Preparation Of Hierarchically Porous Zirconium-based Metal-organic Framework Material With Tunable Mesoporous And Active Site And Study On Arsenic Removal

Nowadays,drinking water safety issues has been plagued and attracted widespread attention.Arsenic contamination has always been one of the most difficult drinking water treatment issue in the world due to its high toxicity and non-degradability.Adsorption is considered to be the simplest and most efficient water purification technology currently.The low arsenic adsorption capacity of traditional adsorbents is hard to satisfy people's needs.Metal-organic frameworks?MOFs?materials shows great application potential in the arsenic adsorption field due to their ultrahigh specific surface area and porosity,and has been widely concerned by researchers.This article focus on the subject of arsenic contamination control,and prepared two metal-organic framework arsenic adsorbents,UIO-66-X%and HP-UIO-66-X%.The arsenic removal performance of the two adsorbents were investigated,and the corresponding arsenic removal mechanism were analyzed.The problems around?1?the efficient removal of pentavalent arsenic in water,and?2?the adsorption mechanism of pentavalent arsenic on the surfaces of the two adsorbents,and?3?the formation mechanism of active sites and mesopores in HP-UIO-66-X%was systematically studied and analyzed.In the first part of this paper,a series of UIO-66-X%arsenic adsorbents with different amounts of NH₂-BDC ligand were prepared,and the effects and reasons of different amounts of NH₂-BDC ligands on arsenic adsorption were studied.Firstly,UIO-66-X%with different amounts of NH₂-BDC ligands has no significant difference in macroscopic morphology,functional group types,specific surface area and pore size distribution.There are differences in the number of defects inside the material,the larger number of NH₂-BDC ligands,the greater amount of internal oxygen vacancies.The greater number of NH₂-BDC ligands,the better arsenic removal performance of UIO-66-X%.Monolayer chemical adsorption playing major role in the adsorption of As???on UIO-66-X%,and adsorption capacity is up to 84.63 mg/g.The pH of the solution has slight effect on the arsenic adsorption,but it has better prformance in acidic condition.The regeneration performance of UIO-66-X%is good,but PO₄^3-and F^-have obvious hindering effect on As?V?adsorption.As?V?mainly substitutes the carboxyl groups at both ends of NH₂-BDC connected to the zirconium atom to form a coordination form of Zr-O-As bond,then attached to the surface of UIO-66-X%.The chief reason of why the introduction of NH₂-BDC ligands can promote the arsenic adsorption is that the introduction of NH₂-BDC ligands reduces the strength of the partial dislocation bond,thus providing more active sites for zirconium atoms that are easy to form coordination bonds with As-O bonds.In the second part,a zirconium-based hierarchically porous metal-organic frame adsorbent,HP-UIO-66-X%,was prepared by selective pyrolysis of NH₂-BDC ligands in UIO-66-X%.The characteristics of HP-UIO-66-X%adsorbent were analyzed by various macro and micro characterization methods.HP-UIO-66-X%is rich in coordination unsaturated zirconium atoms and a large number of mesopores on the surface and inside.The number of coordinated unsaturated zirconium atoms and mesopores are tunable,and the proportion of mesopores can be up to 61%.HP-UIO-66-X%adsorbent can quickly and effectively remove As?V?from solution.The maximum adsorption capacity of HP-UIO-66-X%adsorbent for As?V?at 25?and pH 6 is up to 248.75 mg/g,which is higher than all MOF-based adsorbents reported so far.The applicable pH range of HP-UIO-66-X%is relatively wide?2-11?and the regeneration performance is good.PO₄^3-and F^-has an inhibitory effect on As?V?adsorption.The complex form of H₂AsO₄^-on the surface of HP-UIO-66-X%were obtained by density functional theory calculation?DFT?and X-ray absorption fine structure analysis?XAFS?.The results of adsorption mechanism analysis shows that the arsenate on the complete HP-UIO-66-X%surface mainly depends on the tearing the Zr-O bond to coordinate with the zirconium atom,and on the defect surface,it is through the oxygen vacancies occupying the coordinated unsaturated zirconium atom.Zirconium atom is the main active site for arsenic adsorption.The third part is focus on analyze the formation mechanism of coordination unsaturated zirconium atoms and mesopores in HP-UIO-66-X%by characterization techniques such as electronic paramagnetic resonance spectrometer?EPR?,thermogravimetric analyzer?TGA?,¹H nuclear magnetic resonance spectra?¹H-NMR?,fourier transform infrared spectrometer?FT-IR?.The formation mechanism of the coordinated unsaturated zirconium atom in HP-UIO-66-X%is that the amino group on the NH₂-BDC ligand activates the carboxyl group on both ends connect with the zirconium atom,resulting in the thermal stability difference between NH₂-BDC ligands and the BDC ligands,thus the BDC ligands were retained after thermal treatment at 350°C,while the NH₂-BDC ligands were removed.The first stage of NH₂-BDC ligand removal is that the carboxyl group at one end was thermally released,and then the terminal ligand removed,resulting in a large number of oxygen vacancies and coordination unsaturated zirconium atoms formation.The formation of mesopores in HP-UIO-66-X%is due to the disordered distribution of the thermolabile ligand NH₂-BDC.With the increase of the number of NH₂-BDC ligands,the disorder increases,and the number of mesopores increases accordingly.