Unraveling the Mechanisms of Fe Oxidation and Mn Reduction on Mn Indicators of Reduction in Soil (IRIS) Films

Indicators of reduction in soil (IRIS)
devices are low-cost soil
redox sensors coated with Fe or Mn oxides, which can be reductively
dissolved from the device under suitable redox conditions. Removal
of the metal oxide coating from the surface, leaving behind the white
film, can be quantified and used as an indicator of reducing conditions
in soils. Manganese IRIS, coated with birnessite, can also oxidize
Fe(II), resulting in a color change from brown to orange that complicates
the interpretation of coating removal. Here, we studied field-deployed
Mn IRIS films where Fe oxidation was present to unravel the mechanisms
of Mn oxidation of Fe(II) and the resulting minerals on the IRIS film
surface. We observed reductions in the Mn average oxidation state
when Fe precipitation was evident. Fe precipitation was primarily
ferrihydrite (30–90%), but lepidocrocite and goethite were
also detected, notably when the Mn average oxidation state decreased.
The decrease in the average oxidation state of Mn was due to the adsorption
of Mn(II) to the oxidized Fe and the precipitation of rhodochrosite
(MnCO3) on the film. The results were variable on small
spatial scales (<1 mm), highlighting the suitability of IRIS in
studying heterogeneous redox reactions in soil. Mn IRIS also provides
a tool to bridge lab and field studies of the interactions between
Mn oxides and reduced constituents.