Adsorption Characteristics of Allura Red AC onto Sawdust and Hexadecylpyridinium Bromide-Treated Sawdust in Aqueous Solution

[Image: see text] The Allura red AC (ARAC) dye adsorption onto natural sawdust (NSD) and hexadecylpyridinium bromide-treated sawdust (MSD) was investigated in aqueous solution as a function of contact time, solution pH, particle size, adsorbent dosage, dye concentration, temperature, and ionic strength. The adsorbents were characterized by Fourier transform infrared spectroscopy and X-ray diffraction crystallography. The dye adsorption onto both adsorbents was confirmed by field emission scanning electron microscopy and energy-dispersive X-ray spectroscopy. The maximum dye adsorption was found within 120 min at pH 2.0 for NSD and pH 3.0 for MSD, respectively, with a particle size of 0–75 μm and an adsorbent dosage of 0.07 g/50 mL ARAC dye solution (50 μmol/L). The batch adsorption kinetic data were followed by the pseudo-second-order kinetic model rather than the pseudo-first-order and Elovich kinetic models. Equilibrium adsorption isotherms were explained by the Langmuir isotherm model, and the maximum extent of adsorption was found to be 52.14 μmol/g for NSD and 151.88 μmol/g for MSD at 55 °C. The values of activation energy (E(a)) and thermodynamic parameters (ΔG(⧧), ΔH(⧧), ΔS(⧧), ΔG°, ΔH° and ΔS°) proved that the ARAC dye adsorption onto both adsorbents NSD and MSD is a spontaneous-endothermic physisorption process. ARAC (98–99%) was released from dye-loaded adsorbents in aqueous solution (pH ≥ 12) within 120 min. The adsorbents NSD and MSD were reused for a second time without significant loss of their adsorption efficiency.