Elucidating Details of the Transformation of Birnessite into a Tunnel Structure in the Presence of Tl(I)

The affinity between highly toxic Tl⁺ and birnessite has been a matter of active research because the sorption mechanism involved seems to depend upon the doses of applied Tl⁺ per Mn present, which also affects how birnessite is transformed. Aqueous Tl⁺ is very mobile; therefore, its transformation and strong immobilization by Mn oxides is of high interest for environmental research. Birnessites are laminar Mn oxides that show a complex sorption behavior, which involves binding to internal sites of the structure and external surface groups and a high oxidation potential. In the present short investigation, we report some important issues not resolved over the mechanism involved when small Tl⁺/Mn interacts, in which the laminar structure changes to a tunneled structure. Specifically, because very little Tl⁺ is oxidized (as opposed to when higher ratios are involved), water was experimentally confirmed as the reducing agent and the probable mechanism involved was theoretically worked out. Also, the irreversible nature of Tl(I) sorption inside the transformed tunneled structure was tested, which confirmed the extremely high affinity of the Mn oxide tunnels for dehydrated Tl(I).