Isotopic fractionation of copper and zinc during biogeochemical processes in soils

Biogeochemical cycles of Cu and Zn are deeply influenced by anthropogenic activities, modifying their speciation, bioavailability and mobility. Understanding these cycles is then essential for risk evaluation of these anthropogenic inputs in the environment. Isotopic measurements allow us to better constrain pollution sources of metals and processes modifying their speciation. It is then necessary to determine the isotopic compositions of Cu and Zn sources in the environment and the main processes affecting them. Isotopic fractionation of Cu and Zn was assessed for two main processes: complexation with organic matter (OM) and plant uptake. First, Zn complexation with OM increases with pH. Below pH 6, adsorption of Zn on OM resulted in no measurable isotopic fractionation, while at higher pH, Zn bound to OM is heavier (Δ⁶⁶Zn 0.24 ± 0.06 %o) than free Zn in solution. It is explained by higher -complexation constants and shorter bond lengths for Zn-OM complexes compared to the aqueous free Zn. During hydroponic experiments, Cu and Zn exhibit significant isotopic fractionation but different behavior during uptake by plants. This difference between Cu and Zn during root uptake might be due to different speciation in the solution, reduction of Cu before uptake or difference in uptake strategy. Similar translocation behaviour suggests the implication of comparable internal transporter used by plants to mobilize Cu and Zn. During plant cultivation in soils, Cu isotopic fractionation in root of plants seems more dependent of the Cu speciation in the soils. (author)