J. Phys. Chem. C 2007, 111, 4087-4093
4087
Fe@Fe2O3 Core-Shell Nanowires as Iron Reagent. 1. Efficient Degradation of Rhodamine B by a Novel Sono-Fenton Process Zhihui Ai,† Lirong Lu,† Jinpo Li,† Lizhi Zhang,*,† Jianrong Qiu,‡ and Minghu Wu§ Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China, National Coal Combustion Laboratory, Huazhong UniVersity of Science and Technology, Wuhan 430074, People’s Republic of China, and Department of Biology & Chemistry, Xianning College, Xianning 437100, People’s Republic of China ReceiVed: August 28, 2006; In Final Form: December 11, 2006
In this study, Fe@Fe2O3 core-shell nanowires were first used as a novel Fenton iron reagent. The nanowires were synthesized through the reduction of ferric chloride aqueous solution by sodium borohydride at ambient atmosphere, without protection of inert gases or vacuum. Rhodamine B (RhB) could be efficiently degraded in aqueous media by a novel sonochemical-assisted Fenton (sono-Fenton) system based on these Fe@Fe2O3 core-shell nanowires. The RhB degradation processes were monitored by UV-vis spectroscopy and total organic carbon (TOC) analysis. Fe@Fe2O3 core-shell nanowires showed much higher activity in the sonoFenton system than other iron reagents such as commercial zerovalent iron powders (Fe0), ferrous ions (Fe2+), and ferric ions (Fe3+). It was found that near 100% decoloration and over 60% TOC removal of RhB (5 mg‚L-1) could be achieved in 60 min by this novel sono-Fenton system with 0.018 mol‚L-1 Fe@Fe2O3 coreshell nanowires. This new iron reagents before and after the sono-Fenton reaction were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The characterizations found that the nanowires were transferred to nanotubes/nanoparticles covered with Fe3O4/ Fe2O3 after the sono-Fenton process. A possible mechanism of sono-Fenton degradation of RhB with Fe@Fe2O3 nanowires was proposed on the basis of the experimental results. It involved homogeneous Fenton and heterogeneous Fenton oxidative degradation simultaneously. The high activity of core-shell Fe@Fe2O3 nanowires and the success of their mass production make them attractive for the treatment of organic pollutants in water.
Introduction
Fe(HO2)2+ f Fe2+ + HO2
(3)
In recent years, advanced oxidation processes (AOPs), which involved an in situ generation of highly potent chemical oxidants such as hydroxyl radicals (•OH), have emerged as an important class of technologies to accelerate the nonselective oxidation.1 Among AOPs, Fenton and sonochemical reactions are widely used for the destruction of recalcitrant organic contaminants in wastewater that cannot be eliminated biologically.
O2- + Fe3+ f Fe2+ + O2
(4)
OH + H2O2 f HO2 + H2O
(5)
The classic Fenton reagent is commonly referred to the combination of Fe2+ and H2O2 (Fe2+/H2O2). The combination of Fe3+ and H2O2 is known as Fenton-like reagent (Fe3+/H2O2). As Fe3+ can be produced from Fenton reagent during reactions, Fenton chemistry and Fenton-like chemistry often occur simultaneously.2,3 The following reactions show the mechanism of •OH formed when either Fe2+ or Fe3+ is present (eqs 1 to 5).4,5
Fe2+ + H2O2 f Fe3+ + •OH + OH-
(1)
Fe3+ + H2O2 f Fe(HO2)2+ + H+
(2)
* Address correspondence to this author.
[email protected]. Phone/fax: +86-27-6786 7535. † Central China Normal University. ‡ Huazhong University of Science and Technology. § Xianning College.
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Although Fenton and Fenton-like reagents (Fe3+/H2O2, Fe2+/ H2O2) are efficient to generate oxidative radicals such as •OH in homogeneous reactions to oxidize organic compounds to CO2 and water, they have two main drawbacks for the application on a large scale: high cost of H2O2 and narrow working pH range. It is known that iron in its ferrous and ferric form requires a working pH
