Preparation and optimization of a low-cost adsorbent for heavy metal ions from red mud using fraction factorial design and Box-Behnken response methodology

Abstract

Red mud (RM) is a solid waste generated from the alumina refining industry. The utilization of RM to prepare sorbent has attracted researchers’ attention. This study aimed to prepare a low-cost adsorbent for heavy metal ions by activating RM using Ca2+ and Mg2+ mixed solution neutralization and high-temperature treatment. Pb(Ⅱ) and Zn(Ⅱ) were selected as target contaminants to investigate the possibility of this adsorbent to remove heavy metal ions from the liquid. The Fractional Factorial Design and Box-Behnken design were used to screen the significant preparation parameters and obtain the optimum preparation conditions. The results demonstrated that neutralization time, calcination temperature, and calcination time were determined as significant factors affecting the adsorption capacity of Pb(Ⅱ) and Zn(Ⅱ), and the optimum preparation parameters were the solution concentration of 1.5 mol/L, the liquid-solid ratio of 40 ml/g, the neutralization time of 114 min, calcination temperature of 518 ℃, and the calcination time of 148 min. The physicochemical characterizations of neutralization-calcination RM (NCRM) were determined by X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), and N2 adsorption isotherm analysis. The results revealed that the main mineral phase of NCRM was hematite, calcite, and cancrinite, and NCRM had a loose structure and abundant pores, which contributes to the sorption of heavy metal ions. The adsorption kinetics study referred to satisfactory adsorption of Pb(Ⅱ) via pseudo-first-order model and pseudo-second-order model, while pseudo-second-order model was the best model to describe adsorption of Zn(Ⅱ) onto NCRM. Based on the batch experiment, the maximum adsorption capacities of Pb(Ⅱ) and Zn(Ⅱ) were 218.82 and75.58 mg/g, respectively. The excellent adsorption performance of NCRM on Pb(Ⅱ) and Zn(Ⅱ) implies that this material is a promising material for removing heavy metal ions from contaminated water.