English

• With water pollution mounting up, sewage treatment has attracted growing attention. Especially, wastewater discharged from hospitals and pharmaceutical industries contains residual drugs that can adversely affect the environment [1]. As a typical nonsteroidal anti-inflammatory drug, acetaminophen (ACE) is one of the most commonly used analgesics and antipyretics that has reached the highest detection rate in wastewater and surface water [2, 3]. As a result, ACE at different concentrations have been detected in some aquatic organisms. Although the traditional sewage treatment process can effectively remove ACE from wastewater, the process is neither economically nor environmentally friendly. Therefore, the development of novel methods for ACE removal is crucial. Recently, the sulfate radicals-based advanced oxidation (SR-AOPs) has been developed rapidly in sewage treatment, because of its high selectivity and powerful oxidation capacity [3-5]. SR-AOPs can degrade organic compounds by activating some sulfur-containing organic compounds and producing sulfur-oxygen radicals. Thus far, persulfate has been the most commonly used precursor because of its high stability and strong oxidation ability [6, 7]; however, it is expensive and can easily produce secondary pollution [8-10]. By contrast, sulfite (S(Ⅳ)) as one of the wet flue gas desulfurization (FGD) byproducts is not only cheaper with great production, but also applicable to degrade organic matter [11, 12]. Most of the used sulfites can be activated by transition metal compounds and exhibit favorable progress. Chen et al. attempted to use Fe(Ⅱ) species to catalyze sulfite and found that its degradability was comparable to that of activated persulfate and superior to that of a Fenton system [13]. The Fe(0)/Fe(Ⅲ)-S(Ⅳ) system is well-performed for pollutant degradation under weak acid conditions [14, 15]. Although the acidic environment is conducive to the survival of the Cr(Ⅵ)-S(Ⅳ)-O₂ system [16], Cr(Ⅵ) can be reduced to Cr(Ⅲ) during the activation of sulfites for degrading pollutants [12, 17, 18]. The Cu(Ⅱ)/Co(Ⅱ)-S(Ⅳ) system can afford effective degradation under alkaline conditions [19-21], because a high pH is more conducive for the formation of Co(Ⅱ)-OH/Cu(Ⅱ)-OH, thus producing more SO_x^•− [2, 14]. Despite these cases, the single metal-S(Ⅳ) system has the problems of pH dependency and low efficiency. Meanwhile, because SO₄^•− has high selectivity, it exhibits weak oxidizability for further mineralization, resulting in few intermediates, whereas ^•OH demonstrates a satisfactory mineralization ability with excellent nonselective oxidation [20]. To solve the aforementioned problems, we introduced photocatalysis into an S(Ⅳ) system to establish a heterogeneous catalytic system because the combination of ^•OH and SO₄^•− can efficiently degrade organic compounds.

  TiO₂, an attractive n-type semiconductor, has become a commonly used photocatalyst because of its low price, nontoxicity, environmental protection ability, and satisfactory thermal stability [22-24]. However, its further application is retarded by ineluctable shortcomings, such as wide band gap, poor utilization of solar light, and high recombination rate of photo-generated charge carriers [25]. Therefore, it is advisable to construct a heterojunction for achieving effective charge separation and strong redox ability [26, 27]. Cobalt-based materials (e.g., CoTiO₃, CoO, Co₃O₄ and Co(OH)₂) can be good candidates to integrate with TiO₂ due to their intrinsic activity and low cost [4, 28, 29]. Co₃O₄ is a typical p-type semiconductor which exhibits a strong response to visible light and capacity of activating sodium sulfite [30, 31]. Thus, compositing Co₃O₄ and TiO₂ seems to be an effective way to build advanced catalyst for photocatalytic activation of sodium sulfite.

  In the present study, we constructed a heterogeneous catalytic system by using Co₃O₄ and TiO₂ nanoparticles (Co₃O₄/TiO₂) to form a p-n heterojunction that can serve as a catalyst to degrade ACE through the photocatalytic activation of sodium sulfite. This catalyst could broaden the spectrum utilization range, produce more photogenerated holes and electrons, be beneficial for charge separation, and also efficiently activate sodium sulfite. Importantly, the charge transfer from Co₃O₄ to TiO₂ was verified by X-ray photoelectron spectroscopy analysis and theoretical calculations. We investigated the effect factors of component ratios, sulfite and ACE concentrations, catalyst dosages, and initial pH on the removal efficiency of ACE. Moreover, we identified the predominant active radicals by performing radical quenching experiments and deduced the degradation pathway by monitoring generated intermediate products. The degradation mechanism for ACE removal on Co₃O₄/TiO₂ was proposed via photocatalytic activation of sulfite process. Our work will open up new avenues for the removal of pharmaceutical wastewater by photocatalytic activation of sulfite system, predicting the potential application value in industry.

  Transmission electron microscopy (TEM) images of Co₃O₄/TiO₂ powders (Co/Ti = 1.5) with aggregating some small nanoparticles at different magnifications are shown in Figs. 1a–c to demonstrate the microstructure. The high-resolution TEM (HRTEM) image in Fig. 1c clearly displays the uniform lattice fringes of Co₃O₄ and TiO₂. The fringes with spacing of 0.203 nm correspond to (400) plane of Co₃O₄ phase (shown in the purple dashed frame) [32]. The fringe spacings with 0.351 nm as shown in the yellow part can be attributed to (101) facets of anatase TiO₂ [33]. The results indicate that the composite nanoparticles were consisted of mixed crystallites of Co₃O₄ and TiO₂. The TEM image of pure TiO₂ was shown in Fig. S1 (Supporting information). Additionally, the energy dispersive X-ray spectroscopy (EDS) element mapping images of Co₃O₄/TiO₂ indicated that Ti, Co, and O were the main elements (Fig. S2 in Supporting information). The elements composition and distribution images were performed using EDS of a selected area in Fig. 1d, indicating that three elements were distributed in the hybrid material.

  Figure 1

  

  Figure 1.  (a, b) TEM images, and (c) HRTEM image of Co₃O₄/TiO₂. (d) EDS mapping elements.

  DownLoad: Full-Size Img PowerPoint

  The phase composition of the Co₃O₄/TiO₂ was analyzed by X-ray diffractions (XRD) in Fig. 2a. The blue line represents the XRD pattern of as-synthesized Co₃O₄ without the addition of Ti(SO₄)₂ in the synthesis system, which is consistent with typical Co₃O₄ (JCPDS card No. 42–1467) [34]. The diffraction peaks at 2θ of 31.27°, 36.85°, 38.54°, 44.80°, 55.65°, 59.35°, and 65.23° could be assigned to the (220), (311), (222), (400), (422), (511) and (440) crystal facets of Co₃O₄, respectively. Compared with Co₃O₄, Co₃O₄/TiO₂ (the green line in Fig. 2a) demonstrates a diffraction peak of 36.85° at 2θ, which could be assigned to the (311) crystal face of Co₃O₄ [34]. And other diffraction peaks of Co₃O₄ are ambiguous, which could be due to the relatively low crystallinity of Co₃O₄ in the hybrid materials. Besides, the residual diffraction peaks were well indexed to the JCPDS card No. 21–1272 of anatase TiO₂ [35].

  Figure 2

  

  Figure 2.  (a) X-ray diffractions patterns of Co₃O₄/TiO₂ and Co₃O₄. XPS spectra of (b) full survey spectrum, (c) Ti 2p, (d) Co 2p, and (e) O 1s. (f) UV-DRS spectra of Co₃O₄/TiO₂, TiO₂ and Co₃O₄. (g) (αhν)²-hν curve of Co₃O₄/TiO₂, TiO₂ and Co₃O₄. (h) VB-XPS spectra of Co₃O₄/TiO₂, TiO₂ and Co₃O₄.

  DownLoad: Full-Size Img PowerPoint

  Elemental composition and chemical state of Co₃O₄/TiO₂ was determined by X-ray photoelectron spectroscopy (XPS). All spectra were calibrated by C 1s with binding energy of 284.8 eV. Apparently, the survey shows Ti, O, Co, and C element characteristic peaks (Fig. 2b). High-resolution spectrum of Ti 2p in Fig. 2c is split into two spin-orbit peaks corresponding to 458.8 eV (2p_3/2) and 464.5 eV (2p_1/2), respectively which are corresponding to Ti⁴⁺ [36, 37]. The 2p spectrum of Co in Fig. 2d shows a peak at 779.9 eV which is regarded as the characteristic peak of Co₃O₄ [38]; and the peaks at 781.1 eV and 779.9 eV were regarded as the Co²⁺ and Co³⁺ consistent with the reported Co₃O₄ [32]. As to O 1s spectrum, we have reasonably divided it into three peaks (Fig. 2e), indicating the presence of different types of oxidation species. The O1 and O2 peaks at 530.0 and 531.6 eV can be assigned to lattice oxygen of metal oxide and the hydroxyl group (Ti-OH), respectively [39, 40]. The O 1s spectra of Co₃O₄ confirmed this result (Fig. S3 in Supporting information). The binding energy of O3 is 533.12 eV, which is assigned to adsorbed oxygen of H₂O molecules [40]. Importantly, the binding energies of Co 2p and O 1s in Co₃O₄/TiO₂ positively shifted to 1.0 eV and 0.28 eV, respectively, relative to those of Co₃O₄ (Fig. S3). These phenomena indicated the charge redistribution between Co₃O₄ and TiO₂ at the heterojunction interface [41]. The interfacial charge redistribution of Co₃O₄/TiO₂ may influenced the adsorption energies of HSO₃^‒ or SO₃^2‒ of sulfite activation and the catalytic activity.

  The ultraviolet-visible diffused reflectance spectroscopy (UV-DRS) spectra of the synthesized materials were exhibited in Fig. 2f. The absorption wavelength in the UV region of pure TiO₂ was 387.5 nm [42], and the Co₃O₄ sample displayed an absorption wavelength in the visible-light region, which was similar with the previous studies [43]. Owing to the interfacial charge redistribution and transfer from Co₃O₄ to TiO₂, the absorption bands could widen the visible-light absorption region. In addition, the function (αhν)^1/n = A(hν − E_g) (where α is the absorption coefficient, h represents the Planck's constant, ν is the vibration frequency, E_g is the band gap, A is the proportional constant, and n is the nature of the sample transition). Owing to Co₃O₄ as the direct allowed transition material, n is 1/2, and (αhν)² is plotted against hν in Fig. 2g [44]. The bandgap values of the three materials were 2.37 eV for Co₃O₄/TiO₂, 3.2 eV for TiO₂, and 1.56 eV for Co₃O₄. The maximum valence band (M_VB) value was calculated using valence band XPS (VB-XPS) in Fig. 2h, and the M_VB values of Co₃O₄/TiO₂, TiO₂, and Co₃O₄ were determined to be 2.0, 2.5, and 0.34 eV, respectively.

  Besides, the charge transfer path and the formation mechanism of Co₃O₄/TiO₂ heterojunction was also investigated and presented in Fig. 3. The electronic properties were calculated based on Bader charge approach via density functional theory (DFT) calculations [26, 27]. Firstly, the work function Φ with applying to calculation of electrostatic potential in vertical direction of the monolayer was explored by the formula of Φ = V∞ − E_f. Here, V∞ is the vacuum potential in the vicinity of selected material, and E_f is the Fermi energy level. The computational results and values of TiO₂ and Co₃O₄ were shown in Figs. 3a and b, the calculated work functions of TiO₂ and Co₃O₄ were 3.08 and 5.598 versus vacuum level, respectively. One can know that when TiO₂ and Co₃O₄ form the heterostructure, the electron could flow into TiO₂ from Co₃O₄. The charge density difference in Fig. 3c also proved it. Distinctly, one can see the charge redistribution occurred at the interface of Co₃O₄/TiO₂ heterojunction. The yellow and cyan regions of the construction represented the accumulation and depletion of electrons, respectively. The planar-averaged electron density difference in Fig. 3c revealed the accumulation number of electrons. In addition, the calculated results indicated that the Co₃O₄ part near the interface displayed positively charge, while the TiO₂ part near the interface was negatively charge due to the migration of electrons. By Bader charge approach, it was found that there were electrons (2.041 e⁻) transfer from Co₃O₄ to TiO₂, resulting in the electron accumulation on TiO₂ and hole accumulation on Co₃O₄, which was accord with XPS analysis. Thus, it can be concluded the heterojunction can be efficiently constructed by combining the Co₃O₄ with TiO₂.

  Figure 3

  

  Figure 3.  The calculated work function and corresponding structural model of (a) (101) plane of TiO₂ and (b) (111) plane of Co₃O₄. (c) The planar-averaged electron density difference and side view of the charge density difference over the Co₃O₄/TiO₂ heterojunction. The yellow and cyan regions on the constructed represent accumulation and depletion of electrons, respectively.

  DownLoad: Full-Size Img PowerPoint

  Figs. 4a–c exhibited the kinetic data of the ACE degradation in different systems. In the absence of light, Na₂SO₃ (10 mmol/L) alone did not cause ACE degradation in 30 min. When other conditions remained unchanged, Co₃O₄ and Na₂SO₃ acted together in the system (no light), the pseudo-first-order rate constant (k₁) increased rapidly and the degradation efficiency increased by 69.58%, indicating that Co effectively activated sulfites [4, 20]. When Co₃O₄/TiO₂ and Na₂SO₃ were simultaneously used (no light, Co₃O₄/TiO₂+S(Ⅳ), the red line), the pseudo-first-order reaction rate constant (k₁) increased [45], and the degradation efficiency of the pollutants was 71.66% in 30 min. This may be because the presence of Co₃O₄/TiO₂ can activate Na₂SO₃ to generate SO_x^•‒ [9], thus promoting ACE degradation. In the absence of light, the adsorption efficiency of Co₃O₄/TiO₂ alone was 5.69%, indicating the weak adsorption in the system. Under illumination, Co₃O₄/TiO₂ (light + Co₃O₄/TiO₂) just removed 21.12% of the ACE in 30 min, which was lower than that the red line of Co₃O₄/TiO₂ + S(Ⅳ) (71.66%, no light). These results indicated that the degradation of ACE by O₂^•‒ and ^•OH generated by the separation of electrons and holes in Co₃O₄/TiO₂ under light irradiation was lower than that by SO_x^•‒. Therefore, when Co₃O₄/TiO₂ + S(Ⅳ) under the simulate solar light illumination (Light + Co₃O₄/TiO₂ + S(Ⅳ), the blue line), the degradation efficiency significantly improved up to 96.78%. The degradation efficiency via photocatalytic activation of sulfite of pure Co₃O₄ and TiO₂ was lower than that of Co₃O₄/TiO₂, indicating that the composite demonstrated excellent catalytic performance.

  Figure 4

  

  Figure 4.  (a) Effect of various processes on the degradation efficiency of ACE. (b) Degradation efficiency of various processes. (c) Pseudo–first-order rate constant (k₁) for ACE degradation under different systems. (d) Degradation efficiency of different catalyst under simulate solar light. (e) Pseudo–first-order rate constant (k₁) at different Co/Ti molar ratios. (f) ACE degradation in five continuous batch runs via light + Co₃O₄/TiO₂ + S(Ⅳ) system. Initial conditions: Co₃O₄/TiO₂ = 1 g/L, S(Ⅳ) = 10 mmol/L, ACE = 0.01 mmol/L, and pH 7.

  DownLoad: Full-Size Img PowerPoint

  The effect of different catalyst compositions on ACE degradation was investigated (Figs. 4d and e). The observed reaction rate constant k₁ increased first and then decreased when the Co/Ti molar ratio increased from 0 to 2, and the optimum molar ratio of Co/Ti was 1.5. The ACE removal rate by Co₃O₄/TiO₂ was better than that pure TiO₂ and Co₃O₄, indicating that hybrid structure could effectively improve catalytic performance. To examine the reusability of Co₃O₄/TiO₂ in S(Ⅳ) system, several cycle experiments were performed under optimum conditions. The degradation rate of ACE reached 80% in 20 min after five cycles in Fig. 4f, indicating that ACE still had favorable catalytic activity. However, the cyclic stability of Co₃O₄/TiO₂ decreased. This phenomenon is similar to that reported in the previous [2]. There are two reasons to explain this phenomenon: (1) Excessive ACE might occupy the active sites on the surface of catalyst to weaken the degradation performance [46]. (2) Co₃O₄/TiO₂ had the phenomenon of the metal leaching, which was contributed to shedding of the active site during the reaction process. The ICP results showed that the leaching amount of Ti⁴⁺ was 0.0 mg/L, but that of Co²⁺ ions was 2.0 mg/L. Table S1 (Supporting information) presented the comparison of the ACE removal efficiency under different oxidation processes, and the degradation performance of this study system was found to be superior to that of other processes. Meanwhile, the effect of pH, role of Na₂SO₃, dosage of catalyst, ACE concentration, influence of natural anions, and total organic carbon can be seen in Text S3 and Figs. S4–S7 (Supporting information).

  To elucidate the underlying mechanism for the photocatalytic activation of sulfite degradation of ACE, we added different quenchers to perform scavenging experiments to identify the primary active radicals as described in Figs. 5a and b. On the basis of the findings of previous studies, we used tert–butanol (TBA) and benzoquinone (BQ) as the scavengers of ^•OH and O₂^•−, respectively. Ethanol (EtOH) can act as a scavenger of ^•OH as well as react with SO₄^•− [2, 47]. The addition of TBA, BQ, and EtOH resulted in 14.78%, 36.74%, and 29.6% reductions in ACE degradation efficiency, indicating that species containing ^•OH, O₂^•−, and SO₄^•‒ were generated in the light+Co₃O₄/TiO₂+S(Ⅳ) system. After addition of ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) (as a quencher for h⁺), the ACE degradation efficiency significantly decreased from 96.78% to 32.17%. The above results deduced that ^•OH, h⁺, and SO_x^•‒ played vital roles in the degradation of ACE. Particularly, SO₃^•− could be generated by Co₃O₄/TiO₂ through photocatalytic activation of sulfite process.

  Figure 5

  

  Figure 5.  (a) Inhibition effects of radical scavengers on the degradation of ACE, [EtOH] = 10 mmol/L, [TBA] = 10 mmol/L, [BQ] = 1 mmol/L, [EDTA-2Na] = 1 mmol/L. (b) Pseudo-first-order rate constant (k₁) for the degradation of ACE under different quenching agents. (c) ESR spectra of DMPO-SO₃^•− in a nitrogen atmosphere under dark or light conditions. (d) ESR spectra of DMPO-^•OH and DMPO-SO₄^•− at different reaction times under light conditions. (e) ESR spectra of DMPO-O₂^•− at different reaction times, V_L: V_DMSO = 1:1 (V_L is the sampling volume of reaction solution). Initial conditions: Co₃O₄/TiO₂ = 1 g/L, S(Ⅳ) = 10 mmol/L, pH 7, DMPO = 100 mmol/L, no stirring, and exposed to air.

  DownLoad: Full-Size Img PowerPoint

  ESR was performed to detect the generation of free radicals in the light+Co₃O₄/TiO₂+S(Ⅳ) system with 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) as the capture agent. Fig. 5c displayed the ESR spectra under dark or light conditions and nitrogen purge conditions. A weak signal was detected in the dark, and the peak of DMPO-SO₃^•− was observed under light condition [2, 48]. The peak intensity increased with an increase in the duration of light irradiation, which may be attributable to ^•OH and SO₃^•−. Fig. 5d displayed the ESR spectra of exposed air under light conditions, and the peaks of DMPO-^•OH (the hyperfine coupling constants of ɑN = 1.488 mT and ɑH = 1.488 mT) [49] and DMPO-SO₄^•− (ɑN = 1.417 mT, ɑ_βH = 0.966 mT, ɑ_βH = 0.117 mT, and ɑ_γH = 0.078 mT) were observed [46, 50]. In the presence of DMSO, O₂^•− with the hyperfine coupling constants of ɑN = 1.371 mT, ɑ_βH = 1.005 mT, and ɑ_γH = 0.32 mT was found in Fig. 5e [51]. The results of ESR were in favorable agreement with those of free radical capture experiments. In addition, we also used furfuryl alcohol (FFA) as the scavenger of singlet oxygen (¹O₂). The quenching test indicated that the addition of FFA could not inhibit the degradation of ACE. Therefore, we conclude that ¹O₂ does not play a role in the light + Co₃O₄/TiO₂ + S(Ⅳ) system. And the ESR results showed that there was no signal of ¹O₂ in the system as shown in Fig. S8 (Supporting information). In conclusion, ^•OH, h⁺ and SO_x^•‒ may be the main active species in the degradation process. In addition, the possible removal pathways were proposed in Text S4, Table S2, and Fig. S9 (Supporting information).

  Finally, the ACE removal mechanism by using Co₃O₄/TiO₂ as catalysts via photocatalytic activation of sulfite was proposed according to the XPS analysis and Bader charge approach, which might be due to two aspects: (1) The heterojunction structure can broaden the utilized spectrum range, produce more photogenerated holes and electrons, and facilitate charge separation (Eqs. 1 and 2). The charge transfers induced electron accumulation on TiO₂ near the interface readily react with O₂ molecules to generate more O₂^•− (Eq. 3). And the O₂^•− and h⁺ further react with H₂O or OH to generate ^•OH (Eqs. 4 and 5). The rapid depletion of electrons could stimulate the generation of more holes in the valence band under light illumination, which could react with SO₃^2‒ to produce more SO_x^•‒ (Eqs. 6–9) [2, 14]. (2) Noticeably, the positively charged Co₃O₄ at interface was beneficial for the adsorption of HSO₃^‒ or SO₃^2‒ to generate reactive species (i.e., SO₃^•–, SO₄^•–, SO₅^•–) and further promoted sulfite activation performance.

  On the basis of the aforementioned results, the reaction mechanism of the light + Co₃O₄/TiO₂ + S(Ⅳ) system was deduced as follows (Scheme 1). When Co₃O₄/TiO₂ was added to the system, h⁺ and e⁻ were photogenerated on the surface excited by light illumination, and a series of active substances were directly or indirectly generated by the action of O₂ and H₂O in the solution (Eqs. 1–5) [23, 52]. A series of chain reactions of sulfite could generate SO_x^•−, such as light excitation (Eq. 6) [53, 54] or direct or indirect reaction with h⁺ or ^•OH (Eqs. 7–13) [9, 13, 23]. Under alkaline conditions, complexation occurred on the surface (Eqs. 14–17) [2, 20]. Finally, a series of reactive radicals were generated that could improve the photocatalytic activation of sulfite performance and degrade organic compounds. All equations can be observed as following parts (Eqs. 1–17).

  (1)

  (2)

  (3)

  (4)

  (5)

  (6)

  (7)

  (8)

  (9)

  (10)

  (11)

  (12)

  (13)

  (14)

  (15)

  (16)

  (17)

  Scheme 1

  

  Scheme 1.  Degradation mechanism of the Co₃O₄/TiO₂ through the photocatalytic activation of sulfite process.

  DownLoad: Full-Size Img PowerPoint

  We constructed a heterogeneous system involving Co₃O₄ and TiO₂ nanoparticles to form the p-n heterojunction interface as an efficient catalyst for degrading ACE through photocatalytic activation of sulfite. This catalyst could broaden the utilized spectrum range, produce more photogenerated holes and electrons, facilitate charge separation, and activate sulfite. Charge transfers induced electron accumulation on TiO₂ and hole accumulation on Co₃O₄ were verified by XPS analysis and DFT calculations. Optimal catalytic efficiency could reach 96.78% within 10 min based on the systematic study on experimental parameters (i.e., ACE and sulfite concentrations, catalyst ratio, catalyst dosage, and initial pH). We identified the predominant active radicals by performing radical quenching experiments and using the ESR capture technique which indicated that ^•OH, h⁺, and SO_x^•− were the main active species. The stability and recyclability of Co₃O₄/TiO₂ are confirmed by performing cycling tests. And the degradation pathway is determined by monitoring the generation of intermediate products. Finally, we propose the degradation mechanism of ACE by Co₃O₄/TiO₂ via photocatalytic activation of sulfite. Our work provides a new strategy to design efficient catalyst by engineering heterointerface for sulfite oxidation, which could guide the construction of catalysts. The high-efficient photocatalytic activation system could effectively utilize sulfite to degrade organic pollutants, predicting potential industrial applications for the removal of pharmaceutical wastewater.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (No. 51878273) and the Natural Science Foundation of Hebei Province (No. E2019502199).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.05.044.

  
  
• 1. [1]

     D. Guo, S. You, F. Li, Y. Liu, Chin. Chem. Lett. 33 (2022) 1–10. doi: 10.1016/j.cclet.2021.06.027

  2. [2]

     T. Luo, Y. Yuan, D. Zhou, et al., Chem. Eng. J. 363 (2019) 329–336. doi: 10.1016/j.cej.2019.01.114

  3. [3]

     J. Qi, J. Liu, F. Sun, et al., Chin. Chem. Lett. 32 (2021) 1814–1818. doi: 10.1016/j.cclet.2020.11.026

  4. [4]

     P. Hu, M. Long, Appl. Catal. B 181 (2016) 103–117. doi: 10.1016/j.apcatb.2015.07.024

  5. [5]

     M. Ma, L. Chen, J. Zhao, W. Liu, H. Ji, Chin. Chem. Lett. 30 (2019) 2191–2195. doi: 10.1016/j.cclet.2019.09.031

  6. [6]

     X. Lei, M. You, F. Pan, et al., Chin. Chem. Lett. 30 (2019) 2216–2220. doi: 10.1016/j.cclet.2019.05.039

  7. [7]

     J. Li, Y.J. Li, Z.K. Xiong, et al., Chin. Chem. Lett. 30 (2019) 2139–2146. doi: 10.1016/j.cclet.2019.04.057

  8. [8]

     L. Chen, M. Tang, C. Chen, et al., Environ. Sci. Technol. 51 (2017) 12663–12671. doi: 10.1021/acs.est.7b03705

  9. [9]

     W. Deng, H. Zhao, F. Pan, et al., Environ. Sci. Technol. 51 (2017) 13372–13379. doi: 10.1021/acs.est.7b04206

  10. [10]

      R. Dewil, D. Mantzavinos, I. Poulios, M.A. Rodrigo, J. Environ. Manag. 195 (2017) 93–99. doi: 10.1016/j.jenvman.2017.04.010

  11. [11]

      Z. Wang, F. Bai, L. Cao, et al., Chin. Chem. Lett. 33 (2022) 4766–4770. doi: 10.1016/j.cclet.2022.01.003

  12. [12]

      H. Dong, G. Wei, W. Fan, et al., Chemosphere 196 (2018) 593–597. doi: 10.1016/j.chemosphere.2017.12.194

  13. [13]

      L. Chen, X. Peng, J. Liu, J. Li, F. Wu, Ind. Eng. Chem. Res. 51 (2012) 13632–13638. doi: 10.1021/ie3020389

  14. [14]

      D. Zhou, L. Chen, J. Li, F. Wu, Chem. Eng. J. 346 (2018) 726–738. doi: 10.1016/j.cej.2018.04.016

  15. [15]

      P. Xie, Y. Guo, Y. Chen, et al., Chem. Eng. J. 314 (2017) 240–248. doi: 10.1016/j.cej.2016.12.094

  16. [16]

      Y. Yuan, S. Yang, D. Zhou, F. Wu, J. Hazard. Mater. 307 (2016) 294–301. doi: 10.1016/j.jhazmat.2016.01.012

  17. [17]

      B. Jiang, S. Xin, Y. Liu, et al., J. Hazard. Mater. 343 (2018) 1–9. doi: 10.1016/j.jhazmat.2017.09.015

  18. [18]

      B. Jiang, X. Wang, Y. Liu, et al., J. Hazard. Mater. 304 (2016) 457–466. doi: 10.1016/j.jhazmat.2015.11.011

  19. [19]

      L. Chen, T. Luo, S. Yang, et al., Environ. Chem. Lett. 16 (2018) 599–603. doi: 10.1007/s10311-017-0696-1

  20. [20]

      Y. Yuan, D. Zhao, J. Li, et al., Catal. Today 313 (2018) 155–160. doi: 10.1016/j.cattod.2017.12.004

  21. [21]

      Z. Liu, S. Yang, Y. Yuan, et al., J. Hazard. Mater. 324 (2017) 583–592. doi: 10.1016/j.jhazmat.2016.11.029

  22. [22]

      Q. Pang, G. Liao, X. Hu, Q. Zhang, Z. Xu, J. Inorg. Mater. 34 (2019) 219–224. doi: 10.15541/jim20180379

  23. [23]

      V. Vaiano, O. Sacco, M. Matarangolo, Catal. Today 315 (2018) 230–236. doi: 10.1016/j.cattod.2018.02.002

  24. [24]

      Q. Li, Y. Liu, Z. Wan, et al., Chin. Chem. Lett. 33 (2022) 3835–3841. doi: 10.1016/j.cclet.2021.12.025

  25. [25]

      Z. Wang, Z. Lin, S. Shen, W. Zhong, S. Cao, Chin. J. Catal. 42 (2021) 710–730. doi: 10.1016/S1872-2067(20)63698-1

  26. [26]

      P. Xia, S. Cao, B. Zhu, et al., Angew. Chem. Int. Ed. 59 (2020) 5218–5225. doi: 10.1002/anie.201916012

  27. [27]

      J. Qi, X. Yang, P. Pan, et al., Environ. Sci. Technol. 56 (2022) 5200–5212. doi: 10.1021/acs.est.1c08806

  28. [28]

      H. Wang, Q. Gao, H. Li, et al., Chem. Eng. J. 368 (2019) 377–389. doi: 10.1016/j.cej.2019.02.124

  29. [29]

      H. Li, Q. Gao, G. Wang, et al., Chem. Eng. J. 392 (2020) 123819. doi: 10.1016/j.cej.2019.123819

  30. [30]

      I. Rabani, R. Zafar, K. Subalakshmi, et al., J. Hazard. Mater. 407 (2021) 124360. doi: 10.1016/j.jhazmat.2020.124360

  31. [31]

      H. Zhang, J. Wang, X. Zhang, B. Li, X. Cheng, Chem. Eng. J. 369 (2019) 834–844. doi: 10.1016/j.cej.2019.03.132

  32. [32]

      L. Xu, Q. Jiang, Z. Xiao, Angew. Chem. Int. Ed. 55 (2016) 5277–5281. doi: 10.1002/anie.201600687

  33. [33]

      S. Liu, J. Huang, L. Cao, et al., Mater. Sci. Semicond. Proc. 25 (2014) 106–111. doi: 10.1016/j.mssp.2013.09.021

  34. [34]

      J. Zhong, A. Wang, G. Li, et al., J. Mater. Chem. A 22 (2012) 5656–5665. doi: 10.1039/c2jm15863a

  35. [35]

      Q. Wang, Y. Cui, R. Huang, et al., Chem. Eng. J. 383 (2020) 123142. doi: 10.1016/j.cej.2019.123142

  36. [36]

      Y. Shieh, Y. Chang, Thin Solid. Films 518 (2010) 7464–7467. doi: 10.1016/j.tsf.2010.05.025

  37. [37]

      R. Tang, S. Zhou, Z. Yuan, L. Yin, Adv. Funct. Mater. 27 (2017) 1701102. doi: 10.1002/adfm.201701102

  38. [38]

      J. Yang, H. Liu, W.N. Martens, R.L. Frost, J. Phy. Chem. C 114 (2010) 111–119. doi: 10.1021/jp908548f

  39. [39]

      Q. Yang, H. Choi, D.D. Dionysiou, Appl. Catal. B 74 (2007) 170–178. doi: 10.1016/j.apcatb.2007.02.001

  40. [40]

      X. Dong, B. Ren, X. Zhang, et al., Appl. Catal. B 272 (2020) 118971. doi: 10.1016/j.apcatb.2020.118971

  41. [41]

      G. Yang, Y. Jiao, H. Yan, Adv. Mater. 32 (2020) 2000455. doi: 10.1002/adma.202000455

  42. [42]

      M. Anpo, M. Takeuchi, J. Catal. 216 (2003) 505–516. doi: 10.1016/S0021-9517(02)00104-5

  43. [43]

      D. Barreca, C. Massignan, S. Daolio, et al., Chem. Mater. 13 (2001) 588–593. doi: 10.1021/cm001041x

  44. [44]

      C. Gao, Q. Meng, K. Zhao, et al., Adv. Mater. 28 (2016) 6485–6490. doi: 10.1002/adma.201601387

  45. [45]

      Q. Chen, L. Chen, J. Qi, et al., Chin. Chem. Lett. 30 (2019) 1214–1218. doi: 10.1016/j.cclet.2019.03.002

  46. [46]

      X. Tao, P. Pan, T. Huang, et al., Chem. Eng. J. 395 (2020) 125186. doi: 10.1016/j.cej.2020.125186

  47. [47]

      S. Guo, H. Tang, L. You, et al., Chin. Chem. Lett. 32 (2021) 2828–2832. doi: 10.1016/j.cclet.2021.01.019

  48. [48]

      F. Chen, Q. Yang, F. Yao, et al., Chem. Eng. J. 355 (2019) 624–636. doi: 10.1016/j.cej.2018.08.182

  49. [49]

      Y. Wang, S. Indrawirawan, X. Duan, Chem. Eng. J. 266 (2015) 12–20. doi: 10.1016/j.cej.2014.12.066

  50. [50]

      Y. Chen, G. Zhang, H. Liu, J. Qu, Angew. Chem. Int. Ed. 58 (2019) 8134–8138. doi: 10.1002/anie.201903531

  51. [51]

      C. Diaz-Uribe, M. Daza, F. Martinez, J. Photochem. Photobiol. A 215 (2010) 172–178. doi: 10.1016/j.jphotochem.2010.08.013

  52. [52]

      L. Yang, L.E. Yu, M.B. Ray, Water Res. 42 (2008) 3480–3488. doi: 10.1016/j.watres.2008.04.023

  53. [53]

      X. Yu, D. Cabooter, R. Dewil, Sci. Total Environ. 688 (2019) 65–74. doi: 10.1016/j.scitotenv.2019.06.210

  54. [54]

      M. Entezari, H. Godini, A. Sheikhmohammadi, A. Esrafili, J. Water Process Eng. 32 (2019) 100983. doi: 10.1016/j.jwpe.2019.100983

• 

  Figure 1  (a, b) TEM images, and (c) HRTEM image of Co₃O₄/TiO₂. (d) EDS mapping elements.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  (a) X-ray diffractions patterns of Co₃O₄/TiO₂ and Co₃O₄. XPS spectra of (b) full survey spectrum, (c) Ti 2p, (d) Co 2p, and (e) O 1s. (f) UV-DRS spectra of Co₃O₄/TiO₂, TiO₂ and Co₃O₄. (g) (αhν)²-hν curve of Co₃O₄/TiO₂, TiO₂ and Co₃O₄. (h) VB-XPS spectra of Co₃O₄/TiO₂, TiO₂ and Co₃O₄.

  下载: 全尺寸图片 幻灯片
  

  Figure 3  The calculated work function and corresponding structural model of (a) (101) plane of TiO₂ and (b) (111) plane of Co₃O₄. (c) The planar-averaged electron density difference and side view of the charge density difference over the Co₃O₄/TiO₂ heterojunction. The yellow and cyan regions on the constructed represent accumulation and depletion of electrons, respectively.

  下载: 全尺寸图片 幻灯片
  

  Figure 4  (a) Effect of various processes on the degradation efficiency of ACE. (b) Degradation efficiency of various processes. (c) Pseudo–first-order rate constant (k₁) for ACE degradation under different systems. (d) Degradation efficiency of different catalyst under simulate solar light. (e) Pseudo–first-order rate constant (k₁) at different Co/Ti molar ratios. (f) ACE degradation in five continuous batch runs via light + Co₃O₄/TiO₂ + S(Ⅳ) system. Initial conditions: Co₃O₄/TiO₂ = 1 g/L, S(Ⅳ) = 10 mmol/L, ACE = 0.01 mmol/L, and pH 7.

  下载: 全尺寸图片 幻灯片
  

  Figure 5  (a) Inhibition effects of radical scavengers on the degradation of ACE, [EtOH] = 10 mmol/L, [TBA] = 10 mmol/L, [BQ] = 1 mmol/L, [EDTA-2Na] = 1 mmol/L. (b) Pseudo-first-order rate constant (k₁) for the degradation of ACE under different quenching agents. (c) ESR spectra of DMPO-SO₃^•− in a nitrogen atmosphere under dark or light conditions. (d) ESR spectra of DMPO-^•OH and DMPO-SO₄^•− at different reaction times under light conditions. (e) ESR spectra of DMPO-O₂^•− at different reaction times, V_L: V_DMSO = 1:1 (V_L is the sampling volume of reaction solution). Initial conditions: Co₃O₄/TiO₂ = 1 g/L, S(Ⅳ) = 10 mmol/L, pH 7, DMPO = 100 mmol/L, no stirring, and exposed to air.

  下载: 全尺寸图片 幻灯片
  

  Scheme 1  Degradation mechanism of the Co₃O₄/TiO₂ through the photocatalytic activation of sulfite process.

  下载: 全尺寸图片 幻灯片
•