Adsorptive Removal Of Phosphate Using Zirconia-Functionalized Magnetic Particles

Excessive phosphate discharging may cause the eutrophication of water bodies, and adsorption is considered as one of the most effective methods for the removal of phosphate in water because of its operability, stability and recoverability. Recent research mainly focused on developing highly effective and steady adsorbents. Due to their small particle sizes, high surface areas, strong adsorption capacities, nanosized metal oxides become the preferred material for phosphate adsorption. However, high surface energy of nano-sized particles may lead to aggregation; meanwhile nanomaterials can readily enter into environments, resulting in potential risks. Due to the ease separation with extra magnetic field, magnetic adsorbents have received great attention in environmental pollution control. In order to improve the separation of phosphate adsorption nanomaterials, magnetic composite materials were prepared. In the thesis, magnetic Fe3O4 cores with amorphous SiO2, mesoporous SiO2 and amorphous carbon as respective shells were synthesized. The functionalization of the magnetic materials by ZrO2 was conducted using either deposition precipitation or post-grafting method with organic or inorganic Zr soure as the modification agent. The structure of ZrO2 functionized magnetic materials were investigated by a series of characterization methods. The phosphate adsorption behaviors of the adsorbents were tested by the static and dynamic adsorption, including the influence of water chemistry, ionic strength. The main conclusions are summarized as follows.(1) We prepared silica-coated core-shell magnetic nanoparticles (Fe3O4@SiO2) by the Stober method, which was subsequently coated by ZrO2 via the deposition precipitation method. The adsorbents were characterized by X-ray diffraction, transition electron microscopy, zeta-potential measurement, X-ray photoelectron spectroscopy, and vibration sample magnetometer. The results demonstrated the formation of ZrO2 shell on the surface of Fe3O4@SiO2@ZrO2 via deposition. The adsorbents with a saturation magnetization value of 15.3 emu·g-1 could be readily separated and recovered under an external magnetic field. The batch adsorption tests showed that negligible adsorption for phosphate was identified on Fe3O4@SiO2, while enhanced phosphate adsorption was shown on the Fe3O4@SiO2@ZrO2 upon ZrO2 functionlization, indicating that ZrO2 is effective adsorption sites for phosphate. After normalization by ZrO2 content, the normalized phosphate adsorption amount decreased with ZrO2 content, reflecting a decreased adsorption capacity of ZrO2 due to particle aggregation. Phosphate adsorption on the adsorbents was fitted to the Freundlich adsorption isotherm, and the adsorption kinetics could be well described by the pseudo-second-order model. Phosphate adsorption on Fe3O4@SiO2@ZrO2 increased with the decrease of pH, while negligible influence of the ion strength on phosphate adsorption was observed.(2) In order to improve the adsorption of phosphate, magnetic composite with a mesoporous SiO2 shell was prepared, and ZrO2 functionalized magnetic mesoporous SiO2 (ZrO2-MMS) was obtained using the post-grafting method. The abundant surface Si-OH functional groups of MMS favor organic Zr to form Si-O-Zr to the surface of adsorbents via covalent binding. The materials were characterized and the results showed that ZrO2 moieties with high dispersion as well as particle sizes varied from 10-20 nm on MMS surface. The grafting of ZrO2 resulted in substantially increased surface Zeta potentials. Additionally, the magnetic composite material could be recovered under an external magnetic field. The adsorption behaviors of the materials for phosphate were studied using the batch tests, and the adsorption mechanism was investigated. The results showed that ZrO2-MMS displayed substantially enhanced phosphate adsorption as compared with MMS. The ZrO2 content-normalized adsorption amount was much higher than the Fe3O4@SiO2@ZrO2 prepared via DP method under the same conditions. Phosphate adsorption over MMS and ZrO2-MMS could be well described using the Freundlich model. The adsorption kinetics followed the pseudo-second-order kinetics, and the adsorption rate increased with the decrease of initial phosphate concentration. Phosphate adsorption was controlled by both intraparticle and external diffusion, wherein higher external mass transfer resistance and intraparticle diffusion constant was identified for phosphate adsorption with higher initial phosphate concentration. Additionally, increasing pH led to suppressed phosphate adsorption due to gradually increased expulsive interaction between phosphate anion and adsorbent. Moreover, Phosphate adsorption slightly increased with ionic strength, reflecting an adsorption mechanism of surface complexation model.(3) Due to the excellent stability against erosion of alkali, The Fe3O4@carbon core-shell composite material was prepared using the hydrothermal method, and surface functionalization of the composite by ZrO2 was conducted using the deposition precipitation method. TEM images shows that Fe3O4 core consist of numerous aggregative small Fe3O4 nanoparticles around 5～10nm in diameter. The characterization results showed that the composite material was magnetic and could be recovered under an external magnetic field. Additionally, the adsorption capacity for phosphate increased with the amount of ZrO2 loading, and the adsorption followed the Freundlich adsorption isotherm. The adsorption capacity of the sorbent increased with ZrO2 content, while the ZrO2 content-normalized adsorption was independent on ZrO2 loading. This is probable because there is no effect on the particle size with the ZrO2 loading, likely due to the nearly identical ZrO2 particle sizes of the sorbents with varied ZrO2 contents. Phosphate adsorption on MFC@ZrO2 increased with the decrease of pH, but negligible influence of the ion strength on phosphate adsorption was identified. After nine consecutive adsorption-desorption cycles under alkaline conditions, the sorbent exhibited good adsorption capacity, reflecting an excellent potential for cycling utilization.