Effective photoreduction of a nitroaromatic environmental endocrine disruptor by AgNPs functionalized on nanocrystalline TiO₂

Abstract

Unprecedented photoactivity of silver nanoparticles photodeposited on nanocrystalline TiO₂ for the efficient reduction of 4-nitrophenol at room temperature is reported. The use of Na₂SO₃ as a harmless scavenger agent for the reduction of a nitroaromatic endocrine disruptor yields a valuable 4-aminophenol reagent.

---

Nowadays, many organic pollutants have been targeted as endocrine disrupting chemicals (EDCs). Natural and synthetic hormones, plastics, detergents, pharmaceuticals, pesticides, and industrial chemicals affect the endocrine system of humans and wildlife.¹ Numerous individual biological processes including reproduction, brain function, and the immune system are disrupted by these pollutants.² Phenolic and nitroaromatic compounds, used in industrial and agricultural wastewaters, have exhibited both estrogenic and anti-androgenic activities on humans, animals and plants, even at very low concentrations.^3,4 4-Nitrophenol (4-NP) is a highly harmful compound and has been quoted as a potential endocrine activity disruptor in immature male rats.⁵ 4-NP is present commonly in diesel exhaust and it is also found in industrial and hospital wastewaters since it is a precursor not only of pesticides, but also of certain pharmaceuticals, plasticizers and azo-dyes.^6,7 The permitted level of 4-NP in water is, in general, 30 μg L⁻¹ and its photolysis in aqueous solutions is a very slow and inefficient process.⁷ Currently, the use of hydroxyl radicals (UV-C/H₂O₂) has been of interest in the degradation of 4-NP because of the good photo-oxidation rates,⁸ nevertheless, nanometric sol–gel crystallites of TiO₂ catalysts are more efficient^8,9 than other crystalline TiO₂ catalysts. The use of metallic nanoparticles supported on these metal oxides has enhanced the oxidation or reduction of 4-NP, but partial oxidation of 4-NP and by-products still remain.^9–11 Noble metals can carry out the reduction of 4-NP to 4-aminophenol (4-AP), increasing the reduction rate and selectivity to the valuable 4-AP by the addition of reducing agents, which helps remarkably the reduction reaction. It is generally agreed that the use of the typical sodium borohydride (NaBH₄) or hydrazine (NH₂NH₂) as strong reducing agents can accelerate the 4-NP reduction, but their toxic and dangerous nature compromise their role.^12–16 On the other hand, TiO₂ without noble metals can hardly reduce 4-NP in the presence of these powerful reducing agents. If TiO₂ is modified with polyamine and UV-visible light irradiation is used, the reduction of 4-NP is possible.¹⁷ In this communication, it is demonstrated for the first time that the complete reduction of the endocrine-disruptor 4-NP to 4-AP is possible by using a soft reducing agent such as sodium sulfite (Na₂SO₃) as sacrificing agent at room temperature by means of a photocatalytic process with silver nanoparticles photo-functionalized on the surface of a nanocrystalline TiO₂.

The TiO₂ substrate was synthesized by a previously reported sol–gel technique.⁹ In brief, titanium isopropoxide, n-butanol and an urea/butanol/water solution were added simultaneously dropwise to a 4-neck round bottom flask under continuous stirring. The reactant solution was aged for 72 h at 80 °C. The solvents were removed with a vacuum evaporator at 80 °C, dried overnight under vacuum at 100 °C, and finally annealed at 450 °C for 4 h. Furthermore, practical UV functionalization with 1.5 wt% of AgNPs on sol–gel TiO₂ and commercial TiO₂–P25 surfaces was carried out using a silver nitrate ethanol solution. Either, the sol–gel TiO₂ or commercial TiO₂–P25 was added to the solution and the resultant slurry was irradiated with a UV-lamp (TecnoLite G15T8, 214 nm, 17 W) for 60 min. Under vigorous stirring and finally dried overnight at 80 °C. The photocatalysts were labeled as Ag/TSG and Ag/TP25, respectively. To understand their behaviour in the photoreduction reaction and correlate it with the reducing properties of the materials, structural and optical characterizations were carried out by means of XRD, UV-Vis spectroscopy and TEM microscopy in HRTEM and STEM-HAADF modes. A possible photoreduction mechanism of the 4-NP endocrine disruptor was proposed.

The diffuse reflectance UV-Vis spectra of all the samples (Fig. 1-a) show an absorption peak at 320 nm, which is characteristic of TiO₂ semiconductors.¹⁸ The band-gap energy (E_g) value ranged from 3.05 to 3.28 eV. It has been reported that when the TiO₂ framework is functionalized with AgNPs by different methods, the E_g values are red-shifted, depending on the size and dispersion of particles. In this case, the E_g energies are practically unchanged due to the low silver loading (<2 wt%), however, the dispersion of the AgNPs can be estimated by considering the Surface Plasmon Resonance (SPR), which is correlated with the AgNPs size.¹⁹ Thus, in the Ag/TP25 catalyst, the absorption band centered at 585 nm is attributed to the SPR of AgNPs due to closer AgNPs, but it is detected scarcely in the Ag/TSG catalyst. The sol–gel TiO₂ catalyst has a surface area of 116 m² g⁻¹, while the commercial TiO₂–P25 presents normally an area of 56 m² g⁻¹; therefore, two times more surface area with high dispersion of AgNPs is expected in the Ag/TSG catalyst. Fig. S1 (see ESI†) shows the selected High-Angle Annular Dark-Field (HAADF) images for the sol–gel TiO₂ catalyst obtained in STEM mode in order to identify more clearly the size of the AgNPs. By contrast, in the Ag/TSG catalysts, it is more difficult to identify the AgNPs due to their high dispersion and very small size. The EDX analysis confirmed the presence of silver on the TiO₂ surface. The XRD patterns of both silver functionalized TiO₂ catalysts are shown in Fig. 1-b, where anatase is the main TiO₂ crystalline phase. The peak corresponding to the anatase reflection (25.41 of 2 theta; JCPDF 070-6026) is slightly broad for the Ag/TSG catalyst, showing a smaller crystallite size. In both materials, the AgNPs are no observed due to the high dispersion and nanometric size ≤ 5 nm as it is observed in the HAADF images (see, ESI†). Histograms of the average particle sizes in both samples are presented. For Ag/TSG, the average particle size is 3 nm, while for the Ag/TP25 catalyst the average particle size is 9 nm, which explains the presence of SPR of AgNPs on the P25 surface. Fig. S3† shows the interplanar spacing for TiO₂, ∼0.34 nm (101 plane of anatase), and for Ag, ∼0.21 nm (111 plane 15-3325) and a HAADF image that confirms the presence of AgNPs.

Fig. 1 UV-Vis diffuse reflectance spectra and XRD patterns of: (a) TSG, (b) Ag/TSG, (c) Ag/TP25 photocatalysts.

The photocatalytic reduction test was performed in a home-made, glass reactor system containing 200 mL of an aqueous solution with 7.5 ppm of 4 NP, 8 mM of Na₂SO₃ and 0.025–0.2 g L⁻¹ of the photocatalyst loading, under previously established conditions.²⁰ The suspension was maintained under magnetic stirring (600 rpm) at room temperature and the system was irradiated with 254 nm UV light (UVP-Pen-ray, 4400 μW cm⁻²). The determination of the concentration of the 4-NP endocrine disruptor was made by UV-Vis spectroscopy using a Varian Cary 5000 spectrometer by following the disappearance of the absorption band at 400 nm for the 4-NPhenolate (4-NPate) ions in alkaline media by the presence of Na₂SO₃. When the EDC 4-NPate solution was irradiated with UV light in the presence of the TSG catalyst without noble metals, the absorption band at 400 nm, corresponding to the 4-NPate, got reduced within 3 h and a new absorption band appeared at 295 nm, characteristic of the 4-AP formation, (Fig. 2), suggesting that the EDC 4-NPate was successfully photoreduced.^17,20 A slight decrease in the 4-NPate absorption band was detected for the photocatalytic reaction in the absence of any catalyst (photolysis), suggesting that a certain reduction degree of the EDC 4-NPate took place.

Fig. 2 UV-Vis spectra for the photoreduction of 4-NPate with the TSG photocatalyst under UV light irradiation with 8 mM of Na₂SO₃.

A good linear correlation of the C/C₀ plot versus time for the 4-NPate reductions at low photolysis conversion (<50%) for the TSG and Ag/TSG catalysts (Fig. 3-A) is the proof for a zero order rate kinetics, which indicates that the reduction rate is independent of the EDC 4-NPate concentrations.²¹ The 4-NPate photoreduction with Ag/TSG was completed in ∼1.5 h. The photoreduction rate in the absence of any photocatalyst (photolysis) was very low (k = 0.15 M h⁻¹), but when the TSG catalyst was added, the photoreduction rate was increased as the photocatalyst load was also increased (Fig. 3-B), obtaining a rate constant value of 0.57 M h⁻¹ (∼4 times higher than the one obtained with photolysis) at 0.2 g L⁻¹ of photocatalyst load. In the same way, when the TSG photocatalyst was functionalized with AgNPs (1.5 wt%), the photoreduction rate was also increased as the photocatalyst load was increased, obtaining a rate constant value of 2.7 M h⁻¹ at 0.15 g L⁻¹ of photocatalyst load, which is ∼5 times higher than that for the TSG photocatalyst and ∼18 times higher than that for photolysis.

Fig. 3 (a) Plot of C/C₀ versus time for the photoreduction of the 4-NPate: without, with TSG and Ag/TSG catalyst. (b) Apparent zero order rate constant for the photoreduction of 4-NPate with TSG and Ag/TSG as a function of photocatalyst load.

By comparing the photocatalytic reduction rate of EDC 4-NPate, using commercial TiO₂–P25 (TP25) as reference (Fig. 4), the photoreduction rate of the TSG photocatalyst (0.4 M h⁻¹) was slightly higher than that of TP25 (0.26 M h⁻¹). But, when TP25 was functionalized with AgNPs, the photoreduction rate was increased 12 times until reaching 3.2 M h⁻¹, suggesting that the AgNPs deposited on TiO₂–P25 are the best. These results suggest that the AgNPs deposited on both TiO₂ surfaces enhanced remarkably the electron transfer process from the semiconducting surface to the acceptor reactant (EDC 4-NPate), achieving a high photocatalytic reduction rate. In order to show the unprecedented photoactivity of the AgNPs on TiO₂, an evaluation of substrates with AgNPs functionalization like Ag/ZnS(en)_0.5 hybrid and Ag–Al₂O₃ was carried out confirming the superior photoactivity of the Ag–TiO₂ nanocrystalline composite (see Fig. S4 in ESI†).

Fig. 4 Comparison of the pseudo zero order rate constant value for the photoreduction of the 4-NPate over the TiO₂ photocatalyst and functionalized TiO₂ with AgNPs with 0.15 g L⁻¹ of photocatalyst load.

The photocatalytic mechanism for the reduction of the endocrine-disruptor 4-NPate by using TiO₂ without noble metal follows a similar-previously-reported mechanism,^9,17 where the photogenerated holes on the TiO₂ surface react with the sulfite ions (SO₃²⁻) causing their oxidation until the sulphate is formed, while the photogenerated electrons are transferred to the acceptor molecule (4-NPate) from the nitro group to form the nitrophenolate radical and, as consequence, to form 4-hydroxylaminophenol and the transform it into aminophenol.²² Considering that the redox potential for couples (4-NP/4-AP) and (SO^*₃/SO₃²⁻) in aqueous solution is −0.76 V (ref. 23) and 0.63 V (NHE),²⁴ respectively, in the presence of SO₃²⁻ ions at pH = 9.5, the position of the conduction band of the TiO₂ semiconductor should be favourably negative (<−0.8 V NHE) (Fig. 5). This electron transfer is fast when the AgNPs are deposited onto the TiO₂ surface because the AgNPs act as electron captors and help to transfer electrons from the semiconducting surface to the acceptor molecule through all the AgNPs. The electronic transfer contributes to the stability of the AgNPs during the UV irradiation.

Fig. 5 Schematic representation of the photoreduction of 4-NP in the presence of Na₂SO₃, with AgNPs–TiO₂ photocatalysts.

For recyclability studies the powdered systems need to be immobilised because recuperation from aqueous media implies the loss of the photocatalyst. Further studies will be developed in order to immobilize the material as a coat of foams as we reported previously for the inactivation of marine algae²⁵ and dye solar discoloration.²⁶

In summary, either sol–gel TiO₂ or commercial TiO₂–P25 were tested on the reduction of the endocrine-disruptor 4-NPate to 4-AP in the presence of Na₂SO₃ as a soft reducing agent by means of a photocatalytic process. The AgNPs photodeposited over sol–gel TiO₂ and P25 photocatalysts enhanced around 7 and 12 times the photocatalytic process, respectively. The AgNPs act not only as electron captors, but also as electron transfer agents that enhance the 4-AP yield. AgNPs functionalized over Ag/ZnS(en)_0.5 hybrid or Al₂O₃ substrates did not enhance the 4-NPate photoreduction confirming the unique activity of Ag–TiO₂ materials. For the first time, a practical functionalization of TiO₂ with 3 nm AgNPs has shown potential reduction photoactivity using Na₂SO₃ as a harmless reducing agent, yielding a valuable 4-aminophenol reagent for the analgesic industry. The possible applications of the UV-driven harmless photocatalyst system may be extended to the reduction of other nitro-aromatic compounds to amino-aromatics.

Acknowledgements

The use of the infrastructure of NANOTECH-CIMAV and LINAN are gratefully acknowledged. We thank M. B. A. Rivera-Escoto for her technical assistance. We thank SEP CONACYT-CB-2011/169597 “Desarrollo de Materiales Semiconductores Nanoestructurados para la degradación de contaminantes Disruptores Endócrinos recalcitrantes” for the financial support.

Notes and references

1. C. M. Markey, B. S. Rubin, A. M. Soto and C. Sonnenschein, J. Steroid Biochem., 2002, 83, 235.
2. L. N. Vandenberg, T. Colborn, T. B. Hayes, J. J. Heindel, D. R. Jacobs Jr, D.-H. Lee, T. Shioda, A. M. Soto, F. S. vom Saal, W. V. Welshons, R. T. Zoeller and J. Peterson Myers, Endocr. Rev., 2012, 33, 2011.
3. C. Li, S. Taneda, A. K. Suzuki, C. Furuta, G. Watanabe and K. Taya, Toxicol. Appl. Pharmacol., 2006, 217, 1.
4. J. Lea and A. Adesina, J. Chem. Technol. Biotechnol., 2001, 76, 803.
5. S. Taneda, Y. Mori, K. Kamata, H. Hayashi, C. Furuta, C. Li, K. Seki, A. Sakushima, S. Yoshino, K. Yamaki, G. Watanabe, K. Taya and A. K. Suzuki, Biol. Pharm. Bull., 2004, 27, 835.
6. C. Mei Li, S. Taneda, A. K. Suzuki, C. Furuta, G. Watanabe and K. Taya, Toxicol. Appl. Pharmacol., 2006, 217, 1.
7. G. S. Garbellini, G. R. Salazar-Banda and L. A. Avaca, J. Braz. Chem. Soc., 2007, 18, 1095.
8. A. Di Paola, V. Augugliaro, L. Palmisano, G. Pantaleo and E. Savinov, J. Photochem. Photobiol., A, 2003, 155, 207.
9. A. Hernandez-Gordillo, M. Arroyo, R. Zanella and V. Rodríguez-González, J. Hazard. Mater., 2014, 268, 84.
10. H. Ilyas, I. A. Qazi, W. Asgar, M. Ali Awan and J. Zahir-ud-din Khan, J. Nanomater., 2011, 589185.
11. B. Divband, M. Khatamian, G. R. Kazemi Eslamian and M. Darbandi, Appl. Surf. Sci., 2013, 284, 80.
12. J. Li, C.-Y. Liu and Y. Liu, J. Mater. Chem., 2012, 22, 8426.
13. R. Prucek, L. Kvítek, A. Panacek, L. Vancurova, J. Soukupova, D. Jancıík and R. Zboril, J. Mater. Chem., 2009, 19, 8463.
14. M. J. Fernández-Merino, L. Guardia, J. I. Paredes, S. Villar-Rodil, A. Martínez-Alonso and J. M. D. Tascón, RSC Adv., 2013, 3, 18323.
15. I. H. Abd El Maksod and T. S. Saleh, Green Chem. Lett. Rev., 2010, 3, 127.
16. T. M. Jyothi, R. Rajagopal, K. Sreekumar, M. B. Talawar, S. Sugunan and B. S. Rao, J. Chem. Res., Synop., 1999, 11, 674.
17. K. Urkude, S. R. Thakare and S. Gawande, J. Environ. Chem. Eng, 2014, 2, 759.
18. Y. Shiraishi, Y. Togawa, D. Tsukamoto, S. Tanaka and T. Hirai, ACS Catal., 2012, 2, 2475.
19. Y. Gong, Y. Zhou, L. He, B. Xie, F. Song, M. Han and G. Wang, Eur. Phys. J. D, 2013, 67, 87.
20. A. Hernandez-Gordillo, A. G. Romero, F. Tzompantzi and R. Gómez, Appl. Catal., B, 2014, 144, 507.
21. A. Leelavathi, T. Udaya Bhaskara Rao and T. Pradeep, Nanoscale Res. Lett., 2011, 6, 123.
22. F. Mahdavi, T. C. Bruton and Y. Li, J. Org. Chem., 1993, 58, 744.
23. P. Deng, Z. Xu, Y. Feng and J. Li, Sens. Actuators, B, 2012, 168, 381.
24. P. Neta and R. E. Huie, Environ. Health Perspect., 1985, 64, 209.
25. V. Rodríguez-González, S. Obregón Alfaro, L. M. Torres-Martínez, S.-H. Cho and S.-W. Lee, Appl. Catal., B, 2010, 98, 229.
26. S. Obregón-Alfaro, V. Rodríguez-González, A. A. Zaldívar-Cadena and S. W. Lee, Catal. Today, 2011, 166, 166.

---

Footnote

† Electronic supplementary information (ESI) available: HAADF and HRTEM images and histograms for Ag/TP25 and Ag/TSG; also, comparison of reduction of 4-NP with AgNPs over ZnS and Al₂O₃ substrates and HAADF images of Ag/ZnS. See DOI: 10.1039/c5ra00094g

---