Sequential double chemical activation of biochar enables the fast and high-capacity capture of tetracycline†

The daunting challenge in exploring biochar-based adsorbents to realize both fast adsorption kinetics and high capacity calls for the development of effective approaches for biochar functionalization and activation. Hereby, we present a two-step sequential melamine functionalization and KOH etching of biochar derived from dead-leaf waste biomass. When appropriate melamine functionalization is implemented, violent gas evolution occurs in the 2nd-step KOH activation via etching the C–OH and CO to yield C–O–C sites while chemically reducing −NO_x to other N configurations. The 1st-step melamine functionalization plays a critical role in deepening the KOH-based activation, bringing more N/O-containing groups, enhancing porosity, and boosting the adsorption kinetics and capacity for tetracycline (TC) removal. The optimal sample exhibits a specific surface area (SSA) and pore volume of 1995.03 m² g⁻¹ and 1.190 cm³ g⁻¹, respectively, much superior to the counterpart with only one-step KOH activation (1275.34 m² g⁻¹ and 0.621 cm³ g⁻¹). Occurring over a homogeneous, melamine/KOH-coactivated biochar surface, the adsorption process is found to be driven by both physisorption and chemosorption in a monolayer manner. The prominent SSA and enriched N/O imparted via the double chemical activation render the adsorption kinetics rather fast, with only 30 min required to reach equilibrium. Meanwhile, a superior maximum monolayer adsorption capacity of 433.74 mg g⁻¹ is realized. The adsorbent is also demonstrated to be recyclable and reusable through five cycles of repeated usage. The adsorption process is disclosed to be spontaneous in nature, while TC concentration dictates whether the adsorption is exothermic/entropy-reducing or endothermic/entropy-gaining, with lower TC concentrations leading to the former. Furthermore, we find that hydrogen bonding interactions are the critical driving force for the uptake of TC over the biochar prepared by the double chemical activation. This work sheds light on the exploration of double chemical activation to engineer the architecture and surface functionalities of biochar materials simultaneously for environmental remediation and beyond.