Pyridine-, thiol- and amine-functionalized mesoporous silicas for adsorptive removal of pharmaceuticals
Graphical abstract
Introduction
Adsorption from aqueous phase by solid materials has been used for centuries as one of the most straightforward and effective method of purification of waters and sewages [1,2]. One of the key factors determining the removal efficiency is the proper selection of the adsorbent. An optimum sorbent should have a high pore volume, available adsorptive sites, good mechanical properties, as well as thermal and chemical stability. The adsorption process should additionally proceed quickly and provide the possibility of regeneration and reuse of the adsorbent [3]. Thus, attempts to search for a new type of adsorbents are driven not only by scientific curiosity but also by a real demand for new types of porous structures with properties tailored for specific applications.
Currently, a wide range of adsorbents with diverse porous structures and surface chemistry heterogeneity has been developed. The most commonly utilized classic adsorbents include activated carbons, silica gels, as well as natural and synthetic aluminosilicates (zeolites) [[4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. Unfortunately, their porous structure is often not suitable for the adsorption of pollutants composed of large-sized molecules, which cannot enter the small pores due to the steric restrictions [14,15]. An example of such pollutants are pharmaceuticals, widely present in the natural environment as a result of their constantly increasing usage and emissions [[16], [17], [18], [19]]. Particularly, non-steroidal anti-inflammatory drugs like diclofenac or ibuprofen are of emerging concern due to their widespread use, followed by intensive discards mainly from households and hospitals [20,21].
Removal of pharmaceuticals has been intensively studied in the scientific literature, where considerable attention is paid to the applications of new groups of nano-engineered sorbents including metal-organic frameworks (MOFs) [[22], [23], [24], [25]], mesoporous carbons [14,26], carbon nanotubes [[27], [28], [29], [30]] or ordered mesoporous silicas (OMS) [15,[31], [32], [33], [34], [35]]. Particularly OMS are frequently tested as adsorbents due to a wide range of beneficial properties including high surface areas and pore volumes, tunable pore sizes, and good hydrothermal and mechanical stability. However, the most important advantage of OMS fabrication relies on the possibility of easy and controlled chemical modification by a wide range of functionalities including complexing groups, aliphatic and aromatic organic fragments as well as more sophisticated moieties [33,34,[36], [37], [38], [39], [40], [41], [42]]. Functional groups not only provide favourable adsorption sites for the adsorbed molecules, but also affect other properties of the final silica materials, which are important for their adsorption-related applications. Those properties include morphological features, pore size distribution and pore network, surface charge and hydrophobicity.
Due to their complexing properties amine groups are used most frequently for surface functionalization - aminopropyltriethoxysilane (APTS) is one of the most commonly used functional monomers for surface amination. Amine-functionalized nanoporous silicas are broadly tested as sorbents of heavy metals [43], dyes, carbon dioxide [44], but also pharmaceuticals due to enhanced favourable interactions, both, specific (e.g., formation of hydrogen bonds) and non-specific (e.g. electrostatic interactions) [40,[45], [46], [47], [48]] between the drug molecules and an aminated surface. On the contrary, pyridine groups are rarely used although they can provide enhanced adsorption properties as has been recently shown [34]. In this study three different functional groups were chosen as surface modifiers: amine, thiol, and pyridine, which were incorporated in the silica network on the course of co-condensation between TEOS and proper functional silane. The resulted materials are tested as sorbents of hazardous pharmaceuticals to verify their applicability for removal of hazardous pollutants from aquatic systems.
Section snippets
Reagents
The following reagents were used as received: tetraethoxysilane (TEOS, 99%, Sigma-Aldrich), 2-(2-Pyridyl)ethyltrimethoxysilane (PETS, 95%, Fluorochem), 3-mercaptopropyltrimethoxysilane (MPTS, 97%, Fluorochem), 3-aminopropyltriethoxysilane (АPTS, 98%, Fluorochem), Pluronic F127 (F127, BASF), toluene (POCH), HCl (36%, POCH), NaOH (POCH), NaCl (POCH), ethanol (EtOH, 99.8%, POCH), diclofenac sodium salt (DICL, >98%, Sigma-Aldrich)., ibuprofen sodium salt (IBUP, >98%, Sigma-Aldrich), penicillin G
Results and discussion
The obtained materials were characterized by nitrogen sorption to investigate the porous structure – the obtained isotherms and the derived pore size distributions (PSD) are presented in Fig. 1. Although adsorption isotherms have different shapes, they also have two common features: (i) only very small initial increase of the isotherm at relative pressure < 0.1 and (ii) the presence of extensive hysteresis loop albeit with different shape. The first feature testifies that there is limited
Conclusions
Final morphology and porosity of functionalized mesoporous silicas turned out to be largely dependent on the type of functional monomer used along with TEOS, i.e., amine- and thiol-functionalized silicas were composed of loosely aggregated nanoporous rings/tubes while pyridine-functionalized silica formed consolidated 3D meso-cellular foam. The loose connection of NA nanomoieties resulted in greater susceptibility to hydrolytically-driven dissolution of silica framework, while compact foamy
CRediT authorship contribution statement
Mariusz Barczak: Conceptualization, Methodology, Formal analysis, Investigation, Resources, Writing - original draft, Writing - review & editing, Supervision, Project administration, Funding acquisition, Visualization. Ryszard Dobrowolski: Formal analysis, Writing - review & editing. Piotr Borowski: Methodology, Formal analysis, Resources, Writing - original draft, Writing - review & editing. Dimitrios A. Giannakoudakis: Writing - review & editing, Visualization.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by the Polish National Science Centre under Grant No. DEC-2012/05/D/ST5/03488 titled Synthesis and characterization of modified organosilica sorbents designed for the sorption of biomolecules.
XPS and TEM characterization of the materials was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-06-024/09 Center
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