Sorption Mechanisms Of U(Ⅵ), Th(Ⅳ) On Some Sorbents

The sorption and desorption of tetravalent Th(IV) and hexavalent U(VI) on silica, iron coated sand, silica gel and ACT-attapulgite were studied by using the batch and column techniques in this paper.The dissertation was divided into eight chapters.In the first and second chapter, the study background, basic content and sketch of experimental scheme were introduced in brief, and some basic concepts and principles related to adsorption on solid-liquid surface were described, and the methods, which were used in this paper, were introduced in detail.In the third chapter, Sorption and transport of UO22+ on silica was studied by using the batch and column techniques. The insensitive effect of ionic strength on the UO22+ sorption onto SiO2 and the sorption can be described Freundlich and D-R model very well;There was a positive effect of phosphate on the uranyl sorption onto SiO2;FA will increase and decrease the mobility of uranyl on SiO2 in both the low pH and in the high pH range, respectively;The negative effects of catechol(CA) and salicylate(SA) occured pH range from 2 to 10. The sorption of uranyl onto SiO2 remains unaffected in the presence of phthalate (PH). So, we can not study the effect of FA on the radionuclides sorption on silica by simple organic ligands. On the other hand, from the column experiment, it could be concluded that phosphate or FA exerts a significant control on sorption and transport of U(Ⅵ) on a silica column; in the presence of phosphate and FA sorbed on a silica column preliminarily and simultaneously, there was a the synergic effect on the sorption and transport of U(VI) on a silica column; the U(VI) sorbed on a silica column in the presence and absence of phosphate or FA sorbed preliminarily is not readily desorbed and recoverable. At last, the column experiments have been carried out to investigate the effect of flow rate on the breakthrough curves(BTCs) of phosphate, fulvic acid, and uranium(Ⅵ) onto a silica column. Based on these comparisons of C/Co-V/Vo plots at different flow rates and the theoretical analysis from the Bohart-Adams sorption model, it was found that the right shift (increase in V/Vo of breakthrough), the left shift (decrease in V/Vo of breakthrough), and the nonshift (non-change in V/Vo of breakthrough) of initial parts of BTCs with increasing flow rate are certain to occur instead of only left shift and that three different trends of shifts can be mainly attributed to different rate-controlling mechanisms of sorption process.In the fourth chapter, the sorption and desorption of Th(IV) on silica was studied by using the batch and column techniques. We studied the sorption behavior of Th(IV) on silica and the effects of pH, ionc strength, simple organic compounds, FA, phosphate. The results indicated that the sorption of Th(Ⅳ) on the silica was strongly dependent on pH and weakly dependent on ionic strength. Phosphate exerts a significant control on the Th(Ⅳ) sorption and transport on the silica column.) The breakthrough of Th(Ⅳ) on the silica column is not significantly influenced by the co-injection of Cr3+.A minor proportion of Th(Ⅳ) sorbed on the silica column in the presence and absence of phosphate sorbed preliminarily is not readily desorbed and recoverable. This work suggested that the effects of competing cations and inorganic ligands on the sorption and transport of tetravalent actinides were very important in the assessment of the long term interactions of tetravalent actinides with the environment in radioactive waste storage.The fifth chapter, the adsorption of Th(Ⅳ) on iron coated sand was investigated. The results indicated that the sorption of Th(IV) on the iron coated sand was strongly dependent on concentration of adsorbent and pH, weakly dependent on ionic strength and cation ions. The equilibration of sorption of time was influenced with HA or phosphate added, but could be described accurately by pseudo-second order rate model. The sorption amounts increased significantly in the presence of FA and phosphate.In the sixth chapter, the adsorption of Th(Ⅳ) onto silica gel was studied. The various effects of Th(Ⅳ)sorption onto the silica gel were studied:the contact time, the solid-of-ratio, pH, ionic strength, the soil FA, PO43- and different temperature of the sorptions Th(IV) onto silica gel were also investigated. The adsorption data can be described accurately by pseudo-second order rate model and liquid film diffusion.The results suggested that the adsorption of Th(Ⅳ) onto silica gel increased with increase of the solid-of-ratio, the adsorption was strongly influenced by pH, but the ionic strength have little influence. Different effects of FA on actions adsorption were found at different pH values. The thermodynamic parameters (i.e.;ΔH0,ΔS0, andΔG0) of radionuclide adsorption onto silica gel were calculated from the temperature dependence, and the results suggested that the adsorption reaction was spontaneous and exothermic with a high affinity.In the seventh chapter, the adsorption of Th(Ⅳ) on modified attapulgite was investigated. The samples of three attapulgite were characterized by using SEM, XRD and FTIR in detail. The effect on sorption of Th(IV) on three attapulgites was investigated by using the batch technique as a function of contact time, pH, solid content, ionic strength and temperature. The results showed that sorption processes were influenced by time and could be described accurately by the pseudo-second order rate model. The sorption processes were strongly affected by pH, solid-to-liquid ratio (m/V). The presence of phosphate and FA only caused little effect on the sorption processes. Compared with attapulgite, heat ACT-attapulgite and ACT-attapulgite, the adsorption capacity of ACT-attapulgite and heat-attapulgite to Th(Ⅳ) was improved significantly. The Langmuir isotherm model fit the sorption data better Freundlich than the isotherm model. The thermodynamic parameters of three attapulgite were calculated from the temperature dependence, and the results suggested that the adsorption reaction was spontaneous and endothermic with a high affinity. The adsorption mechanism interpreted from the batch results and indicated that surface complexation model at higher pH and cation competition exchange at lower pH were considered as the main mechanism.In the eighth chapter, the results were summarized and further study proposals were put forward.The results indicated that the sorption capacity of UO22+ and Th4+ on different sorbents was different. The sorption mechanisms of UO22+ and Th4+ on different sorbents were studied by using the batch and column techniques and we have understood the sorbents, especially the sorption mechanisms in the presence of humic acid and phosphate. In order to understand sorption mechanism of radionuclide, the batch adsorption experiment, column experiment and combining with micro-characterizing (micro-characterizing (such as Fourier Transform infrared sorption (FTIR), X-ray photoelectron spectroscopy(XPS), and XRD so on) were used to study the adsorption. At present, I have studied the sorption of radionuclide on oxides and 1 am going to study the sorption of radionuclide and heavy metal ions on the clay minerals.