Zinc (Zn) and aluminum (Al) layered double hydroxide (LDH) precipitates was found to be the main Zn species in Zn2+ or ZnO-contaminated soils, but their formation mechanisms are not well understood. In this work, we used X-ray diffraction (XRD) patterns, Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), 27Al solid-state nuclear magnetic resonance (NMR) and extended X-ray absorption fine structure spectroscopy (EXAFS), to investigate the transformation of ZnO nanoparticles (NPs) into LDH precipitates on nano-γ-Al2O3 under various solution chemistries. Our results indicated the formation of Zn-Al LDH from their oxide mixture at a wide range of solution pH (from 6.5 to 9.5). XRD patterns and EXAFS spectroscopy suggested that: at pH 6.5, the main Zn(II) species of products is LDH (~90%) with a small amount of ZnO NPs (~10%) residual, and at pH 9.5, the main Zn(II) speciation of products are LDH (~68%) with some remains of unreacted ZnO NPs (~32%). NMR spectroscopy showed that the intensity of γ-Al2O3’s tetrahedral AlO4 and octahedral AlO6 was reduced as pH increased, suggesting that the dissolution of γ-Al2O3 was facilitated as pH increased. TEM and SEM images showed that sheet-like LDH particles existed in the reaction products at pH 6.5 (where poor crystalline γ-Al2O3 still remained) and 9.5 (where spherical-shape ZnO NPs still remained). Concentration change of Zn2+ and Al(OH)4− during the reaction suggested that ZnO dissolved at low pH and γ-Al2O3 dissolved at high pH, indicating that LDH formation is via a dissolution-sorption-reprecipitation processes. Results from this work unveil the transformation mechanism for ZnO NPs under Al-rich conditions, which is able to explain the secondary Zn species form in natural environment.

Transformation of ZnO Nanoparticles (NPs) into Layered Double Hydroxide (LDH) precipitates on nano-γ-Al2O3: Implications for the fate of ZnO NPs under Al-enriched environment

253rd National Meeting (2017)

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