Mechanisms of Phosphate Sorption from Aqueous Solutions by Calcined Mg-Al-CO3 Hydrotalcite

The study deals with rehydration of calcined Mg-Al-CO3 hydrotalcite (Mg/Al molar ratio 2:1) occuring in the presence of phosphate ions. The sorption process follows the Langmuir isotherm and may be characterized by a pseudo-second-order kinetic model. The sorption capacity of calcined hydrotalcite (89.3 mg Pig) is determined by various sorption mechanisms. Kinetic tests proved the existence of a fast and slow step. The fast step is characterized by rehydration of the mixed oxide leading to the formation of hydroxide form of hydrotalcite associated with surface sorption of phosphates. This step proceeds within the first 90-120 min and its rate is determined by intra-particleular diffusion. However, if an excess of phosphates is present in solution, the slow step proceeds - intercalation of phosphate ions into interlayer space realized as the ion exchange with OH- anions. Precipitation of Al phosphates may also occur in the slow step. The slow-step processes have a negative impact on the structure-memory effect of calcined hydrotalcite.