Tuesday, 11 July 2006

Synthesized Iron Oxides Used as a Tool for Documenting Reducing Conditions in Soils.

Martin C. Rabenhorst, University of Maryland, Dept. NRSL, 1112 HJ Patterson Hall, College Park, MD 20742, Douglas W. Ming, NASA Johnson Space Center, 2101 NASA Parkway, Mail Code KX, Houston, TX 77058, and Richard V. Morris, NASA -Johnson Space Center, 2101 NASA Parkway, Mail Code KR, Houston, TX 77058.

<>Iron oxides are generally abundant in soils relative to other reducible phases (such as nitrate or manganese oxides). In seasonally wet soils, iron oxides often become segregated as concentration or depletion features due to redox processes, which can lead to recognizable morphological indicators of wetness. In many soils, the iron oxides may cause the redox conditions to become poised in relation to these phases. Interpretation of the soil hydromorphology is somewhat imprecise and concentrations and depletions of iron oxides have been shown to persist as relict features long after environmental conditions have changed. Because the development of anaerobic and reducing conditions in soils can impact a variety of biological and biogeochemical processes, there has been interest in documenting this phenomenon. Common approaches have been the use of Pt electrodes or the use of dyes. One recently introduced approach has been the use of IRIS (Indicator of Reduction In Soil) tubes which are made from 21 mm OD PVC tubing that has been coated with an iron oxide paint. When the tubes are inserted into the soil and reducing conditions develop in the soil, this leads to reduction and dissolution of the iron oxide coating along those portions of the tubes.   

Two issues or questions have emerged that are related particularly to the nature of the iron oxides used in the paint. It was initially proposed by Jenkinson (2002) that ferrihydrite be used to coat the PVC tubing because this poorly crystalline phase was thought to be most similar to the newly formed iron oxides that occur in soils that endure repeated cycles of reduction and oxidation.  Paint derived from pure (or dominantly) ferrihydrite (synthesized similar to method of Schwertmann and Cornell (2000) by titrating 0.2M FeCl2 with 1M KOH to pH 7.5), however, demonstrates poor adhesion to the PVC tubing.  It rubs off easily and is not suitable for use with IRIS tubes.  When ferrihydrite was synthesized by titrating to pH 11 or 12 rather than 7.5, the higher pH enhanced the transformation of the ferrihydrite to goethite.  Within a week 10-40% of the ferrihydrite had been altered to goethite (the higher the pH, the more rapid the alteration) and the transformation to goethite continued over time. Our studies demonstrated that once approximately 30 to 40% of the ferrihydrite was transformed to goethite, the suspension could be effectively used as a durable coating on the IRIS tubes, because paint containing at least 30 to 40% goethite adhered well to the PVC and did not rub off.  The mechanism for this improved adhesion, however, is not fully understood and was explored further by electron microscopy.

The second issue is the phenomenon of partial removal of the synthetic iron oxide coating from the PVC tubing.  When IRIS tubes are extracted from the soil, they commonly exhibit white zones where the iron oxide coating appears to be completely removed (presumably by reduction and dissolution through the activity of soil microorganisms).  At other times (or on other parts of the tubes) there are distinctive patterns or splotches where the iron oxide coating appears to be only partially removed.  Because of thermodynamic constraints, ferrihydrite is considered to be less stable and more easily reduced than goethite. Therefore it has been hypothesized that the apparent partial removal of the iron oxide coating from some IRIS tubes represents the preferential reduction and dissolution of ferrihydrite over goethite under marginally reducing conditions. This hypothesis was evaluated using Mössbauer spectroscopy and XRD techniques.

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