The self-catalysis of ferrate (VI) by its reactive byproducts or reductive substances for the degradation of diclofenac: Kinetics, mechanism and transformation products

•Higher DCF removal by Fe(VI) was observed in NaOH system than in phosphate buffer.•Fe(III) and Fe(II) improved the self-catalysis of Fe(VI) by electron or O transfer.•Reductive substances (e.g., Mn2+ and SO32−) enhanced the decomposition of Fe(VI).•Six transformation pathways were proposed based on the identified seven byproducts.