EDTA溶液中のマグネタイトの溶解速度におよぼす溶存酸素の影響

抄録

The dissolution rate of sintered Fe₃O₄ pellets in 3mol/m³ EDTA (H₄Y) solutions with and without dissolved oxygen (DO) was measured at different pHs (1.1-4.6) at 80°C. The dissolved Fe concentration increases with time in a parabolic manner, exceeding the EDTA concentration in the absence of DO but not exceeding the EDTA concentration in the presence of DO. The rate of dissolution changes with pH, with and without DO. The dissolved Fe concentration vs. pH curves for different dissolution times show maxima at pH 2.4, and the rates without DO are always higher than those with DO. It is assumed that the dissolution reaction involves two coupled processes: (a) the transfer of Fe ions by reaction with EDTA to form Fe^IIIY^- and Fe^IIY^2-; and (b) the transfer of lattice O^2- ions by reaction with H⁺ ions to form H₂O. In the absence of DO, the formed Fe^IIY^2- reacts with Fe³⁺ to dissolve further Fe₃O₄, and can be regarded as equal to free EDTA. The rate equation explaining the dissolution without DO is derived as follows:
d[Fe]/dt=k′k″[H+]^8/3α([Y]_T-2[Fe]/3)/(2^2/3/3)k′[Fe]+k″[H⁺]^8/3 (1)
where [Fe]=[Fe^IIY^2-]+[Fe^IIIY^-], k′ and k″ are composite rate constants, α is the fraction of Y^4- in free EDTA, and [Y]_T is the total concentration of added EDTA. In the presence of DO, the formed Fe^IIY^2- is instantly oxidized to Fe^IIIY^-, and the rate equation for this condition was reported elsewhere. The rate equations for conditions with and without DO reproduce the time-courses of [Fe] well, and the calculated [Fe] vs. pH curves show peaks at pH 2.4. The rate constants k′ and k″ are not affected by DO, and the effect of DO on the dissolution rate is due to different “effective” EDTA concentrations: with DO, the dissolved Fe is Fe^IIIY^-, which does not take part in further dissolution, without DO, one third of the dissolved Fe is Fe^IIY^2-, this can dissolve further Fe₃O₄ to form Fe^IIIY^- and Fe²⁺.