Reverse osmosis solute ionization

The pH of the aqueous solution will determine the extent of ionization of solutes like phenol, 2-chlorophenol or acetic acid in the solution. If such a solute is ionized, the separation of such a solute from water by reverse osmosis through a membrane will be changed since the ionized solute and the nonionized solute have different solute rejections through the membrane. Consider an ionizable acidic solute species 1 ionization leads to a negatively charged species 2 (see equation (5.2.57)). The total feed solute concentration, Cy., is then given by 

We assume now that, other conditions remaining constant, the intrinsic solute rejections of the ionized and unionized species remain independent of pH. Let the solute rejection of the unionized solute 1 be Ri and that of the solute 2 be i 2- Such values for phenol can be determined, for example, by making two measurements with a reverse osmosis membrane (say, the FT-30 membrane manufactured by Filmtec Inc., Minnetonka, MN) at a low pH, phenol is undissociated and so i i is obtained, while, at a very high pH, all phenol is dissociated and present as CeHsO (or, say, as sodium phenolate in caustic solution), yielding i 2- 

The fraction of this solute present in the feed in the ionized form is given by 

Similarly, the permeate concentration of the unionized solute Qp can be obtained using the definition of intrinsic rejection Ri of the unionized species 1, 

The solute rejection under partially ionized condition, 

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Figure 7. Experimental data on the effect of (a) Taft s number for alcohols and (b) interfacial free energy parameter for ionized inorganic solutes on their reverse osmosis separations in systems involving dilute aqueous solutions and cellulose acetate membranes (9 15, 37, 38 ...

A method of producing drinkable water from salt water is reverse osmosis, in which a pressure just in excess of the osmotic pressure is applied to the solution in order to reverse the flow of the solvent (H2O). In principle, what pressure would be required to produce pure water from sea water at 25°C Assume that sea water has a density of 1.021 g/mLand can be considered equivalent to 3.00% by weight NaCl, which is 100% ionized. Express your answer in atm, kPa, and psi. 

Previous work with aqueous solution systems has been successful In treating both completely Ionized salts as well as Incompletely Ionized salts (, 6). This work Incorporates both of these cases In methanol solutions and uses the Klmura-Sourlrajan analysis for the treatment of reverse osmosis data (.7). The surface excess free energy parameters (.-tAG/KT) for the Ions and Ion pairs Involved were determined by the methods established earlier (8). The predictability of membrane performance by the use of data on free energy parameters obtained In this work has been tested. 

The physicochemical criteria approach to reverse osmosis separations Involving the surface excess free energy of solvation for ionized and nonlonized solutes has been demonstrated by this work to include nonaqueous solutions. The parameters and correlations presented in this work permit the prediction of reverse osmosis separations and permeation rates for different alkali metal halides for cellulose acetate OEastman E-398) membranes of different surface porosities from only a single set of experimental data for a sodium chloride-methanol reference feed solution system. 

Reverse osmosis systems now commercially available will remove 95% or more of the dissolved solids normally removed in ion exchange, and as will be discussed later, a few that are not. The ionized solutes are not all removed to the same degree by reverse osmosis any more than ion exchange resins have the same effect on all solutes. Divalent and multivalent ions, such as calcium, magnesium, sulfate, iron and manganese, can be rejected to greater than 99%. So-... 

Electrodialysis (ED) is used to remove ionized substance from hquids through selective ion-permeable membranes. ED is the most widely commercialized electromembrane technology. Desalination of brackish water is the area of electrodialysis application with the largest number of installations. This chemical-free technology competes with reverse osmosis. Electrodialysis shows better resistance to fouling and scaling. It also has an economical advantage in desalination of low-salinity solutions [13]. Also, it should be kept in mind that because of small material consumption ED is the most environmental friendly process for solution desalination [14]. 

We wish to design a reverse osmosis plant to prepare drinking water from water with too high a salt content to drink. If the maximum pressure we can afford is lOOpsig, what is the highest salt content in the water we can tolerate and still produce pure drinking water Assume that the salt is aU sodium chloride, that it is 100% ionized, and that it forms an ideal solution. 

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