Environ. Sci. Technol. 2010, 44, 5496–5502
Copper Isotope Fractionation during Complexation with Insolubilized Humic Acid M O R I T Z B I G A L K E , * ,†,§ S T E F A N W E Y E R , |,‡ AND WOLFGANG WILCKE§ Earth System Science Research Center, Geographic Institute, Johannes Gutenberg University Mainz, Johann-Joachim-Becher-Weg 21, 55128 Mainz, Germany, Institute of Geoscience, University of Frankfurt, Altenho¨ferallee 1, 60438 Frankfurt am Main, Germany, and Geographic Institute, University of Berne, Hallerstrasse 12, 3012 Berne, Switzerland
Received May 25, 2010. Accepted June 1, 2010.
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 ∆65CuIHA-solution ) 0.26 ( 0.11‰ (2SD). Modeled fractionations of Cu isotopes for low- (LAS) and high-affinity sites (HAS) were identical with ∆65CuLAS/HAS-solution ) 0.27. pH did not influence Cu isotope fractionation in the investigated pH range.
Introduction Bioavailability, mobility, and toxicity of Cu depend on Cu speciation in natural systems (1–3). In soils, sediments, and natural waters, at the interface between geosphere and biosphere, organo-Cu complexes are frequently the most important chemical Cu species strongly influencing the fate of Cu in the environment (4–6). Understanding stable isotope fractionation caused by the complexation of Cu by organic matter (OM) may help to interpret Cu isotope signatures in environmental samples and accordingly improve our knowledge about the environmental behavior of Cu. Currently, only a few published studies of Cu isotope ratios in natural biogeochemical systems are available. A first study of Cu isotope fractionation in soils pointed at the need for * Corresponding author phone: +41(0)316314055, e-mail:
[email protected]. † Johannes Gutenberg University Mainz. | Current address: Institute of Geology and Mineralogy, University of Cologne, Zu ¨ lpicher Strasse 49b, 50674 Ko¨ln, Germany. ‡ University of Frankfurt. § University of Berne. 5496
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detailed investigation of Cu isotope fractionation during Cu complexation by organic matter as a prerequisite for appropriate interpretation of Cu isotope ratios in soil (7). A study of Cu speciation in natural waters demonstrated that organo-complexation of Cu influences the partitioning of stable Cu isotopes between particulate and dissolved Cu species (8). These authors observed that particle-bound Cu was isotopically lighter compared to dissolved Cu species (
