Following on earlier work that has shown the value of iron-based IRIS (Indicator of Reduction In Soils) technology, recent efforts have focused on developing a manganese-based IRIS coating. A synthesis procedure developed utilizing a high Na lactate to KMnO4 molar ratio followed by 3 d of dialysis, forms a crystalline birnessite that can be easily applied to PVC tubing and which forms a durable manganese oxide coating. In this study, a series of experiments were run to evaluate the impact of the following parameters on the synthesis of the birnessite: Na/K ratio; lactate/permanganate ratio; lactate source; reaction time; centrifuge washing; dialysis and ageing of the samples. Results confirm that to synthesize birnessite that forms a durable coating both high lactate and high Na/K ratios are required. The high lactate/permanganate ratio (6.7) facilitates a rapid nucleation of triclinic birnessite crystals within 10 min during which time the initial structural template is established, and where some of the manganese (approximately 38%) is reduced to Mn3 . The high Na/K facilitates the ordering of the triclinic birnessite along the c axis following synthesis, and based on analyses of x-ray diffraction (XRD) patterns, the crystallites grow from 2 to 12 nm in direction of c axis over the course of several days. Scanning electron microscopy (SEM) analyses show that oriented structured particles up to 1 mm form. X-ray photoelectron spectroscopy (XPS) analyses demonstrate that there is no difference in the charge structure of the Mn between birnessites that will, or will not, form a durable coating. We conclude that the differences in durability are related primarily to physical interaction and/or interlocking of crystals.
