Molecular Mechanisms of Oxytetracycline and Ciprofloxacin Sorption On Nano-Magnetite.

Nano-magnetite (nano-Fe₃O₄), which has been reported to form on the outer surface of nano-iron, is an important nano-iron species in remediation science. Recent studies demonstrated the potential of nano-Fe₃O₄ to become a good sorbent for certain antibiotics.  However, molecular level investigations of the interaction mechanisms of antibiotics with nano-Fe₃O₄ are lacking. To probe the interaction mechanisms of antibiotics with nano-Fe₃O₄ at the molecular level, we used two commonly prescribed antibiotics, one from the tetracycline group: oxytetracycline (OTC), and another from the fluoroquinolone group: ciprofloxacin (CIP).  Wet chemical, spectroscopic, and surface complexation modeling (SCM) approaches were employed to understand the interaction mechanisms.  Batch Sorption experiments with OTC and CIP and nano-Fe₃O₄ were performed as a function of different solution properties such as pH (~3-10), ionic strength (~0.01-0.5), and initial antibiotic concentrations (0.01-1 mM).  In situ ATR-FTIR experiments were conducted using 5-25 µM OTC/CIP at pH range of 3-9.  Results indicated high affinity of nano-Fe₃O₄ towards both OTC and CIP.  Sorption envelope indicated that retention of OTC and CIP on nano-Fe₃O₄ was highly pH-dependent.  FTIR data showed that amide (-CONH₂), dimethyl amino (-N(CH₃)₂), and phenolic-OH functional groups of OTC participated in the sorption reaction.  For CIP, main functional groups that interacted with nano-Fe₃O₄ surfaces are carboxyl (-COOH) and keto (>C=O).  While no surface complexes could be identified for OTC, a bridging-bidentate surface complex was proposed for CIP.  Analysis of ATR-FTIR data revealed that interactions of OTC and CIP with nano-Fe₃O₄ were inner-sphere.  Our research indicates that nano-Fe₃O₄ has the potential to become a remediation material for antibiotics.