Kinetic and equilibrium modeling of low-cost adsorbent of untreated watermelon peel for adsorption of zinc(II)

Watermelon peel is a bio-waste, lower cost, readily available, environment-friendly, and has high adsorption capacities. This study investigates the feasibility of dried watermelon peel (WP) as a low-cost adsorbent for removing Zn2+ from the aqueous solution. Batch experiments were conducted at different pH, initial concentrations, and contact times to evaluate dried watermelon peels’ maximum adsorption capacity. The removal of Zn2+ was increased gradually with increasing pH, with the maximum removal observed at pH 5.0. Langmuir’s model isotherm can well explain the equilibrium data for Zn2+ adsorption (R2 = 0.9812). The monolayer adsorption capacity was found to be 20.45 mg/g at an optimum contact time of 1 h. Sorption energy analyses were conducted using Temkin and Dubinin–Radushkevich model isotherm. It was depicted that the sorption energies of Zn2+ on WP fitted well with the Dubinin–Radushkevich isotherm with R2 = 0.918, indicating that the adsorption process is probably physical in nature. The adsorption kinetics shows that the pseudo-second-order equation could best explain the removal of Zn2+. This study shows that the WP can potentially be an alternative low-cost adsorbent to remove Zn2+ from an aqueous solution.