Turning waste into adsorbent: Modification of discarded orange peel for highly efficient removal of Cd(II) from aqueous solution

Abstract

The recycling and conversion of discarded pericarp into useful materials are tally with the sustainable development concept. In this work, raw orange peel (OP) was pretreated by alkaline hydrolysis and further decorated by mercaptoacetic acid (MA) to obtain potential adsorbents for capturing toxic Cd(II) from aqueous solutions. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), zeta potential, and N2-BET analyses confirmed the successful preparation of modified OP materials with negatively charged surfaces, decreased specific surface area (SSA), and enlarged pore diameter. The sorption kinetics process of Cd(II) on the modified OP materials was ultrafast and a near 100% removal was achieved after 10 min. The OP-OH and MA@OP-OH composites exhibited higher Cd(II) sorption ratios than raw OP due to the elimination of inert constituents and the introduction of active surface sites. In addition, the sorption of Cd(II) was not interfered with by most of the common aqueous components. In desorption experiments with EDTA-2Na as the extractant, the lower Cd(II) desorption ratio from MA@OP-OH (72%) than those from OP-OH (83%) and OP (87%) provided indirect evidence for tight Cd-sulfhydryl complexation on MA@OP-OH surfaces. The underlying Cd(II) removal mechanism was proved to be dominated by inner-sphere complexation. The experimental findings demonstrated the feasibility of synthesizing and applying modified OP materials for the efficient purification of Cd(II)-bearing wastewater.