Annette Summers Engel
Donald and Florence Jones Professor of Aqueous Geochemistry
University of Tennessee at Knoxville
Department of Earth and Planetary Sciences
Engel Lab Research -
Speciation and Behavior of Sulfur Gases in Biogeochemical Processes
Our ability to reconstruct the past sulfur cycle is hampered by our limited understanding of the nature and behavior of sulfur gases, primarily because it is difficult to measure gas movement into, out of, or through sedimentary systems. To overcome this problem, we produced the first sulfur K-edge X-ray Absorption Near-Edge Structure (XANES) spectra for gases that were evacuated and evolved from simple sulfur-bearing solids (e.g., elemental sulfur, pyrite, gypsum, and other solids) that were incrementally heated to ~400 oC. XANES is one of the most efficient tools to study sulfur speciation in biogeochemical systems. Previously, there were few examples of sulfur gas XANES spectra. The research is done at the LSU Center for Advanced Microstructures and Devices (CAMD) facility.
This new gas-phase XANES technique, which incorporates a furnace or heating pipe on the beamline, expands our ability to evaluate sulfur speciation in all phases. We are also able to study condensed materials formed as a product of heating, gas generation, and cooling. From the condensate we are able to link the volatile and solid phases through diagenetic processes. We work to address three main research questions: (1) What sulfur speciation changes occur when sulfur-bearing inorganic and organic solids are heated? (2) Does the composition of the precursor sulfur-phase (e.g., mineralogy, organic matter sulfur content) influence the type of condensed sulfur phase(s) in a system? (3) Can the signature of microbial processes (e.g., isotopic) be differentiated from low-temperature geological (e.g., diagenetic) processes?  So far, gas compositions from the solids were distinct, and gas species compositions changed when solids were heated to different maximum temperatures. The results provide strong evidence that sulphur gases, as well as solids, change compositionally during diagenesis, even at low (<100o C) temperatures.
X-ray absorption spectra are not isotope sensitive, so in combination with quadrupole mass spectrometry, which we interface with the furnace on the beamline, and conventional mass spectrometry, we are able to determine the isotopic masses of gases and changes in the isotopic composition of solids before and after experiments. After cooling, condensate XANES spectra show different sulfur speies than precursor solids. Isotopic values of delta-34S from condensates also shift (become lighter) compared to starting materials, depending on the maximum temperature applied during an experiment. Our study reveals dynamic processes between volatile and condensed sulfur compounds and may resolve misleading isotopic records in sedimentary rocks.   
Previous Support: National Science Foundation (EAR-0844364) 2009-2013
Project Collaborators:
Dr. F. Josef Hormes, Dr. G. Lisa Bovenkamp, Dr. Alexander Prange (Niederrhein University), Louisiana State University, Center for Advanced Microstructures and Devices (CAMD)
Dr. Yongbo Peng, Louisiana StateUniversity

Related Publications and Conference Abstracts from Group

Engel, A.S., Bovenkamp, G.L., Prange, A., Hormes, F.J. (2014) In situ speciation of sulfur vapors using X-ray absorption near edge structure spectroscopy. Chemical Geology. 380: 1-6. DOI: 10.1016/j.chemgeo.2014.04.019.
Engel, A.S., Prange, A., Bovenkamp, G.L., Peng, Y., Hormes, F.J. Isotopic and compositional characterization of sulphur gases and condensates formed after heating sulphur-containing solids: Implications for sedimentary records. Goldschmidt, June 8-13, 2014. Sacramento, CA. (oral presentation)
Paterson, A.T., Engel, A.S. (2011) Patterns of sulfur speciation from X-ray absorption spectroscopy relate to the distribution of functional genes in microbial mats from Lower Kane Cave, Wyoming (USA). International Symposium for Environmental Biogeochemistry, Istanbul, Turkey, Sept. 25-30, 2011.
Prange, A., Engel, A.S., Hormes, F.J. (2009) XANES spectroscopy of sulfur in the gas phase generated from elemental sulfur and pyrite. 14th International Conference on X-ray Absorption Fine Structure (XAFS14), Camerino, Italy, July 26-31. (poster presentation)
Engel, A.S., Lichtenberg, H., Prange, A., Hormes, J. (2007) Speciation of sulfur from naturally-occurring, filamentous microbial mats from sulfidic cave springs using x-ray absorption near edge spectroscopy. FEMS Microbiology Letters. 269(1): 54-62. DOI:10.1111/j.1574-6968.2006.00600.x.
Engel, A.S., Prange, A., Lichtenberg, H., Hormes, J. (2007) Understanding natural sulfur biosignatures from sulfidic spring microbial mats based on XANES spectroscopy, microbial diversity, and stable isotope systematics. Geological Society of America Abstracts with Programs, 39: 39. (Invited oral presentation).
Engel, A.S., Lichtenberg, H., Prange, A., Hormes, J. (2006) Sulfur speciation in naturally occurring filamentous microbial mats from sulfidic springs: A XANES spectroscopy study. International Symposium on Microbial Sulfur Metabolism, Münster, Germany. Conference Book, p. 134-135. (poster presentation)

"Field" Photographs taken at CAMD

Geochemistry. Geomicrobiology. Geology. Ecology.

Interdisciplinary Research at the University of Tennessee