*2.2. Characterization of AAMs*

Table 3 lists the results of the Brunauer–Emmett–Teller (BET) surface areas of the prepared AAMs.


**Table 3.** BET-specific surface areas (SSA) and pore volumes (PV) of AAMs.

The specific surface area of BFS was negligible, while alkali activation led to the increased surface area of all samples (Table 3). Samples prepared by using the highest amount of NaOH exhibited the highest specific surface area, as well as the highest pore volume. Clearly, a low Si/Na ratio favored the formation of a porous structure in the samples. Sindhunata et al. [43] have reported the highest pore volume for fly-ash-based geopolymers at a SiO2/Na2O ratio < 1. Moreover, the samples cured at 60 ◦C for 24 h exhibited a slightly higher specific surface area, and hence a higher pore volume, than those prepared at room temperature. The higher curing temperature promoted the removal of excess water from the material structure, which in turn increased the porosity of samples further. Furthermore, higher curing temperatures (>50 ◦C) have been reported to particularly increase the amount of mesopores in the material [43].

As can be observed from the surface area results, no significant differences between the AAMs were observed. Therefore, to examine the effect of the Na concentration of samples on the catalytic behavior, samples with the lowest and highest Na concentration (BFS17.5-60 and BFS30-60) were selected as support materials for Fe catalysts. Surprisingly, the surface areas of the Fe catalysts were greater than those of the BFS17.5-60 and BFS30-60 pure supports (Table 3). This result was related to the calcination performed for Fe catalysts. During heat treatment, excess water and carbon dioxide of the support material, and well as traces of Fe salt, evaporated from the AAM structure, enabling the increase in the specific surface area [44]. In addition, the calcination of Fe catalysts led to the decomposition of hydrotalcite (Figure 1, XRD results), which also affected the surface area of materials [44]. Furthermore, as-prepared AAMs mainly exhibited a mesoporous structure (i.e., pore diameter between 2 and 50 nm), with ~10% of pores exhibiting a diameter of less than 2 nm. However, by the addition of Fe to BFS17.5-60 and BFS30-60 via ion exchange, the number of mesopores decreased to 80%, while micropores accounted for only a small percentage of the total pore volume. Moreover, macropores accounted for only ~15% of the total pore volume in Fe/BFS17.5-60 and Fe/BFS30-60, while before Fe ion exchange, pores greater than 50 nm were not detected (i.e., heat treatment enhanced the formation of large pores).

**Figure 1.** X-ray diffractograms of BFS raw material, supports, and Fe catalysts. (#) 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 1.** X-ray diffractograms of BFS raw material, supports, and Fe catalysts. (#) 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> ).

Figure 1 shows the X-ray diffractograms of the BFS raw material; BFS17.5-60, BFS20- 60, BFS25-60, and BFS30-60 supports; and Fe/BFS17.5-60 and Fe/BFS30-60 catalysts. In the X-ray diffractogram of BFS, crystal peaks were not observed, but only one wide halo at 2θ between 22° and 40° was observed, which is characteristic of an amorphous material. After the alkali activation of BFS with NaOH, peaks were observed at 2θ values of 11.3°, 22.8°, 34.5°, and 38.6° (denoted with #), corresponding to hydrotalcite (Mg6Al2CO3(OH)16·4H2O (ICDD file 00-022-0700)), and the high-intensity peak at ~29.4° corresponded to CaCO3 (ICDD file 01-083-4609). However, the broad "hump" observed at 2θ of 28–35° was still present in the X-ray diffractograms of all supports, indicative of a partly amorphous structure. After the ion exchange of BFS17.5-60 and BFS30-60 with the Fe solution, the peaks observed at 2θ values of 24.1°, 33.2°, 35.6°, 40.9°, 49.5°, and 54.1° (denoted by \*) and at 35.7°, 43.4°, 57.4°, and 63.0° (denoted by +) revealed the presence of Fe2O3 (ICDD file 04-015-7029) and Fe3O4 (ICDD file 04-008-8146) phases, respectively. Owing to the heat treatment of Fe catalysts, hydrotalcite was decomposed [44], and peaks corresponding to hydrotalcite were not observed in the X-ray diffractograms of Figure 1 shows the X-ray diffractograms of the BFS raw material; BFS17.5-60, BFS20-60, BFS25-60, and BFS30-60 supports; and Fe/BFS17.5-60 and Fe/BFS30-60 catalysts. In the X-ray diffractogram of BFS, crystal peaks were not observed, but only one wide halo at 2θ between 22◦ and 40◦ was observed, which is characteristic of an amorphous material. After the alkali activation of BFS with NaOH, peaks were observed at 2θ values of 11.3◦ , 22.8◦ , 34.5◦ , and 38.6◦ (denoted with #), corresponding to hydrotalcite (Mg6Al2CO3(OH)16·4H2O (ICDD file 00-022-0700)), and the high-intensity peak at ~29.4◦ corresponded to CaCO<sup>3</sup> (ICDD file 01-083-4609). However, the broad "hump" observed at 2θ of 28–35◦ was still present in the X-ray diffractograms of all supports, indicative of a partly amorphous structure. After the ion exchange of BFS17.5-60 and BFS30-60 with the Fe solution, the peaks observed at 2θ values of 24.1◦ , 33.2◦ , 35.6◦ , 40.9◦ , 49.5◦ , and 54.1◦ (denoted by \*) and at 35.7◦ , 43.4◦ , 57.4◦ , and 63.0◦ (denoted by +) revealed the presence of Fe2O<sup>3</sup> (ICDD file 04-015-7029) and Fe3O<sup>4</sup> (ICDD file 04-008-8146) phases, respectively. Owing to the heat treatment of Fe catalysts, hydrotalcite was decomposed [44], and peaks corresponding to hydrotalcite were not observed in the X-ray diffractograms of Fe/BFS17.5-60 and Fe/BFS30-60.

Fe/BFS17.5-60 and Fe/BFS30-60. Figure 2 shows the DRIFT spectra of BFS, BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60. In the DRIFT spectrum of the BFS raw material, only a few peaks were observed. The band at ~1420 cm−1 corresponded to Na2CO3 [45], and the strong peak at ~1110 Figure 2 shows the DRIFT spectra of BFS, BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60. In the DRIFT spectrum of the BFS raw material, only a few peaks were observed. The band at ~1420 cm−<sup>1</sup> corresponded to Na2CO<sup>3</sup> [45], and the strong peak at ~1110 cm−<sup>1</sup> corresponded to pure silica [46].

cm−1 corresponded to pure silica [46].

AAMs [47].

**Figure 2.** DRIFT spectra of BFS, BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60 samples. **Figure 2.** DRIFT spectra of BFS, BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60 samples.

Alkali-activated samples exhibited several peaks in the analyzed region. The peak at 3730 cm−1 for BFS17.5-60 corresponded to silanol groups, which interact with other atoms—for example, in silanol nests [47]—and the absorption peak at ~3700 cm−1 revealed the presence of four coordinated Al [48]. Moreover, the band at 3610 cm−1 for BFS17.5-60 corresponded to the bridging hydroxyl groups [49]. In the DRIFT spectra of Fe/BFS17.5- 60 and Fe/BFS30-60, the peak centers were shifted to higher wavenumbers than those for the samples without iron, probably due to calcination, and the absorption band corre-Alkali-activated samples exhibited several peaks in the analyzed region. The peak at 3730 cm−<sup>1</sup> for BFS17.5-60 corresponded to silanol groups, which interact with other atoms—for example, in silanol nests [47]—and the absorption peak at ~3700 cm−<sup>1</sup> revealed the presence of four coordinated Al [48]. Moreover, the band at 3610 cm−<sup>1</sup> for BFS17.5-60 corresponded to the bridging hydroxyl groups [49]. In the DRIFT spectra of Fe/BFS17.5-60 and Fe/BFS30-60, the peak centers were shifted to higher wavenumbers than those for the samples without iron, probably due to calcination, and the absorption band corresponding to the silanol groups (3730 cm−<sup>1</sup> ) disappeared by the introduction of iron into AAMs [47].

sponding to the silanol groups (3730 cm−1) disappeared by the introduction of iron into The absorbance bands for BFS17.5-60 and BFS30-60 were observed at 715, 840, 1373, and 1790 cm−1, corresponding to CO32−-containing compounds [46]. Bands at 840 and 1790 cm−1 connected to Na2CO3, and the band at 715 cm−1 corresponded to CaCO3 [46], while that observed at 1373 cm−1 corresponded to hydrotalcite [50], which was also detected in the X-ray diffractograms of these samples (Figure 1). In the DRIFT spectra of Fe/BFS17.5- 60 and Fe/BFS30-60, these peaks were slightly shifted to higher wavenumbers, especially for the band corresponding to hydrotalcite, indicative of its decomposition as a result of heat treatment. Furthermore, the peak at ~1650 cm−1 observed in all samples corresponded The absorbance bands for BFS17.5-60 and BFS30-60 were observed at 715, 840, 1373, and 1790 cm−<sup>1</sup> , corresponding to CO<sup>3</sup> <sup>2</sup>−-containing compounds [46]. Bands at 840 and 1790 cm−<sup>1</sup> connected to Na2CO3, and the band at 715 cm−<sup>1</sup> corresponded to CaCO<sup>3</sup> [46], while that observed at 1373 cm−<sup>1</sup> corresponded to hydrotalcite [50], which was also detected in the X-ray diffractograms of these samples (Figure 1). In the DRIFT spectra of Fe/BFS17.5-60 and Fe/BFS30-60, these peaks were slightly shifted to higher wavenumbers, especially for the band corresponding to hydrotalcite, indicative of its decomposition as a result of heat treatment. Furthermore, the peak at ~1650 cm−<sup>1</sup> observed in all samples corresponded to the H–OH stretching vibrations characteristic of absorbed water [51], the intensity of which slightly decreased due to the heat treatment of iron-containing samples.

to the H–OH stretching vibrations characteristic of absorbed water [51], the intensity of which slightly decreased due to the heat treatment of iron-containing samples. All AAMs exhibited several bands corresponding to the Al and Si bonds. The bands at 435–483 cm−1 corresponded to the Si–O–Si and O–Si–O bending vibrations [52], while All AAMs exhibited several bands corresponding to the Al and Si bonds. The bands at 435–483 cm−<sup>1</sup> corresponded to the Si–O–Si and O–Si–O bending vibrations [52], while the absorption peak at ~600 cm−<sup>1</sup> revealed the presence of Si–O–Si and Al–O–Si symmetric stretching vibrations [53]. The band at ~900 cm−<sup>1</sup> in the spectra of BFS17.5-60 and BFS30-60

corresponded to the Si–O stretching and Si–OH bending modes [53]. Moreover, the band at ~1170 cm−<sup>1</sup> corresponded to the Si–O–Si and Al–O–Si asymmetric stretching vibrations [53], and this band was broadened in the spectra of Fe/BFS17.5-60 and Fe/BFS30-60, due to the calcination of these samples [45]. According to [54–56], Fe2O<sup>3</sup> and Fe3O<sup>4</sup> species should exhibit IR vibrations at 550 and 780 cm−<sup>1</sup> and 571 and 590 cm−<sup>1</sup> , respectively. However, owing to the overlap of the Si and Al vibrations in this wavenumber region, peaks corresponding to Fe cannot be observed in the DRIFT spectra of the prepared samples. the absorption peak at ~600 cm−1 revealed the presence of Si–O–Si and Al–O–Si symmetric stretching vibrations [53]. The band at ~900 cm−1 in the spectra of BFS17.5-60 and BFS30- 60 corresponded to the Si–O stretching and Si–OH bending modes [53]. Moreover, the band at ~1170 cm−1 corresponded to the Si–O–Si and Al–O–Si asymmetric stretching vibrations [53], and this band was broadened in the spectra of Fe/BFS17.5-60 and Fe/BFS30- 60, due to the calcination of these samples [45]. According to [54–56], Fe2O3 and Fe3O4 species should exhibit IR vibrations at 550 and 780 cm−1 and 571 and 590 cm−1, respectively. How-

Table 4 lists the concentrations (as wt %) of Ca, Si, Al, Mg, Fe, and Na of BFS17.5-60, BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60, as determined by ICP-OES analysis. ever, owing to the overlap of the Si and Al vibrations in this wavenumber region, peaks corresponding to Fe cannot be observed in the DRIFT spectra of the prepared samples. Table 4 lists the concentrations (as wt %) of Ca, Si, Al, Mg, Fe, and Na of BFS17.5-60,


**Table 4.** Metal concentrations (as wt %) of selected samples, as determined by ICP-OES analysis. BFS30-60, Fe/BFS17.5-60, and Fe/BFS30-60, as determined by ICP-OES analysis.

The Ca concentrations of the prepared samples were several times lower than those in BFS, while the Si, Al, Mg, and Na concentrations were about the same as those in the raw material (Table 5, experimental). The leaching of Ca probably occurred during the washing of the AAMs using deionized water. BFS contained ~0.5 wt % iron, and ion exchange led to the increase in the iron concentration to 5–7 wt % for Fe/BFS30-60 and Fe/BFS17.5-60, respectively. The theoretical amount of iron by the employed impregnation method was 5.3 wt %, indicating that ion exchange between BFS17.5-60 and the iron salt is slightly better than that between BFS30-60 and the iron salt. The Ca concentrations of the prepared samples were several times lower than those in BFS, while the Si, Al, Mg, and Na concentrations were about the same as those in the raw material (Table 5, experimental). The leaching of Ca probably occurred during the washing of the AAMs using deionized water. BFS contained ~0.5 wt % iron, and ion exchange led to the increase in the iron concentration to 5–7 wt % for Fe/BFS30-60 and Fe/BFS17.5-60, respectively. The theoretical amount of iron by the employed impregnation method was 5.3 wt %, indicating that ion exchange between BFS17.5-60 and the iron salt is slightly better than that between BFS30-60 and the iron salt.

**Table 5.** Elemental composition of the blast furnace slag as determined by ICP-OES analysis <sup>1</sup> . **Table 5.** Elemental composition of the blast furnace slag as determined by ICP-OES analysis. 1


<sup>1</sup> Elements with wt % > 0.01 were reported. 1 Elements with wt % > 0.01 were reported.

Figure 3 shows the FESEM images of BFS, Fe/BFS17.5-60, and Fe/BFS30-60. AAMs clearly exhibited an irregular, non-crystalline shape (Figure 3b,c). According to EDS analysis, the Al and Mg concentrations were ~5 wt %, while on the Fe catalyst surface, the Si and Ca concentrations were a few percent less than those in the bulk, as determined by ICP-OES (Table 4). Figure 3 shows the FESEM images of BFS, Fe/BFS17.5-60, and Fe/BFS30-60. AAMs clearly exhibited an irregular, non-crystalline shape (Figure 3b,c). According to EDS analysis, the Al and Mg concentrations were ~5 wt %, while on the Fe catalyst surface, the Si and Ca concentrations were a few percent less than those in the bulk, as determined by ICP-OES (Table 4).

**Figure 3.** FESEM micrographs of BFS (**a**), Fe/BFS17.5-60 (**b**), and Fe/BFS30-60 (**c**). Dimensions in figures: 1 µm (**a**) and 2 µm (**b**,**c**). **Figure 3.** FESEM micrographs of BFS (**a**), Fe/BFS17.5-60 (**b**), and Fe/BFS30-60 (**c**). Dimensions in figures: 1 µm (**a**) and 2 µm (**b**,**c**).

### *2.3. Oxidation Experiments with AAMs 2.3. Oxidation Experiments with AAMs*

The prepared AAMs, namely BFS17.5-60, BFS20-60, BFS25-60, and BFS30-60, which were first cured at 60 ◦C for 24 h, were examined for the CWPO of a BPA aqueous solution. Figure 4 shows the results of these experiments. The prepared AAMs, namely BFS17.5-60, BFS20-60, BFS25-60, and BFS30-60, which were first cured at 60 °C for 24 h, were examined for the CWPO of a BPA aqueous solution. Figure 4 shows the results of these experiments.

**Figure 4.** Removal of bisphenol A over AAMs as a function of the reaction time. Reaction conditions: concentration [*c*], *c*[BPA] = 60 mg/dm3, *c*[H2O2] = 1.5 g/dm3, *c*[catalyst] = 4 g/dm3, tempera-**Figure 4.** Removal of bisphenol A over AAMs as a function of the reaction time. Reaction conditions: concentration [*c*], *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> , temperature [*T*] = 50 ◦C, initial pH (6–7).

ture [*T*] = 50 °C, initial pH (6–7).

Oxidation reactions were performed at 50 °C at an initial pH of 6–7, a catalyst concentration of 4 g/dm3, and a H2O2 concentration of 1.5 g/dm3. In the absence of a catalyst (not shown), only ~10% of BPA removal was observed, while in the presence of AAMs, BPA removal of 35–39% after 180 min oxidation was observed. Oxidation proceeded during 2.5 h for all samples and stabilized for 3 h. The oxidant H2O2 was added in batches; hence, the final addition was performed at 2 h sampling. The total organic carbon (TOC) was measured from the initial and final samples, and 27–31% of organics were removed. The dissolved oxygen (DO) concentration of the BPA samples changed from ~9.5 mg O2/dm3 to 8.1 mg O2/dm3 during 180 min oxidation, revealing that at the end of the run, oxygen is still present in the samples. Probably, the used reaction temperature (50 °C) was not sufficiently high for the effective decomposition of H2O2 to form active ·OH radicals. In several studies, a higher reaction temperature has been reported to enhance the degra-Oxidation reactions were performed at 50 ◦C at an initial pH of 6–7, a catalyst concentration of 4 g/dm<sup>3</sup> , and a H2O<sup>2</sup> concentration of 1.5 g/dm<sup>3</sup> . In the absence of a catalyst (not shown), only ~10% of BPA removal was observed, while in the presence of AAMs, BPA removal of 35–39% after 180 min oxidation was observed. Oxidation proceeded during 2.5 h for all samples and stabilized for 3 h. The oxidant H2O<sup>2</sup> was added in batches; hence, the final addition was performed at 2 h sampling. The total organic carbon (TOC) was measured from the initial and final samples, and 27–31% of organics were removed. The dissolved oxygen (DO) concentration of the BPA samples changed from ~9.5 mg O2/dm<sup>3</sup> to 8.1 mg O2/dm<sup>3</sup> during 180 min oxidation, revealing that at the end of the run, oxygen is still present in the samples. Probably, the used reaction temperature (50 ◦C) was not sufficiently high for the effective decomposition of H2O<sup>2</sup> to form active ·OH radicals. In several studies, a higher reaction temperature has been reported to enhance the degradation of H2O2, thereby enhancing pollutant removal [57–59].

dation of H2O2, thereby enhancing pollutant removal [57–59]. As all of the AAMs exhibited similar activities for the removal of BPA, samples with the lowest and highest NaOH concentration were selected for further research. Iron was impregnated onto BFS17.5-60 and BFS30-60 samples by ion exchange (Section 3.1), and As all of the AAMs exhibited similar activities for the removal of BPA, samples with the lowest and highest NaOH concentration were selected for further research. Iron was impregnated onto BFS17.5-60 and BFS30-60 samples by ion exchange (Section 3.1), and the prepared Fe catalysts were examined under different reaction conditions.

the prepared Fe catalysts were examined under different reaction conditions. First, the effect of the addition of the active metal on BFS17.5-60 and BFS30-60 was examined at 50 °C at the initial pH, and a catalyst loading of 4 g/dm3. After 3 h oxidation, BPA removal of 42% and 45% for Fe/BFS17.5-60 and Fe/BFS30-60 were observed, respectively (Figure 5). Using the comparison of BPA removal over AAMs without the active First, the effect of the addition of the active metal on BFS17.5-60 and BFS30-60 was examined at 50 ◦C at the initial pH, and a catalyst loading of 4 g/dm<sup>3</sup> . After 3 h oxidation, BPA removal of 42% and 45% for Fe/BFS17.5-60 and Fe/BFS30-60 were observed, respectively (Figure 5). Using the comparison of BPA removal over AAMs without the active metal (Figure 4), the addition of Fe led to the increased activity of both catalysts, namely

BPA removal of 6% and 10% for BFS17.5-60 and BFS30-60, respectively. TOC removal after 3 h oxidation was at the same level for both catalysts compared to that over the pure supports (30% and 33% for Fe/BFS17.5-60 and Fe/BFS30-60, respectively). During oxidation, the DO concentration decreased slightly from ~9 mg/O<sup>2</sup> dm<sup>3</sup> to 6.2–6.6 mg/O<sup>2</sup> dm<sup>3</sup> , indicating that hydrogen peroxide is not consumed completely in the runs. Therefore, Fe/BFS17.5-60 and Fe/BFS30-60 were further examined at higher reaction temperatures. BPA removal of 6% and 10% for BFS17.5-60 and BFS30-60, respectively. TOC removal after 3 h oxidation was at the same level for both catalysts compared to that over the pure supports (30% and 33% for Fe/BFS17.5-60 and Fe/BFS30-60, respectively). During oxidation, the DO concentration decreased slightly from ~9 mg/O2 dm3 to 6.2–6.6 mg/O2 dm3, indicating that hydrogen peroxide is not consumed completely in the runs. Therefore, Fe/BFS17.5-60 and Fe/BFS30-60 were further examined at higher reaction temperatures.

metal (Figure 4), the addition of Fe led to the increased activity of both catalysts, namely

*Catalysts* **2021**, *11*, 664 9 of 19

**Figure 5.** Bisphenol A removal at reaction temperatures of 50 °C, 70 °C, and 100 °C with the Fe/BFS17.5-60 and Fe/BFS30-60 catalysts, at an initial pH of 3.5, a reaction time of 3 h, [BPA] = 60 **Figure 5.** Bisphenol A removal at reaction temperatures of 50 ◦C, 70 ◦C, and 100 ◦C with the Fe/BFS17.5-60 and Fe/BFS30-60 catalysts, at an initial pH of 3.5, a reaction time of 3 h, [BPA] = 60 mg/dm<sup>3</sup> , *c*[H2O<sup>2</sup> ] = 1.5 g/dm<sup>3</sup> , and *c*[catalyst] = 4 g/dm<sup>3</sup> .

mg/dm3, *c*[H2O2] = 1.5 g/dm3, and *c*[catalyst] = 4 g/dm3.

To investigate the effect of temperature on the CWPO of BPA over Fe/BFS17.5-60 and Fe/BFS30-60, oxidation experiments were performed at 70 °C and 100 °C at the initial pH of the BPA aqueous solution. Typically, with the increase in the reaction temperature, the oxidation rate increases. Furthermore, the decomposition rate of H2O2 to active hydroxyl radicals also increases. A higher reaction temperature led to the improved degradation of BPA during 3 h oxidation, with the maximum of 5% over Fe/BFS17.5-60 at 70 °C (Figure 5). The increase in the reaction temperature to 100 °C did not affect BPA removal. During oxidation, the DO concentration decreased from 8.0 mg/O2 dm3 to 5.7 mg/O2 dm3 and from ~10.0 mg/O2 dm3 to 4.2 mg/O2 dm3 at 70 °C and 100 °C, respectively, revealing that hydrogen peroxide is consumed in the reaction. However, owing to the low degradation level of BPA at 100 °C, hydrogen peroxide was probably decomposed directly to H2O without To investigate the effect of temperature on the CWPO of BPA over Fe/BFS17.5-60 and Fe/BFS30-60, oxidation experiments were performed at 70 ◦C and 100 ◦C at the initial pH of the BPA aqueous solution. Typically, with the increase in the reaction temperature, the oxidation rate increases. Furthermore, the decomposition rate of H2O<sup>2</sup> to active hydroxyl radicals also increases. A higher reaction temperature led to the improved degradation of BPA during 3 h oxidation, with the maximum of 5% over Fe/BFS17.5-60 at 70 ◦C (Figure 5). The increase in the reaction temperature to 100 ◦C did not affect BPA removal. During oxidation, the DO concentration decreased from 8.0 mg/O<sup>2</sup> dm<sup>3</sup> to 5.7 mg/O<sup>2</sup> dm<sup>3</sup> and from ~10.0 mg/O<sup>2</sup> dm<sup>3</sup> to 4.2 mg/O<sup>2</sup> dm<sup>3</sup> at 70 ◦C and 100 ◦C, respectively, revealing that hydrogen peroxide is consumed in the reaction. However, owing to the low degradation level of BPA at 100 ◦C, hydrogen peroxide was probably decomposed directly to H2O without the formation of hydroxyl radicals.

the formation of hydroxyl radicals. Typically, homogeneous iron catalysts for CWPO (Fenton process) are used at a pH of ~3, which is known to be optimum for the decomposition of organic compounds [60]. The effect of pH on the degradation level of BPA was investigated at pH 3.5, in addition to the initial pH (6–7) by using Fe/BFS17.5-60 and Fe/BFS30-60 catalysts. The effect of pH was examined at 50 °C, 70 °C, and 100 °C. The pH of the BPA solution was adjusted to 3.5 Typically, homogeneous iron catalysts for CWPO (Fenton process) are used at a pH of ~3, which is known to be optimum for the decomposition of organic compounds [60]. The effect of pH on the degradation level of BPA was investigated at pH 3.5, in addition to the initial pH (6–7) by using Fe/BFS17.5-60 and Fe/BFS30-60 catalysts. The effect of pH was examined at 50 ◦C, 70 ◦C, and 100 ◦C. The pH of the BPA solution was adjusted to 3.5 using 2.0 M HNO<sup>3</sup> before oxidation. At 50 ◦C and pH 3.5, BPA removal increased by 5% over Fe/BFS17.5-60, while over Fe/BFS30-60, it was almost the same after 3 h

oxidation compared to experiments performed at the initial pH of BPA (Figure 5). The DO concentration of the liquid samples was considerably higher (at the end of the run for Fe/BFS17.5-60, 14 mg/O<sup>2</sup> dm<sup>3</sup> ) than that after oxidation at the initial pH. Therefore, acidic pH promotes the formation of hydroxyl radicals during the reaction. However, Fe catalysts did not exhibit considerably higher activity for BPA removal than that at the initial pH, probably due to the basic surfaces of Fe/BFS17.5-60 and Fe/BFS30-60.

At pH 3.5 and 70 ◦C (Figure 5), the DO concentration was the same during tests compared to that in experiments at the initial pH, and the pH change of the BPA solution led to an increase in BPA removal by only 3% and 2% over Fe/BFS17.5-60 and Fe/BFS30-60, respectively. At 100 ◦C and pH 3.5, BPA removal after 3 h was around the same for both Fe catalysts compared with that observed at 100◦C and at the initial pH. However, notably, owing to the basicity of Fe catalysts, the pH of the BPA solution changed to basic during runs in all experiments. The decomposition of H2O<sup>2</sup> to ·OH radicals is the key step in CWPO. However, under a basic reaction pH, the generation of hydroxyl radicals was restricted, thereby further decreasing the degradation of BPA [61]. Thus, the change in pH marginally affects BPA removal.

The adsorption capacity of the Fe catalysts was examined under the severest reaction conditions in this study, i.e., pH of 3.5, a reaction temperature of 100 ◦C, in the absence of the oxidant, and a catalyst concentration of 4 g/dm<sup>3</sup> . For Fe/BFS17.5-60 and Fe/BFS30-60, during the 3 h experiment, 12% and 17% of BPA was adsorbed, respectively, revealing that Fe/BFS17.5-60 is catalytically more active than Fe/BFS30-60. The higher adsorption capacity of Fe/BFS30-60 was related to the higher specific surface area of this sample (Table 3).
