CITRATE-ASCORBATE AS A HIGHLY SELECTIVE EXTRACTANT FOR POORLY CRYSTALLINE IRON-OXIDES

Existing methods for dissolving poorly crytalline Fe oxides (primarily ferrihydrite) in soils and mineral mixtures are simple and rapid but often lack selectivity. In this work, we-tested a more selective, alte rnative ascorbate-based extractant. Ascorbate combined with citrate wa s found to be highly effective in dissolving poorly crystalline Fe oxi des while dissolving little goethite and hematite, Citrate-ascorbate a nd oxalate extracted similar amounts of Fe from most of the soils and Fe-rich materials studied. However, citrate-ascorbate was more selecti ve than oxalate, since it dissolved only negligible amounts of allopha ne or imogolite and magnetite, The recommended citrate-ascorbate extra ction procedure (0.2 M sodium citrate-0.05 M sodium ascorbate, pH 6, 1 6 h) is simple to implement and relatively inexpensive, and uses nonto xic chemicals. Thus, it can be used on a routine basis for estimating poorly crystalline Fe oxides in soils. By subtracting citrate-extracta ble Fe and Al from citrate-ascorbate-extractable Fe and Al values, res pectively, one can estimate Fe and Al contained in poorly crystalline Fe oxides. The citrate-ascorbate extraction, combined with others (oxa late, Tiron, hydroxylamine) can help detect and quantify other soil co mponents such as allophane and magnetite.