Preparation of low-cost activated carbon from Doum fiber (Chamaerops humilis) for the removal of methylene blue: Optimization process by DOE/FFD design, characterization, and mechanism

Abstract

Within the context of a circular economy, this work aims to valorize local lignocellulosic biomass of Doum fiber (Chamaerops humilis), as a highly promising adsorbent material for the removal efficiency of methylene blue (MB) dye via the adsorption process. Thus, this study investigates the potential of both raw (RDF) and activated carbon from Doum fiber (ACDF) as adsorbents. The first task was to enhance the adsorption efficiency of MB onto activated carbon by systematically optimizing its synthesis conditions. To achieve this, a full factorial design (FFD) approach of four factors (activation time, activation temperature, contact time, and concentration of H3PO4) at two levels was employed, using H3PO4 as the activating agent. A series of batch experiments were performed to characterize the adsorption properties of MB onto RDF and ACDF by describing the adsorption conditions, determining the kinetic and isotherm models, and evaluating the thermodynamic parameters. The results obtained revealed that ACDF had significantly higher surface affinity for MB than RDF, with an adsorption capacity of 83.82 mg. g−1. The adsorption process was found to be spontaneous and endothermic for ACDF, while for RDF, it was exothermic. The adsorption data for both ACDF and RDF exhibited a good fit to the Langmuir isotherm model, indicating monolayer adsorption. Furthermore, the kinetic data demonstrated excellent agreement with the pseudo-second-order model, suggesting a three-step rate-controlling mechanism. According to the results of density functional theory (DFT) calculations, the adsorption mechanism of MB mainly involves electrostatic interactions, hydrogen bonds, and n-π, π-π interactions with the two adsorbents.