Uptake of anionic dye from aqueous solution by zinc-aluminium-chloride layered double hydroxides

Fifteen percent of the total world production of dyes is lost during the dyeing process and released in the textile effluents. Removal of textile dyes from wastewater is one of the several major environmental concern because they are difficult to be removed by the conventional wastewater treatment systems (biological, physical and chemical methods) since they are designed to be resistant to degradation or fading by oxidizing agents and light. Adsorption, however, is an effective method for dye removal. In this way we studied the application of layered double hydroxides (LDHs) or hydrotalcite-like compounds as adsorbents to remove a hazardous anionic dye "Indigo Carmine (IC)" from aqueous solutions. LDHs with Zn and Al in the layers and chloride in the interlayer, was prepared following the coprecipitation method at constant pH. The affinity of this material with IC was studied as a function of contact time, initial pH of the solutions and initial dye concentration. It was found that 8 h is enough time for the equilibrium state to be reached in IC adsorption. IC removal was independent of the initial pH. The adsorption isotherm, described by the Langmuir model, is of L-type. The results, demonstrate that the IC adsorption on LDHs occurs by both adsorption on external surface and interlayer ion exchange. A mechanism for removal of IC anion has been confirmed by X-ray diffraction and IR spectroscopy.