Heterogeneous photo-Fenton degradation of bisphenol A over Ag/AgCl/ferrihydrite catalysts under visible light
Graphical abstract
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 (OH) generated in the reactions [1], [2], [3], [4], [5], [6], [7], [8]. The oxidation ability of OH (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 OH 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 OH. 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 (OH) 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.
References (72)
- et al.
Homogeneous oxidation of aqueous solutions of atrazine and fenitrothion through dark and photo-Fenton reactions
Chemosphere
(2009) - et al.
Degradation of nitrobenzene at near neutral pH using Fe2+–glutamate complex as a homogeneous Fenton catalyst
Appl. Catal., B
(2010) - et al.
Water delignification by advanced oxidation processes: Homogeneous and heterogeneous Fenton and H2O2 photo-assisted reactions
Appl. Catal., B
(2008) - et al.
Effect of Fenton and photo-Fenton reactions on the degradation and biodegradability of 2 and 4-nitrophenols in water treatment
Appl. Catal., B
(1994) - et al.
Degradation of organic matter in olive-oil mill wastewater through homogeneous Fenton-like reaction
Chem. Eng. J.
(2011) A comparative study of Fenton and Fenton-like reaction kinetics in decolourisation of wastewater
Dyes Pigments
(2008)- et al.
Oxidation of direct dyes with hydrogen peroxide using ferrous ion as catalyst
Sep. Purif. Technol.
(2003) - et al.
Decolorization of the azo dye Reactive Black 5 by Fenton and photo-Fenton oxidation
Dyes Pigments
(2006) - et al.
Heterogeneous photo-Fenton degradation of methyl orange by Fe2O3/TiO2 nanoparticles under visible light
J. Water Process Eng.
(2016) - et al.
Degradation of Acid Blue 74 using Fe-ZSM5 zeolite as a heterogeneous photo-Fenton catalyst
Appl. Catal., B
(2008)