Nano-Nanocomposite Membrane Based on Cellulose-Nanofiber and Carbon Nanotubes for Low-Pressure Reverse Osmosis Systems

A nano-nanocomposite polymeric membrane based on multiwalled carbon nanotubes (CNT), cellulose nanofibers (CNF), and polyamide was used to produce high purity water in a low-pressure reverse osmosis system. The PA-CNT/CNF membranes were physicochemically characterized in order to track the influence of adding different loads of cellulose nanofiber, the influence of isopropanol (IPA) used as a diluting agent, and a sodium nitrate (NaNO₂) post-treatment applied after membrane synthesis. The mixture of CNF/CNT had a significant influence on membrane roughness, hydrophobicity, and the type of functional groups at the surface compared with a plain PA membrane. The incorporation of CNF improves the water permeation while the CNT structure supports the salt rejection at low pressure (0.75 MPa) RO filtration. A significant amount of experiments showed how the concentration of isopropanol also positively influenced the water permeation and salt rejection at low concentrations but negatively impacted both rejection and permeation at higher concentrations. In the case of the post-treatment with NaNO_2, this improves both water permeation and salt rejection. In the end, a CNF concentration between 3 wt. % to 6 %, IPA concentration between 4% to 6%, and treated with NaNO₂ revealed salt rejections above 99 % and water flux above the 1 m³/m²day. The best performance showed a salt rejection of 99.47% and water permeation of 1.65 m³/m²day, very competitive compared with commercial systems like ES20, ES40 (From Nitto-Denko), and BW60 (Dupont). Rejection analysis of selective ions like Ca, Si, and B salts was performed. The PA-CNT/CNF membrane expands the operation time compared with NSF-certified commercial systems for high-purity water production tested with a Si, Ca, and Na mixed solution. The PA-CNT/CNF membrane maintains a Ca rejection of 97.5% with an increased permeability by 1.8 times and a boron rejection of 50.4%. A 2 inches module was built using the PA-CNT/CNF membrane and was tested in a more realistic environment. Its performance compared with a commercial module (TW30), the PA-CNT/CNF module showed an increase of 14.3% over the water flux production before its saturation. This PA-CNT/CNF polymeric nanocomposite has shown a very competitive performance for the rejections of Si, Na, Ca ions and the production of HPW in a low-pressure RO system.