Performance Study Of Phosphate-induced Uranium Mineralization For The Treatment Of Uranium-containing Wastewater

Promoting the development of nuclear energy actively contributes to energy security and environmental protection,aligning with the goals of the’carbon neutrality’strategy.However,as China vigorously develops its nuclear energy sector,it simultaneously intensifies uranium mining activities.The process of uranium mining results in substantial waste rock and uranium tailings,which release a significant amount of radioactive nuclides through rainwater leaching and natural weathering,causing severe environmental pollution.These pollutants mainly exist in the form of UO₂²⁺（soluble form）,known for their strong migratory capability,posing a serious threat of heavy metal and radioactive toxicity to the surrounding environment and biota.Therefore,it is crucial to employ solidification methods to reduce the migration ability of uranium（VI）.Recent studies have found the presence of uranyl phosphate minerals in soils and aquatic environments affected by uranium contamination.These minerals can persist stably in natural environments and serve as ideal targets for uranium immobilization.Based on the novel concept of ecological restoration using uranium-contaminated minerals,investigating the performance and mechanisms of uranium immobilization through phosphate mineralization and applying them to control and intercept uranium-contaminated wastewater are of paramount importance to ensure the safe development of nuclear energy.This study aims to design solid-phase Ca₃（PO₄）₂+UO₂²⁺and liquid-phase Ca²⁺+PO₄^3-+UO₂²⁺systems to explore the influence of experimental parameters,including the molar ratio of Ca:P:U,solution p H,and reaction temperature,on the immobilization efficiency of uranium（VI）and the stability of immobilization products.The mineralization mechanism of uranium（VI）is elucidated.Visual MINTEQ and PHREEQC software are utilized for simulation calculations,and microscopic characterization techniques such as XRD,SEM,TEM,and EDS are employed to analyze the morphology and phase transformation of uranium immobilization products under different reaction conditions.Additionally,the impact of solidification temperature on mineral phases is investigated to understand the performance of precipitation mineralization in uranium（VI）removal and the induced mechanism of calcium-uranium mineralization.The main research contents and conclusions are as follows:（1）The macroscopic behavior of uranium（VI）immobilization was experimentally investigated in two systems:solid-phase Ca₃（PO₄）₂+UO₂²⁺and liquid-phase Ca²⁺+PO₄^3-+UO₂²⁺.Batch experiments conducted at room temperature determined the optimal reaction conditions for uranium immobilization as p H 5.0 and a Ca:P:U molar ratio of 2:1:1.After 3days of mineralization,the removal efficiency of uranium（VI）reached 99.1%,and the desorption rate of uranium（VI）from the solid phase in 0.1 M sodium bicarbonate solution after24 hours was 2.13%.Kinetic fitting results indicated that the removal of uranium（VI）followed a second-order kinetic model,confirming the dominance of precipitation as the main mechanism.Furthermore,at p H 5.0,the immobilization reaction of uranium（VI）reached equilibrium within 5 minutes.The primary mechanism for uranium（VI）removal in the solid-phase Ca₃（PO₄）₂+UO₂²⁺system was dissolution-precipitation,with a longer-term dynamic equilibrium of dissolution-precipitation leading to reaction equilibrium at around 30 minutes.The main mineral phases produced in the experiment were Ca-autunite（chemical formula:Ca（UO₂）₂（PO₄）₂·x H₂O）,Na-autunite（chemical formula:Na₂（UO₂）₂（PO₄）₂·6H₂O）,and H-autunite（chemical formula:H₂（UO₂）₂（PO₄）₂·8H₂O）.In the hydrothermal ore-forming experiment,the optimal ore-forming conditions were p H=5.0,T=473 K,and a Ca:P:U molar ratio of 2:1:1.Under these conditions,the removal rate of uranium（VI）reached 99.8%,and the desorption rate of uranium（VI）was reduced to less than 1%.Therefore,high-temperature mineralization is beneficial for the removal of uranium（VI）and stabilizes the reaction products.（2）The distribution of uranium（VI）species under various conditions was simulated using Visual MINTEQ software.The results confirmed that the high removal rate in alkaline environments was attributed to the formation of unstable uranyl hydroxide complexes.The PHREEQC software was employed to calculate the solubility product logarithm(log K_sp)and Gibbs free energy（ΔG）of several mineral phases under different reaction conditions,providing insights into the precipitation tendencies of various phosphorous-uranium minerals at different temperatures.The dissolution-precipitation processes were simulated using the EQUILIBRIUM PHASES data module in PHREEQC.By calculating the saturation index（SI）of the precipitate products,the phase transformation relationships among different mineral components and the ore-forming mechanism of calcium autunite were investigated.（3）The precipitate products under different reaction conditions were analyzed for crystal phases and microstructural characteristics using X-ray diffraction（XRD）and scanning electron microscopy（SEM）.The results revealed that the formation of uranyl products in each reaction system was significantly influenced by p H values and ion concentration.Microstructural observations using SEM showed distinct morphologies of uranyl minerals.Ca-autunite（Ca（UO₂）₂（PO₄）₂·x H₂O）exhibited a well-defined rectangular plate-like structure,H-autunite（H₂（UO₂）₂（PO₄）₂·8H₂O）displayed irregular plate-like morphology,and Na-autunite（Na₂（UO₂）₂（PO₄）₂·6H₂O）exhibited stacked plate-like structures.Under high-temperature hydrothermal conditions at 473 K,the XRD diffraction peaks of the uranyl products exhibited reduced background noise and higher intensity,indicating that the hydrothermal conditions were conducive to the formation of mineral phases and improved crystallinity,thus achieving long-term stability of the uranium（VI）mineralization products.（4）Increasing the hydrothermal temperature significantly improves the crystallinity of uranyl products and expands the p H range for the formation of mineral phases,thereby enhancing the removal efficiency of uranium（VI）under alkaline conditions and improving the stability of the products.SEM analysis revealed that the mineral microstructures formed under hydrothermal conditions exhibited more regular structures compared to ambient temperature.The linear fitting between the crystallinity and the desorption rate of uranyl minerals yielded good results,indicating a correlation between mineral crystallinity and stability.