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 (FeS_aq) clusters as a major fraction of the soluble, reduced iron and sulfur pool. FeS_aq 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. FeS_aq 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 FeS_aq 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 FeS_aq clusters and promoting solubility. Dynamic light scattering revealed a concentration-dependent increase in the kinetics of FeS_aq nucleation and cluster aggregation. Increasing temperature promoted FeS_aq cluster nucleation and aggregation while also enhancing dissolution. Alkaline pH also promoted FeS_aq nucleation and cluster aggregation. At 25 °C, pH 7.0, and at reactant concentrations of 30 µM, FeS_aq 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 FeS_aq nucleation and cluster growth that, in turn, influence their bioavailability in anaerobic ecosystems.