PHARMACEUTICAL TRANSPORT IN THE ENVIRONMENT: INTERACTIONS BETWEEN TETRACYCLINE ANTIBIOTIC AND NANOPARTICLE METAL OXIDES IN AQUEOUS SOLUTIONS

Widespread use of pharmaceuticals and their subsequent detection in natural waters is a growing concern. The fate and transport of drug contaminants likely involves nanometer-size (10^-9 m) oxides. This study investigated the interaction of tetracycline (TC), a common polyprotic human and veterinary antibiotic, with several metal oxide nanoparticles (NPs). Batch experiments were performed using 25-50 mg of four NPs (Al₂O₃, Fe₂O₃, SiO₂, and TiO₂) and varying concentrations of TC solutions (0.44 - 444 μM) over a range of pH (2.3 - 8.7). Equilibrium aqueous concentrations were determined by HPLC and removal of TC from solution was quantified by comparison to control samples.

Data indicate that the amount of TC removed from solution (C* = μmol TC removed/mass of NP) is due to adsorption and/or NP-facilitated transformation. Experiments with TC-SiO₂ mixes show that electrostatic attractions are likely controlling the fate of TC. The relationship between the pH-dependent surface charge (point of zero charge = pzc) of SiO₂ and speciation of TC yields strong attraction at pH 5 (C* ~35,000 μmol/kg) (all C* values reported are for initial TC concentrations of 111 μM), where the drug is zwitterionic and the substrate surface is (-). At pH 8.7 TC is (1^-) and SiO₂ is (-), and C* ~9,000 μmol/kg. At pH 2.3, SiO₂ is (+) and TC is (1⁺), and the attraction is comparatively low (C* ~1,700 μmol/kg). Experiments with Fe₂O₃ and TiO₂ indicate that electrostatic interactions are not predictable, and possibly not the primary mechanism for TC removal. In mixes with Fe₂O₃, removal of TC was completely removed at pH 5 (substrate⁺/zwitterionic drug). Greater removal occurred at pH 2 (C* ~ 4400 μmol/kg) (substrate⁺/drug⁺) than at pH 8.7 (C* ~ 2900 μmol/kg) (substrate^-/drug^-). TiO₂ removed all TC from solutions through 111 μM, at all pH conditions investigated. Solubility of TC limited higher concentration (155 – 444 μM) experiments to pH 2.3, where data were fit with Freundlich isotherms. Removal coefficients illustrate that TiO₂ removed the most TC (K_r=9587 L/kg), followed by Fe₂O₃ (K_r =2278 L/kg) and, SiO₂ (K_r =172 L/kg). Because Al₂O₃ increased solution pH, data were considered separately, and TC removal showed an approximate correlation with electrostatic trends. TC was completely removed at pH 5 and pH 8.7. At pH 2.3, TC removal by Al₂O₃ was C* ~ 6,500 µmol/kg .