Adsorption and intercalation of Acid Blue 9 on Mg–Al layered double hydroxides of variable metal composition

Abstract

Layered double hydroxides (LDHs) of the type [ Mg 1 - X Al X ( OH ) 2 ] X + [ ( CO 3 2 - ) X / 2 · m H 2 O ] with variable Mg/Al molar ratios have been prepared by co-precipitation of Mg 2+ and Al 3+ ions at constant pH. The ability of these compounds to adsorb Acid Blue 9 has been evaluated by UV–Vis spectrophotometry, FT-IR spectroscopy, powder X-ray diffraction analysis, N 2 adsorption/desorption analysis, scanning electron microscopy, energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy and atomic absorption spectroscopy. Optimum adsorption of Acid Blue 9 (AB9) was achieved for LDHs produced from mixtures containing Mg and Al in molar ratios varying from 3.1 to 4.4. LDHs with such compositions giving an equilibrium binding constant ( K a) of ca. 1.2 × 10 5 M −1 with a maximum monolayer coverage ( C m) of ca. 7.6 × 10 −2 mmol g −1. This observation was correlated to the surface area, phase purity, pore size and volume, particle size and the surface morphology of the LDHs. Powder XRD analysis after adsorption of 1.85 mM Acid Blue 9 in 20% (v/v) glycerol–H 2O solvent system indicated that the dye was not intercalated between the brucite-like layers as no change was evident in the d (003) spacing. Thus, at this low concentration surface adsorption of the dye is occurring. Powder XRD analysis revealed that intercalation was achieved at 76 mM dye concentration in the same solvent system for materials with Mg:Al molar ratios between 3.1 and 5.6. This results in an expansion of the interlayer spacing from ca. 8 Å to 19 Å to accommodate the AB9 anion. For the sample produced from a 3:1 mixture of Mg/Al, thermal gravimetric and elemental/micro analyses revealed that the new hybrid material has the composition [Mg 3.0Al 1.0(OH) 7.5][(CO 3) 0.62 · 3.2H 2O · 1.5Glycerol · 0.1Acid Blue 9].