Optimization of NaOH Chemical Treatment Parameters for Biomass-Based Adsorbents in Cationic Dye Removal

The aim of this research work was to optimize the chemical treatment of two biomass materials, spent coffee grounds (SCGs) and date pits (DPs), for their use as adsorbents in the removal of methylene blue (MB). The treatment was carried out using sodium hydroxide (NaOH) following a two-level full factorial design, varying the activating solution concentration, activation time, and activation temperature. Only the model based on SCG proved statistically significant. The optimal pretreatment conditions (0.2 M, 5.5 h, 22 °C) yielded an adsorption capacity of 140.23 mg·g−1. The optimal material was characterized by FTIR, BET, SEM, and pHpzc analyses. The effects of pH (1–11), initial concentration (5–300 mg·L−1), adsorbent dose (0.25–2 g·L−1), contact time (0–420 min), and temperature (22–50 °C) on MB adsorption onto the optimal adsorbent were investigated. Adsorption was enhanced by increasing the pH, contact time, and initial concentration, but decreased with higher adsorbent dose and temperature. Kinetic analysis revealed that the pseudo-second-order model best described the data (R2 = 0.96), with a notable contribution from intraparticle diffusion (R2 = 0.98). The Sips model (R2 = 0.99) adequately represented the adsorption isotherm. These findings confirm the strong potential of this biosorbent for the removal of cationic dyes from aqueous solutions.