A novel approach for the synthesis of nanostructured Ag3PO4 from phosphate rock: high catalytic and antibacterial activities

Background Silver orthophosphate (Ag3PO4) has received enormous attention over the past few years for its higher visible light photocatalytic performance as well as for various organic pollutants degradation in aqueous media. Therefore, considerable efforts have been made to the synthesis of Ag3PO4 with high catalytic efficiency, long lifetime, and using low-cost inorganic precursors. Results This article describes our efforts to develop a novel approach to synthesize of nanostructured silver phosphate (Ag3PO4) using phosphate rock as alternative and natural source of PO43− precursor ions. The catalytic experimental studies showed that the nanostructured Ag3PO4 exhibited excellent catalytic activity for reduction of p-nitrophenol in the presence of NaBH4 at room temperature. Furthermore, the antibacterial studies revealed that the obtained Ag3PO4 possess significant effect against E. Coli and S. Aureus bacteria. Conclusion The obtained results make the nanostructured Ag3PO4 prepared from natural phosphate as a highly promising candidate to be used as efficient catalyst and antibacterial agent. Graphic Abstract


Introduction
With industrialization processes and more human activities, the environmental contamination caused by organic pollutants is becoming an overwhelming mystery all over the world [1,2]. The phenolic compounds are one of the most notorious pollutants generated by industrials sources such as synthetic intermediate in the manufacture of pharmaceuticals, plastics, pigments, dyes, pesticides and fungicidal agents, explosives and industrial solvents [3]. Due to their potential to harm human health and living organisms at low concentrations, these compounds were classified as priority materials by the United States Environmental Protection Agency (USEPA) among the top 114 organic pollutants [4,5]. Among the different phenolic compounds, 4-nitrophenol (4-NP) is one of the most frequently occurring products [6]. However, the 4-aminophenol (4-AP) is one of the most important intermediates in the preparing of several analgesic and antipyretic drugs like paracetamol, acetanilide, and phenacetin [7]. Also, it is utilized as corrosion inhibitor in paints, and anticorrosion-lubricating agent in fuels. Further, 4-aminophenol is used efficiently in the dye industry as a wood stain and a dyeing agent for fur and feathers [8]. In view of the harmful effect of 4-NP and the growing demand for 4-AP, the conversion of 4-NP directly to 4-AP via catalytic route becomes greatly desirable. Various metal nanoparticles like as platinum, gold, copper, ruthenium and palladium, are used intensively for the reduction of nitrophenols compounds. Although, all these catalysts are very expensive for industrial use in bulk quantity. Consequently, considerable effort has been devoted to exploit new and efficient photocatalysts for environmental pollution control. Of the well-known photocatalyst materials, silver orthophosphate (Ag 3 PO 4 ) has attracted great attention over the past few years due to its excellent photocatalyst ability for water splitting and degradation of organic contaminants [9][10][11][12][13][14] Several methods have been reported in the literature for the synthesis of Ag 3 PO 4 with variety of commercial reagents to improve of its photocatalytic properties, such as rare metal doping, surface modification, semiconductor coupling and formation of noble metal composites [15][16][17][18]. Nevertheless, some of these synthetic routes suffer from several drawbacks since they involve long and complex steps to produce the desired product. Moreover, many chemical reagents seem to be necessary making the process more costly. However, low-cost fabrication of well-defined Ag 3 PO 4 with superior catalytic properties via a simple process remains a great challenge. On the other hand, natural phosphates are an important natural resource in Morocco, which needs to be valorised. They can be employed not only as fertilizers but also, they have been exploited effectively as catalysts in a wide range of organic transformation [19][20][21]. In continuation of our ongoing program to develop an interesting catalyst at low-cost [22][23][24][25][26][27], we describe in this paper, a novel chemical wet method-based dissolution-precipitation reactions using Moroccan natural phosphate (NP) as phosphorus precursor to synthesize of the nanostructured Ag 3 PO 4 as catalyst for the reduction of 4-NP to 4-AP in the presence of sodium borohydride using aqueous phase and its antimicrobial activity against Escherichia coli and Staphylococcus aureus bacteria. To the best of our knowledge, no studies have been performed on the development of Ag 3 PO 4 from natural phosphate and testing its catalytic activity for the reduction reaction in aqueous solution, and its antibacterial activity.

The preparation of nanostructured Ag 3 PO 4 catalyst
The Ag 3 PO 4 was prepared by a dissolution/precipitation method from a natural phosphate rock coming from the Khouribga region (Morocco). To use this material prior requires initial treatments such as crushing and washing. The fraction of 200-400 μm grain size was washed with distilled water several times to remove the soluble matter. The different elemental constituents of this mineral are given in Table 1. Then, the dissolution process was carried out in a round bottom flask of 500 ml capacity at a rate of 200 rpm. Firstly, about thirty grams of natural phosphate was dissolved in deionized water acidified by HNO 3 acid (65%) to pH 2, under continuous stirring at room temperature. We obtained H 3 PO 4 and Ca 2+ ions as well as the insoluble matter after solid/liquid separation process by centrifugation. Next, the Ag 3 PO 4 catalyst was prepared by a simple precipitation method. In a typical synthesis, 1.5 g of AgNO 3 (8.83 mmol) was dissolved in 80 ml deionized water over 10 min, then the ammonia hydroxide solution 28% (0.1 M) was added with drop by drop under magnetic stirring to above mixture to form a transparent solution. And then, phosphorus precursor prepared previously from natural phosphate were added gradually to the mixture reaction, the resulting precipitate was magnetically stirred at room temperature for one hour. After that, a yellow precipitate of silver phosphate Ag 3 PO 4 is then obtained by centrifugation, which is then washed several times with deionized water to release any unreacted species such as Ca 2+ and NO 3− ions. Finally, the obtained powder Ag 3 PO 4 was dried in desiccator at 80 °C overnight. The schematic illustration of preparation process of nanostructured Ag 3 PO 4 from natural phosphate is exhibited step by step in Scheme 1.

Characterization of the catalyst
X-ray diffraction (XRD) patterns were obtained at room temperature on a Bruker AXS D-8 diffractometer using Cu-Kα radiation in Bragg-Brentano geometry (θ-2θ). Fourier transform infrared (FT-IR) was performed on an ABB Bomem FTLA 2000 spectrometer equipped with a Golden Gate single reflection ATR accessory. The Raman spectra were recorded in the range from 400 to 1400 cm −1 with a Thermo Scientific DXR2 spectrometer. Scanning electron microscopy images were recorded on Specific surface area was determined from the nitrogen adsorption/desorption isotherms (at-196 °C) and measured with a Quantachrome Autosorb-1 automatic analyzer using the BET equation.

Catalytic reduction of 4-nitrophenol to 4-aminophenol
The catalytic activities were evaluated by reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in a quartz cuvette. In a typical procedure, 100 µL of 4-NP (1 mM) and 1 mL NaBH 4 (0.1 M) were placed in a quartz cuvette containing 3 mL of deionised water. After that 5 mg of Ag 3 PO 4 was added into the cuvette to start the reduction reaction. The process of the conversion of 4-NP to 4-AP was followed by UV-Vis spectroscopy at a maximum wavelength of 400 nm.

Antibacterial activity of nanostructured Ag 3 PO 4 powder
The antibacterial activity of the nanostructured Ag 3 PO 4 was studied on Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) by the standard disk diffusion assay on Muller-Hinton agar medium. All disks and materials were sterilized in an autoclave at 120 °C for 20 min before experiments. The disk diffusion assay was performed by placing a 6 mm disk saturated with 10 µL of Ag 3 PO 4 aqueous dispersions (1000-125 µg/ mL) onto an agar plate seeded with E. coli or S. aureus. After 24 h of incubation at 37 °C, the diameters of the inhibition zones were measured.
Scheme 1 Schematic diagram of the preparation of nanostructured Ag 3 PO 4 by dissolution-precipitation process from natural phosphate

Characterization of nanostructured Ag 3 PO 4
The phase structures of the as-prepared nanostructured Ag 3 PO 4 were investigated by XRD and showed in Fig. 1. X-ray diffraction pattern of nanostructured Ag 3 PO 4 clearly showed that all diffraction peaks are a near-systematic superposition of those of standard structure of Ag 3 PO 4 (JCPDS file N o 010-1988). No diffraction peaks for other phases such as calcium phosphate are detected, this indicates that Ag + cations have the best chemical affinity towards PO 4 3− anions compared to that of Ca 2+ ions originating from natural phosphate in which they have been cleaned from the solid by washing treatment. The obtained Ag 3 PO 4 crystalizes in the cubic system with the space group P-43n (No.218) with a unit cell length of 6.002 Å. According to the Debye-Scherrer equation, the average crystallite size of the synthesized Ag 3 PO 4 is about 39 nm. Furthermore, the atomic positions and the lattice parameters collected from Rietveld refinements were used as input data in structural analysis software to model the cubic Ag 3 PO 4 structure as shown in Fig. 2 Figure 3 showed the Fourier transform infrared (FT-IR) spectrum of the Ag 3 PO 4 sample, exhibiting two strong bands in the 1072-732 cm −1 and 586-447 cm −1 ranges related to the molecular vibrations of PO 4 groups in Ag 3 PO 4 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 cantered at 547 cm −1 is related to deformation from the bending mode of the P-O-P bonds.
Furthermore, the Raman spectroscopy was also employed to confirm the FTIR obtained resultants and phase purity of the as-prepared powder. As shown in Fig. 4, the band located at 909 cm −1 is attributed to symmetric stretching vibrations of [PO 4 ] clusters, while the asymmetric stretching vibrations of this cluster were verified at 1040 cm −1 . The bending vibration modes related to [PO 4 ] clusters were found at 428 and 547 cm −1 . The band centered at 716 cm −1 is ascribed to symmetric stretching vibrations of P-O-P bonds. These obtained results are in good agreement with those reported in the literature [28].  In order to support the afore mentioned interpretation 31 PMAS-NMR studies were also investigated. As shown in Fig. 5, the presence of one single crystallographic site of phosphorus at a chemical shift of δ = 28.59 ppm, proves the existence of only one type of phosphorus site in the Ag 3 PO 4 material. Thus, these results are in good accordance with the previously data obtained by XRD analysis and confirm the presence of pure phase of obtained nanostructured silver phosphate [29].
To better elucidate the morphological properties of the prepared Ag 3 PO 4 from natural phosphate, SEM analysis was carried out. In the lower magnification images, the Fig. 6a indicates that the surface of nanostructured Ag 3 PO 4 is formed by a large amount of quasi-spheroid particles having hexagonal and cubic structures, while the higher magnification image, as shown in Fig. 6b, clearly reveals the quasi-spheroid particles through nonuniform diameter polyhedrons. On the other hand, to confirm the chemical composition of the nanostructured Ag 3 PO4, a semi-quantitative elemental analysis was performed, and its EDS spectrum showed in Fig. 6c. The obtained results revealed the presence of O, P and Ag elements without any calcium traces indicating that the Ag 3 PO 4 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.
In order to provide more details about the morphological properties regarding the obtained nanostructured Ag 3 PO 4 , the scanning-transmission electron microscope (STEM) was also used (Fig. 7). The STEM data acquired showed that the Ag 3 PO 4 particles were clustered and formed heterogeneous aggregates of particles that were different in size and irregularly formed.
To further demonstrate the porous structure of nanostructured Ag 3 PO 4 , the specific surface area (S BET ) of the Ag 3 PO 4 powder was calculated from N 2 -sorption measurements and application of the BET method. As shown in Fig. 8a, 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 m 2 /g and average pore size D p calculated from BJH (Barrett-Joyner-Halenda) method was 3.1 nm and 7.3 nm (Fig. 8b). Comparing with the low values given in the literature, a relatively large porous surface of Ag 3 PO 4 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.

Catalytic Reduction of 4-NP to 4-AP
To investigate the catalytic activity of the Ag 3 PO 4 as nanostructured catalyst, the reduction of 4-nitrophenol to its corresponding amino derivatives, 4-aminophenol, in the presence of NaBH 4 in aqueous media was chosen Firstly, the ability of NaBH 4 to reduce 4-NP in absence of our catalyst was examined. As shown in Fig. 9A, the 4-NP in an aqueous solution has a maximum absorption at 317. After added NaBH 4 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 [30]. 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. 9B). Then, when a small amount of Ag 3 PO 4 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. 9C), the presence of an isobestic point at 317 nm indicating that the catalytic reduction of 4-nitrophenol gives 4-aminophenol only without by product [31,32].
To understand the catalytic conversion kinetic of 4-NP, the pseudo-first-order model was used: ln(C t /C 0 ) = ln(A t /A 0 ) = -k app t, where C 0 and C t are the 4-NP concentrations at t = 0 and t = t, respectively and k app 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 NaBH 4 molar concentration on the catalytic efficiency of the reduction of 4-NP into 4-AP was studied. Figure 10a shows the ln(A t /A 0 ) plot as a function of time at different concentration of reducing agent (0.01; 0.05; 0.1 and 0.5 M) 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 NaBH 4 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 BH 4 − ions on the catalyst surface by accelerating the reduction of the diffused 4-NP [33,34]. Note that no significant change in apparent rate constant above 0.1 M of [NaBH 4 ] was observed; thus, an optimum concentration of 0.1 M [NaBH 4 ] was chosen for the future experiments. In addition, the effect of amount of the Ag 3 PO 4 nanostructured on catalytic efficiency was also studied using 0.1 M of NaBH 4 at room temperature. We should mentioned that the reaction was started after adding of Ag 3 PO 4 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. 10b, 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 (Scheme 3). As published elsewhere [35][36][37], the hydrogen atom of BH 4 − 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 -NO 2 is also negatively charged due to its greater electronegativity than carbon from the benzene ring, and the H atoms of the positively charged H 2 O 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 BH 4 − clusters to 4-NP via the Ag 3 PO 4 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. 11, the catalyst was recycled several times with a little loss in catalytic

Antibacterial activity studies
The production of a large quantity of Ag 3 PO 4 through a simple and economical method from natural phosphate can be employed as an antibacterial agent suitable for the biological treatment of wastewater. In this optic, the obtained results of antibacterial activity of Ag 3 PO 4 against E. Coli (Gram-negative) and S. Aureus (Gram-positive) are shown in Fig. 12. The zone of inhibition clearly indicated the significant antibacterial Scheme 3 Mechanism for the reduction of 4-NP by the NaBH4 with nanostructured Ag3PO4 catalyst  Table 2. With 1 mg/mL as the serial concentration of Ag 3 PO 4 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 a result, the nanostructured powder of Ag 3 PO 4 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.
According to some research reports [38][39][40][41], the plausible antibacterial mechanism of the nanostructured Ag 3 PO 4 prepared from natural phosphate was proposed as follows (Fig. 13): (i) Initially, due to electrostatic attraction and affinity to sulfur proteins, our proper material can adhere to the cell wall and cytoplasmic membrane. (ii) The adhered material can enhance the permeability of the cytoplasmic membrane and lead to disruption of the bacterial envelope. (iii) After the destruction of membrane system, a large number of Ag 3 PO 4 , can enter into the cells. (iv) The final stage involves the interference via Ag 3 PO 4 directly binding with proteins, lipids, enzymes, DNA and the oxidation of them by generating reactive oxygen species (ROS). As Known, the generation of ROS and oxidative stress are more significant mechanism owing to the strong destructive power. Subsequently, the integrity of the cell wall/membrane was disrupted, and the intracellular contents leaked out which consequently results in cell death.

Conclusion
This study develops a novel approach for the synthesis of the single phase of the nanostructured Ag 3 PO 4 from natural phosphate as phosphate source via dissolution-precipitation process. The as-prepared sample was successfully characterized by various physicochemical techniques in order to study its structural, textural and morphological properties. Then, the nanostructured catalyst exhibited higher catalytic activity towards the reduction of 4-nitrophenol to 4-aminophenol using NaBH 4 as reducing agent in aqueous solution. In addition, the prepared Ag 3 PO 4 catalyst possesses significant antibacterial activities against E. Coli and S. Aureus bacteria. The operational simplicity, short reaction times, recyclability and antibacterial activity are the outstanding features of the present study.