Antibiofouling performance and mechanisms of a modified polyvinylidene fluoride membrane in an MBR for wastewater treatment: Role of silver@silica nanopollens
Graphical abstract
Introduction
Membrane bioreactors (MBRs) have been widely used in municipal and industrial wastewater treatment, which have prominent advantages over conventional activated sludge (CAS) systems including smaller footprint, less sludge production and improved effluent quality (Ma et al., 2018; Meng et al., 2017). However, membrane fouling, which causes frequent chemical cleaning and ultimately shortens membrane lifespan, remains as a challenge in MBRs (Matin et al., 2011; Le-Clech et al., 2006). Biofouling, due to unwanted deposition, growth and metabolism of microorganisms on membrane surfaces and/or inside membrane pores, is considered as a thorny issue limiting applications of MBRs (Deng et al., 2016; Zhang et al., 2016a).
Membrane surface properties have a great impact on biofouling behaviors, and an attractive strategy for biofouling control is to construct antibiofouling membrane surface that can inhibit bacterial proliferation and retard biofilm formation (Zhang et al., 2016a; Zodrow et al., 2009). Recently, the combination of polymeric materials with silver nanoparticles (AgNPs) has attracted much attention in membrane fabrication due to their strong disinfection capability (Yang et al., 2016; Zhang et al., 2012a). The membranes are usually fabricated by incorporating AgNPs in the membrane directly or immobilizing AgNPs on membrane surfaces (Ben-Sasson et al., 2014; Yin et al., 2013; He et al., 2017; Huang et al., 2016). However, the uncontrollable release of AgNPs often leads to a fast loss of antibacterial ability and thus a short lifetime of AgNPs (Yin and Dang, 2015; Huang et al., 2014). To tackle these problems, extensive efforts have been dedicated to developing nanomaterials to immobilize AgNPs and improve antimicrobial efficiency, including nanozeolite (Wu et al., 2015), silica nanospheres (Huang et al., 2014), graphene oxide (Mahmoudi et al., 2015), and biocarrier (Zhang et al., 2012a).
The initial loading mass of Ag and the rate of Ag+ release hold the key to the efficacy and life-time of the biofouling resistance of Ag-modified membrane (Zhang et al., 2012a). Sufficient Ag loading as well as stable Ag release are both of great significance for long-term performance. Inspired by nanopollens which can easily adhere to the hairy legs of a honey bee for pollination (Song et al., 2016; Barrier et al., 2011; Thorp, 2000) due to their large specific surface area and enhanced interaction with microbes, we developed biomimetic materials, silica nanopollens, as an AgNPs nanocarrier for membrane preparation. Silica nanopollens, which have a structure of nanoscale channels as well as numerous spikes on the outer shell, were supposed to improve silver delivery efficacy, avoid agglomeration and control Ag+ release towards bacteria.
In this study, silica nanopollens loaded with AgNPs were synthesized and used for microfiltration membrane fabrication during phase-inversion process. We systematically investigated the antibiofouling performance and mechanisms in a long-term operated MBR for treating municipal wastewater. The evolution of transmembrane pressure (TMP) and effluent quality was monitored, and the antibiofouling mechanisms were examined by confocal laser scanning microscope (CLSM), real-time PCR and quartz crystal microbalance with dissipation monitoring (QCM-D). Furthermore, Illumina Miseq was employed to analyze the potential influences on microbial communities of the biofilm formed on modified membranes.
Section snippets
Reagents
All chemicals used in this work were of analytical reagent grade unless otherwise stated. Commercial grade PVDF was provided from Solvay Corporation (Solef 6020, Mw = 670–700 kDa). Dimethylacetamide (DMAC) and dimethyl sulfoxide (DMSO) used as solvents and polyethylene glycol (PEG 600) as a pore-forming additive were purchased from Sinopharm (Shanghai, China). Silver nitrate (AgNO3), Tetraethyl orthosilicate (TEOS), resorcinol, formaldehyde, ethanol and ammonia aqueous solution for Ag@silica
Membrane characterization
SEM images (Fig. 1A–C) exhibit the surface morphology of pristine membrane (M0), Ag@silica nanopollens modified membrane (ASNP-M), and Ag@silica nanospheres modified membrane (ASNS-M). In Fig. 1B, spherical Ag@silica nanospheres with uniform size (∼300 nm) were homogeneously distributed over the membrane surface. For the ASNP-M, Ag@silica nanopollens which have numerous spikes on their surface can be clearly observed (Fig. 1C).
Fig. S3A presents wide-scan XPS elemental survey spectra of M0,
Conclusions
In this study, the antibiofouling performance and mechanisms of Ag@silica nanopollens modified membrane (ASNP-M) were systematically evaluated in a long-term operated MBR for treating municipal wastewater. Long-term operation of Ag@silica modified membranes revealed that using silica nanopollens as a carrier matrix for AgNPs and blending them in PVDF membranes offers an effective strategy to mitigate membrane biofouling, mainly due to the enhanced silver loading quantity and controlled Ag+
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.
Acknowledgements
We thank the National Natural Science Foundation of China (Grants 51838009 & 51808395). Dr. Xingran Zhang acknowledges the receipt of Shanghai Talents Development Fund.
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