Synthesis Of Two-dimensional Flaky Carbon Nitride-based Functional Materials And Their Uranium Adsorption Properties

Actively promoting the development of seawater uranium extraction technology is of great significance to the sustainable development of nuclear energy and the construction of national defense.Graphite phase carbon nitride（g-C₃N₄）has been widely used in the field of photocatalytic extraction of heavy metals because of its large specific surface area,suitable band gap,high efficiency,cleanliness and sustainability.Therefore,starting from the construction of g-C₃N₄ materials with high adsorption capacity and high photocatalytic activity,by compounding metal-organic framework（MOFs）nanoparticles,surface manufacturing defects and grafting organic groups such as polyethyleneimine（PEI）and amidoxime group（AO）on g-C₃N₄,unique microstructure,excellent dynamics of ion transport and photocatalytic properties were obtained,so as to meet the requirements of photocatalytic reduction of uranium under light conditions.At the same time,organic functional groups such as carboxyl,amino and amidoxime groups are introduced to enhance the adsorption capacity and selectivity of CN,so as to meet the needs of the process of adsorbing uranium under the condition of no light,so as to improve the extraction ability,extraction speed,cycle stability and selectivity of g-C₃N₄materials for uranium in seawater.It includes the following three aspects:The pristine g-C₃N₄ was synthesized by one-step thermal polymerization of urea,and then a thin layer of g-C₃N₄（CN）was exfoliated by ultrasonic method,and then UiO-66（Ce-UiO-66）in situ growth,synthesized Ce-UiO-66/CN heterojunction semiconductor adsorption materials.The optimal adsorption p H value of Ce-UiO-66/CN was 8,where the maximum adsorption capacity reaches 1151.6 mg/g,which was 2.4 times higher than that of CN.The introduction of Ce-UiO-66 significantly improves the adsorption capacity of CN materials under weak alkaline conditions.The electron transfer between the ligand and metal of Ce-UiO-66 produces the ligand-metal transfer（LMCT）effect that enhances the separation of photogenerated electrons and holes in the composite.This effect effectively enhances the photocatalytic uranium extraction performance of Ce-UiO-66/CN,making it highly suitable for seawater environments.Ce-UiO-66/CN has demonstrated a uranium removal rate of up to 99%.Polyethyleneimine（PEI）grafted defective carbon nitride（NCN）composites（NCN-PEI）were synthesized using two methods-defective and solvothermal.Even after grafting with PEI,NCN-PEI composite retained its excellent photocatalytic efficiency.Under varying light conditions,different forms of uranium were observed to be adsorbed-uranyl superoxide dihydrate（（UO₂）O₂·2H₂O）under light conditions and uranium trioxide dihydrate（UO₃·2H₂O）under darkness conditions.Remarkably,even after five adsorption cycles,the material displayed a consistent 99%uranium removal rate.The maximum adsorption capacity was also greatly enhanced to 1556.9 mg/g at room temperature,which was 3.2 times higher than conventional CN.Uranyl ions(UO₂²⁺)first accumulate on the surface of the material through the adsorption and enrichment of PEI;at the same time,a considerable amount of H₂O₂ is generated at the defect of NCN-PEI;then H₂O₂ diffuses to the vicinity of UO₂²⁺,and reacts with UO₂²⁺to form（UO₂）O₂·2H₂O,realizing（UO₂）O₂·2H₂O grows on the surface of the material,thereby increasing the adsorption capacity of the material.AO-modified defective carbon nitride（NCN）composites（NCN-AO）were synthesized via defect and hydrolysis methods.The adsorption selectivity of uranium by NCN-AO was found to be high as evidenced by the 80%uranium removal rate in the presence of multiple competing cations while the removal rate of other ions was less than 5%.The adsorption cycle stability of uranium by NCN-AO was also good;in the fifth cycle the removal rate of uranium was above 97%and the extraction rate was above 85%.The band gap of the material was significantly reduced upon introduction of AO on CN,with an intermediate energy level appearing near the top of the valence band of the material,mainly contributed by the nitrogen and oxygen atoms on AO.This made possible the separation of the highest occupied molecular orbital（HOMO）and the lowest unoccupied molecular orbital（LUMO）of the material,reduced the recombination rate of photogenerated electron-hole pairs and prolonged the lifetime of photogenerated electrons.