DIRECT FIXATION OF CARBOXYLATE IONS INTO LAYERED DOUBLE HYDROXIDE PRECIPITATES BY COPRECIPITATION

From the viewpoints of removal, recovery and preconcentration of organ ic anions in water, the direct fixation of carboxylate ions into the p recipitates of the layered double hydroxides (LDHs), [M(1-x)(2+)M(x)(3 +) (OH)(2)][A(x/n)(n-). yH(2)O], by coprecipitation method has quantit atively been investigated. When 0.2 mol . dm(-3) mixed solution of MgC l2 and AlCl3 or ZnCl2 and AlCl3 (M(2+)/Al3+=2) was added dropwise into the 2 similar to 200 mmol . dm(-3) carboxylate solution at adjusted p H, the linear saturated aliphatic carboxylate ions with long carbon ch ain such as sebacate, dodecanedioate and laurate, and also the aromati c carboxylate ion such as terephthalate were incorporated highly in th e LDH precipitates. Further, the extent of the coprecipitation became higher with the increase of carbon number of the guest aliphatic carbo xylate ions. The optimum conditions were found as follows; Al3+/A(n-) molar ratio=4 similar to 8, reaction temperature range 25 similar to 4 0 degrees C, pH about 10 (Mg-Al system) and 6 (Zn-Al system). Though t he crystallinity of the LDH precipitates was quite low, the carboxylat e ions were found to be incorporated in a parallel configuration to th e positively charged hydroxide layers when the amount of the guest ani on was small, while those in a tilted configuration to the hydroxide l ayers in the case of large amount of the guest anion. When the initial concentration of the carboxylate ions was quite low or the solution c ontained a lot of inorganic anion, the extent of the coprecipitation o f the carboxlate ions was decreased. However, it could be improved eas ily by increasing the addition of the metal ion solutions. In conclusi on, the possible fixation method based on the new mechanism (coprecipi tation into the interlayer of LDH) can be proposed.