THALLIUM ADSORPTION ONTO NATURAL MANGANESE OXIDES: A COMPARISON WITH SYNTHESIZED ANALOGS

Manganese (Mn) oxides are important hosts of trace metals via adsorption and/or redox processes. Previous sorption studies on synthesized Mn oxides have provided valuable insights on metal adsorption mechanisms, and yet a comparison of synthesized and natural minerals is still rare. Here we focus on thallium (Tl), a paleo-redox tracer with a major sink via Mn oxide burial, to investigate whether Tl adsorption and isotopic fractionation show similar behaviors on natural and synthesized Mn oxides. Sorption experiments were performed on various natural Mn oxides with different structures and Mn oxidation states. Our results confirm that Mn²⁺ and Mn³⁺-rich oxides (hausmannite, feitknechtite, pyrolusite, and todorokite) have limited Tl affinity and isotopic fractionation upon sorption. However, our data show unexpected reversible Tl sorption (e.g., Tl desorption) and rapid isotope exchange on vernadite, leading to a much lower Tl affinity compared to synthesized structurally similar hexagonal birnessite (e.g., δMnO₂). Using an isotope mass balance, we suggest that the observed Tl isotope exchange is likely to be kinetic with minimal fractionation involved, which could be attributed to occupation of oxidative sorption sites and competing adsorption by other trace metals (e.g., Mo, Co, Cr, Cu, Ni, etc). Vernadite experiments also show reversible adsorption of Mo and Cr, raising questions on whether δMnO₂ is an appropriate synthesized analog for vernadite. As Mn oxides play an important role in metal biogeochemical cycles, our results highlight the necessity of additional research on desorption kinetics to better understand trace element cycling in nature.