Experimental Research On Immobilization Of U(Ⅵ) In Solution By Zero-valent Iron And Hematite

Great attention has been paid to the threat of the highly radioactive and the low-level radioactive waste from nuclear industry and the radioactive waste has become a more and more serious problem with the waste from medical industry and research institutions. The uranium(Ⅵ) from uranium minings has caused the pollution of groundwater around. For example,the radioactive radionuclides have polluted the surface water and groundwater to varying degrees after thirty years'business of the large uranium mining in south china.This paper studied the removal effect of uranium(Ⅵ) in solution and its removal mechanism and also studied the removal effect of uranium(Ⅵ) in solution by the byproduct hemallite.The removal rates of uranium(Ⅵ) in solution by zero-valent iron at different pH,reaction time and different temperatures are studied.The experiment results indicated that the removal effect is better when the pH of the solution is bellow 7 and the removal rate rised with the increase of the pH of the solution.The effect of temperature on the chemical reaction was also studied and the conclusion is the impact of temperature is weak.The reaction fitted pseudo-first-order kinetic model well. The removal mechanism of uranium(Ⅵ) was discussed and the immobilization of uranium(Ⅵ) was the reductive precipitation and adsorptive precipitation by the byproduct.Hematite, a type of inorganic sorptive medium, was used for the removal of uranium(Ⅵ) from aqueous solutions. Variables of the batch experiments including solution pH, contact time, initial concentration, temperature ,Ca and Mg ions were studied. The adsorption capacities are strongly affected by solution pH, contact time and initial concentration. Higher pH favors higher uranium(Ⅵ) removal. It was also affected by temperature and Ca and Mg ions and the effect is weak. A two-stage kinetic behavior is observed in the adsorption of uranium(Ⅵ): very rapid initial adsorption in a few minutes, followed by a long period of a slower uptake. It is noted that an increase in temperature results in a higher uranium(Ⅵ) loading per unit weight of the sorbent.. The results indicated that adsorption of uranium by hematite had good efficiency, and the equilibrium time of adsorbing uranium(Ⅵ) was 6h.; The maximum adsorption capacity (qm) only increased from 3.36 mg·g^-1 to 3.54 mg·g^-1 when the temperature increased from 293K to 318K. The isothermal data are well fitted with both Langmuir and Freundlich equations, but the Langmuir isotherm fitted better than that of Freundlich. The pseudo-first-order kinetic model, pseudo-second-order kinetic model and intraparticle diffusion model were used to describe the kinetic data. The thermodynamic parameter, ?G0 were calculated. The negative ?G0 values of uranium(Ⅵ) at different temperatures confirmed the adsorption processes were spontaneous.