Adsorption Of Eu(Ⅲ) On Bentonite And Montmorillonite

Bentonite has been selected as back-filling material for high level nuclear waste repository.Thus the interaction between key radionuclides and betonite as well as its main component montmorillonite is crucial for both design and safty assessment of the repository.A synthetic montmorillonite and Jinchuan bentonite were used in this dissertation.The objectives of the present dissertation were to(1) collect Eu(â…¢) adsorption data on Na-,K- and Ca-bentonite over wide experimental conditions,(2) to set up a proper surface complexation model capable of quatitative description of the adsorption data collected and(3) to compare the adsorption of Eu(â…¢) on dispersed and compacted synthetic Ca-montmorillonite.The Na-bentonite after removing carnonates was characterized in details by analytical techniques such as X-ray diffection(XRD),X-ray fluorescence analysis, infrared spectroscopy(IR),scanning electron microscopy(SEM),partical size analysis and specific surface area analysis.It was found that the purified bentonite can be assumed to be a mixture of montmorillonite(88%) and silica(12%).The calculated structural formula of the montmorillonite is[Si(â…£)₄](Al(â…¢)_3.19,Fe(â…¢)_0.29, Fe(â…¡)_0.38,Mg(â…¡)_0.14)O₂₀(OH)₄(Ca(â…¡)_0.023,Na(â… )_0.35,K(â… )_0.127).The cation exchange capacity(CEC) calculated from this formula is 694 meq/kg,which implies that the CEC of the purified Na-bentonite should be 694×0.88=611 meq/kg.The CEC determined by Ni-en(ethylenediamine) extraction method was found to be 623±12 meq/kg.The results by continuous potentiometric titrations at three ionic strengths (0.01,0.1 and 0.5 mol/L NaCl) at 25℃indicate that The titration curves are parallel and have no common intersection point.The point of zero charge(PZC) of Na-bentonite increases with deceasing ionic strength.The experimental titration curves were successfully interpreted by a double layer model,which involves three sites,i.e.,a layer site and two edge sites.The adsorption of Eu(â…¢) on the Na-,K- and Ca-bentonite was investigated as functions of pH Eu(â…¢) concentration,ionic strength and CO₂ partial pressure by a batch experimental method.It was found that Eu(â…¢) adsorption is significanetly affected by both pH and Eu(â…¢) concentration.The distribution coefficient(K_d) decreases significantly with increasing ionic strength in low pH range(pH＜6), whereas in high pH range(6-9) ionic strength has much less influence on K_d.The desorption results indicated that the adsorption of Eu(â…¢) on Na-bentonite is reversible.In addition,the K_d values of Eu(â…¢) at P_CO2=10^-3.58 atm and those at P_CO2=0 atm are almost identical in low pH range(pH＜7),in contrast the K_d values of Eu(â…¢) decreases rapidly at P_CO2=10^-3.58 atm at pH＞8.The sequence of Eu(â…¢) adsorption on Jinchuan bentonite with different exchangeable cations under same experimental conditions are:Na-bentonite＞K-bentonite＞Ca-bentonite.The adsorption data of Eu(â…¢) on Jinchuan Na-bentonite at ionic strength 0.1 mol/L NaCl were reasonably interpreted by a surface complexation model.Eu(â…¢) adsorption on compacted synthetic Ca-montmorillonite was investigated as functions of Eu(â…¢) concentration and pH by a batch capillary method.It was found that the K_d values in compacted state and those in dispersed state are identical in low pH range,whereas the K_d values in compacted state is lower than those in dispersed state in high pH range.The results of desorption indicated that adsorption of Eu(â…¢) on synthetic montmorillonite in compacted state is reversible.