Enhancing wastewater treatment: a study on steam explosion-biochar derived from Chinese herbal medicine residue for NOR adsorption

Steam explosion technology is gaining attention as an economical, efficient, and eco-friendly industrial pretreatment method. This study used Astragali Radix (AR) residue, activated by steam explosion, to create biochar for removing norfloxacin (NOR) from water. The prepared biochar was analyzed using various techniques, revealing ASB4's rich microporous and mesoporous structure with a surface area of 512 m² g^-1. Kinetic studies showed rapid NOR adsorption best described by the PSO model, with a maximum capacity of 345.7 mg g^-1 at 298 K, according to Langmuir models. Thermodynamic analysis confirmed that the adsorption was exothermic and spontaneous. The adsorption process is mainly driven by pore filling, followed by hydrogen bonding (13.4%), and electrostatic interactions (7.5%), with π-π interactions (3.6%) being the least significant. Moreover, the utilization of response surface modeling (RSM) facilitated the optimization of the adsorption capacity of NOR, yielding a maximum removal capacity of 324 mg g^-1 at 298 K, an initial concentration of 200 mg L^-1, and a pH of 7.83. Economically, the biochar material offers cost benefits with a payback of 1.87 $ per kg. ASB4 demonstrated excellent stability and reusability, retaining a high NOR removal rate even after five cycles. This study highlights the potential of converting AR residues into activated carbons, aligning with circular economy principles and improving wastewater treatment efficiency.