Environmental controls on the kinetics of iron-sulfur cluster nucleation and nanoparticle formation

Abstract

Anoxic, sulfidic conditions have been prevalent since the early Proterozoic and favor aqueous iron-sulfur (FeSaq) clusters as a major fraction of the soluble, reduced iron and sulfur pool. FeSaq cluster formation and nucleation is driven by the high affinity between ferrous iron (Fe(II)) and sulfide (HS−), ultimately yielding particles that precipitate as iron sulfide minerals. FeSaq clusters were recently shown to be bioavailable sources of iron and sulfur for a variety of anaerobes, yet little is known of the factors that influence the kinetics of their formation and nucleation. Here we apply computational and spectroscopic approaches to investigate the dynamics of FeSaq nucleation, cluster growth, precipitation, and redissolution as a function of Fe(II)/HS− concentration, temperature, and pH. Experiments were conducted under excess HS− to mimic euxinic conditions common to contemporary anaerobic aquatic ecosystems and those of the Proterozoic. Density functional theory calculations reveal the key role of water oxygen-iron interactions in stabilizing small FeSaq clusters and promoting solubility. Dynamic light scattering revealed a concentration-dependent increase in the kinetics of FeSaq nucleation and cluster aggregation. Increasing temperature promoted FeSaq cluster nucleation and aggregation while also enhancing dissolution. Alkaline pH also promoted FeSaq nucleation and cluster aggregation. At 25 °C, pH 7.0, and at reactant concentrations of 30 µM, FeSaq clusters < 10 nm in diameter remained in solution for > 2 h. These results underscore the importance of temperature, pH, and reactant concentration in the kinetics of FeSaq nucleation and cluster growth that, in turn, influence their bioavailability in anaerobic ecosystems.