Black bean pod-derived pyrochar for single and binary adsorption of Cu(ii) and direct red 79: adsorption performance, interfacial interactions and regeneration

The interplay between interfacial interactions and pore accessibility during binary adsorption of co-existing metal ions and anionic dyes on biomass-derived pyrochars remains insufficiently understood. Here, a pyrochar derived from black bean pods was investigated for Cu(ii) and Direct Red 79 (DR79) adsorption in single-solute and binary systems. Controlled pyrolysis (300-700 °C, 1-5 h) progressively altered the carbon structure and pore characteristics of the resulting pyrochars. BP7003, prepared at 700 °C for 3 h, exhibited a specific surface area of 517 m² g^-1, a substantial microporous contribution accompanied by mesopore-range features, and oxygen-containing surface functionalities. Cu(ii) showed rapid uptake, reaching a maximum experimentally determined equilibrium adsorption capacity of 117.13 ± 0.95 mg g^-1 at pH 5 under the investigated conditions. Its adsorption behaviour was consistent with contributions from surface electrostatics and oxygen-containing surface functionalities. DR79 exhibited a lower experimental equilibrium adsorption capacity of 66.05 ± 1.15 mg g^-1, with its adsorption behaviour suggesting a stronger influence of pore accessibility within the heterogeneous carbon structure. Kinetic and equilibrium analyses were consistent with heterogeneous adsorption environments for both solutes. Binary adsorption exhibited concentration-dependent behaviour under the investigated binary conditions, with small deviations from the corresponding single-solute reference systems across the tested concentration combinations. These observations are compatible with changes in the interfacial environment and competition for accessible adsorption domains, although a specific Cu-associated interaction mechanism remains unconfirmed. BP7003 retained most of its adsorption performance after four regeneration cycles, although a greater decline was observed for DR79. Overall, the results highlight the combined roles of surface chemistry, interfacial electrostatics, and pore accessibility in shaping Cu(ii)-DR79 co-adsorption across different loading regimes.