| With the rapid development of modern industry, the energy problem is becoming more and more severe. As one of the most important materials of nuclear energy, uranium is considered one of the most promising strategic resources. However, the terrestrial ores of uranium is limited and under over-exploitation. Seawater contains 4.5 billion tons of uranium, which is a thousand times larger than the terrestrial reservations. But the concentration of uranium is fairly low in seawater, known to be 3.3 μg/L. Therefore, the removal and recovery of uranium are of vital importance. Over the past few decades, several methods were developed for the high-efficiency removal of uranium, such as ion-exchange, adsorption, solvent extraction, chemical precipitation, ultrafiltration, nanofiltration and reverse osmosis. Adsorption is considered to be a more effective and economic way with high potential, low cost and easy operation. Therefore, it is vital to find a suitable adsorbent. In recent years, the mesoporous silica materials have attracted much attention due to their superior features such as large surface area, ordered pore sizes with well-defined cylindrical pore shapes, adjustable pore size, abundant hydroxyl in the surface and controllable morphology. Compared with other mesoporous materials, SBA-15 exhibits feathers such as larger pore size and thicker wall thickness which remarkably promote its hydro thermal stability. The traditional method for the synthesis of SBA-15 produces channels paralleled to the long axis of the pores and they restrict the diffusion of ions in the pore. What’s more, it may easily lead to pore blockage. In contrast, SBA-15 with short channels show better performance because of the channels which parallel to the short axis and favors the fast diffusion of molecules, thus have more advantages in application.However, both the pure silica-based SBA-15 and the functionalized SBA-15 are in the powder form, which leads to drawbacks such as difficult to operate and reuse. So it is hard to adapt to the industrialized operation process as well as commercial application. Therefore, we added the powder-formed adsorbents into the casting solution to make membrane-type adsorbent, for their unique characteristics of being easily bent or even twisted, which is very suitable for the industrial application and can overcome the shortcomings of powder-formed adsorbents.Firstly, the mesoporous silica SBA-15 with long channels (L-SBA-15) was prepared by hydrothermal method and added into the solution of polyvinyl chloride (PVC) to form the L-SBA-15-PVC membrane via the phase-inversion process. The results showed that, under the optimized conditions of PVC 10%, L-SBA-15 5% and pH=6, the adsorption capacity can reach 2014.0 mg/m2. The adsorption behavior followed the pseudo-second order rate equation and Langmuir model. Repeated use can be easily achieved by 3 mol/L HNO3 with slightly loss in the adsorption capacity. The comparison between powder-formed L-SBA-15 and the L-SBA-15-PVC membrane showed that there is only 10.9% loss in the adsorption capacity after membrane preparation process. However, the adsorption and desorption procedures are greatly simplified and easy to reuse, which indicates its promising usage in the removal and recovery of uranium from aqueous solutions.Secondly, the mesoporous silica SBA-15 with short channels (S-SBA-15) was also prepared and added into the solution of PVC to form the S-SBA-15-PVC membrane. Under the optimized preparation conditions with 5 (wt%) of S-SBA-15 and 10 (wt%) of PVC, the maximum adsorption capacity of uranium(VI) can reach 2520.0 mg/m2 when pH is 6, which is much higher than the L-SBA-15-PVC membrane. Langmuir model and Freundlich model were both used to fit the adsorption process and the results showed that the adsorption behavior followed the Langmuir model. Kinetic study demonstrated that the pseudo-second order rate equation is more suitable to describe the adsorption behavior. Nine repeated adsorption-desorption cycles were carried out and the adsorption capacity almost kept the same in the last eight cycles.Finally, through organic functional modification, cyano-group modified SBA-15 and amidoxime-group modified SBA-15 were prepared and added into the solution of PVC to form membrane-type adsorbent and then used for the removal of uranium in aqueous solutions. The results showed that the adsorption capacity of SBA-15-PVC membrane was greatly enhanced after organic functional modification.The properties of the membrane made by this method are in positive correlation with the adsorbent powders, which demonstrated that our membrane preparation technics are quite mature and the adsorption property of the membrane depends on the adsorbent itself. As a result, it is of vital importance to seek for adsorbents with superior properties in the future study. |
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