Investigations Of U(?) And Th(?) Sorption On Activated Bentonite And K-feldspar

One of the critical factors for sustainable development of nuclear energy industry was the way to control and dispose the high-level radioactive waste safely and properly.Beishan area in Gansu Province located in northwestern China,where the surrounding rock is granite,is one of potential candidates for the geological repository.Bentonite is determined for the first choice as buffer and backfill material.Therefore,the investigations on the sorption-desorption behavior of radionuclides on activated bentonite and K-feldspar?one major composition of granite?are valuable,and the findings will provide essential and important parameters for the perfomance and safety evaluations of the geological repository.This thesis focused on the sorption behaviors of U???and Th???on activated bentonite and K-feldspar under various conditions.Batch approach was used to investigate effects of general influence factors such as contact time,pH,background electrolyte,ionic strength,solid-liquid ratio,temperature and humic substances.Kinetic and thermodynamic models were adopted to describe Th???and U???sorption processes and to explore the sorption mechanisms.The results were listed as follows:I.U???sorption as a function of contact time tended to become plateau after720 min,and followed the pseudo-second-order model.The sorption of U???on activated bentonite was significantly influenced by pH and ionic strength,whereas slightly affected by background electrolytes.The results indicated that U???sorption mechanism on activated bentonite was mainly the outer-sphere complexation and/or ion-exchange.The increase of temperature was favorable for U???sorption,which indicated the net sorption procedure was endothermic and spontaneous.Freundlich model could decribe U???sorption on acitivated bentonite well,indicating U???sorption was heterogeneous.The presence of humic acid?HA?could inhibit U???sorption under the observed pH range from 1.0 to 10.0;however,fulvic acid?FA?could enhance U???sorption below pH 4.0.II.The sorption of Th???on activated bentonite followed the pseudo-second-order model.Th???sorption exhibited strong pH-and ionic strength-dependence,while weakly affected by background anion ligands and humic substances.The sorption mechanisms were mainly assigned to ion exchange and surface complexation below pH 4.0,while precipitation of Th?OH?₄ was formed at higher pH ranrge.The increasing temperature was favorable to the sorption of Th???on activated bentonite.III.The kinetic process of Th???sorption on K-feldspar followed the pseudo-second-order model.The increase of temperature could efficiently promote Th???sorption capacity and sorption rate,indicating an endothermic sorption process.The presence of sulfate inhibited Th???sorption obviously,while distinct enhancement was observed for phosphate.In addition,slight influence on Th???removal was noticed in the presence of monovalent cations and anions.Th???sorption was tremendously promoted by organic ligands?HA and FA?.Langmuir model could describe Th???sorption on K-feldspar well,suggesting a monolayer coverage.The sorption mechanism was mainly dominated by the outer-sphere surface complexation.