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Precise determination of equilibrium sulfur isotope effects during volatilization and deprotonation of dissolved H_2S

Sim, Min Sub and Sessions, Alex L. and Orphan, Victoria J. and Adkins, Jess F. (2019) Precise determination of equilibrium sulfur isotope effects during volatilization and deprotonation of dissolved H_2S. Geochimica et Cosmochimica Acta, 248 . pp. 242-251. ISSN 0016-7037. http://resolver.caltech.edu/CaltechAUTHORS:20190114-132114245

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Abstract

Sulfide (H_2S, HS^−, and S^(2−)) is ubiquitous in marine porewaters as a result of microbial sulfate reduction, constituting the reductive end of the biogeochemical sulfur cycle. Stable isotopes have been widely used to constrain the sulfur cycle, because the redox transformations of sulfur compounds, such as microbial sulfate reduction, often exhibit sizable kinetic isotope effects. In contrast to sulfate ion (SO_4^(2−)), the most abundant form of dissolved sulfur in seawater, H2S is volatile and also deprotonated at near neutral pH. Equilibrium isotope partitioning between sulfide species can therefore overlap with kinetic isotope effects during reactions involving sulfide as either reactant or intermediate. Previous experimental attempts to measure equilibrium fractionation between H_2S and HS− have reached differing results, likely due to solutions of widely varying ionic strength. In this study, we measured the sulfur isotope fractionation between total dissolved sulfide and gaseous H2S at 20.6 ± 0.5 °C over the pH range from 2 to 8, and calculated the equilibrium isotope effects associated with deprotonation of dissolved H_2S. By using dilute solutions of Na2S, made possible by the improved sensitivity of mass spectrometric techniques, uncertainty in the first dissociation constant of H2S due to ionic strength could be better controlled. This in turn allowed us to close sulfur isotope mass balance for our experiments and increase the accuracy of the estimated fractionation factor. At equilibrium, aqueous H2S was enriched in ^(34)S by 0.7‰ and 3.1‰ relative to gaseous H_2S and aqueous HS−, respectively. The estimated fractionation between aqueous H_2S and HS^− lies between two earlier experimental reports, but agrees within the uncertainty of the measurements with a recent theoretical calculation.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1016/j.gca.2019.01.016DOIArticle
ORCID:
AuthorORCID
Sessions, Alex L.0000-0001-6120-2763
Orphan, Victoria J.0000-0002-5374-6178
Alternate Title:Precise determination of equilibrium sulfur isotope effects during volatilization and deprotonation of dissolved H2S
Additional Information:© 2019 Published by Elsevier Ltd. Received 16 June 2018, Revised 29 December 2018, Accepted 9 January 2019, Available online 14 January 2019. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No.2018R1D1A1B07050970) and an Agouron Geobiology Fellowship to MSS, Gordon and Betty Moore Foundation Grant GBMF 3306 to VJO and ALS, NSF award OCE-1436566 to ALS and NSF award OCE-1340174 to JFA. The authors are grateful to Guillaume Paris for assistance for isotope analysis. We also thank Daniel Eldridge, Boswell Wing, and an anonymous reviewer for constructive comments on an earlier version of this manuscript.
Funders:
Funding AgencyGrant Number
National Research Foundation of KoreaUNSPECIFIED
Ministry of Education (Korea)2018R1D1A1B07050970
Agouron InstituteUNSPECIFIED
Gordon and Betty Moore FoundationGBMF 3306
NSFOCE-1436566
NSFOCE-1340174
Subject Keywords:Sulfur isotopes; Equilibrium; Isotope effect; Hydrogen sulfide; Bisulfide
Record Number:CaltechAUTHORS:20190114-132114245
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20190114-132114245
Official Citation:Min Sub Sim, Alex L. Sessions, Victoria J. Orphan, Jess F. Adkins, Precise determination of equilibrium sulfur isotope effects during volatilization and deprotonation of dissolved H2S, Geochimica et Cosmochimica Acta, Volume 248, 2019, Pages 242-251, ISSN 0016-7037, https://doi.org/10.1016/j.gca.2019.01.016. (http://www.sciencedirect.com/science/article/pii/S0016703719300298)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:92252
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:22 Jan 2019 20:17
Last Modified:29 Jan 2019 16:30

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