Preparation And Uranium Adsorption Performances Of Functionalized Ni-Al Layered Double Hydroxides

As a high-efficient and environmental-friendly energy source,nuclear energy is recognized as the primary energy source to satisfy the world’s economic development in the coming decades.As the key element of nuclear energy,a great demand for uranium can be expected.However,conventional uranium reserves in inland ore are expected to be exhausted in less than a century.Therefore,mining uranium from unconventional resources,such as seawater,has received widespread attention.Layered double hydroxides(LDH)are considered as a promising uranium adsorption material due to their unique layered structure,low price,and high concentration of adsorption sites.However,the adsorption capacity of LDH nanosheet is often limited by their parallel stacking structure.LDH self are often used as a carrier for immobilizing groups though,if the LDH can be immobilized on a suitable carrier,the frontal accumulation can be avoided and the adsorption capacity can be maximized.Towards the development of the high-performance uranium adsorption material,in this thesis we exploited SiO2 microspheres,high specific surface area hollow carbon shells,graphite phase carbon nitride modified hollow carbon spheres,and polydopamine modified melamine sponge as carriers and prepared the novel LDH composite materials.In these LDH composites,a unique spatial structure and a path for ion transportation were constructed to improve the uranium extraction capacity of LDH materials.Specifically,SiO2 microspheres and Ni-Al LDH were used as the carrier and the shell respectively.By varying the amount of urea in the hydrothermal process,a series of SiO2@NiAl LDHx materials with controllable internal structures from core-shell to hollow-sphericalshell was designed and synthesized.TEM,FT-IR and other technologies were used to characterize the structure of SiO2@Ni-Al LDHx.In 100 mg·L-1 uranium solution with the same pH,the adsorption capacity of SiO2@Ni-Al LDH0.2 composite(89.8 mg·g-1)is outperforming other SiO2@Ni-Al LDHx materials and is 5 times higher than that of SiO2 nanospheres(15.2 mg·g-1).The kinetic and thermodynamic behaviors of the SiO2@Ni-Al LDH0.2 composites were investigated,which conform to the pseudo-second-order kinetic and Langmuir isotherm adsorption models.The adsorption equilibrium time is short(40 min)and the maximum adsorption capacity is 435.8 mg·g-1 in 400 mg·L-1 uranium solution at pH=5.0 and 298 K.We synthesized a novel Ni-Al layered double hydroxide adsorbent(C@Ni-Al LDH)with a double-hollow spherical shell structure,by anchoring flake Ni-Al LDH on the surface of the hollow carbon shell(HCS)via in-situ growth.It not only solved the problem of the parallel stack of LDH nanosheets but also avoided the agglomeration of hollow carbon shells in an aqueous solution.Meanwhile,the specific surface area of C@Ni-Al LDH composites was significantly increased,which provides more adsorption sites for uranyl ions.The adsorption capacity of C@Ni-Al LDH composites for uranyl ions reached 545.9 mg·g-1 at pH 5.0.In the multi-element ion competition experiment,the distribution coefficient KdU(14866 mL·g-1)of C@Ni-Al LDH is about twice that of Ni-Al LDH,indicating the strong affinity for uranyl ions.A Ni-Al LDH-coated and graphite phase carbon nitride(g-C3N4)modified double-layer hollow carbon sphere(C3N4-HCS@Ni-Al LDH)was constructed.The adsorption behavior of C3N4-HCS@Ni-Al LDH under different pH,adsorption time,temperature,uranyl ion concentration and interfering ions conditions were investigated.At the optimal adsorption condition of pH=7.0,the adsorption capacity of the C3N4-HCS@Ni-Al LDH composites reached 609.7 mg·g-1.After 5 adsorption-desorption cycles,the removal rate of C3N4HCS@Ni-Al LDH for uranyl ions only decreased from 95%to 81%.The MP@Ni-Al LDH organic-inorganic hybrid materials with high adsorption and easy recovery were successfully prepared by using polydopamine(PDA)as the "bridge".The"bridging" from the LDH to the melamine sponge(MS)macro substrate was conducted through the co-precipitation method.This design addressed the issue of LDH powder that is difficult to recycle in real applications.At the same time,MP@Ni-Al LDH composites showed excellent adsorption capacity because of the characteristics of organic matrix and inorganic particles.The KdU in the ion competition test was as high as 16690 mL·g-1.The maximum adsorption capacity was 559.8 mg·g-1 at the optimal pH=8.0.