A Microbial Induced Synthesis of Hydroxyapatite with High UV Light Photocatalytic Activity for Tetracycline Degradation

More and more new materials have been developed, but the research on the development and utilization of the single-phase materials has been neglected. Assembled from nano-particles, a high specic surface area and porous hydroxyapatite (BI-HA) has been synthesized by feasible bacterial induction. The surface structure and morphology of the nanocomposites were characterized by Brunauer–Emmett–Teller (BET) apparatus, X-ray diffraction (XRD), transmission electron microscopy (TEM). The results suggest the obtained BI-HA powder with porous morphology, which were composed of nanoparticles with (100) crystal plane. The photoactivity of different HA samples was evaluated by the photocatalytic degradation of tetracycline hydrochloride (TC). The HA with (100) crystal plane displayed an obviously enhanced photocatalytic activity (75.33–86.43% for 60 min). Combined with experiments and DFT calculations, for the BI-HA with (100) crystal plane, it displayed better photocatalytic performance for photodegradation of TC. This study provides a viewpoint to fabricate high-performance nonmetal photocatalyst for wastewater treatment.


Introduction
Tetracycline (TC) is often detected from groundwater, surface water and sewage treatment systems as a new source of water pollution. It has caused "pseudo-persistence" pollution to water bodies, leading to an increase in drug-resistant pathogenic microorganisms, which has aroused widespread concern (Zhou et However, the photocatalytic activity of HA is still much lower than expected because of its poor charge transport, slow redox reaction kinetics, and low carrier mobility. Therefore, the development of a modi cation strategy to improve the HA charge kinetics is essential for achieving high performance HA  Here, HA particles were rst prepared by using a novel bacterial induction method. The crystal structure, morphological characteristics, and speci c surface area of HA particles were studied, and the photocatalysis of the prepared HA particles on the degradation of tetracycline (TC) under ultraviolet light was studied. This synthesis method is different from the conventional synthesis methods. Phosphatase can be released by Bacillus subtilis to control apatite mineralization. A large number of ne-pored, high speci c surface area HA crystal materials are surprisingly obtained. HA itself has good photocatalytic activity, and it does not need to add other photosensitive materials to enhance its photocatalytic performance. In addition, the intrinsic relationship between crystal morphological characteristics and photocatalytic activity of HA was preliminarily discussed. This work provided a simple method for improving the photocatalytic activity of HA, and also deepened the understanding of the photocatalytic mechanism of phosphate photocatalysts. 2.2 Synthesis of hydroxyapatite HA was synthesized by following steps. The rst step, Luria-Bertani (LB) liquid medium was prepared and conducted by high pressure steam sterilization. The second step, the activated Bacillus subtilis was inoculated into LB medium (V/V = 10%), and cultured in air bath oscillator under 37 o C for a certain time to obtain bacteria solution with different concentration. The last step, CaCl 2 and C 6 H 5 PO 4 Na 2 (Ca/P = 1.67) were dissolved completely in deionized water at room temperature, and the solution was conducted by sterilization. And the obtained solution was added into the above bacteria solution under a clean bench, and the mixture was cultured in air bath oscillator under 37 o C for 12 h. The obtained products were washed with ethyl alcohol and deionized water, and freeze dried for 24 h.

Characterization
The crystal structures of products were determined through X-ray powder diffraction (XRD; DMAX1400; Rigaku; Japan) with Cu Kα radiation, 2θ = 3-80°. The images of the particle morphology and of the products were obtained using a scanning electron microscope (SEM; TM-4000; Hitachi; Japan) and transmission electron micro-144 scopy (TEM; Zeiss Libra 200FE; Germany). Fourier transform infrared spectroscopy (FT-IR; Nicolet-6700; PerkinElmer; America) in transmittance mode was used within the range of 400-4000 cm -1 to identify the functional groups. Nitrogen adsorption-desorption isotherms were measured with an automatic surface area and porosity analyzer (AUTOSORB-1-C, Quantachrome, America) at the temperature of liquid nitrogen. The pore size distributions were derived from the adsorption branches of the isotherms using BJH theory. The particle size distribution of the assynthesized powder was analyzed by using a laser particle size analyzer (90 plus; Brookhaven Instruments Corporation; America). The values of pH at the point of zero charge (pH pzc) of HA were analyzed using Zeta Potential Analyzer Zeta potential (zetaPALS; Brookhaven; America).

Photocatalytic experiments
The TC degradation activity of the as-prepared HA samples was measured under 300 W mercury lamp.
Typically, 13.0 mg catalyst was dispersed in 50 mL aqueous solution containing TC with an initial concentration of 10 mg L −1 . The mixed solution was placed in the reactor, and the magnetic stirring was carried out for 30 min under the dark environment to reach an equilibrium of adsorption and desorption. At given time intervals, 5 mL solution was taken out every 15 min and centrifuged (4000 rpm, 5 min) to remove the catalyst. The concentration of supernatant was then measured using the UV-vis spectrometer.

Computational details
All DFT calculations were performed by using the Vienna ab initio simulation package (VASP). The generalized gradient approximation (GGA) with Perdew-Burker-Ernzerhof (PBE) functional was applied to address the nonlocal exchange correlation energy. DFT method with the U eff = 5.0 eV was used in all calculations (Yang et al. 2009). The kinetic cutoff energy of 600 eV was adopted, and the Brillouin zone integration was sampled with the 2 × 2 × 3 k-point mesh. Structure optimization was deemed as converged until the force of all atoms less than 0.03 eV/Å, and the criteria of energy convergence was set to 1 × 10 −5 eV.

Material characterization
The mechanism of BI-HA synthesis is presented in Figure 1. The morphologies of the as-prepared samples were investigated by SEM and (HR)TEM techniques. Figure 1 shows The FT-IR spectra of disodium phenyl phosphate dihydrate and the products prepared under different bacterial concentrations are presented in Figure 4. Figure 4a shows the FT-IR spectrum of This phenomenon demonstrated that the addition of Bacillus subtilis solution could favor the formation of BI-HA products.
As presented in Figure 5, the nitrogen adsorption and desorption isotherms and the pore size distribution curves of the product were investigated. According to the IUPAC classi cation, all the isotherms in Figures 5a-5d were assigned to types IV and H3 hysteresis loops, thereby con rming the existence of mesopores.

Photocatalytic degradation of TC
In order to evaluate the photocatalytic activity of different synthetic BI-HA powders, the degradation of TC using BI-HA1, BI-HA2, BI-HA3 and BI-HA4 were tested under UV light irradiation. Firstly, due to the larger speci c surface area of the prepared hydroxyapatite samples, the C/C 0 ratio under dark reaction conditions for 75 minutes was studied. As shown in Fig. 6a, lower than 22% TC was adsorbed onto the as-prepared photocatalysts in the dark, indicating relatively poor adsorption capacity of TC over these samples. Under UV light irradiation, the blank experiment test revealed that the self-degraded of TC is nearly negligible shown in Fig. 6b Under the catalysis of HA, tetracycline will react as follows: We know that the photoinduced electron/hole depends largely on the energy band structure and electronic density of state (DOS) of photocatalyst . As shown in Figures 8a and 8b

Conclusion
The HA photocatalysts had been obtained through a Bacillus subtilis induction method with special ower-like structure were obtained. The obtained BI-HA with porous morphology self-assembled by sheets, which were composed of nanoparticles, had an average particle size of 0.66 µm and a high speci c surface area. For the BI-HA with (100) crystal plane, it displayed better photocatalytic performance for photodegradation of TC. High reactive sites on the HA made it an effective photocatalyst for degradation of TC (above 86%). The BI-HA powder with (100) crystal plane shows dramatic photocatalytic activity, con rming their practical use in water puri cation.

Declarations
The authors declare the following nancial interests/personal relationships which may be considered as potential competing interests: Ethics approval and consent to participate: Not applicable Consent for publication: Not applicable Funding: National Natural Science Foundation of China(No. 21201142) Availability of data and materials Data availability statements can take one of the following forms (or a combination of more than one if required for multiple datasets): The datasets generated and/or analysed during the current study are available in the [NAME] repository, [PERSISTENT WEB LINK TO DATASETS] The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.
All data generated or analysed during this study are included in this published article [and its supplementary information les].
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Ting Zeng Preparation, creation and/or presentation of the published work, speci cally writing the initial draft (including substantive translation). Figure 1 Mechanism of synthesis of the BI-HA products induced by Bacillus subtilis.    Photocatalytic degradation of TC by as-prepared different HA catalysts with different crystallite structures and morphologies.