Manganese Oxides for Environmental Assessment

Abstract

Core Ideas Synthesis of birnessite that forms a durable coating requires both high lactate and high Na to K ratios. A high lactate to permanganate ratio facilitates a rapid (<10 min) nucleation of birnessite crystals. The high Na/K ratio facilitates crystal growth along the c axis because Na+ ions fit well within the triclinic structure. Early‐formed birnessite is initially poorly ordered, but ordering along the c axis occurs over 2 to 3 d. Differences in durability of birnessites are likely related to crystal morphology. 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 μm 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.