Use Of The ESIX For The Separation Of Transition Metal Ions Ni²⁺, Y³⁺ From Aqueous Solution

Heavy metal pollution has become a serious threat to the environment and human health worldwide, on the other hand, these heavy metal ions, including precious metals and rare earth ions belong a serious of important strategic resources, separation and recovery of heavy metal ions is significant both in the environment and resources. However, conventional processing methods such as chemical precipitation, adsorption, electrolysis and ion exchange purification are difficult to meet the technical and economic requirements, especially when dealing with low concentrations of toxic, harmful heavy metal ions from wastewater due to high cost, low processing efficiency and secondary pollution, so the new, highly efficient, low-cost, environment-friendly heavy metal ions separation and recovery technology has always been of great importance. Electrically controlled ion exchange (ESIX) technology in recent years, coupled with electrochemical ion exchange to develop a new type of environment-friendly ion separation technology. As the main driving force of the process is the electrode potential, the ion exchange matrix without chemical regeneration to eliminate the secondary pollution generated by chemical regenerant, which is likely to replace the traditional ion exchange technology and draws the attention of scholars at home and abroad. In this paper, NiHCF films and Ni2+ imprinted PPy/FCN composite films were synthesized as ESIX film electrodes. On the basis of their properties study, Electro-remove and recover rare earth and heavy metal ions in aqueous solutions by ESIX technology, which is a beneficial trial in wastewater treatment.Electroactive nickel hexacyanoferrate (NiHCF) thin films were synthesized by cathodic deposition and investigated as electrochemically switched ion exchange (ESIX) materials for the separation of Y3+ from aqueous solutions. In 0.1 mol·L-1 Y(NO3)3 solution, cyclic voltammetry (CV) combined with electrochemical quartz crystal microbalance (EQCM) technique was used to investigate the electroactivity, reversibility of the film electrodes and the mechanism of ion exchange. The electrochemical behavior of NiHCF film electrodes was also compared with that in Sr(NO3)2 solutions. The ion selectivity of the film was investigated in 0.1 mol·L-1 mixture solution containing [Y(NO3)3 +Sr(NO3)2], respectively. The elementary composition of NiHCF films in reduced and oxidized form were also characterized by X-ray photoelectron spectroscopy (XPS). Experimental results show that the electroactive NiHCF films have reversible electrochemical behavior in aqueous solutions containing Y3+ and Sr2-, respectively. NiHCF film electrodes display a high Y3+ selectivity in Y+3/Sr+ binary mixtures and the Y3+can be separated effectively from aqueous solutions by ESIX processes.In this study, a novel ion imprinted hexacyanoferrate-doped polypyrrole (PPy/FCN) composit film was fabricated by unipolar pulse electro-polymerization (UPEP) methods. The transition metal ion Ni2+ was selected as the imprinting ion, which can be in-situ removed by applying potential pulses during polymerization process. The morphology, structure and elemental composition of the composite film were characterized using scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) and energy dispersive X-ray spectroscopy (EDS), respectively. Electrochemical quartz crystal microbalance (EQCM) combined with cyclic voltammetry (CV) and potential steps (chronocoulometry) techniques was used to investigate the ion exchange behavior of PPy/FCN films in electrolytes containing K+, Ni2+ and Y3+ metal ions. The experimental results indicated that the loading and unloading of cations could be instantly controlled by modulating the potential of Ni2+ imprinted PPy/FCN films. By regulating the redox states of the ion-imprinted composite film, the synergetic effect of PPy and hexacyanoferrate will result in the fast uptake and elution of metal ions, especially for Y3+ ions. This novel composite film can be taken as a promising electrochemically switched ion exchange (ESIX) material for separation of heavy metal ions or rare earth ions.