Structure of Zirconium (IV) in Aqueous Zirconium Chloride Solutions as Studied by Zr-K Edge Extended X-ray Absorption Fine Structure Analysis

It is known that zirconium ion exists in an aqueous chloride solution by taking various structures, depending on the concentrations of hydrogen and chloride ions. The structural change of the zirconium species in solution may affect the functional abilities of the solution, such as a crosslink agent for water-soluble polymers, which is one of the industrial applications of a zirconium solution. Therefore, the influence of the component concentrations in the solution on the structure of the zirconium species must be studied to understand the mechanism of its function and to improve it. X-ray absorption fine structure (XAFS) spectroscopy was used to obtain structural information of the zirconium species. Solutions containing zirconium, hydrogen and chloride ions at various concentrations were measured by the transmission method. The EXAFS (Extended XAFS) analysis indicated that the Zr-Zr coordination number, n(Zr-Zr), decreased with increasing the hydrogen chloride concentration, and did not depend on the zirconium concentration. However, if perchloric acid was used to increase the acidity, n(Zr-Zr) did not change. The results suggest that halide anion plays an important role for the depolymerization of zirconium species in an acidic solution. For 0.1 and 2.0 M zirconium solutions for which the molar ratio [Cl(-)]/[Zr] was kept at about 2, the n(Zr-Zr) values were determined to be 3.7 and 1.9, respectively. However, the same zirconium concentration solutions for which the [Cl(-)]/[Zr] values were around 0.7 showed n(Zr-Zr) values of 4.0 and 3.4, respectively. Thus, the value of 0.7 should be selected to obtain a chemically stable solution. The value of [Cl(-)]/[Zr] = 0.7 agrees with that for the most effective crosslink agent solution for polyvinyl alcohol.