Dissolution of Metal Oxides Accumulated in Nuclear Steam Generators: Study of Solutions Containing Organic Chelating Agents

A study of the reactivity of ethylenediaminetetraacetic acid (EDTA), citric acid, and hydrazine for the dissolution of magnetite particles has allowed some steps of the different mechanisms to be identified. Two mechanisms are suggested: In acidic solutions, the chelating agents are adsorbed at the solid/solution interface followed by desorption of the complexed species FeH_nL^(n+1-4), where HnL is EDTA or citric acid, whereas in alkaline media, direct dissolution of the oxide particles takes place followed by complexation of the species Fe³⁺/Fe²⁺ in solution. The hydrazine apparently reduces the Fe³⁺ ions via a surface complexing reaction involving the ions, a reaction which is in competition with the protonation of the Fe₃O₄ crystal lattice. Finally, regardless of the type of oxide (Fe₃O₄, Fe₂O₃, FeOOH, CuO, or Cu₂O) or the composition of the complexing solutions, suspensions of these particles are highly unstable with respect to agglomeration or settling out, more because of the high concentration of chelating agents than their chemical characteristics.