Study On The Preparation And Phosphorus Removal Performance And Mechanism Analysis Of Iron Or Iron-Cerium Bimetallic Organic Framework

Strict control of phosphorus in water can effectively prevent eutrophication,ensure drinking water safety and alleviate water shortage.The adsorption method is widely studied because of its easy operation,good effect and recoverability.The effectiveness of this technology heavily relies on the functional characteristics of the adsorbent.Metal-organic framework materials（MOFs）exhibit adjustable structure,high specific surface area,and abundant unsaturated sites.They are applied in many fields such as catalysis,gas storage,sensing,drug conduction,etc.And MOFs are also effective in adsorbing cation and organic pollutants.The transition metal Fe is an important raw material for the preparing metal adsorbent due to its high affinity with phosphate,abundant reserves,strong reactivity and high biosecurity.In this study,combining the advantages of MOFs and Fe,Fe MOFs and Fe-Ce bimetallic MOFs were prepared for phosphate removal in water.XRD,SEM,BET,FT-IR and XPS were used to characterize the crystal structure,microstructure and chemical composition of the adsorbent.The performance of phosphorus removal was studied through adsorption isotherm,adsorption kinetics,p H and coexistence ion experiments.The main contents include:（1）Fe（Ⅱ）-MOFs（FSC）and Fe（Ⅲ）-MOFs（FC）were prepared by valence regulation in order to study the effect of valence state of iron on phosphorus removal performance of Fe-MOFs.The adsorption experiments showed that the phosphate adsorption processes of FSC and FC followed the Freundlich isothermal model and pseudo-second-order kinetic model,with chemisorption of multimolecular layers.The adsorption capacities of FSC and FC were 90.51mg/g and 164.43 mg/g,respectively,and Fe（Ⅲ）exhibited better phosphate adsorption capacity than Fe（Ⅱ）.The common coexisting ions Ca²⁺and Mg²⁺in water can increase the adsorption capacity of FSC and FC,while SO₄^2-,NO₃^-,Cl^-and CO₃^2-have little inhibitory effect on phosphate adsorption.In the p H experiment,the phosphate adsorption performance of FSC and FC was different.In the p H range of 3 to 10,the adsorption capacity of FSC in neutral and alkaline solution is higher than that in acidic solution,while the phosphate adsorption capacity of FC in this range was relatively stable.This is related to the p H value and degree of hydrolysis of Fe（Ⅱ）and Fe（Ⅲ）in FSC and FC.（2）In order to improve the adsorption capacity of Fe-MOFs,Fe-Ce-MOFs-1 and Fe-Ce-MOFs-2 were synthesized by hydrothermal blending of cerium salt and ferric salt.Fe-Ce-MOFs-1 and Fe-Ce-MOFs-2 are materials dominated by Fe（Ⅲ）and Ce（III）contents.The adsorption experiments demonstrated that the phosphate adsorption processes of Fe-Ce-MOFs-1 and Fe-Ce-MOFs-2 followed the Langmuir isothermal model and the pseudo-second-order kinetic model,which were single molecular layer chemisorption.The theoretical limit adsorption capacities were 233.22 mg/g and 222.22 mg/g,respectively,showing an increase in adsorption capacity compared to the original FSC and FC.In the coexisting ion experiment,cationic Ca²⁺and Mg²⁺have a positive effect on adsorption capacity,while the reduction degree of anion on adsorption capacity of the adsorbent is CO₃^2->SO₄^2->Cl^->NO₃^-.In p H experiments,Fe-Ce-MOFs-1 and Fe-Ce-MOFs-2 maintained adsorption capacity above 122.0 mg/g when the p H value ranges from 2 to 12,indicating good p H adaptability.Doping Ce salt into Fe-MOFs can effectively enhance its adsorption capacity and p H adaptability.（3）Fe（Ⅱ）-O-Ce and Fe（Ⅲ）-O-Ce were prepared by loading Fe（Ⅱ）salt and Fe（Ⅲ）-O-Ce on Ce-Ui O-66（Ce（IV）-MOFs）using post-loading.Fe（Ⅱ）-O-Ce is a mesoporous material with a high proportion of Fe（Ⅲ）and Ce（III）,and Fe（Ⅲ）-O-Ce is a microporous material with a high proportion of Fe（Ⅲ）and Ce（IV）.The phosphate adsorption capacities of Fe（Ⅱ）-O-Ce and Fe（Ⅲ）-O-Ce were 293.66 mg/g and 246.26 mg/g,respectively,which were higher than those of Ce-Ui O-66（194.48 mg/g）.The phosphate adsorption capacity of Fe（Ⅱ）-O-Ce and Fe（Ⅲ）-O-Ce remained above 118.0 mg/g in the range of p H 2～12,indicating good p H adaptability.In the coexistence experiment,the adsorption capacity of Cl^-,NO₃^-,SO₄^2-and CO₃^2-on Fe（Ⅱ）-O-Ce and Fe（Ⅲ）-O-Ce decreased somewhat.The post-loaded iron-cerium bimetallic MOFs have a clear crystal structure and high adsorption capacity,which is an effective phosphorus adsorbent.