Nanoscale ZnO-adsorbent carefully designed for the kinetic and thermodynamic studies of Rhodamine B

Abstract

Pure ZnO-adsorbent nanoparticles (ZnO-NPs) was carefully designed via calcination of co-precipitated hydroxide of zinc at 500 °C, and used for the kinetic and thermodynamic studies of Rhodamine B (RB) dye adsorption from aqueous solution. The ZnO-NPs was characterized using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM) to determine the composition, crystallinity and morphology of the nanomaterial, while the bonds and functional groups were determined by Fourier-Transform Infrared Spectroscopy (FTIR) method. The adsorption of RB on ZnO-NPs was investigated at varied solution pH (2–10), concentration (10–60 mg/L), adsorbent mass (0.05–0.25 g), temperature (293–315 K), and contact time (5–240 min). The optimum adsorption was established at pH 6 in 150 min, while maximum adsorption capacity of RB on ZnO-NPs was 24.41 mg/g at pH 4 in 180 min using 50 mg/L RB at 313 K. The Langmuir isotherm model fitted well with regression coefficient, R2 = 0.9820. The n value in the Freundlich isotherm model was 1.974, indicating favorable adsorption, while the adsorption process followed pseudo-second-order kinetics with R2 = 0.9874. The change in Gibbs free energy (ΔGo) obtained was negative, signifying spontaneity of the adsorption process, while the positive values of ΔHo (+5.35 kJ/mol) and ΔSo (+0.02 kJ/mol) for RB adsorption on ZnO-NPs indicated endothermic adsorption and randomness at the ZnO-NPs-aqueous interface respectively. Therefore, it can be concluded that the nanoscale ZnO-adsorbent is efficient for the removal of RB from wastewater.