Study On Preparation Of Titanium-based MOFs And Application Of Phosphorus Removal By Adsorption

Phosphorus is not only one of the essential nutrients for humans,but also a natural mineral resource that is difficult to regenerate.The global phosphate rock reserves are far from meeting the needs of human production and life,so the recycling of phosphorus resources needs to be considered.The adsorption and desorption of phosphorus from eutrophic water bodies is one of the possible ways to solve the problem.Traditional adsorbents often have disadvantages such as low adsorption capacity and poor selectivity.Metal organic framework materials（MOFs）have become one of the popular materials due to their large specific surface area and good thermal stability.Titanium-based materials have good selectivity to phosphate and excellent water stability,making them an ideal adsorption material.This topic explores new ideas to solve the problem of eutrophication by preparing new titanium-based MOFs materials and applying them to adsorption and dephosphorization.The main research contents of this paper are as follows:First,in this paper,a traditional hydrothermal synthesis method was used to prepare titanium-based MOFs—Ti-MOFs with titanium tetraisopropoxide as the metal ion and1,4-phthalic acid as the organic ligand.Then the Ti-MOFs were characterized by SEM,TEM,XRD,FT-IR,TGA,N₂adsorption desorption analysis and XPS.The micro morphology and internal structure of the adsorbent were observed,and the phase composition and characteristic functional groups of the adsorbent were analyzed.Secondly,research the performance of Ti-MOFs material by batch adsorption and dephosphorization.The effects of dosage,time,p H,initial concentration of phosphorus solution,temperature,coexisting ions,etc.on the effect of phosphorus removal were analyzed by using a single variable influencing factor experiment.The batch experiment obtained the law that the Ti-MOFs material adsorption phosphorus removal effect changes with a single factor,and determined that the optimal dosing concentration of Ti-MOFs is 1g/L,the reaction equilibrium time is 24h,and the p H range of the adsorbent is 2～10.Increasing the temperature can increase the adsorption capacity and the presence of different concentrations of CI^-,NO₃^-,and SO₄^2-has no significant effect on phosphate adsorption.Then,the mathematical model of Ti-MOFs was analyzed,and it was found that the linear correlation of Freundlich model was better than Langmuir model,which belonged to a multi-layer adsorption process.The Langmuir model was used to calculate the saturated adsorption capacity Qm at 35°C to be 38.17 mg/g.The quasi second order kinetic model is more consistent with the adsorption process of Ti-MOFs.The intra particle diffusion model is used to verify that the intra particle diffusion is the rate limiting step.The adsorption thermodynamic analysis shows that the adsorption entropy?S>0,the Gibbs free energy?G<0,and the adsorption enthalpy?H>0 indicate that the reaction is spontaneous,endothermic,and preferentially occurs on the adsorption surface.As the temperature increases,?G decreases continuously,indicating that at higher temperatures,the feasibility of the adsorption reaction increases.In order to further increase the adsorption capacity of the adsorbent,an amino group was introduced on the basis of the Ti-MOFs material to prepare a functionalized MOFs material—Ti-MOFs-NH₂.The Ti-MOFs-NH₂material was characterized by the same test methods,and compared with the Ti-MOFs material,it was found that the modified adsorbent not only caused the microscopic appearance after successfully introducing amino groups The huge change also greatly increases the specific surface area and pore volume（Ti-MOFs:113.993 m²/g,0.1289 cm³/g;Ti-MOFs-NH₂:593.152 m²/g,0.3853cm³/g）.Through a single variable adsorption phosphorus removal experiment on Ti-MOFs-NH₂material,it is found that the phosphorus removal effect of Ti-MOFs-NH₂material varies with a single factor in the same way as Ti-MOFs material,but under the same conditions,Ti-MOFs-NH₂material The adsorption capacity is further improved（Ti-MOFs:38.46 mg/g;Ti-MOFs-NH₂:46.356 mg/g）.At the same time,the adsorption kinetics results of the modified adsorbent for phosphate indicate that the Ti-MOFs-NH₂material conforms to the quasi-second-order kinetic model,and the intra particle diffusion is the rate limiting step;the adsorption process of the Ti-MOFs-NH₂material is more consistent with Freundlich Isotherm model,physical adsorption and chemical adsorption exist at the same time;the thermodynamic parameters?S>0,?G<0 and?H>0 indicate that the adsorption reaction is endothermic,spontaneous,and preferentially occurs on the adsorption surface with increased entropy process.In order to have a deeper understanding of the adsorption mechanism of titanium-based MOFs materials to phosphate radicals,the comparative characterization of Ti-MOFs and Ti-MOFs-NH₂materials before and after adsorption was carried out,using SEM,XRD,FT-IR and XPS and other test methods,combined with previous mathematical model analysis confirmed that the adsorption mechanism of the two materials can be summarized as their own surface adsorption,electrostatic interaction and coordination exchange interaction.Because the titanium hydroxyl group reacts with the phosphate to form the inner complex Ti-O-P,it not only has a certain phosphorus removal effect under alkaline conditions,but the adsorption capacity is not affected by the ionic strength.Ti-MOFs-NH₂material will have greater self-surface adsorption due to its larger surface area and pore volume;amino groups will be protonated into-NH₃⁺ions under the action of H⁺ions,and produce stronger phosphate anions.Electrostatic attraction,so Ti-MOFs-NH₂material has higher adsorption capacity,better performance and more potential than Ti-MOFs material.Finally,the regeneration effect of Ti-MOFs-NH₂material is studied.After three cycles,the phosphorus removal rate of Ti-MOFs-NH₂material can still reach 78.7%,which proves that the material has certain reusability.Figure[64]table[11]reference[122]...