3.1 Characterization of nanostructured Ag3PO4
The phase structures of the as-prepared nanostructured Ag3PO4 were investigated by XRD and showed in Fig. 1.
X-ray diffraction patterns of nanostructured Ag3PO4 clearly showed that all diffraction peaks are a near-systematic superposition of those of face-centered cubic structure of Ag3PO4 (JCPDS file No010-1988). No diffraction peaks for other phases such as calcium phosphate are detected, this indicates that Ag+ cations have the best chemical affinity towards PO43- anions compared to that of Ca2+ ions originating from natural phosphate in which they have been cleaned from the solid by washing treatment. Without taking into account unreactive species (Ca2+ and NO3-), the process of the reaction can be represented as follows:

Furthermore, according to the Debye-Scherrer equation, the average crystallite size of the synthesized Ag3PO4 is about 39 nm. A view of the structure along the [100] direction is shown in Fig.2. Hence, the polyhedron configuration of Ag3PO4 consists of tetrahedral PO4 and AgO4. It is obvious that one PO4 tetrahedron and three tetrahedral AgO4 are combined with each other through the corner oxygen. Additionally, the average of P-O and Ag-O distance is about 2.37 Å and 1.54Å, respectively.
Fig.3. showed the Fourier transform infrared (FT-IR) spectrum of the Ag3PO4 sample, exhibiting two strong bands in the 1072-732 cm−1 and 586-447 cm−1 ranges related to the molecular vibrations of PO4 groups in Ag3PO4 sample. The former band centered at 941 cm−1 with a very small shoulder at around 1056 cm−1 is assigned to the symmetric and asymmetric stretching mode of the P-O bonds, while the latter centered at 547 cm−1 is related to deformation from the bending mode of the O-P-O bonds.
To better elucidate the morphology properties of the prepared Ag3PO4 from natural phosphate, SEM analysis was carried out. In the lower magnification images, the Fig.4a indicates that the surface of nanostructured Ag3PO4 is formed by a large amount of quasi-spheroid particles having hexagonal and cubic structures, while the higher magnification image, as shown in Fig.4b, clearly reveals the quasi-spheroid particles through non-uniform diameter polyhedrons. On the other hand, to confirm the chemical composition of the nanostructured Ag3PO4, a semi-quantitative elemental analysis was performed, and its EDS spectrum showed in Fig.4c. The obtained results revealed the presence of O, P and Ag elements without any calcium traces indicating that the Ag3PO4 prepared is pure and does not contain any impurities. Note that the presence of carbon and copper peaks is originated from adhesive Cu-carbon tape.
To further demonstrate the porous structure of nanostructured Ag3PO4, the specific surface area (SBET) of the Ag3PO4 powder was calculated from N2-sorption measurements and application of the BET method. As shown in Fig.5a, the sorption isotherm exhibited a type IV isotherm according to the IUPAC classification with a distinct hysteresis loop of H3. Its specific surface area was of 35 m2/g and average pore size Dp calculated from BJH (Barrett-Joyner-Halenda) method was 3.1 nm and 7.3 nm (Fig.5b). Comparing with the low values given in the literature, a relatively large porous surface of Ag3PO4 catalyst could provide more active and beneficial sites for the adsorption of target molecules through the active sites of the catalyst, which would promote the catalytic reaction.
3.2 Catalytic Reduction of 4-NP to 4-AP
To investigate the catalytic activity of the Ag3PO4 as nanostructured catalyst, the reduction of 4-nitrophenol to its corresponding amino derivatives, 4-aminophenol, in the presence of NaBH4 in aqueous media was chosen as a model reaction (Scheme 2). Currently, the reduction of 4-NP to 4-AP is monitored by UV-vis spectra at their specific wavelengths 317 nm for 4-NP and 300 nm for 4-AP.
Firstly, the ability of NaBH4 to reduce 4-NP in absence of our catalyst was examined. As shown in Fig. 6-A, the 4-NP in an aqueous solution has a maximum absorption at 317. After added NaBH4 into solution, the absorbance peak of 4-NP was red shifted from 317 to 400 nm immediately along with a colour change from light yellow to bright yellow. This peak was due to the formation of 4-nitrophenolate ions in alkaline condition caused by the addition of reducing agent, as supported elsewhere [22]. However, in the absence of our catalyst the thermodynamically favorable reduction of 4-nitrophenol was not watched and the absorbance peak corresponding to 4-nitrophenolate ions at 400 nm rest unchanged for a long time (Fig.6-B). Then, when a small amount of Ag3PO4 nanostructured (5 mg) was introduced into reaction solution, the absorbance peak at 400 nm decreases significantly within 32 min and concomitant appearance of a new peak at 300 nm. The new absorption at 300 nm is characteristic peak of 4-AP, revealing the reduction of 4-NP to form 4-AP. In addition, as seen in the UV-Vis spectra (Fig.6.C), the presence of an isobestic point at 317 nm indicating that the catalytic reduction of 4-nitrophenol gives 4-aminophenol only without by product [23-24].
To understand the catalytic conversion kinetic of 4-NP, the pseudo-first-order model was used: ln(Ct/C0)= ln(At/A0)= -kapp t , where C0 and Ct are the 4-NP concentrations at t=0 and t=t, respectively and kapp is the apparent rate constant, which is in good relationship with the disappearance of the nitrophenonlate band at 400 nm versus time. In this light, the influence of the NaBH4 molar concentration on the catalytic efficiency of the reduction of 4-NP into 4-AP was studied. Fig. 7a shows the ln(At/A0) plot as a function of time at different concentration of reducing agent (0.01; 0.05; 0.1 and 0.5M) at room temperature. Based on the results of this study, it is clearly seen that the constant rate conversion of 4-NP to 4-AP increases from 0.024 min-1 to 0.088 min-1 with increasing NaBH4 concentration from 0.01 to 0.5 M, respectively. These results can be interpreted by the presence of an excess of the reducing agent, which favouring the diffusion of BH4- ions on the catalyst surface by accelerating the reduction of the diffused 4-NP [25-26]. Note that no significant change in apparent rate constant above 0.1 M of [NaBH4] was observed; thus, an optimum concentration of 0.1M [NaBH4] was chosen for the future experiments. In addition, the effect of amount of the Ag3PO4 nanostructured on catalytic efficiency was also studied using 0.1M of NaBH4 at room temperature. We should mentioned that the reaction was started after adding of Ag3PO4 as a nanostructured catalyst and the colour of the solution changed gradually from bright yellow to colourless indicated the successive reduction of 4-NP. As showed in Fig. 7b, the conversion reaction seems to be sensitive to the catalyst amount, but from 5 mg of the catalyst, the reaction became uncontrollable and ends very quickly. Thus, the optimum amount of the catalyst was selected to be 5 mg.
Based on the results described above the proposed mechanism for the reduction of 4-NP is given in schematic 3. As published elsewhere [27-29], the hydrogen atom of BH4- is positively charged and could create fine electrostatic attractions with negatively charged oxygen from nitro groups at catalyst surface, facilitating the removal of oxygen and reduction of nitro groups. In addition, the residual nitrogen of -NO2 is also negatively charged due to its greater electronegativity than carbon from the benzene ring, and the H atoms of the positively charged H2O molecules could easily combine with the residual nitrogen of the nitrophenol to form the final aminophenol product. Adding to the proton transfer and deoxygenation, electron transport must occur simultaneously from the BH4- clusters to 4-NP via the Ag3PO4 catalyst substrate to compensate for the charge balance and accomplish the process of reduction.
The reusability of the catalyst is another important factor from economic and environmental point of view, which it is highly desirable to examine in this study. At the end of the reaction, the catalyst was easily separated by filtration from solution, washed with deionized water and ethanol, dried at 80°C and then was reused for the next cycle of catalysis. As shown in Fig. 8, the catalyst was recycled several times with a little loss in catalytic performance after the third cycle. This can be explained by the reduction of silver (Ag+→Ag0) by the excited electrons during catalytic processes, confirmed by the gradual colour change of the Ag3PO4 catalyst (From yellow to dark brown), resulting in decrease in the catalytic efficiency.
3.3 Antibacterial activity studies
The production of a large quantity of Ag3PO4 through a simple and economical method from natural phosphate can be employed as antibacterial agent suitable for the biological treatment of wastewater. In this optic, the obtained results of antibacterial activity of Ag3PO4 against E. Coli (Gram-negative) and S. Aureus (Gram-positive) are shown in Fig. 9. The zone of inhibition clearly indicated the significant antibacterial effect of the nanostructured Ag3PO4 as quantitatively shown in Table 2. With 1 mg/mL as the serial concentration of Ag3PO4 in the biological solution, the maximum diameters of the zones of inhibition are approximately 12.01 mm and 13.50 mm against S. aureus and E. coli, respectively. As result, the nanostructured powder of Ag3PO4 prepared from phosphate rock is in fact an effective antibacterial agent on Gram positive and Gram-negative bacteria such as largely described in the literature.
Table 2. The inhibition diameter zone of the nanostructured Ag3PO4against E. Coli and S. Aureus at different concentrations.
Concentration
(mg/mL)
|
Zone of inhibition (mm)
|
E. coli
|
S. Aureus
|
1
|
13.50
|
12.01
|
0.5
|
12.54
|
11.33
|
0.25
|
10.20
|
10.9
|
0.125
|
10.00
|
9.2
|