Study On The Potential Application Of Graphene Oxide Membranes In High-level Liquid Waste Treatment

To ensure the clean and sustainable utilization of nuclear energy,it is of great necessity to achieve closed nuclear fuel cycle by spent fuel reprocessing.However,a great quantity of high-level liquid waste?HLLW?has been inevitably generated by the current hydrometallurgical separation process.HLLW is a complex aqueous system with high radioactivity and toxicity,in which a considerable number of alpha nuclides such as plutonium and minor actinides exist.These nuclides must decay over hundreds of thousands of years to achieve harmless level to the ecosphere due to the extremely long half-life time.Appropriate management of HLLW is crucially important in order to avoid its potential hazard.Till now,final geological disposal of HLLW after solidification has been the most mature tactics and adopted widely by many countries.The crucial point of this tactics is to achieve selective separation of some composition from HLLW before solidification to improve the economy and safety of geological disposal.Effective removal of non-radioactive water and nitric acid could greatly reduce the volume of HLLW needed to be solidified and geologically disposed.Extraction of high heat-generating radionuclides cesium-137and strontium-90 could effectively improve the utilization efficiency and safety performance of geological repositories.Currently,the mainstream separation methods were evaporation and chemical separation,which were successively applied to extract water,nitric acid,cesium-137 and strontium-90.The main drawbacks of evaporation technique are its massive energy consumption and poor operational safety.The chemical separation methods usually produce massive secondary waste and need sophisticated technology and equipment.Moreover,due to the non-volatility of ¹³⁷Cs and ⁹⁰Sr,the evaporation and chemical separation need to be used successively,greatly increasing the complexity,technical difficulty and operating cost of HLLW treatment.Under the guidance of cleaner production principle,novel separation methods should be developed for HLLW treatment.As a novel separation technology,graphene oxide membranes?GOMs?separation has gained lots of attention and been widely researched in water treatment field such as desalination of seawater,heavy metal and organic dye removal from industrial sewage.GOMs can exhibit extremely high-water flux owing to the nanoscale capillaries and unique water molecules migration behavior.Effective and precise sieving of molecules and ions can be achieved based on size effect,electrostatic interaction and coordination effect.In addition to owning the merits of conventional membrane separation such as simple equipment,easy operation and low energy consumption,GOMs also have advantages of anomaly high water-flux,good selectivity,etc.However,as a subfield of water treatment,HLLW separation based on GOMs were little researched and rarely reported.Based on above consideration,potential application of GOMs in HLLW separation was studied in this work.Firstly,graphene oxide?GO?with high degree of oxidation was synthesized by improved Hummers method.Homogeneous GO aqueous was obtained by dispersion and exfoliation with the aid of sonication.The characterization showed GO possessed abundant oxygen-containing functional groups,great crystal form and laminated structure.The size of GO sheets was micron order and the height was about 1 nm.The face of GO sheets had certain radian and irregular bulges,which resulted in interspace between stacked GO sheets.Subsequently,GOMs with varying thickness were fabricated by vacuum filtration.The layer by layer structure provided pathways for ions and molecules transmission.Secondly,the stability of GOMs in aqueous solutions was systematically researched.When dry GOMs were immersed in water,d-spacings could be greatly enlarged because the electrostatic repulsion between negatively charged GO sheets could overcome van der Waals attraction.As a result,the hydrated GOMs seemed unstable.However,the ions derived from dissociation of electrolytes in solutions could play a positive role in keeping d-spacings from over-expanding.For monovalent cations,high concentrations were required to guarantee the good stability of GOMs,which could be explained by the theory of electrostatic double layer.In contrast,low concentrations of divalent or trivalent cations could also efficiently crosslink the graphene oxide sheets by coordination effect,thus enhancing the stability of GOMs.Based on these results,to ensure the stability in practical application,a method of modifying original GOMs by 1 mmol/L Eu³⁺was proposed and then verified by sucrose permeation experiments.Thirdly,the permeation behavior of typical ions in HLLW through GOMs was systematically investigated.According to the different permeation rates,ions can be divided into three groups:fast group?hydrogen ion and alkali metal elements?,medium group?alkaline earth metal elements?and slow group?f-block elements?.The nitric acid concentration had an insignificant on the permeation of weak coordination cations.However,as the nitric acid concentration decreased,strong coordination cations were generally restrained from migrating through GOMs due to cations hydrolysis and deprotonation of graphene oxide sheets.In the 0.001 mol/L nitric acid medium,the permeation rates of Cs⁺,Na⁺,Sr²⁺,Eu³⁺,UO₂²⁺and Th⁴⁺?initial concentration of each cation:1mmol/L?through GOMs of 1.6 mg GO/cm² were 16.3,9.2,3.0,0.2,0.2 and 0×10^-7 mol/m²/s respectively.The corresponding SF(Sr²⁺/Eu³⁺)value was 17.0.With the thickness of GOMs increased,the migration rates of weak coordination cations linearly decreased and the migration rates of strong coordination cations remained constant first and then linearly decreased at a turning point.The migration of ions across GOMs was indicated as a two-step process,namely entrance into the membrane surface and intramembrane migration,and the permeation rates of ions depend on the lower step.The values of SF(Sr²⁺/Eu³⁺)did not change much as thickness of GOMs decreased from 2.4 mg GO/cm² to 0.4 mg GO/cm².However,thinner GOMs were preferably employed for greater water flux and processing capacity.Finally,based on the experimental results,a novel technical route for HLLW treatment was proposed.In a GOMs separation unit,H⁺,Cs⁺,Na⁺and Sr²⁺along with H₂O would be successively extracted while the alpha nuclides are basically entrapped in the feed end.After the treatment,HLLW could be divided into large volume of non-alpha liquid waste and much smaller volume of alpha liquid waste.The latter can then be solidified for disposal as the traditional route.Because the disposal requirement for non-alpha liquid waste was not as harsh as alpha liquid waste,the bulk of non-alpha liquid gained could be further treated as low or intermediate level waste.The new route could greatly simplify the treating process and lower the cost.Except for HLLW treatment,GOMs separation would also be applied for processing other alpha liquid waste which originated from uranium or thorium mining and metallurgy,nuclear fuel fabrication,etc.