Development of carbon dots-based thin-film nanocomposite membranes for high-performance nanofiltration

• Main Author: Zheng, Han
• Other Authors: Zhou Kun
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/169499
• Institution: Nanyang Technological University

The lack of fresh water is one of the most critical challenges in this century, while the accelerated population growth, industrialization, and global warming further escalate the challenge. During the past decades, pressure-driven membrane-based filtration technologies have gradually occupied the leading position in various water treatment processes to address the exacerbating problem of water scarcity. For instance, nanofiltration, which is capable of removing divalent ions and electroneutral species with a size larger than 1 nm, has been widely applied in recovery of fresh water from ground water, surface water, and different types of wastewater. Among all the developed membranes for nanofiltration, thin-film composite (TFC) membranes comprising a densely crosslinked polyamide selective layer tightly attached on a micro-porous substrate have been recognized as the benchmark because of their satisfactory sieving capability, high operational stability, and ease of fabrication. Unfortunately, despite the extensive research and development effort over the past 50 year, the nanofiltration performances of commercially available TFC membranes are still restricted by the trade-off between water permeability and water–solute selectivity, low solute–solute selectivity, and high fouling propensity. Recent advances in nanotechnology have paved a new way for addressing these challenges by incorporating nanomaterials (as nanofillers) into the polyamide matrix to obtain thin-film nanocomposite (TFN) membranes. These nanomaterials can effectively modify the inherent properties of the selective matrix (e.g., surface hydrophilicity, charge, pore size, surface roughness, and layer thickness) for improved water permeability, water–solute selectivity, solute–solute selectivity, and antifouling capability. Nonetheless, the introduction of nanomaterials in the thin and dense polyamide matrix could incur additional problems such as poor material compatibility and stability, leading to the improvement in some aspects of the nanofiltration performance at an expense of others. Therefore, proper selection and modification of nanomaterials is critical to the development of next-generation TFN nanofiltration membranes with significant improvements compared to conventional TFC membranes on all critical dimensions. Carbon dots (CDs), which are zero-dimensional nanoparticles comprising a carbon core with a typical size smaller than 10 nm and abundant surface functional groups, are one promising candidate as nanofillers for developing high-performance TFN membranes because of their remarkable size and chemical compatibility with the thin and dense polyamide selective layer, excellent stability under harsh operating conditions, as well as highly modifiable surface chemistry. This Ph.D. study is aimed to develop TFN nanofiltration membranes with enhanced nanofiltration performances based on multifunctional CDs nanomaterials. More specifically, this study seeks to 1) design and synthesize CDs with desired properties for the fit-for-purpose development of TFN membranes with improved water permeability, water–solute selectivity, solute–solute selectivity, antifouling capability, and operational stability, 2) acquire a deep understanding of the role of the CDs for the enhanced nanofiltration performances, and 3) optimize the nanofiltration performances of the TFN membranes by establishing a preparation–property–performance relationship. In the first work, zwitterionic CDs are synthesized and embedded in the polyamide selective matrix to enhance the surface hydrophilicity and create additional pathways for faster water permeation. More specifically, CDs grafted with hyperbranched zwitterions, denoted as CDs-ZPEI0.6–10k, are first prepared from the hydrothermal treatment of citric acid in the presence of zwitterionic hyperbranched polyethylenimine (ZPEI0.6–10k) with different molecular weights (0.6, 1.8, and 10 kDa). Subsequently, the synthesized nanoparticles are introduced in the membrane fabrication to form CDs-ZPEI0.6–10k-modified TFN (TFN-CDs-ZPEI0.6–10k) membranes. Characterization results reveal that the grafted shells of super-hydrophilic ZPEI not only increase the chemical compatibility of the CDs in the polyamide layer to suppress the formation of nonselective voids, but also create a zwitterion-based network surrounding the CDs for more efficient water transportation and effective divalent salt rejection. As a result, the TFN-CDs-ZPEI10k membrane overcomes the permeability–selectivity trade-off by demonstrating a 2.8-fold increase in the permeate flux with a higher Na2SO4 rejection rate of 98.1% than the pristine TFC membrane. Moreover, the antifouling property of the polyamide selective layer is improved by virtue of its enhanced surface hydrophilicity. In the second work, CDs with cationic amine groups (PEI-CDs) and those with anionic sulfonate groups (PS-CDs) are prepared and encapsulated into the polyamide selective layer to create charged nanochannels between the CDs and the surrounding polyamide network for high-efficiency nanofiltration. As suggested by the characterization and filtration experiments, the positively charged amine group-based and negatively charged sulfonate group-based nanochannels in the resultant TFN-PEI-CDs and TFN-PS-CDs membranes, respectively, provide alternative pathways for efficient water permeation while effectively rejecting divalent ions via the Donnan and dielectric exclusion effects, thereby overcoming the recurring permeability– selectivity trade-off suffered by dense polymeric membranes. In particular, the creation of negatively charged nanochannels enables the TFN-PS-CDs membrane to achieve one of the best performances among the recently reported nanofiltration membranes, by showing a nearly tripled pure water permeability (30.9 L m–2 h–1 bar–1) together with a higher Na2SO4 rejection rate (99.4%) than the pristine TFC membrane with a pristine polyamide selective layer. Moreover, both the TFN membranes demonstrate greater resistance to foulants with charges of the same sign as the nanochannels. In the third work, an effective approach is developed to systematically expand the pore size of polyamide layers by incorporating the designed ammonium ion-modified CDs into the polyamide network. Experimental analysis shows that the ammonium ions with different alkyl chain lengths attached to the CDs create nanochannels of different sizes in the network to lower the energy barrier for water transportation while maintaining high selectivity to target species. When the alkyl chain length of the ammonium ions reaches eight carbon atoms (i.e., C8 ions), the amphiphilic C8-CDs will induce the formation of a ridged surface nanostructure, hence increasing the membrane filtration area. A deep understanding of the preparation–property–performance relationship for the TFN-C8-CDs membranes is further obtained by manipulating the concentrations of monomers, C8-CDs nanoparticles, and C8 ammonium ions used for the membrane fabrication. The optimized TFN-C8-CDs membrane demonstrates a higher Na2SO4 rejection of 98.9% and NaCl/Na2SO4 selectivity of 83.1 than the pristine polyamide membrane, together with a tripled pure water permeability of 29.0 L m–2 h–1 bar–1. In summary, core–shell structured multifunctional nanoparticles based on CDs factionalized by different groups or compounds (i.e., zwitterions, amine polymers, sulfonate groups, and ammonium ions) are successfully designed and synthesized to improve the membrane hydrophilicity, create charge-tunable and size-tunable nanochannels in the polyamide matrix, and alter the surface nanostructure of the polyamide selective layer, thereby overcoming the permeability– selectivity trade-off, enhancing the solute–solute selectivity, and strengthening the antifouling capability. Attributed to the high stability of the CDs under various working conditions and their desired compatibility with the polyamide matrix, all the developed CDs-based TFN membranes achieve high operational stability. Limitations of the current work are discussed and recommendations and insights for future research are provided in the end. This study sheds light into the development of functionalized CDs nanoparticles to effectively address the challenges encountered by TFC and TFN membranes. This study also contributes to ingeniously designing the interior and surface nanostructure of polyamide membranes for more efficient filtration processes.