Elimination of methylene blue dye from the aqueous solution using waste fig fruit as an activated carbon: a case study of nonlinear adsorption isotherm models and kinetic models

Abstract

Waste fig fruit, an eco-friendly, available, and not toxic, has been used as active carbon for the first time in the elimination of methylene blue dye from its aqueous solution, which was never reported previously. The fig fruit activated carbon (FFAC) has been obtained and then characterized using several techniques, such as field-emission scanning electron microscopes (FE-SEM), X-ray diffraction, Brunauer–Emmett–Teller, and Fourier transform infrared spectroscopy (FTIR). As a part of the practical experiments, temperatures, initial concentrations, FFAC amount, contact time, pH solution, activation agent, and volume solution were studied as factors that can change adsorption parameters. Nevertheless, the pH of the solution was checked at the following values: pH3, pH7, pH9, and pH10, the initial concentration of methylene blue dye was studied at various concentrations: 40, 60, 80, 100, and 120 mg/L. Furthermore, the adsorption temperatures of the solution were checked at the following degrees: 15, 30, 40, and 50 °C. The adsorption capacity value for fig fruit active carbon was 115 mg/g for 0.06 g and 55 mg/g for 0.02 g. Moreover, the correlation coefficient of the Langmuir isotherm model was 0.9911, so the process of adsorption prefers it. It was observed that pseudo-first order kinetics is the preferred to achieve the process with a rate constant of 0.0332 min^˗1. Thermodynamically, the process of adsorption was completely spontaneous, and the Gibbs free energy ranged between − 7.88 and − 15.16 kJ/mol. A monolayer has formed in the adsorption process. Additionally, the maximum capacity of adsorption (Q_m) was 63.38 mg/g. The fig fruit active carbon, an environmentally friendly adsorbent for methylene blue dye, acquired good performance in the adsorption process.