Evidence for tetrahedral AlO4 formation induced by Zn2+ adsorption onto Al(OH)3 gel

27Al magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was used to investigate the effect of Zn2+ ion adsorption on Al(OH)3 gel. While as-prepared Al(OH)3 gel exhibits a single peak around 5 ppm, the same gel with adsorbed Zn2+ has new peaks around 15 and 60 ppm. These peaks correspond to AlO6 octahedral species with adsorbed Zn2+ and AlO4 tetrahedral species, respectively. In order to assess the mechanism for the formation of AlO4 tetrahedral species, hydroxide coprecipitates of Al3+ and Zn2+ were prepared and the chemical states of Al in the solids were observed by 27Al MAS NMR. The coprecipitates were found to have the AlO4 tetrahedral structure, except for those with high Zn/Al ratios. From these results, it can be assumed that adsorption of Zn2+ onto Al(OH)3 gel induces Al3+ dissolution followed by the coprecipitation of Al3+ with Zn2+ to form AlO4 tetrahedral structures on the surface of Al(OH)3 gel.

27Al MAS NMR measurements confirmed the presence of AlO4 species on the surface of Al(OH)3 with adsorbed Zn2+. Zn2+ is first adsorbed onto Al(OH)3 by inner-sphere complex formation. The formation of Zn–Al hydroxide coprecipitate with a zinc aluminate-like local structure then occurred. The change in the chemical state of Al from an AlO6 to AlO4 species indicated that adsorption (inner-sphere complexation) and coprecipitation are closely related to each other and are different stages of the solid liquid interfacial reaction.Figure optionsDownload as PowerPoint slideHighlights
► The presence of AlO4 species was found on Al(OH)3 with adsorbed Zn2+.
► Zn–Al hydroxide coprecipitate with a zinc aluminate-like structure was formed.
► Surface complexation alone is not sufficient to explain heavy metal behavior.