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-borne 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 and a synthetic Na-birnessite, to obtain various Mn{sup 2+} mole fractions f(Mn{sup 2+}). Corundum ({alpha}-Al{sub 2}O{sub 3}) was used as a diluent to prepare mixtures containing 50, 1000, and 100000 mg Mn/kg. 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 Mn{sup 2+} white line peak at 6552.6 eV divided by the intensity of the Mn{sup 2+} white line peak at 6560.9 eV. The white line intensity ratio is more sensitive, with an estimated lower detection limit of 20 mg Mn/kg using a 300 by 300 {mu}m spot, and predicts f(Mn{sup 2+}) with an accuracy of {+-}0.1 mole fraction (95% confidence interval) through most of its range.more » The f(Mn{sup 2+}) 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. 39 refs., 9 figs., 1 tab.« less