Predicting the speciation of ionizable antibiotic ciprofloxacin by biochars with varying carbonization degrees

Sorption mechanisms of ionizable organic pollutants by biochars and approaches for the prediction of sorption are still unclear. In this study, batch experiments were conducted to explore the sorption mechanisms of woodchip-derived biochars prepared at 200-700 °C (referred as WC200-WC700) for cationic, zwitterionic and anionic species of ciprofloxacin (referred as CIP⁺, CIP^± and CIP^-, respectively). The results revealed that the sorption affinity of WC200 for different CIP species was in the order of CIP^± > CIP⁺ > CIP^-, while that of WC300-WC700 remained the order of CIP⁺ > CIP^± > CIP^-. WC200 exhibited a strong sorption ability, which could be attributed to hydrogen bonding and electrostatic attraction with CIP⁺, electrostatic attraction with CIP^±, and charge-assisted hydrogen bonding with CIP^-. Pore filling and π-π interactions contributed to the sorption of WC300-WC700 for CIP⁺, CIP^± and CIP^-. Rising temperature facilitated CIP sorption to WC400 as verified by site energy distribution analysis. Proposed models including the proportion of the three CIP species and sorbent aromaticity index (H/C) can quantitatively predict CIP sorption to biochars with varying carbonization degrees. These findings are vital to elucidating the sorption behaviors of ionizable antibiotics to biochars and exploring potential sorbents for environmental remediation.