Oxidation on Zerovalent Iron Promoted by Polyoxometalate as an Electron Shuttle JAESANG LEE, JUNGWON KIM, AND WONYONG CHOI* School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea Most studies on zerovalent iron (ZVI) were mainly focused on the reductive transformation of halo- or nitro- compounds. Oxidation reactions occurring on ZVI have been recently recognized. In this study, we demonstrate that the oxidation pathways on ZVI can be accelerated by the presence of polyoxometalate (POM: nanosized metal- oxygen cluster anion) serving as an electron shuttle. The ions, SiW 12 O 40 4- and PW 12 O 40 3- , can mediate the electron transfer from the Fe 0 surface to O 2 while enhancing the production of H 2 O 2 , which subsequently initiates the OH radical-mediated oxidation through a Fenton-type reaction. The oxidation reaction was completely quenched by adding methanol as an OH radical-scavenger. On the other hand, PMo 12 O 40 3- completely inhibited the oxidative degradation by irreversibly scavenging an electron and holding it. We systematically investigated the effects of iron loading, the concentration of POM, and pH on the oxidative degradation kinetics of 4-chlorophenol in the POM- mediated ZVI system. The POM-mediated oxidations on ZVI were additionally tested for 12 organic contaminants and the rates were compared. Their oxidative degradation on ZVI was mostly enhanced in the presence of POM (SiW 12 O 40 4- ). The present study provides a good model system upon which the ZVI-based oxidation technologies can be successfully enhanced and modified for further developments. Introduction Zerovalent iron (ZVI) system as a groundwater remediation technology has made great progress in both the fundamental understanding and the practical applications for the past decade since its accidental discovery of the reactivity of the iron-casing used for water sample storage (1). The observed degradation of organic substrates on ZVI surface was attributed mainly to the direct electron transfer from metallic iron to substrates with the oxidative iron corrosion ac- companied (2, 3). Since the corrosion-coupled reduction mechanism on ZVI is now well established (2-4), element iron with the reducing power (E 0 (Fe 2+ /Fe 0 ) )-0.447 VNHE (5)) has been widely applied to the reductive transformation of many aquatic pollutants under the anaerobic condition (6-8). The reductive power of ZVI can be utilized to drive oxidative reactions in the presence of H2O2 that generates an OH radical through Fenton reaction (9-11). Recently, it was also reported that the reduction of O2 on Fe 0 led to in-situ production of H2O2 and Fe 2+ that was responsible for the OH radical-induced oxidation occurring on metallic iron surface (reactions 1, 2) (12-14). The corrosive oxidation of Fe 0 to Fe 2+ can be accompanied by the two-electron transfer to O2 with generating H2O2 [E 0 (Fe 2+ /Fe 0 ) )-0.447 VNHE vs E 0 (O2/ H2O2) )+0.695 VNHE (5)]. By this way, ZVI system can take advantage of not only the reductive but also oxidative reactions, which may make the ZVI technology more versatile. Since both the reductive and oxidative reactions are initiated by an electron transfer on Fe 0 , the enhancement in the electron-transfer rate can increase the efficiency of the overall process. In this respect, the presence of suitable electron shuttling species is needed to accelerate the electron transfer from the surface of Fe 0 to substrates. The presence of quinonoid compounds acting as an electron shuttle, for example, enhanced the reductive reaction of nitro aromatics by H2S(15) and the reduction of CCl4 on Fe 0 surface (16). However, the electron shuttle effect on the oxidative pathway in the ZVI system has not been reported. Polyoxometalate (POM) is a metal-oxygen cluster anion that can undergo reversible reduction without its structural change (17). The reduction and reoxidation of POM (POM T POM - ) can be achieved and utilized in various ways. For example, POM can be photoexcited by absorbing UV photons and then the excited POM can abstract an electron from organic substrates to be reduced. The reduced POM can be reoxidized by dissolved oxygen and the POM is recycled. Such properties of POM have been applied to photocatalysis to trigger a variety of redox reactions under UV-illuminated conditions (17-19). POM can also serve as an electron shuttle between an electron donor and acceptor in both dark and illuminated systems (20-22). In this work, we proposed and tested a hypothesis that POM can play the role of a model electron shuttle in the oxidative pathways occurring on Fe 0 (see Scheme 1). The presence of POM can mediate the transfer of electrons from Fe 0 to O2, which eventually leads to the generation of OH radicals via in-situ production of H2O2. Here, we successfully demonstrated that the proposed POM- enhanced oxidation mechanism indeed works. The perfor- mance and characteristics of this modified ZVI system (Fe 0 /POM/O2) were investigated for the oxidation of 4-chlo- rophenol and other substrates. Experimental Section Chemicals and Materials. Chemicals that were used as received in this study include 4-chlorophenol (4-CP, Sigma), * Corresponding author phone: +82-54-279-2283; fax: +82-54- 279-8299; e-mail: wchoi@postech.ac.kr. SCHEME 1. Proposed Mechanism for the POM-mediated Oxidation in Zerovalent Iron (ZVI) System Fe 0 + O 2 + 2H + f Fe 2+ + H 2 O 2 (1) Fe 2+ + H 2 O 2 f Fe 3+ + HO•+ HO - (2) Environ. Sci. Technol. 2007, 41, 3335-3340 10.1021/es062430g CCC: $37.00  2007 American Chemical Society VOL. 41, NO. 9, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 3335 Published on Web 03/30/2007