**11. Chelating ion exchangers with the bis(2-pyridylmethyl)amine functional groups**

The ion exchange resins with the bis(2-pyridylmethyl)amine (bpa) functional groups also known as bispicolylamine are capable of selective sorption of transition metals, particularly Cu(II) ions due to the presence of donor atoms (nitrogen atoms) which are capable of coordination reaction with Cu(II). Due to this fact, such chelating ion exchange resins can combine ion exchange and complexing reactions and then exhibit high selectivity for metal ions. Dowex M 4195 possessing such functional groups is commercially available. It was synthesized in the early 1970s by Dow Chemical Co. and formerly known as Dowex XFS-4195 or DOW 3N. Also two others: Dowex M4196 (formerly Dowex XFS-4196) N-(2 hydroxyethyl)picolylamine or Dowex XFS-43084 (DOW 2N) with N-(2 hydroxypropyl)picolylamine were recognized (Jones & Pyper, 1979; Grinstead, 1984).

Bis(2-pyridylmethyl)amine (bpa) is an uncharged tridentate ligand having the ability to form charged complexes with most divalent metals. The 1:1 complexes with the metal ions of [M(bpa)]2+ type are stable (Hirayama & Umehara, 1996). Based on the pKa values of bis(2 pyridylmethyl)amine (pK1=0.5, pK2=2.2, pK3=3.4), it can be stated that at low pH values three nitrogen atoms would be protonated, while in the middle range of pH only one. For instance, Cu(II) ions (the coordination number is equal to 4) with the bis(2 pyridylmethyl)amine group and water molecule coordinate to it giving a square planar structure. In the next stage the H2O molecule can be replaced by the anion, which is able to coordinate Cu(II) by a ligand exchange reaction:

$$\text{R-N(bpa)}\text{z} + \text{H}^+ \rightleftharpoons \text{R-NH}^\*\text{(bpa)z}$$

$$\text{R-HN^\*(bpa)}\_2 + \text{M}^{2+} \rightleftharpoons [\text{R-HN^\*(bpa)}\_2]\_2 \text{TM}^{2+} $$

The complexes formed in the resin phase possess the following structure:

The obtained complex ion exchanger provides a new mode for the recognition of ions in the chromatographic analysis. Dowex M-4195 is a weak base ion exchanger and 1 M H2SO4 is in the protonated form (pKa = 3.2). It is also resistant to osmotic shock. Diniz et al. (2000, 2002, 2005) showed that the affinity series of metal ions determined in the one-component system for Dowex M 4195 is as follows: Cu(II) > Ni(II) > Co(II) > Pb(II) > Fe(III) > Mn(II) and it is slightly different from that in the multicomponent system: Cu(II) > Ni(II) > Pb(II) > Fe(III) > Co(II) > Mn(II). The affinity of the transition metal cations for Dowex M 4195 in most cases was in agreement with the Irving-Williams order (Irving and Williams, 1953): Fe(II) < Co(II) < Fe(III) < Ni(II) < Cu(II) > Zn(II). It can be used for purification of chloride solutions after leaching of Mn(II) containing trace amounts of Co(II), Pb(II), Ni(II) and Cu(II). It can be also used for gold recovery (Tuzen, 2008).

On a commercial scale Dowex M 4195 has been used, among others, for separation of Ni(II) ions in the presence of Co(II) at INCO's Port Colborne refinery in Canada and Zambia Chambishi Cobalt Plant (Diniz et al. 2005) for purifying cobalt electrolytes. The efficiency of sorption of both ions is affected not only by pH, but by also by the concentration of sulphate(VI) ions and temperature. It is worth mentioning that separation of the twin pair Co(II)-Ni(II) is one of the most difficult tasks in inorganic chemistry. Contrary to Lewatit TP 207 and Amberlite IRC 718 with the iminodiacetate functional groups, Dowex M-4195 is characterized by the maximum sorption capacity towards Cu(II) ions in the pH range 1-4 (Melling and West, 1984). The sorption process can be presented in the following reaction:

216 Ion Exchange Technologies

coordinate Cu(II) by a ligand exchange reaction:

used for gold recovery (Tuzen, 2008).

Bis(2-pyridylmethyl)amine (bpa) is an uncharged tridentate ligand having the ability to form charged complexes with most divalent metals. The 1:1 complexes with the metal ions of [M(bpa)]2+ type are stable (Hirayama & Umehara, 1996). Based on the pKa values of bis(2 pyridylmethyl)amine (pK1=0.5, pK2=2.2, pK3=3.4), it can be stated that at low pH values three nitrogen atoms would be protonated, while in the middle range of pH only one. For instance, Cu(II) ions (the coordination number is equal to 4) with the bis(2 pyridylmethyl)amine group and water molecule coordinate to it giving a square planar structure. In the next stage the H2O molecule can be replaced by the anion, which is able to

RN(bpa)2 + H+ ⇄ RNH+(bpa)2

2RHN+(bpa)2 + M2+ ⇄ [RHN+(bpa)2]2→M2+

n

The obtained complex ion exchanger provides a new mode for the recognition of ions in the chromatographic analysis. Dowex M-4195 is a weak base ion exchanger and 1 M H2SO4 is in the protonated form (pKa = 3.2). It is also resistant to osmotic shock. Diniz et al. (2000, 2002, 2005) showed that the affinity series of metal ions determined in the one-component system for Dowex M 4195 is as follows: Cu(II) > Ni(II) > Co(II) > Pb(II) > Fe(III) > Mn(II) and it is slightly different from that in the multicomponent system: Cu(II) > Ni(II) > Pb(II) > Fe(III) > Co(II) > Mn(II). The affinity of the transition metal cations for Dowex M 4195 in most cases was in agreement with the Irving-Williams order (Irving and Williams, 1953): Fe(II) < Co(II) < Fe(III) < Ni(II) < Cu(II) > Zn(II). It can be used for purification of chloride solutions after leaching of Mn(II) containing trace amounts of Co(II), Pb(II), Ni(II) and Cu(II). It can be also

N N M2+

n

N N

The complexes formed in the resin phase possess the following structure:

$$\rm RH\_n + Cu^{2+} \rightleftharpoons R-Cu^{2+} + nH^{+} $$

where: R is the resin matrix, n is the stoichiometric ratio, for n2 the SO42- ions sorption occurs.

Partial washing out of copper(II) ions proceeds by means of 4M H2SO4, whereas the total one by means of NH3H2O. In the case of sorption of Cu(II) ions sorption in the presence of Fe(III) ions, the ion exchange mechanism must be assumed. Sorption of both ions is affected not only by pH, but also by concentration of sulphate(VI) ions and temperature. Fe(III) ions sorption increases significantly with the temperature rise from 293 K to 303 K, whereas it does not change for Cu(II) ions.

The ion exchangers with the picolylamine functional groups can be the basis for obtaining the polymeric ligand exchanger (PLE) with the structure presented above (Zhao et al. 1998; Kołodyńska 2009c):

Such kind of ion exchanger consists of the cross linked polystyrene-divinylbenzene matrix, covalently attached bispicolylamine functional groups and Lewis acid cations (such as Cu2+, Ni2+, Fe3+, Co2+ according to the series: Cu2+ > Ni2+ > Fe3+ > Co2+) coordinated to the functional groups and without neutralization of their positive charge so that the anion exchanger is obtained. It is expected to show high affinity not only for phosphates(V) HPO42-, arsenate(V) HAsO42- and chromate(VI) CrO42- ions but also oxalates ox2-, perchlorates ClO4- , tartaric acid as well as simultaneous and selective removal of heavy metal and chromate ions (contrary to other ion exchangers) (Saygi et al. 2008; Dimick, 2008; Du et al. 2008). It was found that for phosphates(V) HPO42- removal, the sorption efficiency is much higher than that for the strongly basic macroporous anion exchanger Amberlite IRA 958 and proceeds according to the reaction:

$$\text{R-M2}^{\ast} \text{(2Cl\cdot)} + \text{HPO} \text{4}^{\ast} \rightleftharpoons \text{R-M2}^{\ast} \text{(HPO} \text{4}^{\ast}) + \text{2Cl}^{\ast}$$

The desorption process can be written as follows:

R–M2+(HPO42-) + 2Cl- + H+ ⇄ R–M2+(2Cl- ) + H2PO4- R–M2+(HPO42-) + 2Cl- ⇄ R–M2+(2Cl- ) + HPO42-

Four chelating ion exchange polymeric resins were tested to remove Ni(II) and Co(II) from synthetic solutions on the commercially available ion exchangers Dowex M4195, Amberlite IRC 748, Ionac SR-5 and Purolite S930. Among the selected resins, Dowex M4195 showed the best results for Ni(II) and Co(II) selective sorption from acid liquors in the whole pH range and with small influence of other elements. Even at pH 1 Dowex M4195 was the most effective (Mendes & Martins, 2004).
