The Secret Ingredient, Cellulose—Exploring Its Potential in Desalination Membranes

Seawater desalination by utilizing a membrane-based technology is a crucial step to overcome the freshwater shortage in some regions of the world. However, the durability and performance of membranes rely on the type of materials and fillers utilized for the fabrication of the membrane. In the efforts towards sustainable, biocompatible, low cost and economical materials, cellulose-based materials or derivatives are the main focus to develop membranes for water desalination. Herein, cellulose is explored as:<br/>Novel material with unique structure to control swelling of reverse osmosis membranes.<br/>Innovative entrapping media for better electrically conductive desalination membranes.<br/>Coating layer to create hydrophilic surface for better desalination of oily wastewater.<br/><b>Anti-swelling desalination membranes. </b>Novel networked cellulose (NC) material is prepared via combination of controlled sulfuric acid hydrolysis and calculated ethanol regeneration of microcrystalline cellulose (MCC). While acid concentration was kept constant at 70%, factors such as hydrolysis time and temperature were varied to obtain the unique structure. The high-yield material has interesting mechanical integrity with tangled networked fibers that make it suitable for improving materials properties. NC was employed as an anti-swelling material that improved the performance of polyvinyl alcohol (PVA) reverse osmosis (RO) membranes. The open structure of NC entraps the PVA polymer inside and restrict PVA from expanding. This in turn imparts it stability against swelling. In addition, the hydrogen bonding formed provides a compact structure and thus prevents the membrane from swelling. The swelling capacity of PVA was reduced from 340% to 150% without compromising desalination performance. In addition, the tensile strength of wet PVA improved 1520% and the elastic modulus improved 1400% upon blending with NC.<br/><b>Electrically enhanced fouling control nanofiltration/RO membranes</b>. Electrically conductive membranes based on NC and carbon nanostructures (CNS) were fabricated via vacuum filtration. The hydrolysis process of cellulose was exploited to better incorporate and entrap CNS within the NC structure. High surface area multi-walled CNTs become trapped in the structure of networked cellulose. Membranes were tested and analyzed for their electrocatalytic activity and explored for their potential in fighting fouling issue. Furthermore, the compaction of pores via incorporation of NC resulted in promising results with respect to nanofiltration of divalent ions, with a rejection of 60% for MgSO₄ and 47% for CaCl₂, while maintaining a high flux of ≥ 100 L m^{− 2} h^{− 1}. Furthermore, NC/CNS/PVA membranes demonstrated enhanced electrocatalytic activity. The RO membranes were tested with 25,000 ppm NaCl as feed, and exhibited 99.8% salt rejection with 93% increase in flux compared to PVA-NC with no CNS implying homogenously mixed CNS. The membrane surface was recovered after fouling via electrolytic cleaning where the membrane was used as the cathode and a potential of −5 V was applied for 20 min.<br/><b>Improved distillation membranes (MD)</b>. A novel concept of stacking two membranes with different wettabilities to obtain a dual-layered membrane was developed. The hydrophilic top layer composed of electrospun PVDF-HFP impregnated with regenerated MCC cellulose while the bottom layer kept hydrophobic. Top layer acted as a hydrophilic barrier preventing oil from passing to the hydrophobic bottom layer during MD process. When tested with saline feed containing 1000 ppm of oil, the stacked dual-layered membrane yielded fresh water flux up to 12.8 kg m⁻² h⁻¹ with complete salt rejection revealing a simple and scalable method for oily wastewater treatment using relatively inexpensive, abundant and ecofriendly materials.