# Application of Modern Research Methods for the Physicochemical Characterization of Ion Exchangers
## 1 Introduction
### Types of Functional Groups  
In the case of ion exchangers, the matrix can be a polyanion (negatively charged) used as a cation exchanger or a polycation (positively charged) used as an anion exchanger. To convert the ST-DVB matrix to an ion exchanger, various functional groups can be attached. The most common functional groups are sulphonic (strongly acidic cation exchangers, SAC), quaternary ammonium of types 1, 2 or 3 (strongly basic anion exchangers, SBA), carboxylic (weakly acidic cation WAC exchangers), amine, polyamine (weakly basic anion WBA exchangers,) and chelating with the iminodiacetic, phosphinic, phosphoric, aminophosphonic, thiol, thiourea, isothiouronium, amidoxime, etc., units [28]. However, among the chelating ion exchangers, those with the iminodiacetic and aminophosphonic groups predominate [29; 30].  
Some of the above-mentioned ion exchangers are characterized as below:  
Strongly acidic cation (SAC) exchangers possess good physical and oxidation stability, variable capacities (5.4 meq/L) depending on the regeneration levels, but small operating efficiency (in the case of water softening in the Na\({}^{+}\) cycle and demineralization) (Figure 4).  
Figure 4: Structure of cation exchanger with the sulfone functional groups.  
Figure 3: SEM images of Lewatit SP112 (**a**–**c**) and Fiban-X (**d**–**f**).  
Weakly acidic cation (WAC) exchangers with a partial exchange capacity, much higher capacity than SAC, high operating efficiency, but the limitations include a relatively high cost, poor kinetics and physical stability (for dealkalization in the H+ cycle and for brackish water softening in the Na+ cycle), weak acid groups such as -COO-, ionise only at high pH. At low pH, that is, a high concentration of hydrogen ions, they form undissociated -COOH [31]. Strongly basic anion exchangers (SBA) with a quaternary ammonium group with good kinetics and short rinses, but there is a trade-off between the exchange efficiency and the resulting quality. They are thermodynamically unstable, prone to organic fouling and cost considerably more than the cation equivalent. Their capacities are equal to 2.5-4.0 meq/g. The SBA ion exchangers of type 1 (trimethylammonium) and type 2 (dimethylhydroxyethyllyammonium) quaternary ammonium functional anion exchangers are obtained by amination of the polymer matrix with trimethylamine (TMA) or dimethylethanolamine (DMEA). Type 1 ion exchangers are either gel with a particularly suitable temperature and oxidation stability or macroporous with higher capacity and resistance against organic fouling (Figure 5).  
They are used for treating waters and wastewaters with a small concentration of free mineral acids, with a large of silica and where a small silica leakage is required. The higher cost of the type 1 ion exchangers is considered acceptable in order to obtain a more efficient and longer-lasting resin. Type 2 ion exchangers provide much better capacity, excellent regeneration efficiency and resistance to fouling; however, they are limited to the lower temperature operation < 35 \({}^{\circ}\)C with a larger SiO2 leakage. Therefore, they are used in small plants. Type 3 ion exchangers with the tri-n-butyl ammonium functional groups are designed to offer the highest selectivity for trace contaminants such as nitrate or perchlorate. Weakly basic anion (WBA) exchangers with a tertiary amine group removes only strong (hydrochloric, sulfuric, nitric) acids. They are characterized by a large operating capacity and regeneration efficiency with good resistance to organic fouling and thermal stability, but longer rinse times, poor kinetics and they cost even more than SBA. Weak base groups lose a proton when the pH value is high, forming uncharged NH2 ions. Consequently, for the resins which are weakly ionic, the exploitable capacity depends on the pH of the liquid being treated. Weak base anion exchangers are particularly effective in handling natural organics [32].  
Figure 5: Structure of anion exchanger of type 1.