Investigation of reaction mechanisms by electrospray ionization mass spectrometry: characterization of intermediates in the degradation of phenol by a novel iron/magnetite/hydrogen peroxide heterogeneous oxidation system Fla ´via C. C. Moura 1 , Maria H. Araujo 1 , Ilza Dalma ´zio 1 , Ta ˆnia M. A. Alves 2 , Leonardo S. Santos 3 , Marcos N. Eberlin 3 * , Rodinei Augusti 1 and Rochel M. Lago 1 1 Department of Chemistry, Federal University of Minas Gerais, Belo Horizonte, MG 31270-901, Brazil 2 Rene ´ Rachou Research Center, Oswaldo Cruz Foundation, Belo Horizonte, MG 30190-002, Brazil 3 ThoMSon Laboratory for Mass Spectrometry, Institute of Chemistry, State University of Campinas, Campinas, SP 13083-970, Brazil Received 24 January 2006; Revised 3 April 2006; Accepted 4 April 2006 Electrospray ionization (ESI) mass spectrometry (MS) and tandem mass spectrometry (MS/MS) were used to monitor the oxidation of phenol by a novel heterogeneous Fenton system based on a Fe 0 /Fe 3 O 4 composite and H 2 O 2 . On-line ESI-MS(/MS) shows that this heterogeneous system pro- motes prompt oxidation of phenol to hydroquinone, which is subsequently oxidized to quinone, other cyclic poly-hydroxylated intermediates and an acyclic carboxylic acid. A peroxide-type inter- mediate, probably formed via an electrophilic attack of HOO . on the phenol ring, was also intercepted and characterized. ESI-MS(/MS) monitoring of the oxidation of two other model aromatic compounds, benzene and chlorobenzene, indicates the participation of analogous intermediates. These results suggest that oxidation by the heterogeneous system is promoted by highly reactive HO . and HOO . radicals generated from H 2 O 2 on the surface of the Fe 0 /Fe 3 O 4 composite via a classical Fenton-like mechanism. Copyright # 2006 John Wiley & Sons, Ltd. The Fenton reaction 1 is one of the most efficient advanced oxidation process 2 for the destruction of organic contami- nants in wastewaters. The classical Fenton system 3 uses a mixture of H 2 O 2 and a soluble Fe(II) salt to generate in situ free hydroxyl radicals (HO . ) according to the Haber-Weiss mechanism (Eqn. (1)). 4 The reaction of the strong oxidizing HO . mineralizes the organic compounds, thus yielding CO 2 and H 2 O as the final harmless products. Fe 2þ þ H 2 O 2 ! Fe 3þ þ OH  þ OH  (1) Recent efforts 5–9 have been made to replace the soluble Fe(II) salt by solid iron compounds in heterogeneous Fenton- like systems. Heterogeneous processes are attractive since they can operate in a fixed-bed reactor, in a near neutral pH, with no sludge and the iron promoter can be recycled. However, most of the iron promoters so far investigated, such as goethite, hematite and ferrihydrite, 5–9 displayed insufficient activity. We recently showed that magnetite-based systems, i.e. Fe 3x M x O 4 (M ¼ Mn or Co) 10,11 and Fe8/Fe 3 O 4 , 12,13 are highly active in promoting Fenton chemistry. Although many heterogeneous Fenton systems have been investi- gated, 14–16 information is scarce on the operating mechan- isms, particularly on whether free radicals are indeed the key oxidative species and how many organic intermediates are involved. We have used a variety of mass spectrometric techniques to reveal the mechanistic details of major water remediation processes. 17–23 Electrospray ionization mass spectrometry (ESI-MS) has became the central technique as it often allows real-time detection and ESI-tandem mass spectrometric (MS/MS) characterization of reactants, intermediates and final products. 24 Major water-treatment processes have been investigated via ESI-MS techniques and even transient reaction intermediates and catalyst complexes have been intercepted and characterized. 25–32 This work aims to apply direct-infusion ESI-MS(/MS) to monitor the oxidation of phenol, a model aromatic compound (as well as benzene and chlorobenzene), in aqueous solution promoted by a novel heterogeneous Fenton system based on a Fe 0 /Fe 3 O 4 composite and H 2 O 2 . RAPID COMMUNICATIONS IN MASS SPECTROMETRY Rapid Commun. Mass Spectrom. 2006; 20: 1859–1863 Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rcm.2523 *Correspondence to: M. N. Eberlin, ThoMSon Laboratory for Mass Spectrometry, Institute of Chemistry, State University of Cam- pinas, Campinas, SP 13083-970, Brazil. E-mail: eberlin@iqm.unicamp.br Contract/grant sponsor: CNPq, CAPES, FAPEMIG and FAPESP. Copyright # 2006 John Wiley & Sons, Ltd.