Structural Approach to Understanding the Solubility of Metal Hydroxides

Abstract

We report the hierarchical structure of zirconium hydroxide after aging at different temperatures to elucidate the factors governing zirconium solubility in aqueous solutions. Zirconium hydroxide solid phases after aging at 25, 40, 60, and 90 °C under acidic to alkaline conditions were investigated using extended X-ray absorption fine structure (EXAFS), wide-and small-angle X-ray scattering (WAXS and SAXS), and transmission electron microscopy (TEM) techniques to reveal the bulk and surface structures of the solid phases from the nanoscale to sub-microscale. After aging at 25 °C, the fundamental building unit of the solid phase was considered to be tetrameric and dimeric hydroxide species. These polynuclear species formed amorphous primary particles that are approximately 3 nm in size, which in turn formed aggregates that are hundreds of nanometers in size. This hierarchical structure was found to be stable up to 60 °C under acidic and neutral conditions and up to 40 °C under alkaline conditions. After aging at 90 °C under acidic conditions and at 60 and 90 °C under alkaline conditions, the WAXS and EXAFS measurements suggested the crystallization of the solid phase. The SAXS profiles and TEM observations supported the existence of crystallized large particles about 60 nm in size, and the appearance of the Guinier region in the SAXS profiles indicated that the crystallization of the amorphous primary particles leads to the reduction of the size of the large aggregates. The transformation of the solid-phase structure by temperature was discussed in relation to the solubility product to understand the solubility-limiting solid phase. The solubility of zirconium hydroxide after aging at different temperatures was governed not only by the size of the amorphous primary particles or crystallized large particles but also by their surface configuration.

Structural Approach to Understanding the Formation of Amorphous Metal Hydroxides

“…24 The SAXS profiles in Figure 1c obtained at I m = 0.5 (NaCl) were observed to be comparable to those at 25 °C in I m = 0.5 M (NaClO 4 ). 24 On the other hand, at lower I m both in NaCl and CaCl 2 solutions, the q dependence of I(q) also changed around q = 0.4 nm −1 , suggesting the possible existence of secondary particles. At I m = 0.5 M (NaClO 4 ), 24 the appearance of secondary particles was only observed at the solid phases aged at elevated temperatures >60 °C, accompanied by the appearance of diffraction peaks.…”

“…24 On the other hand, at lower I m both in NaCl and CaCl 2 solutions, the q dependence of I(q) also changed around q = 0.4 nm −1 , suggesting the possible existence of secondary particles. At I m = 0.5 M (NaClO 4 ), 24 the appearance of secondary particles was only observed at the solid phases aged at elevated temperatures >60 °C, accompanied by the appearance of diffraction peaks. The secondary particles suggested at I m = 0.01 may be precursors of crystalline particles, although no diffraction peaks were confirmed in the WAXS spectra.…”

“…24 Since the theoretical SAXS curves for dimeric and tetrameric hydrolysis species did not reproduce the experimental ones, we have reported that the primary particles consist of fewer stacked multinuclear Zr hydrolysis species. 24 In the present analysis, these trends were confirmed again for those at I m = 0.5, but it was found that the R g2 values slightly depend on the I m and the charge of electrolyte cations. S2 in the Supporting Information), and it was confirmed that the values were within the scattering of the plots in Figure 2.…”

“…At low ionic strengths of I m = 0.1 and 0.01, a possible existence of the secondary particles was suggested. Since the secondary particles with comparable size were observed at I m = 0.5 after aging at elevated temperatures accompanied by crystallization, 24 the secondary particles hinted at 25 °C in the present study might be a precursor of crystallization. The possible formation of secondary particles was shown at lower I m with high ζ potentials; therefore, it could be caused not by the DLVO aggregation but possibly some chemical reactions.…”

“…As the solubility of amorphous M(IV) hydroxide solid phases is considered to depend on the size of the solid-phase particles, 9−11 the overall picture of amorphous M(IV) hydroxide solid phases is needed for the reliable prediction of their solubility. Recently, we investigated the structure of amorphous Zr(OH) 4 solid phases in acidic to alkaline pH regions using a combination of small and wide-angle X-ray scattering (SWAXS) and extended X-ray absorption fine structure (EXAFS) 24 and found that the amorphous Zr(OH) 4 precipitates that aged at 25 °C consisted of primary particles with the size of a few nanometers and aggregates larger than 100 nm. The EXAFS spectra suggested that the primary particles were possibly formed by stacking polynuclear hydrolysis species, as suggested for Zr(IV) polymers and colloids in the acidic pH region in the literature.…”