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Effect of temperature and mass transport on transition metal isotope fractionation during electroplating

Black, Jay R. and John, Seth and Young, Edward D. and Kavner, Abby (2010) Effect of temperature and mass transport on transition metal isotope fractionation during electroplating. Geochimica et Cosmochimica Acta, 74 (18). pp. 5187-5201. ISSN 0016-7037. http://resolver.caltech.edu/CaltechAUTHORS:20100907-100507367

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Abstract

Transition metal stable isotope signatures can be useful for tracing both natural and anthropogenic signals in the environment, but only if the mechanisms responsible for fractionation are understood. To investigate isotope fractionations due to electrochemistry (or redox processes), we examine the stable isotope behavior of iron and zinc during the reduction reaction M^(2+)_(aqueous) + 2e^− = M_(metal) as a function of electrochemical driving force, temperature, and time. In all cases light isotopes are preferentially electroplated, following a mass-dependent law. Generally, the extent of fractionation is larger for higher temperatures and lower driving forces, and is roughly insensitive to amount of charge delivered. The maximum fractionations are δ^(56/54)Fe = −4.0‰ and δ^(66/64)Zn = −5.5‰, larger than observed fractionations in the natural environment and larger than those predicted due to changes in speciation. All the observed fractionation trends are interpreted in terms of three distinct processes that occur during an electrochemical reaction: mass transport to the electrode, chemical speciation changes adjacent to the electrode, and electron transfer at the electrode. We show that a large isotope effect adjacent the electrode surface arises from the charge-transfer kinetics, but this effect is attenuated in cases where diffusion of ions to the electrode surface becomes the rate-limiting step. Thus while a general increase in fractionation is observed with increasing temperature, this appears to be a result of thermally enhanced mass transport to the reacting interface rather than an isotope effect associated with the charge-transfer kinetics. This study demonstrates that laboratory experiments can successfully distinguish isotopic signatures arising from mass transport, chemical speciation, and electron transfer. Understanding how these processes fractionate metal isotopes under laboratory conditions is the first step towards discovering what role these processes play in fractionating metal isotopes in natural systems.


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http://dx.doi.org/10.1016/j.gca.2010.05.014 DOIUNSPECIFIED
Additional Information:© 2010 Elsevier. Received 14 July 2009; accepted 4 May 2010. Associate editor: James Farquhar. Available online 23 July 2010. We thank Professor Edwin Schauble for his input and discussion of equilibrium isotope effects, Professor Jess Adkins for access to facilities at Caltech and Drs. Eric Tonui and Dr. Karen Zeigler for technical support. This work was funded by NASA Exobiology NNG05GQ92G (A.K.).
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NASANNG05GQ92G
Record Number:CaltechAUTHORS:20100907-100507367
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20100907-100507367
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:19795
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:08 Sep 2010 17:13
Last Modified:26 Dec 2012 12:23

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