Floating photocatalysts based on loading Bi/N-doped TiO₂ on expanded graphite C/C (EGC) composites for the visible light degradation of diesel

Abstract

Floating Bi–N–TiO₂ photocatalysts were synthesized using a novel sol–gel method grafted on expanded graphite C/C composites with high adsorption capacity and photocatalytic activity. The Bi–N–TiO₂/EGC were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), the N₂ adsorption/desorption method (BET), UV-vis diffuse reflectance spectroscopy (UV-vis DRS), and X-ray photoelectron spectroscopy (XPS). The SEM and XRD results revealed that all the composites had with mesoporous structures; the crystalline phases were mainly influenced by calcination temperature and anatase transformed into rutile at 800 °C. The N₂ adsorption/desorption method (BET) indicated that the specific surface area and pore size could be changed through adjusting the calcination temperature and Bi dosage. All Bi modified N–TiO₂ composites exhibited higher photocatalytic activity for degradation of diesel (≥53.7%) than N–TiO₂ under visible light irradiation. The composite Bi_1.0–N–TiO₂/EGC calcined at 550 °C with evenly distributed TiO₂ exhibited the highest activity (83.8%), which was attributed to the high surface area, red shift of absorptive light to visible light as well as improved efficient charge separation. The results of influence experiments, under different conditions of pH, salinity, emulsifier and humic acid, also evidenced that Bi–N–TiO₂/EGC composites are stable catalysts for the in situ remediation of diesel contaminated water.

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1. Introduction

Diesel is a complicated mixture of light petroleum products. It consists of alkanes, cycloalkanes and aromatic hydrocarbons.¹ Many components of diesel are insoluble in water and most of these cannot be easily degraded, such as branched and aromatic compounds.² Nowadays intensive transportation has resulted in highly frequent diesel leaks. The compounds of diesel can also be dispersed into the environment through the extraction process and refinery treatments. Diesel is seen as a hazardous fuel due to the negative effect on water quality and the ecological environment.

The control technologies of diesel polluted water have been developed including physical,³ chemical⁴ and biological treatment.^5,6 The most common method is to enclose the diesel polluted water area and pump it up or soak it up with absorbent materials.⁷ However, these technologies are low efficiency with high cost. In recent works, there have been many reports on photocatalytic degradation of diesel.^8–12 TiO₂ has been a hot topic in research for its thorough degradation of organic pollutants in water. It offers low toxicity, a stable structure and performance, ease of preparation. Most organics are thoroughly oxidized under UV lights.^13,14 But the wide band gap (3.2 eV) and rapid recombination of electron/hole pairs have limited the application in the actual treatment of pollutants. Many research groups have endeavored to improve the photocatalytic activity of TiO₂ under visible light including doping with non-metals (N,^15–19 P,^20,21 S,²² C,^23,24 F,^25,26 Cl²⁷) and transition metal ions.^28–35 Among the non-metals, N is a powerful candidate for doping TiO₂ catalysts with high catalytic activities. Asahi et al. proved that doping N into TiO₂ highly enhances its photocatalytic efficiency for the degradation of methylene blue and gaseous acetaldehyde under visible light in 2001.³⁶ Through transition metal ion doping, the separation of photoelectrons from vacancies can be effectively facilitated and the degradation efficiency is significantly increased.

Furthermore, TiO₂ suspends in the water during a typical photocatalytic reaction, and therefore separating photocatalyst from the clean water is required.³⁷ To resolve this problem, many researches have been carried out by immobilizing titanium onto various porous substrates, such as active carbon, fly ash, silica gel, quartz optical fibers.³⁸ Expanded graphite (EG) is a low density, wormlike porous material made from natural flake graphite intercalation and expansion.³⁹ It has a strong adsorption for oil with 86 g/g adsorption capacity within 1–2 minutes due to its unique porous structure. This is much higher than that of activated carbon, cotton, and the current standard linoleum.⁴⁰ Thus, expanded graphite can be a useful choice as a substrate loading TiO₂ photocatalyst for diesel degradation in the polluted water. The present studies have focused on the method of polymer carbonization through dipping EG into a thermosetting resin and carbonizing it under high temperatures to improve the EG mechanical strength and adsorptive ability. The final product is a C/C composite based on EG (EGC).^41–43

In this work, we used a sol–gel method to synthesize a novel floating bismuth/nitrogen co-doped TiO₂ photocatalyst carried on EGC based on its low density and multi-pore structure. The physicochemical property and morphology of Bi–N–TiO₂/EGC was studied through scanning electron microscope (SEM), X-ray diffraction (XRD), adsorption/desorption method, X-ray photoelectron spectroscopy (XPS) and UV-vis diffuse reflectance spectroscopy (DRS). The as-prepared floating photocatalytic composites offered a high and stable degradation rate for diesel under visible light.

2. Experimental

2.1 Materials

Phenol–formaldehyde resin (PF) was made in the lab. no. 0 diesel was bought from Tianbao petrol station of Sinopec in Hongkou district, Shanghai. All other reagents were purchased from Sinopharm Chemical Reagent Beijing Co., LTD (China). Deionized water was used in all experiments.

Expandable graphite (EG, AF99, 325 mesh, 99.995% carbon content) was obtained from Shanghai Human Composite Materials Manufacturing Co., LTD (China). Before usage, the original EG was calcinated at 550 °C in the muffle furnace for 30 s and kept in desiccator for use.

2.2 Synthesis of the photocatalytic composites

EGC was synthesized with 2 g EG and 1.5 g phenol–formaldehyde resin stirred in 50 mL absolute ethyl alcohol at 60 °C for 8 h. This was then heated at 105 °C for 8 h and calcinated at 800 °C for 1 h in the N₂. The phenol–formaldehyde resin was synthesized in the lab with phenol and formaldehyde (1.1 : 1 at%) and heated at 70 °C for 12 h.

The Bi–N–TiO₂/EGC photocatalysts were prepared with a sol–gel method. 18 mL tetrabutyl titanate, Bi(NO₃)₃·5H₂O (Bi : Ti = 0.5, 1.0, 1.5 at%) and 2 g EGC were added to 50 mL absolute alcohol adjusted with 1.0 mL HNO₃ under vigorous stirring to form solution A. 2.4 g urea was dissolved in 3 mL deionized water as the N precursor to form solution B. Solution B was added dropwise into solution A under vigorous stirring to form a gel precipitate. The gel precipitate was aged for 24 h to completely hydrolyze, then heated at 80 °C for 8 h and calcined at different temperatures (400, 550 and 800 °C) for 2 h in the N₂. After cooling, the composites were washed with ethanol and deionized water three times and dried. Then Bi–N–TiO₂/EGC photocatalysts were obtained. In addition, N–TiO₂/EGC was prepared according to the above procedure in the absence of BiNO₃ and calcined at 550 °C. Bi–TiO₂/EGC was prepared according to the above procedure in the absence of urea and calcined at 550 °C. For TiO₂, 18 mL tetrabutyl titanate was added to 50 mL absolute alcohol adjusted with 1.0 mL HNO₃ under vigorous stirring to form solution A. 3 mL deionized water was added dropwise into solution A under vigorous stirring to form a gel precipitate. The gel precipitate was treated following above procedure and finally calcined at 550 °C.

2.3 Characterization

Scanning electron microscopy (SEM, Zeiss Ultra 55) was used to investigate the surface morphology of the photocatalysts. X-ray diffraction (XRD, BrukerD8 ADVANCE) was tested to characterize the photocatalysts phase structure using Cu Kα radiation (40 kV, 40 mA) with a 0.01° step and 2.5 s step time over the range 10° < 2θ < 90°. Nitrogen adsorption–desorption isotherms were used to determine the Brunauer–Emmett–Teller (BET) specific surface areas and pore size distribution (Micrometrics, ASAP 2020). UV-vis diffuse reflectance spectroscopy (UV-vis DRS, Shimadzu UV-2550) was measured in the range 200–800 nm on a UV-vis-NIR scanning spectrophotometer equipped with an integral sphere using BaSO₄ as a reference. X-ray photoelectron spectra (XPS, PHI 5000C ESCA) was performed with Al Kα X-ray radiation. The binding energies were corrected by C 1s level at 284.6 eV as a reference to reduce the relative surface charging effect.

2.4 Photocatalytic activity

The photocatalytic activity of each composite was tested by degradation of diesel. Photocatalyst (0.02 g) was added to a 100 mL quartz photoreactor containing 50 mL of a 500 mg L⁻¹ diesel solution. A 500 W xenon lamp equipped with a UV cutoff filters (λ > 420 nm) was used as the visible light source. The lamp was cooled with a quartz cooling tube to maintain a constant temperature (25 °C) during the photocatalytic reaction. Before the lamp was turned on, the mixture was stirred for 30 minutes in the dark to reach the adsorption–desorption equilibrium. During irradiation, at given time intervals, the diesel samples (20 mL) taken from the mixture were filtered through a 0.45 μm filter to remove the photocatalysts and extracted with dichloromethane (1 : 1). The diesel concentration of sample extracts were analyzed by oil infrared spectrophotometry analyzer (Euro Tech, ET1200). Meanwhile the diesel samples of Bi_1.0–N–TiO₂/EGC (550 °C) (0 h, 3 h, and 5 h) were analyzed with GC/MS (Perkin Elmer, clarus 680-SQ8T).

3. Results and discussion

3.1 Characterization of composites

The SEM was used to characterize the morphologies of Bi–N–TiO₂/EGC composites (Fig. 1). Fig. 1(b–f) show that TiO₂ spherical structures were located at the edge or porous interlayer with particle size of nanometer scale. Fig. 1(c–f) show that dipping into the PF was beneficial to maintain EG porosity structure that could provide more sites for nano-TiO₂ and greatly strengthen photocatalysis. As calcination temperature increased, a reticular structure formed so that the TiO₂ densely distributed on the surface of the EG. As the doping amount of Bi increased, it was conducive to form pore structure, while TiO₂ agglomerated with elevated Bi concentrations.

Fig. 1 SEM of (a) EG (550 °C), (b) N–TiO₂/EG (550 °C), (c) N–TiO₂/EGC (550 °C), (d) Bi_0.5–N–TiO₂/EGC (550 °C), (e) Bi_1.5–N–TiO₂/EGC (550 °C) and (f) Bi_1.5–N–TiO₂/EGC (800 °C).

The XRD pattern of as-prepared composites (Fig. 2) revealed that the composite phase were mainly composed of graphite carbon and anatase phase of TiO₂ with peaks at 2θ values of 25.3, 36.9, 37.8, 48.0, 55.1, 62.7, and 75.0 corresponding to the (1 0 1), (1 0 3), (0 0 4), (2 0 0), (2 1 1), (2 0 4), and (2 1 5) planes, respectively. This indicated that the EGC carrier did not change the phase structure of TiO₂ and had a positive influence on nano-TiO₂ crystal formation. When the calcination temperature reached 800 °C, part of the TiO₂ phase of the composites transformed into the rutile structure with 2θ peak values of 27.5, 36.1, 44.1, 56.7, 62.9, and 69.0 corresponding to the (1 1 0), (1 0 1), (2 1 0), (2 2 0), (0 0 2), and (3 0 1) planes, respectively. According to Scherrer equation: D = Kλ/B cos θ (K = 0.89 and λ = 0.154056 nm), the calculated crystalline size of the photocatalysts are displayed in Table 1. Fig. 2 showed that as the calcination temperature increased from 400 °C to 800 °C for the Bi_0.5–N–TiO₂/EGC, the diffraction peaks became stronger and narrower, which was in accordance with the increased crystalline size of Bi_0.5–N–TiO₂/EGC in the Table 1. The crystalline size were14.7 nm, 10.7 nm and 12.2 nm for Bi_0.5–N–TiO₂/EGC (550 °C), Bi_1.0–N–TiO₂/EGC (550 °C) and Bi_1.5–N–TiO₂/EGC (550 °C), respectively. It indicated that the crystalline size decreased with Bi dosage 1.0% and increased at 1.5%. This found might be ascribed to the formation of Bi₂O₃ on the surface of TiO₂ particle. The difference in crystalline size as Bi dosage increased exhibited that doping Bi had a higher effect on particle growth. In a certain degree, the increase Bi dosage of the Bi–N–TiO₂/EGC photocatalysts inhibited anatase particle growth hence had a reduction in particle size.

Fig. 2 XRD pattern of Bi–N–TiO₂/EGC. A stands for anatase, R stands for rutile and C stands for graphite carbon.

Table 1 Physical structure parameters of floating Bi–N–TiO₂/EGC composites

Photocatalyst	Specific surface area (m^2 g^−1)	Pore size (nm)	Total pore volume (cm^3 g^−1)	Crystalline size (nm)
EG	31.3	12.5	0.098	—
Bi–TiO2/EGC	41.7	9.5	0.112	17.5
N–TiO2/EGC	46.5	8.8	0.102	17.6
Bi0.5–N–TiO2/EGC (400 °C)	70.8	7.3	0.133	13.3
Bi0.5–N–TiO2/EGC (550 °C)	75.4	6.8	0.128	14.7
Bi0.5–N–TiO2/EGC (800 °C)	111.7	5.6	0.154	16.7
Bi1.0–N–TiO2/EGC (550 °C)	109.3	3.7	0.097	10.7
Bi1.5–N–TiO2/EGC (550 °C)	43.7	9.7	0.094	12.2

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The co-affection of doped TiO₂ and EGC on specific surface area and pore-size distribution of the Bi–N–TiO₂/EGC photocatalysts were investigated by the nitrogen adsorption/desorption method (Table 1, Fig. 3). From Fig. 3, it could be found that the N₂ sorption isotherm of the composites were in accordance with type IV of hysteresis loops, indicating a typical mesoporous structure existed in the photocatalysts based on the IUPAC classification.⁴⁴ Based on the pore size distribution data (Table 1), the Bi–N–TiO₂/EGC photocatalysts had narrow distribution of 3–15 nm. According to Table 1, the BET surface area and the total volume of the composites increased with calcination temperature and they also could be improved by adjusting Bi doping amount. Compared to EG and other photocatalysts, the Bi_1.0–N–TiO₂/EGC (800 °C) showed the highest BET specific area (160.877 m² g⁻¹) with the total pore volume of 0.135 cm³ g⁻¹. This was likely because at 800 °C, the EG had a high bulk density;⁴⁵ as for Bi–N–TiO₂/EGC photocatalysts, it is able to increase the BET specific surface area though adjusting Bi doping amount. Table 1 also indicated that at the same temperature excess Bi particles blocked the micropores of the N–TiO₂/EGC. Importantly, the micropores nanostructure and larger specific surface area could beneficial for reactant molecules to the photo-active sites, which favored to strengthen the photocatalytic activity.⁴⁶

Fig. 3 Nitrogen sorption isotherms and the corresponding pore size distribution curves (inset) of the (a) Bi_0.5–N–TiO₂/EGC and (b) Bi–N–TiO₂/EGC (550 °C).

The UV-vis diffuse reflectance spectra of the Bi–N–TiO₂/EGC catalysts was shown in Fig. 4. As shown in Fig. 4, TiO₂ (550 °C) had no significant absorbance in the visible light region (λ > 420 nm) while EG had a weak visible light absorbance. With the introduction of Bi and nitrogen, the composites displayed an absorbance peak at about 400 nm, which showed a little red shift in their absorption above 400 nm. This might because that the formation of N–Ti by doping nitrogen into the nanocrystal structure could cause a lower energy barrier, which made it easier to excite N2pπ electrons into the Ti d_xy than Opπ electrons.^47,48 It was also found that the absorption intensity could be clearly boosted with increased Bi amount, which implied the enhancement of the photocatalytic efficiency.

Fig. 4 UV-vis diffuse reflection spectra of the Bi–N–TiO₂/EGC composites.

The XPS spectra of the Bi–N–TiO₂/EGC composites indicated that Bi_1.5–N–TiO₂/EGC (550 °C) contained C, O, Ti, Bi, and N (Fig. 5(a)). The N 1s signal in Bi_1.5–N–TiO₂/EGC showed that N had integrated into the TiO₂ crystal. The N 1s spectra of Bi–N–TiO₂/EGC had three N peaks in the composite (Fig. 5(b)): anionic N⁻ in Ti–O–N (BE = 398.9 eV); substitutional N in O–Ti–N linkages (BE = 399.5 eV), of which the peak intensity might describe the visible light catalytic activities;^49,50 and N-oxides of pyridinic-N (BE = 403.5 eV) that were bonded to two carbon atoms and one oxygen atom.⁵¹ The Ti 2p of the Bi–N–TiO₂/EGC XPS spectra (Fig. 5(c)) had two peaks at binding energies near 459.1 and 464.9 eV that were attributed to Ti 2p3/2 and Ti 2p1/2, which indicated that Ti existed mainly in the form of Ti⁴⁺.⁴⁶ And a slight shift to lower energy was observed as the doping Bi amount increased in the composites in Fig. 5(c), which expected to enhance the photocatalytic efficiency. The O 1s peaks Fig. 5(d) were 530.4 and 533.4 eV corresponding to two forms of oxygen: Ti–O and surface hydroxyls.⁵² The Bi 4f XPS spectra in Fig. 5(e) presented two peaks with binding energies of 159.6 and 164.8 eV.⁵³ These corresponded to Bi 4f7/2 and Bi 4f5/2, respectively. The two peaks illustrated that the Bi species exists as Bi³⁺ and the shift for Bi 4f7/2 indicated that a small part of the Bi³⁺ changed to higher Bi⁴⁺ to substitute for Ti⁴⁺ in the lattice.

Fig. 5 XPS spectra of the Bi–N–TiO₂/EGC composites.

3.2 Photocatalytic activity

3.2.1 Photocatalytic test of diesel. The Bi–N–TiO₂/EGC photocatalytic activity was evaluated under visible light through degradation of diesel. For the porous EG, as-prepared photocatalysts maintained their lipophilicity, hydrophobicity, and low density—these gave them great absorptive capacity for diesel when they floated on the water surface. During the dark adsorption, the adsorption rate was varied between 28.8 and 68.2 (Fig. 6). This result was mainly attributed to the specific surface area of the Bi–N–TiO₂/EGC photocatalysts, which could be adjusted by changing the calcination temperature and dosage of Bi. According to the BET results, the specific surface areas of Bi_0.5–N–TiO₂/EGC (800 °C) and Bi_1.0–N–TiO₂/EGC (550 °C) were 111.7 and 109.3 m² g⁻¹, respectively, which was 2.6 and 2.5 times compared with Bi_1.5–N–TiO₂/EGC (550 °C) (43.7 m² g⁻¹). Therefore the higher surface area assured the high adsorption for more diesel.

Fig. 6 Diesel dark adsorption of the Bi–N–TiO₂/EGC composites. C_i is the initial concentration of diesel.

The photocatalytic degradation of diesel was well adapted to the Langmuir–Hinshelwood first order kinetics as follows:

where k is the rate constant (min⁻¹), C₀ is the equilibrium concentration of diesel (after dark adsorption), and C is the given t time concentration of the diesel. From Table 2, a good liner relationship between the concentration ratio of diesel and photocatalytic degradation time for Bi–N–TiO₂/EGC could be found hence the photocatalytic reactions of diesel followed the L–H model.

Table 2 The photocatalytic degradation and kinetic rate constants for diesel

Photocatalyst	k (min^−1)	R^2
Bi–TiO2/EGC (550 °C)	0.0019	0.9945
N–TiO2/EGC (550 °C)	0.0027	0.9964
Bi0.5–N–TiO2/EGC (800 °C)	0.0044	0.9957
Bi0.5–N–TiO2/EGC (400 °C)	0.0029	0.9969
Bi0.5–N–TiO2/EGC (550 °C)	0.0042	0.9967
Bi1.0–N–TiO2/EGC (550 °C)	0.0062	0.9977
Bi1.5–N–TiO2/EGC (550 °C)	0.0050	0.9980

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Fig. 7 showed that Bi–N–TiO₂/EGC had a high photocatalytic degradation rate for diesel after 5 hours (≥53.7%). After dark adsorption, the Bi_1.0–N–TiO₂/EGC (550 °C) displayed the highest photocatalytic degradation rate (83.8% with rate constant 0.0062 min⁻¹) compared with other catalysts. As for the calcination temperature, the Bi_0.5–N–TiO₂/EGC (800 °C) exhibited the highest degradation rate among three calcination temperatures but was very close to that calcined at 550 °C. The Bi-doping amount remarkably affected the photo-degradation of diesel. The Bi doping ratio of 1.0 (Bi : Ti, at%) catalysts showed best photocatalytic activity than 0.5 and 1.5 (Bi : Ti, at%). These results could be ascribed to following factors including: (1) the lower charge carrier recombination due to the doping Bi ions. Based on the XPS analyse, Bi mainly existed as Bi³⁺, partially Bi⁴⁺, which contributed to forming a hybridized valence band of Bi 6s and O 2p and narrowing band gap to intensify absorption of visible light absorbance. But as for Bi doping ratio of 1.5, over amount ions would take up the surface of TiO₂ further reduce probability to be excited by photon quantum, which led to the low degradation rate of diesel under visible light;⁵⁴ (2) the increased specific surface area, which normally provided facilitate pathways for reactants accessing to the active centers of photocatalysts. Whereas observed from Table 1, Fig. 6 and 7, the adsorption of the reactant with high specific surface was not well in accordance with the increase of the photocatalytic degradation. The Bi_1.0–N–TiO₂/EGC (550 °C), which was most active compared to other co-doping photocatalysts, displayed a lower specific surface area and Bi_0.5–N–TiO₂/EGC (800 °C) had the highest specific area with lower photocatalytic efficiency. It might be due to that at 800 °C, the agglomerated Bi–N–TiO₂ could not efficiently receive light photons to be excited to generate electron hole pairs even the dominant recombination occurred to decrease the photocatalytic activity.⁵⁵ Moreover, a high adsorption of diesel on to the photocatalysts might highly cover the surface of the Bi–N–TiO₂, which cut off part of the visible light and decrease the yield of electron hole pairs further weaken the photocatalytic activity of the catalyst with high adsorption capacity.⁵⁶ Hence, the reasonable extent adsorption of diesel was beneficial to enhance the photocatalytic activity.

Fig. 7 Photocatalytic activity of the Bi–N–TiO₂/EGC composites for the degradation of diesel.

Fig. 8 showed the GC/MS analysis of diesel sample (reaction time of 3 and 5 h) degraded by Bi_1.0–N–TiO₂/EGC (550 °C). After 3 h of photocatalysis, the small molecular weight alkanes was easily degraded and part of the long alkanes, cyclones, and aromatic hydrocarbons decomposed into short chain hydrocarbons. After 5 h, the characteristic peak intensity of long alkanes, cyclones and aromatic hydrocarbons weakened as the reaction proceed.

Fig. 8 GC/MS spectra for photocatalytic degradation of diesel under visible light.

The enhancement of Bi–N–TiO₂/EGC photo-degradation of diesel under visible light is likely based on the reasons as follows. Firstly, the photo-degradation of catalysts partly depends on the moderate degree of adsorption capacity. Being loaded on EGC, the as-prepared catalysts with higher specific surface area could provide more efficient way for molecules in the diesel close to photocatalytic active sites. Secondly, floating on the surface of water ensures the as-prepared composites efficiently utilize the visible light. Thirdly, in the diesel-polluted water, the separated electron and holes could react finally to form strong oxidizing ˙OH, by which the component of diesel could be degraded to carbon dioxide and water. Substituting O sites with N made it easier for electron to transport from band of N2pπ to the Ti d_xy under visible light. Furthermore, doping Bi facilitated electron separation through capturing electron by Bi⁴⁺/Bi³⁺ energy level.⁵⁴ Then the captured electron on the surface of Bi₂O₃ could react with oxygen to form superoxide radicals. Subsequently the formed superoxide radicals react in water and finally form hydroxyl radicals, which obviously improve the photocatalytic efficiency.

3.2.2 Effect of initial pH. The solution pH is an important factor during TiO₂ photocatalytic degradation. The photocatalysis efficiencies of diesel by Bi–N–TiO₂/EGC under visible light at different pH are shown in Fig. 9. It could be observed the solution pH had an obvious influence on the photocatalytic degradation of diesel. The acid (pH = 2) or alkali (pH = 12) condition reduced the diesel removal rate. It is widely recognized that the point of zero charge (pzc) of TiO₂ is 6.4. During photocatalysis, TiO₂ exists as TiOH₂⁺ at pH < pzc while TiO⁻ at pH > pzc.⁵⁷ During the degradation of diesel, overly acidic or alkaline solution would cause particles to agglomerate finally reduce light intensity. Photocatalytic particles are well dispersed under neutral or even weak acidic conditions so that an improved photocatalysis would happen in the nearly neutral solution by formation of strong oxidation of ˙OH.

Fig. 9 The influence of pH on diesel photocatalytic degradation by Bi–N–TiO₂/EGC with initial diesel concentration of 500 mg L⁻¹.

3.2.3 Effect of salinity. Different sea areas have different salinity with highest (S = 41) of the Red Sea and the mean salinity is 37.⁵⁸ In this work the simulated sea water was prepared according to Table 3. The photocatalytic efficiencies of diesel under visible light in the simulated seawater are shown in Fig. 10. The degradation rate of diesel decreased in the salinity-simulated sea water under visible light (20.0% for Bi_0.5–N–TiO₂/EGC, 12.0% for Bi_1.0–N–TiO₂/EGC, 28.6% for Bi_1.5–N–TiO₂/EGC). With relative high salinity, there might exist the competitive adsorption between cationic ions and diesel on the surface of Bi–N–TiO₂/EGC. The competitive ions might occupy the active sites hence decrease the photocatalytic efficiency of diesel.

Table 3 The compounds of simulated sea water

Salt	Amount of salt (g)/1000 g solution
NaCl	25.122
MgCl2·6H2O	10.738
Na2SO4	4.01
KCl	0.699
SrCl2·6H2O	0.024
KBr	0.1
H3BO3	0.025
NaHCO3	0.18
CaCl2·2H2O	9.27

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Fig. 10 The influence of salinity on the photocatalytic degradation of diesel by Bi–N–TiO₂/EGC with initial diesel concentration 500 mg L⁻¹. (a) Bi_0.5–N–TiO₂/EGC (550 °C), (b) Bi_1.0–N–TiO₂/EGC (550 °C) and (c) Bi_1.5–N–TiO₂/EGC (550 °C).

3.2.4 Effect of polyoxyethylene sorbitan monooleate (PSM). In the production and transportation of diesel, polyoxyethylene sorbitan monooleate (PSM) is used as an emulsifier and dispersant. Fig. 11 shows the effect of PSM on the photocatalytic degradation of diesel by Bi–N–TiO₂/EGC. It indicated that when PSM was added in the diesel, for Bi_0.5–N–TiO₂/EGC (550 °C), Bi_1.0–N–TiO₂/EGC (550 °C), and Bi_1.5–N–TiO₂/EGC (550 °C), the photo-degradation rate first increased to 71.6%, 96.4%, and 65% and then decline to 58.1%, 43.2%, and 50.1%, respectively. This is likely that below the critical micelle concentration (C_PSM < CMC), the photo-degradation had a positive correlation with increased dissolved diesel for the adding of PSM. While at C_PSM ≥ CMC, the reduced active photo quantum which was blocked by excessive amounts of micelle would decrease photo-degradation of diesel.

Fig. 11 The influence of PSM on diesel photocatalytic degradation by Bi–N–TiO₂/EGC with initial diesel concentration 500 mg L⁻¹.

3.2.5 Effect of humic acid (HA). Humic acid exists widely in natural water. It is a significant component of many organisms with concentrations ranging from 0–20 mg L⁻¹.⁵⁹ Now it can serve as a photosensitizer and facilitate photo-degradation of organisms. Fig. 12 shows the influence of HA on diesel photocatalytic degradation by Bi–N–TiO₂/EGC. As shown in Fig. 12, the photo-degradation of diesel was strengthened at a lower concentration of HA (2, 4 and 6 mg L⁻¹), and weakened at ≥8 mg L⁻¹. In one aspect, HA shows a strong absorbance in the visible light region and a relatively high concentration of HA would affect water transmittance; in another aspect, excessive HA might compete with diesel for photocatalytic active sites on the surface of the Bi–N–TiO₂/EGC.

Fig. 12 The influence of HA on diesel photocatalytic degradation by Bi–N–TiO₂/EGC with initial diesel concentration 500 mg L⁻¹.

4. Conclusion

A novel EGC composites loading Bi–N–TiO₂ photocatalyst was prepared via a sol–gel method. The synthesized floating Bi–N–TiO₂/EGC catalysts have enhanced photocatalytic activity and stability for degradation of diesel under visible light. The diesel photo-degradation obeyed the first-order Langmuir–Hinshelwood kinetics model. Bi_1.0–N–TiO₂/EGC (550 °C) with Bi : Ti at 1.0 at% and calcination temperature of 550 °C was the optimized catalyst in this study. The positive synergistic photocatalysis were found between the Bi–N–TiO₂ and the EGC carrier, owing to high adsorption and efficient electron transportation. Considering the flotation and recycling, Bi–N–TiO₂/EGC is a potentially promising catalyst for the in situ treatment of diesel and even other organic pollutants.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21277097, 21377095 and 51179127) and the Fundamental Research Funds for the Central Universities (No. 0400219270).

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