Copper isotope fractionation during complexation with insolubilized humic acid

The bioavailability, mobility, and toxicity of Cu depend on Cu speciation in solution. In natural systems like soils, sediments, lakes, and river waters, organo-Cu complexes are the dominating species. Organo-complexation of Cu may cause a fractionation of stable Cu isotopes. The knowledge of Cu isotope fractionation during sorption on humic acid may help to better understand Cu isotope fractionation in natural environments and thus facilitate the use of Cu stable isotope ratios (δ⁶⁵Cu) as tracer of the fate of Cu in the environment. We therefore studied Cu isotope fractionation during complexation with insolubilized humic acid (IHA) as a surrogate of humic acid in soil organic matter with the help of sorption experiments at pH 2-7. We used NICA-Donnan chemical speciation modeling to describe Cu binding on IHA and to estimate the influence of Cu binding to different functional groups on Cu isotope fractionation. The observed overall Cu isotope fractionation at equilibrium between the solution and IHA was Δ⁶⁵Cu _IHA-solution = 0.26 ± 0.11‰ (2SD). Modeled fractionations of Cu isotopes for low- (LAS) and high-affinity sites (HAS) were identical with Δ⁶⁵Cu_{LAS/HAS-solution} = 0.27. pH did not influence Cu isotope fractionation in the investigated pH range.