Direct Fixation of Carboxylate Ions into Layered Double Hydroxide Precipitates by Coprecipitation

Abstract

From the viewpoints of removal, recovery and preconcentration of organic anions in water, the direct fixation of carboxylate ions into the precipitates of the layered double hydroxides (LDHs), [M²⁺_1-xM³⁺_x(OH)₂][A^n-_x/n⋅ yH₂O], by coprecipitation method has quantitatively been investigated. When 0.2 mol dm^-3 mixed solution of MgCl₂ and AlCl₃ or ZnCl₂ and AlCl₃ ( M²⁺/ Al³⁺= 2) was added dropwise into the 2-200 mmol⋅ dm^-3 carboxylate solution at adjusted pH, the linear saturated aliphatic carboxy late ions with long carbon chain such as sebacate, dodecanedioate and laurate, and also the aromatic carboxylate ion such as terephthalate were incorporated highly in the LDH precipitates. Further, the extent of the coprecipitation became higher with the increase of carbon number of the guest aliphatic carboxylate ions. The optimum conditions were found as follows; Al³⁺ / A^n- molar ratio = 4-8, reaction te mperature range 25-40° C, p H about 10 (Mg-Al system) and 6 (Zn-Al system). Though the crystal linity of the LDH precipitates was quite low, the carboxylate ions were found to be incorporated in a parallel configuration to the positively charged hydroxide layers when the amount of the guest anion was small, while those in a tilted configuration to the hydroxide layers in the case of large amount of the guest a nion. When the initial concentration of the carboxylate ions was quite low or the solution contained a lot of inorganic anion, the extent of the coprecipitation of the carbwdate ions was decreased. However, it could be improved easily by increasing the addition of t he metal ion solutions. In conclusion, the possible fixation method based on the new mechanism (coprecipitation into the interlayer of LHD) can be proposed.