Isotopic fractionations associated with hexavalent chromium cycling in the natural subsurface

Hexavalent chromium, Cr(VI), is a common groundwater contaminant worldwide that can have adverse health effects and impact our groundwater quality. In conjunction with changes in concentration, the variation in isotopic ratios of dissolved Cr(VI) is used to more accurately quantify reactions in the subsurface and underlying hydrogeochemical processes. However, the complexities associated with Cr mobilization through weathering processes and their unknown isotopic signatures significantly challenge the interpretation of field data and limit the use of Cr stable isotope methods. The work presented here seeks to improve our understanding of Cr cycling in the subsurface and identify the processes that can impact the Cr(VI) isotopic ratios. To quantify the isotopic signatures associated with Cr cycling in the natural subsurface, I conducted reduction, adsorption and oxidation experiments and measured the isotopic fractionation associated with each one. While reduction of Cr(VI) and its isotopic fractionation is well studied, reduction with sulfur-bearing species had not been explored. I conducted reduction experiments with hydrogen sulfide at circumneutral pH. The result indicated that the isotopic fractionation induced by this reaction is pH dependent and will lead to isotopically heavy Cr(VI) remaining in the solution with ε reaching up to - 3.0 ±0.1‰ (ε ≈ δproduct flux - δreactant). A major pathway for mobilization of Cr and generation of naturally occurring Cr(VI) contamination is oxidation of Cr(III) bearing solids by manganese oxides. However, the Cr isotopic shifts induced by this reaction are poorly understood. Oxidation experiments conducted with three different Cr(III)-bearing solids (Cr(OH)3, CrFe(OH)3 and chromite) revealed a lack of persistent isotopic offset between the oxidized Cr(VI) and the Cr-bearing solids. Although I have observed ephemeral, significant isotopic fractionation, this fractionation is not sustained through time and its magnitude is too small to explain the range of δ53Cr values reported for geogenic Cr(VI). Another reaction involved in Cr cycle in the subsurface is adsorption of Cr(VI) on manganese oxides. Cr isotopic fractionation during Cr(VI) adsorption has been assumed to be negligible and, therefore, has not been considered during the interpretation of isotopic data in environmental studies. I conducted Cr(VI) adsorption experiments with pyrolusite at varying pH conditions. The results indicate that adsorption of Cr(VI) on manganese oxide surfaces can lead to isotopically heavy Cr(VI) in the aqueous phase. These experiments suggest that both outer-sphere and inner-sphere mechanisms are involved during adsorption of Cr(VI) on pyrolusite. However, isotopic fractionation is related only to the inner-sphere complexes, which form much more slowly. My findings challenge the existing belief that reduction is the only possible pathway for decreasing the concentration of Cr(VI) that is accompanied by large isotopic fractionations. In summary, the results of these studies advance our understanding of fundamental processes that impact Cr isotope ratios. Filling these knowledge gaps improves our ability to detect Cr(VI) reduction and to fingerprint naturally produced Cr(VI) using Cr isotopes.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Geology

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2024

Author dc:creator

• Gholizadeh Ansari, Mahta

Contributors dc:contributor

• Johnson, Thomas Martin
• Lundstrom, Craig
• Sanford, Robert
• McClain, Cynthia

• Chromium
• Isotopes
• Redox

Statement dc:rights

• Copyright 2024 Mahta Gholizadeh Ansari

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/124682

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F124682