Iron Oxyhydroxide reduction in simulated wetland soils: Effects of mineralogical composition of Iris paints

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Reducing conditions in soils can be documented by the voltage measured between Pt and reference electrodes (Eh) or by using dyes like ,-dipyridyl for identifying reduced species (such as Fe2+). Indicator of Reduction in Soils (IRIS) tubes have been recently introduced as an alternative approach for documenting reducing conditions in soils, where a synthetic Fe oxyhydroxide painted on polyvinyl chloride tubes can be reduced, dissolved, and removed by respiring soil microorganisms. In order for the Fe oxyhydroxide paint to adhere to the tubes, it must be synthesized as a mixture of ferrihydrite and goethite. Based on thermodynamics, ferrihydrite is predicted to be more easily reduced at higher redox potentials than goethite. We hypothesized that IRIS tubes made using paint composed of different proportions of ferrihydrite and goethite would perform differently in wetland soils. This experiment evaluated the performance of IRIS tubes created using nine Fe oxyhydroxide paints with different proportions of ferrihydrite and goethite ranging from 15.2 to 70.9 mol % goethite. Tubes were evaluated following placement in mesocosms in the laboratory using two different wetland soil materials for 13 d. An ANOVA indicated that there were highly significant differences (P < 0.0001) in full or intermediate removal of paint due to paint type. In general, the extent of paint removal was inversely related to goethite content. The minimum goethite content needed to successfully manufacture IRIS tubes is approximately 40%, and we recommend that IRIS tubes not be used when the goethite content is beyond the range of 40 to 60 mol %.

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