| Technologies about boron removal and boric acid-based boron isotopic separation have significant influence on the industrial water treatment and 10B production.This research explored the capacity of fuctionalized resins and metal-organic frameworks(MOFs).Through boric acid adsorption under different concentrations,pHs,temperatures and dosages,their boron adsorption and boron isotopic separation behavior was investigated.Further combining the experimental results and Gaussian simulation,the adsorption mechanism between adsorbents and boric acid was preliminary revealed.Based on the conductivity change of the boric acid solution with different functional groups,two pyrocatechol-modified resins(CL-RESIN and NCL-RESIN)were designed and synthesized for effective boron removal,which showed good boron removal capacity at a wide range of pH from 3.4 to 10.9.The optimized boron adsorption occurs at 25? with the maximum adsorption capacity 8.53 mg/g for CL-RESIN and 8.57 mg/g for NCL-RESIN,which were comparable to commercial IRA 743(10.92 mg/g).Furthermore,the boron isotopic separation factors S on two prepared resins are 1.080 for CL-RESIN atpH=6.70 and 1.140 for NCL-RESIN atpH=7.37,which are far higher than all previous results(IRA 743,1.027).Seven water-stable and acid-stable MOFs were examined for boron removal and boron isotopic separation for the first time.They were prepared and characterized by powder X-ray diffraction,scanning electron microscope,nitrogen adsorption/desorption isotherms and thermogravimetric analysis to confirm the structural purity,particle morphology,porosity and thermal stability.All of them demonstrate excellent boron removal performance compared with other boron adsorbents,particularly ZIF-8 with a capacity of 191 mg·g-1 which had the best boron removal capacity so far.The adsorption kinetics,isotherms,thermodynamics,mechanism and recycling of ZIF-8 and UiO-66 were further investigated in batch adsorption process.The highest boron adsorption on ZIF-8 and UiO-66 occurred at 45? with 247.44 mg·g-1 for ZIF-8 and 114.5 mg·g-1 for UiO-66,which can quickly reach equilibrium in one hour.Kinetic fitting with pseudo-second order model indicates chemisorption is significant in the process and intraparticle diffusion is not the only rate-limiting step.Three isotherm models including Langmuir,Freundlich and Henry model were applied for explaining isotherms,among which Freundlich model describes the isotherms best,also suggesting chemisorption plays an important role in this boron adsorption process.Calculated thermodynamic data determined that this process is a spontaneous endothermic process controlled by entropy change.Characterization on exhausted ZIF-8 with 11B MAS NMR and XPS reveals that three interactions at the Zn sites,which was confirmed by the Gaussian simulation.Boron adsorption on UiO-66 came from the coordination with unsaturated Zr sites in the defective UiO-66.11B MAS NMR reveals complex interactions between boric acid and UiO-66.Gaussian simulation confirmed boric acid was adsorbed at the Zr site and benzene rings,which suggested that the coordination and ?-? interactions.11B enriched in all the seven MOFs,which indicated higher separation factors than previous techniques,especially MIL-100(Fe)with a separation factor of 1.75 and MIL-101(Cr)of 2.00.Lower temperature andpH=7 are favorable for the separation process.11B MAS NMR of exhausted MIL-101(Cr)reveals that complex interactions with unsaturated Cr sites.Gaussian simulation confirmed that adsorbed boric acid molecules preferred to locate in the triangular cage where unsaturated Cr sites are denser than pentagonal cages.This research systematically explored boron adsorption and boron isotopic separation from organic polymers to MOFs.The potential of MOFs was realized for the first time,which also pointed out a new direction for this area. |
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