EP composites

Colloids and Surfaces A 583 (2019) 123996

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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa

The photocatalytic degradation of diesel by solar light-driven floating BiOI/ EP composites ⁎

Hongxuan Qiua, Hu Jiwenb, , Run Zhanga, Wenting Gonga, Yichang Yuc, Gao Hongwena,

T

⁎

a

College of Environmental Science and Engineering, Tongji University, Shanghai, 200092, China School of Environmental and Chemical Engineering, Shanghai University, Shanghai, 200444, China c Research Center of Environmental Engineering Technology, Chongqing Research Academy of Environmental Science, Chongqing, 401147, China b

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Expanded perlite BiOI Diesel degradation Photocatalyst

Nowadays, oil spills have occurred frequently, which is seriously harmful to the aquatic wildlife and the ecological environment. The photocatalysis is one important approach to degrade the diesel. The diesel floats on the water surface and is difficult to be removed by direct photocatalysis. However, floating photocatalytic composites can solve this problem. We used one facile solvothermal synthesis method to deposit the metal semiconductor (BiOI) on the expanded perlite (EP) and obtained one efficient photocatalysis nanocomposites, which could float on the water surface with good efficient adsorption and excellent degradation of the diesel. And the experimental results shows a good photocatalytic performance, which is up to 85% of removal rate of the diesel after irradiation for 2 h under simulated sunlight. In addition, this composite is a recycled material (reusing for 5 times) and eco-friendly material with low biotoxicity. As a result, the proposed combination of adsorption and photocatalysis will provide a novel strategy to greatly facilitate the treatment of diesel wastewater.

1. Introduction In recent decades, the massive exploitation of petroleum energy has brought both positives and negatives to human life and environment. For example, the rapid development of petroleum industry could

⁎

provide energy for production and life and drive the fast growth of economy. Meanwhile, petroleum products leakage during the process of exploitation could seriously affect the marine ecosystem balance. According to incomplete statistics, between 2010 and 2015, only 33,000 tons of oil tankers have been leaked worldwide [1], which has

Corresponding authors. E-mail addresses: [email protected] (J. Hu), [email protected] (H. Gao).

https://doi.org/10.1016/j.colsurfa.2019.123996 Received 1 September 2019; Received in revised form 16 September 2019; Accepted 17 September 2019 Available online 18 September 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.

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exceeded the maximum environmental carrying capacity [2], causing serious pollution and damage to the environment [3]. Till now, many ways are applied to remove the oil spill in the natural water, including physical (using oil booms, skimmer or sorbents) [4], chemical (dispersants or elastomizers, burning in site) [5] and biological methods (bacteria, fungi, archaea and algae) [6]. Generally speaking, every method has its pros and cons. The large-scale treatments of spill oil by using physical methods need generally a high unit price and great manpower [7]. And bioremediation by using bacteria, fungi, archaea or algae requires extensive time, and the removal effect are easily influenced by the external environment [8]. The spill oils in the natural water may need one rapid and economically attractive method. Hence, photocatalysis method becomes one common technique to deal with the oil spills because of its fast and excellent efficiency for the degradation of oil. At present, photocatalytic materials used for oil wastewater treatment mainly focus on TiO2 [9,10], ZnO [11], etc. TiO2 has a wide band gap energy (∼ 3.2 eV), which could only convert less than 5% sunlight [12]. If doping modification method is adopted, the consumption cost is too high. Nowadays, bismuth semiconductor photocatalytic materials have attracted attention due to their corresponding properties in visible light, such as Bi2O3 [13], BiVO4 [14], Bi2WO6 [15], BiFeWO6 [16]and BiOX (X = Cl, Br, I) [17–19]. Among these catalysts, BiOI has a narrow band gap (∼ 1.8 eV), which can absorb visible light to the maximum extent. The electron layers of [Bi2O2]2+ and I− in the material facilitate the separation of photogenerated electron-hole pairs, thus showing excellent visible light catalytic performance [20]. Nowadays, most of the photocatalytic materials are mainly powder. It is due to the higher density of it, the powder could sink to the bottom of the natural water bottom. Therefore, the light utilization efficiency will be affected because most of solar light will be blocked by the water and cannot be transmitted to the water bottom (> 0.5 m) [21]. And the metal materials may also cause secondary pollution to the natural environment. As mentioned above, the diesel usually floats on the surface of the water due to its lower density [22], thus reducing dissolved oxygen in water and posing a threat to the life of aquatic animals and plants [23]. As early as 1993, Xing [24] developed TiO2 floating photocatalytic material for the degradation of oil-polluted water. Currently, light and environment-friendly materials are mainly used as carriers, such as expanded graphite (EP) [25], vermiculite [26] and hollow glass beads [27]. Considering the good properties of BiOX, the introduction of light material will extend the application of BiOX in the degradation of oil. For example, Li [28] successfully loaded BiOBr on fly ash cenospheres by the solvothermal method to prepare floatable BiOBr photocatalytic composites, which greatly improved the photocatalytic efficiency of BiOBr materials. Perlite is a kind of natural acidic vitreous volcanic lava. It can expand rapidly 4–30 times under the high temperature of 1000∼1300℃ to form porous materials, so it is called expanded perlite. This material is abundant in China and is a cheap and eco-friendly material commonly used in industry and construction [29]. It is mainly composed of SiO2, Al2O3, Na2O, etc [30]. The surface silanol hydroxyl group is favorable for bonding with the surface of the photocatalytic material to generate hydrogen bonds [31]. Xue H [32] reported one composite by loading B/N-doped TiO2 photocatalyst on the surface of expanded perlite. Its photocatalytic efficiency could reach 99.1% for the degradation of Rhodamine B (RhB) for 5 h under visible light. In this paper, one novel floating photocatalysis, BiOI/EP, was synthesized by a facile and one-pot solvothermal method. The performance of the composite were investigated and the results show that an excellent photodegredation for the treatment of oil-water waste. Therefore, it will become a high-quality remediation material which is not harmful to the environment.

Table 1 X Ray Fluorescence (XRF) of the pure expanded perlite from Huasheng Factory. Compound

Wt%

SiO2 Al2O3 K2O Na2O CaO

74.46 14.47 5.25 3.66 0.834

2. Materials and method 2.1. Instruments and reagents Expanded perlite was purchased from Huasheng Perlite Factory in Xinyang City, China, with an apparent density of 75.3 kg m−3 and an average particle size of 0.15∼1 mm (The components are shown in Table 1). In order to select the best floating, the purchased expanded perlite was placed in a 2 L beaker filled with water, and after standing for 12 h. The upper floating white particles was scooped out and dried under 75 ℃ for 12 h. Bismuth nitrate pentahydrate, ethylene glycol (EG), potassium iodide, absolute ethanol (EtOH) (Aladdin Agents, China), 0# diesel (sinopec). Isopropyl alcohol (IPA), AgNO3, 1,4- benzoquinone (BQ) and potassium iodide (Aladdin Agents, China). All these reagents were used without further treatment. X-ray powder diffraction (XRD) pattern is performed usingEquinoxss/hyperion 2000(Bruker Co.), Scanning electronic microscopy (SEM) images are recorded using Model Quanta 200 FEG (FEI Co.), UV–vis diffuse reflectance spectrum (DRS) is measured using UV2550, (Shimadzu Co.), X-ray photoelectron spectrum is obtained by X-ray photoelectron spectroscopy (XPS, VG ESCALAB MKII XPS system), nitrogen adsorption desorption curve is carried out by Micrometrics (ASAP 2020). X Ray Fluorescence (XRF) spectra are measured by using (Thermo Fisher 3600 mobile X-ray fluorescence spectrometer, Photoluminance (PL) spectra are carried out by PerkinElmer LS 55. 2.2. Preparation of pure BiOI 4 mmol Bi(NO3)3·5H2O was added to 10 mL of ethylene glycol and sonicated for 1 h, which is named solution A; then 4 mmol KI was added to 30 mL of ethylene glycol, stirred and dissolved for 0.5 h, which is considered as solution B; then solution A was added into solution B, stirred for 2 h. Finally, the mixed solution was treated at 140 ℃ for 1 h by solvothermal method at in a Teflon-lined stainless steel autoclave. After cooling to room temperature, the solution was washed three times with absolute ethanol and deionized water, and orange-red powder was generated. 2.3. Preparation of BiOI modified expanded perlite (BiOI/EP) The composite was prepared by a one-pot solvothermal synthesis method. Solutions A, B and C were prepared by dissolving 2, 4 and 6 mmol Bi(NO3)3·5H2O into 10 mL ethylene glycol (EG) respectively and sonicating for 1 h. Then solutions D, E and F were prepared by dissolving 2, 4, 6 mmol KI to 30 mL ethylene glycol for 0.5 h, respectively. The same amount of EP (2 g) is added to solution of D, E and F, followed by the addition of solution of A, B and C respectively with constant stirring at 500 rad min−1 for 1 h and 350 rad min−1 for another 1 h. Finally, they were hydrothermally treated in three Teflonlined stainless steel autoclave at 140 ℃ for 1 h. After cooling down to room temperature, the liquid surface-suspended orange-yellow particles were filtered and collected, washed with absolute ethanol and distilled water for three times. Then the collected particles were placed 2

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in the oven at 75 ℃ for 12 h. At last, The three products were collected and named 1-BiOI/EP, 2-BiOI/EP and 3-BiOI/EP, whose ratios of n (Bi): m (EP) are 1, 2, and 3.

(200), (212), (220), (310) crystal plane [36]. As shown in the Fig. 1, both the characteristic peaks of SiO2 and BiOI can be clearly observed in the 1-BiOI/EP. And the characteristic peaks of BiOI are obvious in the corresponding parts of (102), (110) and (200). As the content of BiOI increases, the characteristic peak of BiOI becomes sharper and sharper, and the peak of SiO2 becomes flatter and flatter. This difference indicates the BiOI was successfully loaded on this EP layer. Furthermore, this apparentness and intensity of the peak clearly shows that a good BiOI tetragonal phase is formed in this BiOI/EP nanocomposite. Fig. 2 depicts the X-ray photoelectron spectroscopy (XPS) of 2-BiOI/ EP. Fig. 2A shows the full spectrum of this composites. It can be seen that the material mainly contains elements such as I, Bi, C, O, Si, and Al. The presence of C 1s is usually used to calibrate the XPS measurements [37]. The Bi 4f spectrum (Fig. 2B) shows two distinct peaks at 158.3 and l63.7 eV, which correspond to the Bi 4f7/2 and Bi 4f5/2, respectively. They are two characteristic peaks of Bi3+, which confirms that the surface of the composite contains BiOI [38]. As shown as in Fig. 2C, the peak of Si 2p is obviously observed. It is the characteristic peak of normal SiO2 or Si-O with binding energy of 102.89 eV [39]. Fig. 2D shows that O 1s of this material is composed of two peaks, one is the characteristic peak of BieO with binding energy at 530.9 eV [40], and another one is the SieO characteristic peak with binding energy at 533.0 eV [41], which indicates that BiOI may be connected with SiO2 through Bi-O-Si and loaded on the surface of EP [42]. Fig. 3 is the SEM images of pure EP, pure BiOI and BiOI/EP, which depicts the microstructure of the material. Fig. 3A shows the 3D hierarchical flower-sphere BiOI microspheres loaded on the surface of expanded perlite. The diameter of the sphere is about 2.5 μm. According to the literature [43], the smaller the size of BiOI, the more the separation and transfer of electron-hole pairs can be promoted. And the size of BiOI loaded on the EP is smaller than that prepared by Mera A [44] (3.28 μm), indicating that BiOI microspheres loaded on the EP surface have better photocatalytic performance. As shown in Fig. 3B, the microstructure of pure EP clearly shows the micro-scale honeycomb-like structure. Moreover, the surface of EP particles has many irregularly shaped-open pores, which can keep it floating on the water surface for a long time and contribute to the absorption and preservation of oil molecules. Fig. 3C–E are the overall topographical views of 1BiOI/EP, 2-BiOI/EP, 3-BiOI/EP; Fig. 3c–e are partial enlarged views of the material. It can be clearly found from the three groups of graphs that, with the increasing content of BiOI, the microspheres on the surface of EP gradually increase, which is also consistent with the results of XRD analysis. Due to the agglomeration of BiOI in 3-BiOI/EP, the overlapping BiOI nanospheres reduced the light utilization rate of the composites [45], thus affected the photocatalytic efficiency to a certain extent. It can be seen from Fig. 3D that the BiOI microspheres are evenly distributed on the surface of EP. As can be seen in Fig. 3d, a mass of BiOI only grow on the surface of the EP, without stack. It indicates that the ratio of the composites reaches the best, which also provides a strong guarantee for the photocatalyst to quickly adsorb oil molecules in the reaction process [46]. Table 2 shows the element contents of Bi and Al in EP and BiOI/EP composites obtained by ICP-MS test, which proves that the composites contain Bi. The higher content of Bi means more BiOI loaded on the surface of expanded perlite, which is beneficial to the photocatalytic effect. The content of Al decrease with the increasing content of Bi. Finally, the content of Bi and Al reaches basically the same value, indicating that a large number of BiOI micropheres do exist on the surface of this composites. The decrease of Bi content in 3-BiOI/EP may be because of the accumulation of a large number of BiOI microspheres, which cannot closely connect with the carrier, consequently result in shedding and affecting the load of EP carrier. The surface areas and pore size diameter of EP and 2-BiOI/EP were measured by N2 adsorption-desorption isotherms and corresponding Barrett-Joyner-Halenda (BJH) pore diameter distribution, respectively. It can be clearly seen in the Fig. 4 where both of them were detected

2.4. Photocatalytic performance test The photocatalytic degradation of diesel oil was carried out in a photocatalytic reactor. In a typical experiment, 30 mg sample was added to 30 mL of oil-water mixture (1000 mg L−1). 300 W Xe lamp with AM 1.5 filter (wavelength ranging from 400 nm to 900 nm) is used as the light source. The output light intensity was 500 mw cm-2. Before the photocatalytic test, mix solution was placed in the dark for 30 min and the light source was turned on for photocatalytic reaction. The residual concentrations of diesel samples in water was measured by the way of ultraviolet spectrophotometry [33]. The changes of different alkane components of diesel in the photocatalytic process were measured by GC–MS. The pictures were shown in the supporting information (Fig. S2). AgNO3, BQ, IPA and KI were selected as scavengers of e−, ·O2-, ·OH and h+ respectively. Typically, 30 mg of the photocatalytic sample was separately added to oil-water mixed solutions (30 mL, 1000 mg L-1) containing 1 mmol L-1 of BQ, IPA, KI, AgNO3. After the photocatalytic reaction for 2 h, the absorbance was measured. 2.5. The acute toxicities method of 2BiOI/EP and BiOI on zebrafish embryo After 2 h (hour post fertilization, hpf), healthy embryos were selected under a microscope and randomly placed in 6-well plates with 5 embryos and 10 mL solution per well. Three groups, each with 3 replications, was settled: the blank group (embryo recombinant water, NC), 2-BiOI/EP (A) and BiOI (B). The exposure concentration of 2-BiOI/ EP and BiOI was selected to be 1 g L−1. The survival rate and healthy situation of embryos at exposure for 8, 24 and 48 h in a constant temperature and light controlled incubator were recorded. 3. Results and discussions 3.1. Characterization of BiOI/EP The XRD patterns of pure BiOI, pure EP, BiOI/EP are shown in Fig. 1. The diffraction peak of pure EP at 21.67°matches the peak of the crystal plane (111) of amorphous Silica [34] (JCPDS 27-0605), which is well proven that the main content of pure EP is SiO2 [35]. After deposition of BiOI, the diffraction peaks of the composites at 24.29, 29.64, 31.52, 45.21, 54.91, 65.95 and 75.08°which suggests representative tetragonal phase BiOI (JCPDS 10-0445) are at (101), (102), (110),

Fig. 1. XRD patterns of as-synthesized samples. 3

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Fig. 2. XPS spectra of the 2-BiOI/EP: survey spectra (A), Bi 4f (B), Si 2p (C), O 1s (D).

Fig. 3. SEM images of BiOI(A), EP(B), 1-BiOI/EP(C,c), 2-BiOI/EP(D,d),3-BiOI/EP(E,e). 4

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Table 2 Elemental percentage of EP and BiOI/EP. Elements

Atomic(%) Bi

Table 3 Physical characteristics of EP and 2-BiOI/EP. Al 2

EP 1-BiOI/EP 2-BiOI/EP 3-BiOI/EP

– 7.41% 20.21% 18.47%

BET Surface Area (m g ) Average Pore Diameter (nm)

24.28% 24.87% 17.77% 16.65%

n= 4)

EP

2-BiOI/EP

1.01 9.10-13.36

3.21 18.90-23.18

Under the excitation wavelength of 360 nm, photoluminescence (PL) spectroscopy was performed to analyze the separation, transfer and capture of photocarriers on the surface of the sample, which directly reflected the recombination efficiency of the charge carriers are performed over EP, BiOI, 2-BiOI/EP [51]. According to the reported reference [52], if the photocatalytic material has a relatively low PL curve, this material has effective electron transfer efficiency and charge trapping. It can be seen from Fig. 5C that all the three curves have the similar peak shape with the highest peak at the wavelength of 467 nm. And the peak of the pure EP is the largest, indicating that the photocatalytic performance is the worst, which is also consistent with the results of DRS. For pure BiOI, most of the photogenerated electrons (e−) and holes (h+) directly at the material surface or volume capture sites, resulting in fluorescence emission, thereby reducing photocatalytic activity [49]. It has been reported in the reference [35], the EP containing Al2O3 and other metal materials can provide an efficient electron transport track to the metal semiconductor photocatalytic material loaded on the surface of EP to inhibit the recombination of photogenerated electron-hole pairs. Thus, it would enhance the photocatalytic activity of BiOI/EP. In order to further study the photocatalytic properties of the material, it can be seen in Fig. 5D that 2-BiOI/EP has the highest photocurrent density compared with pure EP and BiOI within 30 s photocurrent-time (I-t) curves. And it indicates that the composite has a long life of photogenerated carriers and the high separate of electron and hole pairs [53].

into type IV adsorption-desorption isotherms. It means the fact that EP and composites are the typical mesoporous materials, in other words, the presence of BiOI does not change the type of major pores on the surface of EP. Moreover, both of the hysteresis loops without no obvious saturated adsorption platform belong to type H3, which indicates loose combination of particles, no specific shape and incomplete pore structure [47]. The hysteresis loop for EP is much larger than that of composites, indicating that EP has smaller and fewer voids specifically on the surface. As shown in Table 3, specific surface areas of EP and 2BiOI/EP are 1.01 and 3.21 m2 g−1 respectively. The number of the composites is more than 3 times that of the loaded substrate (EP), since the smooth surface of the raw material is changed after BiOI is loaded on the EP surface, making the surface of the composite material become rough. The results can be proved by SEM test. However, the pore diameter of this material is small, indicating that the degradation of diesel by the composites mainly comes from efficient photocatalytic reaction rather than the less effect of adsorption [48]. Fig. 5A, B depicts the UV–vis diffuse reflectance spectra (DRS) of pure EP, BiOI and 2-BiOI/EP. It can be clearly seen in Fig. 5A that pure EP has only one certain absorption in the ultraviolet region at λ < 400 nm and no absorption in the visible region, indicating that the carrier has no photocatalytic activity under visible light. However, the UV diffuse reflectance spectrum of the nanocomposite almost overlaps with that of pure BiOI, which can fully explain that the BiOI microspheres are uniformly loaded on the EP surface, further verifying the result of SEM test. It can also be shown that the photocatalytic properties of the composite under this loading concentration have been significantly improved. In order to study the band gap of materials, the Eq. (1) can be obtained:

α ℏν = A (ℏν − Eg )n/2 (For BiOI,

−1

3.2. Photocatalytic performance of BiOI/EP The adsorption and photocatalytic degradation curves of pure EP and nanocomposites for diesel are shown in Fig. 6A. During the first half hour of the dark reaction, the pure EP particles can absorbed the oil molecules to some extent with the absorption percentage for diesel of 33%. In the light reaction, the absorption percentage for diesel absorbed on the pure EP hardly changed, demonstrating that pure EP had no photocatalytic effect on oil molecules. From the photodegradation curve of nanocomposites, it can be concluded that 2-BiOI/EP has the best photocatalytic effect on diesel, and can achieve the diesel removal percentage of 85% after the photoreaction for 2 h. The validity of this result can be verified by GC–MS which is shown in the Fig. S2. Before

(1)

where α, h, v, A and Eg are the absorption coefficient, Planck constant, the frequency of light, a constant, and the band gap, respectively [49]. As shown in Fig. 5B, direct band gap energy for pure BiOI and 2-BiOI/ EP is 1.81 eV and 1.83 eV, respectively. These results show that optical properties of the synthesized floating photocatalysts were optimal under the visible light irradiation [50].

Fig. 4. N2 absorption-desorption isotherm and pore-size distribution (inset) images of as-synthesized samples:EP(A,a) and 2-BiOI/EP(B,b). 5

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Fig. 5. UV–vis diffuse spectra(A), band gaps(EP excluded) (B),Photoluminescence (PL) (C), and Photocurrent (PC) (D) of Pure EP, Pure BiOI, 2-BiOI/EP.

the photocatalyic reactionIn the absence of light reaction, the diesel sample contains short-chain hydrocarbons with C12-C21 and longchain hydrocarbons with C22- C23. After the photodegradation of 3 h, it can be obviously observed the disappearance of is hard to find the peaks of different hydrocarbon components. This result suggests that most oil components was degraded by the composites in a short time. The photocatalytic efficiency of 3-BiOI/EP is inferior to that of 2BiOI/EP because of the massive accumulation of BiOI microspheres, which influences the light absorption area of the EP [54], which is consistent with previous SEM analysis. Because the surface of 1-BiOI/ EP contained less BiOI load, the pores on the EP surface were not completely blocked, resulting in a good adsorption capacity of the EP to diesel molecules, with an absorption percentage of 26%. As shown in Fig. 6B, the first-order reaction kinetics was used to explain the diesel degradation rate in the photocatalytic process [55],

−

dC = kC dt

(2) −1

where K is the first-order rate constant (min ) and C is the diesel concentration (mg L−1) at time t (min). The K values of 1-BiOI/EP, 2BiOI/EP, 3-BiOI/EP are 0.00721, 0.00986, 0.00851 min−1, respectively, indicating that all three materials have good photocatalytic degradation characteristics for diesel, and the fitting curve better. Among them, 2-BiOI/EP has the largest K value, indicating that the material has the best photocatalytic performance (Table 4). 3.3. The mechanism of photocatalytic reaction and the stability of nanocomposites In order to explore the active species which play a major role in the photocatalytic degradation of oil-water mixtures under simulated

Fig. 6. Adsorption and photocatalytic performance (A) of pure EP and as-synthesis nanocomposites；Conversion in terms of the ratio of the initial concentration (B). 6

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contacts with diesel molecules, it is oxidized to intermediate product (Eq. (6)). And then REDOX reaction is conducted by superoxide radicals generated to generate CO2 and H2O, which are discharged into the air. Some of the remaining electrons in the conductive band or trapped laser electrons accumulated on the catalyst surface may be captured and transferred to the EP, resulting in a reduction in charge recombination [41]. The EP has become electron acceptor and transporter, which is also consistent with the results of Photoluminescence (PL) test. This charge separation process promotes the formation of ·O2− and h+. The oil molecules can be adsorbed in the pores of the EP and directly oxidized by the ·O2− or indirectly oxidized by h+ (Eq. (8)) [60]. Therefore, the excellent performance of 2-BiOI/EP floating photocatalyst is derived from the high visible light absorption rate, suitable band structure, effective charge excitation and transport, buoyant properties and generation of more active free active species [61]. The existence of crucial active species (%O2−) involved in the photocatalytic process was performed by Electron Paramagnetic Resonance (ESR) spin-trap with DMPO [62]. As shown in Fig. 7B, there is no characteristic signals of DMPO-·O2− emerged in the dark. However, it can be seen that the generation of DMPO-·O2− was detected under visible light illumination. And it is the fact that the intensity of it simultaneously expands with the increase of time. These results mean that 2-BiOI/EP can produce ·O2− after being stimulated by light for photocatalytic reaction of diesel oil. And long-time illumination can further enhance the photocatalytic reaction effect [63,64]. In order to study the stability of the material. After the reaction, the composite was washed with water 3 times and soaked with 1 mL petroleum ether for 10 min, so as to remove the diesel molecules in the composite. As shown in Fig. 8A, after three cycles of photocatalytic experiments, the removal rate can still be more than 75% at the reactive time of 2 h. Notably, compared with cycle experiments previously, there is almost no any change in the total removal percentage after five cycles. The results suggest that our composite has good stability and can be reused at least five times. In addition, we also compared the XRD spectra of 2-BiOI/EP before and after the cycle experiments (Fig. 8B). It can be apparently observed that the nanocomposites after cycle experiments still maintain the characteristic peak pattern of BiOI, but with a little bit weakened peak intensity. So, our composite has the potential ability to reutilize in the practical application.

Table 4 First-order kinetic equation fitting results of composite photocatalytic materials for diesel degradation. Photocatalysts

Regression equation

K(min−1)

R2

1-BiOI/EP 2-BiOI/EP 3-BiOI/EP

y = 0.2982 + 0.00721x y = 0.7455 + 0.00986x y = 0.501 + 0.00851x

0.00721 0.00986 0.00851

0.9661 0.9788 0.9753

sunlight, and four different groups of scavengers were added to the solution. It can be seen from Fig. 7A that in addition to the hydroxyl radicals (·OH), the other three reactive species have some influence on the reaction results, indicating that electrons (e−), holes (h+), superoxides (·O2−) all participate in the photocatalytic reaction [56]. The ECB of BiOI calculated by eq3 ( χ = 5.94 eV [57], Ee = 4.5 eV) is 0.525 eV. But according to reference [58], with visible light less than 2.95 eV (λ > 420 nm), e− on the conduction band (CB) of 2-BiOI/EP can be excited to a higher potential edge (−0.6 eV), which is lower than the O2/·O2− redox potential. It suggests that it can react with oxygen on the surface of the material to form superoxide radicals; the EVB to BiOI calculated by Eq. (4) is 2.36 eV, lower than the redox potential of OH-/ ·OH (2.40 eV). It indicates that h+ in the valence band (VB) cannot generate hydroxyl radicals (O%H) with H2O.

ECB = χ − Ee + 0.5Eg

(3)

EVB = ECB + Eg

(4)

In these solutions, the degradation rate containing Potassium iodide (KI) and 1,4-benzoquinone (BQ) decreased most obviously. The addition of superoxide scavenger severely inhibited the degradation of diesel, and the degradation rate of diesel was 85%, which decreased to 33%. This result implies that ·O2− plays a crucial role in the photocatalytic reaction process. However, the loss of h+, e− and ·OH, reduced the degradation rate to only 70%, 73% and 53%, respectively, indicating that the reactive species play a minor role in the reaction. The possible reaction mechanisms are as follows [59]:

2-BiOI/EP + hν → 2-BiOI/EP(e−+h+)

(5)

h+

(6)

e−

+ Diesel →

+ O2 →

Intermediates+

⋅O−2

Intermediate+/ Diesel

(7)

+

⋅O−2

→ Degradation Products(CO2 + H2 O )

3.4. The acute toxicities of 2BiOI/EP and BiOI on zebrafish embryo

(8)

Floating 2-BiOI/EP photocatalytic composite exists between water and air. Under the condition of being excited by sunlight, electrons of VB are transferred to CB, and photogenerated electrons can be generated directly. They can react with the air or the water of oxygen to generate superoxide radicals (%O2−) (Eq. (7)). After h+ in the VB

Fig. 9A shows the survival rate of zebrafish embryos exposed to 8 h and 24 h in the presence of 2-BiOI/EP and BiOI. There are no significant difference in the survival rate between the 2-BiOI/EP group and blank group (NC) after exposure for 8 h and 24 h. After exposure for 24 h, the survival rate of the BiOI/EP group decreased slightly, but was still

Fig. 7. Diesel degradation efficiency at the presence of selected radical scavengers (A); ESR spectra of DMPO-·O2− for 2-BiOI/EP (B). 7

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Fig. 8. Recycling runs (A) and XRD (B) in the degradation of diesel in the presence of 2-BiOI/EP.

Fig. 9. Survival percentage of zebrafish embryos after 8, 24 h exposure (A) and hatching rate after 48 h exposure (B).

and low biotoxicity demonstrate that our floating BiOI/EP photocatalyst is a promising petroleum degrading agent in the natural water.

higher than 80%. However, the BiOI-exposed group shows significant lethal effects at 8 h. During 24 h of exposure, the survival rate of embryo plunged to 17%. Fig. 9B suggests the hatching rate of zebrafish embryos after 48 h exposure to 2-BiOI/EP higher than that of NC. These results demonstrate that this floating photocatalytic composites, BiOI loaded on EP, can greatly subside the lethal effect of the original BiOI on zebrafish embryos and promote the hatching of fish eggs to some extent. The FET test evaluates that this composite is a low acute Toxicity and harmless green material [65].

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. Acknowledgements

4. Conclusion

This work was financially supported by The Key Program for International S&T Cooperation Projects of China (2016YFE0123800); Chongqing research institute performance incentive and guidance special (cstc2018jxjl20017); Chongqing technology innovation and application demonstration special (cstc2018jxxx0020).

In summary, the floating BiOI/EP composites were successfully synthesized by one-pot solvothermal method. The BiOI/EP composites present a honeycomb-like structure with uniform and dense distribution of spherical BiOI on the surface. The performance of BiOI/EP at different ratios of m(Bi) and m(EP) were investigated and the results shows the best property at ratio of 2. SEM images showed that spherical BiOI was uniformly and densely distributed on the surface of EP. XPS data indicate that BiOI and EP are connected by Bi-O-Si. The DRS pattern demonstrates that the band gap of composite approximates that of pure BiOI, which is 1.8 eV. The PL diagram shows that after the two materials are combined, the recombination probability of photogenerated electrons and hole pairs is effectively reduced, and the photocatalytic efficiency is improved. This solar-light driven composites has the excellent photocatalytic activity and achieved the diesel remove percentage of 85% for 2 h reaction. And they could be reutilized for 5 times at least. Finally, biotoxicity research of BiOI/EP composites in the zebrafish embryos shows that BiOI companied with EP could decrease the toxicity of BiOI. Hence, the excellent reusability, stability

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