Adsorption of dye through hydrochar derived from co-hydrothermal carbonization of garden waste and sewage sludge: The adsorption enhancement mechanism of lignin component

In this study, the adsorption performance and behavior for dyes by hydrochar derived from sewage sludge (SS) and lignin-containing waste (garden waste, GW) were investigated. The results indicated that the adsorption capacity for methylene blue (MB) increased as the proportion of GW increased, reaching up to 52.39 mg/g under the mixing ratio of GW and SS 5:5 (5GW:5SS-HC), which represented an enhancement of 22.64 % compared with no addition of GW. The adsorption of 5GW:5SS-HC for MB was significantly influenced by the environmental pH, with enhanced adsorption observed in alkaline conditions due to strengthened electrostatic interactions. Adsorption kinetics and thermodynamics indicated that the adsorption for MB occurred on a heterogeneous surface and was primarily a chemical adsorption process. While the addition of GW did not significantly alter the specific surface area or pore structure of the hydrochar, it increased the presence of hydroxyl and other oxygen-containing functional groups, as well as aromatic ring structures. It strengthened the hydrogen bonding and π-π interactions thereby facilitating the adsorption of MB on the surface of hydrochar. Therefore, co-hydrothermal carbonization of GW and SS can produce hydrochar with excellent adsorption performance for MB, providing a novel technological approach for the resource utilization of SS and new insights into the role of lignin component in promoting the adsorption of hydrochars.