DETERMINING MANGANESE OXIDATION-STATE IN SOILS USING X-RAY-ABSORPTION NEAR-EDGE STRUCTURE (XANES) SPECTROSCOPY

The redox chemistry of soil Mn is important in Mn uptake by plants, the movement of trace elements absorbed on or occluded in Mn-oxides, and the etiology of some soil-home plant fungal diseases. The objectives of this study were to develop calibration curves for determining Mn oxidation states in moist soil samples using micro-x-ray absorption near-edge structure spectroscopy and to test the method on selected soils. Oxidation-state standards were prepared by mixing dry powders of MnSO4 . H2O (Mn2+) and a synthetic Na-birnessite, Na4Mn14O27. 9H(2)O (Mn4+) to obtain various Mn2+ mole fractions, f(Mn2+), where f(Mn2+) = Mn2+/(Mn2+ + Mn4+). Corundum (alpha-Al2O3) was used as a diluent to prepare mixtures containing 50, 1000, and 100 000 mg Mn kg(-1). Quantitative analysis of Mn oxidation state is possible using either the energy-of the pre-edge peak at 6540 eV or a ratio based on the intensity of the Mn2+ white line peak at 6552.6 eY divided by the intensity of the Mn4+ white line peak at 6560.9 eV. The white line intensity ratio is more sensitive, with an estimated loffer detection limit of 20 mg Mn kg(-1) using a 300 by 300 mu m spot, and predicts f(Mn2+) with an accuracy of +/-0.1 mole fraction (95% confidence interval) through most of its range. The f(Mn2+) for four air-dried Indiana surface soils ranged from 0.26 to 0.44 +/- 0.1. Saturation and reduction for 5 d resulted in complete reduction of Mn in the soil matrix.