Development of biochar adsorbents based on walnut shell with magnetic modification for efficient crude oil removal from water

Protecting water from oil spills remains one of the most pressing environmental challenges today. To address this issue, two types of biochar oil adsorbents were developed from walnut shell agricultural waste. The biochar adsorbent was synthesized via carbonization, in which walnut shells were carbonized in a tubular furnace under an argon atmosphere at a rate of 10 °C/min up to 600 °C and held for 1 h. The magnetic composite biochar adsorbent was prepared by introducing magnetite through a co-precipitation method before carbonization. The materials were characterized using thermogravimetric analysis, scanning electron microscopy, energy-dispersive spectroscopy, Brunauer-Emmett-Teller surface area and Barrett-Joyner-Halenda pore size distribution analysis, X-ray diffraction, Fourier transform infrared spectroscopy, and wettability test. These analyses confirmed the formation of uniformly rough surfaces and the successful incorporation of magnetite. Comparative adsorption experiments revealed that the magnetic composite bio-carbon exhibited a higher oil adsorption capacity (6.11 g/g) than the carbonized walnut shell bio-carbon (5.54 g/g), with a removal efficiency of up to 90.0%. Adsorption kinetics followed the pseudo-second-order model, indicating that chemisorption was the dominant mechanism, while the isotherm data were well described by the Langmuir model, suggesting monolayer adsorption on a relatively uniform surface. These findings highlight the potential of walnut shell-derived bio-carbon adsorbents as effective and sustainable materials for oil spill remediation.