3.5.3. Regeneration of the Polymer

Finally, regeneration of the polymer and gaining a polymer-free Cu(II) concentrate in a batch process were investigated. Cu(II) was released from HB-PEI by the addition of hydrochloric acid to the filtration circuit until pH 1.0 was reached. Unbound Cu(II) was then rinsed from the filtration circuit by water. The discharge of Cu(II) (Δ) from the filtration circuit, depending on the volume of rinsing solution used (˾), is described with an adaption of Equation (2):

$$
\Delta = 1 - \frac{c\_{i,\bullet}}{c\_{i,0}} = 1 - e^{-\bullet} \tag{3}
$$

Experimental and calculated data for the Cu(II) discharge are given in Figure 8. Cu(II) was efficiently discharged from the pilot plant by rinsing with water, generating a polymer-free Cu(II) concentrate. The regenerated HB-PEI remained in the filtration circuit and was used in further separation cycles.

**Figure 8.** HB-PEI regeneration by pH decrease and Cu(II) discharge from the filtration circuit by rinsing with water. Cu(II) concentration in cumulative permeate is referenced to initial Cu(II) concentration in the filtration circuit.

Due to the decreased Cu(II) concentration in the filtration circuit, Cu(II) concentration in the accumulated polymer-free permeate steadily decreased (Figure 8, calculated by mass balancing) with increasing volume of rinsing water. As the regenerated polymer solution was supposed to be used for Cu(II) retention again, a sufficient discharge of Cu(II) from the filtration circuit was necessary, as remaining unbound Cu(II) in the polymer solution was recirculated to the retention step, reducing the performance of this step. Depending on the polymer concentration in the filtration circuit, Cu(II) concentrations in a polymer-free concentrate, as given in Table 7, can be achieved in a batch regeneration process. An initial polymer loading of 250 mg Cu(II)/g polymer, no Cu(II) retention during rinsing, and 90% Cu(II) discharge are assumed.

**Table 7.** Final Cu(II) concentration in cumulative permeate collected during polymer regeneration and rinsing of the filtration circuit with water. No Cu(II) retention and 90% discharge (ʤ = 2.3) are assumed. Calculation according to Equation (3) and by mass balancing.


### *3.6. Photometric Control of the Cu(II) Enrichment and Release*

For pilot scale operation, control of the Cu(II) enrichment and release in the filtration circuit is highly beneficial. An inline measurement of the Cu(II) enrichment enables an automated pH decrease by acid addition as soon as the Cu(II) loading of the polymer is completed. Then again, as soon as the Cu(II) release is completed, the pH is automatically increased and a new enrichment cycle can be started.

Therefore, the specific change in light absorption of PEI depending on its complex formation can be utilized. Due to the formation of Cu(II) complexes with HB-PEI (tetraamminecopper(II)) [37] and H2O, photometric control of the enrichment and release during PAUF experiments [23] with fly ash extract was investigated.

Figure 9a shows the increasing Cu(II) loading of HB-PEI at pH 4.0 (feed solutions in cuvettes 1–10), followed by a clearly visible color change representing the Cu(II) release at pH 1.0 (feed solution in cuvette 11). Decreasing the pH value, the feed solutions turned colorless to light blue again, indicating the formation of hexaaquacopper(II) and thus the regeneration of HB-PEI.

**Figure 9.** (**a**) PAUF feed samples of Cu(II) enrichment and final Cu(II) release (far right) from HB-PEI using fly ash extract (KEBAG Zuchwil), HB-PEI = 4.6 g L<sup>−</sup>1. (**b**) Corresponding UV-vis spectra of increasing Cu(II) loading (gray, black, and green graphs) and release (orange graph). Comparable solutions of [Cu(NH3)4] <sup>2</sup><sup>+</sup> (blue graph) and [Cu(H2O)6] <sup>2</sup><sup>+</sup> (red graph) were additionally analyzed.

Figure 9b shows the UV-vis spectra of the PAUF feed samples with increasing Cu(II) concentration from gray to black to green graphs. The prepared solution of [Cu(NH3)4] <sup>2</sup><sup>+</sup> (Figure 9b, blue graph) has an absorption maximum at 605 nm. The copper(II) ammine complex formed with HB-PEI absorbs at very similar wavelengths. The intensity of the absorption bands increases until the maximum loading capacity is reached at around 976 mg L−<sup>1</sup> of Cu(II). Additional Cu(II) forms Cu(II) aqua complexes. Therefore, UV-vis spectra are shifted more to the right toward the absorption maximum of [Cu(H2O)6] <sup>2</sup><sup>+</sup> (810 nm, red graph) and a mixture of ammine and aqua complexes occurs in the feed solution. At pH 1.0, Cu(II) is released from HB-PEI and the Cu(II) aqua complex exists exclusively, showing an absorption maximum at 830 nm (Figure 9b, orange graph).

It is easy to distinguish between the absorption maxima of the ammine and aqua complexes formed within the Cu(II) enrichment and those obtained during the release from HB-PEI. This enables calibrated photometric control of the selective Cu(II) separation from real fly ash extract by PAUF.
