title: Enhanced interlayer trapping of a tetracycline antibiotic within montmorillonite layers in the presence of Ca and Mg.
abstract: The formation of a ternary antibiotic-metal-clay complex is hypothesized as the primary adsorption mechanism responsible for the increased adsorption of tetracycline antibiotics on smectites in the presence of divalent metal cations under circumneutral and higher pH conditions. To evaluate this hypothesis, we conducted a spectroscopic investigation of oxytetracycline (OTC) interacting with Na-montmorillonite in the presence and absence of Ca or Mg salts at pH 6 and pH 8. Despite a two-fold increase in OTC adsorbed in the presence of Ca or Mg, both solid-state nuclear magnetic resonance and infrared signatures of the OTC functional groups involved in metal complexation implied that the formation of an inner-sphere ternary complexation was not significant in stabilizing the adsorbate structures. The spectroscopic data further indicated that the positively-charged amino group mediated the OTC adsorption both in the absence and presence of the divalent metal cations. Focusing on the experiments with Mg, X-ray diffraction analysis revealed that the metal-promoted adsorption was coupled with an increased intercalation of OTC within the montmorillonite layers. The resulting interstratified clay layers were characterized by simulating X-ray diffraction of theoretical stacking compositions using molecular dynamics-optimized montmorillonite layers with and without OTC. The simulations uncovered the evolution of segregated interstratification patterns that demonstrated how increased access to smectite interlayers in the presence of the divalent metal cations enhanced adsorption of OTC. Our findings suggest that specific aqueous structures of the clay crystallites in response to the co-presence of Mg and OTC in solution served as precursors to the interlayer trapping of the antibiotic species. Elucidation of these structures is needed for further insights on how aqueous chemistry influences the role of smectite clay minerals in trapping organic molecules in natural and engineered soil particles.
authors.affiliation: Department of Biological and Environmental Engineering, College of Agricultural and Life Sciences, Cornell University, Ithaca, NY 14853, USA. Electronic address: ludmilla@cornell.edu.
authors.forename: Ludmilla
authors.initials: L
authors.lastname: Aristilde
authors.affiliation: Institut des Sciences de la Terre, ISTerre, Univ. Grenoble-Alpes, CNRS, F-38041 Grenoble, France.
authors.forename: Bruno
authors.initials: B
authors.lastname: Lanson
authors.affiliation: Equipe Matériaux à Porosité Contrôlée, IS2M, CNRS 7361, Université de Haute Alsace, 68093 Mulhouse, France.
authors.forename: Jocelyne
authors.initials: J
authors.lastname: Miéhé-Brendlé
authors.affiliation: Equipe Matériaux à Porosité Contrôlée, IS2M, CNRS 7361, Université de Haute Alsace, 68093 Mulhouse, France.
authors.forename: Claire
authors.initials: C
authors.lastname: Marichal
authors.affiliation: Institut des Sciences de la Terre, ISTerre, Univ. Grenoble-Alpes, CNRS, F-38041 Grenoble, France.
authors.forename: Laurent
authors.initials: L
authors.lastname: Charlet
mesh_terms: D000327:Adsorption; D000900:Anti-Bacterial Agents / Q000737:chemistry*; D001546:Bentonite / Q000737:chemistry*; D002118:Calcium / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008274:Magnesium / Q000737:chemistry*; D008968:Molecular Conformation; D010118:Oxytetracycline / Q000737:chemistry*; D010316:Particle Size; D012964:Sodium / Q000737:chemistry*; D013499:Surface Properties
chemical_list: D000900:Anti-Bacterial Agents; D001546:Bentonite; D012964:Sodium; D008274:Magnesium; D002118:Calcium; D010118:Oxytetracycline
keywords: Antibiotics; Aqueous conditions; Cation-promoted adsorption; Interlayer adsorption; Montmorillonite