Membranes hydroxyl rejection

T.D. Gierke, Ionic Clustering in Nation Perfluorosulfonic Acid Membranes and Its Relationship to Hydroxyl Rejection and Chlor-Alkali Current Efficiency, Paper presented at the Electrochemical Society Fall Meeting, Atlanta, GA (1977). 

Gierke, T.D. "Ionic Clustering in Nafion Perfluorosulfonic Acid Membranes and its Relationship to Hydroxyl Rejection and Chlor-Alkali Efficiency", presented at the 152nd National Meeting of the Electrochemical Society, Atlanta, Ga.,... 

A model for ionic clustering in "Nafion" (registered trademark of E. I. du Pont de Nemours and Co.) perfluorinated membranes is proposed. This "cluster-network" model suggests that the solvent and ion exchange sites phase separate from the fluorocarbon matrix into inverted micellar structures which are connected by short narrow channels. This model is used to describe ion transport and hydroxyl rejection in "Nafion" membrane products. We also demonstrate that transport processes occurring in "Nafion" are well described by percolation theory. 

In this work we propose a model for ionic clustering, which we have called the cluster-network model (2), to account for hydroxyl rejection in nNafionM perfluorinated membranes. In developing this model we have been guided by two requirements 1. the model should be consistent with the available data on the microscopic structure of the polymer (1-5) 2. the model should... 

In the next section we will present the data and arguments on which the cluster-network model is based. We will also discuss the effects of equivalent weight, ion form, and water content on the dimensions and composition of the clusters. In the third section we will present a formalism, which follows from the cluster-network model, based on absolute reaction rate theory (2) and hydroxyl rejection in "Nation perfluorinated membranes. Finally we will outline the concepts of percolation theory and demonstrate that ion transport trough "Nation" is well described by percolation. 

Impurity-Resistant Membranes. Section 4.8 discusses two fundamental issues related to membranes, the possibility of achieving nearly 100% current efficiency and the development of impurity-resistant membranes. Total rejection of the hydroxyl ion by the membrane is possible if the only anode reaction is the discharge of the chloride ion. However, if oxygen evolution, which is favored thermodynamically, also takes place, the principle of electroneutrality makes it impossible for the membrane to exclude the back-migration of the hydroxyl ion. 

Cellulose acetate is an ester of cellulose and acetic acid. Hence, hydrolysis takes place when the pH of the solution with which a cellulose acetate membrane is in contact is too high or too low, lowering the degree of acetylation, defined as the number of hydroxyl groups (total of three in one D-glucopyranose unit) that can be acetylated. Because a degree of acetylation above 2.5 is required for satisfactory salt rejection in seawater desalination, excessive hydrolysis results in poor membrane performance. The pH values between 5 and 7 should be maintained when cellulose acetate membranes are used. ... 

It has been shown that UF PSU membranes treated for 20 sec with oxygen plasma showed increased hydrophilicity. X-ray photoelectron spectroscopy (XPS) analysis proved that this improvement was caused by the presence of the hydroxyl, carbonyl, and carboxyl groups on the surface. For such modified membranes, the flow rate of pure water and gelatin increased and the membranes showed fewer fouling properties (Kim et al. 2002a). By using O2 plasma treatment, the UF property of the PAN (Tran et al. 2007) and PET (Touflk et al. 2002) track membranes could be improved with the enhancement of the membrane flux. Meanwhile, their rejection of albumin and dextrans was almost maintained. 

Current efficiency declines are strictly related to the membrane. Impurities lower the current efficiency by reducing the membrane s ability to reject anions, specifically the ability to prevent hydroxyl ions from migrating from the cathode compartment through the membrane to the anode compartment [144]. This is usually a result of physical damage caused by precipitation and crystallization of impurities inside the membrane. Impurities precipitate because the environment in the membrane changes from an acidic salt solution (pH 2 - 4) to a caustic solution (pH 14 -15) over the 100 - 300 pm thickness of the membrane. 

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