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Article

Enhanced Sulfamerazine Removal via Adsorption–Photocatalysis Using Bi2O3–TiO2/PAC Ternary Nanoparticles

by
Xiaoxuan Zhuang
,
Xing Li
,
Yanling Yang
,
Nan Wang
,
Yi Shang
,
Zhiwei Zhou
*,
Jiaqi Li
and
Huiping Wang
College of Architecture and Civil Engineering, Beijing University of Technology, Beijing 100124, China
*
Author to whom correspondence should be addressed.
Water 2020, 12(8), 2273; https://doi.org/10.3390/w12082273
Submission received: 15 July 2020 / Revised: 8 August 2020 / Accepted: 11 August 2020 / Published: 13 August 2020
(This article belongs to the Section Wastewater Treatment and Reuse)
Browse Figures
Versions Notes

Abstract

:
The presence of sulfonamides (SAs) in water has received increasing attention due to the risk to ecosystems. The adsorption and photocatalysis performance for sulfamerazine (SMZ) of Bi2O3–TiO2 supported on powdered activated carbon (Bi2O3–TiO2/PAC) nanoparticles was evaluated. The amount of doped Bi2O3 not only influenced the photocatalytic performance but also impacted the adsorption capacity. The adsorption mass transfer mechanism of Bi2O3–TiO2/PAC was elucidated and is further discussed in combination with the photocatalytic mechanism. It was indicated that Bi2O3–TiO2/PAC(10%–700 °C) performed best, and the SMZ removal by the adsorption–photocatalysis of Bi2O3–TiO2/PAC(10%–700 °C) reached 95.5%. Adsorption onto active sites was a major adsorption step, and external diffusion was assisted. Superoxide radical (●O2−) and hole (h+) were identified as the major reactive oxygen species (ROS) for SMZ removal. Benzene ring fracture, SO2 extrusion and nitrogenated SMZ were proposed as the main pathways for photocatalysis. Meanwhile, alkaline conditions enhanced photocatalytic performance, while contrary effects were observed for adsorption. The adsorption–photocatalysis removal performance for SMZ in lake water was better than that for river water. It can be generalized for the potential application of photocatalysis coupling with adsorption to remove refractory antibiotics in water.

Graphical Abstract

1. Introduction

Sulfamerazine (SMZ) is one of the most commonly used sulfonamides (SAs) for human and animal infections. The highest levels of SMZ residues have been reported in animal-sourced foods such as eggs, meat and milk [1,2], causing potential adverse effects on food products and ecosystems. SAs and their metabolites cannot degrade completely in traditional wastewater treatment plants, and antibiotic residues also appear in natural waters [3,4]. Various approaches [5] have been developed to remove SAs such as adsorption [6,7], membrane filtration [8,9], electrochemical oxidation [10], biodegradation [11], photocatalysis [12,13], etc. However, adsorption and membrane filtration carry out the transformation of contaminants but cannot achieve the real removal of pollutants. Excessive electrode and energy consumption should be considered for electrochemical oxidation, and the bacteriostatic property of SAs may lead to apparent negative removal during biological wastewater treatment [14]. Among them, solar-driven photocatalysis is widely utilized due to its characteristics of being environmentally friendly, energy-saving and easy to controlling.
Traditional titanium dioxide (TiO2) photocatalysts are stable and demonstrate low toxicity, but they are only activated by ultraviolet light, easy to aggregate and difficult to recycle [15]. The incorporation of heterostructures on TiO2 causes the suppression of the charge carriers’ recombination and expansion of the spectral response to wider wavelengths. Bi2O3 with a narrow band gap exhibits high reactivity in oxidation reactions and could be doped on TiO2 for heterostructures [16,17]. It can enhance visible light utilization by reducing the band gap energy and preventing the recombination of electrons (e−) and h+. Li et al. [18] have prepared Bi2O3 nanoparticles by the atomic layer deposition method deposited on TiO2 films to improve the photocatalytic degradation of methyl blue. Zou et al. [19] have synthesized a Bi2O3/TiO2 photocatalytic film coated on floated glass balls for the efficient removal of organic pollutants. Reddy et al. [20] have revealed that Bi2O3/TiO2 is a promising photocatalyst for enhanced H2 production.
The problems of agglomeration and reuse are the constraints for the sustainable application of TiO2. The immobilization of TiO2 on a substrate is beneficial for recovery [21]. The loading of TiO2 on different supports has been studied [22,23,24]. Since the high adsorption capability of activated carbon can help to enrich the contaminant concentration around the photocatalysts, loading TiO2 on activated carbon has drawn great attention [25,26,27]. It can promote the pollutant transfer process and increase the photocatalytic efficiency. It has been suggested that the integration of adsorption with advanced oxidation process will be a future development trend [5], which demonstrates the significance of adsorption in the oxidation process.
In our latest study [28], the Bi2O3–TiO2 composite supported by powdered activated carbon (PAC) was prepared by two-stage calcination with different calcination temperatures, proposing a solution for the abovementioned limitations of TiO2. Specifically, the effect of the calcination temperature on the photocatalytic properties of the composites was studied to dissolve sulfamethazine (SMT) in water. Likewise, the influence of the Bi2O3 amount has not been clearly characterized, which is another vital condition for preparation. The differences in Bi2O3 can cause the transformation of crystal structures, which induces different optical properties [29].
Herein, SMZ, which is more difficult to degrade than SMT, was applied to verify the effect of different Bi/Ti molar ratios for Bi2O3–TiO2/PAC composites. It was investigated in terms of adsorption and photocatalysis properties, overall adsorption–photocatalysis performance, and characterization. Moreover, the effect of water quality parameters on SMZ removal was demonstrated. Adsorption mass transfer kinetics model analysis, trapping experiments and UPLC–MS analysis were carried out to reveal the mechanism. This work offers a novel adsorption–photocatalysis strategy that can be well applied in micro-polluted water treatment. It can be generalized for application to remove refractory antibiotics due to its excellent efficiency, low cost and environmental friendliness.

2. Materials and Methods

2.1. Chemicals

SMZ (99.0% purity), sodium humate, 2,2-dimethyl-3,4-dihydro-2H-pyrrole 1-oxide (DMPO) and PAC (100 mesh) were purchased from Sigma-Aldrich (DARCO, Saint Louis, MO, USA). Titanium butoxide (98.0% purity), bismuth nitrate pentahydrate (99.0% purity), isopropyl alcohol (IPA), 1,4-benzoquinone (BQ), disodium ethylene diamine tetra acetate (EDTA-2Na), anhydrous ethanol (99.7% purity) and sodium salts (NaCl, NaHCO3 and Na2SO4) were supplied by Aladdin Industrial Corporation (Shanghai, China). Methanol (HPLC grade) was purchased from Fisher Scientific (Fisher Chemical, Atlanta, GA, USA). Other reagents were all at analytical grade and purchased from Beijing Chemical Works (Beijing, China). All aqueous solutions were prepared with ultrapure water (Milli-Q Advantage A10, Millipore, Billerica, MA, USA).

2.2. Synthesis of Bi2O3–TiO2/PAC Composites

The Bi2O3-TiO2/PAC composites were prepared by a sol-impregnation-hydrothermal with two-stage calcination method according to our previous work [28]. The details of the procedures are listed in Table S1. The composites were named as Bi2O3-TiO2/PAC(a-b), whereby “a” denotes the Bi/Ti molar ratio and “b” represents the calcination temperature of the second stage under N2 atmosphere. It was verified that 700 °C was the optimal calcination temperature for SMZ removal, and the data of the calcination temperature for Bi2O3-TiO2/PAC are in the supplementary files. Besides, Bi2O3–TiO2(10%–700 °C) and TiO2(700 °C) without PAC were prepared by the same method. Virgin PAC without calcination was used as a further control.

2.3. Characterization of Bi2O3–TiO2/PAC Composites

X-ray diffraction patterns (XRD) were detected by X-ray diffraction (D8 Advance, Bruke, Germany) with Cu-Kα radiation in the region 2θ = 10–90° to determine the crystal phase composition of the samples. The Brunauer–Emmett–Teller (BET) specific surface area and Barrett–Joyner–Halenda (BJH) pore-size distribution were obtained with surface-area and pore-size analyzers (TRISTAR II 3020, Micromeritics, Micromeritics Corporate, Norcross, GA, USA) at 77 K. The UV-vis diffuse reflectance spectra (UV-vis DRS) were obtained with a spectrophotometer (U-3900, Hitachi, Japan) for the optical properties. A scanning electron microscope (SEM, Hitachi SU-8010) with energy dispersive spectroscopy (EDS) and a high-resolution transmission electron microscope (HRTEM, Tecnai G2 F30, FEI, Hillsboro, OR, USA) were used at an accelerating voltage of 300 kV.

2.4. Experimental Procedures

2.4.1. Adsorption Experiments

The adsorption kinetics and isotherms of SMZ removal by Bi2O3-TiO2/PAC were studied. All adsorption experiments were in the dark, and the dosage of the composites was 1.0 g/L. The adsorption kinetics were determined with an initial SMZ concentration of 20 mg/L at 25 °C, and aliquots were collected at preset times (0, 5, 10, 15, 20, 30, 45, 60, 75 and 90 min). The adsorption isotherms were investigated at different SMZ concentrations of 5, 10, 15, 20, 25 and 30 mg/L in a constant-temperature vibration incubator set at 25 °C. The kinetic and isotherm models [30,31,32,33,34,35,36,37] (seen in Table S2) were used to fit the adsorption data. All aliquots were filtered with 0.22 μm syringe filters (Beihua Dawn, Beihua Dawn Membrane Separation Technology, Beijing, China) and then analyzed.

2.4.2. Photocatalytic Experiments

Two sets of photocatalytic experiments were conducted. One was performed from the beginning of solar light irradiation, and the other was carried out after adsorption equilibrium in the dark and then irradiation under solar light. A 300 W xenon lamp (CEL-HXF300E7, Ceaulight, Beijing, China) was used to generate simulated solar light irradiation, the light intensity of which was 443.5 mω2/cm2, measured with an optical power density meter (NP2000, Ceaulight, Beijing, China). A concentration of 20 mg/L of SMZ was continuously stirred in a 0.25 L batch reactor surrounded by a water-cooling device. The distance between the surface of the reaction liquid and light source was approximately 15 cm. Composites at 1.0 g/L were added into the solution for the 120 min photocatalytic experiments. The aliquots were sampled at intervals of 20, 40, 60, 80, 100 and 120 min. The photocatalytic kinetics of the Bi2O3–TiO2/PAC composites followed a pseudo-first-order kinetic model [38], as shown in Equation (1).
ln C 0 C t = k t
where C0 (mg/L) and Ct represent the concentration of SMZ at adsorption equilibrium and at time t, respectively; t (min) means the reaction time, and k (min−1) means the kinetic rate constant.
The experiments of trapping and studying the influences of water quality parameters on photocatalysis were conducted at adsorption equilibrium. Hydroxyl (●OH), ●O2− and h+ were captured with IPA, BQ and EDTA-2Na, respectively. Scavenging agents at 2.0 and 5.0 mM were added into the solution for the trapping experiment. Electron paramagnetic resonance (EPR) spectrum measurements for ●OH and ●O2− were investigated using DMPO and methanol, respectively, with sampling through a 100 μL capillary. The initial pH of the solution was adjusted with diluted 0.1 M HCl and NaOH. Three concentrations of anions including HCO3−, Cl− and SO42− were set at 0.5, 2.0, and 5.0 mM. The humic acid (HA) concentrations were 0.5, 2.0 and 5.0 mg/L. Surface water was collected from Tonghui River (39°92′ N, 116°58′ E, Beijing, China) and Lianshi Lake (39°89′ N, 116°17′ E, Beijing, China). The main water quality of the surface water is shown in Table S3. All aliquots were filtered with 0.22 μm syringe filters. All trials were conducted three times, and average values are reported.

2.5. Analytic Methods

The concentration of SMZ was detected with a high-performance liquid chromatograph (Agilent 1260 LC) equipped with a C18 column (Waters XBridge, 4.6 mm × 250 mm, 5 μm). The detection wavelength was 270 nm. The mobile phase was methanol and ultrapure water with a 50:50 ratio, and the flow rate was 1 mL/min. The EPR spectra were collected on an electron spin resonance spectrometer (JEOL JES-FA200). The intermediates were identified by UPLC/MS (Agilent 6460 Triple Quad LC/MS) with a C18 column (Agilent Zorbax Eclipse Plus, 2.1 mm × 50 mm, 1.8 μm). The mobile phase of the UPLC/MS was a 70:30 mixture of methanol and ultrapure water (0.1% formic acid) at a flow rate of 0.2 mL/min. Positive electrospray ionization (ESI+) and negative electrospray ionization (ESI−) modes were both used for analysis.

3. Results

3.1. Adsorption Characteristics

3.1.1. Adsorption Kinetics

The adsorption capacity of the Bi2O3–TiO2/PAC composites increased sharply in the first 10 min, and all reached adsorption equilibrium after 30 min (Figure S1). In the three models, the correlation coefficients (R2) of the pseudo-second-order kinetics were above 0.99 (Table 1), suggesting that the adsorption rate was proportional to the square of the SMZ concentration and the chemical reaction was the vital factor for SMZ adsorption. For the different Bi/Ti molar ratios of the composites, the adsorption rate gradually decreased with an increase in the Bi/Ti molar ratio. Although doping Bi improved the utilization of visible light to enhance the photocatalytic performance, excessive Bi2O3 reduced the adsorption efficiency of the SMZ, which was detrimental to the enrichment of the SMZ at low antibiotic concentrations in the water environment [39]. Comparing with the adsorption kinetics of Bi2O3-TiO2(10%–700 °C) and TiO2(700 °C), likewise, the recombination of Bi2O3 could reduce the adsorption capacity of TiO2. Therefore, the addition of PAC effectively improved the adsorption performance of Bi2O3–TiO2, which further promoted the photocatalytic efficiency. For Bi2O3-TiO2/PAC composites with different second stage calcination temperatures, as shown in Table S4, the adsorption performance of the second stage calcination at 500 °C was the best. It is worth noting that the difference between the adsorption properties of Bi2O3-TiO2/PAC(10%–600 °C) and Bi2O3-TiO2/PAC(10%–700 °C) was negligible, indicating the effectiveness of N2 atmosphere for preventing the loss of PAC in the process of high-temperature calcination [40].

3.1.2. Adsorption Isotherm

The Langmuir and Freundlich isotherm models were used to describe the adsorption process, the R2 of which, both above 0.95, well described the monolayer adsorption and multilayer adsorption simultaneously existing, as shown in Figure 1. To be specific, the R2 of the Langmuir model was larger, indicating that monolayer adsorption and surface adsorption predominated. Consistent with the adsorption kinetics, the larger the molar ratio of Bi/Ti, the lower the adsorption capacity of the Bi2O3–TiO2/PAC composites on SMZ. The adsorption performance of Bi2O3–TiO2 was less than that of TiO2, indicating Bi2O3 occupied part of the lattice gap of TiO2. Compared with the adsorption isotherm models of PAC, Bi2O3–TiO2(10%–700 °C) and TiO2(700 °C), the adsorption capacity of the Bi2O3–TiO2/PAC composites was lower than that of PAC but higher than that of Bi2O3–TiO2(10%–700 °C) and TiO2(700 °C). This revealed that the adsorption capacity of PAC was manifested in the Bi2O3–TiO2/PAC composites. Additionally, the higher the second stage calcination temperature, the lower the adsorption capacity (Figure S2). A high calcination temperature can improve the adsorption capacity of TiO2 because of the higher crystallinity [41]; however, the adsorption performance of Bi2O3–TiO2/PAC was contrary. This is another piece of evidence for the adsorption dominance of PAC, since a high temperature would lead to PAC loss.

3.2. Photocatalytic Characteristics

3.2.1. Photocatalytic Performance

The photocatalytic performance of the Bi2O3–TiO2/PAC composites at adsorption equilibrium was studied. The sum of the adsorption and photocatalysis removal efficiency of Bi2O3–TiO2/PAC(10%–700 °C) for SMZ reached 96.9%, which performed the best among the as-prepared composites (Figure 2). With different Bi/Ti molar ratios, the photocatalytic removal efficiency of Bi2O3–TiO2/PAC(8%–700 °C), Bi2O3–TiO2/PAC(10%–700 °C) and Bi2O3–TiO2/PAC(12%–700 °C) in the 120 min photocatalytic reactions was 63.4%, 67.9% and 74.2%, respectively. Compared with Bi2O–TiO2(10%–700 °C) and TiO2(700 °C), the photocatalytic efficiency of Bi2O3–TiO2/PAC(10%–700 °C) improved by 42.5% and 53.2% (Figure S3). Bi doping resulted in the generation of heterogeneous structures of Bi2O3 and TiO2, impeding e− and h+ recombination. At different second stage calcination temperatures, the photocatalytic removal efficiency of Bi2O3–TiO2/PAC(10%–500 °C) and Bi2O3–TiO2/PAC(10%–600 °C) was 51.2% and 63.0%, respectively. Elevating the second stage calcination temperature had a significant impact on the crystallite formation and size of TiO2 [40], which greatly improved the photocatalytic efficiency.

3.2.2. Photocatalytic Kinetics

It can be observed in Figure 3 that the photocatalytic process for SMZ conforms to the pseudo-first-order kinetic model. The maximum kinetic constant of Bi2O3–TiO2/PAC(10%–700 °C) was 0.0242 min−1, which was 1.14, 1.15, 2.70 and 5.12 times higher than that of Bi2O3–TiO2/PAC(8%–700 °C), Bi2O3–TiO2/PAC(12%–700 °C), Bi2O3–TiO2(10%–700 °C) and TiO2(700 °C), respectively. The addition of PAC provided a photocatalytic reaction with a continuous higher concentration of SMZ than Bi2O3–TiO2; therefore, the photocatalysis could degrade SMZ more promptly. Additionally, the photocatalytic kinetic constant of Bi2O3–TiO2/PAC(10%–700 °C) was 1.84 and 1.27 times higher than that of Bi2O3–TiO2/PAC(10%–500 °C) and Bi2O3–TiO2/PAC(10%–600 °C), respectively (Figure S4).

3.3. Overall Adsorption–Photocatalysis Performance

SMZ removal was enhanced initially by increasing the Bi/Ti molar ratio and decreased thereafter, while the Bi/Ti proportion reached a high level. Bi2O3–TiO2/PAC(10%–700 °C) exhibited the best removal efficiency from the beginning of solar light irradiation within 120 min, as shown in Figure 4a. When the Bi/Ti molar ratio was lower than the optimal amount, the Bi-O polyhedral was increased with an increase in the Bi/Ti molar ratio. It can prolong the life of carriers and increase the numbers of active sites and substrates adsorbed due to the smaller crystal size. However, when the Bi/Ti molar ratio was higher than its optimum amount, higher concentration of Bi increased the particle agglomeration of Bi2O3, which caused severe nanocrystal heterojunction decline and then affected the adsorption–photocatalysis performance [42].
The coupling removal efficiency for SMZ was affected by co-existing organic matter. The same concentration of SMZ in two bodies of water as in ultrapure water was removed by Bi2O3–TiO2/PAC(10%–700 °C) in 120 min of solar light irradiation without pre-adsorption and solution pH adjustment. It can be observed in Figure 4b that the coupling adsorption–photocatalysis efficiencies for SMZ in the river water and lake water were lower than those of ultrapure water. The removal performance for SMZ in the lake water was better than that in the river water, probably because the organic matter of the river water was more abundant than that of the lake water. The inhibition was mainly caused by HA, fulvic acid (FA), high-molecular-weight polysaccharides and proteins [43]. It has been reported that the inhibition effect was higher for FA than that for HA [44]. They not only competed with SMZ for the reactive oxygen species (ROS) in photocatalysis but also for the adsorption-active sites of Bi2O3–TiO2/PAC [45].

3.4. Effect of Water Quality Parameters

3.4.1. Effect of Initial pH

SMZ adsorption on Bi2O3–TiO2/PAC(10%–700 °C) was significantly affected by the pH of the solution, which governed the dissociation degree of the functional groups on the composite and changed the surface potential of the material. It was shown that at pH 7.0 of the solution, SMZ was more easily adsorbed on Bi2O3–TiO2/PAC(10%–700 °C) (Figure 5a). Because of two acid–base dissociation points of SMZ, there are three forms of SMZ at different pH values [46]. The neutral molecular form is more readily adsorbed than the other forms [47]. When the pH of the solution was 3.0–7.0, the adsorption removal of SMZ gradually increased, which was due to H+ and SMZ competitive adsorption. With the continuous augmentation of pH, the adsorption removal of SMZ declined. The electrostatic repulsion between the anionic forms of SMZ and the Bi2O3–TiO2/PAC(10%–700 °C), as well as the hydrophobic effect on the surface groups of the composite, affected the adsorption performance of the SMZ.
The effect of pH on the photocatalysis of Bi2O3–TiO2/PAC(10%–700 °C) was distinguished from that on adsorption. The SMZ photocatalytic efficiency increased from 61.3% to 67.9% as the initial pH increased from 3.0 to 7.0 (Figure 5b). When the initial pH further increased to 9.0, the removal efficiency continued rising but then dropped at pH 11. H+ can consume ●O2−, which was the main reaction species for the Bi2O3–TiO2/PAC(10%–700 °C) photocatalysis of SMZ; therefore, a high concentration of H+ resulted in a lower ●O2− yield (Equations (2) and (3)). The dissociation constant (pKa) of SMZ is pKa,1 = 1.75 and pKa,2 = 7.35 [48]. When the initial pH of the SMZ solution was higher than 7.35, the ability of sulfonyl-N to donate electrons to SMZ was enhanced due to deprotonation, which was conducive to the degradation of SMZ. However, when the concentration of OH− was higher, the photo-induced carriers h+ reacted with OH− and created ●OH (Equation (4)). The generated ●OH contributed less than h+ to the SMZ photocatalytic degradation by Bi2O3–TiO2/PAC(10%–700 °C), reducing the removal efficiency for SMZ.
O2 + e− → ●O2−
●O2− + 2H+ + e− → H2O2
h+ + OH− → ●OH

3.4.2. Effect of Inorganic Anions

The addition of 0.5 mM HCO3− improved the photocatalytic efficiency for SMZ. When the concentration of HCO3− increased above 0.5 mM, the photocatalytic efficiency decreased but remained higher than that without HCO3− (Figure 6a). HCO3− is an important scavenger for ●OH (Equations (5) and (6)) [49]. Since ●OH was not the dominant factor for photocatalysis and the resulting ●CO3− may be involved in SMZ degradation, the photocatalytic performance for SMZ was enhanced by adding a small amount of HCO3−. Meanwhile, the addition of HCO3− increased the pH of the solution from 7 to 8.5, which also promoted the photocatalytic degradation of SMZ by Bi2O3–TiO2/PAC(10%–700 °C).
In the presence of Cl− and SO42−, as shown in Figure 6b,c, the removal efficiency for SMZ of Bi2O3–TiO2/PAC(10%–700 °C) slightly decreased with the different concentrations of Cl− and SO42−. Both these anions can react with h+ and ●OH (Equations (7) and (8)), resulting in a decrease in photocatalytic performance. It should be stated that Cl− was less influential than SO42− for photocatalysis. The 5.0 mM addition of SO42− led to a 17.8% decrease in photocatalytic efficiency compared to 0 mM SO42− in the solution at the initial 20 min. A previous study has reported that SO42− could be attached to the surface of TiO2 via van der Waals forces and hydrogen bonds, and replace some of the ●OH of TiO2 by a ligand exchange mechanism [50].
HCO3− + ●OH → ●CO3− + H2O
CO32− + ●OH → ●CO3− + OH−
Cl−/SO42− + h+ → ●Cl/●SO4−
Cl−/SO42− + ●OH → ●Cl/●SO4− + H2O

3.4.3. Effect of HA

The presence of HA inhibited the photocatalytic degradation of SMZ; with an increase in the HA concentration from 0 to 5.0 mg/L, the photocatalytic efficiency for SMZ decreased by 9.1% (Figure 6d). Firstly, HA competed with SMZ for the ROS in the photocatalysis. Furthermore, the photons were contended by HA and Bi2O3–TiO2/PAC(10%–700 °C), resulting in the production of fewer active oxygen species. However, the SMZ photocatalytic efficiency was up to 58.8%, even at a 5.0 mg/L HA concentration, indicating the photocatalytic stability and availability of Bi2O3–TiO2/PAC(10%–700 °C).

3.5. Characterization of Bi2O3–TiO2/PAC Composite

3.5.1. XRD Analysis

The sharp peaks with high intensity identified that Bi2O3–TiO2/PAC was well crystallized, which showed anatase (JCPDS 21-1272) and rutile (JCPDS 76-0323, JCPDS 88-1173) crystalline phases for TiO2 [51], and a Bi2O3 (JCPDS 74-1633, JCPDS 74-1373) crystalline phase [52] existed, as shown in Figure 7a. The percentages of rutile in the anatase–rutile mixtures for Bi2O3–TiO2/PAC(8%–700 °C), Bi2O3–TiO2/PAC(10%–700 °C) and Bi2O3–TiO2/PAC(12%–700 °C) were ca. 28, 31 and 34 wt%, respectively, which were determined using Spurr and Myer’s equation [53]. It has been shown that a mixture of these two crystals exhibited higher activity than pure anatase or rutile [54], especially the TiO2 crystal type composed of 30% rutile and 70% anatase, exhibiting the highest activity [55]. The average crystal size was calculated using the Scherrer equation [56], and the effect of the instrument broadening effect was deducted. As shown in Table 2, the average crystal size of Bi2O3–TiO2/PAC(10%–700 °C) was larger than that of Bi2O3–TiO2/PAC(8%–700 °C) and Bi2O3–TiO2/PAC(12%–700 °C); meanwhile, it was 2.1 and 1.9 times larger than that of Bi2O3–TiO2(10%–700 °C) and TiO2(700 °C). Due to doping Bi2O3 to TiO2, the intensity of the anatase phase in TiO2 decreased (Figure 7b), which promoted a rutile phase shift. The addition of PAC obviously enhanced the formation of the crystalline phase.

3.5.2. BET Surface Area and Pore Size Distribution Analysis

According to the IUPAC classification, the BET isotherm of the Bi2O3–TiO2/PAC matches the type IV isotherm (Figure 8). The majority pore size of the composite was 2–50 nm, which indicated that substantial mesoporous structures exist. The pore size distribution range of Bi2O3–TiO2/PAC(12%–700 °C) was significantly larger than that of Bi2O3–TiO2/PAC(8%–700 °C) and Bi2O3–TiO2/PAC(10%–700 °C). As the Bi/Ti molar ratio increased, the pore volume decreased (Table 2), which further proved that Bi2O3 influenced the formation of pores. Bi2O3–TiO2/PAC(10%–700 °C) exhibited a larger BET surface area than Bi2O3–TiO2(10%–700 °C) and TiO2(700 °C), which was due to the addition of PAC. The pore volumes of PAC and Bi2O3–TiO2/PAC(10%–700 °C) were 0.654  and 0.277 cm3/g, respectively, demonstrating that some of the Bi2O3–TiO2 nanoparticles were deposited into the PAC pores. From the N2 absorption–desorption isotherm analysis and pore size distribution in Figure 9, it can be inferred that parts of the pores in PAC were occupied by Bi2O3–TiO2, causing the aperture range to move to the right.

3.5.3. UV-Vis Diffuse Reflectance Spectrum Analysis

The absorption spectrum of Bi2O3-TiO2/PAC exhibited significant red-shifts to the visible−light region compared with Bi2O3-TiO2(10%–700 °C) and TiO2(700 °C), indicating the combination of Bi2O3-TiO2/PAC extended light absorption to visible light region and improved the light harvesting. The photophysical properties of Bi2O3–TiO2/PAC(10%–700 °C) revealed a stronger photo-absorption band in both ultraviolet and visible light than the others (Figure 10). The band gap of Bi2O3–TiO2/PAC(10%–700 °C) was 2.58 eV as calculated by the Tauc-plot method [57], which was the minimum among all. The formation of the heterostructure caused by the Bi doping and PAC loading reduced the band gap energy of Bi2O3–TiO2/PAC(10%–700 °C) [58]. It has been proposed that the energy band lies between the Ti4+ 3d band and Bi3+ 6s band [59].
The surface morphology and microstructure of Bi2O3–TiO2/PAC(10%–700 °C) was revealed by SEM-EDS and HRTEM images, shown in Figure S5. Bi2O3 and TiO2 particles were attached to the surface and pores of the PAC. PAC accounted for a larger proportion in the Bi2O3–TiO2/PAC(10%–700 °C). It was obvious that Bi2O3 and TiO2 were reasonably well supported on PAC, and parts of distinct pores manifested the retained adsorption properties of the PAC. The coexistence of Bi2O3, anatase TiO2 and rutile TiO2, by matching to the lattice fringes in the as-synthesized composite, was certified.

4. Discussions

4.1. Adsorption Mechanism

The adsorption mass transfer mechanism includes external diffusion, internal diffusion and adsorption on active sites. The mass transfer mechanism can be summarized by an adsorption kinetic study [60]. The pseudo-second-order kinetic model was related to the availability of adsorption sites on the surface of the adsorbent [31]. It was found that the mass transfer adsorption of Bi2O3–TiO2/PAC(10%–700 °C) met the Boyd’s external diffusion equation and Langmuir kinetic model, as shown in Table 3, while the k value of the Langmuir kinetics model was lower. In the assumptions of these two models, the slowest adsorption step for each is required. Therefore, it is comprehensively judged that the adsorption mechanism of Bi2O3–TiO2/PAC(10%–700 °C) is mainly adsorption onto active sites as well as assistance by external diffusion, and internal diffusion rarely exists. The adsorption process was influenced with the interaction controlled by the exchange and share of e− between Bi2O3–TiO2/PAC composites and SMZ [61,62].

4.2. Photocatalytic Mechanism

Trapping experiments show that both ●O2− and h+ played a greater role, while ●OH was weaker than these two radicals (Figure 11). As for SMZ removal, the experiments of trapping similarly identified ●O2− and h+ as the main radicals. Moreover, the SMZ removal under solar light irradiation without composite was only 3.6%; thus, the photolysis by solar light irradiation can be neglected. The mineralization degree indicated by TOC/TOC0 reached 0.351 in 120 min of solar irradiation at adsorption equilibrium (Figure S6), confirming that SMZ decomposed into small fragments.
Possible SMZ photocatalytic pathways for Bi2O3–TiO2/PAC(10%–700 °C) were proposed. The specific chemical formulas and SMZ chromatographic data are shown in Table S5 and Figure S7. A speculative reaction mechanism is shown in Figure 12—that the first path was the structural damage caused in the benzene ring fracture, which agrees well with the findings of others [63,64]. The second path was SO2 extrusion, leading to the appearance of intermediate product B, which is a common phenomenon in the degradation process for SAs [65]. Further attack of the sulfonamide bond of B produced E. The third path was the nitrogenated SMZ being formed by the oxidation of the amino group of the benzene ring. After that, the intermediates were mineralized into fracture rings and simple compounds, such as CO2, H2O and NH4+ [66].

4.3. Adsorption–Photocatalysis Mechanism

The coupling adsorption–photocatalysis mechanism of Bi2O3–TiO2/PAC in removing SMZ under solar light irradiation is illustrated in Figure 13. For the Bi2O3–TiO2/PAC system, SMZ was mainly adsorbed on the active sites of the Bi2O3–TiO2/PAC. Meanwhile, the ROS produced by the composite were non-selective and attacked functional groups of SMZ. A heterogeneous junction was formed by doping Bi2O3 with TiO2, which greatly reduced the band gap of the photocatalysts and provided good conditions for the separation of photo-generated carriers, greatly improving the photocatalytic performance under visible light. The adsorption capacity of PAC ensured persistent high levels of pollutants for the photocatalytic system [5]. Additionally, the combination of PAC could improve the adsorption performance and recovery rate of photocatalysts, which was of great theoretical and engineering significance for an appropriate adsorption–photocatalysis composite.

5. Conclusions

Under a controlled Bi/Ti molar ratio, Bi2O3–TiO2/PAC ternary nanoparticles exhibiting adsorption–photocatalysis are capable of removing SMZ in water. The adsorption capacity of PAC ensured persistent high levels of pollutants for the photocatalysis system. Adsorption onto active sites assisted by external diffusion is the main mechanism of adsorption. ●O2− and h+ were identified as the major ROS in photocatalysis. Bi2O3–TiO2/PAC(10%–700 °C) exhibited the best SMZ removal performance, at 95.5% under directly solar light irradiation. The crystal size of Bi2O3–TiO2/PAC(10%–700 °C) was larger than that of Bi2O3–TiO2/PAC(8%–700 °C) and Bi2O3–TiO2/PAC(12%–700 °C). The visible light response was enhanced because of heterostructures, a narrowed band gap and an excellent mixture ratio of anatase to rutile in the phase of TiO2. As the Bi/Ti molar ratio increased, the pore volume decreased. The removal performance for SMZ in lake water was better than that for river water. Under acidic conditions, photocatalysis performance was greatly reduced. When the pH was above 7, the photocatalytic effect was enhanced while the adsorption performance declined. Besides, the increase in HCO3− improved the photocatalytic efficiency for SMZ, while the addition of Cl−, SO42− or HA had negative effects on the photocatalytic efficiency for SMZ. Benzene ring fracture, SO2 extrusion and nitrogenated SMZ were proposed as the main pathways of SMZ degradation. Antibiotics have been identified as a particular category of trace chemical contaminants. The Bi2O3–TiO2/PAC adsorption–photocatalysis system has exhibited great efficiency for the removal of one of the typical refractory antibiotics. The outcomes of this study provide significant insights into the adsorption–photocatalysis system and potentially benefits the control of antibiotic pollutants in water.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4441/12/8/2273/s1, Figure S1: Plot of qt-t for the adsorption of SMZ onto Bi2O3-TiO2/PAC composites. Figure S2: Adsorption isotherm of SMZ adsorption on Bi2O3-TiO2/PAC with different calcination temperature. Figure S3: Photocatalytic efficiencies, Bi2O3-TiO2/PAC with different calcination temperature. Figure S4: Photocatalytic efficiencies of Bi2O3-TiO2/PAC with different calcination temperature. Figure S5: Bi2O3-TiO2/PAC(10%–700 °C) microstructure image. FE-SEM image (a), SEM-EDS spectra (b), HRTEM micrograph (c) and the lattice parameters for composite (d). Figure S6: TOC removal in the photodegradation of SMZ by Bi2O3-TiO2/PAC(10%–700 °C). Figure S7: TIC and Mass chromatography of SMZ on Bi2O3-TiO2/PAC system. Table S1: Preparation method of Bi2O3-TiO2/PAC composites. Table S2: Adsorption kinetic and isotherm models used for data analysis. Table S3: Main water quality of surface water. Table S4: Adsorption kinetic parameters of Bi2O3-TiO2/PAC composites with different calcination temperatures. Table S5: Chemical formulas and main fragments (m/z) of intermediate products.

Author Contributions

X.Z., J.L. and H.W. were responsible for the experimental work. X.L. and Y.Y. supervised the laboratory work. Z.Z. led the research. N.W. and Y.S. provided technical support for this work. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (51978006).

Acknowledgments

Sincerely thanks to Teacher Tang for the help on material characterization.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Adsorption isotherm of sulfamerazine (SMZ) adsorption on Bi2O3–TiO2/PAC with different molar ratios (a), Bi2O3–TiO2(10%–700 °C), TiO2(700 °C) and PAC (b).
Figure 1. Adsorption isotherm of sulfamerazine (SMZ) adsorption on Bi2O3–TiO2/PAC with different molar ratios (a), Bi2O3–TiO2(10%–700 °C), TiO2(700 °C) and PAC (b).
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Figure 2. Photocatalytic efficiencies, Bi2O3–TiO2/PAC with different Bi/Ti molar ratios (a) and Bi2O3–TiO2 and TiO2 (b).
Figure 2. Photocatalytic efficiencies, Bi2O3–TiO2/PAC with different Bi/Ti molar ratios (a) and Bi2O3–TiO2 and TiO2 (b).
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Figure 3. Photocatalytic kinetics, Bi2O3–TiO2/PAC with different Bi/Ti molar ratios (a) and Bi2O3–TiO2 and TiO2 (b).
Figure 3. Photocatalytic kinetics, Bi2O3–TiO2/PAC with different Bi/Ti molar ratios (a) and Bi2O3–TiO2 and TiO2 (b).
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Figure 4. The contribution of Bi2O3-TiO2/PAC with different molar ratio (a), and the effect of real waters (b) on overall adsorption-photocatalysis performance for SMZ removal.
Figure 4. The contribution of Bi2O3-TiO2/PAC with different molar ratio (a), and the effect of real waters (b) on overall adsorption-photocatalysis performance for SMZ removal.
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Figure 5. Effect of initial pH on adsorption (a) and photocatalysis (b) of SMZ removal by Bi2O3–TiO2/PAC(10%–700 °C).
Figure 5. Effect of initial pH on adsorption (a) and photocatalysis (b) of SMZ removal by Bi2O3–TiO2/PAC(10%–700 °C).
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Figure 6. Effects of HCO3− (a), Cl− (b), SO42− (c) and HA (d) on the photocatalytic degradation of SMZ by Bi2O3–TiO2/PAC(10%–700 °C).
Figure 6. Effects of HCO3− (a), Cl− (b), SO42− (c) and HA (d) on the photocatalytic degradation of SMZ by Bi2O3–TiO2/PAC(10%–700 °C).
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Figure 7. XRD patterns of Bi2O3–TiO2/PAC with different Bi/Ti molar ratios (a), and PAC, TiO2(700 °C) and Bi2O3–TiO2(10%–700 °C) (b).
Figure 7. XRD patterns of Bi2O3–TiO2/PAC with different Bi/Ti molar ratios (a), and PAC, TiO2(700 °C) and Bi2O3–TiO2(10%–700 °C) (b).
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Figure 8. N2 absorption–desorption isotherm analysis (a) and pore size distribution (b) of Bi2O3–TiO2/PAC with different Bi/Ti molar ratios.
Figure 8. N2 absorption–desorption isotherm analysis (a) and pore size distribution (b) of Bi2O3–TiO2/PAC with different Bi/Ti molar ratios.
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Figure 9. N2 absorption–desorption isotherm analysis (a), and pore size distribution (b) of PAC, TiO2(700 °C) and Bi2O3–TiO2(10%–700 °C).
Figure 9. N2 absorption–desorption isotherm analysis (a), and pore size distribution (b) of PAC, TiO2(700 °C) and Bi2O3–TiO2(10%–700 °C).
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Figure 10. UV-vis diffuse reflectance spectra (DRS) spectra of Bi2O3–TiO2/PAC, Bi2O3–TiO2(10%–700 °C) and TiO2(700 °C).
Figure 10. UV-vis diffuse reflectance spectra (DRS) spectra of Bi2O3–TiO2/PAC, Bi2O3–TiO2(10%–700 °C) and TiO2(700 °C).
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Figure 11. 2 mM (a) and 5 mM (b) of scavenging agents concentration for ROS detection for SMZ removal by Bi2O3–TiO2/PAC(10%–700 °C).
Figure 11. 2 mM (a) and 5 mM (b) of scavenging agents concentration for ROS detection for SMZ removal by Bi2O3–TiO2/PAC(10%–700 °C).
Water 12 02273 g011
Water 12 02273 g012 550
Figure 12. Proposed pathways of SMZ catalysis by Bi2O3–TiO2/PAC(10%–700 °C).
Figure 12. Proposed pathways of SMZ catalysis by Bi2O3–TiO2/PAC(10%–700 °C).
Water 12 02273 g012
Water 12 02273 g013 550
Figure 13. Schematic diagram of the coupling adsorption and photocatalysis mechanism of Bi2O3–TiO2/PAC composites irradiated under sunlight.
Figure 13. Schematic diagram of the coupling adsorption and photocatalysis mechanism of Bi2O3–TiO2/PAC composites irradiated under sunlight.
Water 12 02273 g013
Table
Table 1. Adsorption kinetic parameters of Bi2O3–TiO2/PAC composites with different Bi/Ti molar ratios, Bi2O3-TiO2(10%–700 °C) and TiO2(700 °C).
Table 1. Adsorption kinetic parameters of Bi2O3–TiO2/PAC composites with different Bi/Ti molar ratios, Bi2O3-TiO2(10%–700 °C) and TiO2(700 °C).
CompositesPseudo-First-OrderPseudo-Second-OrderIntra-Particle Diffusion
k1
(h−1)		qe
(mg·g−1)		R2k2
(mg·g−1·h−1)		qe
(mg·g−1)		R2Kd
(mg·g−1·h−1)		CR2
Bi2O3–TiO2/PAC
(8%–700 °C)		0.0251.3880.6670.0845.4501.0000.4322.1470.631
Bi2O3–TiO2/PAC
(10%–700 °C)		0.0521.7600.8200.0384.8170.9970.4741.0870.818
Bi2O3–TiO2/PAC
(12%–700 °C)		0.0521.5570.9820.0283.7230.9930.3420.5240.889
Bi2O3–TiO2
(10%–700 °C)		0.1432.1510.9510.0323.3970.9900.3180.5630.813
TiO2(700 °C)0.0811.3790.9010.0685.2910.9940.4651.7040.636
Table
Table 2. Physico-chemical parameters of the prepared catalysts.
Table 2. Physico-chemical parameters of the prepared catalysts.
SamplesBET Surface Area (m2/g)Pore Volume (cm3/g)Average Pore Size (nm)Average Crystal Size (nm)
Bi2O3–TiO2/PAC(8%–700 °C)123.0440.28014.40746.9
Bi2O3–TiO2/PAC(10%–700 °C)130.7360.27713.74048.2
Bi2O3–TiO2/PAC(12%–700 °C)103.4450.24815.27647.8
Bi2O3–TiO2(10%–700 °C)84.7850.35116.54023.3
TiO2(700 °C)61.9210.1227.85925.3
PAC928.7670.6544.899-
Table
Table 3. Adsorption mass transfer kinetics parameters of Bi2O3–TiO2/PAC(10%–700 °C).
Table 3. Adsorption mass transfer kinetics parameters of Bi2O3–TiO2/PAC(10%–700 °C).
TypesModelskqeR2
External diffusionBoyd’s external diffusion equation0.1114.520 (q∞)0.995
Internal diffusionWeber and Morris model0.6123.4550.787
Adsorption onto active sitesLangmuir kinetics model0.006 (ka), 0.0004 (kd)4.5020.993

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Zhuang, X.; Li, X.; Yang, Y.; Wang, N.; Shang, Y.; Zhou, Z.; Li, J.; Wang, H. Enhanced Sulfamerazine Removal via Adsorption–Photocatalysis Using Bi2O3–TiO2/PAC Ternary Nanoparticles. Water 2020, 12, 2273. https://doi.org/10.3390/w12082273

AMA Style

Zhuang X, Li X, Yang Y, Wang N, Shang Y, Zhou Z, Li J, Wang H. Enhanced Sulfamerazine Removal via Adsorption–Photocatalysis Using Bi2O3–TiO2/PAC Ternary Nanoparticles. Water. 2020; 12(8):2273. https://doi.org/10.3390/w12082273

Chicago/Turabian Style

Zhuang, Xiaoxuan, Xing Li, Yanling Yang, Nan Wang, Yi Shang, Zhiwei Zhou, Jiaqi Li, and Huiping Wang. 2020. "Enhanced Sulfamerazine Removal via Adsorption–Photocatalysis Using Bi2O3–TiO2/PAC Ternary Nanoparticles" Water 12, no. 8: 2273. https://doi.org/10.3390/w12082273

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