Zinc Ions adsorption from aqueous solution using raw and acid-modified orange peels: Kinetics, Isotherm, Thermodynamics, and Adsorption mechanism

In this study, the potential application of raw orange peels (ROP) and treated with sulfuric acid (H2SO4) (TOP) was studied to remove Zn2+ from aqueous solution by varying parameters time, adsorbent dose, pH, particle size, and temperature. The adsorbents were characterized FTIR, SEM, XRD, and point of zero charge. Equilibrium time and optimum adsorbent dosage were found to be 120 min and 2.6 g/L solutions. Adsorption was highly pH-dependent and maximum removal occurred at a pH of 8. Adsorption percentage decreased with an increase in particle size. The pseudo-second-order model best fitted the kinetics adsorption study and boundary film diffusion is the rate-determining step (RDS) for ROP and both boundary and film diffusion is RDS for TOP adsorption. Isotherm study predicted a maximum adsorption capacity of 119.05 and 29.5 mg/g for ROP and TOP from the Langmuir plot, respectively, and the adsorption process was physisorption in nature. Thermodynamic parameters ΔG and ΔH indicated that adsorption process was spontaneous and exothermic in nature. Desorption studies showed that adsorption is by weak electrostatic attraction.