Adsorption Of Np(Ⅴ), Am(Ⅲ) And Cs(Ⅰ) On Na-kaolinite And Na-illite

A quantitative understanding of key radionuclides’ sorption behavior is of vital importance for reliable long-term safety assessment of proposed high-level nuclear waste（HLW）geological repository.The present study is focused on the sorption of three key radionuclides（237Np,241 Am and 137Cs）,with two domestic clay minerals,namely Maoming kaolinite（Guangdong province）and Zhangzhou illite（Fujian province）as adsorbent substrate.The effect of contact time,solid-to-liquid ratio,p H,ambient CO₂,humic acid（HA）and initial metal cation concentration on the sorption behavior is investigated through batch sorption experiments under different experimental conditions.Modeling and analysis have been conducted for partial adsorption data.The results obtained are briefly given below.（1）Na-kaolinite and Na-illite were characterized with SEM,N2–BET,XRD,XRF and FT–IR in detail.The surface properties of Na-kaolinite and Na-illite were evaluated by continuous potentiometric titration.The comparison with titration curves of KGa-1 kaolinite,Zettlitz kaolinite and Puy-en-Velay illite samples indicated that Maoming kaolinite and Zhangzhou illite have a different surface acid-base chemistry from these samples.（2）In the absence of ambient CO₂,Np（V）adsorption on Na-kaolinite and Na-illite was strongly influenced by p H,the Np（V）uptake increasing continuously with increasing p H.A relative constant Kd value was determined in the range of the soild-to-liquid ratio from 4 to 20 g/L.No dependence on ionic strength in the range of 0.01 – 0.1 M Na Cl of Np（V）adsorption was observed,indicating the formation of inner-sphere surface complexes of Np（V）with Na-kaolinite and Na-illite.When the initial Np（V）concentration was in the range of 8.49 × 10-11 to 5.50 × 10-7 M,the sorption isotherm of Np（V）on Na-kaolinite and Na-illite could be well fitted with a Freundlich-type isotherm equation.In general,Na-kaolinite showed stronger Np（V）sorption ability than Na-illite.In the presence of ambient CO₂ or HA,Np（V）adsorption was further increased in low p H range but decreased in higher p H range.（3）In the absence of ambient CO₂,the adsorption behaviors of Am（Ⅲ）at trace level（6.02 × 10-10 M）on Na-kaolinite and Na-illite were very similar.Am（Ⅲ）adsorption was rapid and the corresponding kinetics agrees with the pseudo-second order rate equation.A relative constant log Kd value was observed for a soild-to-liquid ratio between 1 and 10 g/L.The adsorption of Am（Ⅲ）was strongly p H dependent,and increased with increasing p H with peak adsorption occurring between p H values of 5 and 6.5.Under acid condition（p H = 4.0）,Am（Ⅲ）adsorption decreased sharply with increasing ionic strength,indicating that Am（Ⅲ）adsorption is controlled by cation exchange reaction.At near neutral p H（p H = 6.60）,the sorption was almost independent of ionic strength due to the formation of inner-sphere surface complexation.The presence of HA somewhat enhanced Am（Ⅲ）adsorption at low p H,whereas it drastically decreased Am（Ⅲ）adsorption at higher p H.Under the conditions from acid p H to near neutral p H,the sorption isotherm of Am（Ⅲ）on Na-kaolinite and Na-illite could be well fitted to a Freundlich-type isotherm equation in the concentration range of 5.38 × 10-11 to 5.44 × 10-9 M.（4）In the absence of ambient CO₂,the adsorption of Cs at trace level（2.00 × 10-8 M）on Na-kaolinite and Na-illite were relatively quick,and the equilibrium was reached in about 7 days.Cesium log Kd values on Na-kaolinite had little to do with p H change,but log Kd values on Na-illite were somewhat enhanced with increasing p H.The adsorption of trace Cs（Ⅰ）（[Cs（Ⅰ）]0 < 10-8 M）on Na-kaolinite and Na-illite were almost independent of the initial Cs（Ⅰ）concentrations.However,the log Kd values of Cs（Ⅰ）at higher concentrations decreased gradually with increasing initial Cs（Ⅰ）concentrations.In contrast to Na-kaolinite,Na-illite exhibited stronger sorption ability for low concentration Cs（Ⅰ）（[Cs（Ⅰ）]0 < 1.98 × 10-5 M）,but sorption ability for higher concentration Cs（Ⅰ）decreased relatively.The sorption isotherms of Cs（Ⅰ）on both caly minerals（especially Na-illite）were nonlinear on the whole,indicating that there was more than one kind of adsorption sites in the minerals.Therefore,in order to get a better data fitting,a multi-site cation exchange model might be employed.（5）In order to deal with the interference of 233 Pa,a short-lived β active daughter nuclide of 237 Np,to the liquid scintillation counting（LSC）measurement of 237 Np,a method for the activity determination of 237 Np with a Wallac 1414-003 Guardian liquid scintillation counter based on pulse shape analysis（PSA）function,getting rid of chemical separation or sample processing has been developed using.Experimental results indicated that the optimum PSA level for α/β events discrimination could also be well determined by 237Np/233 Pa mixed solutions no matter there was radioactive secular equilibrium or not.At PSA level of 38,the activity measurement results of 237 Np by LSC were in good accordance with those obtained with 2πα grid ionization chamber.This method could be used not only to satisfy the efficient 237 Np relative measurement requirements of batch sorption experiments,but also to determine the activity of 237 Np and 233 Pa in solution simultaneously.（6）2SPNE SC/CE model,developed for Puy-en-Velay illite（France,Na form）,cannot interpret the titration or Np（V）adsorption data of Zhangzhou Na-illite.A new adsorption model was constructed with the framework of 2SPNE SC/CE model to interpret the data.Comparing with 2SPNE SC/CE model,the site capacity of this model is much lower,and the corresponding constants of protolysis and complexation reactions are quite different as well.The reason for these differences may be:（1）the specific surface area（SSA）of Zhangzhou illite is lower;（2）the ratio of isomorphous substitution（e.g.content of Fe2O3 and Mg O）in Puy-en-Velay illite is higher than that of Zhangzhou illite.（7）The potentiometric titration curves of Na-kaolinite could be interpreted well with both the double layer model（DLM）and a non-electrostatic model（NEM）.However,Np（V）adsorption on Na-kaolinite was quantitatively better interpreted with a NEM considering only surface complexes（≡SWONp O₂,≡SWONp O₂ OH and ≡SSONp O₂）compared with DLM.In addition to three surface complexes（≡SSOAm2+,≡SSOAm OH+ and ≡SSOAm（OH）2）,one cation exchange species（≡X3Am）had to be included in NEM to quantitatively interpret the Am（Ⅲ）adsorption data.