Molecular sorption binding energy and spatial site occupancy determine the competitive sorption of antibiotic clusters on graphene

The formation of hydrophobic organic clusters and their impacts on sorption have been widely reported, the clustering of hydrophilic organics has generally been overlooked. However, hydrophilic organic clusters can theoretically form and play a significant role in sorption. This study explored the formation of sulfonamide antibiotic clusters (hydrophilic organics) and their competitive sorption on graphene using experimental and theoretical approaches. Three selected sulfonamides, sulfamethazine (SMT), sulfamethoxazole (SMX), and sulfamethizole (SMZ), can form clusters due to stronger interactions among themselves than with H 2 O, being adsorbed on graphene as clusters rather than as single molecules. The multi-stage sorption process was observed as a result of graphene aggregation, which could be eliminated by controlling the graphene concentration (1 mg/mL). SMZ exhibited higher equilibrium solid-phase concentrations (qe) than SMT and SMX in both single-component and double-component systems. Additionally, SMT exhibited lower reduction rates of q e (22 % and 24 %) compared to the competitive components (36 % and 30 %) in double-component systems. The interaction between clusters and graphene led to their dissociation. The stronger sorption binding energy of SMZ (−2.52 eV) promoted interaction with graphene, both for clusters and single molecules, compared to SMX (−2.44 eV) and SMT (−2.38 eV). SMZ displaced SMX and SMT on graphene, leading to higher apparent sorption. Meanwhile, the lower spatial site occupancy for SMT contributed to its stronger competitive strength compared to the other components. This study emphasized the formation of antibiotic clusters and elucidated their competitive sorption processes on graphene, advancing our understanding of their environmental behavior and risks.