Influence of Hydrothermal Carbonization Parameters on Surface Properties and Methylene Blue Adsorption Performance of Rose Stem-Derived Hydrochar

This research investigated the effect of the synthesis conditions on the adsorption capacity of hydrochar (HC) derived from Rosa rubiginosa stems via hydrothermal carbonization. HC samples were prepared at 180, 200 and 220 °C using masses with three different raw-material-to-water ratios (i.e., 5, 7.5 and 10 g of biomass in 80 mL of water), obtaining yields between 43 and 57%. The point of zero charge (PZC) of all samples was determined, obtaining the highest value (5.36) for the sample prepared at 180 °C using 5 g of biomass: 80 mL water ratio (HC5_180). This sample exhibited a crystallinity index value (ID/IG, determined by Raman Spectroscopy) of 1.73, which is indicative of a higher degree of structural disorder, related to a greater presence of defects generated by surface functional groups, which, in turn, was identified by infrared spectroscopy. Adsorption tests were conducted using methylene blue (MB), achieving 76.3% removal at an initial concentration of 2.5 mg/L. Adsorption isotherms were also obtained for each HC type, which were fitted to the Langmuir and Freundlich models to elucidate the adsorption mechanism, while kinetic studies were performed on the HC5_180 sample. The findings demonstrate that HC adsorption capacity is governed by the distribution and type of surface functional groups. Thus, the valorization of floral industry by-products into biosorbents, as HC, promotes the circular economy, converting operational waste into a sustainable raw material with added commercial value.