Hydrothermal synthesis of magnetic-biochar nanocomposite derived from avocado peel and its performance as an adsorbent for the removal of methylene blue from wastewater

In this work hydrothermally prepared magnetic iron oxide coated biochar nanocomposite (Fe₃O₄-BC), from raw avocado peel (raw AVP) and ferric chloride hexahydrate, effectively adsorbed aqueous methylene blue (MB) dye. Successful Fe₃O₄-BC formation was confirmed by TEM, FT-IR, XRD, and SEM. BET analyses measured a higher surface area (25.98 m²/g vs. 18.89 m²/g) and smaller-pore diameter (8.25 nm vs. 13.01 nm) for Fe₃O₄-BC compared to raw AVP. The Tempkin and Langmuir isotherm models successfully modeled the Fe₃O₄-BCMB adsorption equilibrium data, indicating uniform adsorption binding energy and homogeneous single layer adsorption. The maximum Langmuir adsorption capacity q_max = 62.1 mg/g for MB compared well with previously reported values for low cost carbonaceous adsorbents. The temporal experimental data was best represented by the pseudo-second order and two-phase pseudo-first-order kinetic models suggesting that chemisorption dominated. Thermal studies indicated spontaneous endothermic adsorption (ΔG < 0, ΔH > 0). Fe₃O₄-BCshowed remarkable stability and reusability during four consecutive adsorption–desorption cycles. Compared to literature, Fe₃O₄-BC adsorption was markedly faster requiring between 1 and 3 orders of magnitude less time to reach equilibrium. Consequently, a significantly lower treatment time would be required industrially which, coupled with magnetic separation, reusability, and relatively high adsorption capacity of the adsorbent, highlights the unprecedented industrial potential of Fe₃O₄-BC nanocomposite as an adsorbent for the treatment of MB polluted waters.