Zn-Al layered double hydroxides intercalated with surfactant: Synthesis and applications for efficient removal of organic dyes

Abstract

As the presence of organic dyes is recognized as the common cause of many environmental problems, it is important to develop an effective strategy to efficiently remove diverse dyes from wastewater. To this end, dodecylsulfate-intercalated layered double hydroxides (DS-Zn/Al LDHs) were synthesized at a Zn/Al molar ratio (2:1) based on urea-hydrolysis followed by ion exchange. The surface of the LDHs was made hydrophobic to increase the adsorption affinity for organic pollutants. The prepared DS-Zn/Al LDHs were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). As observed in UV-spectrophotometric studies, the DS-Zn/Al LDHs exhibited excellent adsorption capacity toward organic dyes (e.g., methyl orange (MO), Congo red (CR), and brilliant green (BG)) compared to previously reported LDHs due to the enhanced surface area provided by the surfactant and its porous nature. The intercalation of DS greatly increased the affinity for organic dyes because of hydrophobic interactions between dye molecules and surfactants. The sorption was observed to depend on concentration of dye, agitation time, and temperature while being less dependent on pH (for 5 <= pH <= 10). The maximum adsorption capacities for CR, MO, and BG were 149.3, 114.9, and 87.0 mg/g, respectively with their corresponding partition coefficients of 102.3, 62.8, and 17.3 mg/g/mu M, respectively. The adsorption isotherms for the dyes were better explained by the Langmuir model than by the Freundlich model. The results further demonstrated the significance of a pseudo-second-order kinetic model. As such, adsorption primarily occurred on a mono layer in the homogeneous surface of the composite. The reusability characteristics of DS-Zn/Al LDHs are also found as their advantageous feature to make the process more economical with the least waste generation. The cost of the DS-Zn/Al LDHs is approximately US$ 2.0 x 10(2)/ton (e.g., based on the material ratio used for their synthesis). The overall results of this study demonstrated the great potential of DS-Zn/Al LDHs for treatment of diverse dyes in effluent systems with high cost-effectiveness.