Magnetic biochar nanocomposites of coffee husk and khat (Catha edulis) leftover for removal of Cr (VI) from wastewater

Abstract

In this study, iron oxide biochar nanocomposites made from khat leftover (KL) and coffee husk (CH) are investigated as possibly useful adsorbents for the removal of Cr (VI) from water. Biochar-based iron oxide nanocomposites were synthesized by pretreating 25 g of biomass with a 1:1 M ratio of FeS to FeCl₃ and pyrolyzing at 300 °C for 1 h. Pristine biochar, synthesized through biomass pyrolysis of CH and KL at 300 °C, removed 74.98% and 84.78% of Cr (VI) from aqueous solutions containing 20 mg L⁻¹, respectively. The corresponding nanocomposites showed a maximum removal efficiency for Cr (VI) of 99.83% with the iron oxide-coffee husk biochar nanocomposite (Fe₃O₄–CHBNC) and 99.86% with the iron oxide-khat leftover biochar nanocomposite (Fe₃O₄-KLBNC). A pseudo-second-order model and the Langmuir isotherm are both well-fitted by the adsorption process, suggesting advantageous monolayer adsorption. The Fe₃O₄–CHBNC and Fe₃O₄-KLBNC demonstrated satisfactory removal efficiencies even up to six cycles, indicating their potential effectiveness for large-scale use for treating wastewater contaminated by Cr (VI).