Adsorption of methyl orange using gel beads derived from polysaccharides extracted from dragon fruit stems waste in Binh Thuan province, Vietnam

ABSTRACT Schematic illustration of the adsorption mechanism of methyl orange onto polysaccharide gel beads (PS) derived from dragon fruit stems, showing electrostatic attraction, n–π interaction, dipole–dipole and hydrogen-bonding interactions, and Yoshida hydrogen-bonding interaction between the dye molecules and functional groups on the gel beads. In this study, polysaccharide gel beads (PS) extracted from dragon fruit stems were used as an adsorbent for the removal of methyl orange (MO) dye from aqueous solutions. The obtained PS were characterized using scanning electron microscopy, energy-dispersive spectrometry, Fourier-transform infrared spectroscopy, and Brunauer–Emmett–Teller analysis. Batch adsorption experiments were conducted to evaluate the effects of pH (2–10), contact time (20–360 min), adsorbent dosage (2–10 g/L), and initial MO concentration (10–60 mg/L). The results showed that equilibrium was reached after 120 min of contact. The maximum adsorption capacity of MO, calculated using the Langmuir isotherm model, was 11.99 mg/g, obtained at room temperature, pH 2, adsorbent dosage of 4 g/L, and an MO concentration of 50 mg/L. Adsorption kinetics followed the pseudo-second-order model (R2 > 0.99), and the Langmuir isotherm fitted the data well (R2 > 0.96). Thermodynamic analysis indicated that the adsorption process was slightly spontaneous, endothermic, and dominated by physical interactions. These findings demonstrate that PS is a promising, low-cost, and environmentally friendly adsorbent for the efficient removal of MO from wastewater.