among the R4N+ sites of the ion exchange resin and the Cu(CN)2
− and
Cu(CN)3
2 − complexes, which require one and two adsorption sites,
respectively, can be given by:
where R represents the organic radical and X− the counter-ion (Cl−).
The loading of Cu(CN)4
3 − species was considered negligible, since
under the investigated conditions (CN/Cu = 3) the tetra-coordinated
complex is not the predominant species, as previously demonstrated.
Thus, on the basis of the stoichiometry suggested by Eqs. (9) and (10),
the expected loading of copper as Cu(CN)2
− species is twice that obtained
for Cu(CN)3
2− species. The similar copper loadings obtained at both pH
values indicate that the loaded species on the resin may be represented
by Cu(CN)3
2 − species, which was detected in the loading solutions
(Fig. 5) and on the loaded resins by Raman.
Raman spectra were obtained for the resin after loading
(0.008 mol•L−1 Cu; CN/Cu = 3; and pH 5.4–5.8 and 10.5), as shown
in Fig. 10. The spectra are deconvoluted into two peaks by means of
Lorentzian line shapes. The resin analysis after loading with solution
pH 5.4–5.8 indicated that only vibration mode of Cu(CN)3
2 − species
(at 2108 cm−1) is well-distinguished, and a shoulder at lower Raman
shift (about 2094 cm−1) can be visualized. After loading with solution
pH 10.5, the Raman analysis indicated symmetric and asymmetric
vibration modes of the Cu(CN)3
2− species at 2108 and 2094 cm−1,
respectively. Therefore, only the Cu(CN)3
2− complex was identified in
the resin after loading at pH 5.4–5.8 and 10.5.
Fig. 10. Raman spectra of the resin after loading with fitting results.
Solution: [Cu] = 0.008 mol•L−1; CN/Cu=3; and pH 5.4–5.8 and
10.5. The black markers represent the experimental data, while the
solid lines and dotted lines the fitted curves.
3.5. Effects of ionic strength and type of “spectator” cation
The effects of ionic strength and type of “spectator” cation on the
adsorption density of the copper cyanocomplexes on the activated carbon
were investigated at pH 10.5 for CN/Cu = 3 (Fig. 11) and CN/Cu = 4
(Fig. 12) aiming at evaluating the role of these parameters in the performance
of the material used for gold adsorption, such as the C1 sample.
Two situations will be considered: Condition (Fig. 11) where Cu(CN)3
2−
species predominates, and condition (Fig. 12) where Cu(CN)4
3 − species
is the predominant one due to the high ionic strength of the solution