Kinetic, equilibrium, and thermodynamic studies of untreated watermelon peels for removal of copper(II) from aqueous solution

In this study, the watermelon peels (WP) were used as an adsorbent without any chemical or physical treatment for Cu2+ removal from the aqueous solution. Removal of Cu2+ has been evaluated with respect to initial metal concentration, contact time, pH, and temperature using batch-adsorption techniques. The maximum adsorption process was favorable at pH 5. As for the effects of concentration on WP, the maximum adsorption for Cu2+ has shown at 70 mg/L. Sorption equilibrium reach rapidly with 79.5% Cu2+ removal in 75 min. The adsorption isotherms could be fitted well by the Langmuir model with a maximum adsorption capacity of 357.14 mg/g. The R-L value in the present investigation was less than one, which indicates that the adsorption of the Cu2+ onto WP is favorable. Sorption energies analyses were conducted using Dubinin-Radushkevich (D-R) and Temkin isotherm models. It was found that the sorption energies of Cu2+ on WP obeyed D-R isotherm with R-2 = 0.998, mean adsorption energy, b = 0.0018 mol(2)/J(2), and mean free energy, E = 0.0167 kJ/mol which vividly proved that the adsorption process was physisorption in nature. The kinetics models fitted with pseudo-second-order suggesting that the adsorption might involve chemical sorption. The positive value of Gibbs free energy shows the reaction is non-spontaneous and the positive value of enthalpy shows the adsorption process to be exothermic in nature. Fourier transform infrared analysis showed that carboxyl and hydroxyl functional groups were involved in the adsorption of Cu2+. The present study showed that WP is an easy synthesis, economic, and an effective adsorbent for Cu2+ removal from aqueous solutions.