Hierarchical factors governing the removal of ionizable pharmaceuticals by cassava peel–derived biochar: Bayesian modeling and DFT approaches

The problem of ciprofloxacin (CFX) pollution in the aquatic environment has attracted widespread attention. Finding low-cost, efficient separation materials and studying their sorption behavior remain important. In this work, cassava peel–derived biochar (CP) was produced for CFX removal, and sorption behavior was analyzed using Bayesian inference and DFT methods. The effects of pyrolysis conditions (500 and 700°C), pH (3, 7, 10), and temperature (20, 26, 32°C) were statistically investigated. Results showed that increasing the pyrolysis temperature enhanced the specific surface area from 29.45 m² g⁻¹ (CP500) to 278.43 m² g⁻¹ (CP700). Kinetic sorption was described by the two-stage intraparticle diffusion model, with first-stage diffusion rates of 1.31 and 1.67 mmol kg⁻¹ min ⁻½ for CP500 and CP700, respectively. Maximum adsorption capacity ( Q ma x ) based on the Langmuir model was primarily affected by pH, following the order: pH 7 (11.6–12.3 mmol kg⁻¹) > pH 3 (8.6–10.0 mmol kg⁻¹) > pH 10 (4.6 mmol kg⁻¹). Moreover, biochar type affected binding affinity ( K L ), with the posterior distribution of K L consistently higher for CP500 compared to CP700. At pH 7, the sorption was attributed to hydrogen bonding (CP500) and π–π EDA interactions (CP700). The strong electron-accepting properties of CFX facilitated π–π EDA interactions with graphitic carbon. Thermodynamic analysis confirmed that sorption was spontaneous at all conditions. Overall, sorption was governed hierarchically by pH, biochar type, and temperature, yet both biochar contributed effectively to CFX removal. Future studies should evaluate sorption in complex water matrices to enhance real-world applicability.