SYNTHESIZED IRON OXIDES USED AS A TOOL FOR DOCUMENTING REDUCING CONDITIONS IN SOILS

Abstract

Recently, a new technology known as indicator of reduction in soils (IRIS) has emerged for documenting reducing conditions in soils. An iron (Fe) oxide paint is applied to polyvinyl chloride tubing that is subsequently placed within the soil. Under reducing conditions, the Fe oxides gain electrons, become soluble, and are stripped from the tubing. This study was initiated to address two questions that have emerged regarding the manufacture and use of IRIS tubes: (i) Why is a mixture of goethite (Gt) and ferrihydrite (minimum 30%-40% Gt) required for the Fe oxide paint to adhere and remain durable when applied to polyvinyl chloride tubing? and (ii) What is represented by the development of pale yellow areas that sometimes are observed on the IRIS tubes? To address these questions, Fe oxide coatings were examined using X-ray diffraction and Mössbauer techniques, by differential chemical extractions, and using scanning electron microscopy. Based on X-ray diffraction and scanning electron microscopy, we conclude that the transformation of ferrihydrite to Gt during paint synthesis results in large Gt crystals with an elongated lath-shaped habit that appear to produce an interlocking network of particles that enhances the durability of the paint. Differential extractions and Mössbauer data indicate that the development of pale yellow zones on the IRIS tubes represents a preferential dissolution of ferrihydrite over Gt, which is consistent with thermodynamic predictions of the relative stability of these two minerals in wetland soil environments.