John Wiley & Sons, Ltd.

Complexes of the antimicrobial ciprofloxacin with soil, peat, and aquatic humic substances

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Natural organic matter (NOM) is implicated in the binding of antibiotics by particles in soils and waters. Our previous computational study revealed structural re‐arrangement of both hydrophilic and hydrophobic moieties of NOM to favor H‐bonding and other intermolecular interactions, as well as both competition with ion‐exchange reactions and bridging interactions by NOM‐bound divalent cations. We investigated the importance of these interactions using fluorescence‐quenching spectroscopy to study the adsorption of ciprofloxacin (Cipro), a fluoroquinolone antibiotic, on four reference humic substances (HS): Elliott soil humic acid (HA), Pahokee peat HA, and Suwannee river HA and fulvic acid. We developed a simple affinity‐spectrum HS model to characterize the cation‐exchange capacity and the amount of H‐bond donor moieties as a function of pH. Our Cipro adsorption results stress the influence of both pH conditions and the type of HS: both soil HA and peat HA exhibited up to 3 times higher sorption capacity than the aquatic HS at pH • 6; normalizing to the aromatic C content accounted for the differences among the terrestrial HS; and increasing the concentration of divalent cations led to a decrease in adsorption on aquatic HA but not on soil HA. And, our pH‐dependent speciation models of the Cipro‐HS complexes illustrate an increase in complexation due to an increase in deprotonation of HS ligands with increasing pH and, at circumneutral and alkaline pH, enhanced complexation of zwitterionic Cipro only in the presence of soil HA and peat HA. Our findings imply that, in addition to electrostatic interactions, van der Waals interactions as facilitated by aromatic structures and H‐bond donating moieties in terrestrial HS may facilitate a favorable binding environment. Environ. Toxicol. Chem. © 2013 SETAC

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