*2.4. Stability of the Used Catalysts*

The possible leaching of the elements from the prepared AAMs was examined by ICP-OES in detail, in addition to the leaching tests (Section 2.1) after oxidation. The oxidized water samples were immediately filtered after 3 h CWPO using a 0.45 µm cellulose nitrate filter to remove the solid catalysts. The Al, Si, and Ca concentrations were determined from the oxidized water samples catalyzed by BFS17.5-60, BFS20-60, BFS25-60, and BFS30-60, and in addition to these elements, Fe was analyzed from the filtered samples catalyzed by Fe/BFS17.5-60 and Fe/BFS30-60. According to the results, the leaching of Ca and Si was observed under all of the utilized reaction conditions with all catalysts. In all of the oxidized water samples, the Ca concentration was 25–50 mg/dm<sup>3</sup> , and the Si concentration was 11–19 mg/dm<sup>3</sup> . However, notably, the Ca concentration was slightly lower in the water samples catalyzed by AAMs without iron. Therefore, the heat treatment of Fe catalysts (Section 3.1) led to the increased dissolution of Ca in the water phase during oxidation treatment. The leaching of Ca was around the same level as that detected in stability tests (Section 2.1) with BFS17.5-60, BFS20-60, BFS25-60, and BFS30-60, revealing that these samples also can be used at reaction temperatures > 100 ◦C and pressures ≥ 2 MPa.

The Al concentration of aqueous BPA samples oxidized at the initial pH was 1.0–1.4 mg/dm<sup>3</sup> . The dissolution of Al was slightly higher at 150 ◦C and 2.0 MPa (Table 1), i.e., under conditions of the stability test, than that under CWPO reaction conditions. However, in the case of oxidation experiments performed at a pH of 3.5, and at temperatures 50 ◦C, 70 ◦C, and 100 ◦C over that of Fe/BFS17.5-60 and Fe/BFS30-60, 0.6–1.2 mg/dm<sup>3</sup> of Al was leached from the catalysts in the obtained effluents. Therefore, the dissolution of Al from the prepared AAMs is more dominant in the CWPO of BPA, which is conducted at the initial pH. Onisei et al. [41] have investigated the leaching behavior of several elements (e.g., Si, Pb, Ca, Zn, Al) from fly ash-based geopolymers. The study was performed in the pH range of 6–13. According to their results, the leaching of Al increased slightly in the pH range of 10.5–13.0. In the CWPO of BPA, the initial pH of the BPA solution was 6–7. However, at the end of the run, the effluent pH was ~11, due to the basic character of the Fe catalysts. Moreover, in CWPO experiments, which were started at a pH of 3.5, the pH of the BPA solution was ~10 in the oxidized water sample. Therefore, the adjustment of

the pH at the start of the CWPO of BPA did not considerably affect the removal of BPA, but it decreased the leaching of Al from the Fe catalysts. the pH at the start of the CWPO of BPA did not considerably affect the removal of BPA, but it decreased the leaching of Al from the Fe catalysts.

of the BPA solution was ~10 in the oxidized water sample. Therefore, the adjustment of

*Catalysts* **2021**, *11*, 664 11 of 19

However, the leaching of Ca, Si, and Al was not related to the removal of BPA, because those elements were not active in CWPO. The stability of the material is a key characteristic of the catalyst; therefore, the preparation method of AAM-based catalysts should be carefully considered. Moreover, the leaching of iron was rather negligible (maximum of 0.2 mg/dm<sup>3</sup> at 70 ◦C and at the initial pH) using Fe/BFS17.5-60 and Fe/BFS30-60 at the employed reaction temperatures and pH. Therefore, the CWPO of BPA with these catalysts proceeded via a heterogeneous reaction. However, the leaching of Ca, Si, and Al was not related to the removal of BPA, because those elements were not active in CWPO. The stability of the material is a key characteristic of the catalyst; therefore, the preparation method of AAM-based catalysts should be carefully considered. Moreover, the leaching of iron was rather negligible (maximum of 0.2 mg/dm3 at 70 °C and at the initial pH) using Fe/BFS17.5-60 and Fe/BFS30-60 at the employed reaction temperatures and pH. Therefore, the CWPO of BPA with these catalysts proceeded via a heterogeneous reaction.

The activity and durability in consecutive tests and the effect of heat treatment as a regeneration method were examined using Fe/BFS7.5-60 at 50 ◦C, at the initial pH, and at H2O<sup>2</sup> and catalyst concentrations of 1.5 g/dm<sup>3</sup> and 4.0 g/dm<sup>3</sup> , respectively. To have sufficient material for consecutive tests and regeneration, 12 runs were performed in total, and the catalysts used in these experiments were collected and combined. Between consecutive experiments, the used catalyst was filtered from the effluent and dried at 105 ◦C for the subsequent runs. The activity and durability in consecutive tests and the effect of heat treatment as a regeneration method were examined using Fe/BFS7.5-60 at 50 °C, at the initial pH, and at H2O2 and catalyst concentrations of 1.5 g/dm3 and 4.0 g/dm3, respectively. To have sufficient material for consecutive tests and regeneration, 12 runs were performed in total, and the catalysts used in these experiments were collected and combined. Between consecutive experiments, the used catalyst was filtered from the effluent and dried at 105 °C for the subsequent runs.

After the first oxidation reaction, BPA removal of 41% was observed, which decreased to ~6% after the second run using the same catalyst (Figure 6). Furthermore, BPA removal after the third experiment, which used Fe/BFS17.5-60 twice, was practically the same (34%) as that observed in the second run, indicative of the catalyst's stability for multiple cycles in the CWPO of BPA. However, the removal of BPA after three cycles using Fe/BFS17.5-60 was around the same as that using BFS17.5-60 for one cycle, revealing that the addition of Fe does not significantly affect catalytic activity due to the basic reaction pH. Moreover, TOC results confirmed the reusability of Fe/BFS17.5-60, while BPA removal was the same during the three consecutive tests. After the first oxidation reaction, BPA removal of 41% was observed, which decreased to ~6% after the second run using the same catalyst (Figure 6). Furthermore, BPA removal after the third experiment, which used Fe/BFS17.5-60 twice, was practically the same (34%) as that observed in the second run, indicative of the catalyst's stability for multiple cycles in the CWPO of BPA. However, the removal of BPA after three cycles using Fe/BFS17.5-60 was around the same as that using BFS17.5-60 for one cycle, revealing that the addition of Fe does not significantly affect catalytic activity due to the basic reaction pH. Moreover, TOC results confirmed the reusability of Fe/BFS17.5-60, while BPA removal was the same during the three consecutive tests.

**Figure 6.** Consecutive tests using Fe/BFS17.5-60 for the CWPO of BPA. Reaction conditions: *c*[BPA] = 60 mg/dm3, *c*[H2O2] = 1.5 g/dm3, *c*[catalyst] = 4 g/dm3, *T* = 50 °C, initial pH (6–7). **Figure 6.** Consecutive tests using Fe/BFS17.5-60 for the CWPO of BPA. Reaction conditions: *c*[BPA] = 60 mg/dm<sup>3</sup> , *c*[H2O<sup>2</sup> ] = 1.5 g/dm<sup>3</sup> , *c*[catalyst] = 4 g/dm<sup>3</sup> , *T* = 50 ◦C, initial pH (6–7).

The regeneration of once-used Fe/BFS17.5-60 was examined by heat treatment at 250 ◦C and 500 ◦C. The procedure was performed by increasing the temperature at a rate of 1 ◦C/min to the reaction temperature, at which the catalyst was kept for 2 h. After oxidation, 34% and 32% BPA removal was observed at 250 ◦C and 500 ◦C, respectively, using Fe/BFS17.5-60. Therefore, the regeneration procedure is not effective at returning the activity of the catalysts to the original level. In addition, carbon deposition is confirmed to not be responsible for the activity decrease of Fe/BFS17.5-60, because heat treatment at elevated temperatures is a typical regeneration procedure for catalysts with carbon deactivation [62]. The regeneration of once-used Fe/BFS17.5-60 was examined by heat treatment at 250 °C and 500 °C. The procedure was performed by increasing the temperature at a rate of 1 °C/min to the reaction temperature, at which the catalyst was kept for 2 h. After oxidation, 34% and 32% BPA removal was observed at 250 °C and 500 °C, respectively, using Fe/BFS17.5-60. Therefore, the regeneration procedure is not effective at returning the activity of the catalysts to the original level. In addition, carbon deposition is confirmed to not be responsible for the activity decrease of Fe/BFS17.5-60, because heat treatment at elevated temperatures is a typical regeneration procedure for catalysts with carbon deactivation [62].

### *2.5. Characterization of the Used Catalysts 2.5. Characterization of the Used Catalysts*

Figure 7 shows the X-ray diffractograms of BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60 after oxidation at the initial pH and at a reaction temperature of 50 ◦C. According to XRD analysis, the hydrotalcite phase (denoted by #, ICDD file 00-022-0700) was still present in BFS17.5-60 and BFS30-60, and the CaCO<sup>3</sup> phase (¤, ICDD file 01-083- 4609) was observed in all samples. Moreover, in the X-ray diffractograms of Fe/BFS17.5-60 and Fe/BFS30-60, the Fe3O<sup>4</sup> and Fe2O<sup>3</sup> iron phases (denoted by +: ICDD file 04-008-8146 and \*: ICDD file 04-015-7029, respectively) were still present, but the high Ca concentration of samples led to the overlap of the CaCO<sup>3</sup> peaks with those of Fe3O<sup>4</sup> and Fe2O<sup>3</sup> at 2θ of 36◦ and Fe3O<sup>4</sup> at 2θ of 43◦ . Figure 7 shows the X-ray diffractograms of BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60 after oxidation at the initial pH and at a reaction temperature of 50 °C. According to XRD analysis, the hydrotalcite phase (denoted by #, ICDD file 00-022-0700) was still present in BFS17.5-60 and BFS30-60, and the CaCO3 phase (¤, ICDD file 01-083-4609) was observed in all samples. Moreover, in the X-ray diffractograms of Fe/BFS17.5-60 and Fe/BFS30-60, the Fe3O4 and Fe2O3 iron phases (denoted by +: ICDD file 04-008-8146 and \*: ICDD file 04-015-7029, respectively) were still present, but the high Ca concentration of samples led to the overlap of the CaCO3 peaks with those of Fe3O4 and Fe2O3 at 2θ of 36° and Fe3O4 at 2θ of 43°.

**Figure 7.** X-ray diffractograms of used BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60. (#) ICDD file 00-022-0700 (Mg6Al2CO3(OH)16·4H2O, hydrotalcite); (¤) ICDD file 01-083-4609 (CaCO3); (\*) ICDD file 04-015-7029 (Fe2O3); (+) ICDD file 04-008-8146 (Fe3O4). **Figure 7.** X-ray diffractograms of used BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60. (#) ICDD file 00-022-0700 (Mg6Al2CO<sup>3</sup> (OH)16·4H2O, hydrotalcite); (¤) ICDD file 01-083-4609 (CaCO<sup>3</sup> ); (\*) ICDD file 04-015-7029 (Fe2O<sup>3</sup> ); (+) ICDD file 04-008-8146 (Fe3O<sup>4</sup> ).

Furthermore, acidic pH and a higher reaction temperature did not affect the phase structure, and hydrotalcite was still observed in the X-ray diffractograms of BFS30-60 and BFS17.5-60 (results not shown).

The specific surface area of samples was analyzed after the oxidation of BPA at the initial pH and at reaction temperatures of 50 ◦C for BFS17.5-60 and BFS30-60, as well as reaction temperatures of 50 ◦C and 70 ◦C for Fe/BFS17.5-60 and Fe/BFS30-60. The BET results for used BFS17.5-60 and BFS30-60 revealed that the specific surface areas were 10.4 and 18.5 m2/g, respectively, revealing that the surface area of BFS30-60 decreases to ~30%, while a rather negligible change in the surface area of BFS17.5-60 was observed (Table 3). For Fe catalysts, the specific surface areas increased after oxidation. In case of Fe/BFS17.5- 60, the surface area was ~45% higher, and in case of Fe/BFS30-60, it doubled compared to that of the fresh catalyst (Table 3). Clearly, during oxidation, the Fe catalyst surface is refined by H2O2. For example, Han et al. [63] and Liu et al. [64] have used hydrogen peroxide to modify surface properties, i.e., to increase the surface area and porosity of materials. However, the larger specific surface area did not improve the removal of BPA in consecutive tests using Fe/BFS17.5-60 (Figure 6); therefore, the CWPO of BPA is not a surface area-specific reaction, as is the case for the catalytic wet air oxidation of BPA [65].
