Enhanced adsorption of Congo red on microwave synthesized layered Zn-Al double hydroxides and its adsorption behaviour using mixture of dyes from aqueous solution

Abstract

The adsorption of Congo red (CR) dye on to the ZnAl-layered double hydroxides (LDHs) nanoadsorbents synthesized using conventional hydrothermal (HT) and microwave-assisted hydrothermal (MW) methods were investigated. Thorough characterization of LDHs were done by using X-ray diffraction, Fourier-transform infrared spectroscopy, BET surface area, Transmission electron microscopy and CHNS elemental analysis methods. Among the microwave synthesized LDHs, ZnAl-NO3 has shown superior adsorption capacity for Congo red (CR) in aqueous solution than its counter ZnAl-CO3 LDH which was attributed to enhanced porosity and acidic property of the ZnAl-NO3 LDH. Adsorption capacity of the LDH adsorbents has been tested using various concentrations of CR ranging from 100 to 1200 ppm in water and the monolayer adsorption capacity of ZnAl-NO3 was observed as 571.43 mg g−1. It is important to note that adsorption studies were performed at ambient temperature and without maintaining pH of the solution. We have also studied the adsorption performance of the ZnAl-NO3-LDH for mixture of various anionic and cationic dyes in water using 25–100 ppm concentration of each dye in water. It has showed good adsorption affinity for anionic dyes over cationic dyes. In addition, the adsorption rates of CR onto ZnAl-NO3-MW are rather fast and CR adsorption equilibrium data obeys Langmuir adsorption isotherm model and kinetics data fitted to pseudo second-order kinetic model with correlation coefficients ranging from 0.998 to 0. 999.