摘要
利用溶胶-凝胶技术通过分相法和超临界干燥法分别合成了多孔SiO2凝胶,借助TG-DTA、SEM、FT-IR、压汞仪、气体吸附仪等分析手段,对合成的多孔SiO2凝胶的结构性能进行了表征；采用间歇法研究了多孔SiO2凝胶对溶液中铈的吸附性能,筛选出分相法制备的多孔SiO2凝胶,然后采用间歇法和连续法分别研究了其对溶液中铀的吸附性能；以铈为模拟元素,采用低温烧结多孔Si02凝胶的方法合成了高硅玻璃陶瓷固化体,并借助于TG-DTA、XRD、SEM及抗浸出性能测试等分析测试方法,对烧结的多孔SiO2凝胶固化锕系核素进行了初步研究。研究成果如下：
(1)通过分相法和超临界干燥法均成功制备了多孔SiO2凝胶,其中,分相法制备的多孔SiO2凝胶内部是由连续的孔结构构成,是一种轻质多孔网络结构材料；具有较高的孔隙率和骨架密度,较低的表观密度；孔径分布以大孔和中孔为主,微孔较少,因此能够以不同的物理吸附方式吸附分离溶液中的离子及离子基团；基本结构单元为Si-O-Si,表面有大量的羟基基团和有机基团,对其进行高温处理,其基本结构特征均未发生明显变化,因此该法制备的多孔SiO2凝胶具有良好的热稳定性。超临界干燥法制备的多孔SiO2凝胶是具有海绵状网络结构的多孔材料,高温处理后,网络结构发生坍塌,热稳定性较差。由以上所述可知,分相法制备的多孔SiO2凝胶的性能明显优于超临界干燥法制备的多孔Sio2凝胶,因此,在锕系核素的分离固化研究中,主要研究了分相法制备的多孔SiO2凝胶对铈和铀的吸附分离性能和固化包容性能。
(2)多孔SiO2凝胶(分相法)对溶液中铈和铀均具有良好的吸附性能,但对铈的吸附平衡时间较长；在初始浓度一定时,对铈和铀的吸附性能受固液比的影响较大,而且对铈和铀的吸附体系的最大固液比是不同的；对铈和铀的吸附主要以物理吸附和化学吸附两种方式进行的,而且在吸附反应的不同阶段,主要的吸附方式是不同的；多孔SiO2凝胶对铀的吸附性能随着溶液pH值的不同而有所不同,在中性和弱酸性溶液中,吸附性能较好,而在pH<5的酸性较强的溶液中吸附性能较差；溶液的初始浓度不同时,吸附性能是不同的,平衡吸附量随着初始浓度的增加而逐渐升高,平衡吸附率和平衡吸附比则逐渐降低；溶液中所含离子的种类和价态不同时,多孔SiO2凝胶对铀的吸附性能也不同,三价离子对铀的吸附影响较大,而二价及一价离子对铀的吸附基本无影响；多孔SiO2凝胶对铀的吸附符合Langmuir等温吸附方程,吸附动力学符合准二级反应动力学方程,因此铀在多孔凝胶上是单分子层吸附,吸附反应不是简单的表面反应过程,而是多种反应过程的组合,吸附反应不仅发生在多孔凝胶的表面,还发生在颗粒内部；动态吸附性能受流入液流速的影响的较大,动态平衡吸附量随着流速的降低逐渐增大,而迁移速率则逐渐减小。
(3)通过低温烧结而不是高温熔融工艺可以成功制备包容铈的高硅玻璃陶瓷固化体,固化体的外观特征、致密度及密度与烧结温度的高低有关,温度过高或过低均不利于固化体的烧结,较佳烧结温度约为1150-1250℃；包容的模拟元素铈是以CeO2晶体的形式均匀分布在高硅玻璃基质中,抗浸出性能测试也显示低温烧结制备的高硅玻璃陶瓷固化体的元素归一化浸出率明显低于目前已经工业化应用的硼硅酸盐玻璃固化体,对铈具有良好的包容性能。
Sol-Gel technology was applied to synthesize porous silica gel by phase separation and supercritical drying. The structure property of synthesized porous silica gel were characterized by means of TG-DTA, SEM, FT-IR, Press Mercury Analyzer, Gas Adsorption Analyzer and so on. Discontinous process was applied to study the adsoptive property of porous silica gel to cerium ions, while the adsorptive property to uranium was studied using discontinuous process and dynamic adsorption test. High silica glass-ceramic solidification forms wraping the simulating element cerium are synthesized by sintering porous silica gel at lower temperature, which was analyzed by TG-DTA, XRD, SEM, leaching test and so on. Research results are as follows:
Porous silica gel could be successfully fabricated by phase separation and supercritical drying. That one fabricated by phase seperation is a kind of light porous material which has the structure of contimous pore and network with high porosity, high skelen density, low apparent density, the pore size distribution of main mesopore and micropore, the basic structure unit of Si-O-Si and surface of a large number of hydroxyl groups and organic groups. Porous silica gel synthesized by phase separation has good thermal stability as the basic structure features are not changed significantly after heat-treatment at high temperature, while porous silica gel prepared by supercritical drying has sponge-like network structure and poor thermal stability. In summary, porous silica gel synthesized by phase separation has better performance than that synthesized by supercritical drying, so that our works were focoused on the adsorption-separation properties and solidification-enbeding properties of porous silica gel fabricated by phase separation in the studing of the separation-solidification of simulative High Radioactive Level of Liquid Waste.
Porous silica gel synthesized by phase separation has fine adsorptive capacity to cerium and uranium, while a longer equilibrium adsorptive time to cerium was obtained. The adsorptive properties of porous silica gel to cerium and uranium are affected by sorbents' quantity for a given initial concentration of cerium and uranium. Adsorptive mechanism of cerium and uranium on porous silica gel was more physic adsorption than chemical adsorption, and the main adsorption mechanism are different in different stage of the adsorptive reaction. The adsorptive properties of porous silica gel to uranium are affected by the pH value in solution, which are better in neutral and weak acid solution, while poorer in strong acid solution at pH<5.0. The adsorptive property parameters of porous silica gel to uranium are different when the initial concentration of uranium in solution is changed. The equilibrium adsorptive capacity increase with increasing the initial concentration of uranium in solution, while the equilibrium adsorptive rate and ratios decrease. When the species and states of other ions in solution are different, the adsorptive capacity of porous silica gel are also changed. Trivalent ions have greater influence on the adsorptive capacity of porous silica gel, while divalent and monovalent ions no influence. It keeps to Langmuir equation orderliness and quasi-Second order kinetic equation that porous silica gel adsorbed uranium. Therefore, uranium in the surface of porous silica gel is monolayer adsorption, and adsorption reaction is not a simple surface reaction process but the combination of a variety of reactions, which not only occurs on the surface of porous silica gel but also occurs in the grains. Dynamic adsorptive property are greatly influenced by the flow rate of solution.The dynamic equilibrium adsorptive capacity increases with decreasing the flow rate, while the migration rate of uranium is gradually reduced.
High silica glass-ceramic solidification forms immobilizing cerium could be successfully synthesized by sintering in lower temperature not melting at higher temperature. Appearance character, compaction and density of solidification forms relate with the sintering temperature. Solidification forms are conducive to being sintered and the optimum sintering temperature is about 1150℃～1250℃. Cerium in the form of CeO2 crystals uniformly distribute in the high silica glass matrix. The results of leaching test show that high silica glass synthesized by sintering porous silica gel at the lower temperature has a good immobilizing performance. The normalized leaching rate of elements is significantly less than that of the current borosilicate glass solidification forms which have been applied to industry now.
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