Continuous Separation Of Cesium Based On NiHCF/PTCF Electrode By Electrochemically Switched Ion Exchange

Electrochemically switched ion exchange (ESIX) is a separation technology as an alternative to conventional ion exchange for removing ions from wastewater. In ESIX, ion loading and unloading can be easily controlled by modulating the redox states of ion exchange thin films formed on conductive substrates to separate ions from mixed solutions and regenerate the matrix.Nickel hexacyanoferrate (NiHCF), an inorganic coordination compound with an open zeolite-like structure, is an excellent candidate for ESIX of alkali metal cations, especially for the ESIX of cesium because of its especial affinity for cesium (Cs+>Rb+>K+>Na+>Li+). Conductive substrate with large surface area enhances obviously the cation-exchange capacity of thin-film NiHCF electrodes. The utilization of porous three-dimensional carbon felt (PTCF) as an ESIX substrate is an effective way to synthesize NiHCF electrodes with a higher cation-exchange capacity due to its high area/volume ratio, high porosity, good conductivity, chemical and electrochemical stability, and excellent machinability.In the present study, electroactive films of NiHCF were synthesized on PTCF substrate using chemical deposition. The composite films were characterized by scanning electron microscopy (SEM). In two-electrode system, the cesium ions adsorption and regeneration performance of NiHCF film was investigated by ion chromatography. The adsorption of cesium ions as a function of contact time, applied voltage, initial concentration of cesium was also investigated. Ion separation ability of NiHCF films was used to develop an electrochemically switched ion exchanger for cesium separation. A cell arrangement containing two ion exchanger electrodes was simply developed to provide continuous operation of the cesium separation process. The results showed that NiHCF/PTCF electrode possessed an excellent ESIX property and applied potential played an important role in ESIX process. The adsorption ratio of cesium was95%and the regeneration ratio was about70%when the NiHCF/PTCF electrode was applied an8V potential and the initial concentration of cesium was9mg/1.Continuous operation:NiHCF/PTCF electrode was used as ESIX electrode in a packed bed for continuous separation for cesium ions. The effects of solution concentration on the ion-exchange capacity of the electrodes were investigated by CVs. Cycling stability and long-term storage stability of NiHCF/PTCF electrodes in water was also studied. The NiHCF/PTCF electrodes with excellent ion-exchange ability were used to assemble a diaphragm-isolated ESIX reactor for cesium separation. Continuous separation of cesium and regeneration of NiHCF/PTCF electrode based on the diaphragm-isolated reactor were performed in a laboratory-scale two-electrode system. The rusults showed that the ion exchange capacity of NiHCF/PTCF electrodes was approximately160times higher than that of NiHCF films prepared on nickel foam electrodes with the same volume. The results in the diaphragm-isolated ESIX reactor with the NiHCF/PTCF electrodes revealed that the cesium concentration of the simulated wastewater almost decreaseed to0after4h absorption, with the absorption efficiency of cesium higher than96%and some cycles100%and the the desorption efficiency of cesium was much lower than the absorption efficiency, about60～70%only. This process was suitable for simultaneous absorption and desorption of cesium in a continuous cesium separation. Adsorption kinetic:Electrochemically switched ion exchange of cesium ion from simulation solution using a diaphragm-isolated reactor with two identical nickel hexacyanoferrate/carbon felt electrodes as working electrodes was reported. Applied potential dependent adsorption and desorption of cesium ion on the electrode and effects of initial concentrations and pH values of the simulation solutions on the adsorption of cesium ion were investigated. The adsorption rate of cesium ion in electrochemically switched ion exchange process was fitted by pseudo-first order rate model. Also, the rate-determining steps of cesium adsorption were analyzed based on the experiment results of cesium adsorption. It was found that desorption of Cs+was impossible to be realized by a single ion exchange process in NiHCF/PTCF electrodes and introduction of applied potential on the electrode greatly enhanced the adsorption/desorption rate of Cs+and increased the separation efficiency. The adsorption rate constants were almost not influenced by the initial concentration of Cs+. H3O+was found to play a dual role of electrolyte and competitor, and the value of adsorption rate constants showed a curve diversification with the increase in pH. The electrochemically switched adsorption process of Cs+by NiHCF/PTCF electrodes was found to mainly proceed in two steps, i.e., an ion diffusion step with a slow diffusion rate and an ESIX step with a fast adsorption rate.