Removal Of Uranium From Aqueous Solution Using Montmorillonite-Supported Zero-Valent Iron Nanoparticles

Actinides are of great interest in terms of radioactive waste disposal because of their longevity. Uranium occurs naturally as U isotopes,238U(99.28%),235U(0.711%),234U(0.006%), and they exist as hexavalent uranyl complex in the natural environment. The use of uranium at industrial and military sites has resulted in very high uranium contamination, and dangerous to human healthy and environmental protection because of its long half-life (such as238Ut1/2=4.51×109years).Uranium, an actinide element, has been released into the environment through mining operations, nuclear testing, nuclear fuel, nuclear weapons production sites and accidental spill, and therefore, it is a major contaminant in soils, sediments, and groundwater. A provisional drinking water maximum contaminant level (MCL) for uranium of15μg/L has been established by the World Health Organization (WHO2004). However, it is discussed worldwide that10or5μg/L would be more reasonable. Geochemical processes occurring naturally, including dissolution/precipitation, redox reactions, and sorption/desorption reactions at the water-rock interface, control the mobility and transport of uranium in the subsurface system, such as aquifer sediments, soils, and groundwater.Montmorillonite-supported zero-valent iron nanoparticles (M-nZVI) was synthesized by sodium borohydride reduction and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FE-SEM). The interaction of uranium with montmorillonite, zero-valent iron nanoparticles and M-nZVI were studied using batch technique under different experimental conditions such as pH, ionic strength, initial U(Ⅵ) concentration, solid-to-liquid ration (m/V), EDTA, humic acid, fulvic acid and temperature. An effluent solution with a low level of uranium, i.e.,4.2x10-7mol/L(100μg/L) was used in the experiments to avoid precipitation of amorphous uranium-hydroxides. Uranium occurs naturally in low concentrations below100μg/L in soil, rock, as well as in surface and groundwater. The research contents of this thesis are summarized as follows: 1. Removal of uranium from aqueous solution using montmorilloniteThe SSA for montmorillonite was10.23m2/g and the layer spacing was1.28nm. The chemical composition was determined by X-Ray Fluorescence spectrometer:Al2O319.0%, SiO259.5%, MgO3.9%, Fe2O31.7%, K2O0.6%, Na2O3.6%and CaO2.3%.The removal efficiency of U(Ⅵ) using montmorillonite was strongly dependent on the pH values. The removal rate increases with the increase of pH value when the pH range was about2-6.5. The removal efficiency of U(Ⅵ) using montmorillonite was48.05%at pH6.5. The removal rate decreases with the increase of pH value when the pH range was about6.5-9. The concentration of NaNO3had significant effect on the U(Ⅵ) adsorption using the montmorillonite. The removal rate decreases with the increase of ions strength when the pH range was about2-7. The removal rate increases with the increase of ions strength when the pH range was about7-9. The removal percentage of U(Ⅵ) from the aqueous solution using montmorillonite increases with increasing solid content. The results show that the adsorption amount of U(Ⅵ) using the montmorillonitel was increased with increasing the initial U(Ⅵ) concentration at C[U(Ⅵ)]initial<100ppb. The isotherm of U(Ⅵ) on montmorillonite were fitted to non-linear models of Langmuir and Freundlich, and the equilibrium data were best described by the Langmuir isotherm model. The results showed that temperature has a significant effect on the reduction of U(Ⅵ) using montmorillonite in aqueous solution. The U(Ⅵ) removal percentage increased when the temperature increased initially. Hence, it is proposed that the mechanism of U(Ⅵ) removal efficiency using montmorillonite should include both ion exchange and surface complexation. When the pH<7, the main mechanism of U(Ⅵ) removal efficiency using montmorillonite is ion exchange adsorption. When the pH>9, the main mechanism of U(Ⅵ) removal efficiency using montmorillonite is inner-sphere surface complexation.2. Removal of uranium from aqueous solution using zero-valent iron nanoparticles (nZVI)The SSA for nZVI was26.6m2/g. The particles are oxidation resistant well with iron core-iron oxide shell structure. The core, consisting of zero-valent iron, forms an electron source that might reduce ions possessing higher standard reduction potential than that of iron. Attempts to form shell coating iron core have also been applied to protect the iron core from further oxidation. The removal efficiency of U(Ⅵ) using nZVI was strongly dependent on the pH values. The removal efficiency of U(Ⅵ) using nZVI was strongly dependent on the pH values. Results indicated that when pH is pH2, it produced less removal efficiently of U(Ⅵ). The removal efficiency of U(Ⅵ) using nZVI was73-78%at pH3-5. The results indicated that the optimum removal efficiency of U(Ⅵ) using M-nZVI was78%at pH5.0. The removal rate decreases with the increase of pH value when the pH range was about6-9. The concentration of NaNO3had a insignificant effect on the U(Ⅵ) adsorption using the nZVI. The removal percentage of U(Ⅵ) from the aqueous solution using nZVI increases with increasing solid content at m/v<0.1g/L. The removal efficiency of U(Ⅵ) using0.125g/L of nZVI was99%. The results show that the adsorption amount of U(Ⅵ) using the nZVI was decreased with increasing the initial U(Ⅵ) concentration at C[U(Ⅵ)]initial<100ppb. The isotherm of U(Ⅵ) on montmorillonite were fitted to non-linear models of Langmuir and Freundlich, and the equilibrium data were best described by the Langmuir isotherm model. The results showed that temperature has a insignificant effect on the reduction of U(Ⅵ) using nZVI in aqueous solution. These results are consistent with the hypothesis that the removal of U(Ⅵ) from aqueous solution is not only an adsorption process but also a reduction process in which U(Ⅵ) ions are reduced concomitantly by nZVI. The pH did not change after the reaction at the pH2. The pH was5.57after the reaction at the initial pH3. The pH was about9after the reaction at the initial pH4-8. The pH did not change after the reaction at the pH9. The iron content of the reaction solution was65.8mg/L at pH2. The iron content of the reaction solution was18.9mg/L at pH3. The iron content of the reaction solution was4.22mg/L at pH4. The iron content of the reaction solution was22-37ug/L at pH5-9. The main mechanism of U(Ⅵ) removal efficiency using nZVI is redox mechanisms, namely, the oxidation of iron, adsorption of U(VI) to nZVI, formation of oxide and hydroxide precipitates of U(Ⅳ) and Fe(Ⅲ) that coated the surface of the nZVI.3. Removal of uranium from aqueous solution using montmorillonite-supported zero-valent iron nanoparticles (M-nZVI)The SSA for as-synthesized M-nZVI was91.42m2/g. The isoelectric point (IEP) of M-nZVI was at pH5.6. These images clearly demonstrate that the aggregation of nZVI was eliminated and the nZVI was well dispersed on the M surface. The results indicate that the removal efficiency of U(Ⅵ) using M-nZVI was strongly dependent on the pH values. Results indicated that when pH is low (pH2), it produced less removal efficiently of U(Ⅵ). The removal efficiency of U(Ⅵ) was97.8%at pH3.0. The optimum pH values were in the range3.0to5.0. Therefore, pH3.0was selected for subsequent experiments. It can be seen that the pH solution did not significantly influence on the removal of U(Ⅵ) at pH of3-5. The removal rate decreases with the increase of pH value when the pH range was about6-9. The removal efficiency of U(Ⅵ) using M-nZVI was9.5%at pH9. The concentration of NaNO3had insignificant effect on the U(Ⅵ) adsorption using the M-nZVI. The removal percentage of U(Ⅵ) from the aqueous solution using M-nZVI increases with increasing solid content at m/v<0.1g/L. The removal efficiency of U(Ⅵ) using0.1g/L of M-nZVI was97.8%. However, the removal efficiency of U(Ⅵ) reached a steady state above0.1g/L of M-nZVI. The results show that the adsorption amount of U() using the M-nZVI was increased with increasing the initial U(Ⅵ) concentration at C[U(vi)]initiai<100ppb. The isotherm of U(Ⅵ) on M-nZVI were fitted to non-linear models of Langmuir and Freundlich, and the equilibrium data were best described by the Langmuir isotherm model. The results showed that temperature has a insignificant effect on the reduction of U(Ⅵ) using M-nZVI in aqueous solution.The presence of EDTA decreases U(Ⅵ) sorption at pH2-9. After pre-equilibrium of EDTA sorbed on U(Ⅵ), and subsequent sorption of M-nZVI. The presence of EDTA decreases U(Ⅵ) sorption on M-nZVI at pH2-9. The EDTA-U(Ⅵ) complexes is formed in solution and thereby reduce U(Ⅵ) sorption pH2-9. After pre-equilibrium of U(Ⅵ) sorbed on M-nZVI, and subsequent sorption of EDTA. The presence of EDTA decreases U(Ⅵ) sorption on M-nZVI at pH2-9. Different concentration of EDTA and EDTA to join order of M-nZVI removal of uranium pH change after the reaction is the same. The pH of0.001mol/L EDTA is higher than0.01mol/L EDTA after reaction at pH2-9. The U(Ⅵ)-HA complexes is formed in solution and thereby reduce U(Ⅵ) sorption pH2-7and enhances U(Ⅵ) sorption on M-nZVI at pH8-9. After pre-equilibrium of U(Ⅵ) sorbed on M-nZVI, and subsequent sorption of HA.The U(Ⅵ)-FA complexes is formed in solution and thereby reduce U(Ⅵ) sorption pH2-6and enhances U(Ⅵ) sorption on M-nZVI at pH8-9.Hence, it is proposed that the mechanism of U(Ⅵ) removal efficiency using M-nZVI was redox mechanisms, namely, the oxidation of iron, adsorption of U(Ⅵ) to M-nZVI, formation of oxide and hydroxide precipitates of U(Ⅳ) and FeOOH that coated the surface of the M-nZVI. It can be concluded that the iron hydroxide were formed as a result of Fe0corrosion reaction, where Fe0first oxidizes to Fe(Ⅱ) and then to Fe(Ⅲ). This resulted in the formation of U(Ⅳ) hydroxide precipitates which gradually coated the surface of the M-nZVI particles. The Fe content in the M-nZVI after reacting with U(VI) decreased from43.13to11.90wt%, while the oxygen content increased from41.18to63.28wt%. This result may be explained on the basis of the corrosion of Fe0to Fe(Ⅱ), Fe(Ⅲ), iron oxide or hydroxide on the surface of M-nZVI, and therefore, a decrease in the Fe content and increase in the oxygen content were observed.There is one novel point of this theses:(1) Preparation of Al-montmorillonite-supported zero-valent iron nanoparticles (M-nZVI).(2) Development of M-nZVI for uranium removal.