Characterization and Quantification of Iron Oxides Occurring in Low Concentration in Soils

Abstract

Iron (Fe) oxides control plant nutrient and contaminant redistribution in soils, but their phase identification and quantification is complicated by their low concentration and poor crystallinity and by the high detection limits of the current techniques. Cyclic voltammetry using a carbon paste electroactive electrode was tested to characterize soil iron oxides. Twenty‐eight clays (<2 µm) from eight soils varying in parent material and pedological development were separated, and Fe phases were determined by x‐ray diffraction (XRD) before and after preconcentration treatment with 5M sodium hydroxide (NaOH). Morphology of Fe phases was determined by electron microscopy, and total extractable Fe phases in soil were determined by citrate bicarbonate dithionite. The preconcentration treatment was effective in phase identification. Goethite and hematite were found, depending upon origin of the soil clays. The samples with less than 2% hematite needed preconcentration. Three percent goethite in clays was on the edge of the XRD limit. The electrochemical signals of re‐oxidation of Fe2+ released from reductive dissolution of free Fe oxides was quantitatively related to their content in the sample. Voltammetry effectively quantified goethite and hematite or both in a mixture with detection limit as low as 0.025 mg 100−1 mg in clay. The technique can be used for quick estimate of low content of Fe oxides in clays and can be adapted for free Fe estimation in soils.