INTERACTION OF A FLUOROQUINOLONE ANTIBIOTIC WITH NANOMETER-SIZE SOIL OXIDE PARTICLES: ADSORPTION AND BREAKDOWN

Antibiotic contamination is a concern in soil-water systems because of the potential for promoting antibiotic resistance. The transport of drugs in these systems may involve nanometer (nm)-size particles, such that there is a need for data on both the adsorption of drugs to nanoparticle surfaces and the fate of drugs after adsorption. To assess these questions, batch mixing experiments were done with ofloxacin (OFX), a common veterinary antibiotic, and nm-size particles of Al₂O₃, SiO₂, Fe₂O₃, and TiO₂. Initial OFX concentrations ranged from 2.8E-6 to 6.9E-4 mol/L in solutions buffered at pH=6.2. Sorption was quantified by HPLC analysis and OFX breakdown products were analyzed by LC/MS.

OFX is an amphoteric fluoroquinolone (pKa₁ = 5.97, pKa₂ = 8.28). Al₂O₃ has a net (+) surface charge at pH < 8.7 (pzc), while the pzc values for the other particles are: SiO₂ < 2.8, Fe₂O₃ < 6.5-7.0, and TiO₂ < 7.2. OFX sorption to Al₂O₃ generates linear isotherms of mass sorbed/mass nano vs. liquid concentration (R²>.96). Maximum OFX sorbed was 16 mmol/kg, with a K_d of 441 L/kg (± 35). OFX sorption to SiO₂, TiO₂ and Fe₂O₃ is logarithmic, with the parameters: SiO₂ Ln K_f=3.0 (± .52), n=.57 (± .10); TiO₂. Ln K_f = 8.6 (±3.5), n= .82 (±.51); Fe₂O₃ Ln K_f = 5.5 (± 1.2), n=.89 (± .17). Maximum OFX sorbed was 10 mmol/kg to SiO₂, 370 mmol/kg to TiO₂, and 11 mmol/kg to Fe₂O₃. Substrates with sorbed OFX were rinsed with OFX-free solutions in successive desorption experiments to remove weakly attached molecules. Results show ~ 7% of the initially-adsorbed OFX could be removed from Al₂O₃, 89% from SiO₂, < 1% from TiO₂, and 27% from Fe₂O₃. Smaller percentages indicate stronger adsorption mechanisms.

Breakdown of zwitterionic OFX was observed in 80-hr timed mixing experiments with 25nm-TiO₂ when samples analyzed at intervals were compared to OFX controls. Decrease in OFX was accompanied by an increase in at least 2 product compounds. Mass spectral data suggest these breakdown reactions may involve the N-methyl, carbonyl, and carboxyl groups. Concentrations of the two products (m/z = 336-337 and m/z = 348-349) increase at a linear rate of ~ 1.5%/hr. Comparison of data from isotherm and timed experiments will quantify the amount of sorption vs. breakdown. Preliminary results from SiO₂, Fe₂O₃ and Al₂O₃ timed experiments indicate less breakdown of OFX compared to the effects of TiO₂.