Formation of Crystalline Zn–Al Layered Double Hydroxide Precipitates on γ‑Alumina: The Role of Mineral Dissolution

• Published in: Environmental science & technology Vol. 46; no. 21; pp. 11670 - 11677
• Main Authors: Li, Wei, Livi, Kenneth J. T, Xu, Wenqian, Siebecker, Matthew G, Wang, Yujun, Phillips, Brian L, Sparks, Donald L
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 06.11.2012
• Subjects: Adsorption; Aluminum - chemistry; Aluminum Oxide - chemistry; Applied sciences; Biological and physicochemical properties of pollutants. Interaction in the soil; Chemical Precipitation; Crystallization; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Hydroxides - chemistry; Magnetic Resonance Spectroscopy; Microscopy, Electron, Transmission; Minerals - chemistry; Pollution; Pollution, environment geology; Soil and sediments pollution; Solubility; X-Ray Absorption Spectroscopy; X-Ray Diffraction; Zinc - chemistry; Chemical precipitation; Pollutant behavior; Soil pollution; X ray diffraction; Dissolution; EXAFS spectrometry; Zinc; Heavy metal; Sorption; Trace element; Transmission electron microscopy; Layered double hydroxide; Alumina; Synchrotron radiation
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es3018094

To better understand the sequestration of toxic metals such as nickel (Ni), zinc (Zn), and cobalt (Co) as layered double hydroxide (LDH) phases in soils, we systematically examined the presence of Al and the role of mineral dissolution during Zn sorption/precipitation on γ-Al2O3 (γ-alumina) at pH 7.5 using extended X-ray absorption fine structure spectroscopy (EXAFS), high-resolution transmission electron microscopy (HR-TEM), synchrotron-radiation powder X-ray diffraction (SR-XRD), and 27Al solid-state NMR. The EXAFS analysis indicates the formation of Zn–Al LDH precipitates at Zn concentration ≥0.4 mM, and both HR-TEM and SR-XRD reveal that these precipitates are crystalline. These precipitates yield a small shoulder at δAl‑27 = +12.5 ppm in the 27Al solid-state NMR spectra, consistent with the mixed octahedral Al/Zn chemical environment in typical Zn–Al LDHs. The NMR analysis provides direct evidence for the existence of Al in the precipitates and the migration from the dissolution of γ-alumina substrate. To further address this issue, we compared the Zn sorption mechanism on a series of Al (hydr)oxides with similar chemical composition but differing dissolubility using EXAFS and TEM. These results suggest that, under the same experimental conditions, Zn–Al LDH precipitates formed on γ-alumina and corundum but not on less soluble minerals such as bayerite, boehmite, and gibbsite, which point outs that substrate mineral surface dissolution plays an important role in the formation of Zn–Al LDH precipitates.