Differential chemoreductive dissolution of iron oxides in a Brazilian Oxisol

Stabilities of naturally occurring hematite and goethite in the Bol horizon of a well-drained Brazilian Oxisol, and their rate of reductive dissolution by sodium dithionite, were investigated. The soil has codominant hematitic and goethitic Fe oxide mineralogy and a reddish hue (2.5YR). There was no apparent foreign-ion substitution in the hematite, but the goethite contained 34 mole percent of AlOOH substituting for FeOOH. Results of limited additions of sodium dithionite and measured redox-potential (Eh) development during the reaction showed that hematite and Al goethite were metastable at Eh = 98 mV, pH = 7.3, and 25°C. Aluminum goethite was metastable at Eh = -2 mV, pH = 7.3, and 25°C. Hematite dissolution at low Eh occurred readily at 25°C. Aluminum-goethite dissolution at low Eh was imperceptible at 25°C, but occurred readily at 75°C. -Authors