Separating Soil Iron‐ and Manganese‐Oxide Fractions for Microelement Analysis

Abstract

Abstract Since sodium dithionite (Na 2 S 2 O 4 ) is often contaminated with zinc (Zn) and can form metal sulfide precipitates, it is not suitable for solubilizing iron (Fe) oxides in fractionation schemes for soil microelements. The objective was to find an alternate method to solubilize crystalline Fe oxides that would fit into a scheme including extractants for amorphous Fe and manganese (Mn) oxides. Three soils were extracted with seven reagents designed to remove amorphous and/or crystalline Fe or Mn oxides [0.1 M Na 4 P 2 O 7 , pH 10.0; 0.2 M (NH 4 ) 2 C 2 O 4 in 0.2 M H 2 C 2 O 4 , pH 3.0 (oxalate); 0.1 M NH 2 OH‐HCl, pH 2.0; 1.0 M NH 2 OH‐HCl in 25% acetic acid; 0.1 M ascorbic acid in the oxalate solution; 0.1 g SnCl 2 per gram of soil in the oxalate solution; and 1.0 g dithionite per gram of soil in citrate buffer]. The Na 4 P 2 O 7 , an extractant for elements associated with the organic fraction, extracted amounts of Fe similar to that for the oxalate solution. The two NH 2 OH‐HCl extractants solubilized very little Fe (<1% total), but NH 2 OH‐HCl alone solubilized as much Mn as most of the other extractants indicating that it is specific for Mn oxides. The ascorbic acid‐oxalate and SnCl 2 ‐oxalate experimental methods extracted amounts of Fe similar to the dithionite method and amounts of Al higher than the dithionite method. Of the amorphous Fe‐oxide extractants, the oxalate solution solubilized the most Zn and Cu, whereas of the crystalline Fe‐oxide extractants, the ascorbic acid‐oxalate solubilized the highest amounts of Zn and Cu. The extractants suggested for a fractionation scheme are NH 2 OH‐HCl for Mn oxides, oxalate solution shaken with the soil in the dark for amorphous Fe oxides, and ascorbic acid‐oxalate for crystalline Fe oxides.