Polyamide composite membranes degradation

Polyamide, composite membranes are very sensitive to free chlorine (recall from Chapter 4.2.1 that cellulose acetate membranes can tolerate up to 1 ppm free chlorine continuously). Degradation of the polyamide composite membrane occurs almost immediately upon exposure and can result in significant reduction in rejection after 200 and 1,000-ppm hours of exposure to free chlorine (in other words after 200-1,000 hours exposure to 1 ppm free chlorine). The rate of degradation depends on two important factors 1) degradation is more rapid at high pH than at neutral or low pH, and 2) the presence of transition metals such as iron, will catalyze the oxidation of the membrane. 

Initially, polyamide composite membranes that have been degraded due to chlorine attack will exhibit a loss in flux.4 This drop in flux is followed by an increase in flux and salt passage. 

In theory, the tolerance of polyamide composite membranes to chloramines is about 300,000 ppm-hrs. However, chloramines are usually in equilibrium with free chlorine, making it difficult to use chloramine in RO pretreatment, as the free chlorine will degrade polyamide composite membranes. 

A typical recipe for an interfacially formed aromatic polyamide composite membrane comprised a 2.0% aqueous solution of the aromatic diamine and a 0.1% nonaqueous solution of trimesoyl chloride. This recipe was extraordinarily simple, and ran quite contrary to experience with piperazine-based membranes. For example, surfactants and acid acceptors in the aromatic diamine solution were generally not beneficial, and in many cases degraded membrane performance by lowering salt rejection. In contrast, surfactants and acid acceptors were almost always beneficial in the NS-300 membrane system. In the nonaqueous phase, use of isophthaloyl chloride as a partial replacement for trimesoyl chloride had relatively little effect on flux, but tended to decrease salt rejection and increase susceptibility to chlorine attack. 

Monochloramine is approximately 200 times less effective than free chlorine as a disinfectant. However, it is still used as an alternative to chlorine because chloramines do not react as readily with organic materials to form trihalomethanes (THMs). In theory, due to the less aggressive nature of chloramines, the tolerance of polyamide composite membranes to chloramine is about 300,000 ppm-hrs. However, chloramines are usually in equilibrium with free chlorine, making it difficult to use chloramine in RO pretreatment, as the free chlorine will degrade polyamide composite membranes. 

Nonetheless a few commercially successful noncellulosic membrane materials were developed. Polyamide membranes in particular were developed by several groups. Aliphatic polyamides have low rejections and modest fluxes, but aromatic polyamide membranes were successfully developed by Toray [25], Chemstrad (Monsanto) [26] and Permasep (Du Pont) [27], all in hollow fiber form. These membranes have good seawater salt rejections of up to 99.5 %, but the fluxes are low, in the 1 to 3 gal/ft2 day range. The Permasep membrane, in hollow fine fiber form to overcome the low water permeability problems, was produced under the names B-10 and B-15 for seawater desalination plants until the year 2000. The structure of the Permasep B-15 polymer is shown in Figure 5.7. Polyamide membranes, like interfacial composite membranes, are susceptible to degradation by chlorine because of their amide bonds. 

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