Utilization of low-cost watermelon rind for efficient removal of Cd(II) from aqueous solutions: Adsorption performance and mechanism elucidation

Abstract

Recycling of agricultural waste as adsorbents for pollutant removal realizes “Waste to Resource”. This study utilized an abundant fruit waste material, watermelon rind (WR), to remove Cd(II) from aqueous solutions (Cd(II) solution in distilled deionized water) in a batch mode, and the underlying adsorption mechanism was deciphered by complementary methods. The Cd(II) adsorption process rapidly reached equilibrium in 60 min, with 90% of adsorption finished in 10 min (50 mg/L Cd(II), 0.5 g/L WR). The maximum Cd(II) adsorption capacity of WR was 102.0 ± 0.1 mg/g at pH 7.0 (25–500 mg/L Cd(II), 1.0 g/L WR), and the Dubinin-Radushkevich isotherm and pseudo-second-order kinetic models provided the best description to the adsorption process. Spectroscopic analysis using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) showed hydroxyl, carboxyl, ether, and amine groups as the binding sites, probably via complexation. Physicochemical analysis such as zeta potential measurement and scanning electron microscopy coupled with electron dispersive X-ray spectroscopy (SEM-EDS) revealed the occurrence of electrostatic interaction and microprecipitation. Moreover, ion exchange of Cd(II) with Mg2+and Ca2+ bound to WR contributed to 83% of the Cd(II) adsorption. The findings indicate the great potential of WR for Cd(II) removal from wastewater. The mechanism elucidation uncovered the interactions between Cd(II) and the WR surface, providing insights to the development and optimization of biosorbents derived from agricultural waste.