Preparation Of Magnetic Lanthanum-based MOFs For Selective Phosphate Removal From Wastewater

Excessive discharge of phosphorus in sewage not only leads to eutrophication of water body,but also causes waste of phosphorus resources.The traditional method is difficult to recover and reuse the phosphorus.The adsorption method has been applied to the removal of phosphate in water in recent years because of its fast reaction rate and high removal efficiency.In recent years,organic metal frameworks（MOFs）have been widely concerned because of their stable structure,large specific surface area and easy surface modification.However,there are few studies on phosphate adsorption with lanthanum as metal ligands.In this paper,two lanthanum metal organic frameworks,La MOFs and CE MOFs,were synthesized by solvothermal method.Then,NH₂-La-MOFs and NH₂-Ce-MOFs were obtained by adding amino group to enhance improve the ability of phosphate adsorption.Finally,Fe₃O₄ was embedded into NH₂-La-MOFs to compose Fe₃O₄@NH₂-La-MOFs.Then,the paper studied the effect of influencing factors on phosphorus removal and regeneration performance of Fe₃O₄@NH₂-La-MOFs.The main research contents are as follows:（1）La-MOFs and Ce-MOFs were synthesized by solvothermal method and characterized by SEM,BET and FTIR.The two materials are composed of layered crystals.The p H value has little effect on the adsorption capacity of the material.The adsorption capacity are relatively stable in the range of p H=3-12 and p H=3-11,respectively.Meanwhile,they reach the maximum value of 114.23 and 92.86 mg P/g respectively at p H=8.The results of the pseudo-second kinetic model are better than those of the pseudo-first-order kinetic model,indicating that the chemical reaction occurred in the adsorption process of phosphate.Combining with the fitting results of the in particle diffusion model,it suggests that the rate limiting steps of La-MOFs and Ce-MOFs are chemical action and in particle diffusion.By analyzing the isotherm experimental data,it is found that Langmuir model has the best fitting result,which proves that the adsorption process is mainly monolayer adsorption.According to Langmuir model,the adsorption capacities of La-MOFs and Ce-MOFs are 100.13 mg P/g and 88.78 mg P/g respectively.According to the calculation of adsorption energy by D-R model,it is inferred that the adsorption process is mainly chemical adsorption,which is consistent with the kinetic analysis.（2）NH₂-La-MOFs and NH₂-Ce-MOFs,which was characterized by SEM,BET and FTIR,were synthesized by solvothermal method.These two materials are suitable for a wide p H range.With the increase of p H,the adsorption capacity showed a trend of increasing first and then decreasing,reaching the maximum at p H=8 and 6,respectively.Under the condition of the coexistence of anion and humic acid,NH₂-La-MOFs and NH₂-Ce-MOFs still have high selective adsorption properties.The results of the pseudo-first-second kinetic model are better than those of the pseudo-first-order kinetic model.Combining with the fitting results of the in particle diffusion model,it suggests that the rate limiting steps of La-MOFs and Ce-MOFs are chemical action and in particle diffusion.Langmuir model and D-R model can well fit the isotherms of NH₂-La-MOFs and NH₂-Ce-MOFs.According to Langmuir model,the adsorption capacity is 141.58 mg P/g and 106.04mg P/g respectively.Compared with La-MOFs and Ce-MOFs,The adsorption capacity of the modified material was greatly improved.According to the adsorption energy calculated by D-R model,it can be concluded that the adsorption process is mainly chemical adsorption.（3）Fe₃O₄@NH₂-La-MOFs was prepared by embedding method and then was characterized by SEM,BET,FTIR,XPS and VSM.It was confirmed that Fe₃O₄ was loaded successfully and could be separated effectively by external magnetic field.The adsorption capacity of Fe₃O₄@NH₂-La-MOFs reaches the highest value when p H=8.Under the co-existence of Cl^-,NO₃^-,HCO₃^-,SO₄^2-and humic acid,the phosphorus removal agent still has high selectivity for phosphate.Comparing with the pseudo-first-order kinetic model,the pseudo-first-second kinetic model can better describe the adsorption process.Combined with the fitting results of the in particle diffusion model,it can be inferred that the rate limiting steps of Fe₃O₄@NH₂-La-MOFs adsorption of phosphate are chemical action and in particle diffusion.Langmuir model and D-R model can well fit the isotherm test results,which proves that the phosphate adsorption is mainly single-layer adsorption.Langmuir model and D-R model can well fit the isotherm test results,which proves that the phosphate adsorption is mainly single-layer adsorption.According to the D-R model,the maximum adsorption capacity is 111.22 mg P/g,and the adsorption energy E is 10.10 k J/mol,which can be inferred that the reaction is chemisorption.After 10 times of adsorption desorption,the adsorption capacity of Fe₃O₄@NH₂-La-MOFs decreased to 81.48%of the initial value,showing a good potential for phosphate removal and recovery.Through in-depth analysis,it is shown that the mechanism of phosphate adsorption by Fe₃O₄@NH₂-La-MOFs is ligand exchange and electrostatic attraction.