Role of iron surface oxidation layers in decomposition of azo-dye water pollutants in weak acidic solutions

While decomposition of water pollutants in the presence of metallic iron can be strongly influenced by the nature and structure of the iron surface layer, the composition and structure of the layer produced and transformed in the decomposition process, have been meagerly investigated. The studies presented here establish strong relationships between the composition and structure of the iron oxidized surface layer and the kinetics and reaction pathways of orange II decomposition. The most striking observation is a dramatic difference between dye decomposition at pH 3 and 4. Orange decomposition at pH 2 and 3 is a very fast process with pseudo-first-order kinetics, with a surface normalized rate constant k(SA) = 0.18 L/m(2) min at pH 3 and 30 C. Whereas at pH 4 and 5 the rate is lower with pseudo-zero-order kinetics, with normalized rate constant k(SA) = 1.4 x 10(-5) mol/m(2) min at pH 5 and 30 C. At pH 3 the iron surface is covered by a polymeric Fe(OH)(2) mixed with FeO very thin layer whose thickness remains almost constant with reaction time. There is a slow formation of an additional surface product with akaganeite-like structure. At pH 3 almost all oxidized iron is detected in solution, whereas at pH 5 almost total oxidized iron is cumulated on iron surface in the form of a lepidocrocite, gamma-FeOOH, layer. The thickness of the layer increases continuously with time. The quantitative evaluation of the produced surface lepidocrocite and its surface distribution were performed by means of infrared reflection spectroscopy and spectral simulation methods. At higher temperature 40-50 degrees C, other surface products such as goethite, alpha-FeOOH, and feroxyhite, beta-FeOOH, are also observed. Decomposition of orange is a multi-step process, at pH 3 the orange molecule is at first adsorbed on the very thin iron oxidized layers through SO3 group and then undergoes reduction. Discoloration of orange II in aerobic solution takes place by reduction of the -N=N- bond at the iron surface. The major intermediate is 1-amino-2-naphtol, which undergoes further decomposition without forming any aromatic species. The previously suggested sulfanilic acid as intermediate was not detected in solution. At pH 3 orange reduction and reduction of intermediates are governed by the combination of an electron transfer reaction, with the thin oxide surface layer as a mediator, and the catalytic hydrogenation reaction. At pH 4 and 5 continuous growing of lepidocrocite surface layer demonstrates the importance of the layer as a mediator in the electron transfer reaction. The layer shows a good conductivity, which results from adsorption and absorption of iron ions in the surface structure. It is observed that the decomposition reaction becomes significant at open circuit potential (OCP) below -120 mV (SHE). At pH 3 this condition is fulfilled almost immediately after introduction of iron to aqueous solution, whereas at pH 4 and 5 the OCP of iron decreases very slowly. Iron surface layer composition and structure can be modified by an addition of Fe2+ to solution, which increases the dye decomposition rate. The performed observations make the treatment of waste water in the presence of metallic iron a promising environmental solution. (c) 2004 Elsevier B.V. All rights reserved.