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. 2021 Nov 5;6(45):30698–30707. doi: 10.1021/acsomega.1c04604

Mechanism of Photodegradation of Organic Pollutants in Seawater by TiO2-Based Photocatalysts and Improvement in Their Performance

Hengtao Xu †, Zhe Hao †, Weihua Feng †, Ting Wang ‡,*, Yao Li ‡
PMCID: PMC8600626  PMID: 34805697

Abstract

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The mechanism of photodegradation of organic pollutants in seawater by TiO2-based catalysts irradiated by visible light was first explored by adding holes and free radical traps. The results showed that the photogenerated holes formed by the catalyst played a key role in the degradation of organic pollutants, regardless of whether the photodegradation occurred in seawater or pure water. Considering that the Yb-TiO2-rGO catalyst has a strong adsorption for organics, the salt ion almost did not interfere with the adsorption of pollutants by Yb-TiO2-rGO. Therefore, the degradation performance of Yb-TiO2-rGO did not remarkably change in the two water systems. For P25-ZN with a weak adsorption capacity for organics, several salt ions in the seawater hindered the contact of pollutants with the catalyst surface. Thus, the degradation rate of P25-ZN for phenol was significantly reduced. After the solvothermal reduction treatment for catalysts using ethylene glycol (EG) as the solvent, the increase in the Ti3+ content in the catalyst improved the visible-light response and activity of the catalyst. In addition, a small amount of EG grafted on the catalyst surface promoted the photocatalytic reaction process on the catalyst surface, thereby effectively resisting the interference of salt ions.

Introduction

Zero emission for pollutants in the industrial field is the main goal of environmental governance projects. This goal is achieved through the development of recent environmental science and technology.1,2 Zero emission (i.e., the discharged water should contain no pollutants) is required for wastewater discharge in China’s coal chemical industry.1,2 However, meeting the zero-emission requirements is still difficult for pollutant emissions in most industrial fields. For example, the textile industry and its upstream related chemical industries are among the main sources of industrial wastewater; the COD in textile wastewater is mainly required to be below 80 mg·L–1 to meet the discharge standard, and the goal of zero discharge has not been achieved.3,4 However, considering that benzene-ring-containing dye molecules are difficult to degrade, few dyes exist in the marine system because of the continuous action of photochemistry. The continuous accumulation of benzene-ring-containing compounds in the marine system may further form polycyclic aromatic hydrocarbons under long-term photochemistry.5,6 Therefore, a thorough treatment of wastewater containing low concentrations of refractory pollutants and treatment of refractory pollutants in seawater are the biggest challenges facing environmental governance.

TiO2-based heterogeneous semiconductor photocatalysts are promising materials for the degradation of highly toxic organic pollutants at low levels, and their removal could often exceed 90% because of thorough mineralization and nonselective decomposition.7−9 The application of heterogeneous photocatalysis for removing pollutants in seawater is challenging because of the interference of salt ions in seawater.10,11 The heterogeneous photocatalysis process by TiO2 in liquid systems is divided into three steps.12,13 First, pairs of photogenerated electrons and holes are formed by TiO2 irradiated by a light source. Then, the photogenerated electrons and holes combine with surface hydroxyl and adsorb water and oxygen molecules to form various free radicals. Finally, the holes and various free radicals complete the decomposition for organic matter.12,13 If the photocatalytic reaction occurs in seawater, a large number of salt ions interfere with the photodegradation of organic pollutants. Our previous works showed difficulty in avoiding ion interference with the photodegradation of TiO2-based catalysts in high-salinity wastewater, such as seawater, when the organic concentration in the wastewater was high or the adsorption capacity of the catalyst for organics was limited.14,15 The photodegradation mechanism of pollutants in seawater and other high-salt wastewater should be determined and the influence of inorganic ions on the photocatalytic reaction should be understood for the application of heterogeneous photocatalysis technology to remove pollutants in seawater.

Based on the results of previous works15−17 and literatures,12,13 two visible-light-responsive photocatalysts, namely, reduced P25 and Yb-doped TiO2-rGO, are employed in the present work. The photodegradation mechanism of phenol and methyl orange in seawater under visible-light irradiation is explored. Phenol and methyl orange are commonly employed as pollutants.12−14 The former represents solvents or intermediate products widely used in the chemical industry, and the latter is a typical dye in the textile industry. In addition, the wastewater from the chemical or textile industry is the main source of industrial wastewater, and it might enter the marine environment to generate pollution through land discharge and other methods. Then, the influence of salt ions in seawater on the photodegradation of pollutants that occurred on the catalyst surface or in bulk is investigated. Ethylene glycol (EG) with dihydroxyl was employed as a solvent for the solvothermal reduction of TiO2-based catalysts to improve their degradation efficiency in seawater.

Results and Discussion

Photodegradation Mechanism of Phenol in Water and Seawater by TiO2-Based Catalysts

The influence of different scavengers on phenol photodegradation excited by visible light was studied. Benzoic acid (as a scavenger for •O2–), tert-butanol (as a scavenger for •OH), and formic acid (as a scavenger for h+) were employed in the present work to explore the effects of scavenger addition on the photodegradation in pure water and seawater under visible-light irradiation. The photodegradation curves in Figures S1 and S2 (SI) show that the influence of scavenger addition on the photodegradation of phenol by the two catalysts is similar. After formic acid was added as a scavenger for h+, the photodegradation of phenol by the two catalysts was severely inhibited, followed by the addition of tert-butanol as a scavenger for •OH. The addition of benzoic acid as a scavenger for •O2– slightly inhibited the photodegradation. This finding indicated that the photogenerated holes generated by the two catalysts under the visible-light irradiation affected the phenol degradation in seawater. This conclusion is different from the belief that hydroxyl radicals are the key reactants in many heterogeneous photocatalysis studies.18,19

For further comparison, the photodegradation rate constants obtained by plotting ln(C/C0) against irradiation time t and the removal rate of phenol within 5 h are shown in Figure 1. The photodegradation rate constants of phenol by the two catalysts in pure water after different scavenger additions are also listed in these two figures for comparison. The photogenerated holes have a decisive effect on the pollutant degradation by the two catalysts, whether in pure water or seawater. Based on the comparison of the photodegradation in pure water and seawater, the activity of the two catalysts shows different changes. Under similar conditions, the activity of the Yb-TiO2-rGO catalyst in the two water systems does not differ much, indicating that the salt ion interferes less with the photodegradation process, thus supporting the results of the previous works.16,17 However, the P25-ZN catalyst shows a remarkable difference in the degradation of phenol in pure water and seawater. Therefore, the different interferences of salt ions in the organic degradation by the two catalysts are caused by the different adsorption properties of the two catalysts affecting the transfer of photogenerated holes and electrons.

Figure 1.

Figure 1

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Effect of scavenger addition on the photodegradation rate constant for phenol by TiO2-based photocatalysts: (a) Yb-TiO2-rGO and (b) P25-ZN.

The photodegradation mechanism of organic pollutants in seawater by TiO2-based catalysts is discussed here. During the heterogeneous photocatalysis process, the photocatalysts (like TiO2) first adsorb some pollutants to form an interaction with the catalyst surface, before irradiation.20,21 After light excitation, TiO2 first forms photogenerated holes and electrons.12,13,20 Then, the photogenerated holes directly decompose organic matter on the catalyst surface, or the holes react with adsorbed water and hydroxyl groups to form •OH and degrade organic matter.20,21 These two processes could be considered as surface reactions, as shown in Figures 2 and 3. In addition, the photogenerated holes and electrons may diffuse into the bulk phase to react with water or oxygen to generate •OH and •O2–, which further decomposes organic pollutants in the bulk phase of the wastewater. The salt ions interfere with the photodegradation in seawater mainly by inhibiting the contact between the organic pollutant and the catalyst surface or depressing the reaction of •O2– and •OH in the bulk phase with pollutants. The results from Figures S1, S2 and Figure 1 show that the bulk reaction is not the main reaction pathway. Thus, salt ions may interfere with the photodegradation by preventing the interaction with pollutants or adsorption for pollutants of the catalyst surface.

Figure 2.

Figure 2

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Photodegradation mechanism of phenol by Yb-TiO2-rGO irradiated by visible light.

Figure 3.

Figure 3

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Photodegradation mechanism of phenol by P25-ZN irradiated by visible light.

The Yb-TiO2-rGO catalyst has a very strong adsorption for organics because of the reduced GO as a carrier.16,17 Whether in pure water or seawater, most of the phenol is preferentially adsorbed onto the Yb-TiO2-rGO surface, thus directly participating in the subsequent surface reaction (Figure 2). The salt ion almost does not interfere with the adsorption of phenol (the adsorption rates of Yb-TiO2-rGO for phenol in pure water and seawater are 37.2 and 34.4%, respectively). Therefore, the degradation performance of Yb-TiO2-rGO does not remarkably change in the two water systems. P25-ZN has a weak adsorption capacity for organics (the adsorption rate of P25-ZN for phenol in pure water is 7.8%), but its small particle size (25 nm in diameter) could be well dispersed in pure water. Thus, phenol could make good contact with the P25-ZN surface to participate in the photodegradation reaction. However, a large number of salt ions in seawater hindered the contact of phenol with the catalyst surface (Figure 3). Thus, the degradation rate of P25-ZN for phenol is remarkably reduced, in which the adsorption rate of P25-ZN for phenol in seawater is 3.4% and decreases by more than half than that in pure water.

The photodegradation mechanism by TiO2-based catalysts in seawater is verified. Figure 4 shows the effects of scavenger addition on the degradation of methyl orange as a second pollutant in pure water and seawater by the two catalysts. The effects of scavenger addition on the photodegradation by the two catalysts were the same whether methyl orange or phenol was used. That is to say, the photogenerated hole excited by visible light is also the main reactive substance in the photodegradation of methyl orange by the two catalysts. A large amount of methyl orange is also preferentially adsorbed on the catalyst surface because of the good adsorption capacity of Yb-TiO2-rGO for organics (the adsorption rates of Yb-TiO2-rGO for methyl orange in pure water and seawater were 33.2 and 31.4%, respectively). Therefore, the performance of Yb-TiO2-rGO for degrading methyl orange in seawater was similar to that in pure water. However, the difference in the degradation activity of P25-ZN against methyl orange in seawater and pure water was significantly smaller than the change in the performance of P25-ZN to degrade phenol (the adsorption rates of P25-ZN for methyl orange in pure water and seawater are 8.1 and 7.7%, respectively). The photodegradation activity of P25-ZN for methyl orange in seawater is only slightly weaker than that for methyl orange in pure water. This finding may be attributed to the molecular diameter of methyl orange being approximately 5–8 nm, which is much larger than the diameter of each salt ion in seawater. Therefore, the salt ions in seawater cannot easily prevent the methyl orange molecules from contacting the P25-ZN surface effectively. Phenol molecules have a similar size as each salt ion, and many salt ions in seawater hindered the phenol molecules from effectively contacting the P25-ZN surface.

Figure 4.

Figure 4

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Effect of scavenger addition on the photodegradation rate constant for methyl orange by TiO2-based photocatalysts: (a) Yb-TiO2-rGO and (b) P25-ZN.

The photodegradation of methyl orange and phenol in some high-salinity wastewater by the two catalysts was determined to confirm the above-mentioned mechanism, and the corresponding photodegradation rate constants are listed in Figures S3 and S4 (SI). The results indicated that the different ions in the wastewaters showed similar effects on the photodegradation for the two pollutants and confirmed that the photodegradation of pollutants by the catalyst in seawater is mainly prevented by enhancing the adsorption capacity of the catalyst for organics. In the following experiments, the TiO2-based catalysts were treated by solvothermal reduction using EG as the reducing agent to enhance the adsorption capacity of the catalyst for organics, especially the adsorption for organics of reduced P25 catalysts.

Improvement on the Performance of Reduced P25 Composite Catalysts

The structural characterization of P25 composite catalysts treated by EG solvothermal reduction was performed. The transmission electron microscopy (TEM) images and high-resolution TEM (HRTEM) images of different reduced P25 in Figure S5 (SI) revealed that all reduced P25s by EG solvothermal process exhibited thinner disorder layer on the P25 surface because EG solvothermal reduction transformed some crystalline TiO2 to disordered amorphous TiO2 on the P25 surface. Comparison of the EG-reduced P25 under three temperatures illustrated that the thinner disorder layer on P25-EG-160 and P25-EG-170 were similar, whereas the disordered layer became obscure on the P25-EG-180 surface. The phenomenon was observed because some disordered amorphous TiO2 transferred back to crystalline TiO2 as the temperature in the EG solvothermal process increased. These findings were consistent with the results in the former works on P25 reduced by alcohol.14 The X-ray diffraction (XRD) patterns in Figure S6 indicate that the peaks of these EG-reduced P25 catalysts can be well indexed to anatase TiO2 (JCPDS No. 21-1272) and rutile TiO2 (JCPDS No. 21-1276), and no other crystal peaks are observed except those of TiO2, similar to the peaks of pristine P25.22,23

Figure 5 shows the Fourier transform infrared (FTIR) spectra of different P25 photocatalysts reduced by the EG solvothermal process. The FTIR spectra of pristine P25 are also illustrated in this figure for comparison. The absorption peaks corresponding to oxygenated functional groups on the P25 surface significantly decreased after solvothermal reduction by EG. The broadband in 3450 cm–1 was due to the stretching mode of hydrogen-bonded adsorbed water molecules or OH groups. The absorption peak at 1630 cm–1 was attributed to the bending mode H–O–H from the adsorbed molecular water.24−26 In addition, a new absorption peak (at 1560 cm–1), corresponding to the C–O–O stretching vibration and indicating the introduction of a few EG grafted on the P25 surface, appeared in the three EG thermal reduction samples.27,28

Figure 5.

Figure 5

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FTIR spectra of different P25 composite catalysts treated by EG solvothermal reduction.

Similar to previous results of P25 reduction,14,17 the X-ray photoelectron spectroscopy (XPS) profiles for Ti 2p in the three EG-reduced P25s and their corresponding deconvolution results (Figure S7 and Table 1) showed the peaks attributed to Ti 2p1/2 and Ti 2p3/2 of Ti4+ (at approximately 464.3 and 458.5 eV, respectively) and to Ti 2p1/2 of Ti3+ (at approximately 463.5 eV). The Ti3+ peak in P25-EG-180 slightly decreased because of the back transformation from amorphous TiO2 to crystal TiO2. The XPS profiles of O 1s and their corresponding deconvolution results are listed in Figure S8 and Table 1. The XPS profiles of O 1s in pristine P25 are also illustrated in Table 1 for comparison. EG solvothermal reduction decreases the peak intensities of the crystal lattice and nonlattice oxygen in the catalysts. The decrease in the nonlattice oxygen must be caused by the removal of some oxygenated functional groups on the catalyst surface and a few EG grafting. The decrease in crystal lattice oxygen in the reduced P25 could be attributed to the transformation of crystal TiO2 to amorphous TiO2, which would facilitate the visible-light response of reduced P25 catalysts according to the literatures29,30 and our previous works.14,15 The recrystallization of some amorphous TiO2 in P25-EG-180 increased the peak intensity of the crystal lattice oxygen.

Table 1. Data from the XPS Fitting Results of P25 Composite Catalysts Treated by the EG Solvothermal Reduction.

      O 1s
  Ti 2p
   nonlattice O
catalyst Ti3+ Ti4+ lattice O surface O adsorbed O
P25 0 100 92.2 4.5 3.3
P25-EG-160 8.3 91.7 71.8 15.6 12.6
P25-EG-170 8.9 91.1 83.7 8.7 7.6
P25-EG-180 6.8 93.2 89.2 10.8 0
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The UV–vis differential reflectance spectroscopy (DRS) of different reduced P25 catalysts and the corresponding plots of (αhν)1/2 versus band gap obtained from DRS are demonstrated in Figure 6. EG solvothermal reduction caused the formation of Ti3+. Thus, EG of the reduced P25 remarkably decreased, especially those of P25-EG-160 and P25-EG-170 (2.55 and 2.77 eV, respectively), indicating an obvious visible-light absorbance. The decrease in Ti3+ in P25-EG-180 resulted in weak visible-light absorbance.

Figure 6.

Figure 6

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UV–vis DRS (left) and the corresponding plots of (αhν)1/2 versus photon energy (right) of different reduced P25 catalysts treated by EG solvothermal reduction.

The photodegradation of tetracycline by EG of the reduced P25 composite catalysts in water and seawater is studied.

The degradation curves in Figure 7 show that the removal of hydrophilic oxygen groups and the grafting of EG on the P25 surface enhanced the capacity of the surface to adsorb phenol in pure water and seawater. For comparison, the degradation curves for pure P25 in water and seawater are listed in Figure 7. Considering the absence of visible-light response, the visible light could not irradiate pure P25 to degrade phenol both in pure water and seawater. Based on the comparison of the curves in Figure 1, the removal rates of P25-EG-160 and P25-EG-170 for phenol are significantly higher than that of P25-ZN catalyst, especially that in the seawater. This finding suggests that the interference in the degradation of phenol by P25-EG-170 and P25-EG-160 due to a large number of salt ions is obviously weakened. The P25-EG-180 catalyst has a strong phenol adsorption capacity, but its degradation performance is the weakest among the catalysts because of its weak visible-light response. Interestingly, the performance of P25-EG-180 for phenol in the seawater exhibits a slight difference from that in pure water. Therefore, improving the adsorption capacity of the catalyst for organics is a key factor to avoid the interference of salt ions in seawater in the photodegradation of pollutants with small molecules.

Figure 7.

Figure 7

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Photodegradation curves of different P25 composite catalysts treated by EG solvothermal reduction for phenol in water and seawater.

Improvement on the Performance of Yb-TiO2-rGO

The structure characterization for Yb-TiO2-rGO treated by EG solvothermal reduction is performed. As shown in the TEM images of Yb-TiO2-rGO catalysts by EG solvothermal reduction in Figure S9 (SI), a gray translucent yarnlike morphology indicates the rGO morphology and black dots attached to rGO are the TiO2 morphology. Similar to the previous results on Yb-TiO2-rGO reduced by alcohol solvothermal process, the TiO2 particles on the rGO surface can maintain a small particle size (about 10 nm) after the EG thermal reduction under 160 and 170 °C. However, some agglomeration of TiO2 on the rGO surface can be observed in Yb-TiO2-rGO-EG-180. This phenomenon may be caused by the grafting of more EG onto the rGO surface at high temperatures, which destroyed the binding between TiO2 and rGO and thus formed some TiO2 agglomerate.

Figure S10 (SI) shows the XRD patterns of the Yb-TiO2-rGO catalyst after EG solvothermal reduction under different temperatures. The diffraction peaks of the XRD patterns of the three catalysts were similar, and the diffraction peaks of anatase and rutile crystalline TiO2 appeared in all Yb-TiO2-rGO catalysts, consistent with the results in the previous work on Yb-TiO2-rGO reduced by the alcohol solvothermal process.16,17 This finding indicates that the EG solvothermal process could also promote the transformation of TiO2 from an amorphous state to a crystalline state.

The FTIR spectra in Figure 8 indicate that the absorption peaks in Yb-TiO2-rGO after EG solvothermal reduction under different temperatures are similar. A broad absorption peak at 3450 cm–1 could be attributed to −OH stretching on the Yb-TiO2-rGO surface. The other obvious peaks corresponded to C–C stretching at 1630 cm–1, C–OH stretching at 1420 cm–1, and Ti–O stretching at 500–650 cm–1. The appearance of the characteristic peak of C–O–O at 1560 cm–1 and the enhancement of the characteristic peak of C–O–C at 1160 cm–1 in the catalyst after EG solvothermal reduction suggested that a few EG could be grafted on the Yb-TiO2-rGO surface during the process.23,27 Apart from these, the IR band at 1100 cm–1 also corresponded to the stretching vibration of the −C–O– group caused by the grafted poly(ethylene glycol) (PEG) backbone. The higher the thermal reduction temperature, the more obvious the characteristic peaks are and the higher the amount of EG grafted onto the catalyst surface.

Figure 8.

Figure 8

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FTIR spectra of different Yb-TiO2-RGO catalysts treated by EG solvothermal reduction.

The XPS profiles of Ti 2p and O 1s and their corresponding deconvolution results are listed in Figures S11 and S12 (SI). The XPS profiles of Ti 2p in Figure S11 show that the peaks at 463.5 eV, which could be attributed to Ti 2p1/2 in the chemical valence state of Ti3+, are observed in the three catalysts after EG solvothermal reduction, apart from the characteristic peak of Ti4+ (at 464.3 and 458.5 eV, respectively). Yb3+ causes lattice distortion structures and anionic vacancies in the TiO2 lattice. The anionic vacancies (oxygen vacancies) might generate Ti3+ in catalysts, thus enhancing visible-light response, due to the impurity levels introduced by Yb3+. Figure S12 shows very obvious fitting peaks of the adsorbed and surface oxygen in the three catalysts.14,31 The corresponding deconvolution results of the three EG-reduced catalysts are listed in Table 2. The catalysts of alcohol solvothermal reduction are also listed in the table for comparison.

Table 2. Data from XPS Fitting Results of Yb-TiO2-rGO Catalysts Treated by EG Solvothermal Reduction.

      O 1s
  Ti 2p
   nonlattice O
catalyst Ti3+ Ti4+ lattice O surface O adsorbed O C–O
Yb-TiO2-rGO 8.6 91.4 11.5 18.4 27.0 43.1
Yb-TiO2-rGO-EG-160 10.2 89.8 12.1 21.6 28.1 38.2
Yb-TiO2-rGO-EG-170 14.7 85.3 11.8 23.7 27.8 36.7
Yb-TiO2-rGO-EG-180 9.2 90.8 13.6 25.8 23.1 37.5
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Table 2 shows that the Ti3+ content and the surface oxygen of EG-reduced catalysts are all higher than those of alcohol-reduced catalysts. The increase in the Ti3+ content was caused by the two hydroxyl groups in EG, thus showing a stronger reduction for catalysts than that for ethanol. The hydroxyl groups from EG grafted on the rGO surface increased the surface oxygen of the catalysts. The higher Ti3+ content in the catalysts favored the visible-light response of the catalyst. The grafting of EG on the catalyst surface could enhance the adsorption for organic pollutants of the EG-reduced catalysts. The data in Table 1 also show that when the temperature of EG solvothermal reduction reaches 180 °C, the Ti3+ content in the catalyst decreases, which may be caused by the formation of TiO2 agglomeration on the rGO surface.

Most works agree that the Ti3+ content in the catalyst determines its visible-light response and visible-light activity.32,33Figure 9 indicates the photocurrent response curves of different EG-reduced Yb-TiO2-rGO under visible-light excitation. Yb-TiO2-rGO-EG-170 with the highest Ti3+ content had the strongest photocurrent response under visible-light excitation, followed by Yb-TiO2-rGO-EG-160. The Ti3+ content in Yb-TiO2-rGO-EG-180 decreased because of the TiO2 agglomeration, thus weakening its photocurrent response.32,33

Figure 9.

Figure 9

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Photocurrent of different Yb-TiO2-rGO catalysts treated by EG solvothermal reduction.

The photodegradation of tetracycline by EG-reduced Yb-TiO2-rGO in water and seawater is studied. The photodegradation curves for phenol in Figure 10 show that the EG-reduced Yb-TiO2-rGO shows a significant increase in the adsorption capacity for phenol. Thereafter, the EG grafted on the catalyst surface improves the affinity between the catalyst and organics. The higher the grafting amount of EG is, the stronger the adsorption for the phenol of catalysts. In addition, Yb-TiO2-rGO-EG-170 and Yb-TiO2-rGO-EG-160 show improved catalytic activity in pure water and maintain high catalytic activity in seawater, in which the removal rates for both are above 95%, indicating a significant resistance to salt ion interference. This finding can be attributed to the increase in the Ti3+ content of the EG-reduced catalysts and the enhancement of the adsorption of phenol due to the grafting of EG on the catalyst surface. However, the TiO2 that agglomerated in Yb-TiO2-rGO-EG-180 reduces the Ti3+ content. Thus, its photocatalytic activity is significantly weaker than that of the two other catalysts, even if it has the strongest adsorption for phenol. To further confirm the removal rate of the spectrophotometric method, we measure the total organic carbon (TOC) in wastewater before or after photocatalysis by Yb-TiO2-rGO-EG-170 in pure water and seawater. The TOC removal rates for phenol by this catalyst in pure water and seawater are 91.8 and 93.4%, respectively, which are close to the removal rate obtained using the spectrophotometric method. This could confirm that the thorough mineralization of phenol forms CO2 and H2O.

Figure 10.

Figure 10

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Photodegradation curves of different Yb-TiO2-rGO catalysts treated by EG solvothermal reduction for phenol in (A) water and (B) seawater.

To confirm the stability and reusability of catalysts, we carried out five cycles of photodegradation experiments of phenol by TiO2-rGO-EG-170 under identical conditions. After each test, the powders were washed with deionized water and dried before the next test, and the results are listed in Figure S13. Based on the degradation curves in Figure S13, TiO2-rGO-EG-170 exhibited nearly no loss of activity in the five cycles of photodegradation experiments of phenol. This result highlighted the reusability of TiO2-rGO-EG-170 mesoporous materials and confirmed the grafted EG on the catalyst surface exited after five cycles of photodegradation for phenol.

Conclusions

The photodegradation mechanism of organic pollutants by two kinds of TiO2-based catalysts in pure water and seawater was studied by adding different free radical trapping agents under visible-light excitation. Whether the photodegradation occurred in seawater or pure water, the photogenerated holes formed by the catalyst played a key role in the degradation of organic pollutants. In seawater, salt ions mainly interfered with the contact between the pollutants and the catalyst surface, thus depressing the catalyst performance. The Yb-TiO2-rGO catalyst demonstrated very strong adsorption for organics caused by the reduced GO as the carrier. Whether in pure water or seawater, most phenols were preferentially adsorbed onto the Yb-TiO2-rGO surface. The salt ion almost did not interfere with the adsorption of phenol. Therefore, the degradation performance of Yb-TiO2-rGO did not change significantly in the two water systems. For P25-ZN with a weak adsorption capacity for organics, many salt ions in seawater hindered the contact of phenol with the catalyst surface. Thus, the degradation rate of P25-ZN for phenol was significantly reduced. When methyl orange had a molecular diameter of approximately 8 nm, which was larger than that of salt ions, the salt ions in seawater could not easily prevent the methyl orange molecules from contacting the P25-ZN surface effectively. The phenol molecules were of similar size as each salt ion, and many salt ions in seawater hindered the phenol molecules from effectively contacting the P25-ZN surface.

When the TiO2-based catalysts were treated by solvothermal reduction with EG as the solvent, the increase in the Ti3+ content in the catalyst after EG thermal reduction improved the visible-light response and photocatalytic activity. In addition, a small amount of EG grafted on the catalyst surface promoted the photocatalytic reaction process on the catalyst surface, thus effectively resisting the interference of salt ions. Moreover, improving the adsorption capacity of the catalyst for organics is a key factor to avoid the interference of salt ions in seawater on the photodegradation of pollutants with small molecules.

Experimental Section

Materials

Graphite powder (G, 8000-mesh) was purchased from Reagent Chemical Manufacturing (Shanghai, China). H2SO4, KMnO4, NaNO3, NaCl, Na2SO4, MgCl2, and CaCl2 were obtained from Shanghai Reagent Factory (Shanghai, China). Tetrabutyl titanate, EG, and Yb(NO3)3·5H2O were of analytical grade and obtained from Aladdin Shanghai Pure Crystalline Chemical Reagent (Shanghai, China). Gas nanometer TiO2 without porous P25 (21 nm, 50 m2·g–1) was obtained from Degussa, Germany. Analytical-grade absolute ethanol was purchased from Reagent Chemical Manufacturing (Shanghai, China), and it was distilled and then stored over 4 Å molecular sieves prior to use.

Preparation of TiO2-Based Powder Photocatalysts

Yb-Doped TiO2-rGO Powder Photocatalysts

The ANS preparation process was in accordance with the previous works.15,16 TiO2-rGO catalysts doped with 1.0 wt % Yb3+ achieved the optimal performance for the phenol degradation in seawater when 1.5 mL of water and 2.15 g of tetrabutyl titanate were used during the preparation of the doped TiO2-rGO samples. Therefore, these conditions were used to synthesize Yb-doped TiO2-rGO powder photocatalysts in the present work. Then, solvothermal treatment using ethanol as the solvent for the TiO2-GO catalyst was conducted at 170 °C for 24 h following a previously described method, and the as-prepared catalyst was designated as Yb-TiO2-rGO (the specific surface area is 50 m2·g–1, the particle size of TiO2 in this catalyst is about 20 nm). The Yb-doped TiO2-rGO powder photocatalysts treated by solvothermal reduction using EG as the solvent was designated as Yb-TiO2-rGO-EG-X, where X refers to the temperature in solvothermal reduction.

P25 Composite Powder Photocatalysts

The preparation process followed the previously described method in the former work.16 The P25 catalyst treated by hydrothermal reduction using Zn metal as a reducing agent at 160 °C for 6 h was designated as P25-ZN (the specific surface area was 50 m2·g–1, the particle size of TiO2 was about 25 nm). The catalysts treated by solvothermal reduction using EG as a solvent was designated as P25-EG-X, where X refers to the temperature in solvothermal reduction.

Characterization

The particle morphologies were characterized by transmission electron microscopy (TEM) using a Tecnai G2 F20 device. The crystallinity of the sample was obtained by X-ray diffraction (XRD) using a D/max-rA XRD instrument (XD-98) with Cu Kα radiation (1.5406 Å). The accelerating voltage and the applied current were 40 kV and 30 mA, respectively. The surface properties of the samples were investigated by X-ray photoelectron spectroscopy (XPS) with Al Kα X-ray (hm = 1486.6 eV) radiation operated at 150 W (Thermo ESCALAB 250). Diffuse reflectance ultraviolet spectra were taken using a Shimadzu UV-2450 spectropolarimeter fitted with a DRUV apparatus. Photoelectrochemical analysis was performed on a CHI660E workstation in a standard three-electrode configuration, with 0.5 M Na2SO4 solution as the electrolyte.

Photocatalysis Experiments

Simulated seawater composed of 2.5% NaCl, 1.1% MgCl2, 0.40% Na2SO4, and 0.16% CaCl2 dissolved in distilled water without CO2 was used as seawater.16,17

Photocatalysis experiments on powder catalysts were carried out by photodegrading phenol or methyl orange (concentration of 5.0 mg·L–1) in aqueous solutions under visible-light irradiation (a 30 W LED lamp with a 400 nm cutoff filter as the visible-light source).17 Following the procedures described in the literature,17 before irradiation, the suspension was stirred in the dark for 30 min to ensure the establishment of the adsorption/desorption equilibrium. At 30 min intervals, the sampled suspension was centrifuged, and the upper clear solution was extracted. A UV–vis spectrophotometer (Beijing, China) was used to measure the changes in tetracycline concentration (wavelength 357 nm) in the solution.

Acknowledgments

The authors gratefully acknowledge the financial support from the Ecological Environment and Fishery Resources Survey of Jiaojiang River Estuary Water Control Project, Investigation of Damaged Coastline in Ningbo City and Research on Remediation Countermeasures and the Natural Science Foundation of Zhejiang Province (Contracts LY19B060004).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c04604.

  • Effect of scavenger addition on the photodegradation of phenol by Yb-TiO2-rGO irradiated by visible light; effect of scavenger addition on the photodegradation of phenol by P25-ZN irradiated by visible light; photodegradation rate constants for phenol and methyl orange in different methyl orange wastewaters by Yb-TiO2-rGO; photodegradation rate constants for phenol and methyl orange in different wastewaters by P25-ZN; HRTEM images of different P25 composite catalysts treated by EG solvothermal reduction; XRD patterns of different P25 composite catalysts treated by EG solvothermal reduction; XPS profiles of Ti 2p in different P25 composite catalysts treated by EG solvothermal reduction; XPS profiles of O 1s in different P25 composite catalysts treated by EG solvothermal reduction; TEM images of different Yb-TiO2-rGO catalysts treated by EG solvothermal reduction; XRD patterns of different Yb-TiO2-RGO catalysts treated by EG solvothermal reduction; XPS profiles of Ti 2p in different Yb-TiO2-rGO catalysts treated by EG solvothermal reduction XPS profiles of O 1s in different Yb-TiO2-rGO catalysts treated by EG solvothermal reduction; and five cycles of photodegradation for phenol for 5 h using TiO2-rGO-EG-170 under visible-light irradiation are listed in Figures S1–S13 (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c04604_si_001.pdf (1.6MB, pdf)

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Supplementary Materials

ao1c04604_si_001.pdf (1.6MB, pdf)

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