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.