Circumventing Limitations to Selectivity

The selectivity between two cations of a chelating resin should in principle be equal to that of the chelat-ing ligand itself, but this is seldom true in practice. For example, iminodiacetate resins are very much less

Table 5 Comparison of an iminodiacetate resin with the iminodiacetate iona

Ion

Ca2 +

Fe2 +

Co2 +

Cd2 +

Zn2 +

Nl2 +

Pb2 +

Cu2 +

Hg2 +

K"

1.00

4.0

6.7

15

17

57

1200

2300

2800

Km

1.00

1600

2.3 x104

600

2.8 x 104

4.7 x105

7 x 104

9 x 107

1.5 x 109

aSelectivities for divalent cations (relative to Ca2 + ) of an iminodiacetate chelating resin (K") and the iminodiacetate ion (Km).

aSelectivities for divalent cations (relative to Ca2 + ) of an iminodiacetate chelating resin (K") and the iminodiacetate ion (Km).

selective than the free iminodiacetate ion, as shown in Table 5, which compares equilibrium constants for the reaction:

with the corresponding values from Table 3 for iminodiacetate resins. There are several reasons for the discrepancy. In the first place the resin is heterogeneous on a molecular scale. Its chelating groups are located in environments that may vary in polarity, dielectric constant or steric crowding, depending for example on their closeness to cross-links. This variation in chemical environment affects the binding constant for different metal ions to differing extents, degrading the selectivity of the resin. Secondly, the high local concentration of ligands in the resin may lead to a mixture of 1 : 1, 2 : 1 and higher complexes. At loadings less than 100%, the number of ligands coordinated to each metal ion is almost impossible either to control or to measure. Thirdly, the exchange reaction itself changes the properties of the resin. Selectivities of simple ion exchangers are well known to vary with the extent of loading, and the same will be true of chelating resins, especially when adsorbed metal ions are coordinated by ligands attached to different polymer chains, reversibly creating additional cross-links. The result of these factors is not merely reduced selectivity of polymeric resins relative to low molecular weight ligands, but in some cases the order of selectivity is reversed (Table 5).

The mixed composition and additional cross-link effects can be eliminated by designing the resin to accommodate the maximum coordination number of the cation with a single multidentate ligand. Resins containing EDTA moieties are an example. These were synthesized by Takeda et al. at Asahi Chemical in 1985, with rn-divinylbenzene as the essential building block. Addition of bromine across one double bond before polymerization allowed the introduction of an iminodiacetate group on each carbon of the pendant C2 chains. However, even in these resins, ligands still experience a range of chemical environments.

Oligoamine weak base resins suffer an additional source of heterogeneity since the amine (diethylenet-

riamine (DETA), for example) can attach to the precursor resin (chloromethylated polystyrene-co-divinylbenzene) through either primary or secondary nitrogens, and can react with multiple chloromethyl groups, introducing additional covalent cross-links. This effect was eliminated in experimental resins: the primary nitrogens of DETA were blocked by forming Schiff base adducts, followed by amination exclusively through the secondary nitrogen and subsequent hydrolysis of the Schiff base groups to recover the primary amino groups (Figure 7). The latter were then converted by further reaction to iminodiacetate, aminophosphonate or other chelating groups. The selectivity sequence of the fozs(iminodiacetate) derivative is:

Fe(III) > In(III) > Ga(III) + Cu(II) > Zn(II) > Al(III)

with distribution coefficients decreasing from & 10 Lg"1 at pH 1 for Fe(III) to & 10mLg" for Al(III) at pH 2. The resin readily separates Ga(III) and In(III) from Zn(II) and Al(III) in acid solution, but Fe(III) and Cu(II) must first be removed (by precipitation as sulfides, for example). For most trivalent rare earth cations, D + 100-500 mL g"1 at pH 2, with the highest values being observed for the medium atomic

Figure 7 Synthesis of diethylenetriamine resin coupled exclusively through the secondary nitrogen, and conversion to a ¿»«(iminodiacetic acid) derivative.

Figure 8 Lysine-N",N"-diacetic acid resin.

Figure 8 Lysine-N",N"-diacetic acid resin.

weight lanthanides (Sm-Dy). The selectivity is great enough to permit chromatographic separation of pairs of lanthanides (La/Pr or Nd/Sm).

The effect of inhomogeneity can be minimized by introducing spacer groups between the chelating groups and the polymer backbone. A highly selective nitrilotriacetate-like resin was synthesized by func-tionalizing a polystyrene matrix with lysine-Na,Na-diacetic acid (Figure 8). This resin shows high selectivity for trivalent ions such as Ga(III) and In(III) over divalent cations. When loaded with Fe(III) it can be used to adsorb As(III) and As(V) oxyanions by ligand exchange.

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