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.