| The safe disposal for high-level radioactive waste（HLW）is one of the most significant issues that directly affects the sustainable development of nuclear energy in China.Deep geological disposal is currently recognized as the safest and most reliable way of disposal of HLW in China and many countries.Beishan（Gansu province,China）is preliminarily selected as a site to build a HLW repository in China,and its underground bedrock is granite.The first underground research laboratory is constructing at Beishan site.Granite is an igneous rock composed of minerals such as mica,feldspar,chlorite and quartz.Thus,the study on the sorption characteristics of granite components and minerals on radionuclides is helpful for comprehensive understanding the migration behavior of key radionuclides in granite media,which is of great significance for engineering design and safety assessment of a HLW repository.In this work,the sorption behaviors of U（Ⅵ）,Cs（Ⅰ）and Sr（Ⅱ）on biotite and muscovite were studied by batch method.The influences of p H,ionic strength,radionuclide concentration,fulvic acid（FA）and CO2 partial pressure were investigated.XPS technology was used to explore the sorption differences of radionuclides on different types of mica minerals.The results mainly included the following aspects:（i）A combination of characterization methods including XRD,XRF,FT-ⅠR,SEM,N2-BET,and potentiometric titration showed that the contents of Fe and Mg,specific surface area,CEC value（Cation Exchange Capacity）,and surface hydroxyl site capacity in biotite were significantly higher than those in muscovite.Therefore,biotite has stronger sorption capacity than muscovite.（ii）The sorption of U（Ⅵ）on both biotite and muscovite are significantly affected by p H but independent of ionic strength,indicating that the surface complexation reaction mainly controlled the sorption of U（Ⅵ）on mica minerals.The structure Fe（Ⅱ）in biotite has the ability to reduce U（Ⅵ）to U（ⅠV）in acidic p H range.XPS analysis showed that the reduction ability of Fe（Ⅱ）in biotite decreased with the increase of p H,and the ratio of Fe（Ⅱ）/Fe（ⅡⅠ）on biotite surface increased with the increase of p H,while the ratio of U（ⅠV）/U（Ⅵ）showed an opposite trend.The complexation reaction between CO32-and U（Ⅵ）in solution competes with the sorption reaction of U（Ⅵ）on mica minerals,resulting in an obvious decrease U（Ⅵ）sorption when p H>7.5.The effect of FA on U（Ⅵ）sorption was p H-dependent.It promotes U（Ⅵ）sorption at low p H but inhibits U（Ⅵ）sorption at high p H.（iii）The sorption of Cs（Ⅰ）on both biotite and muscovite is greatly affected by ion strength,and the sorption mechanism is cation exchange reaction.The sorption of Sr（Ⅱ）on mica minerals was mainly dominated by ion exchange reaction in low p H range,whereas surface complexation reaction occurred at higher p H.The sorption isotherm data of Cs（Ⅰ）on mica can be fitted by Freundlich sorption model,while the sorption data of Sr（Ⅱ）is more consistent with Langmuir sorption model.The competitive sorption experiments of Cs（Ⅰ）and Sr（Ⅱ）showed that the sorption affinity of mica minerals for Sr（Ⅱ）is stronger than that of Cs（Ⅰ）.The presence of Sr（Ⅱ）significant decreased the sorption percentage of Cs（Ⅰ）,while Cs（Ⅰ）has no obvious influence on the sorption of Sr（Ⅱ）in the concentration range of 0.5-5.0 mg/L. |
