AMPICILLIN ADSORPTION TO NANOMETER-SIZE Al2O3 IN WATER: MACROSCOPIC EXPERIMENTS AND MECHANISTIC INVESTIGATIONS

A current research focus is contamination of soil-water systems by veterinary and human pharmaceuticals. Interest is also growing in the interactions between these drugs and nanometer-size particles in water. Batch adsorption experiments were performed with ampicillin (AMP), a common antibiotic, and 50 nm-Al₂O₃ to assess the effect of nanoparticles on the fate and transport of drugs in water. Sorption was determined by LC/MS analysis of equilibrium solutions. AMP has two dissociation centers (pKa₁ = 2.5, pKa₂ = 7.3), and Al₂O₃ surface charge is positive over the range of pH investigated. Also, a preliminary investigation of attachment mechanisms was done by a multiple rinsing technique and with Raman spectroscopy and powder X-ray diffraction (XRD).

AMP sorption can be described by linear isotherms (R² = 0.96-0.99) in the initial solution concentration range of 2.9 µmol/L to 2.9 mmol/L. Mass sorbed at pH 2 ranges from 0.203-27.8 mmol/kg with an estimated K_d of 11.08 (+/- 0.32) L/kg. At pH 4-8, maximum AMP sorbed ranges from 1.5-46 mmol/kg. Distribution coefficients are: K_d = 0.55 (+/- .04) L/kg at pH 4; K_d = 21.9 (+/-.85) L/kg at pH 6; and, K_d = 39.5 (+/- 2.2) L/kg at pH 8. Samples with adsorbed antibiotic were rinsed with AMP-free solutions of the same pH in three subsequent mixing experiments in an attempt remove weakly-attached molecules. At pH 2, approximately 47 % of the initially adsorbed drug was removed, and at pH 4 virtually 100% was removed. Only 7% of the drug could be removed at pH 6, and 3% at pH 8. Apparently, weak electrostatic forces dominate at pH< 4, but at higher pH the drug is adsorbed by stronger attachment mechanisms, such as surface complexation.

Paste from experiments at pH 8 was examined with micro-Raman spectroscopy. Results were compared to spectra collected from AMP precipitates and mechanical AMP-Al₂O₃ mixtures. In comparison, the paste analyses showed systematic shifts for wavenumbers (cm^-1) associated with CH₃-group vibrational modes. Dried portions of the same samples (pastes, precipitates and mixes) were also analyzed with XRD. Diffraction patterns showed a decreased intensity or absence of the (420) and (123) peaks for AMP in the experimental sample. Correlation between XRD and crystal chemistry is underway to corroborate Raman data that suggest a methyl-group role in ampicillin adsorption to Al₂O₃.