**4. Chelating and special ion exchangers**

204 Ion Exchange Technologies

**Figure 4.** The monodisperse and hetrodisperse ion exchangers.

also described in the paper by Ivanov (1996).

The influence of temperature on the equilibrium properties of ion exchange resins was studied extensively. The decrease of the capacity of the cation exchange resins based on the polystyrene matrix due to the operation temperature is not a significant problem. However, the relatively slight decomposition gives enough decomposition products to cause significant problems elsewhere. This may be decomposition of the bone polystyrene matrix, resulting in styrene sulphonic acid derivatives or as a substitution of the sulphonic group giving sulphate. Further decomposition of styrene sulphonic acid derivatives will also result in sulphate as one of the end products (desulphonation). The amount of sulphate produced is sometimes high. The information on the stability of the ion exchange resins mainly deals with the anion exchange resins. The mechanism of the degradation of quaternary ammonium salts and tertiary anions is well-known (Reynolds, 1982; Fernandez-Prini, 1982; Fisher, 2002). The effect of temperature on the properties of chelating ion exchangers was

**3. Application of ion exchangers for heavy metal ions removal** 

metals (PGM), chromium, copper, zinc, nickel, cobalt and tungsten.

Ion exchange technique can remove traces of ion impurities from water and process streams and give a product of desired quality. Ion exchangers are widely used in analytical chemistry, hydrometallurgy, antibiotics, purification and separation of radioisotopes and find large application in water treatment and pollution control (Clifford, 1999; Luca et al. 2009). The list of metals which are recovered and purified on an industrial scale by means of ion exchange include: uranium, thorium, rare earth elements (REEs), gold, silver, platinum

In some of these cases, the scale of operations is relatively small, for instance in the rare earth elements or noble metals, but the values of recovered metals are very high. Ion exchange process is particularly suitable for purification of metal ions with a high value and low processing. The alternative is also a process of large-scale recovery of trace amounts of metals from waste streams, such as cadmium and mercury, chromium, or copper and zinc. The use of ion exchange processes in hydrometallurgy is high and every year continues to grow. It is associated mainly with the progress of what is observed in the synthesis of new selective chelating ion exchangers containing complexing ligands (Minczewski, et al. 1982;

Typical disadvantage of lack a of the selectivity towards heavy metal ions and alkali and alkaline earth metal ions of most widely used functionalized ion exchangers such as Chelex 100 is overcome by introducing chelating ligands capable of removing selective metal ions. It exhibits high affinity for heavy metal ions: **Cu2+ > Hg2+ > Pb2+ > Ni2+ Zn2+ Cd2+ > Co2+ > Fe2+ > Mn2+ > Ca2+ > Mg2+ > Sr2+ > Ba2+ >>> alkali ions > H+,** whereas for sulphonic ones the analogous affinity series can be as presented earlier (for Lewatit SP 112).

Generally, the functional group atoms capable of forming chelate rings usually include oxygen, nitrogen and sulphur. Nitrogen can be present in a primary, secondary or tertiary amine, nitro, nitroso, azo, diazo, nitrile, amide and other groups. Oxygen is usually in the form of phenolic, carbonyl, carboxylic, hydroxyl, ether, phosphoryl and some other groups. Sulphur is in the form of thiol, thioether, thiocarbamate, disulphide groups etc. These groups can be introduced into the polymer surface by copolymerization of suitable monomers, immobilization of preformed ligands, chemical modification of groups originally present on the polymer surface. However, the last two are most often used (Warshawsky, 1987). Chelating resins with such type of ligands are commonly used in analysis and they can be classified according to Fig.1. (Kantipuly et al. 1990). The choice of an effective chelating resin is dictated by the physicochemical properties of the resin materials. These are the acid-base properties of the metal species and the resin materials, the polarizability, selectivity, sorptive capacity, kinetic and stability characteristics of the resin. The sorption capacity of chelating ion exchangers depends mainly on the nature of functional groups and their content as well as solution pH as for their selectivity it depends on the relative position of functional groups, their spatial configuration, steric effects, and sometimes their distance from the matrix and to a lesser extent on the properties of the matrix. Their use allows the recovery of valuable metals from ores and sludge, sea water and industrial effluents. They are used as flotation agents, depressants, flocculants and collectors.

It is worth emphasizing that these resins are invaluable wherever it is necessary to concentrate or remove elements present in very low concentrations.

With a range of well known chelating ion exchangers only a few types are produced on an industrial scale. Among the most important ones these with the functional groups: amidoxime, dithiocarbamate, 8-hydroxychinoline, iminodiacetate, aminophosphonic, bispicolylamine, diphosphonic, sulphonic and carboxylic acid groups, thiol, thiourea as well as isothiourea should be selected (Sahni & Reedijk, 1984; Busche et al. 2009). Among them the chelating ion exchangers possessing methylglucoamine, bis(2pirydylmethyl)amine also known as bispikolilamine, thiol etc. are used for special applications such as removal of precious metal ions, heavy metal ions from the acidic medium, boron and special oxoanions removal. A separate group are ion exchangers of solvent doped type used for In, Zn, Sn, Bi, etc. separation. The advantages of ion exchangers from these groups include good selectivity, preconcentration factor, binding energy and mechanical stability, easy regeneration for multiple sorption-desorption cycles and good reproducibility in the sorption characteristics.
