Calcined Layered Double Hydroxides for Decolorization of Azo Dye Solutions: Equilibrium, Kinetics, and Recycling Studies

This work presents results on the sorption of the azo dyes Remazol Yellow GR110 and Remazol Golden Yellow RNL onto calcined MgAl-CO3 hydrotalcite under different pH (7 and 11) and temperature (25°C and 40°C) conditions. In addition to isotherm and kinetic data, this work also shows results on two techniques (thermal and ion exchange) for hydrotalcite regeneration. The Langmuir isotherm and the pseudo-second-order kinetics fit best the experimental data for both anionic dyes, resulting in a qmax of 106.3 mg/g (0.18 mmol/g) for Remazol Yellow GR110 and a qmax of 657.2 mg/g (1.1 mmol/g) for Remazol Golden Yellow RNL at the best adsorption conditions (25°C and pH=7). Results on layered double hydroxides (LDH) characterization (X-ray diffraction, Fourier-transformed infrared Spectroscopy (FTIR) indicate that the azo dyes did not intercalate into the LDH but were rather adsorbed onto its surface. Thermal recycling reduced the LDH adsorption capacity by 20% to 30% per cycle due to incomplete dye decomposition during the treatment at 500°C. Likewise, the recycling of hydrotalcite via ion exchange was fairly ineffective, as the efficiencies for dye recovery were 15%, 20% and 30% when chloride, hydroxyl and carbonate anions were respectively used as exchangers.