Enhanced Adsorptive Removal of Anionic Dyes by using Calcined Zinc Aluminium Layered Double Hydroxide CZA-LDH from Aqueous Solution

Abstract

Removal of anionic dyes namely Acid Violet-17(AV-17) and Direct Red-7 (DR-7) from aqueous solution were investigated by calcined zinc aluminium layered double hydroxide (CZA-LDH). Batch adsorption studies were carried out by varying the parameters such as contact time, adsorbent dose, initial dye concentration, temperature and pH. The equilibrium adsorption capacity of AV-17 (adsorbent dose 0.5g/L) and DR-7(adsorbent dose 0.125g/L) dyes were increased from 48.08 mg/g to 172.55 mg/g and 200 mg/g to 683.33 mg/g respectively with increase of initial dye concentration from 25 mg/L to 100 mg/L at 30°C. Similarly the percentage of dye removal was decreased from 100% to 85.42% and 96.15% to 86.27% for DR-7 and AV-17 respectively with increase of initial dye concentration from 25 mg/L to 100 mg/L. The adsorption kinetics of both the anionic dyes AV-17 and DR- 7 by CZA-LDH followed the pseudo second order kinetic model with high correlation coefficient values (R2) than the pseudo first order. The experimental data were well fitted with Freundlich isotherm model rather than Langmuir model. The adsorption equilibrium was more favourable at acidic pH than alkaline pH for both dyes. The thermodynamic parameters like ΔGo and ΔHo calculated for AV-17 and DR-7 revealed that the adsorption process is spontaneous and endothermic in nature. The XRD pattern of CZA-LDH before and after the adsorption of AV-17 and DR-7 proved the intercalation of anionic dyes between the layers with memory effect by the reconstruction of its original layered structure. This was further evidenced by slight increase in the interlayer distance. The adsorption processes were further supported by Langmuir dimensionless separation factor RL and n value of the Freundlich Equation. Thermal regeneration and reusability of CZA-LDH was performed for the removal of AV-17 & DR-7. The recycling and reusability for the dye removal up to second cycle for both anionic dyes were remarkable and suggested that the CZA-LDH was an effective adsorbent for the removal of anionic dyes AV-17 and DR-7 from aqueous solution.