Removal of bisphenol A by mesoporous BiOBr under simulated solar light irradiation

Abstract

3D mesoporous BiOBr microspheres were fabricated via a facile, rapid and environmentally friendly one-step solvothermal process without using templates. The physiochemical properties of BiOBr were characterized by XRD, FESEM, TEM and nitrogen adsorption techniques. The photodegradation behaviors of bisphenol A (BPA) catalyzed by BiOBr were investigated under simulated solar light irradiation. The photocatalytic activities of the BiOBr were superior to that of commercial Degussa P25 TiO₂. Particular attention was paid to the identification of intermediates and acute toxicity of photocatalytic degradation samples of BPA by GC–MS and bioluminescence bacteria, respectively. Deducing from the results, BiOBr can be a good kind of catalyst irradiated by visible light even under sunlight.

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Introduction

Recent years the contamination of various endocrine disrupting chemicals (EDCs) has caused increasing public concern.^1–6 It has been reported that EDCs are related to various adverse effects on aquatic organisms and mankind, such as reproductive and sexual abnormalities, even at low exposure levels.^7–10 Bisphenol A [2,2-bis(4-hydroxyphenyl)propane, BPA] which was first described as a synthetic estrogenic agent in 1936 now is characterized as one of the representative EDCs.^11,12 BPA has been widely used as a raw material for the production of polycarbonate plastics and epoxy resin, such as baby bottles, dental sealants and lining of food cans. As an important industrial chemical, a large amount of BPA is being produced every year, for example about 3.2 million metric tons in 2008.^13,14 Inevitably, a great quantity of BPA has been released into the aquatic environment.^15–18 Recent studies alert that EDCs including BPA have been detected in surface water, wastewater and even drinking water.^19–21 Various methods have been applied to remove BPA from water, such as physical moethods,^22,23 biological methods,²⁴ chemical oxidation,^25,26 electrochemical oxidation,²⁷ and photocatalytic method.^28–30 Physical technologies, such as activated carbon adsorption, can quickly adsorb BPA from water.²³ But there is a potential threat to the environment from BPA molecules that remain intact. Biodegradation methods are usually feasible for eliminating organic micropollutants at a level of ng L⁻¹–μg L⁻¹, but the efficiencies are very low.^31,32 Among these methods mentioned above, photocatalytic oxidation is one of the most promising technologies due to its high speed degradation efficiency.³³ For example, titanium dioxide (TiO₂) has found a wide application in the cleanup of organic chemicals in water.³⁴ However, TiO₂ can only be excited by ultraviolet irradiation, which only covers ∼4% of the solar spectrum.^35,36 Due to this inherent limitation, solar energy (visible light accounting for 43% energy)³⁷ can't be utilized efficiently. In an attempt to efficiently harvest solar energy, intensive efforts have focused on the design and development of novel visible-light-responsive photocatalysts.^38,39

Bismuth oxybromide (BiOBr) is a type of ternary compound belonging to the family of main group multicomponent metal oxyhalides, which possesses unique and excellent electrical, magnetic, optical, and luminescent properties.^40,41 Recently BiOBr has been found to be a promising photocatalyst for practical application due to its high absorption coefficient in a broad UV–visible–NIR spectral range.^42,43 BiOBr has a unique sandwich-like crystal structure. Two Br slices sandwich the tetragonal [Bi₂O₂] slices and forms a [Bi₂O₂Br₂] layer unit. The layer units are stacked together one by one in an orderly way by nonbonding (van der Waals) interactions through the Br atoms along the anisotropic growth direction, namely, the c-axis.⁴⁴ The internal electric fields in sandwich-like crystals are believed to favor charge separation and the conduction of the photogenerated electron/hole pairs, which could facilitate redox reactions on the semiconductor surface and be beneficial to enhance the photocatalytic activity.^45,46 Some research has suggested that the photocatalytic activity is closely related to the configuration of the photocatalysts. Therefore, the shape and the structure of the photocatalysts have been tuned to improve their activities.^47,48 In recent years, three-dimensional (3D) nanostructured materials, such as nanoplates and nanosheets, have attracted more attention because of their large surface areas, nearly perfect crystallinity, structural anisotropy, and quantum confinement effects in the ultrathin thickness. It is considered that this structure could favor the decrease of recombination opportunities of the photoexcited electron/hole pairs and favor their transfer to the surface to react with organic molecules.⁴⁹ Moreover, mesoporous materials with ordered open pore frameworks and high surface areas can effectively harvest visible light due to multiple scattering within the solid framework.^50,51

The present work extensively evaluates the photocatalytic performances of BPA removal by mesoporous 3D BiOBr microspheres under the simulated sunlight irradiation.

Materials and methods

Materials and reagents

BPA (purity >99%) was purchased from Sigma-Aldrich Co. A stock solution of BPA (20 mg L⁻¹) was prepared by dissolving a certain amount of BPA in purified water. TiO₂ particles (P25, Degussa) were purchased from Degussa Corporation. Bismuth nitrate, sodium bromide and absolute alcohol were purchased from Tianjin Chemical Company. All the other reagents were of analytical grade and used as received. The chemicals used for the mobile phase of HPLC were HPLC-grade methanol and acetonitrile from Dikma Chemical (China) and Mill-Q ultrapure water.

Preparation of samples

In a typical process, 1.0 mmol Bi(NO₃)₃·5H₂O was added into 16 mL absolute alcohol, and the suspension was stirred vigorously for 15 min at room temperature. Then 8 ml absolute alcohol containing stoichiometric amounts of NaBr was dropwise added into the former suspension. After stirring for 15 min, the mixture was then transferred to a 30 mL Teflon-lined stainless autoclave. The autoclave was allowed to be heated at 150 °C for 12 h under autogenous pressure, and then cooled down to room temperature. The precipitates were collected and washed with absolute alcohol and deionized water and dried at 60 °C, separately. X-ray diffraction (XRD) patterns were obtained on a Rigaku D/Max-2500 diffractometer with Cu-Kα radiation (λ = 1.54178 Å). The morphology and size of the synthesized catalysts were recorded by field emission scanning electron microscope (FESEM, FEI nanosem 430) and high-resolution transmission electron microscope (HRTEM, Tecnai G²F 20), respectively. The UV–vis diffuse reflectance spectra (UV–vis DRS) were performed at room temperature by a SHIMADZU UV-3600 spectrophotometer equipped with an integrated sphere and using BaSO₄ as reference. The specific surface area was calculated using a Quantachrome NOVA 2000e sorption analyzer at 77 K with samples degassed at 120 °C under vacuum prior to test.

Degradation experiments

The photocatalytic degradation experiments were carried out in a XPA-2 photochemical reactor (Xujiang Co., Nanjing, China). Simulated solar light irradiation was provided by an 800 W xenon lamp (Institute of Electric Light Source, Beijing), which was positioned in the cylindrical quartz cold trap. The system was cooled by circulating water and maintained at room temperature. Prior to the irradiation, the suspension was magnetically stirred in the dark for 30 min to ensure adsorption/desorption equilibrium of BPA to/from the catalysts. About 5 mL of reaction solution was taken at selected time intervals and separated through centrifugation (1500 rpm, 5 min). The BPA in samples was detected by Agilent 1200 HPLC with a reversed phase C18 column and UV detector. The detecting wavelength was set at 217 nm. The mobile phase was 70 : 30 (v/v) acetonitrile and Milli-Q water with a flow rate of 1.0 mL min⁻¹. The intermediates of BPA photodegradation samples were condensed by SPF and then detected by a GC-MS (Agilent 7890A-5975C, USA) equipped with an HP-5 capillary column (30 m × 0.25 mm i.d.) with helium as carrier gas at a flow rate of 1 mL min⁻¹. The GC was operated through the temperature programming: holding for 10 min at 50 °C, then rising at a rate of 15 °C min⁻¹ towards 300 °C for 5 min.

Toxicity of BPA photodegradation samples

All photodegradation samples were diluted into one-tenth of the initial concentration with 0.85% NaCl (wv⁻¹). Acute toxicity was determined by the bioluminescent bacterium Vibro qinghaiensis measured at 15 min incubation times with luminescence at 490 nm by BHP9514-Drinking water safety detector (Shanghai, China).

Results and discussion

Characterization of the catalyst

The X-ray diffraction (XRD) pattern provides phase information of the as-synthesized BiOBr product (Fig. 1). All the detected peaks can be indexed to the tetragonal phase of BiOBr (JCPDS No.73-2061), with no other characteristic peaks observed. This indicates that a well-crystallized high-purity product formed. The SEM images of the as-prepared BiOBr product are illustrated in Fig. 2(a) and (b). It is found that the product is composed of a large quantity of relatively uniform microspheres with diameters ranging from 1 to 3 μm (Fig. 2(a)). The SEM image at high magnification reveals that the BiOBr microsphers are constructed by nanoplates with thickness of 20–25 nm (Fig. 2(b)). The numerous nanoplates grew radically to form hierarchical microspheres with an open porous structure. The morphological information of the as-prepared product was also detected by TEM characterization. The TEM images shown in Fig. 2(c) further indicate that the as-prepared microspheres are composed of nanoplate-based nanostructures. The HRTEM image reveals that the building units of the BiOBr are nanostructured in nature (Fig. 2(d)). The UV–vis diffuse reflectance spectrum of the as-prepared BiOBr sample was measured using a UV-visible spectrophotometer shown in Fig. 3. From Fig. 3, the photoabsorption of BiOBr sample is from UV light to visible light and the wavelength of the absorption edge is 446 nm. The band gap of the BiOBr microspheres is estimated to be 2.64 eV. It is in the visible light region and is close to the value reported in other literature compared with that of P25 about 3.2 eV. The porous structures and Brunauer–Emmett–Teller (BET) specific surface areas of the BiOBr microspheres were conducted by using nitrogen adsorption and desorption isotherms. Fig. 4 shows the N₂ adsorption/desorption isotherm and the pore-size distribution of BiOBr product. The type-IV isotherm indicates that the product has a mesoporous structure. All the mesopores fall within the size range of 7 to 30 nm. These mesopores are believed to be attributed to the interplate spaces of the aggregated BiOBr nanoplates. The BET specific surface area of the sample is 7.69 m² g⁻¹ calculated from the adsorption data.

Fig. 1 XRD pattern of the as-synthesized BiOBr product.

Fig. 2 (a) Low magnification SEM image, (b) High magnification SEM image, (c) Low magnification TEM image, (d) High magnification TEM image.

Fig. 3 UV-visible diffuse reflectance spectra of BiOBr microspheres.

Fig. 4 Nitrogen adsorption-desorption isotherms and Barrett–Joyner–Halenda (BJH) pore size distribution plot (inset) of BiOBr powders.

Degradation efficiency of the catalysts

The degradation efficiency of BPA by BiOBr is illustrated in Fig. 5. It is clearly seen that after 90 min adsorption of BPA on the BiOBr microspheres in the dark, the concentration of BPA hardly decreased. The photolysis of BPA without any photocatalysts under simulated solar light irradiation is much slower and only 25.6% of BPA could be degraded after 90 min irradiation. However, the photodegradation efficiency of BPA is nearly 100% in the presence of BiOBr microspheres. For comparison, we also investigated the photocatalytic performance of Degussa P25. The degradation rate of BPA by Degussa P25 was only 50.3%, which is much less efficient than that of BiOBr microspheres under the same conditions.

Fig. 5 The photocatalytic efficiency of BPA under simulated solar irradiation by BiOBr as compared to the results by P25, only with irradiation and BiOBr without irradiation at room temperature. [BPA]_o = 20 mg L⁻¹, [BiOBr] or [P25] = 0.5 g L⁻¹.

BiOBr has a unique sandwich-like crystal structure. The internal electric fields in sandwich-like crystals are considered to favor the conduction of the photogenerated electron/hole pairs and the charge separation, which could be beneficial to enhance the photocatalytic performance. Previous studies discovered that a suitable conformation of pores allows incident light waves to penetrate deep inside the photocatalyst and leads to a high mobility of charges.^52,53 It is conjectured that the mesopores in the catalyst favor the penetration of light waves and bring the target compound molecules (BPA) deep inside the photocatalyst, which would lead to high photocatalytic activity.⁵⁴

Toxicity evaluations

Relative luminosity of luminous bacteria is a commonly used indicator of biological acute toxicity. Luminous bacteria (Vibro qinghaiensis) were used to evaluate the toxicity of samples after different irradiation time. Relative luminosity is the ratio of the bioluminescence intensities when luminous bacteria are exposed to the samples vs. the bioluminescence intensities when luminous bacteria are exposed to the control solutions.⁵⁵ Relative luminosity is calculated from the equation below:
Where E₀ and E are normalized bioluminescence intensities in the absence and presence of each sample after 15 min of exposure, respectively. Fig. 6 demonstrates the acute toxicity reflected by the luminosity of BPA photodegradation samples at different irradiation intervals.

Fig. 6 The relative luminosity of BPA samples at different irradiation intervals under simulated solar light. [BPA]_o = 20 mg L⁻¹, [BiOBr] = 1.0 g L⁻¹.

The luminosity rate of the photodegradation sample containing 20 mg L⁻¹ BPA was about 35% and the luminosity rate of the sample after irradiation was increased, reflecting a declined toxicity. It indicated that the more irradiation time the lower acute toxicity of BPA photodegradation samples whether catalyzed by TiO₂ or BiOBr. It can be seen that BPA had been completely decomposed into the smaller and less toxic chemicals after 90 min reactions. This result was consistent with GC–MS in Fig. 7. The detection of intermediates was carried out through the use of the BPA samples after 15 min irradiation. These samples were concentrated 600-fold by solid phase extraction (SPE) and then detected by GC–MS with an identification program obtained from NIST. Fig. 7 detected only one peak of BPA without other peaks of the intermediates. It shows that BPA can be completely photodegraded without producing more or less toxic intermediates under simulated sunlight. It can be seen that BiOBr could be used for the removal of EDCs using visible light.

Fig. 7 GCMS of BPA sample after 15 min irradiation under simulated sunlight. [BPA]_o = 20 mg L⁻¹, [BiOBr] = 1.0 g L⁻¹.

Conclusions

The visible light responsive catalyst BiOBr was synthesized through a hydrothermal method. As a model EDC, BPA can be photodegraded by BiOBr under simulated sunlight. Comparing with that of the classic catalyst Degrussa P25, BiOBr shows higher photoactivity in removing BPA under current conditions. The toxicity of photodegradation intermediates of BPA decreased with the irradiation time, showing the technology without producing secondary pollution. BiOBr under visible light exposure could be beneficial for EDC removal and for practical applications using visible light.

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