Hydrochar magnetic adsorbent derived from Chinese medicine industry waste via one-step hydrothermal route: Mechanism analyses of magnetism and adsorption

The annual production of Chinese medicine industry waste (CMIW) is extremely large, and their improper disposal can lead to resource wastage and environmental pollution. CMIW is rich in lignocellulose, which can be converted into a porous carbon-based adsorbent for pollutant removal. In this study, hydrochar magnetic adsorbents were prepared from CMIW via hydrothermal carbonization (HTC) without adding magnetic substances. The mechanism of magnetic formation and adsorption properties was investigated. The results showed that the saturation magnetization of hydrochar was significantly higher than that of CMIW. This hydrochar could be easily separated and recycled from mixed solution by external magnetic fields. The existence of iron in CMIW was the root of magnetism formation. The Fe3+ in CMIW was reduced to Fe3+/Fe2+ in hydrochar during HTC process. XRD spectra elucidated the presence of Fe3O4 phase in hydrochar. XPS spectra exhibited the lattice oxygen, Fe2p3/2 and Fe2p1/2 and SEM showed the granular aggregates were on the surface of hydrochar, which indicated the presence of Fe3O4. The formation of Fe3O4 caused hydrochar to exhibit strong magnetism. HTC230-3 hydrochar exhibited both the strongest magnetism and the highest methylene blue (MB) removal rate at an adsorption temperature of 30 °C. The surface structure of hydrochar was rough and porous, resulting in an abundance of adsorption sites and a high adsorption capacity. The adsorption of MB on CMIW hydrochar was not only attributed to the physical adsorption caused by micropores and mesopores but also mainly related to abundant oxygen-containing functional groups. The adsorption of MB was in accordance with the pseudo-second-order kinetics and Freundlich models. This work provided an innovative insight for high value and efficient utilization of CMIW, achieving the goal of environmentally friendly and treating waste with waste.