What specific impact do the material properties of seawater reverse osmosis membrane have on its salt rejection rate?
Release Time : 2025-09-03
The influence of seawater reverse osmosis membrane material properties on its salt rejection rate is essentially a function of the membrane material's chemical structure, physical morphology, and separation mechanism. Membranes of different materials exhibit significant differences in molecular sieving capacity, charge interactions, and anti-fouling properties, and these properties directly determine salt rejection.
The chemical structure of the membrane material determines its molecular sieving capacity. For example, cellulose acetate membranes, whose microporous structure, formed by their natural polymer chains, primarily relies on the size-exclusion effect to retain ions and small molecules. Due to its wide pore size distribution, cellulose acetate typically achieves a salt rejection rate of 95%-98% for monovalent ions (such as sodium and chloride), while achieving a 98% rejection rate for organic compounds with a molecular weight greater than 100. In contrast, polyamide membranes, formed through interfacial polymerization, form an ultra-thin desalination layer with a denser cross-linked structure. Their pore size can be controlled between 0.5 and 1 nanometer, resulting in salt rejection rates exceeding 99% for high-valent ions (such as calcium and magnesium), and a stable rejection rate of over 98% for monovalent ions. This difference stems from the stronger steric hindrance provided by the aromatic ring structure of polyamide, making it more difficult for small ions to pass through.
The surface charge characteristics of seawater reverse osmosis membranes significantly influence ion retention. Polyamide membranes typically have a negative surface charge, which results in a stronger repulsion of positively charged ions (such as calcium and magnesium), thereby improving salt rejection. However, this charge also results in slightly lower retention of negatively charged ions (such as chloride and sulfate) than positively charged ions. Furthermore, the surface charge density of the membrane also affects its antifouling performance. For example, cellulose acetate membranes, due to their neutral surface charge, are less susceptible to the adsorption of charged colloidal particles, allowing them to maintain high salt rejection in low-pollution water. However, over long-term operation, the surface charge of polyamide membranes may change due to the adsorption of organic matter, leading to fluctuations in salt rejection.
The antifouling properties of seawater reverse osmosis membrane materials indirectly affect the stability of salt rejection. Cellulose acetate membranes, due to their strong hydrophilicity, are less susceptible to biofilm and inorganic scale formation on their surfaces. Therefore, they can maintain high salt rejection rates even when seawater pretreatment is inadequate. However, they have poor heat resistance and are susceptible to hydrolysis at high temperatures (>35°C), resulting in a decrease in salt rejection. While polyamide membranes offer greater chemical stability, they are sensitive to residual chlorine, which breaks down amide bonds, causing pores on the membrane surface and significantly reducing salt rejection. Therefore, when using polyamide membranes, it is necessary to strictly control the residual chlorine content of the influent or to use chlorine-resistant modified materials (such as polyhydrazide membranes).
The membrane's structural design also influences salt rejection. Composite membranes combine an ultrathin desalination layer with a porous support layer, achieving both high salt rejection and improved water flux. For example, the desalination layer of a polyamide composite membrane is typically less than 0.2 microns thick, while the support layer has pores ranging from 10 to 50 nanometers. This structure allows for rapid passage of water molecules while effectively retaining salt. In contrast, asymmetric membranes (such as cellulose acetate hollow fiber membranes) have thicker desalination layers (approximately 0.5-1 micron). While their desalination rates are slightly lower, they offer lower manufacturing costs and are suitable for large-scale desalination projects.
The pressure resistance of the seawater reverse osmosis membrane material also affects desalination efficiency. Under high-pressure operating conditions (6-8 MPa), polyamide membranes, due to their excellent mechanical strength, maintain the structural integrity of the desalination layer and maintain stable desalination rates. Cellulose acetate membranes, on the other hand, are susceptible to compaction and deformation under high pressure, resulting in a smaller pore size and reduced water flux. However, this narrower pore size may slightly improve desalination efficiency.