was carried out for four cycles to test the reusability of OCMK-3, with the results shown in Fig.S4. A slight decrease in CIP removal efficiency could be observed after each cycle, which might be attributed to the oxidation of the surface during the reaction, which changed its chemical composition. Moreover, the interaction between PS and the active sites was hindered owing to the adsorption of oxidation intermediates, thus imposing effects on the surface chemistry of OCMK-3 and the electron transfer from OCMK-3 to PS. However, the CIP removal by regenerated OCMK-3 after four cycles was 89.2% of that by the fresh OCMK-3, demonstrating that OCMK-3 could be regenerated and reused(Jya and Xh et al., 2020).
To test the practicability of the OCMK-3/PS system, real pharmaceutical wastewater was used as a target to test the degradation efficiency of the OCMK-3/PS system in the complex environment, with the results shown in Fig. 9. Multiple peaks could be observed from the spectrum of the original sample in Fig. 9 (a), showing complex components of the water with high concentrations. The high fluorescence intensities could also be identified in Fig. 9 (a), which were related to fulvic-like compounds, indicating the poor biodegradability and serious pollution of the wastewater(Wang and Yang et al., 2016). After the adsorption, the number and area of peaks in Fig. 9 (b) were reduced, indicating that most fulvic-like compounds were adsorbed by OCMK-3. After two hours of reaction with OCMK-3/PS (Fig. 9 (c)), peaks were almost disappeared, indicating that the dissolved organic matter was almost removed after treatment, and the biodegradability of water was greatly improved(Yu and Feng et al., 2020). In addition, to further evaluate the mineralization of the organics, TOC contents of the wastewater were evaluated, with the results shown in Fig. 9 (d). It could be observed that only 5% of TOC were adsorbed, while about 75% of TOC were removed in OCMK-3/PS system, indicating the high TOC removal efficiency of the system. The results further convinced the high removal efficiency of the OCMK-3/PS system in both laboratory-prepared polluted water and practical wastewater.