Goethite-Bound Copper Controls the Fate of Antibiotics in Aquatic Environments

Ciprofloxacin (CIP), a commonly used antibiotic, is today found in natural waterways and terrestrial environments alongside trace heavy metal contaminants. CIP however has a weak affinity for iron (oxy)­hydroxide minerals, which often control contaminant transport in nature. This weak affinity is caused by the electrostatic repulsion between positively charged mineral surfaces and the CIP piperazine ring. Using goethite (α-FeOOH), a representative iron (oxy)­hydroxide nanomineral, we show that the presence of Cu­(II) greatly enhances CIP adsorption while at the same time catalyzes CIP oxidation to byproducts, which are new to nature. The CIP uptake was greatest at circumneutral pH and in saline conditions, where Cu­(II), CIP, and mineral surface charges were the least repulsive. Vibrational spectroscopy and molecular simulations revealed that the enhanced uptake of CIP was caused by the the coordination of metal-bonded Cu­(II)–CIP surface complexes on goethite. The inner-sphere Cu­(II)–CIP complex also facilitated CIP oxidation into a series of new products, which we identified by mass spectrometry. Finally, to predict Cu­(II) and quinolone loadings prior to redox-driven reactions, we propose a multisite surface complexation model using Cu­(II)–CIP ternary surface complexes, alongside an ion pair to account for the ionic strength dependence on loadings. The information developed in this work will help tracking the fate of CIP in contaminated aquatic environments.