3.6.2. As(III)-PBN@SiO<sup>2</sup> and As(V)-PBN@SiO<sup>2</sup>

The cation As(III) and the oxidized species As(V) detected on the PBN@SiO<sup>2</sup> substrate with XPS after a decontamination process are shown in Figure 11. The binding energy values (in eV) for O (1s), Si (2p), Fe (2p), and N (1s) in PBN@SiO<sup>2</sup> and As-PBN@SiO<sup>2</sup> are listed in Table 3. The XPS survey scan as shown in Figure 11D shows peaks at a binding energy of 49.03 eV, which are associated with the presence of As(V) and indicate the successful sorption of As(V) by PBN@SiO<sup>2</sup> [31]. The other peak located at 43.23 eV is associated with the adsorption of As(III) over SiO<sup>2</sup> prior to the oxidation process [32]. However, the peak positions observed for the Fe+3 and Fe+2 core level (2p) spectra of PBN@SiO<sup>2</sup> are shifted slightly to a lower binding energy relative to the unreacted and unabsorbed PBN@SiO<sup>2</sup> species. This shift in the peaks for Fe2+ 2p3/2 (binding energy of 708.19 eV) and Fe2+ 2p1/2 (binding energy of 721.42 eV) may be attributed to arsenic adsorption [33]. The shift in the peak position (with a reduction in intensity) of Fe+3 2p3/2 (binding energy of 712.86 eV) relative to pure PBN@SiO<sup>2</sup> suggests the reduction of the material during arsenic oxidation [34]. The position of the characteristic peak of Fe+2 (binding energy of 55.04 eV, 3p) remained unchanged throughout the As(III) oxidation and adsorption process [31]. A peak emerged at a binding energy of 398.99 eV, which was attributed to the presence of nitrogen in the environment. Alterations in the peak position of PBN@SiO<sup>2</sup> relative to that of As-PBN@SiO<sup>2</sup> were associated with chemical adsorption by PBN@SiO<sup>2</sup> of arsenic species.

.


**Table 3.** XPS data of PBN@SiO<sup>2</sup> and Arsenic-PBN@SiO<sup>2</sup>

**Figure 11.** Figure 11 X-ray photoelectron spectrum of As-PBN@SiO2 after arsenic exposure. (**A**) Identified Si (IV) chemical states in SiO2. (**B**) Fe+2 and Fe+3 species XPS peak in PBN@SiO2. (**C**) Arsenic species (As(V) and As (III)) at PBN@SiO2. (**D**) (**1a-b**) EDX line scan measurement comprised of an inner SiO2 and an outer ferric hexacyanoferrate with As(III) enrichment over the PBN@SiO2 surface. (**2**) EDX analysis shows all of the anticipated elements. **Table 3.** XPS data of PBN@SiO2 and Arsenic-PBN@SiO2. **Figure 11.** X-ray photoelectron spectrum of As-PBN@SiO<sup>2</sup> after arsenic exposure. (**A**) Identified Si(IV) chemical states in SiO<sup>2</sup> . (**B**) Fe2+ and Fe3+ species XPS peak in PBN@SiO<sup>2</sup> . (**C**) Arsenic species (As(V) and As(III)) at PBN@SiO<sup>2</sup> . (**D**) (**1a**,**1b**) EDX line scan measurement comprised of an inner SiO<sup>2</sup> and an outer ferric hexacyanoferrate with As(III) enrichment over the PBN@SiO<sup>2</sup> surface. (**2**) EDX analysis shows all of the anticipated elements.

### **Sample Si(2p) O Fe+3 Fe+2 Fe+2 N As(III) As(V)**  *3.7. Effect of pH on Arsenic Removal*

**1s 2p3/2 2p1/2 2p3/2 1s 3d 3d PBN@SiO2** 103.63 532.62 712.12 721.27 708.34 397.07 - - **As-PBN@SiO2** 103.49 531.99 712.86 721.42 708.19 397.05 and 398.99 43.23 49.03 *3.7. Effect of pH on Arsenic Removal*  The results in Figure 10 illustrate the effects of pH on the removal of As(III) using PBN@SiO2. As can be observed, As(III) removal was dependent on pH; the greatest removal efficiency occurred under moderate pH (pH = 6–9) and was found to diminish at highly acidic pH (pH < 3). As reported earlier, surfaces of silica beads were positively charged in highly acidic conditions and acquired a negative charge in the pH range of 3– 10 [35]. Subsequently, moderate pH was found to be favorable for a sorbent surface since The results in Figure 10 illustrate the effects of pH on the removal of As(III) using PBN@SiO2. As can be observed, As(III) removal was dependent on pH; the greatest removal efficiency occurred under moderate pH (pH = 6–9) and was found to diminish at highly acidic pH (pH < 3). As reported earlier, surfaces of silica beads were positively charged in highly acidic conditions and acquired a negative charge in the pH range of 3–10 [35]. Subsequently, moderate pH was found to be favorable for a sorbent surface since decreased protonation is supposed to enlarge the attraction force between the negatively charged PBN@SiO<sup>2</sup> surface and the positively charged As(III) cationic species. This result is similar to earlier findings by Gupta et al. who reported a significant increase in As(III) adsorption onto iron oxide-coated quartz sand with an increase in pH from 4.5 to 7.5 [36]. At a highly acidic pH (<3), repulsion occurred between the positively charged adsorbent sites and the adsorbate species (As+3), which prevented the adsorption and arsenic oxidation processes. No substantial rise in As(III) removal efficiency was observed with an elevation in pH.

decreased protonation is supposed to enlarge the attraction force between the negatively

### *3.8. Analysis of PBN@SiO<sup>2</sup> Surface through SEM-EDX after As(III) Remediation*

After the As(III) removal process, the PBN@SiO<sup>2</sup> surface was analyzed using SEM. The result showed the change in morphology of PBN (cubic to spherical) after arsenic interaction (Figure 11E). The EDX results suggest that the material is comprised of an inner SiO<sup>2</sup> chemistry and an outer ferric hexacyanoferrate (Fe+3[Fe+2(CN)6]) chemistry, with some As(III) enrichment over the PBN@SiO<sup>2</sup> surface. The presence of the anticipated elements was confirmed through EDX analysis.

### *3.9. Recyclability and Proposed Mechanism*

It has been well established that Prussian blue is comprised of Fe metal in Fe+2 (low spin) and Fe+3 (high spin) states, which are linked via CN bridges. Prussian blue can undergo reduction to what is known as Prussian white (FeII–C≡N–FeII) or oxidation to what is known as Prussian yellow (FeIII–C≡N–FeIII) [37,38]. Reduction of Prussian blue to Prussian white on the surface of silica gel was found to facilitate the As(III) oxidation to As(V) and their subsequent removal. Conversion of Fe3+ to Fe+2 was shown during the decontamination in PBN and validate the altered chemical environment due to arsenic interactions. The addition of ferric chloride to the white-blue colored arsenite-treated PBN@SiO<sup>2</sup> residue instantly generated a blue color; this phenomenon is attributed to the conversion of hexacyanoferrate <sup>h</sup> FeII(CN)<sup>6</sup> i species of <sup>K</sup>2[FeIIFeII(CN)<sup>6</sup> ] into Prussian blue <sup>K</sup>[FeIIIFeII(CN)<sup>6</sup> i ) through the interaction with ferric species (ferric chloride). The reaction during Prussian blue synthesis has been shown as:

$$4\text{K}\_2[\text{Fe}^{\text{II}}\text{Fe}^{\text{II}}(\text{CN})\_6] + 4\text{FeCl}\_34\left(\text{K}[\text{Fe}^{\text{III}}\text{Fe}^{\text{II}}(\text{CN})\_6]\right) + 4\text{KCl}$$

The variation of surface charge of SiO<sup>2</sup> with a change in pH was found to be the fundamental framework for PBN activity over the course of arsenic removal.

### *3.10. Mechanism of PBN Based Fluorescence Sensing of As(III)*

The findings as shown in Figures 4–6 revealed an analyte-dependent intervalence transition in iron hexacyanoferrate [FeIII <sup>4</sup>[FeII(CN)6]3] between Fe2+ and Fe3+ as shown below:

$$\text{Fe}^{2+} + \text{Fe}^{3+} + \text{light energy} \rightarrow \text{Fe}^{3+} + \text{Fe}^{2+} $$

The intervalence transition may be evaluated based on changes to the absorption spectrum. The fluorescein–PBN interaction is associated with fluorescence resonance energy transfer as recently described [39,40]; this material is capable of quenching the emitted fluorescence of fluorescein. When PBN undergo interaction with As(III), there is a conversion of PBN into Prussian white nanoparticles, followed by a conversion of As(III) to As(V); thus, the quenching ability is lost. The Prussian white nanoparticles can further be converted into PBN after treating the same with acid as discussed above. This scheme provides an effective and inexpensive method for PBN-mediated removal of As(III) udermvisible light.
