As an environment-friendly activator, iron ions (Fe(II) and Fe(III)) have been extensively studied in homogeneous activation of persulfate (PS) to remove organic pollutants from water. However, the slow reduction of Fe(III) to Fe(II) and the subsequent low activation efficiency limit the wide applications of PS/Fe(II) or PS/Fe(III). In this study, the roles of four complexing agents, including gallic acid (GA), ethylene diamine tetraacetic acid, (S,S)-ethylenediamine-N,N'-disuccinic acid trisodium salt and citric acid in the activation of PS/Fe(III) were comparatively investigated with iopamidol (IPM) used as a model organic pollutant. Results indicate that GA was the most effective with an increase in the observed pseudo first-order rate constant (k(obs)) by 9.2 folds for IPM degradation by PS/Fe(III). Through radical scavenger (ethanol and tert-butanol) tests and electron paramagnetic resonance analysis, HO center dot and SO4 center dot- were identified to be responsible for the accelerated degradation of IPM, and Ha played a more important role. Intrinsically, the promoted Fe(III) reduction and PS decomposition accelerated the degradation of IPM. The transformation by-products of IPM in the PS/Fe(III)/GA system were identified and potential degradation pathways were proposed. Besides, the accelerated degradation of other three organic pollutants (methyl orange, congo red, and diclofenac) further demonstrated the applicability of the PS/Fe(III)/GA system to water treatment. (C) 2017 Elsevier B.V. All rights reserved.