Cork-derived magnetic composites: a preliminary study†

Materials based on graphitic carbon are used for environmental remediation, due to their high surface area and their capacity to adsorb pollutants in liquid environments. Carbonaceous materials derived from residues are particularly interesting, as their synthesis has a smaller impact on the environment. In the present work, we report a preliminary study on the preparation of graphitic carbon made from cork waste powder modified with magnetic nanoparticles (MNPs). This is the first time such composites were prepared using pyrolysed/carbonised cork, from a powder residue of cork stopper production. This makes the process sustainable and in line with the circular economy. The composites were prepared by vacuum infiltration of the MNPs on pyrolysed cork powder, with a successive thermal treatment, resulting in a carbon material that retained the porous microstructure of the original cork, ideal for the absorption of pollutants or separation of oils and water, while also being magnetically separable afterwards. It was seen that post-infiltration heating was better in air than under nitrogen, with the nitrogen atmosphere and presence of highly porous carbon possibly partially reducing magnetite to FeO, with a reduction in magnetic properties. MNPs with different chemical compositions were tested – zinc ferrite (ZnFe₂O₄) and magnetite (Fe₃O₄) – with the magnetite composites showing the highest magnetisation. Moreover, magnetite particles of different dimensions were considered: 6, 9 and 15 nm; results indicated that the 9 nm magnetite NPs were the most easily infiltrated; the magnetisation, however, was higher for the composites with the 15 nm magnetite NPs (about 9 emu g⁻¹), despite the oxide component comprising only around 12 wt% of the composite, due to their greater initial magnetisation. This value is higher than those of similar composites prepared using carbon from other natural sources. SEM analysis showed the presence of MNPs on the surface of the material, with the particles being on the nanometric scale and showing no aggregation on the micron scale. Composites prepared with these 15 nm MNPs also showed greater stability in both water and an organic solvent (chloroform) and were demonstrated to be magnetically separable from suspensions, making them the most suitable for environmental remediation applications.