Preparation Of La-based Two Dimensional Metal Organic Framework Nanosheets And Its Application In Efficient Phosphorus Removal

Eutrophication refers to the deterioration of water quality caused by excessive nitrogen and phosphorus content in the water body.High concentration of phosphorus in water bodies will lead to environmental pollution,reduction of aquatic organisms,and even severely threaten and affect human health.The concentration of total phosphorus in natural water bodies higher than 0.02 mg P/L will lead to eutrophication of water bodies.Therefore,effective control of phosphorus concentration in water becomes the key to eutrophication control.Due to the strong affinity of rare earth elements（La,Ce）for phosphate,lanthanum based metal organic frameworks（Ln-MOFs）have been widely studied as adsorbents for phosphorus removal in water.Compared with three-dimensional bulk materials,two-dimensional metal organic frameworks nanosheets exhibit unique properties due to their large surface area and high surface energy,which can expose a large number of metal active sites and reduce mass transfer resistance.However,how to obtain a large number of 2D Ln-MOFs in a simple and rapid way and use them as phosphorus removal adsorbents is still a huge challenge.Two-dimensional La-based metal organic frameworks 2D Ln-TDA（La,Ce）nanosheets and 2D La-TDC nanosheets were rapidly synthesized by microwave-ultrasonic assisted synthesis method in Na OH aqueous solution,using La（NO₃）₃·6H₂O or Ce（NO₃）₃·6H₂O as metal source and 2,2’-thioformic acid（H₂TDA）and 2,5-thiodicarboxylic acid（H₂TDC）as organic ligand.XRD,SEM,FTIR,BET,XPS,zeta potential and TG were employed to characterize the phase composition,micro morphology,functional group structure,pore size,specific surface area,chemical state,surface charge and phase transition process of the two types adsorbents.The adsorption properties of the two adsorbents for phosphate were investigated,and the adsorption mechanism was analyzed.The specific content is as follows:1.Two-dimensional Ln-TDA（Ln=La,Ce）nanosheets were prepared by microwave-ultrasonic assisted synthesis method,and bulk Ln-TDA（Ln=La,Ce）were prepared by hydrothermal method.The optimum preparation conditions of microwave-ultrasonic assisted synthesis method were as follows:microwave reaction temperature was 90℃,reaction time was 10 min.The four adsorbents prepared by the two methods were used for phosphate adsorption test.The results show that the adsorption capacity of 2D Ln-TDA（Ln=La,Ce）prepared by the two methods is similar,but the adsorption efficiency of 2D Ln-TDA（Ln=La,Ce）nanosheets prepared by microwave-assisted ultrasonic synthesis method is significantly improved.The kinetic process of 2D Ln-TDA（Ln=La,Ce）nanosheets conforms to the pseudo-second-order kinetic model,and the adsorption energy reaches adsorption equilibrium within 15 minutes.The Langmuir model is used to estimate the maximum adsorption capacity to be 253.5 mg/g and 259.5 mg/g,respectively.The adsorption performance is not affected by p H value and coexisting ions,and humic acid has a slight effect on the adsorption performance;The La（OH）₃/Ln-TDA（Ln=La,Ce）composite can be obtained by etching 2D Ln-TDA（Ln=La,Ce）nanosheets with an appropriate amount of Na OH solution,and the adsorption capacity for phosphate can be further increased to about 308.9 mg/g;In the actual water body,the concentration of phosphate can be controlled at about 0.02 mg P/L,which can basically effectively inhibit the occurrence of eutrophication.2.The 2D La-TDC nanosheets were prepared by the microwave-ultrasonic assisted synthesis method.The optimized preparation conditions were:microwave heating temperature was 90℃,microwave reaction time was 15 min,and the ratio of La³⁺/TDC was 1.5:1.The 2D La-TDC nanosheets were used for the adsorption test of phosphate in aqueous solution.The adsorption test results show that the adsorption process of 2D La-TDC nanosheets conforms to the pseudo-second-order kinetic model,and the adsorption equilibrium time is about 60 minutes.The Langmuir model is used to evaluate the maximum adsorption of 2D La-TDC nanosheets,and the maximum adsorption capacity is 223.0 mg/g.The adsorption performance is not affected by p H and coexisting anions.In the actual water body,the concentration of total phosphorus can be controlled at about0.02 mg P/L,which can basically inhibit the occurrence of eutrophication in the water body.