Effects of particle size and mineral crystallinity on formation of Zn[sbnd]Al layered double hydroxides (LDH) on aluminum (oxyhydr)oxides

Abstract

The widespread usage of zinc oxide nanoparticles (ZnO NPs) inevitably leads to its release in nature and interaction with environmental minerals. To reveal ZnO NP retention by natural aluminum (Al) minerals as layered double hydroxide (LDH) in the environments, the effects of Al (oxyhydr)oxide particle size and crystallinity on ZnAl LDH formation were investigated during ZnO NP interaction with seven different Al (oxyhydr)oxides at neutral and alkaline pHs. Dissolution kinetics displayed a decrease of dissolved Zn2+ and Al(OH)4− concentrations in a dual-solid system (ZnO and Al minerals) relative to a single-solid one (ZnO or Al minerals), suggesting a potential transformation of mixture solids. Diffraction, spectroscopic, and microscopic analyses indicated that Al (oxyhydr)oxide particle size and crystallinity had obviously impacted on ZnAl LDH formation. The composition percentage of ZnAl LDH in solid products followed the order of amorphous Al hydroxides (89.90% and 86.41% Zn species as ZnAl LDH at pH 7 and 10, respectively) > 5 nm γ-Al2O3 (41.62% and 49.62%) > 35 nm γ-Al2O3 (14.77% and 19.03%) > large size Al (oxyhydr)oxides (below 13.00%), including 70 nm γ-Al2O3, corundum, boehmite, and gibbsite. ZnAl LDH formation was regulated mainly by Al (oxyhyro)oxide crystallinity and particle size via changing Al oxide dissolution at pH 10 and Zn2+ adsorption at pH 7, whereas solution pH controlled the crystallinity of the formed ZnAl LDH. A dissolution-interaction-coprecipitation pathway was proposed to elucidate the mechanism of ZnAl LDH formation. The findings highlight the importance of substrate mineral size and crystallinity for ZnAl LDH formation in environmental settings.