Zinc recovery from waste zinc ash - A new “green” route for the preparation of Zn-Al layered double hydroxide used for molybdate retention

Abstract The aim of this paper is to study the synthesis of a Zn-Al layered double hydroxide using a waste as an alternative source of zinc, namely the zinc ash resulted from hot dip galvanizing process. The zinc ash was treated with hydrochloric acid to obtain zinc chloride solution, which was subjected to a purification process. The layered double hydroxide was synthesized by a co-precipitation method at low oversaturation. The as-obtained (LDH) and calcined at 450 °C (CLDH) adsorbents were used in the removal process of molybdate from water. Both materials were subjected to structural and morphological characterization. The as-synthesized product has characteristics close to those of the material obtained from analytical grade reagents. The kinetic study of the adsorption showed that the process followed accurately a pseudo-second order kinetics (correlation coefficients > 0.99). The results concerning the process equilibrium showed that CLDH exhibited higher efficiency (maximum adsorption capacity of 39.8 mg/g) in the adsorption of Mo(VI) from aqueous solutions than LDH (13.7 mg/g), which is in accordance with their specific surface area. The mechanism of molybdate uptake follows inner-sphere surface complexation. These materials obtained from a secondary resource are quite efficient in the removal process of molybdate anions from aqueous solutions, as compared to other adsorbents obtained from expensive analytical grade reagents reported in the literature.