Density Functional Theory Modeling of the Oxidation Mechanism of Tl(I) by Birnessite

This study establishes a theoreticalfoundation for theoxidationpathway of monovalent thallium (Tl(I)) to trivalent thallium (Tl(III))on birnessite, which is responsible for the over million-times enrichmentof Tl in marine ferromanganese deposits over seawater concentration.Tl(I) oxidation occurs on vacant Mn(IV) sites located on the basalplanes of the birnessite layers and on the edge sites, in agreementwith experiment. Two Mn(IV) atoms are reduced to Mn(III) when Tl(I)gives up two electrons in two one-electron steps with formation ofan intermediate Tl(II) inner-sphere complex. Tl(I) oxidation is facilitatedat pH > 4-5 by the partial hydrolysis of the Tl(III) inner-sphereproduct on the reactive basal and edge sites. Oxidation by O-2 is thermodynamically unfavorable. Although density functional theoryhas predictive power for an intermediate Tl(II) complex, it wouldbe difficult to characterize it as Tl(II) is highly reactive and thereforeprobably short-lived. These findings provide the first atomic-scaledescription of the oxidation of Tl(I) by a manganese oxide and fillgaps in our understanding of global thallium sequestration in naturalsystems.