Water purification ultrafiltration membranes using nanofibers from unbleached and bleached rice straw

www.nature.com/scientificreports OPEN Water purification ultrafiltration membranes using nanofibers from unbleached and bleached rice straw Mohammad L. Hassan1*, Shaimaa M. Fadel1, Ragab E. Abouzeid1, Wafaa S. Abou Elseoud1, Enas A. Hassan1, Linn Berglund2 & Kristiina Oksman2,3* There has been an increasing interest in recent years in isolating cellulose nanofibers from unbleached cellulose pulps for economic, environmental, and functional reasons. In the current work, cellulose nanofibers isolated from high-lignin unbleached neutral sulfite pulp were compared to those isolated from bleached rice straw pulp in making thin-film ultrafiltration membranes by vacuum filtration on hardened filter paper. The prepared membranes were characterized in terms of their microscopic structure, hydrophilicity, pure water flux, protein fouling, and ability to remove lime nanoparticles and purify papermaking wastewater effluent. Using cellulose nanofibers isolated from unbleached pulp facilitated the formation of a thin-film membrane (with a shorter filtration time for thin-film formation) and resulted in higher water flux than that obtained using nanofibers isolated from bleached fibers, without sacrificing its ability to remove the different pollutants. Membranes are used in a wide variety of industries for removing undesirable materials from different media. Among them, those that depend on separation based on the size of materials to be removed, e.g., micro-, ultra- and nano-filtration membranes, are manufactured using different polymers; generating pores within these membranes with targeted size is the key for their u se1. Using cellulose and its derivatives for making membranes for use in micro- and ultrafiltration is well known at the commercial level2. Cellulosic membranes are usually prepared using casting technique, which depends on the dissolution of cellulosic materials in suitable solvents, followed by film formation by immersion in a non-solvent. This technique requires use of large amounts of solvents; their recovery is important for economic and environmental reasons. Since the emergence of nanocellulosic materials, e.g., cellulose nanofibers and cellulose nanocrystals, increasing research is ongoing regarding their use in the area of membranes. Two approaches have been investigated for use of nanocellulose in that area so far3. The first approach is through their incorporation in other polymer matrices to improve the performance of prepared membranes. Dissolving the polymers in suitable solvents and also good dispersion of nanocellulosic materials in the polymer solution are necessary before film casting. The second approach studied, which is more favorable and interesting, is to make membranes from a layer of nanocellulose with adequate porosity over other polymeric supports without the need to dissolve the cellulose and to use the film casting technique to generate porosity. The second approach was first introduced by Ma et al.4; it depends on the correlations between pore size distribution of a nonwoven layer structure and bulk porosity. According to this approach, the pore size of a film made from fibers can be adjusted by controlling the fiber diameter. It was also proved through a study of the pore size of films made from electrospun nanofibers that the mean pore size was found to be a factor of ~ 3 ± 1 greater than the mean fiber diameter and that the maximum pore size was a factor of ~ 10 ± 2 greater than the mean fiber diameter4. Because cellulose nanofibers and nanocrystals have diameters within a few nanometers, generally from 4 to < 100 nm, formation of membranes suitable for ultra- and 1 Cellulose and Paper Department and Centre of Excellence for Advanced Sciences, National Research Centre, 33 El‑Buhouth street, Dokki 12622, Giza, Egypt. 2Department of Engineering Sciences and Mathematics, Luleå University of Technology, 97187 Luleå, SE, Sweden. 3Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, ON M5S 3G8, Canada. *email: ; Scientific Reports | (2020) 10:11278 | https://doi.org/10.1038/s41598-020-67909-3 1 Vol.:(0123456789) www.nature.com/scientificreports/ Figure 1.  TEM images of RSNF isolated from (a) unbleached neutral sulfite pulp and (b) bleached soda pulp. nanofiltration purposes could be expected. Cellulose nanofibers can be isolated from cellulose pulp with much higher yield because their isolation depends on applying a high mechanical shear force onto the pulp fibers. Cellulose nanocrystals are prepared by acid hydrolysis of pulps using a relatively high concentration of acids, and the yield of the isolated nanocrystals is relatively low. The use of cellulose nanofibers has been investigated much more than that of cellulose nanocrystals to make films for ultrafiltration membranes because the nanofibers have much better ability to form thin films with good flexibility and less aggregation than the nanocrystals5–8. The published works so far regarding making films from cellulose nanocrystals for ultrafiltration applications have focused on their use as self-standing nanopaper sheets5, thin films over cellulose n anofibers6, thin films over filter paper and comparing them to cellulose n anofibers7, and thin-layer films over polyethersulfone and comparing them to cellulose nanofibers8. However, cellulose nanofibers were used for making thin-film membranes in more s tudies3,9–11, in addition to their use as additives in other polymeric matrices to improve the properties of prepared membranes3,12. The literature survey mentioned above reveals that cellulose nanofibers used in making membranes were extracted from bleached cellulose pulp fibers, i.e., fibers mostly from cellulose and some hemicelluloses but without or with very little l ignin3,9–11. A common challenge associated with membrane preparation from cellulose nanofibers is formation of compact films of these nanofibers upon drying as a result of extensive hydrogen bonding between hydroxyl groups at their surfaces. This results in low-flux membranes, especially when filtering suspensions containing particles with small size or high-molecular-weight materials. To overcome this problem, researchers have studied different solutions, such as exchanging water in the wet nanofiber film after formation of membranes by using other solvents such as a lcohol10,13. This resulted in formation of less dense films and improving the porosity and thus the flux of the membranes. Others studied the addition of other particles to the nanofiber suspension before membrane preparation and drying; this added gaps between the nanofibers and thus improved the porosity and flux10,14. Others studied chemical modification of nanofiber’ surfaces by spacers. Upon membrane formation, these spacers act as gaps between the nanofibers, leading to improved water flux of the m embranes11. All of the aforementioned solutions definitely add to the cost of mak (...truncated)