Transformation of Hexagonal Birnessite upon Reaction with Thallium(I): Effects of Birnessite Crystallinity, pH, and Thallium Concentration

We examined the uptake of Tl(I) by two hexagonal birnessites and related phase transformations in laboratory experiments over 12 sequential additions of 0.01 M Tl(I)/Mn at pH 4.0, 6.0, and 8.0. The Tl-reacted Mn oxides were characterized for their structure, Tl binding, and morphology using X-ray diffraction, X-ray photoelectron and X-ray absorption spectroscopies, and transmission electron microscopy. Very limited Tl oxidation was observed in contrast to previous works, where equal Tl(I)/Mn was added in a single step. Instead, both birnessites transformed into a 2 x 2 tunneled phase with dehydrated Tl(I) in its tunnels at pH 4, but only partially at pH 6, and at pH 8.0 they remained layered. The first four to nine sequential Tl(I)/Mn additions resulted in lower residual dissolved Tl+ concentrations than when the same amounts of Tl(I)/Mn were added in single steps. This study thus shows that the repeated reaction of hexagonal birnessites with smaller Tl(I)/Mn at ambient temperature triggers a complete phase conversion with Tl(I) as the sole reacting cation. The novel pathway found may be more relevant for contaminated environments and may help explain the formation of minerals like thalliomelane [Tl+(Mn7.54+Cu0.52+)O-16]; it also points to the possibility that other reducing species trigger similar Mn oxide transformation reactions.

Automated summary

In laboratory experiments, the uptake of Tl(I) by hexagonal birnessites was observed to trigger a complete phase conversion with Tl(I) as the sole reacting cation, resulting in lower residual dissolved Tl+ concentrations compared to single-step additions. This novel pathway may have implications for contaminated environments and the formation of minerals like thalliomelane.