**2. Ion exchange (IX)**

Ion exchange may be defined as the exchange of ions between the substrate and surrounding medium. The most useful ion exchange reaction is reversible. When the reaction is reversible, the ion exchanger can be reused many times. Generally resins are manufactured in the spherical, stress and strain free form to resist physical degradation. They are stable at high temperatures and applicable over a wide pH range. Ion exchange resins, which are completely insoluble in most aqueous and organic solutions, consist of a cross linked polymer matrix to which charged functional groups are attached by covalent bonding (Sherrington, 1998). The ion exchangers which contain cations or anions as counterions are called cation exchangers or anion exchangers, respectively. The usual matrix is polystyrene cross linked for structural stability with 3 to 8 percent of divinylbenzene (3-8 % DVB) (Kunin, 1958; Helfferich, 1962). The resins of higher cross linking (12-16% DVB) are more costly, both to make and to operate and they are specially developed for heavy duty industrial applications. These products are more resistant to degradation by oxidizing agents such as chlorine, and withstand physical stresses that fracture lighter duty materials. Typical ion exchangers are produced with a particle size distribution in the range 20-50 mesh (for separation of anions from cations or of ionic species from nonionic ones). For more difficult separations, materials of smaller particle size or lower degrees of cross linking are necessary. Moreover, when the separation depends solely upon small differences in the affinity of the ions, a particle size of 200-400 mesh is required and when the selectivity is increased by the use of complexing agents, the particle size in the 50-100 mesh is adequate. The ion exchangers finer than 100 mesh are employed for analytical purposes and for practical applications on the commercial scale the materials finer than 50 mesh are used.

Depending on the type of functional groups of exchanging certain ions, the ion exchangers with strongly acidic e.g., sulphonate -SO3H, weakly acidic e.g., carboxylate -COOH, strongly basic e.g., quaternary ammonium -N+R3 and weakly basic e.g., tertiary and secondary amine -N+R2H and -N+RH2 should be mentioned. The strong acidic cation exchangers are well dissociated over a wide pH range and thus reaching its maximum sorption capacity. On the other hand, weak acidic cation exchangers containing, for example, carboxylic functional groups reach the maximum sorption capacity at pH> 7.0 as presented in Fig.1.

**Figure 1.** The sorption capacity of ion exchangers depending on pH.

198 Ion Exchange Technologies

**2. Ion exchange (IX)** 

all types of water, and its content is subject to large variations (Barceloux, 1999). The natural content of copper in the river water ranges 0.9-20 g/dm3 and for saline waters 0.02-0.3 g/dm3. Copper is an essential nutritional element being a vital part of several enzymes. It is one of the components of human blood. The estimated adult dietary intakes are between 2 and 4 mg/day. The demand for copper is increased in pregnant women, children and the elderly. Good dietary sources of copper include animal liver, shellfish, dried fruit, nuts and chocolate. In some cases drinking water may also provide significant levels of copper. Copper in the body is involved in oxidation-reduction processes, acts as a stimulant on the amount and activity of hemoglobin, in the process of hardening of collagen, hair keratinization, melanin synthesis as well as affects on lipid metabolism and properties of the myelin sheath of nerve fibers. In animal cells it is mainly concentrated in the mitochondria, DNA, RNA, and the nucleus. Copper readily forms a connection with various proteins, especially those of sulphur. Although copper is an essential metal, it can, in some circumstances, lead to toxic effects including liver damage and gastrointestinal disturbances. Such as Wilson's disease (also known as hepatolenticular degeneration), Indian Childhood Cirrhosis (ICC) which are characterised by an accumulation of copper-containing granules within liver cells. Ingestion of high levels of copper salts is known to cause gastrointestinal upsets. Additionally, absorption of copper compounds by inhalation causes congestion of the nasal mucosa, gastritis, diarrhea and toxic symptoms such as chronic lung damage. Copper compounds act on the intact skin, causing it to itch and inflammation. They can cause conjunctivitis, ulceration and corneal opacity, nasal congestion and as well as sore throat and nasal septum. The upper limit recommended by WHO for copper is less than 1.3

mg/dm3. The maximum limit in drinking water is 0.05 mg/dm3 (Fewtrell et al. 1996).

Ion exchange may be defined as the exchange of ions between the substrate and surrounding medium. The most useful ion exchange reaction is reversible. When the reaction is reversible, the ion exchanger can be reused many times. Generally resins are manufactured in the spherical, stress and strain free form to resist physical degradation. They are stable at high temperatures and applicable over a wide pH range. Ion exchange resins, which are completely insoluble in most aqueous and organic solutions, consist of a cross linked polymer matrix to which charged functional groups are attached by covalent bonding (Sherrington, 1998). The ion exchangers which contain cations or anions as counterions are called cation exchangers or anion exchangers, respectively. The usual matrix is polystyrene cross linked for structural stability with 3 to 8 percent of divinylbenzene (3-8 % DVB) (Kunin, 1958; Helfferich, 1962). The resins of higher cross linking (12-16% DVB) are more costly, both to make and to operate and they are specially developed for heavy duty industrial applications. These products are more resistant to degradation by oxidizing agents such as chlorine, and withstand physical stresses that fracture lighter duty materials. Typical ion exchangers are produced with a particle size distribution in the range 20-50 mesh (for separation of anions from cations or of ionic species from nonionic ones). For more difficult separations, materials of smaller particle size or lower degrees of cross linking

Additionally, ion exchangers possess: the iminodiacetate functional groups (- N{CH2COOH}2), phenol (-C6H4OH), phosphonic (-PO3H2) and phosphine (-PO2H) functional groups. These groups are acidic in nature and are dissociated with the exchange of H+ or Na+ ions for other cations from the solution. Negative charge of the functional groups is offset by an equivalent number of mobile cations so-called counter ions. Counter ions can be exchanged for other ions from the solution being in the contact with the resin phase.

There are also amphoteric exchangers, which depending on the pH of the solution may exchange either cations or anions. More recently these ion exchangers are called bipolar electrolyte exchange resins (BEE) or zwitterionic ion exchangers (Nesterenko & Haddad, 2000). The aminocarboxylic amphoteric ion exchangers AMF-1T, AMF-2T, AMF-2M, ANKB-35 as well as the carboxylic cation exchanger KB-2T were, for example used for recovery of Ni(II) from the Mn(NO3)2–H2O system (Kononowa et al. 2000).

The individual ions present in the sample are retained in varying degrees depending on their different affinity for the resin phase. The consequence of this phenomenon is the separation of analyte ions, such as metal ions, however, the nature and characteristics of the resin phase determine the effectiveness of this process (Fritz, 2005). The affinity series which for various types of ion exchangers are as follows:

### **2.1. Cation exchangers with the sulphonic functional groups**

It is well known that the affinity of sulphonic acid resins for cations varies with the ionic size and charge of the cation. The affinity towards cation increases with the increasing cation charge:

$$\mathrm{Na^{+}\leq Ca^{2+}\leq Al^{3+}\leq Th^{4+},}$$

and in the case of different cations with the same charge the affinity increases with the increasing atomic number:

$$\mathrm{Li^{\cdot}\leqslant H^{\cdot}\leqslant \mathrm{Na^{\cdot}\leqslant}\leqslant \mathrm{NH}\mathrm{d^{\cdot}}\leqslant \mathrm{K^{\cdot}\leqslant \mathrm{Rb^{\cdot}}\leqslant \mathrm{Cs^{\cdot}}\leqslant \mathrm{Ag^{\cdot}}\leqslant \mathrm{Tl^{+}}}$$

$$\mathrm{Mg^{2+}}\leqslant \mathrm{Ca^{2+}}\leqslant \mathrm{Sr^{2+}}\leqslant \mathrm{Ba^{2+}}$$

$$\mathrm{Al^{3+}}\leqslant \mathrm{Fe^{3+}}.$$

Generally, the affinity is greater for large ions with high valency.

For the strong acidic cation exchanger the affinity series can be as follows:

Pu4+ >> La3+ > Ce3+ > Pr3+ > Nd3+ Sm3+ Eu3+ Gd3+ Tb3+ Dy3+ Ho3+ Er3+ Tm3+ Yb3+ Lu3+ > Y3+ > Sc3+ > Al3+ >> Ba2+ > Pb2+ > Sr2+ > Ca2+ > Ni2+ > Cd2+ > Cu2+ > Co2+ > Zn2+ > Mg2+ > UO22+ >> Tl+ > > Ag+ > Cs+ > Rb+ > K+ > NH4+ > Na+ > H+ > Li+

and for Lewatit SP-112 it is as: Ba2+ > Pb2+ > Sr2+ > Ca2+ > Ni2+ > Cd2+ > Cu2+ > Co2+ > Zn2+ > Fe2+ >Mg2+ > K+ >NH4+ > Na+ >H+.

### **2.2. Cation exchangers with the carboxylic functional groups**

Cation exchangers with the carboxylic functional groups show the opposite the affinity series for alkali and alkaline earth metal ions. Noteworthy is the fact that the cations exhibit a particularly high affinity for H+. The affinity of this type of cation is therefore as follows:

$$\mathrm{Mg^{+}>Mg^{2+}>Ca^{2+}>Sr^{2+}>Ba^{2+}>Li^{+}>Na^{+}>K^{+}>Rb^{+}>Cs^{+}.$$

#### **2.3. Anion exchangers with the quaternary ammonium functional groups**

The charge of the anion affects its affinity for the anion exchanger in a similar way as for the cation exchangers:

citrate > tartrate > PO43- > AsO43- > ClO4- > SCN- > I- > S2O32- > WO42- > MoO42- > CrO42- > C2O42- > SO42> SO32-> HSO4- >HPO42- > NO3- > Br- > NO2- > CN- > Cl- > HCO3- > H2PO4- > CH3COO-

$$\text{>IO3} \text{ > HCOO} \text{ > BrO3} \text{ > ClO3} \text{ > F} \text{ > OH4}$$

for *Dowex 1* (type 1):

200 Ion Exchange Technologies

increasing atomic number:

>Mg2+ > K+ >NH4+ > Na+ >H+.

cation exchangers:

> SO42> SO32-> HSO4-

citrate > tartrate > PO43- > AsO43- > ClO4-

>HPO42- > NO3-

> IO3-

for various types of ion exchangers are as follows:

**2.1. Cation exchangers with the sulphonic functional groups** 

Generally, the affinity is greater for large ions with high valency.

For the strong acidic cation exchanger the affinity series can be as follows:

**2.2. Cation exchangers with the carboxylic functional groups** 

separation of analyte ions, such as metal ions, however, the nature and characteristics of the resin phase determine the effectiveness of this process (Fritz, 2005). The affinity series which

It is well known that the affinity of sulphonic acid resins for cations varies with the ionic size and charge of the cation. The affinity towards cation increases with the increasing cation charge:

Na+< Ca2+<Al3+<Th4+, and in the case of different cations with the same charge the affinity increases with the

Li+< H+< Na+< NH4+ < K+< Rb+< Cs+< Ag+< Tl+

Mg2+ < Ca2+ < Sr2+ < Ba2+

Al3+ < Fe3+.

Pu4+ >> La3+ > Ce3+ > Pr3+ > Nd3+ Sm3+ Eu3+ Gd3+ Tb3+ Dy3+ Ho3+ Er3+ Tm3+ Yb3+ Lu3+ > Y3+ > Sc3+ > Al3+ >> Ba2+ > Pb2+ > Sr2+ > Ca2+ > Ni2+ > Cd2+ > Cu2+ > Co2+ > Zn2+ > Mg2+ > UO22+ >> Tl+ > > Ag+ > Cs+ > Rb+ > K+ > NH4+ > Na+ > H+ > Li+ and for Lewatit SP-112 it is as: Ba2+ > Pb2+ > Sr2+ > Ca2+ > Ni2+ > Cd2+ > Cu2+ > Co2+ > Zn2+ > Fe2+

Cation exchangers with the carboxylic functional groups show the opposite the affinity series for alkali and alkaline earth metal ions. Noteworthy is the fact that the cations exhibit a particularly high affinity for H+. The affinity of this type of cation is therefore as follows:

H+ > Mg2+ > Ca2+ > Sr2+ Ba2+ Li+ > Na+ > K+ > Rb+ > Cs+.

The charge of the anion affects its affinity for the anion exchanger in a similar way as for the

> NO2-

> BrO3-

> I-

> CN-

> ClO3-

> S2O32- > WO42- > MoO42- > CrO42- > C2O42-

> H2PO4-

> CH3COO-

> Cl- > HCO3-

> F- > OH-

**2.3. Anion exchangers with the quaternary ammonium functional groups** 

> SCN-

> Br-

> HCOO-

$$\text{C}\\ \text{ClO}\\ \text{r} > \text{I} \vdash \text{HSO}\\ \text{r} > \text{NO}\\ \text{r} > \text{Br} \vdash \text{NO}\\ \text{r} > \text{Cl}\\ \vdash \text{HCO}\\ \text{r} > \text{CHx}\\ \text{COO} > \text{OH}\\ \vdash \text{F}\_2$$

for *Dowex 2* (type 2):

ClO4- > I- > HSO4- > NO3- > Br- > NO2- > Cl- > HCO3- > OH- > CH3COO- > F-

### **2.4. Anion exchangers with the tertiary and secondary amine functional groups**

Only with the exception of the OH- ion, the affinity of the anion exchangers with the tertiary and secondary functional groups is approximately the same as in the case of anion exchangers with the quaternary ammonium functional groups. These medium and weakly basic anion exchangers show very high affinity for OH ions.

Anion exchange materials are classified as either weak base or strong base depending on the type of exchange group. These are two general classes of strong base anion exchangers e.g. types 1 and 2 depending on chemical nature. The synthesis of the weak base anion exchangers with the tertiary amine groups is usually provided by the chloromethylation of PST-DVB followed by the amination by secondary amine (Drăgan & Grigoriu, 1992). Weak base resins act as acid adsorbers, efficiently removing strong acids such as sulphuric and hydrochloric ones. They are used in the systems where strong acids predominate, where silica reduction is not required, and where carbon dioxide is removed in degasifiers. Preceding strong base units in demineralizing processes, weak base resins give more economical removal of sulphates and chlorides. The selectivity for the bivalent ions such as SO42- depends strongly on the basicity of the resin, the affinities of various functional groups following the order: primary > secondary > tertiary > quaternary. Therefore among the factors affecting the sorption equilibrium the most important are: first of all nature of functional groups and the concentration of the solution (Boari et al. 1974). At low concentration the resin prefers ions at higher valency and this tendency increases with solution diluting. It should be also mentioned that obtaining resins with the primary amine functional groups is difficult by chemical reactions on polystyrene-divinylbenzene copolymers. Weakly basic anion exchangers can be used, for example for zinc cyanide removal from the alkaline leach solutions in the Merrill Crowe process (Kurama & Çatlsarik, 2000).

### **2.5. Gel and macroporous resins**

The development in polymerization technique has provided novel matrices for a series of new ion exchangers. They differ from the earlier corresponding copolymers that are characterized by being essentially cross linked gels of polyelectrolytes with pore structure defined as the distance between polymeric chains.

It is well known that the fouling of the resin by organic compounds and mechanical stress imposed by plant operating at high flow rates are the most important problems encountered in the use of the ion exchange resins (De Dardel & Arden, 2001). To overcome these problems the ion exchangers with a high degree of cross linking containing artificial open pores in the form of channels with diameters up to 150 nm were introduced (Fig. 2).

**Figure 2.** The structure of gel and macroporous ion exchangers (http://dardel.info/IX/index.html)

The first macroporous ion exchanger was a carboxylic resin made by Rohm and Haas, which covered a wide variety of acrylic compounds copolymerized with polyvinyl cross linking agents to make insoluble, infusible weakly acidic resins. By 1948 AmberliteTM IRC-50, made by the copolymerization of methacrylic acid and divinylbenzene was in production and possessed the 'sponge structure' (Abrams & Milk, 1997). According to the definition by Stamberg and Valter (1970) the macroporous resin should be characterised by measurable inner surface by any suitable method resulting from pores 5 nm, even in the completely dried state. In contrast, the gel materials did not show any porosity in the dry state. Then the term 'macroreticular' (sometimes abbreviated to MR) was selected to distinguish resins with a particular type of porosity obtained by application of precipitating diluents such as t-amyl alcohol. In 1979 Amber-Hi-Lites stated that 'macroreticular' resins are those made by a copolymerization technique which brings about precipitation during the polymerization, thus resulting in a product which has two phases, a gel phase in the form of microspheres formed during the phase separation and the pore phase surrounding the microspheres (Kunin, 1979). Later when quantitative porosity measurements were used it was shown that other methods of preparation gave products similar to those declared as 'macroreticular'. Therefore classification of resins should be based on their properties and function (Ion exchange resins and adsorbents, 2006).

During last decades the great progress was made by the development of the macroporous ion exchange resins. It should be mentioned that macroporous resins can also perform as adsorbents because of their pore structure. For organic ion exchange resins based on cross linked polystyrene the porosity was originally selected by the degree of cross linkage. These gel type resins are able to sorb organic substances from water according to their degree of porosity and the molecular weight of the adsorbate. They not only allow for large molecules or ions to enter the sponge like structure but also to be eluted during the regeneration. Therefore they perform two functions: ion exchange by means of the functional groups and the reversible adsorption and elution due to the macroporous structure. They are also resistant against organic fouling which results in a longer resin life compared with the conventional gel type ion exchangers as well as the quality of the treated water is much better because of the adsorption of organic species by the macroporous structure. The SEM scan of the macroporous anion exchanger Lewatit MonoPlus MP 500 is presented in Fig. 3.

Selective Removal of Heavy Metal Ions from Waters and Waste Waters Using Ion Exchange Methods 203

**Figure 3.** SEM scans of macroporous resins.

202 Ion Exchange Technologies

exchange resins and adsorbents, 2006).

problems the ion exchangers with a high degree of cross linking containing artificial open

pores in the form of channels with diameters up to 150 nm were introduced (Fig. 2).

**Figure 2.** The structure of gel and macroporous ion exchangers (http://dardel.info/IX/index.html)

The first macroporous ion exchanger was a carboxylic resin made by Rohm and Haas, which covered a wide variety of acrylic compounds copolymerized with polyvinyl cross linking agents to make insoluble, infusible weakly acidic resins. By 1948 AmberliteTM IRC-50, made by the copolymerization of methacrylic acid and divinylbenzene was in production and possessed the 'sponge structure' (Abrams & Milk, 1997). According to the definition by Stamberg and Valter (1970) the macroporous resin should be characterised by measurable inner surface by any suitable method resulting from pores 5 nm, even in the completely dried state. In contrast, the gel materials did not show any porosity in the dry state. Then the term 'macroreticular' (sometimes abbreviated to MR) was selected to distinguish resins with a particular type of porosity obtained by application of precipitating diluents such as t-amyl alcohol. In 1979 Amber-Hi-Lites stated that 'macroreticular' resins are those made by a copolymerization technique which brings about precipitation during the polymerization, thus resulting in a product which has two phases, a gel phase in the form of microspheres formed during the phase separation and the pore phase surrounding the microspheres (Kunin, 1979). Later when quantitative porosity measurements were used it was shown that other methods of preparation gave products similar to those declared as 'macroreticular'. Therefore classification of resins should be based on their properties and function (Ion

During last decades the great progress was made by the development of the macroporous ion exchange resins. It should be mentioned that macroporous resins can also perform as adsorbents because of their pore structure. For organic ion exchange resins based on cross linked polystyrene the porosity was originally selected by the degree of cross linkage. These gel type resins are able to sorb organic substances from water according to their degree of porosity and the molecular weight of the adsorbate. They not only allow for large molecules or ions to enter the sponge like structure but also to be eluted during the regeneration. Therefore they perform two functions: ion exchange by means of the functional groups and the reversible adsorption and elution due to the macroporous structure. They are also resistant against organic fouling which results in a longer resin life compared with the conventional gel type ion exchangers as well as the quality of the treated water is much better because of the adsorption of organic species by the macroporous structure. The SEM scan of the macroporous anion exchanger Lewatit MonoPlus MP 500 is presented in Fig. 3.

Ion exchange applications can be performed by either column (flow continuous) and batch technique. In column operations the ion exchange resin is placed in the vertical column to form a bed. Once the application is completed, the resin can be regenerated to use in another cycle. In batch operations the resin is shacked in a vessel with the solution to be treated. After the application is completed, the resin can be regenerated in place or transferred to a column for regeneration.

While the main aims in the production of conventional ion exchangers were focused on obtaining a high ion exchange capacity and improved chemical resistance and thermal and mechanical strength, in the case of monodisperse ion exchange resins, these efforts directed towards improvement of kinetic parameters. Heterodisperse ion exchangers are usually characterized by a standard grain size of 0.3-1.2 mm and uniformity coefficient (UC) within the limits of 1.5-1.9. In the case of monodisperse ion exchange resins during the manufacturing process the grain size from 0.6 mm and uniformity coefficient within the limits 1.1-1.2 is usually achieved. In addition, monodisperse ion exchangers, due to the uniform packing of the column, show more than 12% higher ion exchange capacity, faster kinetics of exchange and a much higher mechanical strength, which is extremely important from the economical point of view. As the particle size of the ion exchanger material and its uniformity are the most important parameters influencing the hydraulics and kinetics of the ion exchange therefore the monodisperse ion exchangers provided better flow characteristics in column applications in comparison to the conventional heterodisperse ion exchangers (the flow rate decreases with the decreasing particle size, however, smaller particles have larger outer surface, but cause larger head loss in the column processes) (Scheffler, 1996; Krongauz & Kocher, 1997). The visualization of the monodisperse and hetrodisperse ion exchangers is presented in Fig. 4a-b.

For example the research carried out by Zainol & Nicol (2009a) shows that in the the sorption process of Ni(II) and other metal ions the monodisperse resin (Lewatit MonoPlus TP 207) proved to be superior to the conventional heterodisperse ones in terms of loading capacity for Ni(II) and also the kinetics of adsorption. This makes it a preferred choice for different applications.

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

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 also described in the paper by Ivanov (1996).
