Amino Functionalization And Defect Tuning To Enhance The Adsorption Performance Of MIL-100(Fe) For Low Concentrations Antimony In Water

Considering that the concentration of antimony（Sb）（7.3～163μg/L）contamination in local drinking water sources of mining areas,heavily exceeds the standards of drinking water（5μg/L）,and the dominative form of Sb is Sb（Ⅴ）in waterbodies,a water stable and low-cost Fe-based metal-organic framework（MIL-100（Fe））was selected for the efficient adsorption of Sb（Ⅴ）from water,to ensure the drinking water safety in Sb mining areas.To improve the adsorption capacity of MIL-100（Fe）for Sb（Ⅴ）,the amino group（-NH₂）was grafted to improve the affinity and the surface defect was tuned to increase the adsorption sites to decrease the residual Sb（Ⅴ）concentrations in water after treatment to meet the national drinking water standard（5μg/L）.The main research contents and results were as follows:（1）Synthesis of amino-modified MIL-100（Fe）and study of its adsorption performance on Sb（Ⅴ）:The-NH₂ groups were successfully grafted onto MIL-100（Fe）by post-synthetic modification using ethylenediamine.Compared with MIL-100（Fe）,the removal efficiency of Sb（Ⅴ）by MIL-100（Fe）-NH₂ treatment was increased from 47.33%to 98.12%at the solution p H value of 6 and the adsorbent dosage of 1.0 g/L,and the residual concentration in the solution just3.52μg/L,was lower than the national drinking water standard（5μg/L）.The adsorption of Sb（Ⅴ）on MIL-100（Fe）-NH₂ fitted better with pseudo-second-order model and Langmuir model.Compared with that of MIL-100（Fe）(K_L=4.75×10^-4 L/μg),the higher Langmuir constant K_L value(1.16×10^-2 L/μg)confirmed that-NH₂ groups significantly enhanced the affinity between adsorbent and Sb（Ⅴ）.The study of adsorption mechanism revealed that Sb（OH）₆^-replaced the terminal-OH groups of the adsorbents and coordinated with Fe nodes to generate inner-sphere complexes,and the-NH₂ groups on MIL-100（Fe）-NH₂ could enhance the coordination between Sb（Ⅴ）and Fe nodes to improve the adsorption performance of the adsorbent.The hydrogen bonding between the C=O,-OH and-NH₂ groups on MIL-100（Fe）-NH₂ and Sb（OH）₆^-also contributed to the adsorption.MIL-100（Fe）-NH₂ could work effectively in a wide p H range,with varied ionic strength and coexisting ions,and even in the real water matrix.The initial p H,ionic strength and coexisting anions of the solution negligibly affected the adsorption of Sb（Ⅴ）on MIL-100（Fe）-NH₂ and the removal efficiency of Sb（Ⅴ）in the real water body achieved 90.94%.（2）Synthesis of defective MIL-100（Fe）s and study of their adsorption performance on Sb（Ⅴ）:A series of defective MIL-100（Fe）s were prepared by varying the type of modulators,the ratio of organic ligand and metal precursor,and the ratio of organic ligand and modulator.Compared with acetic acid as the modulator,the defective MIL-100（Fe）prepared with ammonium acetate had a typical hierarchical porous structure with more abundant meso-and macro-pores,as well as larger pore size（5.19 nm>3.25 nm）.When the ratio of organic ligand and metal precursor was at 5:7,the defective MIL-100（Fe）prepared at the ratio of organic ligand and modulator of 0.75:1 possessed the highest removal efficiency of 97.85%,and it could reduce 200μg/L Sb（Ⅴ）solution to 4.75μg/L,meeting the national drinking water standard.The isothermal adsorption of Sb（Ⅴ）on defective MIL-100（Fe）could fitted well with the pseudo-second-order model and Langmuir models.The adsorption of Sb（Ⅴ）on defective MIL-100（Fe）was significantly influenced by the initial p H value of the solution,and the removal efficiency of Sb（Ⅴ）increased with increasing p H as p H value increased from 2 to 6 and decreased as p H value further increased,suggesting electrostatic interactions between the defective MIL-100（Fe）and Sb（Ⅴ）.The presence of CO₃^2-,SO₄^2-,PO₄^3-and HA in the solution all inhibited the adsorption of Sb（Ⅴ）.On the contrary,the increasing ionic strength in solution could promote the adsorption,indicating that the adsorption process was driven by the inner-sphere complexation.