Reductive transformation and mineralization of an azo dye by hydroxysulphate green rust preceding oxidation using H 2 O 2 at neutral pH Tiangoua Kone, Khalil Hanna * , Mustapha Abdelmoula, Christian Ruby, Cédric Carteret Laboratoire de Chimie Physique et Microbiologie pour l’Environnement (LCPME), UMR 7564 CNRS-Nancy Université, 405, Rue de Vandoeuvre, 54600 Villers-Lès-Nancy, France article info Article history: Received 2 July 2008 Received in revised form 1 December 2008 Accepted 2 December 2008 Available online 14 January 2009 Keywords: Reactivity Green rust Fenton-like Oxidation Reduction abstract In this study, the reactivity of hydroxysulphate green rust (GRðSO 2- 4 Þ) toward reductive transformation, oxidative degradation and mineralization of organic compounds was evaluated using Methyl Red (MR) as model pollutant. The GRðSO 2- 4 Þ was synthesized by co-precipitation method and characterized by X- ray diffraction (XRD), Mössbauer spectroscopy and Fourier Transform Infrared (FTIR) analyses. Reductive decolourization of MR solution occurred in the presence of GRðSO 2- 4 Þ, while no total organic carbon (TOC) decay was observed during the equilibration time. Significant TOC removal (87%) was noted when H 2 O 2 was added to the GRðSO 2- 4 Þ=MR mixture after the preliminary reduction step. UV–Vis analysis, dissolved iron and H 2 O 2 concentration measurement, and batch sorption test showed that the heterogeneous Fen- ton-like reaction is the main mechanism by which the pollutant was mineralized. Increasing of H 2 O 2 / Fe(II) ratio did not affect significantly the mineralization rate of MR. However, slight decolourization of MR and absence of TOC abatement were noted when both MR and H 2 O 2 were simultaneously mixed with the GRðSO 2- 4 Þ. XRD analysis, Mössbauer spectroscopy and FTIR spectroscopy revealed that the oxidation end-products of GRðSO 2- 4 Þ were mainly a poorly crystallized goethite when GR was oxidized after equil- ibrating with MR in solution. However, a badly crystallized iron oxide was formed when GR was imme- diately oxidized. In all cases, the interlayer anion ðSO 2- 4 Þ was ejected from GR structure to aqueous solution. These results suggest that the GRðSO 2- 4 Þ=H 2 O 2 system could be used to promote the reduc- tion/oxidation reaction of organic pollutants. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The reactivity of Fe(II)-bearing minerals toward reductive trans- formation of organic pollutants has been widely investigated (Erbs et al., 1999; Lee and Batchelor, 2002; O’Loughlin and Burris, 2004; Chun et al., 2007; Choi and Lee, 2008). In these works, it has been demonstrated that abiotic reactions with Fe(II)-containing miner- als dominate the long-term behavior of reducible pollutants in the subsurface and affect their overall attenuation in the environ- ment. On the other hand, the Fe-species are used in Fenton process to catalyze the decomposition of hydrogen peroxide and generate hydroxyl radical. Å OH radical is a very strong reactive oxidant (E h = 2.7 V) which is able to degrade most of the organic com- pounds in aqueous matrices. In order to maintain iron under dis- solved form, the Fenton process works at pH values below 4, which is harmful for the environment. Because of this disadvan- tage, some Fe-bearing minerals such as iron oxides and hydroxides have been used in modified Fenton working at circumneutral pH (Lin and Gurol, 1998; Kwan and Voelker, 2003; Matta et al., 2007; Hanna et al., 2008). Since Fe(II) was found to be more effec- tive than Fe(III) for the generation of hydroxyl radicals (Valentine and Wang, 1998; Hanna et al., 2008; Matta et al., 2008), Fe(II)- bearing minerals phase such as Fe(II)-containing clays, magnetite, siderite, pyrite and green rusts (GRs) could be the best promoters of Fenton-like oxidation. GRs are Fe(II)–Fe(III) hydroxysalts belonging to the general class of layered double hydroxides family (Hansen, 2001). GRs can be found in soils and sediments under suboxic and anoxic conditions (Génin et al., 1998). They are characterized by a crystalline struc- ture consisting of the stacking of brucite-like layers carrying positive charges and interlayers constituted by anions and water molecules (positively charged hydroxide layers ½Fe II ð1xÞ Fe III x ðOHÞ 2  xþ alternate with negatively charged interlayers of anions A n and of m water molecules per anion) (Hansen, 2001; Génin et al., 2002). Thus their general formula can be written as ½Fe II ð1xÞ Fe III x ðOHÞ 2  xþ ½ x n A n ; mH 2 O x where A n is the intercalated anions ðA n ¼ Cl  ; SO 2- 4 ; CO 2- 3 ...Þ and x is the Fe(III) molar fraction (Génin et al., 1998; Hansen, 2001). In comparison with stoichiom- etric magnetite Fe II Fe III 2 O 4 , GRs contain about two times more Fe(II) species (x is generally situated between 0.25 and 0.33) (Hansen, 2001). Because of their structural Fe(II), GRs are very reactive com- pounds and play a central role in the redox cycling of iron in many 0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.12.002 * Corresponding author. Tel.: +33 (0)3 83 68 52 42; fax: +33 (0)3 83 27 54 44. E-mail address: khalil.hanna@lcpme.cnrs-nancy.fr (K. Hanna). Chemosphere 75 (2009) 212–219 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere