Fractionation of Cu and Fe isotopes in metal-rich mine sites biotic and abiotic processes

Abstract: After mineral exploitation the residual grinded and milled material, rich in sulphide minerals and heavy metals, is often left exposed to the atmospheric variables. This weathered mine waste material can lead to the formation of acid mine drainage (AMD) which has negative effects to the environment. The fractionation of stable isotope of metals such as Cu and Fe can be measured using innovative analytical techniques developed recently and could offer a detailed hindsight of the geochemical processes occurring in mine contaminated sites. Tailings profiles from Northern Sweden with high content of Cu and Fe sulphides and in different stages of weathering and/or remediation, along with plant and soil samples from a phytoremediation test site in Ronneburg, Germany were analysed using MC-ICP-MS to measure the isotope ratios of 65Cu/63Cu and 56Fe/54Fe. The analytical method used requires anion exchange chromatography to extract Cu and Fe from a complex matrix prior to the proper isotope ratio measurement. The samples from the tailings profile were useful to interpret the geochemical processes that can lead to a fractionation of Cu and Fe in the field, since redox-driven reactions such as rock oxidation and mineral precipitation are present in such environment. This study shows that precipitation of covellite in a redox-boundary zone in a mine tailings can cause a clear fractionation of Cu (Δ65Curock-covellite= -5.66±0.05‰) and a depletion of the lighter Cu isotope in the oxidised areas of the tailings due to dissolution of the remaining Cu-sulphides. Precipitation of Fe(oxy)hydroxides as a result of the oxidation process of sulphide-bearing rocks can also fractionate Fe, being the precipitated mineral slightly enriched in 56Fe.The influence of soil bacteria and plant uptake in the fractionation of Cu and Fe was investigated in pot and field experiments at the Ronneburg site, where organic amendments were used. The results showed that the plant material was enriched in the lighter Fe isotope compared to the substrate used in the pot and field experiments, in spite of the application of a bacterial consortium. Cu isotope fractionation is more susceptible to the changes in the amendments used, being those bacterial consortium, mychorriza or compost than Fe isotope fractionation. There are differences in the fractionation values in pot and field trials, regardless of the type of organic amendment applied. As an overall view, leaves are enriched in the heavier Cu isotope compared to the soils, regardless of the amendment usedThe application of the results obtained in this work would help not only to offer a view in the cycle of Fe and Cu in the surface environment, and the understanding of the (bio)geochemical processes occurring in sulphide soil surfaces. But also in the way that current remediation techniques of metal contaminated sites could be evaluated, having in mind that simplified systems show a different Cu and Fe fractionation compared to natural systems where more variables are needed to take into account.