Interaction Mechanism And Adsorption Performance Of Radionuclide U(?) On Functional Layered Double Hydroxides(LDHs-x) Clay

Layered double hydroxides?LDHs?clay mineral materials,based on its excellent thermo-stability,unique “memory effect”,easily-exchanged intercalated layer anions and opened porous structures,have been applied in nano-catalysis,energy storage,environmental remediation,sensor and other fields.Especially in environmental remediation,due to it existed in nature,clean-cheap and another advantages,it has been used as adsorbents for the removal and degradation of organic dyes,heavy metals and radionuclides.However,clay mineral materials also possess some drawbacks,such as poor acid resistance and less kind of surface oxygen-containing functional groups that limit their application in the treatment of radionuclides.Hence,this study was focused on designing and assembling high acid-resistance LDHs materials with abundant functional groups,and it has proposed to synthesis glycerol-modified nanoscale rod-like Ca/Al layered double hydroxides?Ca/Al LDH-Gl?,flocculent Ni/Al layered double hydroxides?Ni/Al LDH-Gl?,rod-like ternary Ca-Mg-Al layered double hydroxides?Ca-Mg-Al-LDH?and its derivatives?Ca-Mg-Al-LDOx?,and graphitic carbon nitride @ layered double hydroxides nanocomposites?g-C3N4@Ni-Mg-Al-LDH?.In order to overcome these drawbacks of LDHs applied in the remediation of radionuclides,various characterization techniques?e.g.,SEM,TEM,element mapping,BET,EDS,FT-IR,XRD and XPS?have been used to analyze the physicochemical properties and microstructure.Advanced EXAFS technique and DFT theoretical calculations were employed to explore the removal mechanisms and reaction interaction of U???on the surface of LDHs after functionalization.Dependent on batch adsorption experiments,the adsorption behavior and removal efficiency of U???under various environmental conditions(e.g.,solid content,pH,ionic strength,contact time,temperature and C[CO₃^2-])were simulated systematacially.One-step hydrothermal synthesis technique was adopted to synthesis Ca/Al LDH-Gl and Ni/Al LDH-Gl,and it was confirmed the existence of abundant oxygen-containing functional groups?e.g.,C-O,O-C=O and C=O?on the surfaces of both adsorbents,which could provide enough active sites and reaction interface.The saturated adsorption capacities of U???calculated from Sips model were 266.5 mg·g^-1 for Ca/Al LDH-Gl and 142.3 mg·g^-1 for Ni/Al LDH-Gl at 298.15 K,and the higher adsorption capacity of Ca/Al LDH-Gl might be due to more functional groups and abundant high-activity “Ca-O” groups.The EXAFS analysis confirmed the non-transformation of U???to U?IV?on solid particles,and stable inner-sphere complexes were not formed by reduction interaction but by chemical adsorption.DFT calculations further evidenced that the interaction mechanism were mainly attributed to hydrogen bonds and electrostatic interactions.Under low concentration of CO₃^2-(C[CO₃^2-] < 0.04 mol·L^-1),Ca/Al LDH-Gl was dependent on the concentration of CO₃^2-,and it indicated that CO₃^2-should be avoid in the natural adsorption of application process.Hydrothermal synthesis and calcination techniques were applied to fabricate rod-like Ca-Mg-Al-LDH and Ca-Mg-Al-LDDx?x: 200,300,400,500 and 600 °C?,and the adsorption capacity of U???on various adsorbents could be controlled and adjusted through changing the calcination temperature.The monolayer saturated adsorption capacities of U???on these adsorbents were in the order of Ca-Mg-Al-LDO500(486.8 mg·g^-1)> Ca-Mg-Al-LDO600(373.4 mg·g^-1)> Ca-Mg-Al-LDO400(292.5 mg·g^-1)> Ca-Mg-Al-LDO300(260.0 mg·g^-1)> Ca-Mg-Al-LDO200(223.5 mg·g^-1)> Ca-Mg-Al-LDH(132.5 mg·g^-1).It was attributed to the calcined Ca-Mg-Al-LDDx possessed unique “memory effect”,nano-“size effect” and the different contents of various metal-oxide bonds?e.g.,Ca-O,Mg-O and Al-O?.The results of kinetic and thermodynamic studies demonstrated that the adsorption was a spontaneous and endothermic chemical process.Two-step hydrothermal polymerization and one-step calcination techniques were used to prepare grid-like g-C3N4@Ni-Mg-Al-LDH composites,and comparing the results of before and after polymerization,it could be found that the maximum adsorption capacities of U???on g-C3N4@Ni-Mg-Al-LDH(99.7 mg·g^-1)was 1.5 times higher than monomer Ni-Mg-Al-LDH(59.8 mg·g^-1)and 3.0 times higher than monomer g-C3N4(31.1 mg·g^-1).It indicated that the synergistic effects of various functional groups in the surface of composites have improved the complex ability for uranyl ions.In addition,at whole pH range?2.0<pH<11.0?,the adsorption of U???on g-C3N4@Ni-Mg-Al-LDH was dependent on ionic strength,which demonstrated that the adsorption process was mainly due to outer-sphere surface complexation and/or ion-exchange.The results of kinetic and thermodynamic studies demonstrated that the adsorption of U???on various materials was a chemical reaction rather than physical process,and the adsorption of U???on g-C3N4@Ni-Mg-Al-LDH was a spontaneous and endothermic process,and increasing temperature was beneficial for the adsorption reaction.This paper discussed and analyzed the environmental behavior and interaction mechanism of U???on LDHs composites from macroscopic,microcosmic and molecular levels,and these as-prepared LDHs functional materials would as potential and promising materials for the removal and enrichment of radionuclides,which could also provide reference frame for the synthesis functional composites materials and the remediation of pollutants.