Study On The Immobilization Effect Of Uranium-Containing Silica Gel Waste With Boron-Based Glass System

Uranium containing wastewater is a type of liquid waste with the largest amount and volume generated in uranium mining and various nuclear facilities research and processing,and has a high degree of mobility and biological toxicity.The use of silica gel adsorption for the treatment of uranium-containing wastewater is the main tool used to purify uranium-containing wastewater in China.The increase in uranium-containing wastewater directly leads to an increase in the volume of the silica gel waste,resulting in increasing storage pressure and the risk of uranium re-analysis into the environment.And the disposition of uranium-containing silica gel（UCS）using safe and efficient treatment methods is a matter of urgency.After considering completely the waste composition of uranium-containing silica,the boron-based glass system was selected as matrix for immobilizing uranium-containing silica waste,which was investigated in this study as follows:1)The direct sintering of uranium-containing silica gel showed that it cannot achieve the"adsorption-curing"effect.Moreover,uranium-containing silica gel lost water to form Si O₂at 1300 and 1500℃,and powdered samples with no mechanical properties indicated that the addition of a secondary substrate was an inevitable choice.2)Silica gel can lose water to from Si O₂under certain conditions.Taking the construction of a borosilicate glass network as a starting point,B₂O₃was selectively added for the auxiliary immobilizing UCS.When the sintering temperature was 500°C,the physical phase composition of the sample was only B₂O₃.And the main physical phases in sintering body were B₂O₃and UB₂O₆at 600-900°C.Meanwhile,SEM-EDS results showed that the morphology of the sintering body sintered at 600°C showed a tendency of silica gel wrapped by B₂O₃substrate.And the uranium elements were uniformly distributed on both boron glass and silica gel.The leaching tests showed that the generated UB₂O₆was a soluble phase,and all samples showed different degrees of dissolution in the leaching tests,exhibiting poor resistance to dissolution and leaching.The above results fully demonstrate that immobilizing using only B₂O₃substrate was not able to meet the requirements.3)Bi₂O₃,a modifier capable of building glass networks,was added.Both the physical pictures and the XRD test results of samples showed the sintering body could form completely amorphous and transparent glass at 900°C and 1100°C.SEM-EDS results show that the surface of the sample B-Bi6 was homogeneous and dense and the matrix elements B,Bi,Si,U and O were uniformly distributed in the sintering body,which proved that silica gel can completely dissolve in the B-Bi glass matrix.The deconvolution results of FT-IR indicated that the absorption peaks caused by Si-O-Si in silica gel had a tendency to weaken or even disappear with high bismuth content.The elemental spectrum of Si and B revealed the formation of B-Si bond characterized by XPS which could be used as a reference for the dissolution of silica gel in B-Bi glass system.The leaching test results indicate that the samples B-Bi1,B-Bi2 and B-Bi3 were more suitable for the solidification of UCS,showing a lower leaching rate(10^-4～10^-3g·m^-2·d^-1).However,it was not conducive to the dissolution of silica gel,and there was also a white silica gel covering layer on the surface of these samples.4)The sample with low leaching rate was selected for the solubility experiment of silica gel.The SEM-EDS results demonstrated that the white surface layer was formed by the silica gel without cracks but filled with a large number of holes under the role of B-Bi as a binder.Taking the formation of completely transparent glass as the basis for determining the dissolution of silica gel in the B-Bi matrix,and the solubility of sample B-Bi1-Si for silica gel was less than 1 wt.%.The solubility of silica gel in sample B-Bi2 was between 1 and 5wt.%and the maximum solubility of sample B-Bi3-Si for silica gel was 6 wt.%.5)Uranium-containing wastewater with a concentration of 100 mg/L was adsorbed by300-400 mesh silica gel at a ratio of adsorbent to mother liquor of 1:500.Under this condition,0.6 g of silica gel containing uranium with an adsorption capacity of 16.242 mg/g was successfully produced.And it was used to verify the immobilizing UCS with a mass fraction of 6 wt.%by sample B-Bi3-Si.The physical image and XRD results were highly consistent,indicating that the sintering body was a completely transparent glass.And its surface showed uniform,tight and flat characteristics under SEM test,and the EDS results also exhibited no aggregation of elements.B-Si bond were found in the elemental spectra of both boron and silicon characterized by XPS,which was consistent with the previous findings.And the presence of B-Si bond was the reason for the complete dissolution of silica gel in the B-Bi matrix.The leaching results showed that the release rates of the matrix elements B and Bi were maintained at 10^-4g·m^-2·d^-1and 10^-6g·m^-2·d^-1respectively,while the waste element U was released at 10^-7g·m^-2·d^-1.