Valorization of Cagaita (Eugenia dysenterica) Pulp through Torrefied Biomass Production for Methylene Blue Removal from Aqueous Solutions

A low-severity torrefaction route was employed to convert cagaita ( Eugenia dysenterica ) pulp into an efficient carbonaceous adsorbent for methylene blue (MB) removal from aqueous solutions. The torrefied biomass (TBCP), prepared at 250°C for 1 h under air, exhibited a strong pH-dependent adsorption behavior, with the adsorption capacity increasing from 4.4 mg g⁻¹ at pH 2 to 20.7 mg g⁻¹ at pH 11. Kinetic studies revealed rapid MB uptake under alkaline conditions, well described by the pseudo-first-order model (R² = 1.000 at pH 11), while Elovich analysis highlighted surface heterogeneity effects at acidic and neutral pH. Thermodynamic evaluation at pH 7 confirmed a spontaneous (ΔG° < 0) and endothermic adsorption process (ΔH° = +5.30 kJ mol⁻¹) driven by entropy gain (ΔS° = +110.6 J mol⁻¹ K⁻¹). These results demonstrate that mild torrefaction is sufficient to generate an effective, low-cost adsorbent from fruit pulp waste, offering a sustainable alternative for dye-contaminated wastewater treatment.