Ion rejection

Common membrane processes include ultrafiltration (UF), reverse osmosis (RO), electro dialysis (ED), and electro dialysis reversal (EDR). These processes (with the exception of UF) remove most ions RO and UF systems also provide efficient removal of nonionized organics and particulates. Because UF membrane porosity is too large for ion rejection, the UF process is used to remove contaminants, such as oil and grease, and suspended soHds. 

Vourch et al49 studied the applicability of the RO process for the dairy industry wastewater. The treated wastewater total organic carbon (TOC) was <7 mg/L. It was found that in order to treat a flow of 100 m3/d, 540 m2 of the RO unit is required with 95% water recovery. Dead-end NF and RO were studied for the treatment of dairy wastewater.50 Permeate COD, monovalent ion rejection, and multivalent ion rejection for the dead-end NF were reported as 173-1095 mg/L, 50-84%, and 92.4-99.9%, respectively. When it comes to the dead-end RO membranes, the values for permeate COD, monovalent ion removal, and multivalent ion removal were 45-120 mg/L, >93.8%, and 99.6%, respectively. Membrane filtration technology can be better utilized as a tertiary treatment technology and the resultant effluent quality will be high. There can be situations where the treated effluents can be reused (especially if RO is used for the treatment). 

Loeb-Sourirajan membranes based on sulfonated polysulfone and substituted poly(vinyl alcohol) produced by Hydranautics (Nitto) have also found a commercial market as high-flux, low-rejection membranes in water softening applications because their divalent ion rejection is high. These membranes are also chlorine-resistant and have been able to withstand up to 40 000 ppm h of chlorine exposure without degradation.1 The structures of the polymers used by Hydranautics are shown in Figure 5.8. 

Fig. 14.7. Chloride ion rejection (Rob.) during the diafiltration of NaCl from a solution of red dye. During this process the charge properties of the membrane change continuously. The bold line shows the prediction allowing for this change in properties. The upper line is the prediction if it is assumed that the membrane charge remains at its initial value and the lower line is the prediction if it is assumed that the membrane charge remains at its final value. (Xa is the membrane charge.)...

T. Tsuru, S. Nakao and S. Kimura, Calculation of ion rejection by extended Nernst-Planck equation with charged reverse osmosis membranes for single and mixed electrolytes. J. Chem. Eng. Japan 24 (1991) 511-517. 

The striations in the common. Type I grains of belite are zones into which impurity ions rejected during the transition from a to a have been concentrated (Y1,01,C1). In some cases, they appear to have retained the a-CjS structure (Y 1,01,FI). In contrast, a study on synthetic belites showed that the material exsolved between the lamellae or at grain boundaries was often... 

The polyelectrolyte multilayers consist of an alternating sequence of molecular layers of cationic and anionic polyelectrolytes and can therefore be regarded at as a multi-bipolar membrane on a molecular level, which likewise should be useful for ion separation. A corresponding model for the ion rejection of the multibipolar membrane [75] is shown in Fig. 20. The model... 

Due to Dorman exclusion principle [29] charged membranes can reject inorganic salts even though they have pores much larger than the salts and this ion rejection is known to decrease with increasing feed ionic strength. The example of 1.1 electrolyte filtration through different pore sizes at a pH far from the... 

Fig. 12.12. Influence of zeta-potential (Stem-layer thickness 1) and Streaming-potential (electrokinematic flow) on ion rejection and volume flux for porous ceramic membranes exhibiting negatively charged pore walls. Cases of micropores (nanofiltration), mesopores (ultrafiltration) and macropores...

Results concerning filtration studies with 0.2 pm titanium dioxide membranes supported on stainless steel or ceramic porous tubes were recently reported by Porter et al. [47,48]. Solutions containing sodium nitrate alone and in the presence of anionic, direct and acid dyes were filtered with adjusted solution pH. Electrolyte rejections and colour rejections were measured at pH values from 4 to 10. They showed that the charged membrane was responsible for ion rejection at low ionic concentration while rejection decreased to near 0% as the salt concentration was raised to 5000 ppm. These results are consistent with long range forces associated to Debye-length which can reach several hundred Angstroms in the solution for very low ionic concentrations. 

The electric field intensification in the surfactant-mediated separation processes is shown in Table In the feed, surfactant and metal ion concentrations are denoted by Csf and Cmf. respectively, while the corresponding concentrations in the permeate under steady-state conditions are denoted by C p and Cmp. Surfactant and metal ion rejections at a steady state are defined as Rs = (1-Csp/Csf) and Rm = (1-Cmp/Cmf)- The molar ratio of metal ion to surfactant is denoted by Fms- The separation of the electrodes is 3 mm. In Table 2, the initial current, pH, and solution conductivity are also given. It shows that both metal and surfactant are separated effectively under an electric field in which the permeate flux, surfactant, and metal ion rejections are enhanced. Economic analysis of the process indicates that some 20- to 50-fold efficiency increase is achieved compared with the no electric field case. For the process to be economical, low-solubility surfactants that can form multilamellar droplets should be used as carriers. 

While these membranes exhibit sodium ion transport numbers as high as 0.98 mol F-1 (i.e. only 2% of the electrolysis current is carried by hydroxide ion through the membrane) no comprehensive theoretical treatment of this unusually high permselectivity has yet emerged. The variation of permselectivity as a function of various cell parameters is also of interest, not only for practical reasons but also because of the insight that may be gained into the nature of hydroxide ion rejection. This research is directed at the latter problem, that is the characterization of membrane permselectivity... 

Surface treatment has also been employed to generate membranes with improved hydroxide ion rejection capability for chlor-alkali applications. In this procedure, one surface of a sulfonyl fluoride XR resin film is treated with an amine such as ethylene-diamine. After hydrolysis, a thin barrier layer of weakly acidic sulfonamide exchange sites is formed. When this treated surface faces the cathode solution, improved hydroxide rejection is realized in a membrane chlor-alkali cell. 

A recent study (12) has shown that Nafion is also suitable for use in water electrolyzers with alkaline solution as the supporting electrolyte. The major charge carrier is the alkali metal ion because of the negligible IT " ion concentration in alkaline solution and the Off ion rejection capability of the cation exchange membrane. The current efficiency of the cell is related to the inhibition of the transport of gaseous products across the separator. Thus, the ionic groups of the membrane are not important, in this case, because alkaline solution is the major electrolyte, and the migration of any ionic species across the membrane would not affect the current efficiency of the cell (33). 

The Liquid Column is Infinite. Practically, this condition holds until the diflFusion of ions rejected from the phase boundary reaches the end of the liquid column. 

Physical sieving applies to colloids and large molecules. Apart from that, rejection is a function of the relative chemical affinity of the solute to the membrane material. Ion rejection follows the tyotropic series, which means that rejection is increased with the increased hydrated radius of the ion. The order of the ions, however, may change due to ion pairing, complexadon, or other solute-solute interactions, and it is, therefore, difficult to predict rejection for mixtures of ions. The rejection behaviour in the presence of organics, or even of organics themselves is poorly understood and only trends can so far be noted. Rejection is usually evaluated with NaCl or MgS04 solutions. 

In summary, key parameters to ion rejection are the membrane pore size, charge, pH, ion charge and size, flux and pressure, concentration, solute-solute interactions, composition of mixtures, and speciation. While models have been successful in explaining some results, the entire rejection mechanism is still poorly understood. 

Rejection of oiganics may be determined by size and charge as well as the same parameters that govern ion rejection. In addition, factors such as molecular conformation and stmeture may play a role. 

Ion rejection studies are not an objective of this study. However, the ion rejection of the system used and the variation of ion rejection due to fouling are important issues. Improvement in our understanding of boundary layer mechanisms seems more appropriate at this stage. Issues worthy of further investigation are. 

UF membranes are generally believed not to retain ions. However, some authors have reported ion rejection (Kiichler and Miekeley (1994)). Cations, especially multivalent cations and trace metals do interact with humic substances (Klein eta/. (1990)) and these inorganics would consequently be retained... 

The TFC-ULP and TFC-S membranes have a very high ion rejection of >80% for sodium and >90% for calcium. Values are comparable to those obtained in the absence of organics. 

The CA-UF membrane has the lowest rejection of organics, and the ion rejection is slightly increased compared to the absence of organics. This may indicate interactions between the retained organics and the cations. This membrane shows a very interesting flux behaviour, with no decline at all over the experiments and a higher pure water flux after the experiments. This indicates the lack of concentration polarisation or osmotic pressure effect at the low salt rejection. The smooth membrane surface would also influence this. The adsorption of ions or organics render the membrane more hydrophilic. 

The total score for each parameter suite (colloids, DOC, and ions) is 100. The columns are subdivided into equal fractions, resulting in total points of 25 for the individual colloid fraction, for the organic fraction, and 50 for the major cations (Na+, Car ). For colloids stable primary colloids (OPS system) are assumed as these appear most abundant in a natural water. Ion rejection is represented by sodium and calcium rejection in the absence of organics and DOC rejection of solutions containing 0.5 mM CaCb (except for MF where calcium concentration is 2.5 mM). As a sum criterion, the water quality parameter (WQP) is introduced. The maximum score for WQP is 300. 

The metal-ion permselective properties were examined as follows metal ion solutions containing Fe(N03)3 and FeCh with 2 ppm concentration were prepared, respectively. Before and after the formation of ESA layers, filtration measurement of the metal ion solution through sample membranes was carried out by using an ultrafiltration cell (Amicon 8010, 50 ml volume). The metal-ion rejection (R) was defined by the following equation ... 

Diffusion dialysis (DD) is an ion-exchange membrane-separation process driven by a concentration gradient across the membrane, i.e., ion transport is driven by the concentration gradient with Donnan criteria of co-ion rejection and preservation of electrical neutrality [17, 40]. Since it is a spontaneous process, DD results in an increase in entropy and a decrease in Gibbs free energy hence it is thermodynamically favorable. 

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