Adsorption Of Strontium Ion By Nanostructured Transition Metal Compounds Under Electric Field Coupling

With industrialisation of society,human need for energy has increased and the massive use of fossil fuels has caused global climate change.Nuclear energy is widely exploited because it does not produce greenhouse gases（carbon dioxide）during its application.However,nuclear waste,nuclear power plant operation processes and nuclear accidents all produce radioactive wastewater,in which the radionuclide ⁹⁰Sr has a long half-life,is highly radioactive,exists as a cation in aqueous solutions,is easily transferred in the food chain and is highly hazardous to the environment and organisms.Therefore,it is urgent to treat ⁹⁰Sr in radioactive wastewater.Adsorption has become a common method due to its simple operation and equipment,but it is difficult to separate the adsorbent from the contaminated liquid after adsorption,and the process of regenerating the adsorbent requires the use of chemicals,which can easily bring secondary contamination.Electrosorption is a method combining adsorption with an applied electric field,which has the advantages of high treatment efficiency,easy desorption and regeneration of the adsorbent and without secondary pollution.It has broad application prospect in the treatment of wastewater containing strontium.Transition metal compounds sodium titanate,sodium vanadate and zirconium phosphate all have octahedral crystal structures and free ions or moieties.Of these,sodium titanate has a Ti O₆ octahedral crystal form,sodium vanadate has a VO₆ octahedral crystal form and both have interlayer-filled Na⁺;zirconium phosphate has a Zr O₆ octahedron and free hydroxyl groups.Based on the characteristics that the free ions in the crystal structure of transition metal compounds are easily exchanged with Sr²⁺,this thesis uses three nanostructured transition metal compounds as ion exchangers and investigates their adsorption properties on Sr²⁺under the action of electric field.The main studies are as follows.（1）Sodium titanate/carbon nanotubes（NaTNS/CNTs）materials are prepared by hydrothermal method and the effect of voltage on adsorption is investigated.The adsorption amount is 91.27 mg g^-1 at 1.2 V,which is approximately three times higher than that without electric field adsorption(32.03 mg g^-1),and the adsorption equilibrium time is also greatly reduced.The experiments show that the NaTNS/CNTs adsorption material can effectively improve adsorption effect by electric field coupling.Multiple cycles of adsorption and desorption experiments demonstrated the reusability and stability of the adsorbent.The kinetics,isothermal model and thermodynamics of the adsorption process are investigated by varying the conditions of adsorption time,initial concentration of solution and reaction temperature.（2）Sodium vanadate（NVO）nanomaterial is prepared by hydrothermal method combined with calcination,and its adsorption performance to positively charged Sr²⁺in solution is investigated by adjusting the applied voltage.Under the voltage of 1.2 V,the adsorption capacity of Sr²⁺is 89.34mg g^-1 when adsorbed for 120 min.At the same time,the adsorption capacity is 45.92 mg g^-1 without applied voltage,indicating that applied electric field greatly improved the adsorption performance of NVO nanomaterials.Theoretical adsorption amount obtained by fitting adsorption kinetics and adsorption isotherm model is similar to the experimental value.Thermodynamics reflects that the adsorption is endothermic.（3）Zirconium phosphate/graphene oxide（Zr P/GO）nanosheets are synthesised by a one-step hydrothermal method.By comparing their adsorption performance on Sr²⁺with and without an electric field(q_{1.2 V}=45.28 mg g^-1,q_{0 V}=22.25 mg g^-1),it is shown that the adsorption effect of Zr P/GO adsorbent could be effectively improved under the coupling effect of electric field.The conditions of adsorption time,initial concentration and reaction temperature are varied to explore the adsorption kinetics,isotherms and thermodynamics.Adsorbent material is subjected to cyclic experiments of adsorption and desorption and still maintains 88%of the first adsorption capacity,demonstrating the excellent stability of the adsorbent.Cyclic voltammetry test confirmed that NaTNS/CNTs,NVO and Zr P/GO are double-layer capacitors.Specific capacitors reflecting the electroadsorption properties are calculated by the specific capacitance formula.This thesis combines material adsorption with bilayer capacitive adsorption,where electroadsorption moves charged ions in solution towards the electrode with the opposite voltage and stores the charged ions in the bilayer.The ions in the crystal structure of the electrode material are adsorbed with Sr²⁺by ion exchange,which facilitates the rate of adsorption and enhances the adsorption of Sr²⁺as the concentration of Sr²⁺enriched in the bilayer is much higher than the concentration in solution.