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Abstract
Characterization of oxide and oxyhydroxide iron (Fe) and aluminum (Al) forms in highly-weathered Ultisols of the southeastern United States is necessary to develop further understanding of colloidal-facilitated transport of pollutants, sorption of contaminants, erosion, and soil genesis. The objective of this study was to examine Fe and Al oxides from several highly-weathered Ultisols and evaluate their relationships with particle size fractions and other soil chemical and physical properties. Samples contained in the argillic horizon of 13 highly-weathered pedons were examined. These pedons either contained kandic horizons, were in a kaolinitic mineralogical family, or were in a siliceous mineralogical family with a subactive cation exchange capacity activity (CEC) class. Standard characterization analyses were performed on all pedons. Samples were fractionated into a coarse (2 to 2000 μm) and fine (< 2 μm) fraction, and ammonium oxalate (Feo and Alo) and dithionite-citrate-bicarbonate (Fed and Ald) extractable Fe and Al were quantified in each. Selective dissolution treatments were conducted on < 2 μm fractions prior to mineralogical analyses. A modified differential XRD (DXRD) was used for further Fe oxide characterization. Pretreatments worked well for concentrating crystalline oxide forms. All Fe and Al extracted forms were higher in the fine than the coarse fraction. The Fe oxides were highly crystalline, but a significantly higher ratio of Feo/Fed was found in the fine as compared to the coarse fraction (0.041 to 0.020, respectively). Both goethite and gibbsite quantities were negatively correlated with effective cation exchange capacity (ECEC) (r = −0.54 and −0.80, respectively). The Feo/ Fed ratio in the < 2 μm fraction was positively correlated (r = 0.49) with hematite content, while it was negatively correlated with goethite quantities (r = −0.61), suggesting a ferrihydrite precursor is associated with hematite formation.
