Mobilization and isotope fractionation of uranium, copper and iron in the environment - implications for (bio)remediation of contaminated sites and mine tailings

Abstract

The change of the oxidation state of redox-sensitive elements is frequently associated with a change in solubility and mobility. Understanding these mobilization processes and developing monitoring tools is crucial to limit environmental pollution in the future and may even support the identification of prospection-worth mineralization. Specifically, this thesis explores the use of the isotope systems of copper (Cu), iron (Fe), and uranium (U) as potential tools for tracing environmental processes related to the impact of mine tailings and (bio)remediated sites. In the first part, sulfidic mine tailings were investigated since they have a high potential for contamination of the environment by releasing acid mine drainage. An optimized and tested sequential extraction method of Cu and Fe isotopes was applied to samples from two depth profiles of porphyry copper mine tailings from the Atacama Desert (Chile). Iron isotope fractionation was traced to oxidative sulfide weathering and secondary enrichment like (re-) precipitation. The Cu isotope signature was interpreted to result from capillary Cu rising due to arid climate conditions, and/or, by the preferential adsorption of heavy Cu isotopes to the surface of Fe(hydr-)oxides. These findings showed that the composition of Cu and Fe isotopes can be used to trace mobilization processes and secondary element enrichment. In the second part, U isotope fractionation as a monitoring tool for the long-term stability of non-crystalline U(IV) - an important U host in sedimentary U deposits and the dominant product of (bio)remediation - was examined. Laboratory anoxic complexation experiments were performed in which the ligands EDTA, citrate, and bicarbonate were found to preferentially mobilize heavy 238U. These findings demonstrated that heavy U isotope signatures are not necessarily the result of U reduction but may also be generated during U mobilization. They may potentially be used to distinguish between anoxic ligand complexation and oxidative U mobilization or U adsorption to oxides. In the last part, oxidative mobilization (biotic and abiotic) of non-crystalline U(IV) was explored, which might also affect the U isotope composition. No significant U isotope fractionation was observed during oxidation with both Fe(III) or Acidithiobacillus ferrooxidans. Isotope fractionation during all involved reactions was thus either very small or different isotope effects cancelled each other out, implying that oxidative mobilization can be neglected in the interpretation of natural and anthropogenic U isotopic signatures. In conclusion, the main findings of this thesis are (1) sequential extraction in combination with Cu and Fe isotope analysis can trace mobilization and enrichment processes in mine tailings and (2) a differentiation between oxic mobilization and abiotic complexation in the subsurface is trackable using U isotope systematics, which helps to assess the long-term stability of non-crystalline U(IV) after in-situ leaching or (bio)remediation.