Síntese e caracterização das hidrotalcitas de Mg/Al com aplicação na remoção do ácido 2,4-diclorofenoxiacético (2,4-D)

Abstract

Samples of Lamellar Double Hydroxides (HDLs) of Mg and Al were synthesized by the coprecipitation method, varying the pH and Mg/Al molar ratio of the synthesis mixture, thus obtaining different precursors with structures of the hydrotalcite type that, after being calcined, presented as different mixed oxides. The samples were characterized using inductively coupled plasma optical emission spectroscopy, X-ray diffraction, Fourier transform infrared attenuated total reflectance spectroscopy, thermogravimetric analysis, N2 physisorption and scanning electron microscopy. All synthesized samples showed diffraction peaks characteristic of the hydrotalcite phase, while the mixed oxides showed a typical structure of the periclase MgO phase. Different values of the basal spacing were found in the samples varying pH in 7, 9 and 11, respectively equal to: 7.69 nm, 7.76 nm and 7.80 nm. This is because the pH affects the hydrotalcite composition and consequently affects the basal spacing. Furthermore, when the Mg/Al molar ratios were changed to 2.3 and 1, the corresponding values were: 7.91 nm, 7.80 nm, 7.66 nm respectively. Infrared spectroscopy confirmed the bands corresponding to the vibration of carbonate ions occupying the interlamellar space. Mass losses characteristic of the elimination of hydroxyl and carbonate ions were found in thermogravimetric analyses. It was verified that morphology influences the adsorption of the pesticide as well as the charge density of the brucite-type layers. The samples formed clusters of particles with lamellar shapes. After being calcined, the one that presented a more regular and defined morphology, varying the pH, was synthesized at pH 11. The increase in aluminum content favored a greater removal capacity, due to the mixed oxides, which have a higher charge density.