This finding is based on advanced spectroscopic and microscopic analyses that reveal the immobilization of rhenium as single atoms or small polynuclear clusters on the surface of magnetite nanoparticles.

It is supported by spectro-microscopic investigations conducted at the LPS Orsay, ESRF Grenoble, ALS Berkeley and IMMM, Le Mans within a METSA and a CNRS-MITI projects. The results of this study will be published in the journal Science Advances on May 16, 2025.

The results indicate an abiotic transition pathway in which aqueous Fe2+ in the presence of pure or pre-oxidized magnetite (Fe3O4) serves as an electron source to reduce very mobile Re(VII) anion to individual Re(IV) atoms or small polynuclear species immobilized on the particle surface. Furthermore, these analyses provide a fundamental understanding of redox processes governing the fate of electrons in natural batteries and that of Re and its transport in the environment.

The new results from this study provide insights into natural battery discharge and recharge processes that control the fate of redox active critical or radioactive elements – but potentially also carbon - during Earth's paleoenvironments, and also today in recovery processes essential in the circular economy to recycle these critical elements. This discovery, which focuses on the geochemical analyses of nano-magnetite conducted by ISTerre scientists, was complemented by spectro-microscopic techniques conducted nationally and worldwide.

The new findings demonstrate how critical (or very toxic) elements, which are very mobile entities, remaining in the ocean for thousands of years, can be trapped in a very dynamic way at the contact with naturally recharged batteries. The battery is consequently slowly discharged by electrons travelling from the core of the nano-battery to its periphery. This interpretation is significant because a considerable portion of the scientific community believes that the discharge of magnetite occurs in a centripetal mode, iron oxidation occurring first on the surface before being propagated inward, a process that stops the electron transfer by creating an insulating layer.

In contrast, the new results show that Fe2+ recharged pre-oxidized magnetite nanoparticles exhibit a maghemite core and a magnetite shell, challenging the traditional core-shell magnetite-maghemite model. The electrons thus travel through the non-discharged conductive part of the magnetite battery to its inner discharged part. This type of electron dynamic activity also appears to be more conducive to the emergence of life on Earth than the previously imagined static configuration for the Hadean.