Hydrochar Loaded with Nitrogen-Containing Functional Groups for Versatile Removal of Cationic and Anionic Dyes and Aqueous Heavy Metals

Developing novel sorbents for effective removal of heavy metals and organic dyes from industrial wastewater remains a central theme for water research. We modified hydrochar derived from the hydrothermal carbonization of wheat straw at 180 °C with 3-Aminopropyl triethoxysilane (APTES) to enhance its versatile adsorption of Pb(II), Cu(II), methylene blue (MB), and reactive red (Red). Pristine and modified hydrochar (HyC and APTES-HyC) were characterized and tested for sorption performance. Characterization results revealed an enriched presence of N-functional groups, mainly -NH₂ and C-N, on APTES-HyC, in addition to an increased specific surface area from 1.14 m²/g (HyC) to 4.51 m²/g. APTES-HyC exhibited a faster adsorption rate than HyC, reaching equilibrium approximately 4 h after initiation. The Langmuir adsorption capacities of APTES-HyC were 49.6, 14.8, 31.7, and 18.3 mg/g for Pb(II), Cu(II), MB, and Red, respectively, about 8.5, 5.0, 1.3, and 9.5 times higher than for HyC. The enhanced adsorption performance of APTES-HyC is attributed to the increased N-functional groups, which facilitated adsorption mechanisms specific to the pollutant of concern such as formation of frustrated Lewis pairs and cation-π interactions for metal ions and π-π interactions and hydrogen bond for dyes. This study offers a novel and facile approach to the synthesis of N-doped carbon materials for practical applications.