Chelating Ion Exchangers

The cation binding of transition, heavy and noble metals can be performed by an ion exchange process with strong and weak acid cation exchangers as well as with strong base exchangers; the latter are used if the metal cations are present in the form of complex anions.

However, the most promising technique for the binding of metal cations is the use of ion exchangers which contain chelating functional groups. Thus, exchangers with iminodiacetate groups can remove several ppm of Ca2 + , Mg2+ or Sr2+ from brine, in contrast to cation exchangers containing the

Figure 14 Preparation of an acrylic-type weak base anion exchanger from acrylonitrile-DVB copolymer.

(P is a polymer structure unit that contains an acid or base group.) Scheme 2 Some distinctive ion exchange reactions.

Table 2 Some chelating acid and base functional groups

Functional group

Chemical structure

Utilizations

Amine

-nh-(-c2h4-nh-)x-h

Removal of transitional metals

Iminodiacetic

-N-(-CH2-COOH)2

Selective removal of heavy metals

Isothiouronium

-S-C(NH2) = nh

Selective removal of mercury and noble metals

Aminophosphonic

-CH2-NH-CH2-P(O)(OH)2

Especially for decalcification of brine solutions

Phosphonic acid

-P(O)(OH)2

Preconcentation of uranyl ions

Hydroxamic acid

-CO-NHOH

Selective retention for Fe(III) ions

Hydroxyamine

-CH2-N(CH3)-CH2-(CHOH)4CH2OH

Selective retention for boric acid

p — CH2 — NH — (C2H4NH) — H + Mn p—ch2—nh—(ch2—ch2—nh) —h o

CH2-Cn o

Figure 15 Classical binding of metal cations by some chelating functional groups. (P is a polymer structure unit that contains an acid or base group.)

non-chelating common acid groups. The latter exchangers show a higher selectivity for divalent cations than for Na +, but the difference is not very large, so that they will not be able to bind Ca2 +, Mg2 + or Sr2 + cations present at very low concentrations.

The high selectivity of chelating ion exchangers is attributed to the establishment of much stronger bonds than the simple electrostatic attractive forces present in the case of the common ion exchangers. Because of these bonds, the chelating processes show a high degree of irreversibility.

It must be mentioned that the chelating groups, attached to polymer networks, are groups with two or more electron donor elements, such as N, S, O and P, and they can function like the model low molecular weight chelating agents. Their preparation takes place in two main ways - polymerization or polycon-densation.

Table 2 shows some functional groups, which are capable of chelating. The binding of metal cations by some chelating functional groups is illustrated in Figure 15.

The nature of metal cation binding can be modified by changing the pH of the solution to cause chemical modification of the chelating groups. Thus, amine and isothiouronium groups can take part in the equilibria, shown in Figure 16. These groups will bind metal cations by a coordination process in neutral or base media via the free base forms while in an acid medium the same cations will be bound as complex anions by an anion exchange process via the salt forms.

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