Use of red mud from amazon region as an adsorbent for the removal of methylene blue: process optimization, isotherm and kinetic studies

The aim of this study is to evaluate the potential of red mud applied as an adsorbent in the removal of dyes from the textile industry; the adsorption process was optimized using both response surface methodology and desirability function. Scanning electron microscopy (SEM), X-ray diffraction (XRD), point of zero charge (pHPZC), nitrogen physisorption and thermogravimetric analysis techniques were used to characterize the adsorbent. Adsorption process was based on an experimental design, where contact time (5 and 60 min), pH of the medium (4 and 6.5) and adsorbent concentration (5 and 10 mg L−1) were the independent variables. In addition, the kinetic mechanism and adsorption equilibrium were investigated. The results indicated that a higher removal efficiency of methylene blue was obtained at higher adsorbent concentrations (8–10 mg L−1) and lower pH ranges (4.0–5.0), while increasing the adsorptive capacity is achieved by decreasing the adsorbent concentration and pH. The optimum adsorption conditions were: 60 min, a pH of the medium at 4.0 and adsorbent concentrations of 8.66 and 10 g L−1. The equilibrium data were fit to the Langmuir equation (R2 = 0.993) with a maximum adsorption capacity of 6.541 mg g−1. Experimental measurements showed that the adsorption kinetics corresponded to a pseudo-second-order reaction (R2 = 0.998) with the chemisorption as the process controlling step. The results obtained in this work indicated that red mud can be an alternative for the removal of pigments in wastewater treatment processes in an efficient and economical way.