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Experimental alteration of vivianite to lepidocrocite in a calcareous medium

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Vivianite [Fe3(PO4)2·8H2O] is easily oxidized in the presence of air. In this work, we studied the oxidation and incongruent dissolution of vivianite in a calcareous medium containing an anion-exchange resin (AER) that acted as a sink for phosphate. Freshly prepared vivianite suspensions with calcite sand and an AER membrane were oxidized and stirred by bubbling air or CO2-free O2. Experiments were finished when oxidation rate and P removal by the AER became slow, which was at 53 days (air system) or 28 days (CO2free O2 system). At these times, the respective values of the Fe(II)/total Fe ratio were 0.29 and 0.12, and the respective values of the atomic P/Fe ratio were 0.15 and 0.11. The final product of the oxidation was poorly crystalline lepidocrocite in the form of thin (1­5 nm), irregular lamellae that were soluble in acid oxalate. The unit-cell edge lengths of this lepidocrocite were a = 0.3117, b = 1.2572, and c = 0.3870, vs. a = 0.3071, b = 1.2520 and c = 0.3873 nm for the reference lepidocrocite. The lepidocrocite lamellae contained occluded P (P/Fe atomic ratio = 0.03 ­0.04).The results of this and a previous study made us hypothesize that this occluded P is structural and occupies tetrahedral sites adjacent to the empty octahedral sites in the sheets extending on {010}.


Document Type: Research Article


Publication date: 2002-12-01

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