Study On The Selective Adsorption Behavior Of Capacitive Deionization In High-Salt Radioactive Wastewater And Its Application

The Fukushima Nuclear Accident（FNA）in 2011 caused leakage of a large amount of radioactive nuclide,mixing with groundwater and seawater around the environment,which aroused widespread public concern and worries about how to safely treat and dispose of these radioactive fluids.This thesis focuses on applying a highly efficient,clean and low-cost water treatment technology,capacitive deionization（CDI）,to the treatment of radioactive waste and exploring various electrode materials for the selective separation of nuclides in radioactive waste liquids containing high concentrations of salt ions.The specific research contents and results are as follows:1.SPAC,a porous activated carbon with-SO3-functional group modification,was synthesized by treating chestnut shell fiber with carbonization,pore-structuring,and surface modification.SPAC electrode exhibited high electroadsorption capacity(33.11 mg g^-1)and high adsorption rate(2.89 mg g^-1 min^-1)for Sr（Ⅱ）in CDI.The maximum Sr（Ⅱ）adsorption capacity of 58.21 mg g^-1 was measured in the isothermal adsorption experiment in a solution with an initial Sr²⁺ concentration of 150 mg L^-1,which was in accordance with the Langmuir model（R2=0.991）.The SPAC electrode exhibited a high Sr（Ⅱ）adsorption capacity in a mixed Na⁺-Sr²⁺ solution（molar ratio Na（Ⅰ）:Sr（Ⅱ）=20:1）with a selectivity of 70.65.Combined with DFT simulations,electrochemical analysis and spectroscopic analysis,the selective mechanism of ion exchange during the electroadsorption of Sr²⁺by SPAC was further demonstrated.2.Carbon nanotubes interspersed with potassium zinc ferrocyanide（KZnFC-CNT）synthesized by co-precipitation method were used as CDI electrode for selective separation of Cs（Ⅰ）after electrochemical treatment.The activated KZnFC-CNT electrode showed excellent removal ability for Cs（Ⅰ）,and the kinetic fitting results showed that the adsorption capacity for Cs（Ⅰ）could reach up to 420.17 mg g^-1.The batch experiments revealed that the maximal selectivity of 172.5 was achieved at the initial molar concentration ratio of Na（Ⅰ）:Cs（Ⅰ）of 1:1 and the applied voltage of 1.0 V.Mechanism-investigation experiments showed that KZnFC after electrochemical treatment will form K（Ⅰ）vacancies inside its crystal,and both Cs（Ⅰ）and Na（Ⅰ）can occupy this site during the electrosorption.After undergoing a crystallographic transformation,Cs（Ⅰ）will be locked inside the crystal lattice and achieving the selective separation of Cs（Ⅰ）.3.To achieve the safe disposal of high-salt waste liquid after the removal of radionuclides,olivine phase O-FePO₄@C was successfully synthesized by electrochemical de-lithiation of LiFePO₄@C.O-FePO₄@C as a cathode exhibited excellent Na（Ⅰ）removal capacity(adsorption capacity:61.3 mg g^-1;maximum adsorption rate:3.4 mg g^-1 min^-1)and cycling stability.According to the electrochemical and XRD results,Na⁺and Mg²⁺ions can be embedded in the lattice of O-FePO₄.According to the DFT simulation results,the tight hydrated layer of Mg²⁺ions and the strong repulsive force with lattice ions severely inhibit the embedding process of Mg²⁺ions.Therefore,O-FePO₄@C has good selective electrosorption behavior for Na（Ⅰ）,and the order of selective adsorption of ions is as follows:Na⁺>Mg²⁺>Ca²⁺,which provides a new idea for the application of treated high-salt waste streams in agricultural irrigation.