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Microscopic Mechanism Of Interaction Between Radioactive Cesium And Micas

Posted on:2019-07-23Degree:DoctorType:Dissertation
Country:ChinaCandidate:H Y WuFull Text:PDF
GTID:1311330566964546Subject:Radiochemistry
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Since the last century,several nuclear tests and accidents,and nuclear industrial production had released huge amounts of the radioactive pollution to the environment.It poses worldwide hazards and long-term threats to the eco-environment and human health.Radioactive cesium(RCs)is a major contributor to the radioactivity in polluted area because of its high yield in nuclear fission.It is key to evaluating and predicting radioactive contamination,since its strong mobility.It is well known that micaceous minerals play a vital role in the migration,transformation and distribution of RCs.However,how weathering controls the adsorption mechanism and speciation of RCs to micas is still unclearThe interaction between micas(muscovite,biotite and phlogopite)and RCs were studied using batch methods,microscopic spectroscopy(X-ray absorption fine structure spectroscopy,EXAFS)and modeling calculations(general adsorption mode,GAM)at a molecular level.The adsorption,speciation,bioavailability of RCs and the structural changes of micas were systematically studied.The characterization shows that the response to weathering follows the order of muscovite<biotite<phlogopite,and weathering degree induced follows the order of SCa->Ca->Na-method.The interlayer distance of micas after weathering increased(from 10 A to 15 A).Along with the enhanced weathering degree,cation exchange capacity(CEC),radiocesium interception potential(RIP)increased,also the capacity of FES(frayed edge sites)and IS(interlayer sites).The experiments were conducted in Cs+ concentration of 10-11 5?10-2 5 mol/L,where the dominant sites controlled Cs+ adsorption as the affinity order for Cs+ as FES,IS,TIIS(Type ? sites)and PS(Planar sites).EXAFS results showed that strong weathered micas could trap RCs as stable inner-sphere complexes(ISC)through interlayer collapse(interlayer distance partially or completely reduced to?10 A).On the other hand,fresh micas would adsorb RCs as weak outer-sphere complexes(OSC),resulting in the extremely mobile and migratory RCs species in the environment.The four-site GAM can describe and predict the adsorption behavior of RCs quantitatively,which is applicable to the effects from micas weathering.RCs and Th(?)/Eu(?)present an antagonistic effect on interlayer weathering and ions adsorption when interacting with phlogopite.Th(?)/Eu(?)with the high hydration energy and large ionic radius can accelerate phlogopite weathering.XRD patterns confirmed that the interlayer distance of phlogopite would increase gradually(14?15 A)with the increased Th(?)/Eu(?)concentration and temperature.The relative concentration of Cs+ and Th(?)/Eu(?)will control the interlayer weathering of phlogopite.When[Cs+]>>[Th(?)/Eu(?)],phlogopite interlayer would be collapsed preferentially by Cs+,forming a more stable structure.When[Th(?)/Eu(?)]>>[Cs+],phlogopite interlayer would be expanded preferentially by Th(?)/Eu(?),which cannot be collapsed by Cs+.The major mineral in RCs retardation is biotite in Beishan granite,the host rock for the high-level radioactive waste repository.The results of electron probe micro-analyzer(EPMA)and ?-X-ray fluorescence(?-XRF)affirmed that Cs mainly concentrated in the marginal area of the biotite(FES-rich area).In addition,Cs+ as a form of OSC adsorbed on feldspar group minerals,suggesting that the inhomogeneity of lithology may increase the mobility and environmental risk of RCs.The study of the microscopic mechanism of interaction between RCs and typical mica minerals is helpful for understanding the migration,transformation and distribution behaviour of RCs in the ecological environment,especially taking into account the impacts of geological processes(such as weathering,erosion,transport,deposition)on mica minerals.These findings can also support scientific solutions and assessment for the environmental remediation in radioactively contaminated areas,and for the evaluation of high-level radioactive waste management.
Keywords/Search Tags:Radioactive cesium(RCs), Mica, EXAFS, GAM, Weathering
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