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, alternative ascorbate-based extractant. Ascorbate combined with citrate was found to be highly effective in dissolving poorly crystalline Fe oxides while dissolving little goethite and hematite, Citrate-ascorbate and oxalate extracted similar amounts of Fe from most of the soils and Fe-rich materials studied. However, citrate-ascorbate was more selective than oxalate, since it dissolved only negligible amounts of allophane or imogolite and magnetite, The recommended citrate-ascorbate extraction procedure (0.2 M sodium citrate-0.05 M sodium ascorbate, pH 6, 16 h) is simple to implement and relatively inexpensive, and uses nontoxic chemicals. Thus, it can be used on a routine basis for estimating poorly crystalline Fe oxides in soils. By subtracting citrate-extractable Fe and Al from citrate-ascorbate-extractable Fe and Al values, respectively, one can estimate Fe and Al contained in poorly crystalline Fe oxides. The citrate-ascorbate extraction, combined with others (oxalate, Tiron, hydroxylamine) can help detect and quantify other soil components such as allophane and magnetite.