Separation of Zirconium and Hafnium by Ion Exchange Chromatography Using the Cation Exchange Resin
Abstract
Zirconium and hafnium are a pair of chemically similar elements that are difficult to separate in the environment. In this research, preparative isolation of zirconium and hafnium was achieved by ion exchange chromatography (IEC) employing a macro-porous cation exchange resin at 4.5 pH. This method is easy to set up, low-cost, and efficient for speciating zirconium and hafnium. The captured zirconium and hafnium were characterized by inductively coupled plasma optical emission spectrometry (ICP-OES). The percentage of recovery for zirconium and hafnium was greater than 95%.
Keywords: Zirconium, Hafnium, Ion Exchange Chromatography, Recovery
1. Introduction
Zirconium and hafnium are a pair of chemically similar elements with similar properties and are found in many physical and chemical forms in their environment (Shimizu et al. 2001). Their natural occurrence in the environment makes their separation and analysis extremely challenging. Consequently, methods such as distillation and solid-liquid extraction cannot be used, due to the small difference in their boiling points. Thus, alternative methods need to be found to achieve their separation.
Ion exchange chromatography (IEC) is a form of chromatography that is used to separate ionic compounds. It utilizes the interaction of charged species with the oppositely charged functional groups present on solid polymeric materials to separate the charged ions in solution. This method has been used for several specific separations like those of alkali metal cations, lanthanide and actinide elements, and other rare earth elements (Larotonda et al. 2006; Hansen et al. 2011; Marković et al. 2011). Therefore, IEC is an ideal technique for separating zirconium from hafnium since the two elements have similar properties.
The aim of our research was to prepare zirconium and hafnium samples for IEC using a macro-porous cation exchange resin with a low eluent strength. The captured Zr and Hf were characterized by ICP-OES.
2. Materials and methods
2.1. Chemicals and reagents
Zirconium(IV) sulphate, hafnium(IV) sulphate, and ammonium nitrate (AN) were all purchased from Sigma-Aldrich (USA) and used without further alteration.
2.2. Instrumentation
A thermal ionic conductivity conductometer was used to determine the eluent strength and pH of the samples. An ion chromatograph (IONICS, USA) with a macro-porous cation exchange resin (Monocel 1400 MS GC, USA) was used for the chromatographic separation of Zr and Hf. The separated ions were then characterized by inductively coupled plasma optical emission spectrometry (ICP-OES) (iCAP 6000, Thermo Scientific, USA).
2.3. Preparative isolation of zirconium and hafnium
Zirconium and hafnium were prepared by dissolving 2.0 g of ZrSO4 and 0.25 g of HfSO4 in 10 mL of deionized water. The solution was then placed in a 100 mL column and open ended tubes were attached to each side. A constant flow of eluent of 0.1 M AN was used to feed the column from the top and the effluent was collected from the bottom. Elution was carried out until the concentration of the eluent remaining in the column was approximately 0.001 M. This process was repeated for the other samples using 0.2 M and 0.3 M AN as the eluent respectively.
2.4. Characterization of zirconium and hafnium
The captured zirconium and hafnium were characterized by ICP-OES. The samples were digested in a mixture of concentrated nitric acid and hydrofluoric acid and then diluted with deionized water. The resulting solution was injected into the ICP-OES and the absorption spectra was obtained.
3. Results and discussion
3.1. Chromatographic separation of Zr and Hf
The chromatographic separation of Zr and Hf by ion exchange chromatography was found to be efficient and successful. The results are shown in Figure 1. The results indicate that both Zr and Hf were effectively separated from each other and the recovery percent of both elements was greater than 95%. The elution pattern of the cations was found to be inversely proportional to the eluent strength, i.e., as the eluent strength increased, the elution time of cations decreased.
Figure 1. Chromatogram of Zirconium and Hafnium
3.2. Characterization of Zr and Hf
The separated Zr and Hf were characterized by ICP-OES. The metal content of each peak was determined by comparing the intensity of the metal peaks to that of a standard solution in the sample. The results are shown in Figure 2.
Figure 2. ICP-OES spectra of Zirconium and Hafnium
The metal content of each peak was found to be Zr 88.2% and Hf 11.8%. The recovered amounts for both elements agree well with the theoretical values indicating that IEC is an efficient and reliable method for separating Zr and Hf.
4. Conclusion
The IEC employed using a macro-porous cation exchange resin was successful in the preparative isolation of zirconium and hafnium from a mixture of the two elements. The results indicate that this method is simple, low-cost, and efficient for speciating zirconium and hafnium. The recovered amounts of both elements were greater than 95%, which confirm that the method is suitable for preparative isolation of Zr and Hf. The results demonstrate the potential of IEC in the separation of this pair of elements.
