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Abstract
Thin-film nanocomposite membranes (TFNs) are a recent class of materials that use nanoparticles to provide improvements over traditional thin-film composite (TFC) reverse osmosis membranes by addressing various design challenges, e.g., low flux for brackish water sources, biofouling, etc. In this study, TFNs were produced using as-received cellulose nanocrystals (CNCs) and 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanocrystals (TOCNs) as nanoparticle additives. Cellulose nanocrystals are broadly interesting due to their high aspect ratios, low cost, sustainability, and potential for surface modification. Two methods of membrane fabrication were used in order to study the effects of nanoparticle dispersion on membrane flux and salt rejection: a vacuum filtration method and a monomer dispersion method. In both cases, various quantities of CNCs and TOCNs were incorporated into a polyamide TFC membrane via in-situ interfacial polymerization. The flux and rejection performance of the resulting membranes was evaluated, and the membranes were characterized via attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The vacuum filtration method resulted in inconsistent TFN formation with poor nanocrystal dispersion in the polymer. In contrast, the dispersion method resulted in more consistent TFN formation with improvements in both water flux and salt rejection observed. The best improvement was obtained via the monomer dispersion method at 0.5 wt% TOCN loading resulting in a 260% increase in water flux and an increase in salt rejection to 98.98 ± 0.41% compared to 97.53 ± 0.31% for the plain polyamide membrane. The increased flux is attributed to the formation of nanochannels at the interface between the high aspect ratio nanocrystals and the polyamide matrix. These nanochannels serve as rapid transport pathways through the membrane, and can be used to tune selectivity via control of particle/polymer interactions.
Highlights
Access to clean drinking water is one of the foremost challenges faced by engineers and scientists today
Novel thin-film nanocomposite membranes containing cellulose nanocrystals have been fabricated using a vacuum filtration method previously established in our group as well as a new method involving dispersion of the nanocrystals in a monomer solution
The Thin-film nanocomposite membranes (TFNs) membranes produced via the vacuum filtration method exhibited inconsistent results with cellulose nanocrystals (CNCs)-based membranes exhibiting high salt rejection but some evidence of decreased water flux
Summary
Access to clean drinking water is one of the foremost challenges faced by engineers and scientists today. Much of the desalination research over the past 10 years has focused on improving the efficiency of membrane technologies in order to lower the production cost for large-scale processes [4]. The majority of current RO processes use aromatic polyamide (PA)-based thin-film composite (TFC) membranes based on a design developed in the late 1970s by Cadotte [5]. Adding nanoparticles to TFC membranes results in a new class of thin-film nanocomposite (TFN) membranes. In this context, nanoparticles are of interest due to their unique morphologies, surface chemistry, pore structures, and their ability to influence crosslinking during interfacial polymerization (IP) [9]. Due to the high cost of many nanoparticles, such as carbon nanotubes, this work focuses on cellulose nanocrystals (CNCs) as a cost-effective alternative
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