Thallium oxidative fractionation on birnessite-like phases: insights from mineralogical and isotope analyses of adsorption time series experiments

The thallium (Tl) isotope system has become a promising tracer for global ocean oxygen by tracking Mn oxide burial, because Tl(I) has a significant isotopic fractionation upon oxidative adsorption on birnessite-like phases with hexagonal symmetry. However, lab experiments have yet to reproduce theoretical isotopic fractionation for Tl(I) oxidation. Here we conducted adsorption experiments using a range of initial Tl concentrations over a time series of up to ∼ 218 days in duration on two synthetic hexagonal birnessite-like phases (δ-MnO 2 and acid birnessite). We show that both δ-MnO 2 and acid birnessite are similarly efficient in scavenging Tl(I) (via non-oxidative adsorption), but Tl(I) oxidation only occurs on δ-MnO 2 . This finding is likely a consequence of the larger surface area for δ-MnO 2 and subsequent higher abundance of accessible Tl(I) oxidation sites on the layer edges and vacancies. Our results reveal rapid Tl(I) oxidation on δ-MnO 2 from X-ray absorption near edge structure (XANES) measurements, but net Tl isotopic fractionation may not be significant until isotope exchange between the adsorbed and aqueous Tl allows 205 Tl to accumulate in Tl(III) complexes. We also observed Tl(III) reduction on δ-MnO 2 and Tl desorption in the long time series experiments. Tl(III) reduction on the mineral is likely associated with reactions with Mn(III), which is produced by Mn(IV) reduction in the Tl(I) oxidation process. Tl reduction/electron transfer, however, does not necessarily impact net Tl isotopic fractionation. Preservation of the original Tl isotopic fractionation signal, even with dynamic Tl electron transfer and Tl desorption, further strengthens the validity of Tl isotopes as a proxy in ancient sedimentary records. Furthermore, from the correlation between net Tl isotope fractionation and Tl(III) proportion, isotopic fractionation for Tl(I) oxidation is calculated to be 1.0028–1.0050, overlapping with the range of theoretical calculations. The maximum net fractionation observed in our experiments (∼1.0014) is also consistent with observed fractionation between authigenic Mn oxides in pelagic clays and seawater, which could be representative of the magnitude of Tl isotope fractionation associated with Mn oxide burial in the mass balance. In total, this study provides new insight into the controls on Tl redox reactions and fractionation at the surface of Mn oxides. Additional research is still required to investigate the complex interactions of mineralogy, Tl oxidation, and Tl bounding environments to obtain a more robust understanding of the dynamic Tl isotope behavior on birnessite-like phases.