MECHANISMS AND RATES OF IRON DISSOLUTION FROM BRAZILIAN PLINTHUSTALFS BY DILUTE HYDROCHLORIC AND ASCORBIC ACIDS

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Abstract

Although the dissolution of Fe oxides has been studied extensively, information regarding the dissolution of Fe from tropical plinthic soils is limited. This study examined the influence of acidity, temperature, and chemical reduction on extraction from Fe-rich water-stable aggregates and associated matrices. The two soils studied were Plinthustalfs, developed from the clayey sandstone sediments of the Inferior Cretaceous Itapecuru Formation in Maranhão state, Brazil, and represent an immature Groundwater Laterite soil. Pedon 2 contains more free Fe oxides than Pedon 1. All extractions performed dissolved less Fe than acid ammonium oxalate. Larger apparent reaction orders for solution (H+) were determined for A and E than for B horizons and are attributed to desorption of organic compounds. Smaller activation energies and larger reactive surface areas (frequency factors) in the A and E horizons could be related to larger organic carbon concentrations. Larger activation energies and smaller reactive surface areas in the water-stable aggregates corresponded with apparently smaller Al substitution in the Fe oxides. The influence of chemical reduction was also stronger in fractions containing larger amounts of organic carbon and greater (apparent) isomorphic substitution of Fe3+ by Al3+. With progressive dissolution, there is a decrease in apparent reaction order and an increase in activation energy as well as in reactive surface area.

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