Formation of Organic Anion-pillared Layered Zinc Hydroxides by Coprecipitation

Abstract

From the viewpoints of removal, recovery and preconcentration of organic anions (A^n-) in water, the direct fixation into the precipitates of layered zinc hydroxide by a coprecipitation has quantitatively been investigated. When the solution of 8-200 mmol dm^-3 Zn (NO₃)₂ was added dropwise into the solution of 1-250 mmol dm^-3 organic anions at 25°C with stirring, the saturated aliphatic carboxylate ions with long carbon chain such as sebacate and dodecanedioate, the aromatic carboxylate ions such as terephthalate and also the anionic surfactants such as dodecylbenzenesulfonate and dodecyl sulfate were incorporated highly in the layered zinc hydroxide precipitates. The optimum conditions were found as follows; Z n²⁺/ A ^2- molar ratio:>1.6, Zn²⁺/A^- molar ratio> 3.2, and pH 6-7. The pecipitates were also found to have the layer structure with large basal spacings, which crystallinity being quite high in spite of the low reaction temperature. The guest organic anions were thought to be incorporated in a perpendicular or tilted configuration to the zinc hydroxide layers or botallackite type layers as a monolayer or bilayer by coordinating their anionic functional group, to Zn²⁺ ion in the host layers. Therefore, if the solutions contained a lot of inorganic anions, the coprecipitation of the organic anions was not influenced and the excellent selectivity for organic anions was observed. In conclusion, the possible fixation method based on the new mechanism (coprecipitation as pillared-layered zinc hydroxides) can be proposed.