Elsevier

Chemical Engineering Journal

Volume 346, 15 August 2018, Pages 567-577
Chemical Engineering Journal

Heterogeneous photo-Fenton degradation of bisphenol A over Ag/AgCl/ferrihydrite catalysts under visible light

https://doi.org/10.1016/j.cej.2018.04.073Get rights and content

Highlights

  • Novel Ag/AgCl/Fh photo-Fenton catalysts were synthesized.

  • Plasmonic Ag/AgCl particles can significantly accelerate the redox cycling of Fe2+/Fe3+.

  • The degradation rate constant of BPA over 6%Ag/AgCl/Fh is nearly about 5.1 times as high as that of pure Fh.

  • Ag/AgCl/Fh exhibit very high photo-Fenton catalytic activity even at pH 6.

  • Mechanisms for enhanced photo-Fenton catalytic activity of Ag/AgCl/Fh were investigated.

Abstract

The conventional heterogeneous Fenton reaction is often confined by the lower regeneration of Fe2+, which then inhibits the decomposition of H2O2 and the generation of radical dotOH. Here we propose a novel idea to significantly accelerate the redox cycling of Fe2+/Fe3+ in heterogeneous Fenton reaction by introducing photo-generated electrons from plasmonic particles. Towards this aim, novel plasmonic Ag/AgCl nanoparticles coated ferrihydrite (Ag/AgCl/Fh) have been synthesized. Compared with pure Fh, a remarkable enhancement in the photo-Fenton degradation towards bisphenol (BPA) can be observed for all Ag/AgCl/Fh samples under visible light. Noticeably, the rate constant of 6%Ag/AgCl/Fh is 0.0506 min−1, which is about 5.1 times as high as that of pure Fh (0.0099 min−1). Moreover, the catalytic activity of 6%Ag/AgCl/Fh on the degradation of BPA remains quite efficient with a low Fe leaching after 4 recycles. The results of the effect of initial pH indicates that Ag/AgCl/Fh samples exhibit relatively high photo-Fenton catalytic activity even at pH 6. The electron spin resonance (ESR) analysis reveals that radical dotOH plays a significant role in the photo-Fenton reaction. The concentration of radical dotOH can even reach 267.6 μmol/L after 60 min, which is much higher than that of pure Fh (69.2 μmol/L). The measurement of Fe2+ concentration and the XPS Fe2p spectra of Ag/AgCl/Fh before and after the degradation of BPA indicate the loading of Ag/AgCl can accelerate the conversion of Fe3+/Fe2+ by the photo generated electrons from Ag nanoparticles due to the surface plasmon resonance (SPR) effect under the visible light irradiation.

Introduction

In recent years, homogeneous Fenton reactions have been applied in the field of environmental remediation due to a large amount of hydroxyl free radical (radical dotOH) generated in the reactions [1], [2], [3], [4], [5], [6], [7], [8]. The oxidation ability of radical dotOH (φo = 2.73 V) is much stronger than most other active species, such as H2O2 (φo = 1.78 V) and ozone (φo = 2.08 V), which makes radical dotOH powerful to degrade most organic contaminants to the mineral end products in a non-selective way [9], [10]. Conventional homogeneous Fenton process occurs in an acidic Fe2+/H2O2 aqueous system [2], [11], [12], [13]. However, its applications are limited mainly due to the following drawbacks: (i) low regeneration rate of Fe2+, (ii) tight range of optimum pH (2.5–3.5), and (iii) excessive amounts of generated ferric hydroxide sludge [14], [15].

In order to solve above problems, heterogeneous Fenton-like reactions have been developed which show great potential to replace the conventional homogeneous Fenton reactions [10], [16], [17]. Recently, a great deal of attention has been paid to the heterogeneous Fenton-like catalysts such as Fe3O4 [10], [18], α-Fe2O3 (Liu et al., 2017), γ-Fe2O3 [19], α-FeOOH [20], β-FeOOH [21], and ZnFe2O4 [22]. Among them, ferrihydrite (Fh) nanoparticles have attracted much research interests recently as it is a naturally occurring Fe (III) hydroxide nanomineral with a large specific surface area (SSA > 200 m2/g) [23], [24], [25], [26]. Nevertheless, the catalytic efficiency of the Fh nanoparticles awaits further improvement due to the relatively low regeneration rate of Fe2+ (by reducing Fe3+ to Fe2+) during the Fenton reaction.

For the purpose of improving the regeneration rate of Fe2+, recently many studies have focused on introducing semiconductors to the photo-Fenton system, such as BiVO4@Fh [23], Fe3O4@rGO@TiO2 [10], SiO2/Fe3O4/C@TiO2 [9], and BiOIO3/Fe2O3 [27]. The photogenerated electrons from these semiconductors accelerate the conversion of Fe3+/Fe2+, which then promote the decomposition of H2O2 into radical dotOH. These studies enlighten us that introducing in-situ generated electrons to the heterogeneous Fenton catalysts might be an effective strategy for enhancing their photo-Fenton reactivity.

In recent years, the Ag/AgX (Cl, Br, I) catalysts have become the focus of research in the field of photocatalysis because of the surface plasmon resonance (SPR) effect under visible light [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39]. These photocatalysts can nearly absorb entire visible light and generate electrons and holes through the collective oscillations of the surface electrons [40]. However, pure Ag/AgCl composites usually aggregate to micrometer-scale particles size, resulting in low surface areas and high recombination rate of the photo generated charge carriers [41]. In this regard, we propose that combining Ag/AgCl with Fh may achieve exciting novel composites with high photo-Fenton reactivity, as the generated electrons from Ag/AgCl can help in reducing Fe3+ on Fh, and Fh may have a tailoring effect on Ag/AgCl to inhibit its aggregation (because of the large surface areas and plenty of reactive surface sites of Fh).

In this study, we have synthesized Ag/AgCl/Fh heterogeneous photocatalysts via a multistep route by loading the different content of Ag/AgCl on the surface of Fh. The structural and photoelectrochemical characteristics of Ag/AgCl/Fh samples have been studied. The obtained Ag/AgCl/Fh catalysts exhibited remarkably enhanced photo-Fenton catalytic activity for the degradation of BPA under visible light. It may result from the SPR effect of Ag nanoparticles and the fast carrier transfer between the active materials in the composites.

Section snippets

Materials

Fe (NO3)3·9H2O (AR), AgNO3 (AR), NaCl (AR), NaOH (AR), HCl (AR), and hydrogen peroxide (30 wt%) were obtained from Shanghai Chemical Reagent Corporation, China. Bisphenol A was purchased from Macklin Reagent Company. Benzoic acid (99.5%) and p-hydroxybenzoic acid (99%) were obtained from Aladdin Industrial Corporation (Shanghai, China). All labware were cleaned by soaking overnight in dilute HCl solution and washed in ultra-pure water (>18 MΩ/cm) before experiments.

Synthesis of Fh

Two-line Fh was synthesized

Structural characterization

The XRD patterns of Ag/AgCl, Fh, and Ag/AgCl/Fh were investigated (Fig. 1). AgCl shows four distinct reflections at 27.8°, 32.2°, 46.2°, and 54.8°, which can be ascribed to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) diffraction planes, respectively, well consistent with the crystalline phase of cubic AgCl (JCPDS 31-1238). The reflection at 38.1° (1 1 1) can be assigned to the reduced Ag nanoparticles (JCPDS 04-0783). The patterns of all Ag/AgCl/Fh samples show two broad reflections at 35° and

Conclusion

Plasmonic Ag/AgCl nanoparticles coated Fh with high photo-Fenton catalytic activity were successfully synthesized via a deposition–precipitation-photo reduction method. The photo-Fenton catalytic activities of Ag/AgCl/Fh composites for the degradation towards BPA are improved evidently compared with pure Fh. The concentration of dominant radicals (radical dotOH) can even reach to 267.6 μmol/L after 60 min, which is much higher than that of pure Fh (69.2 μmol/L). The loading of Ag/AgCl can accelerate the

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (41572031), National Program for Support of Top-notch Young Professionals, Guangdong Provincial Program for Support of Top-notch Young Professionals (2014TQ01Z249), and China Scholarship Council.

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