Desalination membranes with ultralow biofouling via synergistic chemical and topological strategies
Graphical abstract
Introduction
Biofouling is a persistent problem for all the materials and processes related to the aquatic environment [[1], [2], [3]] and is of particular concern for membrane-based desalination and water treatment processes because it significantly reduces process efficiency [4,5]. Unlike other foulants, the self-replicating characteristics of microorganisms require the frequent cleaning of the membrane to maintain acceptable performance levels [6,7].
Biofouling occurs in a series of sequential steps, starting with the initial attachment of the microorganism, followed by growth and reproduction [2,3,7]. Biofouling can thus be mitigated by passively minimizing the surface adhesion of microorganisms (i.e., anti-adhesion action) or by actively inactivating microorganisms and preventing their growth (i.e., antimicrobial action). While significant efforts to create water treatment membranes with inherent biofouling resistance have been made [4,5,8], most proposed strategies rely on chemical modifications. For example, anti-adhesion characteristics can be implanted by modifying the membrane surface with hydrophilic materials, including polyvinyl alcohol, polyethylene glycol (PEG) and polydopamine [[9], [10], [11], [12]], which can reduce the hydrophobic interaction between microorganisms and the surface. In addition, antimicrobial ability can be imparted to the membrane by incorporating or depositing biocidal materials, including organic biocides, carbon nanotubes and silver nanoparticles (AgNPs) [[13], [14], [15], [16]]. Although these chemical modifications improve the anti-biofouling performance of the membrane, their effects are relatively weak and short-lasting due to subsequent alteration of the surface chemistry and the loss of incorporated materials during membrane operation [4,17].
Recently, the topological modification of the membrane surface by introducing well-defined patterns, including lines, prisms and pillars [[18], [19], [20]], has proven effective in alleviating biofouling by preventing the surface attachment of microorganisms [[21], [22], [23], [24], [25]]. In particular, the biomimetic Sharklet pattern has been demonstrated to offer superior biofouling resistance when compared with other simple artificial patterns [26,27]. Although topological modifications can ensure relatively long-lasting biofouling resistance, their lack of microbial inactivation ability limits anti-biofouling performance. To overcome the intrinsic drawbacks of topological and chemical anti-biofouling approaches, combining the two has been proposed [[28], [29], [30]]. However, most combination strategies are based on the use of chemical modifications to impart both anti-adhesion and antimicrobial functions to the membrane [[29], [30], [31]], restricting the degree to which antifouling performance can be enhanced. Weinman et al. employed a combination of chemical and topological modifications on nanofiltration membranes by fabricating trench surface patterns via nanoimprinting followed by coating with a PEG derivative [32]. However, this strategy impaired membrane performance while leading to only a minor improvement in fouling resistance.
Furthermore, from a fundamental perspective, the anti-biofouling effects of chemical and topological modifications have not yet been directly compared. From a practical perspective, dual chemical–topological modification has yet to be reported for desalination membranes because extremely delicate engineering techniques are required. Hence, it is imperative to explore a more effective combinatorial anti-biofouling strategy that can systematically and significantly improve the antifouling performance of a membrane without compromising its separation performance.
Here, we demonstrate that the combination of topological Sharklet patterning and anti-adhesive and antimicrobial chemical modifications can significantly and synergistically improve the biofouling resistance of a desalination membrane while maintaining its separation ability. A Sharklet-patterned thin-film composite (TFC) membrane was fabricated by creating a patterned support via phase separation (PS)-micromolding, followed by layered interfacial polymerization (LIP) to produce a polyamide (PA) permselective top layer. Tannic acid (TA) was coated on the patterned TFC membrane to reinforce a chemical anti-adhesion function due to its high hydrophilicity and to simultaneously facilitate the subsequent deposition of AgNPs by in-situ reducing the Ag precursor (silver nitrate, AgNO3) [33,34]. AgNPs were deposited on the TA-coated TFC membrane via simple exposure to a TA/AgNO3 solution due to their excellent antimicrobial activity and high hydrophilicity, which can also suppress microbial attachment [16,35]. The hydrophilic nature of TA and AgNPs can minimize the reduction in the permeation of the membrane caused by the surface coating. The surface properties and desalination performance of Sharklet-patterned membranes with sequential chemical modifications were characterized. The biofouling behavior of the proposed membranes was also characterized using a model biofoulant (Pseudomonas aeruginosa) under both static and dynamic conditions and compared with flat control membranes to elucidate the antifouling effects of each modification and their combination.
Section snippets
Materials
The following chemicals were all used as received: polyacrylic acid (PAA, Mw = 100 kg mol−1, Sigma-Aldrich), branched polyethyleneimine (PEI, Mw = 270 kg mol−1, Sigma-Aldrich), polyacrylonitrile (PAN, Mw = 150 kg mol−1, Sigma-Aldrich), polyurethane acrylate (PUA) resin (MINS™ 311RM, MINUTA Tech.), m-phenylenediamine (MPD, TCI), trimesoyl chloride (TMC, TCI), sodium hydroxide (NaOH, Daejung Chem.), sodium chloride (NaCl, Daejung Chem.), TA (Sigma-Aldrich), AgNO3 (Daejung Chem.), nitric acid (HNO3
Membrane structures
Fig. 2 presents the structures of the fabricated Sharklet-patterned support and TFC membranes with sequential chemical modifications. All of the support and membranes preserved the Sharklet pattern from the original mold. Specifically, the patterned support prepared via PS micromolding exhibited a cross-section with a dimension close to that of the original Sharklet pattern (width × height = 2 × 3 μm), representing a high pattern fidelity of ~75% (inset of Fig. 2a). The pattern valley (bottom)
Conclusions
In this study, a combinatorial anti-biofouling strategy was employed for a desalination membrane by fabricating a Sharklet pattern on the membrane surface and subsequently depositing hydrophilic TA and biocidal AgNPs. The fabricated membranes exhibited high RO performance with good water permeance and NaCl rejection. Importantly, the combined surface modification strategies endowed the membrane with superior biofouling resistance compared to topological or chemical modification only. The
Credit author statement
Wansuk Choi: Methodology, Validation, Formal analysis, Investigation, Writing - Original Draft. Min Gyu Shin: Methodology, Validation, Formal analysis, Investigation. Cheol Hun Yoo: Methodology, Validation, Formal analysis, Investigation. Hosik Park: Validation, Investigation. You-In Park: Validation, Investigation. Jong Suk Lee: Conceptualization, Methodology, Writing - Review & Editing. Jung-Hyun Lee: Conceptualization, Methodology, Writing - Review & Editing, Supervision.
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 National Research Foundation of Korea grant funded by the Korean government (2019R1A2C1002333 and 2019M3E6A1064103) and the Technology Innovation Program (20010914) funded by the Ministry of Trade, Industry and Energy (MOTIE, Korea).
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