Semi-interpenetrating polymer network microspheres with superior dimensional stability as multifunctional antibacterial adsorbent materials
Graphical abstract
Introduction
In fact, contaminations of water resources by positively-charged toxins (such as Pb2+, Cu2+, Cd2+, methylene blue (MB) and etc.) and pathogenic microorganisms are two major water quality concerns which have attracted great deal of attention in scientific communities around the world [1,2]. For one thing, the intake of the positively-charged toxins could cause serious health problems, including allergy, renal insufficiency, liver failure, metabolic disorders and etc [[3], [4], [5]]. Meanwhile, they are insusceptible, non-biodegradable and thus have a risk of accumulation in the food chain [1]. For another, contacting with pathogenic microorganisms during the wound healing process would lead to severe wound infection and even cause life threatening complications [6]. Pathogenic microorganisms such as extraintestinal pathogenic Escherichia coli are serious causes of infections including urinary tract infections, neonatal meningitis and septicemia. [7] Thus, it is of great importance to concurrently solve these water quality concerns.
Fixed-bed column has been widely used for water remediation because of its simple operation, high efficiency and cost-effective implementation [8]. The removal of positively-charged toxins in wastewater using the fixed-bed column has been extensively studied. However, none of the aforementioned columns could simultaneously inhibit the growth of microorganisms. Hence, it is crucial to develop multifunctional fixed-bed column which can concurrently remove positively-charged toxins and inhibit the growth of microorganisms.
The functionalities of the fixed-bed column depend on the properties of the materials filled in the column. At present, adsorption is deemed as an economical method for water treatment owing to its easy operation, low cost and general applicability for various pollutants [9]. Various adsorbents have been utilized to endow the column with adsorption function for the removal of positively-charged toxins. In fact, although it has not been reported before, antibacterial materials could also be filled in the column to endow it with antibacterial function. Furthermore, the column filled with the antibacterial adsorbent was envisaged to concurrently remove positively-charged toxins and inhibit the growth of microorganisms. Nevertheless, not all adsorbents and antibacterial materials are qualified for the fixed-bed column because there are several restrictions on the stationary phase in the column. For one thing, the size of the filling materials should be suitable and easy to control. Adsorbents in the form of powder or nanoparticles are not qualified for the column because the they are too small and thus have the risk of leakage [10]. For another, dimensional stability is also demanded for the filling materials in the column [11]. Insufficient dimensional stability could lead to the rupture of materials and increase the resistance in the continuous-flow contact vessels.
As widely used adsorbents filled in the column, hydrogel microspheres (HMs) have been studied intensively due to their high density of functional groups and 3D cross-linking network [[12], [13], [14]]. Owing to their abundant functional groups, HMs tend to exhibit satisfactory adsorption performance and have good potentials for further multi-functionalization. Considering the facile preparation procedure, a wide variety of natural polysaccharides such as cellulose, sodium alginate, chitosan and carrageenan have been used to prepare HMs via phase separation technique, ionic cross-linking, hydrogen bonding or conformational transition technique [15,16]. For example, Liang et al. synthesized chitosan/carrageenan ampholytic composite HMs via emulsification procedure in LiOH/KOH/urea aqueous system, and the obtained HMs exhibited highly efficient adsorption capacity towards heavy metal ions in wastewater [17]. Song et al. prepared carrageenan-based HMs via conformational transition technique while Pan et al. designed a cellulose-based composite cryogel by the microchannel liquid-flow focusing and freezing method [18,19]. However, the use of polysaccharides-based HMs has been hindered by their poor mechanical properties and inadequate dimensional stabilities due to their unstable physical cross-linking structure [[19], [20], [21]]. Additionally, polysaccharides-based HMs usually exhibited no antimicrobial effects since their composition and structure were somewhat similar to nature tissue [22,23]. Some of them could even be degraded and constitute a carbon source for bacteria [24]. Synthetic polymer-based hydrogels containing functional groups (e.g. -NH2, -SO3H, −COOH and −CONH2) were also reported as column fillers for water remediation [25,26]. The characteristics of polymeric hydrogels varied from the composition and more specifically the monomers/types of monomers. Generally speaking, polymeric hydrogels were easy to modify to produce hydrogels with desirable properties. Nevertheless, polymeric hydrogels with spherical morphology are hard to obtain and the size of spheres are hard to control for satisfying the practical industrial water remediation [27].
Herein, HMs with a semi-IPN structure were prepared by combining PES chains with PAA 3D cross-linked network. The introduction of rigid and hydrophobic PES matrix was envisaged to enhance the dimensional stability of the PES/PAA HMs, thereby facilitating the practical applications under high operation pressure. Electrospraying technology coupled with subsequent in-situ crosslinking polymerization was utilized to realize the dimensional control of the PES/PAA HMs. Furthermore, Ag nanoparticles (NPs) were immobilized on the microspheres via ion exchange and in-situ reduction to form the multifunctional antibacterial adsorbents Ag@PES/PAA HMs. Pb2+, Cu2+, Cd2+ and MB were chosen as representative positively-charged toxins while Escherichia coli (E. coli, Gram-negative) and Staphylococcus aureus (S. aureus, Gram-positive) were utilized as model bacteria for investigating the multifunctional capabilities of the Ag@PES/PAA HMs.
Section snippets
Materials
PES (Ultrason E6020 P) was obtained from BASF chemical company. Acrylic acid (AA), N,N’-methylidenebis (acrylamide) (MBA), 2,2-azobisisobutyronitrile (AIBN), ammonium persulfate (APS), silver nitrite (AgNO3) and VC were purchased from Aladdin Reagent Co. Ltd. N,N’-Dimethylacetamide (DMAc) and sodium dodecyl sulfate (SDS) were obtained from Chengdu Kelong Chemical Reagent Co. Ltd. MB and the salts of heavy metal ions (Cu2+, Pb2+ and Cd2+) were purchased from Aladdin reagent Co.Ltd. Deionized
Preparation and characterization of Ag@PES/PAA HMs
In this study, electrospraying method was used to prepare microspheres in the first step. During the process of electrospraying, liquid flowed out from a nozzle with a high voltage and was subjected to an electric field. The droplets obtained via electrospraying could be smaller than 1 mm and the size distribution of the droplets was usually narrow. Meanwhile, the size of the droplets could be easily controlled by adjusting the flow rate and voltage applied to the nozzle [28,29]. In this case,
Conclusions
In this study, we synthesized Ag@PES/PAA HMs via electrospraying and cross-linking polymerization. The as-prepared microspheres exhibited high dimensional stability owing to the formation of semi-IPN structure, thereby facilitating their practical applications under high operation pressure. Additionally, the adsorption column filled with the Ag@PES/PAA HMs was found to be exceptionally efficient in the removal of the positively-charged toxins (The removal ratios reached 98.03%, 99.57%, 99.42%
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
The National Natural Science Foundation of China (No.51433007, 51673125, 51773127, 51803131, 51803134 and 51873115), the State Key Research Development Programme of China (Grant Nos. 2016YFC1103000 and 2018YFC1106400), Science and Technology Program of Sichuan Province (2017SZ0011 and 2019YJ0132) financially sponsored this work. We also thank Ms. H. Wang, of the Analytical and Testing Center at Sichuan University, for the SEM micrographs.
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