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

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Abstract

Mesoporous and nonporous SiO2 and Al2O3 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 SiO2 surfaces occurs slightly below the pKa2 (pH 8.28) of the antibiotic and sorption diminishes rapidly at pH > pKa2. For Al2O3, 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 pKa values of ofloxacin. The effects of pH on adsorption and ATR–FTIR spectra suggest that the zwitterionic compound adsorbs to SiO2 solids through the protonated N4 in the piperazinyl group and, possibly, a cation bridge; whereas the antibiotic sorbs to Al2O3 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 SiO2 and nonporous Al2O3, relative to their counterparts. It is hypothesized that decreased ofloxacin sorption to mesoporous Al2O3 occurs due to electrostatic repulsion within pore confines. In contrast, it appears that the environment within SiO2 mesopores promotes sorption by inducing formation of ofloxacin–Ca complexes, thus increasing electrostatic attraction to SiO2 surfaces.

Introduction

Fluoroquinolone carboxylic acids (FQCAs) are a class of chemotherapeutic agents with antibacterial activity used in human and veterinary medicines. Although absorptivity of orally administered FQCAs is high [1], a portion of the dose passes through the body into human and animal excrement. Thus, FQCAs have been detected in wastewaters insufficiently treated by sewage treatment plants [2], [3], liquid animal manures [4], and streams [5]. Recent studies in the United States and Europe have documented the presence of FQCAs in wastewaters and streams with concentrations typically reported in the range of ng L−1 to μg L−1 [2], [5], [6], [7]. Due to the land application or discharge of wastes to streams and our limited knowledge of the fate and interactions of FQCAs in aquatic and terrestrial environments, these compounds are of significant environmental concern [3], [5], [8].

Within the large class of FQCAs, ofloxacin is used to treat urinary and respiratory tract infections in humans and animals [9]. Although a significant number of studies have investigated aqueous ofloxacin–metal complexation reactions [9], [10], [11], [12], [13], [14], much less work has been done on the sorption of ofloxacin to minerals and soil. Djurdjevic et al. [13] determined that sorption of ofloxacin to Al2O3 solids exhibited S-shaped isotherms when experiments were conducted from 19 to 140 μmol L−1 in neutral and very acidic (pH 1) aqueous background solutions, and isotherms were L-shaped (Langmuir) in basic solutions (pH 11). The greatest extent of sorption occurred at neutral pH (0.7 mmol g−1) followed by sorption at pH 1 (0.5 mmol g−1) and sorption at pH 11 (0.38 mmol g−1) [13]. However, ofloxacin sorption onto Al(OH)3 gel exhibited a C-shaped (linear) isotherm and 21% of adsorbed ofloxacin was released from the mineral surface during desorption reactions [15]. Al dissolution and changes in solution pH as a function of ofloxacin adsorption were not measured in either study. Thus, it is unclear to what extent aqueous Al may compete with Al surfaces for ofloxacin complexation, and the multifunctionality of this compound may equate to several possible bonding mechanisms.

Nowara et al. [16] investigated the sorption of several FQCAs, including levofloxacin (an active optical isomer of ofloxacin), to soil, soil clay fractions, and soil minerals. This study reported that adsorption of FQCAs to soil, soil clay fractions, and layer silicates is very high (95–99% removal from initial aqueous concentrations ranging from 0.28 to 28 μmol L−1) and desorption in 0.01 M CaCl2 is very low (<2.6% of adsorbed amount was released into solution). Infrared spectra and microcalorimetry data were interpreted by Nowara et al. [16] to suggest that FQCAs are bound to clays via a cation bridge between charged basal surfaces and the carboxylate functional group of FQCAs [16]. However, cation bridging is a relatively weak sorption mechanism that is not associated typically with irreversible adsorption as observed by Norwara et al. [16]. In addition, experimental pH values were generally equal to or less than the pKa1 of the FQCAs, thus cationic forms of the compounds may have been adsorbed to mineral surfaces. High Koc values (40,000–71,000) suggest that sorption was also influenced by the amount of organic carbon present in the soil [16], and others have reported sorption of FQCAs to dissolved humic acids [17], [18].

An additional factor that should be considered when studying the fate of organic compounds in soils and sediments is substrate surface morphology. Recent studies have demonstrated that mineral mesoporosity (2–50 nm in pore diameter), as occurs in naturally weathered geosorbents [19], can impact organic compound sorption. Zimmerman et al. [20] observed that nitrogenous organic compounds smaller than one-half the average mesopore diameter exhibited significantly greater surface area-normalized adsorption to mesoporous alumina and silica, relative to nonporous analogues. Sorption of larger compounds was inhibited due to compound exclusion from the internal mesopore surfaces. Goyne et al. [21] documented increased adsorption of the pesticide 2,4-D to alumina sorbents with increased mesoporosity. However, it should be noted that porosity, in and of itself, is not always the most important governing factor. For instance, 2,4-D did not adsorb to mesoporous or nonporous silica, presumably because of electrostatic repulsion [21].

In this work, studies were conducted to investigate the sorption of the FQCA ofloxacin to nonporous and mesoporous Al2O3 and SiO2 mineral sorbents. The objectives of this study were (a) to investigate differences in ofloxacin sorption to Al2O3 and SiO2 surfaces as a function of pH and initial concentration, (b) to determine if mesoporosity influences the amount of ofloxacin sorbed to Al2O3 and SiO2 solids, and (c) to determine the mechanism(s) through which ofloxacin binds to Al2O3 and SiO2 surfaces.

Section snippets

Properties of the adsorbate

Ofloxacin (9-fluoro-2,3-dihydro-3-methyl-10-(4-methyl-1-piperanzinyl)-7-oxo-7H-pyrido[1,2,3-de]-1,4-benzoxazine-6-carboxylic acid; 98% minimum purity) was purchased from Sigma–Aldrich Co. (St. Louis, MO) and used as received. The compound was stored at 4 °C in the dark to minimize photolytically induced degradation [22]. Ofloxacin is a zwitterionic compound with acid dissociation constants of 6.08 (pKa1) and 8.25 (pKa2) (Fig. 1) [12], [23]. As shown in Fig. 1b, the antibiotic is primarily

Ofloxacin adsorption as a function of pH

The effects of pH on ofloxacin adsorption to SiO2 surfaces are shown in Fig. 2a. Above pH 5.0, Si-P700 adsorbs significantly more ofloxacin than does Si-NP8. Maximum ofloxacin sorption to these minerals (80.3 and 67.2% for Si-P700 and Si-NP8, respectively) occurs slightly below the pKa2 (pH 8.28) of the antibiotic and diminishes rapidly at pH > pKa2. Thus, we presume that cationic and zwitterionic ofloxacin are adsorbed to the negatively charged silica surfaces (i.e., triple bondSiO functional groups) via

Conclusions

Intraparticle mesoporosity in SiO2 solids was found to result in increased uptake of ofloxacin when adsorption was normalized to sorbent surface area. Relative to the nonporous solid, the presence of intraparticle porosity resulted in a statistically significant sorption enhancement throughout the isotherms and over most of the sorption edge for the porous silica adsorbent (Si-P700). Observations of proton release in association with ofloxacin sorption and sorption in excess of the surface site

Acknowledgments

The authors thank Mary Kay Amistadi for laboratory assistance, Sridhar Komarneni, Bharat Newalkar, and Stephen Stout for mineral synthesis and preparation, and Chad Trout for assistance with molecular modeling. Financial support was provided by the Penn State Biogeochemical Research Initiative for Education (BRIE) sponsored by NSF (IGERT) Grant DGE-9972759 and by the Penn State Materials Research Science and Engineering Center (MRSEC) sponsored by NSF Grant DMR-0080019. Andrew Zimmerman

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