2007 GSA Denver Annual Meeting (2831 October 2007)
Paper No. 63-9
Presentation Time: 10:15 AM-10:30 AM

FRACTIONATION OF CU AND ZN ISOTOPES DURING ADSORPTION ONTO AMORPHOUS FE(III) OXIDES: EXPERIMENTAL MIXING OF ACID-ROCK DRAINAGE AND AMBIENT RIVER WATER

BALISTRIERI, Laurie S., US Geological Survey, University of Washington, School of Oceanography, Box 355351, Seattle, WA 98195, balistri@usgs.gov, BORROK, David M., School of Geosciences, University of Louisiana at Lafayette, Lafayette, LA 70504, WANTY, Richard B., US Geological Survey, PO Box 25046, MS 964 Denver Federal Center, Denver, CO 80225, and RIDLEY, W. Ian, US Geological Survey, PO Box 25046, MS 973 Denver Federal Center, Denver, CO 80225

Fractionation of Cu and Zn isotopes during adsorption onto amorphous ferric oxides is examined in experimental mixtures of metal-rich acid-rock drainage and relatively pure river water, and during batch adsorption experiments using synthetic ferrihydrite in complex synthetic acid-rock drainage and simple NaNO3 solutions. Metal adsorption as a function of pH is well described using a diffuse double layer model. Isotopic measurements of dissolved Cu (65Cu/63Cu) and Zn (66Zn/64Zn) show systematic changes that indicate significant fractionation of the stable metal isotopes during sorption. The isotopic data are best described by a closed system, equilibrium exchange model. The fractionation factors (αsoln solid) are 0.99927 + 0.00008 for Cu and 0.99948 + 0.00004 for Zn or, alternately, the separation factors (Δsoln-solid) are -0.73 + 0.08 for Cu and -0.52 + 0.04 for Zn. These results indicate that the heavier isotope preferentially adsorbs onto the oxide surface, which is consistent with shorter metal-oxygen bonds and lower coordination number for the metal at the surface relative to the aqueous ion. Fractionation of Cu isotopes also is greater than that for Zn isotopes. Limited isotopic data for adsorption of Cu, Fe(II), and Zn onto amorphous Fe(III) oxide from this and other studies suggest that isotopic fractionation is related to the intrinsic equilibrium constants that define the strength of aqueous metal interactions with oxide surface sites. Greater isotopic fractionation occurs with stronger metal binding by amorphous Fe oxides with Cu > Zn > Fe(II). This process-based understanding of stable metal isotopic fractionation can be used to successfully design field programs, interpret field results, and improve our understanding of the transport and fate of elements in the environment.

2007 GSA Denver Annual Meeting (2831 October 2007)
General Information for this Meeting
Session No. 63
Sources, Transport, Fate, and Toxicology of Trace Elements in the Environment II
Colorado Convention Center: 503
8:00 AM-12:00 PM, Monday, 29 October 2007

Geological Society of America Abstracts with Programs, Vol. 39, No. 6, p. 177

© Copyright 2007 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.