The Ion-Exchange Process

Handbook of Ion Chromatography, Third, Completely Revised and Enlarged Edition. Joachim Weiss Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-28701-9 

When the counter ion of the ion-exchange site is replaced by a solute ion, the latter is temporarily retained by the fixed charge. The various sample ions remain for a different period of time within the column due to their different affinities towards the stationary phase and, thus, separation is brought about. 

For example, if a solution containing bicarbonate anions is passed through an anion exchange column, the quaternary ammonium groups attached to the resin are exclusively in their bicarbonate form. If a sample with the anions A and B is injected onto the column, these anions are exchanged for bicarbonate ions according to the reversible equilibrium process given by Eqs. (32) and (33)  

The separation of the anions is determined by their different affinities towards the stationary phase. The constant determining the equilibrium process is the selectivity coefficient, K, which is defined as follows  

The selectivity coefficient can be determined experimentally by adding a certain amount of resin material to a solution with known concentrations of X and HCO3. The resulting concentration of the exchanged ions is determined in the mobile and stationary phase, respectively, after equilibrium is achieved. To precisely calculate the selectivity coefficient, the activities Oi have to be used instead of the concentrations c,. As a prerequisite, the determination of the activity coefficient according to Eq. (35) is required, which is difficult to perform in the matrix of an ion-exchange resin. 

In ion chromatography, the concentration of ions in the solutions to be analyzed is small, thus they may be equated with the activity coefficient in the first approximation. For the sake of simplicity, we only use the concentrations Ci within the scope of this discussion. 

Resin- NR3 HCO3 + A Resin- NRsA + HCO Resin- NR3 HCO3 -I- B Resin- NRsB -l- HCO3 

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Ion exchange equilibria. The ion exchange process, involving the replacement of the exchangeable ions Ar of the resin by ions of like charge Bs from a solution, may be written ... 

Investigations in aqueous systems have established many of the fundamental principles of ion exchange as well as providing useful applications. The scope of the ion exchange process has, however, been extended by the use of both organic and mixed aqueous-organic solvent systems.32,33... 

If the preference for hydrogen ion exchange shown by lime-soda glasses can be reduced, then other cations will become involved in the ion exchange process and the possibility of an electrode responsive to metallic ions such as sodium and potassium exists. The required effect can be achieved by the introduction of aluminium oxide into the glass, and as shown in Table 15.2, this approach has led to new glass electrodes of great importance to the analyst. 

Often, ion exchangers are made into membranes (flat sheets, rolled sheets, capillaries, hollow fibers) separating different solutions. On the one hand, this can serve to make the ion-exchange processes continuous on the other hand, further technological opportunities arise (discussed in Section 26.2.3). 

The ion exchange process involves the ability of hexavalent uranium as the uranyl ion, UO+, to form anionic complexes with sulfate ions, SO2-, and carbonate ions, CO2-. In a general way, it may be mentioned that the uranyl ion exits in dynamic equilibrium with its sulfate complexes,... 

The ion-exchange process consists of adsorbing these anionic complexes selectively and quantitatively on an anion-exchange resins as illustrated in the following reactions ... 

The ion-exchange process is applicable for removing a broad range of ionic species from water containing all metallic elements, inorganic anion such as halides, sulfates, nitrates, cyanides, organic acids such as carboxylics, sulfonics, some phenols at sufficiently alkaline pH conditions, and organic amines at sufficiently acidic conditions. 

Some innovating treatment technologies may be introduced in the treatment of wastewater generated in the aluminum fluoride industry to make its effluent safer. The ion exchange process can be applied to the clarified solution to remove copper and chromium. At a very low concentration, these two pollutants can be removed by xanthate precipitation.24 A combination of lime and ferric sulfate coagulation will effectively reduce arsenic concentration in the wastewater. 

On the other hand, optionally added co-ions of the eluent may also interfere with the ion-exchange process through competitive ion-pairing equilibria in the mobile phase. The effect of various amines added as co-ions to the polar-organic mobile phase was systematically studied by Xiong et al. [47]. While retention factors of 9-fluorenylmethoxycarbonyl (FMOC)-amino acids were indeed affected by the type of co-ion, enantioselectivities a and resolution values Rs remained nearly constant. For example, retention factors k for FMOC-Met decreased from 17.4 to 9.8 in the order... 

The interlayer space of LDHs can be expanded to some extent in a suitable solvent medium, which favors the ion exchange process. An aqueous medium, for example, favors the exchange by inorganic anions, whilst an organic solvent favors exchange by organic anions [14]. 

The chemical composition of the LDH sheets influences the charge density of the sheets and the hydration state, thereby affecting the ion exchange process. 

Some other factors such as temperature also have an impact on the ion exchange process. It is usually accepted that higher temperatures favor ion exchange [89]. It should be noted, however, that too high a temperature might have an adverse effect on the structural integrity of the LDHs. 

Figure 6. The illustration depicts the ion-exchange process that occurs in the oven.

Ion exchange involves the formation and breakage of bonds between ions in solution and exchange sites in a zeolitic adsorbent. The reaction equilibrium of the ion exchange process depends most significantly on contact time, operating temperature and ionic concentration. 

The ion selectivity of a membrane can be established by measuring the potential difference between two identical reference electrodes. One electrode is immersed in the specimen solution, the other in a reference solution, and the membrane is interposed between them. The composition of solution 2 is constant, and, if both solutions contain monovalent ions, the ion-exchange process can be described as follows ... 

Although neodymium is the 28th most abundant element on Earth, it is third in abundance of all the rare-earths. It is found in monazite, bastnasite, and allanite ores, where it is removed by heating with sulfuric acid (H SO ). Its main ore is monazite sand, which is a mixture of Ce, La, Th, Nd, Y, and small amounts of other rare-earths. Some monazite sands are composed of over 50% rare-earths by weight. Like most rare-earths, neodymium can be separated from other rare-earths by the ion-exchange process. 

Samarium is the 39th most abundant element in the Earths crust and the fifth in abundance (6.5 ppm) of all the rare-earths. In 1879 samarium was first identified in the mineral samarskite [(Y, Ce U, Fe) (Nb, Ta, Ti )Ojg]. Today, it is mostly produced by the ion-exchange process from monazite sand. Monazite sand contains almost all the rare-earths, 2.8% of which is samarium. It is also found in the minerals gadolmite, cerite, and samarskite in South Africa, South America, Australia, and the southeastern United States. It can be recovered as a byproduct of the fission process in nuclear reactors. 

Gadohnium is the 40th most abundant element on Earth and the sixth most abundant of the rare-earths found in the Earths crust (6.4 ppm). Like many other rare-earths, gadolinium is found in monazite river sand in India and Brazil and the beach sand of Florida as well as in bastnasite ores in southern California. Similar to other rare-earths, gadolinium is recovered from its minerals by the ion-exchange process. It is also produced by nuclear fission in atomic reactors designed to produce electricity. 

Of all the 17 rare-earths in the lanthanide series, terbium is number 14 in abundance. Terbium can be separated from the minerals xenotime (YPO ) and euxenite, a mixmre of the following (Y, Ca, Er, La, Ce, Y, Th)(Nb, Ta, Ti O ). It is obtained in commercial amount from monazite sand by the ion-exchange process. Monazite may contain as much as 50% rare-earth elements, and about 0.03% of this is terbium. 

Holmium is the 12th most abundant of the rare-earths found in the Earths crust. Although it is the 50th most abundant element on Earth, it is one of the least abundant lanthanide metals. It is found in gadolinite and the monazite sands of South Africa and Austraha and in the beach sands of Florida and the Carolinas in the United States. Monazite sand contains about a 50% mixture of the rare-earths, but only 0.05% by weight is holmium. Today, small quantities of holmium are produced by the ion-exchange process. 

Lutetium is the 60th most abundant element on Earth, and it ranks 15th in the abundance of the rare-earths. It is one of the rarest of the lanthanide series. It is found in monazite sand (India, Australia, Brazil, South Africa, and Florida), which contains small amounts of all the rare-earths. Lutetium is found in the concentration of about 0.0001% in monazite. It is difficult to separate it from other rare-earths by the ion-exchange process. In the pure metallic form, lutetium is difficult to prepare, which makes is very expensive. 

The result of the ion-exchange process in which the +3 ion of lutetium combines with the -1 ion of chlorine to form the binary compound 3LiCl is written as follows ... 

A perspective based on kinetics leads to a better understanding of the adsorption mechanism of both ionic and nonionic compounds. Boyd et al. (1947) stated that the ion exchange process is diffusion controlled and the reaction rate is limited by mass transfer phenomena that are either film diffusion (FD) or particle diffusion (PD) controlled. Sparks (1988) and Pignatello (1989) provide a comprehensive overview on this topic. 

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