Preparation of biomass activated carbon and its adsorption performance for anionic/cationic dyes(10mg/L): A case study of coffee grounds, hemp stalks and macadamia nut shells

Biomass-derived activated carbons were prepared from coffee grounds (CHA-C), hemp stalks (HSA-C), and macadamia nut shells (MSA-C) via H₃PO₄ activation and pyrolysis. Adsorption tests with methylene blue (MB, cationic) and methyl orange (MO, anionic) followed a pseudo-second-order kinetic model and Langmuir isotherms. The Langmuir adsorption capacities for MB were 45.25 mg/g (MSA-C) > 44.46 mg/g (CHA-C) > 42.76 mg/g (HSA-C), while for MO they were 38.45 mg/g (HSA-C) > 31.61 mg/g (CHA-C) > 30.77 mg/g (MSA-C). MB uptake was mainly driven by electrostatic attraction and π–π interactions, whereas MO removal relied on pore filling and hydrogen bonding. These results confirm the feasibility of converting biomass wastes into effective carbon adsorbents and highlight the role of precursor type in dye removal [3] .This study conducted a comparative analysis of multiple precursors under uniform benchmark conditions, revealing the coupling relationship between "selective switching" of MB/MO and pore structure-surface chemistry. The study successfully integrated characterization, mechanism, and material selection into a closed-loop, reusable design framework. • Agricultural wastes (coffee grounds, hemp stalks, macadamia shells) were valorized into activated carbons via controlled H₃PO₄ activation and pyrolysis, yielding tailored micro–mesoporous structures.The prepared carbons exhibited high specific surface areas (≈900–1200 m²/g) and balanced pore hierarchies, enabling effective adsorption of both cationic (methylene blue) and anionic (methyl orange) dyes. • Adsorption followed pseudo-second-order kinetics and Langmuir isotherms, revealing dye-specific mechanisms: electrostatic/π–π interactions for methylene blue and pore filling/hydrogen bonding for methyl orange. • This work establishes a closed-loop design linking precursor type, activation chemistry, and adsorption performance, advancing sustainable wastewater treatment and biomass valorization.