ANTIBIOTIC ADSORPTION TO NANOMETER-SIZE Fe2O3 AND TiO2

Pharmaceutical contamination of water is a growing area of research. The fate and transport of drugs, such as antibiotics, are controlled in part by their adsorption to nano-size particles in natural soil-water systems. To assess these interactions, batch experiments were done on TiO₂ and Fe₂O₃, with ampicillin (AMP), a common antibiotic used in human and animal medicine. AMP sorption to 50 nm-Fe₂O₃ (maghemite) at different pH was determined by HPLC analysis of equilibrium solutions. AMP sorbed to 25 nm-TiO₂ (anatase) was determined by LC/MS analysis of solutions.

AMP is amphoteric, with pKa₁ = 2.5, and pKa₂ = 7.3. Fe₂O₃ has a net positive surface charge at pH less than ~6.5 (ZPC ~ 6.5). AMP sorption to Fe₂O₃ at pH 2 and pH 6-8 can be described by linear isotherms (R² > 0.93) in the initial solution concentration range of 0.29 to 2.9 mmol/L. Maximum AMP sorbed ranges from 25-58 mmol/kg. At pH 2, the estimated K_d = 394 (+/- 43) L/kg. At pH 6, K_d = 551 (+/-37) L/kg, and at pH 8 K_d = 256 (+/- 37) L/kg. AMP sorption at pH 4 can be described by a Freundlich isotherm (Log K_f = 3.27 (+/- 0.19 ), n = 0.57 (+/- 0.15)), with a maximum drug sorbed of 48 mmol/kg.

Samples with adsorbed AMP were rinsed with AMP-free solutions of the same pH in 3 subsequent mixing experiments to remove weakly-attached molecules. 8% of the drug was removed from the Fe₂O₃ at pH 4, < 1% was removed at pH 6, and about 2% at pH 8.

AMP sorption to TiO₂ can be described by linear isotherms (R² >0.97) from pH 2 to 8, over initial liquid concentrations of 0.072-2.9 mmol/L. Maximum AMP sorbed varies from 94 to 114 mmol/kg. The ZPC of TiO₂ is 7.2. At pH 2 the K_d = 3028 (+/- 267) L/kg, and at pH 4 the K_d = 11,533 (+/- 823) L/kg. At pH 6, K_d = 12,712 (+/- 672) L/kg, and at pH 8, K_d = 1941 (+/- 342) L/kg.

TiO₂ samples with adsorbed AMP were also rinsed with AMP-free solutions of the same pH in 2 subsequent experiments to remove weakly-attached molecules. Only 2% of the drug was removed from the anatase surface at pH 2, <1% at pH 4 and 6, and approximately 1% was rinsed off at pH 8.

Observed AMP adsorption to nano-Fe₂O₃ and TiO₂ is highest in the pH 4-6 range, coincident with the dominant AMP species being zwitterionic, and the mineral surfaces being positively charged. Low yields in rinse experiments indicate strong attachment mechanisms, such as surface complexation. LC/MS evidence for AMP breakdown is also being investigated.