Characterization and adsorption capacity of four low-cost adsorbents based on coconut, almond, walnut, and peanut shells for copper removal

Agricultural wastes (AWs) are available abundantly at no or low costs; however, in most cases, not used reasonably. Despite their interesting chemical properties, coconut shells (CS), almond shells (AS), walnut shells (WS), and peanut shells (PS) are usually burned in the fields or discharged without any valorization. These AWs were investigated as low-cost bio-adsorbents to remove copper ions (Cu2+) from aqueous solutions. The four adsorbents were characterized using X-ray diffraction (XRD), the Fourier transform infrared spectra (FTIR), nitrogen adsorption/desorption measurements, scanning electron microscopy, and energy-dispersion X-ray spectroscopy (SEM-EDS). Characterization results revealed that the materials under investigation had porous surfaces, rich in fibers, and several potential adsorption sites. Therefore, their adsorption capacity for Cu2+ removal was evaluated under different operating conditions. Results showed that the CS had the best adsorption capacity among tested AWs. Under optimized parameters, the highest adsorption capacity was found 25, 18, 10, and 5 mg/g for WS, CS, PS, and AS, respectively. The adsorption of Cu2+ on the four adsorbents followed the second-order rate equation and the Langmuir adsorption isotherm model. After the adsorption process, the characterization of studied materials revealed no structural changes, proving the physical adsorption of Cu2+ on shells through long-range interactions between Cu2+ and reactive sites of adsorbents. The high adsorption capacity of the selected adsorbent was attributed to the presence of high content of cellulose compared to lignin.