Study On The Performance And Mechanism Of Uranium（Ⅳ） Removal By Artificially Synthetic Fluorapatite And Natural Fluorapatite

This thesis aims to solve the problem that it is difficult to have both superior stability and excellent adsorption performance of uranium（Ⅳ）adsorption materials in the strong acidic environment（p H<5）.In this thesis,the artificially synthetic fluorapatite[solid phase method（atmospheric pressure calcination method）and liquid phase method（precipitation method）]and Guizhou natural fluorapatite are selected as uranium（Ⅳ）adsorption materials.The reasons for selection of fluorapatite are as follows:fluorapatite is the most abundant and stable apatite mineral in nature,and fluorapatite has strong anti-irradiation stability and excellent chemical stability.At the same time,fluorapatite still has certain solubility in the strong acidic solution.Calcium ion(Ca²⁺)and phosphate ion(PO₄^3-)released by fluorapatite[FA,Ca₁₀（PO₄）₆F₂]can combine with uranyl ion(UO₂²⁺)to form meta-autunite type or autunite type uranium ore,which can realize the core concept of"deriving from nature（uranium ore）and attributing to nature（uranium ore）"in this thesis.Under the conditions of multiple factors[p H,solid-liquid ratio,adsorption time,initial uranium（Ⅳ）concentration and adsorption temperature],the uranium（Ⅳ）removal performances of artificially synthetic fluorapatite and natural fluorapatite were systematically studied.The adsorption experimental data were fitted by means of adsorption thermodynamic model,adsorption kinetic model（pseudo-first-order and pseudo-second-order）and isothermal adsorption model（Langmuir and Freundlich）.Combined with XRF,XRD,FTIR,BET,SEM,TEM,XPS and ICP-OES,the artificially synthetic fluorapatite（solid phase method and liquid phase method）and Guizhou natural fluorapatite before and after adsorption of uranium（Ⅳ）were systematically characterized,and the uranium（Ⅳ）removal mechanisms of abovementioned fluorapatites were revealed.The main contents of this thesis include the following three parts:（1）CaF₂,Ca₂P₂O₇ and Ca（OH）₂ were selected as the reaction raw materials.5 wt.%Ca F₂ was additionally added on the basis of the original stoichiometric molar ratio.Fluorapatite was synthesized by the solid phase method（atmospheric pressure calcination method）at 900℃for 4 hours via the"secondary calcination"process.The synthesized fluorapatite was mechanically milled for 4 hours.The average particle size and specific surface area of acquired fluorapatite powder were 1.38μm and 8.67 m~2/g respectively.The adsorption capacity and removal rate of uranium（Ⅳ）by fluorapatite synthesized by solid phase method（atmospheric pressure calcination method）reached 655.17 mg/g and 78.62%respectively.The adsorption process conforms to pseudo-second-order kinetics model（chemical adsorption）and Langmuir isothermal adsorption model（monolayer adsorption）,and the adsorption process is a spontaneous and endothermic process.The uranium（Ⅳ）removal mechanism by fluorapatite synthesized via solid phase method（atmospheric pressure calcination method）is surface mineralization.After being adsorbed with uranium（Ⅳ）,the Ca（UO₂）₂（PO₄）₂·6H₂O is formed on the surface of FA.The Ca（UO₂）₂（PO₄）₂·6H₂O has superior stability under the condition of p H≥3.（2）Fluorapatite was synthesized by liquid phase method（precipitation method）on the basis of the original stoichiometric molar ratio.C₁₆H₃₆FN,Ca（NO₃）₂·4H₂O and（NH₄）₂HPO₄ were selected as the reaction raw materials.After the slurry which synthesized by liquid phase method suffered the vacuum drying,the dried slurry was calcined at 300℃for 3 hours.The synthesized fluorapatite powder was rod-like crystal,the diameter range and length range of synthesized fluorapatite were 20-70 nm and 100-500 nm respectively.The specific surface area of synthesized fluorapatite was 67.9 m~2/g.The adsorption capacity and removal rate of uranium（Ⅳ）by fluorapatite which synthesized via liquid phase method（precipitation method）reached 716.58 mg/g and 85.99%respectively.The adsorption process conforms to pseudo-second-order kinetics model（chemical adsorption）and Langmuir isothermal adsorption model（monolayer adsorption）,and the adsorption process is a spontaneous and endothermic process.The uranium（Ⅳ）removal mechanism by fluorapatite synthesized via liquid phase method（precipitation method）is surface mineralization.After being adsorbed with uranium（Ⅳ）,the Ca（UO₂）₂（PO₄）₂·6H₂O is formed on the surface of FA.The Ca（UO₂）₂（PO₄）₂·6H₂O has superior stability under the condition of p H≥3.（3）The Guizhou natural fluorapatite minerals in this thesis are composed of fluorapatite[Ca₁₀（PO₄）₆F₂,75.66 wt.%],silica（Si O₂,16.04 wt.%）and calcium phosphate[Ca₃（PO₄）₂,6.36 wt.%].The natural fluorapatite powder after ball milling for 4 hours is irregular block.The average particle size and specific surface area of natural fluorapatite powder were 12.4μm and 4.7 m~2/g respectively.The adsorption capacity and removal rate of uranium（Ⅳ）by natural fluorapatite reached 620.08 mg/g and 74.41%respectively;The adsorption process conforms to pseudo-second-order kinetics model（chemical adsorption）and Langmuir isothermal adsorption model（monolayer adsorption）,and the adsorption process is a spontaneous and endothermic process.The uranium（Ⅳ）removal mechanism by Guizhou natural fluorapatite is mainly surface mineralization.After being adsorbed with uranium（Ⅳ）,two kinds of meta-autunites[Ca（UO₂）₂（PO₄）₂·2H₂O,Ca（UO₂）₂（PO₄）₂·6H₂O]are formed on the surface of natural fluorapatite.The Ca（UO₂）₂（PO₄）₂·6H₂O and Ca（UO₂）₂（PO₄）₂·2H₂O have superior stability under the condition of p H≥3.In this thesis,artificially synthetic fluorapatite[solid phase method（atmospheric pressure calcination method）and liquid phase method（precipitation method）]and Guizhou natural fluorapatite are used as uranium（Ⅳ）adsorption materials,which not only achieve the utilization the resource of natural fluorapatite as uranium（Ⅳ）removal materials,but also realize the mineralization treatment of radionuclide uranium（Ⅳ）in the uranium（Ⅳ）containing waste water.The related research results show that fluorapatite has considerable industrial application prospects,which can be used as the potential adsorption materials.