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Title:Interaction of fluroquinolone antibiotics with nanophase metal oxide photocatalysts and soil minerals
Author(s):Paul, Tias
Director of Research:Strathmann, Timothy J.
Doctoral Committee Chair(s):Strathmann, Timothy J.
Doctoral Committee Member(s):Mariñas, Benito J.; Werth, Charles J.; Machesky, Michael L.
Department / Program:Civil & Environmental Eng
Discipline:Environ Engr in Civil Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Metal oxides
Surface Complexation
Surface Complexation Model
Ion Pairing
Charge Distribution
Abstract:The contamination of surface waters with pharmaceutical compounds has been recognized as a problem in many regions of the world. These xenobiotic compounds originate from point sources, such as treated wastewater effluent, and also from non-point sources, such as runoff from animal rearing operations. A major goal of current studies is to understand the impact these typically sub-therapeutic levels of pharmaceuticals can have on receiving environments. Assessing their risk involves predicting how long they will be retained in a given system, the transformations they may undergo, and resultant potency of transformation products. Both transport and chemical transformation processes can be strongly influenced by the adsorption properties of the pharmaceuticals. This study focuses on a select group of antibacterial agents, fluoroquinolones (FQ). They are important from the therapeutic standpoint as they are often our last line of defense against highly resilient bacterial infections. Dissemination of fluoroquinolones in the environment has raised concerns about favoring the accumulation of antibacterial resistance genes in exposed bacteria, which can be transferred to pathogenic bacteria. The overall objective of this work is to characterize important interactions between fluoroquinolone antibiotics and nanophase metal oxides that contribute to their adsorption, transformation, and deactivation in engineered and natural aquatic systems. While the transformation of FQs under engineered treatment scenarios have been investigated, few studies have quantified the concomitant decrease in pharmaceutical activity and linked the deactivation with specific molecular transformations. Furthermore, no mechanistic models currently exist to quantify the adsorption of zwitterionic FQs to charged metal oxides surfaces. The elucidation of oxidation-induced deactivation mechanisms and adsorption mechanisms has implications for the risks associated with fluoroquinolone release into the environment. Supplementing our previous work on photocatalytic transformation of FQ with nano-anatase under acidic conditions, we first investigate the transformation of FQs under more environmentally relevant conditions. In these studies, we undertake the identification of transformation products using mass spectrometric techniques. We further quantify the decrease in antibacterial activity of photo(cata)lytically treated suspensions of FQs using bacterial cell growth inhibition assays. Our findings corroborate previous evidence indicating that FQ molecular transformations in the visible light photocatalytic systems are mainly localized to transformations to the piperazine moiety. Contrary to expectations, the antibiotic potency of the FQ transformation products is negligible in comparison to that of the parent FQ, even though no evidence for transformation to the core quinoline structure was found. These findings imply that deactivation of FQ potency can be achieved through chemical processes that are not sufficiently oxidative to mineralize the core of the FQ molecule. In additional work, we characterize the trends in adsorption of FQs to anatase surfaces, a process that is proposed to be essential for the progress of visible light-TiO2 photocatalysis. Using batch adsorption studies and Fourier Transform Infrared (FTIR) spectra, we probe the ligand adsorption sites and potential modes of adsorption applicable to FQ adsorption. A spectroscopically consistent surface complexation model (SCM) based on the theory of ligand charge distribution (CD) is then developed to account for FQ adsorption trends. The SCM includes complexation of the FQ molecule through inner-sphere bonding of the carboxylate group and hydrogen bonding interactions involving the adjacent carbonyl oxygen to neighboring Ti adsorption sites. This model fits adsorption data collected over a wide range of pH (3-11), ionic strength (1-100 mM), and FQ concentration (20 - 500 μM) conditions. The SCM also suggests that FQ adsorption is enhanced by ion pairing of non-bonding groups on the FQ molecule with perchlorate anions. Inner-sphere bonding between FQ and TiO2 surfaces is also consistent with our understanding of the visible light-initiated charge transfer mechanism resulting in oxidation of FQs. Finally, we test the applicability of the CD SCM formulation to characterize the adsorption of zwitterionic and non-zwitterionic FQs to two other environmentally relevant metal oxides, goethite (α-FeOOH) and boehmite (γ-AlOOH). Previous research suggests that the adsorption of FQs to goethite is a pre-condition for their subsequent oxidation by electron transfer processes. In the current study, batch adsorption experiments conducted with a FQ analogue indicate that the carboxylic acid group is necessary for adsorption to goethite surfaces. Model fitting combined with FTIR spectroscopy indicate that zwitterionic FQ adsorption to goethite is well-characterized by the presence of four adsorbed complexes, one hydrogen bonded complex and three inner-spherically adsorbed species similar in structure to the adsorbed species formed in the FQ-TiO2 system. On the other hand, adsorption of a non-zwitterionic FQ to goethite is well predicted by a single adsorbed complex in which the molecule is involved in bidentate bonding through the carboxylate group only. Adsorption of FQs to boehmite is well-fit with a three species model consisting of molecules bonded inner-spherically through one carboxylate oxygen and one carbonyl oxygen atom. Overall, FQ adsorption to all three metal oxides appears to be dominated by inner-sphere complexation through the carboxylate group, with the adjacent carbonyl playing a supporting role either through inner-sphere bonding or hydrogen-bonding interactions. This type of complexation has previously been proposed by other researchers, but this is the first study that can support it through a formalized model based on complex structure that is capable of predicting adsorption trends over a broad range of solution conditions. The successful modeling results suggest that the CD SCM formalism can be an appropriate tool to bridge the gap between macroscopic adsorption data and molecular scale observations from spectroscopy.
Issue Date:2012-06-27
Rights Information:Copyright 2012 Tias Paul
Date Available in IDEALS:2012-06-27
Date Deposited:2012-05

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