Quantifying Fenton reaction pathways driven by self-generated H2O2 on pyrite surfaces.

Oxidation of pyrite (FeS2) plays a significant role in the redox cycling of iron and sulfur on Earth and is the primary cause of acid mine drainage (AMD). It has been established that this process involves multi-step electron-transfer reactions between surface defects and adsorbed O2 and H2O, releasing sulfoxy species (e.g., S2O32-, SO42-) and ferrous iron (Fe2+) to the solution and also producing intermediate by-products, such as hydrogen peroxide (H2O2) and other reactive oxygen species (ROS), however, our understanding of the kinetics of these transient species is still limited. We investigated the kinetics of H2O2 formation in aqueous suspensions of FeS2 microparticles by monitoring, in real time, the H2O2 and dissolved O2 concentration under oxic and anoxic conditions using amperometric microsensors. Additional spectroscopic and structural analyses were done to track the dependencies between the process of FeS2 dissolution and the degradation of H2O2 through the Fenton reaction. Based on our experimental results, we built a kinetic model which explains the observed trend of H2O2, showing that FeS2 dissolution can act as a natural Fenton reagent, influencing the oxidation of third-party species during the long term evolution of geochemical systems, even in oxygen-limited environments.

Amorphous calcium carbonate biomineralization in the earthworm's calciferous gland: pathways to the formation of crystalline phases.

In this study, we investigated the microstructural transformations that take place during carbonate formation in the earthworm's calciferous gland by analysing the evolution from the precursor fluid of the solid phases (spherulites) to the final carbonate concretions released by the gland. Results from HREM and electron diffraction showed that the spherulithic deposits merely consisted of ACC partially transformed to vaterite. Furthermore, comparisons of the diffraction spectra and microstructural analyses allowed the identification of the transition sequences to more stable carbonates. And thus, transformations of ACC to calcite were observed on the surfaces of these amorphous globular aggregates as their smooth characteristic surface became rougher with time. This transition path was not unique, and the presence of aragonite, as an intermediate phase, has also been found. In this particular case, the transition process followed a completely different pathway with the crystallization starting in the centre of the sphere and progressively extending to the periphery, leading to the formation of radial aggregates. In situ experiments performed on the freshly extracted precursor fluid and analysed by FT-IR spectroscopy showed that ACC is the main constituent and is probably stabilised by macromolecules such as proteins and sugars. Furthermore, the Debye-Scherrer diffraction experiments showed that the carbonate phase present in this fluid remains stable as ACC for more than a week. All these features are indicative of this entire process being biologically controlled by the earthworms. The analysis of the amorphous structure factor of this ACC indicates that these transformations are preceded by short-range order modifications of the amorphous precursor phase.