2.4.3. Pilot Plant Filtration Data

The permeate flux and specific power consumption (pressure and cross-flow pump) of the pilot plant were determined at 40 ◦C and a transmembrane pressure (TMP) of 5 bar, depending on the polymer concentration and tangential velocity in the filtration circuit. The pilot plant was filled with fly ash extract and operated in recirculation mode (permeate was recirculated into the feed reservoir). Pretreated HB-PEI solution was stepwise added to the feed reservoir and fed into the filtration circuit.

### 2.4.4. Selective Cu(II) Retention

The filtration circuit was filled with a 4 g L−<sup>1</sup> solution of pretreated HB-PEI in water. To start the retention experiment, fly ash extract was continuously fed into the filtration circuit, with the pH value kept at 3.0. Samples were taken from the filtration circuit and analyzed by ICP-OES. Retention experiments were ended after 53 L (extract with 0.13 g L−<sup>1</sup> Cu) or 21 L (extract with 0.7 g L−<sup>1</sup> Cu).

### 2.4.5. Rinsing of Cu(II) Loaded Preconcentrates with Water

Rinsing of the Cu(II) loaded polymer concentrate was performed using concentrates from the Cu(II) retention experiments (Section 2.4.4). The filtration circuit was filled with polymer containing fly ash extract (4 g L−<sup>1</sup> HB-PEI, 2.0 g L−<sup>1</sup> Zn, 0.7 g L−<sup>1</sup> Pb, 4.7 g L−<sup>1</sup> Ca) and the pilot plant was operated at pH 3.0 with water as feed for 1 h. Samples were taken from the filtration circuit and analyzed by ICP-OES.

Thickening of the polymer solution was achieved by feeding fly ash extract with 4 g L−<sup>1</sup> HB-PEI to the filtration circuit until a polymer concentration of 25 g L−<sup>1</sup> HB-PEI was reached.

Cu(II) retention during rinsing was optimized using 25 g L−<sup>1</sup> HB-PEI loaded with 200 mg of Cu(II)/g polymer. Rinsing was carried out with water for 1 h at pH 4.0, 3.5, and 3.0. Samples were taken from the filtration circuit and analyzed by ICP-OES.

### 2.4.6. Recovery of Cu(II) and Regeneration of Polyethyleneimine

Recovery of Cu(II) from the filtration circuit was performed using a concentrate received after rinsing (4 g L−<sup>1</sup> HB-PEI, 0.7 g L−<sup>1</sup> Cu(II)). The pH was decreased to 1.0 and the pilot plant was operated with water. Samples were taken from the filtration circuit and analyzed by ICP-OES.

### *2.5. Monitoring Cu(II) Enrichment and Release by UV-Vis Spectroscopy*

Polyethyleneimine and Cu(II) form stable tetraamminecopper(II) complexes, which absorb light within the visible spectral range and appear in blue. Therefore, we investigated a possible photometric control of the Cu(II) enrichment and release.

UV-vis spectra of feed samples resulting from the Cu(II) loading experiments with HB-PEI (see Section 2.2.5) were recorded with a Lambda 35 UV-vis spectrometer (Perkin Elmer, Waltham, MA, USA) using polymethylmethacrylate sample cuvettes (VWR, Darmstadt, Germany). Solutions of Cu(II) nitrate trihydrate in ammonium hydroxide and in ultrapure water were prepared and investigated, functioning as comparative samples representing Cu(II) ammine complexes without polymer.

### **3. Results and Discussion**

### *3.1. Metal Retention of Di*ff*erent PEIs in MSWI Fly Ash Extracts*

Three different polyethyleneimines were investigated regarding their retention behavior towards all metal species contained in the fly ash extracts from MVA Ingolstadt and KEBAG Zuchwil. Composition of the fly ash extracts was previously analyzed; the resulting main metal components and chloride concentrations are shown in Table S1.

Figure 2 shows the retention of Cd(II), Cu(II), Ni(II), Pb(II), Zn(II), Fe(III), and Sb(V) using HB-PEI (Figure 2a,b), PE-PEI (Figure 2c,d), and MOD-PEI (Figure 2e,f). HB-PEI contains primary, secondary, and tertiary amino groups. Each PEI was investigated using the fly ash extracts from both MVA Ingolstadt (Figure 2b,d,f) and KEBAG Zuchwil (Figure 2a,c,e).

**Figure 2.** Influence of pH on the retention of Cd(II), Cu(II), Ni(II), Pb(II), Zn(II), Fe(III), and Sb(V) using fly ash extracts from KEBAG Zuchwil and MVA Ingolstadt: (**a**,**b**) hyperbranched polyethyleneimine (HB-PEI) = 10.0 g L−<sup>1</sup> (Zuchwil), 7.9 g L−<sup>1</sup> (Ingolstadt); (**c**,**d**) partially ethoxylated polyethyleneimine (PE-PEI) = 9.4 g L−<sup>1</sup> (Zuchwil), 7.7 g L−<sup>1</sup> (Ingolstadt); (**e**,**f**) modified polyethyleneimine (MOD-PEI) = 9.7 g L−<sup>1</sup> (Zuchwil), 8.0 g L−<sup>1</sup> (Ingolstadt).

As illustrated in Figure 2a–f, Cu(II) is the only metal ion retained at approximately 100% from pH 3.0 upwards. At this pH, Cd(II) and Pb(II) are retained at about 10% using PE-PEI (Figure 2c,d), and 15–30% using HB-PEI (Figure 2a,b). The fly ash extract from MVA Ingolstadt additionally contains Fe(III) and Sb(V), which are increasingly retained by the membrane starting from pH 3.0 on (Figure 2b,d,f).

As shown in Table 2, both fly ash extracts contained many more metal ions than depicted in Figure 2. The retention of Li, Na, K, Rb, Mg, Ca, Sr, Si, Al, and Mn was also investigated in the same PAUF experiments described in Figure 2. All were barely retained by the membrane, as can be seen in Table 2. Regarding the monovalent alkali metal ions that was to be expected, however, the nonbinding especially of divalent alkaline earth metal ions is a crucial result regarding the target Cu(II) selectivity in the treatment of highly saline fly ash extracts.


**Table 2.** Retention of Li(I), Na(I), K(I), Rb(I), Mg(II), Ca(II), Sr(II), Al(III), Si(IV), and Mn(II) using fly ash extracts from KEBAG Zuchwil and MVA Ingolstadt and HB-PEI, PE-PEI, and MOD-PEI polymers.

The high Cu(II) retention observed is based on the interaction of Cu(II) with the respective PEI. Cu(II) forms tetraamminecopper(II) complexes with amine groups of the three PEIs. Ni(II) and Zn(II) are retained by forming ammine complexes as well but only at pH > 3.0.

To clarify the observed retention of additional metal ions (Figure 2a–f), ultrafiltration experiments were carried out using fly ash extract without the addition of PEI. The results obtained are shown in Figure S2.

To some extent, Pb, Fe, and Sb were retained in the PAUF experiments described above, but they also showed similar retention in the experiments without the addition of PEI (Figure S2). During these experiments, visible turbidity of the feed solutions was observed; the feed samples were centrifuged and the supernatants were analyzed. A continuously decreasing Fe concentration above pH 3.0 was observed: Between pH 3.0 and 5.0 Fe(II) and Fe(III) may both exist, but Fe(II)-hydroxide requires a pH > 7.0 for precipitation [3]. Therefore, Fe(III) was precipitated as solid iron(III)oxide hydrate and retained by the ultrafiltration membrane in the PAUF experiments (Figure 2b,d,f).

The Sb concentration of the centrifuged feed samples also decreased above pH 3.0. This could occur due to the precipitation of Na[Sb(OH)6], formed with sodium ions contained in the fly ash extract and also resulting from adjusting the pH with sodium hydroxide. Furthermore, Sb(V) and Sb(III) may coexist [31], enabling coprecipitation of Sb(OH)3 with Fe(III) oxide hydrate. Similar retention progression of antimony and iron, shown in Figure 2b,f, indicates this.

Pb(II) was retained to different degrees using HB-PEI, PE-PEI, and MOD-PEI (Figure 2) and also without the addition of PEI (Figure S2). Dissolved Pb2<sup>+</sup> forms lead chloro complexes such as [PbCl]<sup>+</sup>, [PbCl3] <sup>−</sup>, [PbCl4] <sup>2</sup><sup>−</sup> and/or hardly soluble PbCl2 in the presence of 60 g L−<sup>1</sup> chloride [32], as contained in the fly ash extracts. Weibel et al., thoroughly investigated Pb-chloro complex formation in fly ash extract and identified [PbCl3] <sup>−</sup> and [PbCl4] <sup>2</sup><sup>−</sup> as the mainly present species [3,6]. Presumably, a certain percentage of the Pb(II) binds to protonated amino groups of the PEIs via these negatively charged chloro complexes. Especially tertiary amino groups of polyethyleneimine may act as anion exchangers and therefore bind negatively charged chloro complexes. The lead retention observed in the experiments without PEI, however, may arise from lead precipitated as PbCl2.

As shown in Figure 2a–d, Cd(II) was also retained to a small extent. This may be due to the formation of stable negatively charged Cd(II) chloro complexes binding to polyethyleneimine analogous to Pb(II) [33]. Weibel et al., found [CdCl4] <sup>2</sup><sup>−</sup> and [CdCl3] − to be the dominant Cd-chloro complexes formed in fly ash extracts [6]. Cadmium, however, did not show any retention without PEI (Figure S2), because CdCl2 is water-soluble, in contrast to PbCl2.

Conclusively, at pH 3.0, Cu(II) was the only metal ion being bound to the investigated PEIs via metal complex formation with amine groups.

After enrichment, Cu(II) has to be released from the polyethyleneimine in order to achieve selective Cu(II) separation. At the same time, a complete metal release means regeneration of the polymer, which can then be reused in further enrichment cycles.

We investigated the Cu(II) release by decreasing the pH of the feed solution at the end of each PAUF experiment. For all six feed solutions shown in Figure 2, Cu(II) retention at pH 1.0 was 0% using fly ash extract from KEBAG Zuchwil (Figure 2a,c,e) and 4–5% using fly ash extract from MVA Ingolstadt (Figure 2b,d,f). Therefore, Cu(II) was successfully released from all investigated PEIs at pH ≤ 1.

Summarizing the sorption behavior of MOD-PEI, Cu(II) was selectively separated at pH 3.0 from 16 different metal ions, including additional heavy metals, alkaline and alkaline earth metals. Moreover, the very high chloride concentration of 60 g L−<sup>1</sup> in the highly saline fly ash extracts did not influence the pH-dependent Cu(II) chelating behavior. This also applies for HB-PEI [19] and PE-PEI [20]. The selective Cu(II) separation from real fly ash extract is even more exceptional, as the Cu(II) concentration in fly ash extracts is up to 380 times lower compared to interfering ions, such as alkaline (earth) metals or zinc. The highest selectivity regarding Cu(II) separation was achieved with MOD-PEI (Figure 2e,f). Compared to HB-PEI and PE-PEI MOD-PEI provides a lower number of amino groups. This leads to an early displacement of other competing metal ions by Cu(II) because Cu(II) forms the most stable transition-metal complex compounds among all metal ions present in the investigated fly ash extracts [34].
