Study On The Synthesis Of Zirconium Phosphonates And Their Properties Of Sr²⁺ Removal

Metal phosphonates have been extensively developed since the report of the first crystalline zirconium phosphate by Clearfield et al.in the 1970s.Among them,zirconium phosphonates have been widely applied in many fields such as ion-exchange,proton conductivity,separation,and catalysis.However,it is difficult to synthesize large single crystals of the zirconium phosphonates,because of the high tendency of hydrolysis of Zr⁴⁺ions and the extremely strong affinity between phosphonate ligands and Zr⁴⁺ions.This also constrains the utilizationof zirconium phosphonates.On the other hand,along with the depth of fossil energy crisis,nuclear power has been developed as an alternative clean energy source.However,this also leads to the massive accumulation of radioactive spent fuel,including actinides such as U,Pu and the fission element of Sr-90,and Cs-137.It is estimated that 8700 tons of spent nuclear fuel will be produced by 2020.Large amount of materials that can be used for spent fuel reprocessing and nuclear emergency treatment is required.The zirconium phosphonates itself exhibits excellent water,heat and radiation stability,and has good ion exchange ability and separation performance.Therefore,the synthesis of new crystalline zirconium phosphonates has positive significance for promoting spent fuel treatment and nuclear energy development.In this work,two new zirconium phosphonates were synthesized,and their related properties,especially the Sr²⁺uptake capacity werefurther studied.The concrete working is islisted as follows:1.Two novel zirconium phosphonates[（CH₃）₂NH₂]₂[Zr（CH₂（HPO₃）（PO₃））₂]（SZ-5）and[（CH₃）₂NH₂][Zr（CH₂（PO₃）₂）F]（SZ-4）have been successfully synthesized by solvothermal method as large single crystals,using the methylenediphosphonate ligand.Single-crystal X-ray diffraction（XRD）analysis reveals that SZ-5 shows a 1-D chain structure and SZ-4 shows a 2-D layered structure.Additional,both compounds were characterized with TGA,PXRD,FT-IR,SEM/EDS and proton conductivity techinique.Proton conductivity data shows that SZ-5 exhibitsa moderate proton conductivity,which is due to the existence of OH^-group and[（CH₃）₂NH₂]⁺cations residing in the free space.2.The stability measures show that SZ-4 and SZ-5 have excellent radiation resistance and hydrolytic stability in a large pH range.A phase to phase transformation of SZ-4occurred after the treatment of strong acidic solution of pH=0,resulting good proton conductivity of the treated SZ-4 sample.3.The Sr²⁺uptake experiments reveal that SZ-4 and SZ-5 can efficiently selective remove Sr²⁺from acidic solutions that hashigh salinity.The real seawater removal experiments show that SZ-4 has an excellent radioactive strontium exchange capability when compared with SZ-5.X-ray Absorption Fine Structure（XAFS）analysis of Sr uptake SZ-5was performed.The single crystal X-ray diffraction and further DFT calculations on Sr²⁺uptake SZ-4 reveal a distinctive two-step mechanism:1)Sr²⁺first outcompetes protons to coordinate toboth SZ-4 framework and the dimethylammonium ions as the intermediate state;2)then water molecules substitute dimethylammonium cations to reach a thermodynamically favourable stage.