Differential Chemoreductive Dissolution of Iron Oxides in a Brazilian Oxisol

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Abstract
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.
Funding Information
  • U.S. Department of Agriculture

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