| Radioanalysis has been long recognized as the essential of experimental radiochemistry,environmental monitoring and radiopharmaceuticals.Normally,it is prerequisite to establish separation protocol to acquire precise distribution of target radionuclides in complex matrices or in vivo.The major challenge of light actinide separation chemistry is the real application of materials with excellent target selectivity towards ultratrace element,reliable physicochemical stability in corrosive media and acceptable reproducibility in upscale fabrication.Based on a comprehensive evaluation of state-of-the-art separation materials for light actinides(mainly for U,Th),we focus on the low-cost high-valency metal alkylphosphonate frameworks with large affinity for actinide,outstanding chemical stability in acidic media.We investigate the hierarchical porosity,amorphous backbone of tin(?)alkylphosphonates,and niobium(?)alkylphosphonates as representative unconventional metal-phosphonate frameworks.Based on the sequestration discrepancy towards a multitude of cations,it is feasible to utilize these materials to enrich and purify the tetravalent actinides from simulated complex matrices.The shift from fabricating new motifs to exploring intrinsic stability of metal-organic frameworks has clearly emerged in last decade.To gain the governing factors of chemical stability of these materials for the real applicability of radionuclide separation,we inspect the element leaching,microstructure evolution and speculate the degradation mechanism.Since irons will affect the sequestration of tetravalent actinides for metal-phosphonate frameworks,we have carried out the optimization of batch parameters and uncovered the preliminary sequestration mechanism in order to enhance the affinity for tetravalent actinide and establish the separation protocols.The primary results are briefly summarized as follows:Firstly,we prepare the tin or niobium alkylphosphonate via nonionic templating method in water-ethanol.Subsequently,the sols are treated solvothermally and the F127 templates are removed by Soxhlet extraction or percholorated oxidation.Eight categories are obtained with each having six different molar ratio of phosphorus to metal.One optimal has been selected by the largest distribution coefficient of thorium or lanthanides in 20-element solutions.The elevation of molar ration will increase the amount of free phosphonate moieties thus enhance the affinity for surrogate ions.By characterizing these eight optimal samples,one can observe that the tin alkylphosphonates are largely amorphous aggregates while the only exception is the Sn-EDTMP has nanocrystalline regions that referred to tin oxides by X-ray diffraction and electron diffraction analysis.In other words,this nanocomposite is essentially the amorphous hybrids impregnated by tin oxides.All niobium alkylphosphonates are long-range disordered,agglomerated particles with vesicular voids.The tin or niobium alkylphosphonates will stack in massif-like with predominant external surface areas,wide-distributed mesopores driven by the principle of minimization of surface energy.The structural model has been deduced as the metal-oxygen-phosphorus layers pillared by alkyl chain where water molecules,phosphonates moieties and counterions will reside between the layers.These inorganic-organic hybrids have free or confined phosphonate moieties,alkyl chains and metallic defect sites that render three stepwise weight losses:the first is the evaporation of physic-adsorbed water;the second is the decomposition of alkyl chains;and the third is the phase-change of inorganic backbone.It is revealed that a large amount of phosphonate moieties and a strong metal-oxygen bonding will contribute to the greater extent of thermal stability.They readily transform into pyrophosphate around 700-900?.Secondly,both have "charge-discrimination" for 20-element surrogates that display large affinity for highly charged cations while barely adsorb the low-valency ions,among which the distribution coefficients of Th,Fe,U,Ln,Sr and Cs are?104-105 mLg-1,?103-104 mLg-1,?102-105mLg-1,?102-103mLg-1?101-102mLg-1 and?102-103mLg-1,respectively.The former three will be captured by the tin alkylphosphonates within 30 minutes while it takes 8 hours to achieve the saturated sequestration of Ln,Sr,and Cs.The discrepancy under different acidity will dictate the establishement of two-step thorium separation process:the first is loading the sample in 0.1or 1.0 molL-1 HNO3;the second is elution by 0.5 molL-1 EDTA-NaCO3.Thirdly,the normalized leaching rate under different acidity,temperature and immersion duration will collectively describe the elemental leaching behavior of tin alkylphosphonates.The leaching kinetics is divided into two regions:the first is the rapid dissolution from 0.5 hours to 2 days;the second is near saturation from 2 to 7 days.The protonation degree of metallic defect sites is greater than that of phosphonates,rendering the incongruent leaching of tin,phosphorus,the mean values of which are?10-2-10-5 gm-2d-1,?10-2-10-4 gm-2d-1.The kinetics of amorphous materials fit into first order equation while the nanocrystalline tin oxide will leach after the saturated dissolution of peripheral amorphous regions of Sn-EDTMP.This category will basically keep the micro-meso-macro porosity of hybrid backbone intact in 3.0 molL-1 HCl while the whole ion affinity correlates inversely with the selectivity towards Th.A plausible degradation mechanism has been proposed as triple steps:the first is the diffusion of nucleophiles from bulk solution to the boundary of aggregated particles;the second is the dislocation of boundary renders the interparticle voids increase and the nucleophiles gather at metal-oxygen-phosphorus layers;the third is dual-step nucleophilic degradation of inorganic layers that triggers dynamic "elimination-exposure" of phosphonate moieties.The aggregated intensity of inorganic layers will determine the acidic stability of powdered materials.Thus,it becomes feasible to enhance the acidic stability by transforming them into pellets or polymer composites.Fourthly,the free phosphonate moieties and niobium sites of different protonated states will collectively sequester multiple speciation of Th,Fe.The predominant capture mode of Fe is electrostatic attraction-proton exchange.The sequestration of Th incorporates multidentate complexation accompanied by nitrate ions besides the "charge discrimination"effect.The Fe,Th are captured in the form of Th(NO3)x,FeOOH by the niobium alkylphosphonates.It will be appropriate to utilize them to enrich trace tetravalent actinide under moderate acidity,large liquid-solid ratio and ionic strength.The first-case study of metal(?,?)alkylphosphonate frameworks as radioanalytical separation candidate has been demonstrated by focusing on the isolation of light actinides.We have expounded a preliminary structure-property-functionality relationship,underlining the real application of metal-organic frameworks in radioanalytical separation. |
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