| Developing advanced nuclear energy and renewable energy technologies to optimize the energy structure and gradually replace fossil energy is important for future energy strategy.The utilization of nuclear energy promotes the development of national economy and society,but leads to the intensification of radioactive pollution,as mining and nuclear energy production will produce a large amount of waste rock and uranium tailings,the exposed waste rock and uranium tailings released radionuclides and toxic elements through rainwater leaching and weathering.Uranium in wastewater performs radioactive and chemical toxicity,causing damage to human health and bio-organism due to the easy migration of UO22+once discharged into the environment without efficient treatment.Therefore,it is urgent to immobilize uranium and reduce its migration to avoid environmental hazards.Uranyl phosphate minerals found in natural uranium ores,uranium-contaminated soils and sediments can be stabilized under natural conditions.Based on a new concept of ecological remediation of uranium contamination to natural minerals,a new strategy of artificially metallogenic of uranyl phosphate minerals is hypothesized in this work to control uranium contamination,which is consistent with the view of returning uranium pollutant into natural mineral to avoid the leaching risk.In this work,a new insight of artificial metallogenic of saleeite is reported for immobilizing U(VI).Three systems such as Mg3(PO4)2+UO22+,Mg2++PO43-+UO22+,Mg2++PO43-+Mg3(PO4)2+UO22+were designed to investigate the artificial metallogenic process of UO22+with phosphate.The macroscopic adsorption and precipitation solidification behaviors of U(VI)in experimental systems were investigated by adjusting the molar ratio of Mg:P:U,p H value of solutions,temperature and simulating the hydrothermal mineralization process in batch experiments.The morphology and metastable phases of the secondary ores in these systems were characterized by using XRD and SEM analysis.Theoretical calculations of dissolution and precipitation equilibrium were conducted using PHREEQC and Visual MINTEQ models.The stability of saleeite in geochemical environments were investigated using desorption experiments.The main research contents and conclusions are as follows:(1)Experimental comparision were conducted to investigate the macroscopic behavior of U(VI)removal in three systems.The kinetic fitting results showed that the removal process of U(VI)in the experimental systems was consistent with the pseudo-second-order model,the mechanism of dissolution-precipitation contributed to the favorable U(VI)removal efficiency of 93.86%in the Mg3(PO4)2+UO22+under p H=5.0,and the hydrothermal mineralization process increased the dissolution rate of Mg3(PO4)2,so that the highest U(VI)removal efficiency was increased to 97.51%.In the Mg2++PO43-+UO22+system,the U(VI)removal efficiency increased to 95.86%by direct precipitation under optimal conditions with the molar ratio Mg:P:U=0.5:1:1,the stoichiometric ratio of saleeite(Mg(UO2)2(PO4)2·10H2O)and p H=5.0,and the process of hydrothermal mineralization increase the U(VI)removal efficiency to be 98.48%.Both precipitation and surface adsorption contributed to the highest U(VI)removal efficiency of about 96%under p H=3.0-9.0.Therefore,the precipitation effect is more favorable to the removal of U(VI)and the significant effect of PO43-in the system.Because a variety of uranyl phosphate compounds formed in the system,and the presence of Mg2+facilitates the stabilization of saleeite.(2)The XRD and SEM characterization of the precipitates of each system showed that chernikovite(H2(UO2)2(PO4)2·8H2O)formed under p H=3.0,the microscopic morphology performed as nanorod structure;saleeite(Mg(UO2)2(PO4)2·10H2O)formed under p H=5.0,showed as regular nanosheet structure.The formation of saleeite and other minerals also confirmed by the PHREEQC calculation of saturation index.The microscopic morphology of the formed minerals from hydrothermal mineralization system showed clearer as diamond lamellar structure.And the XRD diffraction peaks are less spurious and more intense,proving that the high temperature conditions are favorable for the formation of saleeite with stronger crystallinity.While precipitates formed under p H value of 7.0 and alkaline environment varying degrees of peak broadening characteristic of amorphous phases.(3)Desorption rate was used to assesse the stability of formed crystalline mineral.The U(VI)desorption rate was low and less than 30%in each system under p H=3.0and p H=5.0,and the U(VI)desorption rate was reduced to less than 10%in the hydrothermal mineralization system.Calculating log Kspand Gibbs free energy variation?G of several precipitates at 298 K,373 K and 473 K proved that the increase of temperature is favorable to the stabilization of chernikovite(H2(UO2)2(PO4)2·8H2O)and saleeite(Mg(UO2)2(PO4)2·10H2O),which achieved long-term stable immobilization of U(VI)and avoiding leaching risk.(4)Theoretical calculations of the molar ratios P/U and Mg/U in the precipitation of the experimental system were perfoemed by SOLUTION data of PHREEQC,which was used to quantify the relationship between the conversion of mineral components and investigate the mineralization mechanism.The conditions of p H=3.0 and p H=5.0is favorable for the formation of the crystalline chernikovite(H2(UO2)2(PO4)2·8H2O)and saleeite(Mg(UO2)2(PO4)2·10H2O)respectively,the experimental and theoretical values of molar ratios P/U and Mg/U are close to the stoichiometric ratios of the major mineral,the molar ratio P/U is close to 1:1 and Mg/U is close to 0.5:1.While the molar ratios P/U and Mg/U deviate from the stoichiometric ratios due to the surface adsorption of the precipitates,the transformation of the physical phases caused by p H and ion ratios,and the changes in the lattice parameters of the mineral during hydrothermal mineralization. |
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