Controllable Preparation Of Magnetic Hydroxyapatite Microspheres And Their Photocatalytic Performance After Loading With Ag₃PO₄

Semiconductor photocatalysis has been a new technology for wastewater treatment in recent years, and it has wide application potentials in the field of industrial wastewater treatment. Semiconductor photocatalyst is the nucleus of the semiconductor photocatalysis. At present, a lot of semiconductor photocatalysts with excellent performances have been developed, including TiO2, ZnO, Bi2WO6, Ag₃PO₄, etc. Ag₃PO₄ can utilize sunlight effectively for the photocatalytic reactions because it can be excited by visible light. Furthermore, Ag₃PO₄ also has the advantage of high activity. Its photocatalytic activity is higher than most of the known semiconductor photocatalysts when illuminated with visible light. However, the chemical property of Ag₃PO₄ is not stable, which shortens its service life greatly as a photocatalyst. Therefore, it has been one of important research problems for researchers in the field of Ag₃PO₄ photocatalyst to search for a method to retain its high catalytic activity and improve its stability.This dissertation mainly focuses on the improvement of the stability and photocatalytic activity of the Ag₃PO₄ photocatalyst, and the solution is the preparation of magnetic hydroxyapatite（HAp）/Ag₃PO₄ composite photocatalyst. The specific research programme of this dissertation are as follows: Firstly, the CaCO3 microspheres（CCMs） were synthesized with a precipitation method, and the HAp microspheres（HMs） were obtained via a microwave-assisted preparation method. Secondly, the magnetic HMs were prepared with the addition of magnetic particles during the preparation process based on the systematic study of the above two kinds of microspheres. Finally, a kind of magnetic photocatalyst was obtained after the growth of Ag₃PO₄ particles on surface of the magnetic HMs via an in-situ ion-exchange method, and its photocatalytic performance was also studied. The research results of this dissertation would provide the theoretical basis for the application and development of the Ag₃PO₄ photocatalyst.The main contents and conclusions of this dissertation are shown as follows:（1） The CCMs have been prepared with Ca（NO3）2·4H2O, Na2CO3 and pure water via a facile precipitation method without any other additives. The results showed the existence of two kinds of microspheres in different sizes in CCMs. The diameters of them were about 5 μm and 2 μm, respectively. Both of them were assembled by nanoparticles which were less than 100 nm in diameter. Various CaCO3 samples with different morphologies including microspheres, aggregates of microspheres and microcubes, aggregates of nanorods and nanocubes, dumbbell-like ordered structures of microrods, and monodisperse microrods could be obtained by changing the reaction conditions. The morphologies and crystal phase compositions of CaCO3 samples were closely related. All of the CaCO3 samples with microsphere morphologies were composed of vaterite and calcite, and the content of vaterite was very high. If the content of vaterite was relatively low and the content of calcite was relatively high, microcubes would appear in the CaCO3 samples. If the content of aragonite was relatively high in the CaCO3 samples, nanorods or microrods would appear.（2） The HMs have been prepared rapidly by the treatment of CCMs with the help of microwave. The results showed that the HMs were about the same size as the CCMs, and they were composed of nanorods or nanosheets. The microwave power and reaction time could affect the conversion efficiency of CCMs to HMs, while the phosphate and water dosages could affect the morphologies of the obtained HMs. Various HMs composed of nanorods, spindle-like nanorods, large nanosheets or small nanosheets could be obtained by changing the reaction conditions. The key factor that affected the morphologies of the HMs was the initial phosphate concentration in the reaction system. The HMs were composed of nanorods when the initial phosphate concentration was relatively low. The nanorods would turn to nanosheets under opposite condition. Furthermore, this microwave-assisted method had obvious advantages when compared with the hydrothermal method.（3） Based on the systematic study of the CCMs and HMs, the magnetic Fe3O4/CaCO3 composite microspheres（FCCMs） have been prepared with the addition of Fe3O4 submicron particles during the preparation process of the CCMs. Then, the Fe3O4/HAp composite microspheres（F3HMs） have been prepared by the conversion of FCCMs with the help of microwave, which similar to the preparation process of HMs. Ultimately, stable γ-Fe₂O₃/HAp composite microspheres（F2HMs） were obtained after the heat treatment of F3 HMs. The results showed that the FCCMs were composed of CCMs with the Fe3O4 submicron particles attached onto their surfaces. The addition of Fe3O4 would not affect the structure and size of CCMs. Various Fe3O4/CaCO3 composite materials composed of Fe3O4 submicron particles and CaCO3 with different morphologies could be obtained by changing the reaction conditions. These included microspheres, aggregates of microspheres and microcubes, aggregates of nanorods and nanocubes, dumbbell-like ordered structures of microrods, and dendritic microrods. The F3 HMs were composed of HMs with Fe3O4 submicron particles attached onto their surfaces, and the existence of Fe3O4 would inhibit the aggregation of microspheres effectively. The morphologies of F2 HMs remained substantially unchanged, but the saturation magnetization of F2 HMs decreased when compared with that of F3 HMs.（4） Based on the successful synthesis of F2 HMs, different γ-Fe₂O₃/HAp/Ag₃PO₄ composite microspheres（FHAMs） photocatalysts have been prepared after the growth of Ag₃PO₄ particles on surfaces of the F2 HMs via an in-situ ion-exchange method. The results showed that these photocatalysts were composed of HMs with γ-Fe₂O₃ submicron particles and Ag₃PO₄ particles attached onto their surfaces. The saturation magnetization of FHAMs decreased when compared with that of F2 HMs. When the content of Ag₃PO₄ was higher, the saturation magnetization was lower, but all of the FHAMs samples could be separated from water with the help of an external magnetic field.（5） The photocatalytic performance of FHAMs photocatalysts was also studied. The results showed that all of the photocatalysts with different contents of Ag₃PO₄ could degrade the organic dyes in water under visible light irradiation. When the content of Ag₃PO₄ was higher, the photocatalytic performance was better. Many factors in photocatalytic experiments would affect the photocatalytic performance of the composite photocatalysts. In a certain range, a high dosage of photocatalyst, low concentration of pollutant, and strong intensity of light were beneficial for the enhancement of their photocatalytic performance. Furthermore, a weak acid or weak alkaline reaction system could also enhance their photocatalytic performance. The synergistic reactions between Ag₃PO₄ and HAp could promote the separation and transfer of the photogenerated electrons and holes. As a result, the photocatalytic performance and stability of the FHAMs photocatalyst were better than that of the pure Ag₃PO₄ photocatalyst. The FHAMs photocatalyst was magnetic and stable. It could be recycled with the help of an external magnetic field, and it could be reused for many times.