LAUSR

Abstract In this study, the formation constant (log10β10) for FeB(OH)4+ as expressed in the following reaction, Fe 2 + + B OH 4 − ⇌ FeB OH 4 + is determined as 3.70 ± 0.10 (2σ) at 25 °C based on our long-term solubility experiments up to 2121 days on Fe(OH)2(cr) in the presence of borate ranging from 0.01 to 0.03 mol·kg−1. In our solubility measurements, the equilibrium was attained from the direction of supersaturation. In the experimental design, we used Fe2(OH)3Cl (pure iron end member of hibbingite) as the starting material. When Fe2(OH)3Cl was in contact with solutions without background concentrations of chloride, it was completely converted to Fe(OH)2(cr). The stability constant of FeB(OH)4+ is expected to find applications in many areas of study. For instance, FeB(OH)4+ may have played an important role in transport of ferrous iron in primitive oceans at the surface of the Earth. In the near-field of geological repositories, the formation of FeB(OH)4+ can sequestrate soluble borate, lowering borate concentrations available to the formation of the Am(III)-borate aqueous complex.