Coupling experiments with calculations to understand the thermodynamics evolution for the sorption of zwitterionic ciprofloxacin on oxidizing-aged pyrogenic chars in the aquatic system

Abstract

Oxidized aging due to the long-term exposure can significantly alter the sorption of pyrogenic chars (i.e., biochar, BC) towards antibiotics, which determined their fates in natural environments. In this study, the sorption of ciprofloxacin (CIP) on the oxidizing-aged BCs was studied linking the experimental thermodynamics and theoretical calculations. Results revealed that Q0 of CIP negatively correlated with their average site energies (Em), while pore-normalized Q0 on aged BCs were 2–6 folds higher than fresh BCs. From competitive sorption, it is proposed that the transformation of CIP± to CIP+ occurred and the π+–π electron donor-acceptor interaction and Coulombic attraction onto the aged BCs played a critical role. These two specific interactions with CIP were thermodynamically improved when aging degree increased and favored the free energies (ΔaG) of sorption by 2–5 kJ mol−1. Based on the identified relationship between experimental ΔOA-ΔG0 with Ea through DFT calculations, the contributions of the specific interactions to antibiotic sorption on aged BCs were quantified. This study provided an in-depth understanding of how the aging process affects the sorption of zwitterionic antibiotics on BCs and also possibilities to predict the fate of antibiotics in the presence of BCs over a long-term period.