| NSF Org: |
EAR Division Of Earth Sciences |
| Recipient: |
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| Initial Amendment Date: | July 26, 2013 |
| Latest Amendment Date: | July 26, 2013 |
| Award Number: | 1324791 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Enriqueta Barrera
EAR Division Of Earth Sciences GEO Directorate for Geosciences |
| Start Date: | September 1, 2013 |
| End Date: | August 31, 2018 (Estimated) |
| Total Intended Award Amount: | $399,730.00 |
| Total Awarded Amount to Date: | $399,730.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
1608 4TH ST STE 201 BERKELEY CA US 94710-1749 (510)643-3891 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
307 McCone Hall Berkeley CA US 94720-4767 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | SURFACE EARTH PROCESS SECTION |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
(1) Technical Abstract
Light-driven redox interactions between semiconducting sulfide minerals and organic molecules may have initiated the abiotic carbon chemistry needed for the appearance of life on Earth. In particular, the tricarboxylic acid (TCA) cycle has been proposed as a primordial metabolism for emerging organisms. Testing this proposal requires knowledge of the products and mechanisms of candidate mineral-organic reactions. We will determine the rates, mechanism, and intermediate species in one light-driven reaction from the TCA cycle. Specifically, we will study the two-electron reduction of fumarate to succinate on colloidal zinc sulfide (ZnS) surfaces, using time-resolved transient optical and infrared absorption spectroscopy and Fourier transform electron paramagnetic resonance spectroscopy to probe reaction progress from the subnanosecond to the microsecond timescales. These complementary techniques will allow correlating the temporal evolution of the charge distribution in the semiconductor and the breaking and formation of bonds in the organic reactant. This research will contribute new constraints on the geochemical conditions of the early Earth that could have permitted the establishment of a prebiotic carbon cycle driven by solar energy.
(2) Broader significance and importance
The absorption of light by metal sulfide minerals can initiate photochemical reactions between the mineral surface and adsorbed organic molecules. This kind of semiconductor photochemistry must have been occurring under the conditions known to exist in the early Earth and it is speculated that they played an important role in synthesis of complex organic molecules required for the development of life. One particular reaction that transforms one small organic acid molecule, fumarate, into another, succinate, occurs readily on the surface of zinc sulfide (ZnS) particles under ultraviolet (UV) illumination. In this process, the surface transfers two electrons to fumarate, which also acquires two protons from water, to form two new carbon-hydrogen bonds. The precise pathway of the reaction, including the ordering of electron and proton transfer steps, is currently unknown. In particular, we hypothesize that the binding of fumarate to the ZnS surface stabilizes the intermediate chemical species and enables this relatively complex redox reaction to proceed without forming side products. In our proposed research, we plan on using time-resolved spectroscopic techniques to capture chemical signatures of the different intermediate species and thereby determine the reaction pathway. By subsequently investigating how the reaction pathway and rate changes in response to changing ZnS or solution chemistry, we expect to be able to determine the role of the sulfide surface in influencing this reaction. This work will put constraints on the geo chemical conditions of the early Earth that could have permitted solar-driven, abiotic organic reactions. This work has implications not only for understanding the origins of life, but also the development of materials for solar energy applications based upon Earth-abundant elements. This work also addresses fundamental science at the basis of mineral-biomolecule interactions, which may provide further insights into topics in medical geology.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Light-driven redox interactions between semiconducting sulfide minerals, metals and organic molecules may have initiated the abiotic carbon chemistry needed for the appearance of life on Earth. Graduate student David Mangiante completed a range of experimental studies that provided quantitative constraints on the feasibility of abiotic photoreduction of fumarate, a small organic acid, on zinc sulfide minerals under the likely geochemical conditions of the early Earth.
This research provided new understanding of the redox reactivity of photoexcited zinc sulfide (ZnS). The experimental data permitted us to construct a new model for the excited electronic states in ZnS that are characterized by different relaxation timescales. We showed that photoexcited ZnS can reduce adsorbed molecules from (1) hot electrons in the conduction band, (2) electrons that have relaxed to the bottom of the conduction band and (3) from an electron trap state the has a lifetime exceeding 20 ns. These findings provide a basis for understanding all environmental and technological uses of ZnS photocatalysis (Mangiante et al., 2019).
The research provided new understanding of how one-electron excitations in ZnS are able to drive two-electron organic redox reactions. The most reducing photoexcited states are the shortest lived and can only transfer an electron to surface-bound molecules. Longer-lived trap states are able to reduce unbound, diffusing molecules, but do not have the reducing power to transfer an electron to a negatively charge one-electron reduction product.
A secondary goal of the project was the development of time-resolved infrared spectroscopy to determine the mechanism of organic redox reactions. To establish this method we studied the photolysis of ferric oxalate, a sunlight driven chemical reaction that is responsible for carbon dioxide release from surface waters containing organic carbon. We resolved a long-standing debate as to the mechanism of this process (Mangiante et al., 2017).
Through a no-cost extension, we extended the work on ZnS to iron disulfide (pyrite; FeS2). In contrast to ZnS this mineral is electrically conductive and we were able to use ambient pressure X-ray photoelectron spectroscopy to characterize the surface electronic structure in the presence of water (Carrero-Romero et al 2018 and in preparation).
This project fully supported the graduate studies of Dr. David Mangiante, who successfully graduated in 2017 and partially supported the postdoctoral research studies of Dr. Sergio Carrero-Romero.
This Research Project produced the following Journal Articles and Conference Presentations
David Mangiante, Richard D. Schaller, Jillian F. Banfield and Benjamin Gilbert (2019). Pathways for the photoreduction of fumarate on ZnS. ACS Earth and Space Science. SUBMITTED
David. M. Mangiante, Richard D. Schaller, Piotr Zarzycki, Jillian F. Banfield, and Benjamin Gilbert (2017). Mechanism of Ferric Oxalate Photolysis. ACS Earth and Space Chemistry. 1 270.
Benjamin Gilbert, David. M. Mangiante, Richard D. Schaller, Piotr Zarzycki, Jillian F. Banfield (2018). Mechanism of ferric oxalate photolysis from ultrafast infrared spectroscopy. American Chemical Society Spring Meeting 2018. New Orleans.
Sergio Carrero Romero, Hendrik Bluhm Michael Whittaker, Benjamin Gilbert (2018). Molecular Characterization of the Hydrated Pyrite 001 Surface by Ambient Pressure XPS. Goldschmidt. Boston.
D. M. Mangiante, J. F. Banfield and B. Gilbert (2015). Pathways of Fumarate Photoreduction on ZnS Nanoparticles. Josep Comas i Solà International Summer School in Astrobiology, Santander, Spain. Santander, Spain.
Last Modified: 01/18/2019
Modified by: Jillian F Banfield
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