关键词: 非织造布, 相转化法, 聚砜, 聚酰胺, 反渗透膜, 水处理

Abstract:

Objective The structure of polyamide composite reverse osmosis (RO) membranes primarily consists of a nonwoven fabric base layer, a polysulfone ultrafiltration (PSF) support layer, and a polyamide (PA) separation layer. The microstructure and properties of the nonwoven fabric directly influence the structure of the PSF support layer, which in turn affects the structure and performance of the RO membrane. A series of PSF base membranes and RO membranes were prepared based on nonwoven fabrics with different structural characteristics. The structure-performance relationships between the characteristics of the nonwoven fabric and the the PSF base membranes and RO membranes were investigated.

Method A series of polysulfone ultrafiltration base membranes were prepared using polyethylene terephthalate (PET) nonwoven fabric through a phase inversion process, and the corresponding RO membranes were fabricated by interfacial polymerization (IP). The influences of structural parameters, such as fiber packing density, water contact angle, and Wenzel roughness, of the nonwoven fabric on the structure of the polysulfone ultrafiltration base membranes were studied. Additionally, the RO membranes prepared were characterized using scanning electron microscope, and their separation performance was tested.

Results During the preparation of PSF-A base membranes, the surface was relatively hydrophilic (water contact angle of about 60°) with moderate Wenzel roughness because of the uniform fiber packing and moderate average cross-sectional density of approximately 0.75 g/(m²·μm) of nonwoven fabric A. This facilitated the casting of the PSF membrane solution and effectively permeated half of the nonwoven fabric A, resulting in a PSF-A base membrane with a moderate pore size (of about 35 nm) and a high surface porosity (of about 3.8%). This contributed to the effective permeation and uniform dispersion of the m-phenylenediamie (MPD) aqueous solution on the PSF membrane surface, leading to the formation of a uniform and dense RO-A membrane. Nonwoven fabric B exhibited severe fiber adhesion and the highest average cross-sectional density of approximately 0.85 g/(m²·μm), with a hydrophobic surface (water contact angle of about 84°) and the lowest Wenzel roughness. This reduced the effective permeation depth of the PSF casting solution (about one-third of nonwoven fabric B) while accelerating the phase inversion rate on the PSF membrane surface, resulting in a PSF-B base membrane with the largest pore size (about 45 nm), but fewer and unevenly distributed surface pores. During the preparation of the RO-B membrane, the MPD aqueous solution struggled to evenly disperse on the surface of the PSF-B membrane, causing the PA layer of the RO-B membrane to be uneven with more significant defects. Nonwoven fabric C had the loosest fiber packing with the lowest average cross-sectional density of about 0.50 g/(m²·μm), a strongly hydrophilic surface (water contact angle of about 12°), and the highest Wenzel roughness, which promoted the effective permeation (about two-thirds of nonwoven fabric C) of the PSF casting solution. The resulting PSF-C base membrane had the smallest pore size (of about 30 nm) and relatively concentrated surface pores. This led to the MPD aqueous solution being distributed only at the membrane pores of the PSF-C base membrane, resulting in an uneven distribution of the PA layer in the prepared RO-C membrane.

Conclusion In this study, a series of PSF base membranes were prepared by phase inversion process with three different types of nonwoven fabrics possessing varying properties. The influences of nonwoven fabric characteristics on the pore structure and performance of PSF base membranes were investigated. Additionally, the relationship between the pore structure of the PSF base membrane and the structure-performance of the polyamide layer in the RO membrane was explored. The results showed that when the fiber packing density of the nonwoven fabric is moderate (average cross-sectional density of approximately 0.75 g/(m²·μm)), the hydrophilicity is suitable (water contact angle of about 60°), and the surface Wenzel roughness is optimal (Wenzel roughness of about 1.15), it favors the permeation of the PSF casting solution (permeation depth of approximately half the thickness of the nonwoven fabric). The PSF base membrane prepared from this nonwoven fabric exhibited uniform pore size and distribution, which facilitated the orderly dispersion of MPD and promoted the interfacial polymerization (IP) reaction, resulting in a uniform and dense polyamide layer in the RO membrane, achieving the desalination rate of over 97%. In summary, by adjusting the pore structure and surface properties of the PSF base membrane, the nonwoven fabric effectively controlled the IP process. This study provides new insights into the development of high-performance RO membranes using nonwoven fabrics and PSF ultrafiltration base membranes.

Key words: nonwoven fabric, phase transition method, polysulfone, polyamide, reverse osmosis membrane, water treatment