Electron-rich biochar enhanced Z-scheme heterojunctioned bismuth tungstate/ bismuth oxyiodide removing tetracycline

: Photocatalytic treatment of antibiotics in aqueous ecosystems has become a promising method. However, the low efficiency photogenerated charge separation and slow kinetics of the catalyst severely limit its deployment for industrial applications. Here, the three-dimensional bismuth tungstate (Bi 2 WO 6 )/ bismuth oxyiodide (BiOI) loaded on biochar (BC/BWI) composite catalyst was designed for the efficient removal of tetracycline (adsorption capacity: 227.09 mg/g, removal rate: 99.8 %). Via construction of Z-scheme heterojunctions at the interface of Bi 2 WO 6 and BiOI, the built-in electric field promotes the directional separation of photogenerated carriers to achieve efficient separation and utilization of photogenerated charges. Meanwhile, the introduction of electron-rich biochar (BC) effectively enhances the adsorption performance, photogenerated electron migration capacity and mass transfer process of the material. The introduction of BC and the building of Z-scheme heterojunctions effectively achieve the spatially synergistic separation of photogenerated charges. The • O 2-dominates the photocatalytic process, according to the mechanistic studies. The degradation intermediate product testing revealed that tetracycline is efficiently degraded through two main pathways. This work provides ideas for the design of catalysts for the efficient removal of antibiotics from water bodies.


Introduction
Tetracycline is the second most used antibiotic in the world and is usually released directly into the environment through human and animal feces. 1-3 Tetracyclines in the environment lead to the emergence of antibiotic-resistant bacteria and antibioticresistant genes. 4 The emergence of drug resistance causes a decline in the therapeutic potential of humans and animals. 5,6 According to statistics, 0.7 million people die each year due to antibiotic resistance. Meanwhile, this number will increase to 10 million with the continuous rise in the use of antibiotics in 2050. 7 In addition, tetracycline in water ecosystems will affect the ecological balance, which in turn will seriously affect the environment. [8][9][10] In the face of current antibiotic pollution, finding an efficient and green water treatment method has become the key to address these problems.
Among the reported many antibiotic and organic removal methods, [11][12][13] photocatalysis has received widespread attention for its advantages of environmental protection, high efficiency and no secondary pollution. 14-16 Bi 2 WO 6 stands out in antibiotic removal with its excellent light absorption properties and suitable band gap structure. 17 Many works have demonstrated the excellent performance of Bi 2 WO 6 in the antibiotic removal process. 18 Construction of heterojunction at the interface can effectively improve Bi 2 WO 6 properties. Bi 2 WO 6 /BiOI exhibits excellent photocatalytic performance. Construction of heterojunction effectively separates photogenerated charges and reduces the recombination rate of photogenerated carriers 19 . However, the photogenerated charge recombination rate still greatly limits the performance of the catalyst due to carrier recombination during the migration process.
Various biochars have been prepared and applied in energy storage, analysis and other fields. [20][21][22] The electron-rich biochar can now significantly increase the reactive sites, improve adsorption performance, and accelerate photogenerated electron migration. 23,24 These properties of biochar make it a suitable carrier for photocatalysts.
To address the problems of low photogenerated charge separation and slow reaction kinetics, the construction of heterojunctions 25 and the introduction of electron-rich biochar became the solution strategy. 26 In order to achieve effective separation of photogenerated charges and increase the reaction kinetics, a space charge synergistic separation strategy was created. BC/BWI composites were prepared by the calcination method and solvothermal method.
Photocatalytic removal of tetracycline shows excellent performance. The Z-scheme heterojunctions at the interface of Bi 2 WO 6 and BiOI were constructed for the effective separation of photogenerated charges while the built-in electric field promoted the directional migration of photogenerated carriers. The introduction of electron-rich biochar effectively increases the active site and adsorption performance, accelerates the photogenerated electron migration and increases reaction kinetics. Synergistic charge separation of electron-rich carriers and Z-scheme heterojunctions increase the photocatalytic removal of tetracycline. Mechanistic studies indicated that •O 2is the main photoactive species. Analysis of reaction intermediates revealed that tetracycline is mineralized through two main degradation pathways. This research provides promising ideas for designing efficient antibiotic removal.

Photocatalytic removal of tetracycline
Tetracycline was used as a pollutant model to evaluate the adsorptionphotocatalytic performance of catalysts. Typically, 10 mg of catalyst was distributed in 100 mL of 50 mg/L tetracycline solution. The adsorption-desorption equilibrium was achieved by stirring for 40 minutes in the dark. The 300 W xenon lamp (λ ≥ 420 nm, Hxuv300 Visref, Beijing China Education Au-light Co) was then turned on to start the photocatalytic degradation process. The photocatalytic degradation time was 60 min, and 3 mL of the solution was filtered at 10 min intervals. UV spectrophotometer is used to detect the tetracycline while the maximum absorption wavelength was 358 nm.
(1) = ln C 0 / where K, C 0 and C t are the degradation rate constants, the concentration of tetracycline at initial and time t, respectively. q e , V, M and C% are adsorption capacity, reaction liquid volume, catalyst mass, and degradation rate, respectively.

Supplementary
Information includes detailed information and photoelectrochemical measurements and synthesis of Bi 2 WO 6 , BC and BiOI.

Material structure characterization
In this study, biomass waste corncob was used as raw material to prepare porous biochar (BC) by high temperature carbonization and alkaline activation. Bi 2 WO 6 and BiOI Z-type heterojunction were constructed on BC surface by hydrothermal method.
The synthesis process of BC/BWI is shown in Fig. 1. According to the band structure of Bi 2 WO 6 and BiOI, the constructed heterojunction is more consistent with the Z-type heterojunction, which can better promote photo-generated charge separation and retain strong oxidation and reduction capabilities. Meanwhile, in-situ construction of BC/BWI Z-type heterojunction on porous BC can also improve the photocatalytic efficiency, and further convert TC into small molecule substances. In addition, the introduction of electron-rich BC accelerates the reaction mass transfer process, promotes the migration of photogenerated charges and enhances the photocatalytic performance Detailed morphology and microstructure studies were conducted on the prepared samples using SEM and TEM. The SEM and TEM images of BC prepared by hightemperature carbonization and alkaline activation methods are shown in Fig. S1(a-f).
The TEM images demonstrate the 2D flat and thin layered structure of typical biomass carbon materials, which is also an important reason for the increase in specific surface area. 27,28 As shown in Fig. 2a and Fig. S1(g-i), Bi 2 WO 6 is a flower like structure formed by crossing a large number of nanosheets. The pure BiOI is assembled from a large number of irregular nanosheets (Fig. 2b), which are loose, rough, and disordered. Fig.   2c shows a BCs/BWI 0.2 composite catalyst well covered by Bi 2 WO 6 and BiOI nanofragments. Unlike BC, it is clearly observed that these fragments are more loosely and loosely attached to BC.
To further demonstrate its details, Fig. 2c shows a large and distinct sheet-like structure in the lower part of the image, while the upper part is mostly composed of loose nano fragments with uncovered carbon voids. These findings demonstrated that the production of BiOI is an in situ growing process on Bi 2 WO 6 nanoflowers, which can be regarded as a topological chemical conversion. 29 Due to the abundant attachment points provided by carbon materials, the composite of the two aggregates on BCs forms the structure as shown in the figure. In theory, after adding KI and Na 2 WO 6 solutions, BiOI is generated in the reaction system. As the reaction progresses, BiO + is gradually released, and then BiO + reacts with WO 4 2to generate Bi 2 WO 6 based on the topological chemical reaction, which is in close contact with the continuously generated BiOI. 30 As a result, a heterostructure was formed between Bi2WO6 and BiOI, enhancing charge separation in photocatalytic processes. With the increase of KI, Bi 2 WO 6 Nanoflower gradually collapses and decreases, and more and more BiOI is generated. The ideal molar ratio of Bi 2 WO 6 to BiOI was therefore determined to be 20%, taking into account that stable heterostructures can show excellent photocatalytic performance.  (e, f) shows a locally enlarged view, both of which exhibit distinct spine-like structures.  W-O tensile vibration and translational mode, respectively. 35 For BC-BWI 0.2 , apart from the above peak positions, the observed only one peak located at 147 cm -1 is attributed to the Bi-I vibration of BiOI. 36 These characterization analyses indicate that the synthesis of BC/BWI 0.2 catalyst was successful, consistent with the XRD analysis.
The nitrogen adsorption measurement evaluated the specific surface area and pore structure properties. In Fig. 3c, a H 3 hysteresis loop type IV with a P/P 0 range of 0.8 to 1.0 is clearly visible in BC/BWI. 37 As shown in Fig. 3d, all samples prepared in this study have a large number of micropores and mesoporous structures (pore size 0-2 nm). Furthermore, more-small sized BiOI fragments enter the pore structure of BC, resulting in a smaller pore size. During the growth process, the composite enters the pores, cross aggregates, and accumulates on the surface of carbon materials, which may be the reason for the increase in pore volume. This will be beneficial for providing more adsorption centers and improving the catalytic performance of the complex.
The UV-Vis diffuse reflectance spectra were tested to examine the series of optical properties of photocatalysts, as shown in Fig. 3e. In the above experiment, the absorption edge of pure Bi 2 WO 6 was at 440 nm. In Fig. 1e, the light absorption edge of  of C1s at 284.6 eV can be assigned as the carbon signal used for calibration in the instrument. 40,41 In the spectrum of Bi 4f, as shown in Fig. 4b   The LSV data demonstrates that BC/BWI has a minimum overpotential, which indicates that BC/BWI can effectively separate photogenerated charges (Fig. 5d). The Mott Schottky method was used to measure the flat band (FB) potential of the catalyst, and Fig. 5(e, f)

Removal of tetracycline
The adsorption performance of the material will effectively affect the subsequent photocatalytic process and also have a significant role in the removal rate of tetracycline.
The adsorption properties of the catalyst were studied using tetracycline as a pollutant model (Fig. 6a). The introduction of BC significantly increases the adsorption performance of the catalyst. The adsorption capacity of BC/BWI 0.2 is up to 227.09 mg/g (Table S2). This reveals that BC not only disperses the catalyst, but also effectively increases the adsorption sites, enhances the mass transfer process of TC molecules, and improves the accessibility of the reaction. On this base, the BC/BWI adsorption kinetics were investigated. The adsorption process of TC is divided into two main stages. In the 30 min, the adsorption process has a large mass transfer driving force because of the high TC concentration in the solution. Meanwhile, during the initial stage of adsorption, the material has a large number of free adsorption sites, allowing TC to adsorb rapidly on the material surface or in the pore structure. 49 The adsorption rate gradually decreases as the adsorption site is occupied and the TC concentration in the solution decreases. At 40 min, the adsorption-desorption process is in dynamic balance. 50 The results of the adsorption kinetic fit are shown in Table S2. The correlation results indicate that the pseudo-second-order kinetic equation is more appropriate to explain the adsorption method of tetracycline in BC/BWI (Fig. 6b-c, Eq(S1)-(S2)). This reveals that the whole adsorption process may be dominated by chemical adsorption. Also, chemical interactions between the adsorbent and tetracycline may occur through exchange of electrons or sharing of electrons. 51,52 The intra-particle diffusion model was accustomed to describe the adsorption behavior (Fig. 6d, Eq(S3)). In the first diffusion phase, K dif 1 values are relatively large and TC is controlled by molecular diffusion and membrane diffusion. In the second diffusion phase, the K dif 2 value is smaller, indicating that TC diffuses from the outer surface of the catalyst into the pore structure. Also, the fitted curve does not pass through the origin. This indicates that the intra-particle diffusion model is not the only rate-controlling mechanism, but that other diffusion mechanisms are also involved in the process. 53 within 1 hour. In addition, we also tested that the degradation efficiency of BWI 0.2 on TC was about 51.84%, which is 1.5 times that of Bi 2 WO 6 and nearly 3 times that of BiOI. However, it is evident in the figure that the adsorption capacity of Bi 2 WO 6 , BiOI, and BWI 0.2 is almost zero. With the increase of BC, the adsorption capacity of the composite catalyst for TC is greatly improved. At the same time, the photocatalytic efficiency has also slightly improved, thanks to the fact that BC can serve as receptors  (Table S4). This study shows a significant advantage in the removal of TC, which also proves the efficiency of the space charge synergistic separation.   With these data ( Fig. 7 and Fig. 8), the generation process of photoactive species is proposed. Under visible light irradiation, the semiconductor undergoes an energy band jump (Eq (4) - (5)). eand h + are produced on the band gap structure of the semiconductor, respectively. Based on the photoelectric characterization data (Fig. 3ef and Fig. 5e- in the photocatalytic process, which is not consistent with the experimental reality ( Fig.7e and Fig. 8). (4) The formation of type Ⅱ heterojunction is inconsistent with the experimental situation, which indicates that the formation of Z-scheme heterojunction. E CB1 electrons leap to E VB2 with holes complex (Eq (6)). The photogenerated electrons on E CB2 also efficiently migrate to BC. Meanwhile, E VB1 and E CB2 generate photoactive radicals with environmental factors (H 2 O and O 2 ), respectively (Eq (7)- (13)). The formation of Zscheme heterojunction effectively separates the photogenerated charge and also preserves the redox ability of the material. 58 BC acts as an electron-rich carrier to accelerate the migration of photogenerated electrons and further enhance photocatalytic performance. [59][60][61][62][63] This work introduces electron-rich carriers and constructs Z-scheme heterojunction is to achieve efficient removal of TC. Charge synergistic separation strategy is a favorable idea for designing efficient photocatalysts. Photoactive species were involved in the degradation of TC, which was generated into small molecule products through two main pathways.

Declarations
These authors have no conflict of interest.

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