*3.4. As(III) Decontamination from Aqueous Solution Using PBN@SiO<sup>2</sup>*

The heterogeneous PBN@SiO<sup>2</sup> system was studied in order to understand the dynamic interaction occurring between As(III) and PBN. Accordingly, inexpensive and non-reactive silica beads were used for the modulation of active PBN in the formulation of the heterogeneous matrix. Heterogeneous methods are considered to play an influential role in catalysis due to their straightforward separation and large-scale applicability. For As(III) decontamination, the as-synthesized PBN@SiO<sup>2</sup> (0.05 g) was successfully packed in a column of 10 mm diameter. The standard As(III) solution (10 ppm) was prepared via adding an appropriate amount of sodium arsenite salt in Milli-Q water; 10 mL of the solution was passed through the PBN@SiO<sup>2</sup> enclosed column. The fluorescence analysis of separated supernatant (PBN@SiO<sup>2</sup> processed) was performed using Flo (0.2 mM) under similar conditions. In this study, 10 µL of as-eluted supernatants (PBN@SiO<sup>2</sup> processed and unprocessed As(III) solution) were added separately with Flo and left to stand at room temperature for 2 min. Their emission spectra were recorded to understand PBN@SiO<sup>2</sup> interactions with As(III). Unprocessed As(III) solution was observed to enhance the emission intensity of Flo many-fold (Figure 9A) as compared to the PBN@SiO<sup>2</sup> processed As(III) solution (Figure 9B).

PBN@SiO<sup>2</sup> was shown to significantly remove the As(III) from the contaminated solution. The ICP analysis of PBN@SiO<sup>2</sup> processed As(III) aqueous solution was performed to quantify the arsenic concentration in the solution. The result showed 0.0018 ppm arsenic (As) content for the PBN@SiO<sup>2</sup> processed As(III) solution. In addition, 0.13 ppm Fe content was also detected in the processed As(III) solution. The ICP analysis indicated that some of the iron species of PBN ([FeIII[FeII(CN)6]) leached out with the eluent during interaction with the As(III) species. To investigate the presence of active iron species in the eluent, we studied the addition of ferrous sulfate with active ferrous species (Fe+2) to the colorless supernatant eluent. During this process, we added the ferric chloride-containing active ferric species to the colorless supernatant eluent. We observed that colorless supernatant changed immediately to an intense blue color (resembling the Prussian blue color) when ferric chloride was added. However, no such changes were observed when ferrous sulfate (containing Fe+2) was added.

We performed a fluorometric experiment in which supernatant (SN) was employed to observe its modulation of the Flo fluorescence properties. Fluorescent emission spectra were recorded after adding Flo (10 µL) to 10 µL of PBN@SiO<sup>2</sup> processed supernatant (SN). A small change in intensity (I<sup>o</sup> = 102.23) was observed with respect to the Flo (I<sup>o</sup> = 98.74) as seen in Figure 9C (1 and 2). A study that involved adding ferric chloride to the Flosupernatant (Flo-SN) mixture showed that the Flo fluorescence property was quenched (I = 31.73) when compared to Flo (I = 98.74), as shown in Figure 9C (3). Supernatant-

*Nanomaterials* **2021**, *11*, x FOR PEER REVIEW 12 of 20

containing ferrocyanide species had an instant interaction with the added ferric chloride, which instantly converted into PBN. PBN@SiO2 interactions with As(III). Unprocessed As(III) solution was observed to enhance the emission intensity of Flo many-fold (Figure 9A) as compared to the PBN@SiO2 processed As(III) solution (Figure 9B).

similar conditions. In this study, 10 μL of as-eluted supernatants (PBN@SiO2 processed and unprocessed As(III) solution) were added separately with Flo and left to stand at room temperature for 2 min. Their emission spectra were recorded to understand

**Figure 9.** Fluorescence emission spectra of Flo with unprocessed (**A**) and PBN@SiO2 processed (**B**) As(III) solution. Identification of ferrocyanide species in supernatant on the addition of ferric chloride via fluorescence quenching (**C**). Study of the effect of ferrocyanide species (1–4 mm) over Flo emission spectra (**D**). **Figure 9.** Fluorescence emission spectra of Flo with unprocessed (**A**) and PBN@SiO<sup>2</sup> processed (**B**) As(III) solution. Identification of ferrocyanide species in supernatant on the addition of ferric chloride via fluorescence quenching (**C**). Study of the effect of ferrocyanide species (1–4 mm) over Flo emission spectra (**D**).

PBN@SiO2 was shown to significantly remove the As(III) from the contaminated solution. The ICP analysis of PBN@SiO2 processed As(III) aqueous solution was performed to quantify the arsenic concentration in the solution. The result showed 0.0018 ppm arsenic (As) content for the PBN@SiO2 processed As(III) solution. In addition, 0.13 ppm Fe content was also detected in the processed As(III) solution. The ICP analysis indicated that some of the iron species of PBN ([FeIII [FeII(CN)6]) leached out with the eluent during interaction with the As(III) species. To investigate the presence of active iron species in the eluent, we studied the addition of ferrous sulfate with active ferrous species (Fe+2) to the colorless supernatant eluent. During this process, we added the ferric chloride-containing active ferric species to the colorless supernatant eluent. We observed that colorless supernatant changed immediately to an intense blue color (resembling the Prussian blue color) when ferric chloride was added. However, no such changes were observed when ferrous sulfate (containing Fe+2) was added. To analyze the role of the residual ferrocyanide species in the supernatant over the emission spectra, a fluorescence experiment centered on the ferrocyanide concentration was conducted. We prepared and added different amounts (1 mM to 4 mM) of ferrocyanide solution to a constant amount of Flo (0.2 mM) to analyze the influence of the solution over the Flo emission intensity. Ferrocyanide acted as a weak enhancer (Figure 9D). These results indicate that the As(III) was supposed to undergo oxidation into arsenate in the presence of PBN. The iron species in the PBN undergo reduction into Fe+2 throughout the As(III) removal process. On the addition of active ferric species to the supernatant, an immediate reaction leads to the formation of PBN after the interaction with residual ferrous species. The collected PBN@SiO<sup>2</sup> was characterized with XPS to observe the significance of arsenic treatment over the PBN@SiO<sup>2</sup> phase (as discussed in a subsequent section). Moreover, the resultant eluent was collected into separate vials and underwent HPLC analysis for the detection of arsenic species.

### We performed a fluorometric experiment in which supernatant (SN) was employed to observe its modulation of the Flo fluorescence properties. Fluorescent emission spectra *3.5. HPLC Results on PBN@SiO<sup>2</sup> Treated Arsenic(III)*

were recorded after adding Flo (10 μL) to 10 μL of PBN@SiO2 processed supernatant (SN). A small change in intensity (Io = 102.23) was observed with respect to the Flo (Io = 98.74) as seen in Figure 9C (1 and 2). A study that involved adding ferric chloride to the Flo-supernatant (Flo-SN) mixture showed that the Flo fluorescence property was quenched (I = 31.73) when compared to Flo (I = 98.74), as shown in Figure 9C (3). Supernatant-containing ferrocyanide species had an instant interaction with the added ferric chloride, which instantly converted into PBN. All the separated species were noted in the ion-chromatogram at their respective retention time such as arsenobetaine (AsB) at 2.17/2.42/2.55 min, dimethylarsinic acid (DMA) at 3.57 min, As(III) at 3.8/3.9 min, and As(V) at 7.7 min. The chromatogram shown in Figure 10A–D was obtained as the result of HPLC separation of the arsenic species after treatment with PBN@SiO<sup>2</sup> at different pH values (2.2–8.5). HPLC analysis illustrates that the removal efficiency of As(III) ((Figure 10A) by PBN@SiO<sup>2</sup> increased from 33.52% (Figure 10B) to 59.90% (Figure 10C) with a pH increase from 2 to 6.5; this improved to 95.13% (Figure 10D) under a mild alkaline condition (pH-8.5). The ion chromatogram results also showed an insignificant peak at a retention time of 7.35 min ((Area% = 1.4) at pH = 6.5 and (Area% = 9.09) at pH = 8.5), which was associated with leaching of As(V) in an aqueous solution during the oxidation–adsorption process. All of the arsenic

tion during HPLC.

species (As(III), DMA, AsB) identified at various retention times along with their relative concentration in a HPLC environment are shown in Table 2. *Nanomaterials* **2021**, *11*, x FOR PEER REVIEW 14 of 20 *Nanomaterials* **2021**, *11*, x FOR PEER REVIEW 14 of 20 *Nanomaterials* **2021**, *11*, x FOR PEER REVIEW 14 of 20

*Nanomaterials* **2021**, *11*, x FOR PEER REVIEW 14 of 20

**Figure 10.** Ion chromatogram obtained during HPLC speciation of species present in the sample**.**  (**A**) Standard As(III) (5 ppm) solution at pH=6.5). (**B**) Standard As(III) sample (5 ppm) after PBN@SiO2 treatment in acidic medium (pH=2). (**C**) As(III) standard solution (5 ppm) after PBN@SiO2 treatment in neutral medium (pH=6.5). (**D**) As(III) standard sample (5 ppm) after PBN@SiO2 treatment in alkaline medium (pH=8.5). (**E**) XPS analysis of PBN@SiO2. (**1**) A complete survey scan with all recognized species. (**2**) Fe+2 and Fe+3 species XPS peak in PBN@SiO2. (**3**) Identified Si (IV) chemical states in SiO2. **Figure 10.** Ion chromatogram obtained during HPLC speciation of species present in the sample. (**A**) Standard As(III) (5 ppm) solution at pH = 6.5). (**B**) Standard As(III) sample (5 ppm) after PBN@SiO<sup>2</sup> treatment in acidic medium (pH = 2). (**C**) As(III) standard solution (5 ppm) after PBN@SiO<sup>2</sup> treatment in neutral medium (pH = 6.5). (**D**) As(III) standard sample (5 ppm) after PBN@SiO<sup>2</sup> treatment in alkaline medium (pH = 8.5). (**E**) XPS analysis of PBN@SiO<sup>2</sup> . (**1**) A complete survey scan with all recognized species. (**2**) Fe2+ and Fe3+ species XPS peak in PBN@SiO<sup>2</sup> . (**3**) Identified Si(IV) chemical states in SiO<sup>2</sup> . **Figure 10.** Ion chromatogram obtained during HPLC speciation of species present in the sample**.**  (**A**) Standard As(III) (5 ppm) solution at pH=6.5). (**B**) Standard As(III) sample (5 ppm) after PBN@SiO2 treatment in acidic medium (pH=2). (**C**) As(III) standard solution (5 ppm) after PBN@SiO2 treatment in neutral medium (pH=6.5). (**D**) As(III) standard sample (5 ppm) after PBN@SiO2 treatment in alkaline medium (pH=8.5). (**E**) XPS analysis of PBN@SiO2. (**1**) A complete survey scan with all recognized species. (**2**) Fe+2 and Fe+3 species XPS peak in PBN@SiO2. (**3**) Identified Si (IV) chemical states in SiO2. **Figure 10.** Ion chromatogram obtained during HPLC speciation of species present in the sample**.**  (**A**) Standard As(III) (5 ppm) solution at pH=6.5). (**B**) Standard As(III) sample (5 ppm) after PBN@SiO2 treatment in acidic medium (pH=2). (**C**) As(III) standard solution (5 ppm) after PBN@SiO2 treatment in neutral medium (pH=6.5). (**D**) As(III) standard sample (5 ppm) after PBN@SiO2 treatment in alkaline medium (pH=8.5). (**E**) XPS analysis of PBN@SiO2. (**1**) A complete survey scan with all recognized species. (**2**) Fe+2 and Fe+3 species XPS peak in PBN@SiO2. (**3**) Identified Si (IV) chemical states in SiO2. (**A**) Standard As(III) (5 ppm) solution at pH=6.5). (**B**) Standard As(III) sample (5 ppm) after PBN@SiO2 treatment in acidic medium (pH=2). (**C**) As(III) standard solution (5 ppm) after PBN@SiO2 treatment in neutral medium (pH=6.5). (**D**) As(III) standard sample (5 ppm) after PBN@SiO2 treatment in alkaline medium (pH=8.5). (**E**) XPS analysis of PBN@SiO2. (**1**) A complete survey scan with all recognized species. (**2**) Fe+2 and Fe+3 species XPS peak in PBN@SiO2. (**3**) Identified Si (IV) chemical states in SiO2. **Table 2.** All arsenic species (As(III), DMA, AsB) identified at different retention times along with their relative concentra-

15.81 16.21 6.8 3.57 min Figure 10C 13.45 11.79 8.5 3.57 min Figure 10D

15.81 16.21 6.8 3.57 min Figure 10C 13.45 11.79 8.5 3.57 min Figure 10D

15.81 16.21 6.8 3.57 min Figure 10C 13.45 11.79 8.5 3.57 min Figure 10D

13.45 11.79 8.5 3.57 min Figure 10D

**Table 2.** All arsenic species (As(III), DMA, AsB) identified at different retention times along with their relative concentration during HPLC. **Table 2.** All arsenic species (As(III), DMA, AsB) identified at different retention times along with their relative concentration during HPLC. **Table 2.** All arsenic species (As(III), DMA, AsB) identified at different retention times along with their relative concentration during HPLC. tion during HPLC. **Species Description Molecular Height Area System Retention** 

**Table 2.** All arsenic species (As(III), DMA, AsB) identified at different retention times along with their relative concentra-


AsB, which frequently existed in the zwitterionic form due to the interaction between the positively charged arsenic and the negatively charged carboxylic group, starts to migrate immediately after interacting with the hydrophobic C18 Shim-pack column. However, As(III) is a neutral species (pKa = 9.2) up to a pH of 8, which eluents slowly with the solvent front. Consequently, negatively charged DMA and As(V) species feasibly eluent by a variety of interactions (e.g., H bonding and ion-exchange) along with hydrophobic effects. The obtained result was acquired after a total run time of 25 min and repeated twice to minimize the experimental error.
