Batch adsorptive removal of Cr (VI) from aqueous phase using magnetic activated carbon nanocomposite prepared from walnut shells by ZnCl ₂ -FeCl ₃ activation

Abstract

This work aims to assess the physicochemical and adsorptive ability of magnetic activated carbon nanocomposites (MACN) developed from walnut shells (WS) through single-step and dual-step preparation routes utilising ZnCl2-FeCl3 as an activator and magnetising agent for Cr (VI) elimination from the aqueous phase. Synthesis of MACN via a one-step route (MACN1) was achieved through direct activation of WS with ZnCl2-FeCl3. In contrast, the preparation of MACN by dual-steps route (MACN2) was accomplished by carbonising WS at 450°C for 1 h, followed by ZnCl2-FeCl3 activation of the resulting biochar. The N2-adsorption-desorption isotherms, X-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive X-ray (EDX), Transmittance Electron Microscope (TEM), Fourier Transform Infra-Red spectroscopy (FTIR), and Vibrating Sample Magnetization (VSM), were implemented to characterise the MACN samples. BET surface area of MACN1 was 1780.20 m2/g with an average pore diameter of 1.95 nm compared to 283.62 m2/g and 1.91 nm for MACN2. The VSM, FESEM, and TEM measurements ensured the presence of iron oxide nanoparticles on the surface of the composites. Investigating various operating parameters disclosed that the highest adsorptive elimination (AE) of Cr(VI) from its aqueous phase by MACN1 was 99.66% compared to 97.43% for that achieved using MACN2. The adsorption results were best fitted to the Langmuir isotherm using both composites with maximum adsorption capacity of 233.12 mg/g for MACN1 compared to147.55 mg/g for MACN2, suggesting that the adsorption is homogonous and chemical. Moreover, the pseudo-2nd-order kinetics model exhibited the best match for the adsorption of Cr(VI) by both adsorbents, demonstrating that chemisorption was involved in eliminating Cr(VI) from its aqueous phase. After five repetitions, the regeneration experiments confirmed that the MACN adsorbents maintained a high AE. Based on the findings, the as-prepared composites have extensive potential to be applied on the industrial scale.