The Effect of the Carbon Nanotubes’ Surface Area On the Morphology and Performance of Cellulose Acetate Nanocomposite Membranes in Nanofiltration Applications

Population increase and climate change have significantly affected the availability of fresh water for consumption. Studies by UNESCO showed that more than 28% of the world population didn’t have access to clean safe drinking water in 2020. This situation is expected to get more serious over the coming 30 years with the limited availability of natural fresh water sources. The need to find solutions from alternative non-freshwater resources for drinking or irrigation is becoming a necessity. Within the context of desalination technologies, thousands of investigations are conducted to find the cheapest most efficient way of water treatment aiming at expanding the availability of fresh water. The most widely used approach entails membranes technology that has a 75% market share of water production. This approach proved to be cost effective with low energy consumption. However, just like any other technology, it faces several limitations and challenges including the several pretreatment steps of raw water from different sources to reduce fouling and scaling on the surface of the membranes, membranes degradation from pH or chlorine disinfection, or change in performance due to different feed temperatures. The wide variety of operational conditions and limitations opened vast areas of research for membranes’ development. A special class of filtration membranes, nanocomposite membranes, utilizes the incorporation of nanomaterials (nanofillers) in polymeric matrices aiming at developing membranes that need the lowest possible pressures to give the highest water permeation rates, salt rejection rates, maximum resistance to different fouling factors and long durability, all with minimal cost of production. This entails a thorough understanding of the effect of different preparation conditions on the morphology and porosity of the nanocomposite membranes. Our research team was the first to report in the literature the incorporation of functionalized multiwalled carbon nanotubes (CNT), as nanofillers, to cellulose acetate (CA) polymer nanocomposite membranes for water desalination applications. The study entailed using different concentrations of the CNT and different preparation conditions to reach an optimum performance for NaCl rejection in comparison to blank CA membranes. To the best of our knowledge, studying the effect of different surface areas of CNT on nanocomposite membranes in filtration applications has not been thoroughly investigated. Therefore, as a continuation of our previous work, the current research aimed at addressing this research gap. In this respect, seven different surface areas of functionalized CNT were incorporated, at the same concentrations (0.0010 wt%), to 15 wt% CA (50,000 Da) nanocomposite membranes prepared using phase inversion with acetone as a solvent and deionized water as a non-solvent. Quantification of the carboxylic functional groups on the surface of the CNT was carried out using Boehm acid base back-titration and was found to range from ~0.7 to ~0.2 mmol/g of the CNT. On the other hand, characterization of the CNT/CA cast solutions was carried out using Rheology measurements, and the viscosity of the cast solutions were found in the range of ~5 to ~7 Pa.s, without showing any correlation with the change in the CNT surface area. Characterization of the morphology, nanofiller dispersion, porosity, hydrophilicity, and tensile properties of the CNT/CA nanocomposite membranes was carried out using SEM, AFM, porosity % test, water uptake %, BET, contact angle, and tension stage. It was found that, as a general trend, as the surface area of the CNT increased, the overall porosity of the nanocomposite membranes decreased in terms of micropores and macrovoids’ sizes, as well as the overall porosity percentage. Roughness and contact angles measurements showed a slight decrease in values with the increase in CNT surface areas. The water uptake and tensile properties of the membranes were found to show no correlation with the change in CNT surface areas. The performance analysis of the nanocomposite membranes in nanofiltration application was evaluated by measuring the pure water flux as well as the permeation and rejection rates of 5000 ppm Na2SO4 salt solution at 15 bars. Two different trials were carried out in this essence. The first was evaluating open capped CNT with closed capped CNT of the same types of CNT with different surface areas, and it was found that the open capped CNT had higher salt permeation rates (\u3e60% increase) with a slight adverse effect on the salt rejection (2h at 15 bars. This was due to the direct impact of the change in porosity on performance of the membranes