Phosphorus Adsorption Properties And Mechanism Study Of Zr-based Metal-organic Frame Composites

Phosphorus is seen as a major contributor to eutrophication in water bodies,making the removal of phosphorus from water particularly important.Zirconium metal organic frameworks（Zr-MOFs）have shown great potential for the remediation of phosphorus-contaminated water.Among them,UiO-67 MOFs have received much attention for phosphorus removal due to their good crystal configuration and large specific surface area,as well as their good stability.However,the phosphorus adsorption performance and mechanism of UiO-67 MOFs have not been fully investigated,and their adsorption sites have not been fully explored.In addition,as polluted water often contains many other co-existing ions,such as the most abundant inorganic ion calcium and some organic humic acids,the effect of co-existing materials on the phosphorus adsorption performance of UiO-67 is also worth investigating.Based on this,firstly,we synthesised UiO-67 and a series of ammonia-modified UiO-67 MOFs was synthesized to investigate their action and mechanism for phosphate removal from contaminated water.Secondly,the Zr-MOF material UiO-67 was prepared using a solvothermal method to explore the adsorption performance and mechanism of Zr-MOF on phosphorus under the coexistence of calcium ions and humic acid.The effect of coexisting ions on the adsorption of phosphorus by MOFs was also investigated by varying the conditions of phosphorus concentration,reaction time,pH,temperature,individual substances and mixed system.The results of this paper show that.（1）The adsorption of phosphorus by UiO-67 was promoted by the addition of calcium ions and humic acid,but the promotion effect of humic acid was not significant.The adsorption amount of the adsorbent could reach about 101 mg/g after the addition of calcium ions alone,while the adsorption amount would be higher when the two were added together.The analysis of the MOF material before and after the addition of coexisting ions using Fourier Infrared Transform Spectroscopy（FT-IR）,X-ray Photoelectron Spectroscopy（XPS）,Electron Paramagnetic Resonance（EPR）and X-ray Powder Diffraction（PXRD）showed that the calcium ions can undergo a substitution reaction with the carboxyl group in UiO-67,and Ca²⁺will replace the hydrogen atom in-COOH to form O-C=O-Ca,thus enabling more phosphorus to be adsorbed,while humic acid,which contains a large amount of-OH and-COOH,will formπ-πstacking and hydrogen bonds with UiO-67 resulting in more adsorption sites for phosphorus.（2）Secondly,in a study of all the UiO-67 series,it was found that the defective UiO-67 modified by an amino group（D-UiO-67-NH₂）was strongly associated with the highest adsorption affinity and the shortest uptake equilibrium time for phosphate.The results show that the phosphate uptake performance was enhanced by the combinations of the defects,carboxyl and amino groups in D-UiO-67-NH₂,which is further confirmed by Fourier transform infrared（FTIR）,nuclear magnetic resonance（NMR）,XPS and density functional theory（DFT）calculations.The abundant Zr-OH and oxygen vacancy active sites was provided by the defects in the Zr node,resulting in the formation of Zr-O-P inner sphere coordination,the carboxyl group in the D-UiO-67-NH₂ ligand linker interacts with the phosphate via hydrogen bonding,while the amino group not only provides additional adsorption sites to interact with the phosphate via hydrogen bonding,but also further strengthens the formed hydrogen bond and Zr-O-P coordination.This paper therefore provides an emerging and promising approach to phosphorus adsorption by designing Zr-MOFs adsorbents with high performance for phosphorus adsorption through the action of amino and carboxyl groups,as well as defectively generated oxygen vacancies and Zr-OH groups.