Preparation Of Magnetic Porous Calcium Silicate Composites With Core-Shell Structure And Evaluation Of Their Adsorption Properties For Heavy Metal Ions

The presence of heavy metals in wastewater has led to a major global environmental problem,because heavy metals have significant toxicity to humans,animals and plants and microorganisms even at low level,and cannot be degraded by microorganisms.Adsorption is widely applied in the treatment of heavy metals because of its simple operation,high efficiency,and low cost.Adsorbents play a crucial role in the adsorption process;hence,the development of environment-friendly and effective adsorbents should be a key point in removing heavy metals from wastewater.In recent years,porous calcium silicate?PCS?has elicited considerable research interest in the field of heavy metal wastewater treatment because of its large specific surface area,high porosity,good chemical stability,controllable structure,and negative charge strength.The ferroferric oxide magnetic particles themselves have poor adsorption capacity for heavy metals,but they can be separated by a magnetic field,which can make the process continuous,simplify the separation operation,and improve the separation efficiency.In this paper,considering the advantages of the above two,two kinds of core-shell magnetic porous calcium silicate composites are designed and constructed.The main results of this dissertation are summarized as following:Fe₃O₄ magnetic particles were synthesized through a modified solvothermal reaction.A two-component material Fe₃O₄@CaSiO₃ with Fe₃O₄ magnetite core and layered porous CaSiO₃ shell was synthesized via precipitation using calcium nitrate and sodium silicate as raw materials.The morphology,structure,magnetic properties,and composition of this material were characterized using scanning electron microscopy?SEM?,Fourier-transform infrared spectroscopy?FT-IR?,vibrating sample magnetometer?VSM?,energy-dispersive X-ray spectrometry?EDX?,ect,and the adsorption performance,adsorption kinetic characteristics,and reutilization property of Fe₃O₄@CaSiO₃ toward Cu²⁺,Ni²⁺,and Cr³⁺were studied.Results showed that the Fe₃O₄@CaSiO₃ magnetic composite has a 2D core-layer architecture with cottony morphology,a specific surface area of 41.56 m²·g^-1,pore size of 16nm,and pore volume of 0.25 cm³·g^-1.The measured magnetization saturation?Ms?values of Fe₃O₄@CaSiO₃ magnetic composite was 57.1 emu·g^-1.The adsorption data of the three heavy metal ions adsorbed by Fe₃O₄@CaSiO₃ fitted well with the Redlich-Peterson and pseudo-second-order models,and all the adsorption processes attained equilibrium within 150min.The maximum adsorption capacities of Fe₃O₄@CaSiO₃ toward Cu²⁺,Ni²⁺,and Cr³⁺were427.10,391.59,and 371.39 mg·g^-1 at an initial concentration of 225 mg·L^-11 and a temperature of 293 K according to the fitted curve with the Redlich-Peterson model,respectively.After five cycles of recycling,the removal percentages of Cu²⁺,Ni²⁺,and Cr³⁺by Fe₃O₄@CaSiO₃decreased by 25.56%,27.22%,and 40.13%,respectively.In this work,in order to further increase the specific surface area of Fe₃O₄@CaSiO₃,firstly,a thin silica layer was coated on Fe₃O₄ microspheres through a modified St?ber method to prepare Fe₃O₄@SiO₂ core-shell microspheres;Secondly,a three-component material Fe₃O₄@SiO₂@CaSiO₃ with a Fe₃O₄@SiO₂ magnetite core and a layered porous CaSiO₃ shell was synthesized via precipitation and used as the adsorbent to remove heavy metal ions from wastewater.The structure,morphology,magnetic properties,and composition of this material were analyzed and discussed by scanning electron microscopy?SEM?,Fourier-transform infrared spectroscopy?FT-IR?,vibrating sample magnetometer?VSM?,energy-dispersive X-ray spectrometry?EDX?,ect,and the adsorption efficiency,adsorption thermodynamic,kinetic characteristics,and reutilization property of Fe₃O₄@SiO₂@CaSiO₃for Cu²⁺,Ni²⁺,Cr³⁺,and Mn²⁺were investigated over a wide range of experimental conditions.Results showed that the Fe₃O₄@SiO₂@CaSiO₃ magnetic composite had a core-shell structure with cottony morphology,a specific surface area of 75.97 m²·g^-1,pore size of 11.4 nm,and pore volume of 0.36 cm³·g^-1.The measured magnetization saturation?Ms?value of Fe₃O₄@SiO₂@CaSiO₃ was 56.5 emu·g^-1.The adsorption equilibrium data of the four heavy metal ions adsorbed by Fe₃O₄@SiO₂@CaSiO₃ fitted well with Redlich-Peterson and pseudo-second-order models,and all the adsorption processes attained equilibrium within 200min.The maximum adsorption capacities of Fe₃O₄@SiO₂@CaSiO₃ toward Cu²⁺,Ni²⁺,Cr³⁺,and Mn²⁺were 376.64,306.59,282.24,and 160.49 mg·g^-1 at the temperature of 293 K,respectively,which were superior to those reported in the literature.After five adsorption-desorption cycles,the removal rates of Cu²⁺,Ni²⁺,Cr³⁺,and Mn²⁺by Fe₃O₄@SiO₂@CaSiO₃ decreased by 31.02%,34.52%,46.48,and 34.50%,respectively.In addition,the adsorption performance of Fe₃O₄@SiO₂@CaSiO₃ on Ag⁺ions was also investigated.The results show that the Fe₃O₄@SiO₂@CaSiO₃ composite also has excellent adsorption performance for Ag⁺ions.The adsorption equilibrium data was in accordance with the Redlich-Peterson model.Based on the fitting curve of this model,the maximum adsorption capacity of Fe₃O₄@SiO₂@CaSiO₃to Ag⁺ions was 127.84 mg·g^-1 at 293 K at an initial concentration of 225 mg·L^-1,which was higher than the adsorption amount reported in the literature.At the same time,the adsorption of Ag⁺ions by the composite reached equilibrium within about 150 minutes.The kinetic data was more in line with the pseudo-second-order model,and the q_e value of this model fitted very close to the experimental value(q_ed).After five adsorption-desorption cycles,the removal efficiency percentage of Ag⁺ions by Fe₃O₄@SiO₂@CaSiO₃ tended to be stable and decreased by34.90%.The above adsorption thermodynamics results show that the adsorptions of Cu²⁺,Ni²⁺and Cr³⁺by Fe₃O₄@CaSiO₃ and the adsorptions of Cu²⁺,Ni²⁺,Cr³⁺,Mn²⁺and Ag⁺by Fe₃O₄@SiO₂@CaSiO₃ were spontaneous endothermic processes featuring an entropy increase.The binding mechanism was studied by X-ray photoelectron spectroscopy analysis,which implied that chemical adsorption resulting from the surface complexation reaction of active groups of-O-and-OH plays a dominant role in the adsorption process.In addition,the adsorption mechanism also included ion exchange resulting from the Ca²⁺on the surface and edge of the magnetic composite with heavy metal ions,and physical adsorption resulting from a net force field originating from the polar surface and edge and high specific surface energy.Simultaneously,two kinds of magnetic composites could be easily separated from the solution by applying an external magnetic field.Therefore,the synthesized Fe₃O₄@CaSiO₃ and Fe₃O₄@SiO₂@CaSiO₃ magnetic composites exhibit excellent adsorption performance,magnetic separation characteristics,and regeneration performance,which is easy to realize the continuous automatic process of wastewater treatment,showing great application potentials for removing heavy metal ions.