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Wetland Soils

Synthetic Iron Oxides as an Indicator of Reduction in Soils (IRIS)

Dep. of Natural Resource Sci., Univ. of Maryland, College Park, MD 20742-5821

^* Corresponding author (mrabenho{at}umd.edu)

Various circumstances in wetland delineation or mitigation strategies require the documentation of reducing conditions in soils. An innovative approach for assessing reduction in soils known as Indicator of Reduction in Soils (IRIS) has been recently introduced. IRIS tubes are manufactured by coating polyvinylchloride (PVC) pipe with a paint prepared from an Fe oxide (mainly ferrihydrite) suspension. When the iron oxide is synthesized by adding KOH to a FeCl₃ solution, the mineral phase is essentially pure ferrihydrite. Over time, and under particular conditions, the ferrihydrite is transformed to more crystalline forms such as goethite and hematite. The objectives of this study were (i) to assess the mineralogical composition of iron oxide paint synthesized for the manufacture of IRIS tubes and (ii) to assess the significance of mineralogical composition of the Fe oxide paint on desirable qualities for the construction of IRIS tubes. The effects of pH and storage time on the mineralogical alteration of ferrihydrite are examined and compared with performance of the paint by a scale for adhesion and durability. Paint composed of nearly pure ferrihydrite shows poor adhesion and durability. Data suggest that for paint to adhere well to the PVC tubing, the Fe oxide suspension must contain a minimum of 30 to 40% goethite. This desired paint composition can be facilitated during synthesis by titrating the Fe oxide suspension to pH 11 or 12 rather than pH 7.5, which is typically prescribed for the formation of synthetic ferrihydrite.

Abbreviations: IRIS, Indicator of Reduction in Soils • PVC, polyvinylchloride • TS, Technical Standard