Sorption of the antibiotic ofloxacin to mesoporous and nonporous alumina and silica

Mesoporous and nonporous SiO 2 and Al 2O 3 adsorbents were reacted with the fluoroquinolone carboxylic acid ofloxacin over a range of pH values (2–10) and initial concentrations (0.03–8 mM) to investigate the effects of adsorbent type and intraparticle mesopores on adsorption/desorption. Maximum ofloxacin adsorption to SiO 2 surfaces occurs slightly below the p K a 2 (pH 8.28) of the antibiotic and sorption diminishes rapidly at pH > p K a 2 . For Al 2O 3, maximum sorption is observed at pH values slightly higher than the adsorbent's point of zero net charge (p.z.n.c.) and less than midway between the p K a values of ofloxacin. The effects of pH on adsorption and ATR–FTIR spectra suggest that the zwitterionic compound adsorbs to SiO 2 solids through the protonated N 4 in the piperazinyl group and, possibly, a cation bridge; whereas the antibiotic sorbs to Al 2O 3 solids through the ketone and carboxylate functional groups via a ligand exchange mechanism. Sorption edge and isotherm experiments show that ofloxacin exhibits a higher affinity for mesoporous SiO 2 and nonporous Al 2O 3, relative to their counterparts. It is hypothesized that decreased ofloxacin sorption to mesoporous Al 2O 3 occurs due to electrostatic repulsion within pore confines. In contrast, it appears that the environment within SiO 2 mesopores promotes sorption by inducing formation of ofloxacin–Ca complexes, thus increasing electrostatic attraction to SiO 2 surfaces.