Reductive sorption of vanadium by green rust in seawater

Vanadium (V) is a redox-sensitive trace metal that typically exists in one of three oxidation states (+3, +4 and +5) in natural waters; a feature increasingly used in paleoredox studies of ancient marine sediments. However, our knowledge of V geochemistry in low-oxygen marine environments is still limited, especially regarding interactions of V with reduced iron minerals such as green rust. Carbonate green rusts (GR_CO3) are mixed Fe^II/Fe^III-phases found in some modern ferruginous settings, such as Lake Matano (Indonesia), and were likely abundant in ancient ferruginous marine systems where they may have played an essential role in authigenic V enrichments in sediments. Here, we present an abiotic pathway of V removal from seawater via reduction and adsorption onto amorphous GR_CO3. Suspensions of the freshly precipitated GR_CO3 (1 g L⁻¹) were added to vanadate (1 mg V^V L⁻¹ initial concentration) in anoxic synthetic seawater solutions. Vanadium removal by GR_CO3 was rapid and efficient, with 92 – 99% of V removed in under 20 seconds. Synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy showed that V^V adsorbed by GR_CO3 was partially reduced to a mixture of V^V and V^IV, with the average oxidation state of adsorbed V increasing (+4.3 to +4.7) with increasing solution pH (7.5 to 8.5). Upon subsequent exposure to aerated seawater, V-bearing GR_CO3 oxidized to lepidocrocite [γ–FeO(OH)] within 24 h, with concomitant reduction of all solid-phase V^V to V^IV. During oxidation, V was not released back into solution; rather, extended X-ray absorption fine structure (EXAFS) modeling revealed that V^IV was structurally incorporated into lepidocrocite as octahedral vanadyl (VO²⁺). Our work further constrains the aqueous geochemistry of V, which has implications for understanding V cycling and removal mechanisms in both modern and ancient marine systems.