Removal of amaranth from aqueous solutions by Fe2+/H2O2 and Fe3+/H2O2 electro-oxidation systems was comparatively studied in an undivided electrochemical cell with anthraquinonedisulfonate (AQDS)/polypyrrole (PPy) composite film modified graphite as cathode. The modified cathode allowed the formation of strong oxidant hydroxyl radical ((OH)-O-center dot) in the medium via Fenton's reaction between cathodically generated H2O2 and added or regenerated Fe2+. The effects of solution pH and iron catalyst concentration on dye degradation by the two systems were studied and compared. Degradation intermediates were analyzed by FTIR and GC-MS, and a tentative reaction pathway is proposed. Dye decay reaction always obeys pseudo-first-order kinetics for Fe3+/H2O2 process, while dye degradation follows a two-stage process with each stage obeying pseudo-first-order kinetics in the case of Fe2+/H2O2 system. In addition, the long-term stability and structural relaxation of the composite film during multiple experimental runs are also examined. (C) 2009 Elsevier B.V. All rights reserved.