Adsorption Of U(?),Th(?)and Eu(?)/Am(?) On Beishan Granite And Gaomiaozi Bentonite:Effects Of Temperature And Humic Substances

Adsorption studies of key radionuclides on buffer/back-filling materials and host rock are the basis for engineering design and safety assessment of a high-level radioactive waste(HLW)repository.Aiming at Gaomiaozi bentonite and Beishan granite,host rock and buffer/back-filling material of the most potential HLW repository in China,this dissertation focused on the effects of temperature and humic substances(HSs)on adsorption of U(?),Th(?),Eu(?)/Am(?).Based on macroscopic adsorption data of these radionuclides on Beishan granite and Gaomiaozi bentonite covering a wide range of experimental conditions,surface complexation models(SCMs)were constructed.The result of this dissertation provided a part of basic data and corresponding models necessary for the engineering design and safety assessment of the Beishan HLW repository.The manuscript is divided into seven parts.The chapter 1 described the research background,the open questions and the aims of this study.In the chapter 2,temperature effect on acid-base properties and complexation behaviors of HSs was studied.The following chapters 3-4 presented the adsorption data and corresponding model work for Eu(?)/Am(?)and U(?)on Beishan granite at different temperatures.The chapter 5 described the adsorption behavior of Am(?)on Gaomiaozi bentonite.The adsorption of U(?)and Th(?)on Gaomiaozi was studied in chapter 6.The chapter 7 summarized the general conclusions and perspectives.The obtained results of this dissertation are briefly described below.I.Proton binding to HSs from different sources(two humic acids and two fulvic acids)were investigated by using continuous potentiometric titrations at 0,25 and 40 oC.The complexation of Eu(?)with humic acid(HA)was studied using ion exchange method.It was found that the bindings of proton and Eu(?)to HSs were strongly dependent on pH and ionic strength.The absolute value of the charge density(?Qcharge,mol/Kg)for the proton bindings of HSs increased with increasing temperature.The acid-base properties at different temperatures and Eu(?)complexation of HSs can be quantitatively described by the NICA-Donnan model.The standard enthalpy changes of proton binding for each humic and fluvic acid were evaluated from fitting parameters of NICADonnan model obtained at different temperatures via the van't Hoff equation,and can be used to predict the acid-base chemistry at other temperatures.II.Effects of background electrolyte,CO2,temperature and other factors on Eu(?)/Am(?)and U(?)adsorption on Beishan granite were studied using batch-type experiments.X-ray photoelectron spectroscopy(XPS)characterization was used to constrain the parameters of the generalized composite(GC)model.Electron probe micro analyses(EPMA)indicated major materials that controlled Eu(?)adsorption on granite.The GC models were applied to quantitatively interpret the adsorption data and was used to predict the literature data of U(?)on granite originated from different areas.A GC model,based on XPS analysis,can quantitatively interpret Eu(?)and Am(?)adsorption in both NaCl and CaCl2 solution in the absence/presence of CO2.Temperature change in the range of 25?80 oC does not affect Eu(?)adsorption on Beishan granite.The presence of fulvic acid(FA)of 2?20 mg/L significantly inhibited the Eu(?)and Am(?)adsorption on Beishan granite.EPMA indicated that biotite has higher affinity for Eu(?)than other major minerals,oligoclase,quartz and orthoclase.U(?)adsorption on granite is endothermic.U(?)adsorption data at different temperatures could be described by a GC model with three surface complexes,?SOUO2+,?SO(UO2)2(OH)2+ and ?SO(UO2)3(OH)5.The enthalpy changes(?H)of the surface reactions were evaluated from the equilibrium constants obtained at three temperatures via the van't Hoff equation.Blind modeling predictions were performed by using the proposed model and ?H,and a satisfactory agreement with the literature data was obtained.?.The adsorption of Am(?)on Gaomiaozi bentonite was studied as a function of pH,Am(?)concentration,ionic strength,temperature and the composition of background electrolytes by using batch-type experiments.The distribution coefficient(Kd,mL/g)of Am(?)on Gaomiaozi bentonite was dependent on pH,ionic strength and Am(?)concentration.It was found that temperature has a negligible effect on Am(?)adsorptionat 25?80 oC.Under possible disposal conditions at pH~8.0,bentonite exhibited a strong affinity to Am(?)with trace concentrations,and corresponding Kd could reach the magnitude of 106 mg/L.Am(?)adsorption on bentonite could be quantitatively interpreted by a non-electrostatic model with a cation exchange reaction and two surface complexation reactions.?.The acid-base chemistry of Gaomiaozi Na-bentonite(Na?GMZ BNT)was studied by using continuous potentiometric titrations at temperature of 25,35 and 45 oC.The adsorption of U(?)and Th(?)on Na?GMZ BNT in the presence of humic acid(HA)was studied at different temperatures.It was found that the temperature does not affect the acid-base chemistry of Na?GMZ BNT as well as the HA adsorption on Na?GMZ BNT.However,adsorption of U(?)and Th(?)increases with the increasing temperature.In the ternary adsorption system,the presence of HA enhances U(?)and Th(?)adsorption on Na-GMZ BNT at low pH,whereas it inhibits at high pH.In the presence of HA,U(?)and Th(?)adsorption on Na?GMZ BNT increases with the temperature increase.