Adsorption of sulfonamide antibiotics into high silica zeolites from water: A combined structural, spectroscopic and computational approach

Introduction Sulfa drugs belong to the class of synthetic antibiotic sulfonamides, massively used for the treatment of infections in human therapy, livestock production and aquaculture [1]. Owing to their environmental diffusion, sulfa drugs are known to induce high level of resistance in bacteria through by-pass mechanism [2]. In order to limit the diffusion of resistance determinants, it is of utmost importance to identify adsorbents for this antibiotic family to be used for water clean up purpose. In this study, high silica zeolites have been tested for their capability to extract sulfa drugs from water. Kinetics, capacity, and reversibility of the adsorption process have been studied along with the localization of the sulfa drugs into zeolite porosity systems. The main host-guest (& guest-guest, when existing) interactions along with the tautomeric form embedded into sorbents have been defined. When present, reaction products have been identified and the transformation mechanism proposed. Regeneration procedures were finally investigated. Experimental Adsorption kinetics and capacity of zeolites Y, mordenite (MOR), and ZSM-5 (SiO2/Al2O3 = 200, 200, and 500, respectively) towards ten sulfa drugs (sulfonamide, sulfadiazine, sulfamethazine, sulfachloropyridazine, sulfathiazole, sulfapyridine, sulfamerazine, sulfadimethoxine, sulfadoxine, and sulfamethoxazole) were defined by thermogravimetric and HPLC analyses. The location of sulfonamides into zeolite channel systems has been obtained by Rietveld refinements from X-ray powder diffraction data (XRPD). The zeolite-sulfonamide host-guest interactions have been investigated by IR analysis performed on zeolites at the highest adsorption capacity and augmented by computational analysis at the DFT level. Regeneration (extraction with solvents, thermal, photolysis, Fenton, photo-Fenton, thermal treatments) have been performed on exhausted zeolites. In situ HT XRPD experiments were performed at GILDA beamline at ESRF (Grenoble), to test the regeneration capacity of zeolites Y and ZSM-5. Results and Discussion The adsorption of sulfamethoxazole by high silica zeolites was in the order: zeolite Y (24% zeolite dry weight - DW) > ZSM-5 (8%) > MOR (6%). With the exception of sulfanilamide, which showed scarce affinity for zeolite Y (3% zeolite DW), the other sulfa drugs irreversibly adsorbed at ca. 26% on average and with the process equilibrium reached in less than 1 min [3]. As shown by IR analysis, weak H-bonds and van der Waals type interactions between single molecules of sulfamethazine, sulfachloropyridazine, sulfamerazine, sulfadimethoxine, sulfadoxine, or sulfamethoxazole and zeolite Y were responsible for the irreversible extraction of sulfa drugs from water [4,5]. The occurrence of intramolecular medium strength H-bond in sulfadiazine, sulfathiazole, or sulfapyridine upon adsorption revealed the formation of dimeric species inside zeolite cage [4,5]. The energy associated to the imide-amide tautomeric forms of monomer and dimer sulfa drugs and their occurrence into zeolite Y cage have been elucidated by a combined DFT computational modelling and IR study: the imide form for dimer sulfathiazole and amide form for both momomer and dimer species for the other sulfonamides [5]. Among the regeneration strategies approached, the thermal treatment and the solvent extraction gave the best results. As far as zeolite MOR is concerned, Rietveld structure refinement revealed that sulfachloropyridazine or sulfamethoxazole incorporation of caused a remarkable change in the channels' dimension when compared to the parent zeolite and a close vicinity of the heterocycle ring to the MOR side pocket oxygens [6] (See Figure). Sulfachloropyridazine locatization in MOR It is proposed that the H-bond between mordenite silanols and sulfachloropyridazine heterocycle nitrogen atom induces partial positive charges in the heterocycle ring favouring at 65°C the displacement of the chlorine leaving group by an OH- via nucleophilic aromatic substitution mechanism with the formation of the reaction product with 100% selectivity [6]. In situ HT synchrotron XRPD experiments demonstrated that the treatment at 575°C assures a complete regeneration of both exhausted Y and ZSM-5 zeolites. The removal of sulfamethoxazole induces framework deformations which do not affect the crystallinity of sorbent materials. Conclusions The notable and irreversible adsorption capacity for sulfa drugs shown by high silica zeolites Y, MOR, and ZSM-5, selected on the basis of their high hydrophobicity and peculiar pore architectures, along with their regenerability, clearly indicates these sorbents as affordable and recyclable materials for clean up waters polluted with this antibiotic class. The arrangement of sulfa drugs inside zeolite porosities, the zeolite-antibiotic host-guest interactions, the tautomeric form involved of embedded monomeric and dimeric species could be defined by a multidisciplinary structural, spectroscopic and computational approach. These materials are very interesting also for drug delivery issues. Acknowledgements Research co-funded by Ministry of Education, University and Research within the Project: Zeolites as nano-reactors for the environment: efficiency, selectivity and stability in the adsorption of drugs from contaminated waters. 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