Preparation and characterization of Camellia oleifera nut shell-based bioadsorbent and its application for heavy metals removal

As a renewable agricultural solid waste, Camellia oleifera nut shell (CONS) is often discarded or burned, causing adverse environmental impact and a waste of resources. The purpose of this work was to develop a CONS-based bioadsorbent for the removal of heavy metals. Both CONS and ethanol/NaOH-modified CONS (MCONS) were prepared. The specimens were characterized using physiochemical composition, Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX). The effects of pH, initial metal concentration, adsorbent dosage, adsorption time, and temperature on the Cr(VI) and Cu(II) removal were evaluated. The adsorption kinetics, isotherms, and thermodynamics were determined. The MCONS sample had a higher carboxyl group content and surface area than the CONS sample, which helped explain its enhanced adsorption performance of heavy metals. The maximum uptake capacity of Cr(VI) and Cu(II) was 16.39 mg/g and 27.26 mg/g for MCONS, compared with 6.34 mg/g and 9.89 mg/g for CONS. The adsorption kinetics for CONS and MCONS fit well with the pseudo-second-order kinetic model. The adsorption isotherms fit well to the Langmuir model. The thermodynamic analyses revealed that the adsorption process was spontaneous and exothermic.