Tellurium Goes for a Ride on the "Ferrous" Wheel: Reduction of Te(VI) and Te(IV) by Iron(II)-Bearing Minerals

Iron(II)-bearing minerals (magnetite (Fe3O4), siderite (FeCO3), green rust (a mixed Fe(II)/Fe(III) layered double hydroxide), etc.) are common products of microbial Fe(III) reduction, and they provide a reservoir of reducing capacity in many subsurface environments that may contribute to the reduction of redox active elements such as tellurium (Te), which can exist in multiple valence states (-II, -I, 0, I, II, IV, V, and VI)—however, only -II (telluride), 0 (native Te), IV, and VI are relevant under conditions commonly found at the Earth's surface. Compared to other, more abundant metalloids (e.g., As, Se, and Sb), little is known regarding the biogeochemistry of Te in aquatic and terrestrial environments. To better understand the redox behavior of Te under ferrugenic/sulfidogenic conditions, we examined the interactions of Te(VI) and Te(IV) (500 µM) in aqueous suspensions containing 25 mM Fe(II) as either magnetite, siderite, vivianite [Fe3(PO4)2•8H20], green rust, or mackinawite (FeS), using X-ray absorption spectroscopy at the Te K-edge to determine changes in the valence state of Te. In the FeS systems, complete reduction of both Te(VI) and Te(IV) to Te(0) was observed within 12 h and near complete reduction (> 90%) was observed within 2.5 days in the green rust systems. In the presence of siderite, complete reduction of both Te(VI) and Te(IV) to Te(0) occurred within 60 days. We observed >60% reduction of Te(VI) to Te(IV) within 2.5 days in the magnetite system, however, Te(0) was not observed until 120 days; complete reduction to Te(0) was observed at day 120 in the Te(IV) system. With vivianite there was >80% reduction of Te(VI) to Te(IV) within 0.5 days; however, no further reduction of Te(VI) to Te(IV) was observed over the duration of the experiment (120 days) and in the Te(IV)-vivianite system there was no evidence of Te(IV) reduction within 120 days. The reduction of Te(VI) and Te(IV) in soils and sediments has been largely attributed to microbial activity, presumably involving direct enzymatic reduction; however, the reduction of Te(VI) and Te(IV) by Fe(II)-bearing minerals suggests that abiotic or coupled biotic-abiotic processes may also play a critical role in Te redox chemistry and thus need to be considered in modeling the global biogeochemical cycling of Te.