Diphonix

A similar coordination mechanism operates in the adsorption of some metal ions from highly acidic solution on phosphonic acid resins, for example ac-tinide ions on Diphonix®, one of very few new chelat-ing resins to be introduced commercially in the 1990s. The chelating groups in this resin are contributed by vinylidene-1,1-diphosphonic acid monomer units, but the resin also contains acrylic acid and styrenesulfonic acid units (Figure 6). As with the amidoxime resins described above, hydrophilic monomers are essential for high reaction rates. Diphonix is the outcome of work at the Argonne National Laboratory on 1,1-diphosphonic acid ligands and at the University of Tennessee on ion exchangers possessing two or more types of functional group, operating by different mechanisms. It was developed primarily for actinide separations in highly acidic media, but also strongly adsorbs divalent transition metal cations at pH 1-2 and trivalent cations at pH 0-2.

CONH(C2h4NH)nH Figure 5 Acrylic oligoamine resin.

P03H2

P03H2

Figure 6 Structure of Diphonix®.

In pH 1 nitric acid distribution coefficients on Diphonix decrease in the order:

from ~3Lg~1 to ~500mLg~1. For these ions a double-logarithmic plot of the distribution coefficient against [H + ] has slope — n, where n is the cation charge. This indicates that adsorption occurs by simple exchange of Mn + for H + . In 1 mol L~1 nitric acid, the order of selectivity is:

Th(IV) > Fe(III) + U(VI) > Np(VI) + Pu(IV) > Bi(III)

with D as high as 900 L g^1 for Th(IV) and 200 L g^1 for Fe(III) and U(VI), decreasing to 400 mLg-1 for Al(III) and 80mLg_1 for Cr(III). Uptake of the strongly preferred ions shows little pH dependence, indicating adsorption without displacement of H+ on phosphonic acid groups that are not significantly ionized under these conditions. To maintain electroneutrality, the complexed metal ions must be associated with anions from solution, either free or in mixed-ligand complexes containing phosphonic acid groups and small ion ligands such as Cl~. The less preferred ions show simple ion exchange behaviour over a limited pH range, but uptake increases at high acid concentrations, indicating a change in mechanism.

The very high affinity of Diphonix for Fe(III) makes it very suitable for removing iron from acidic copper electrolytes in copper production by electrowinning. The process has been demonstrated in pilot-scale trials at copper refineries, with no decrease in resin performance over hundreds of cycles. Fe(III) is desor-bed by reducing it to Fe(II).

Solar Panel Basics

Solar Panel Basics

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