FORMATION OF ORGANIC ANION-PILLARED LAYER ED ZINC HYDROXIDES BY COPRECIPITATION

From the viewpoints of removal, recovery and preconcentration of organ ic 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 (NOe)(2) was added dropwise into the solution of 1-250 mmol dm(-3) organic anions a t 25 degrees C with stirring, the saturated aliphatic carboxylate ions with long carbon chain such as sebacate and dodecanedioate,the aromat ic carboxylate ions such as terephthalate and also the anionic surfact ants such as dodecylbenzenesulfonate and dodecyl sulfate were incorpor ated highly in the layered zinc hydroxide precipitates. The optimum co nditions were found as follows; Zn2+/A(2-) molar ratio: >1.6, Zn2+/A(- ) molar ratio >3.2, and pH 6-7. The pecipitates were also found to hav e the layer structure with large basal spacings, which crystallinity b eing quite high in spite of the low reaction temperature. The guest or ganic anions were thought to be incorporated in a perpendicular or til ted configuration to the zinc hydroxide layers or botallackite type la yers as a monolayer or bilayer by coordinating their anionic functiona l group to Zn2+ ion in the host layers. Therefore, if the solutions co ntained a lot of inorganic anions, the coprecipitation of the organic anions was not influenced and the excellent selectivity for organic an ions was observed. In conclusion, the possible fixation method based o n the new mechanism (coprecipitation as pillared-layered zinc hydroxid es) can be proposed.