Hydrochar production from co-hydrothermal carbonization of corn straw and Brewer's spent grain: Nitrogen behavior and mechanisms of Cd2+ adsorption for environmental remediation

This study investigated the co-hydrothermal carbonization (co-HTC) of corn straw (CS) and brewer's spent grain (BSG) to produce nitrogen-rich hydrochars for enhanced Cd 2+ immobilization. Experiments were conducted at temperatures of 180-260 °C and residence times of 2-6 h. Results show that increasing reaction severity leads to the enrichment of carbon and nitrogen, reaching maximum contents of 59.59% and 2.72%, respectively, at 260 °C for 4 h. The peak specific surface area (5.96 m 2 /g) was achieved under moderate conditions (220 °C, 2 h). Mechanistically, XPS and FTIR characterizations confirm that co-HTC facilitates the transformation of proteinaceous nitrogen into stable aromatic heterocycles, including pyridinic-N, pyrrolic-N, and quaternary-N. These nitrogen-containing functional groups serve as crucial active sites, enhancing Cd 2+ binding via surface complexation and ion exchange. Adsorption analysis indicates that the process follows the Langmuir model (R 2  = 0.9468-0.9902) and pseudo-second-order kinetics (R 2  = 0.9994), signifying a monolayer chemisorption mechanism. The hydrochar produced at 220 °C for 6 h exhibited the highest adsorption capacity of 7.63 mg/g. This research provides a theoretical foundation for tailoring nitrogen functionalities in hydrochars to optimize heavy metal remediation in contaminated soils.