This paper described the formation and mechanism of nano-iron oxide from air oxidation of Fe(OH)₂ under irradiation by visible light in the presence of ethylenediamine, oxalic acid and citric acid. The experimental result showed that the pure γ-FeOOH phase was generated at room light when a certain amount of ethylenediamine, oxalic acid and citric acid was added respectively. And the crystalline degree of γ-FeOOH became poor with the increasing of light intensity. We thought that the reaction systems were added ethylenediamine, citric acid and oxalic acid under visible light irradiation, not only made intermediate green rust GR(II) crystal weak and speeded up the GR(II) to dissolve, also the complex of complexing agent with the Fe^2＋ could absorb part of the visible light to generate strong oxidizing free radicals, improving the rate of oxidation reaction and oxidizing Fe^2＋ to Fe^3＋ rapidly. The rapid oxidation speed was favorable for the formation of the low crystalline γ-FeOOH.

Key words: complexing agent, visible light, nano-iron oxide, mechanism