Professor Schoonen's research interests aim at understanding the interaction between water and rocks at surface and near-surface conditions. Schoonen's research group is currently working on three major projects: heterogeneous catalysis involving mineral surfaces; surface chemistry of iron sulfides; and the hydrogeochemistry of Long Island.

Iron Sulfides
Iron sulfides are found in nearly all reduced sedimentary environments and are the most common mineral in metal sulfide ores. In sedimentary environments as well as in ore-forming environments, iron sulfides scavenge other metals from solution. For example, much of the gold mined today is so-called "invisible" gold, which forms through gold sorption onto iron sulfides. Scavenging of heavy metals in sewage outfall, harbor sediments, wetlands, and tidal flats are examples that illustrate the environmental importance of iron sulfides. However, despite their economic and environmental importance, the formation and surface chemistry of iron sulfides are far from understood. Schoonen's research on iron sulfides focuses on understanding the mechanisms and kinetics of iron sulfide formation as well as the surface chemistry of iron sulfides. As an integral part of this research effort, Schoonen and his student Yong Xu are studying the stability and reactivity of metastable aqueous sulfur species, such as thiosulfate, which may play an important role in the formation of pyrite and marcasite, the two most abundant iron sulfides.

Heterogeneous Catalysis on Mineral Surfaces
While catalysis, or promotion of reactions, is extensively studied in the chemical community, geochemist are only now addressing the importance of catalysis in geochemical systems. For example, our group has recently shown how pyrite can catalyze a reaction between thiosulfate, S₂O₃ ^2-, and dissolved molecular oxygen. This reaction does not proceed at a significant rate in homogeneous systems, but is catalyzed by semiconducting metal-sulfide minerals, such as pyrite, sphalerite and galena. Subsequently, our group has demonstrated that this reaction occurs in Cinder Pool, Yellowstone National Park.

While our initial work in this area of research was limited to studying catalytic reactions involving metal sulfides, we are now exploring a wide array of semiconducting minerals, including anatase, rutile, ilmenite, and iron oxides. The applications range from decomposing solvents to understanding the role these minerals may have played on an early Earth.

Surface Chemistry of Iron Sulfides
Iron sulfides are found in nearly all reduced sedimentary environments and are the most common minerals in metal sulfide ores. In sedimentary environments as well as in ore-forming environments, iron sulfides scavenge other metals from solution. For example, much of the gold mined today is so-called "invisible" gold, which forms through gold sorption onto iron sulfides. Scavenging of heavy metals in sewage outfall, harbor sediments, wetlands, and tidal flats are examples that illustrate the environmental importance of iron sulfides. However, despite their economic and environmental importance, the formation and surface chemistry of iron sulfides are far from understood. Schoonen's research group and Dr. Daniel Strongin's group at Temple University have initiated a research project to understand the structure, chemical composition, and reactivity of the surface of pyrite, the most abundant iron sulfide mineral. In this DOE-supported project, modern surface science probes as well as low-temperature techniques are combined to study the surface. Complementary to this project student Joachim Bebie is investigating the interactions of various simple organic compounds with the pyrite surface. The purpose of this reaserch is to determine if pyrite could have concentrated compounds such as amino acids and carboxylic acids on an early Earth. This last project is supported by NASA-Exobiology.

Long Island Groundwater
Long Island provides an excellent natural laboratory for the study of hydrogeochemical processes. The Long Island aquifer system is perhaps the best-monitored and most studied coastal aquifer system in the world. As such it represents a unique opportunity to study hydrogeological processes relevant to most of the Atlantic coastal plain. Schoonen's groundwater research focuses mainly on the geochemistry of the Central Pine Barrens on eastern Long Island. This pine barrens forest represents an ecosystem that once dominated much of the Atlantic coastal plain. It is a unique ecosystem with coastal ponds and groundwater-fed rivers. In a collaborative effort, Professors Schoonen and Hanson and their students are studying the hydrogeology and cycling of major and minor elements in this ecosystem. Besides the studies in the Central Pine Barrens, Schoonen has supervised a number of research projects of the Long Island aquifer system by part-time students enrolled in the department's M.S. in Hydrology program.

Selected Publications 

Schoonen, M.A.A., Fisher, N.S., and Wente, M.A. (1992). Au sorption onto pyrite and goethite: a radiotracer study. Geochim. Cosmochim. Acta 55, 1,801-1,814.

Staudt, W., Oswald, E.J., and Schoonen, M.A.A. (1993). Na, Cl, SO₄ concentrations in dolomite: a new technique to constrain the composition of dolomitizing fluids. Chem. Geol. 107, 97-109.

Dekkers M.J. and Schoonen M.A.A. (1993) An electrokinetic study of pyrrhotite and greigite. Geochim. Cosmochim. Acta 58, 4147-4153.

Staudt W., Reeder R.J., Schoonen M.A.A. (1994) Surface structural control on compositional zoning of SO₄²⁺ and SeO₄²⁺ in synthetic calcite single crystals. Geochim. Cosmochim. Acta 58, 2087-2098.

Schoonen M.A.A. (1993) Calculation of the zero point of charge of metal oxides between 0 and 350C. Geochim. Cosmochim. Acta 58, 2845-2851.

Poulson S.R. and Schoonen M.A.A. (1994) Variations of the oxygen isotope fractionation between NaCO^3- and water due to the presence of NaCl at 100-300C. Chem Geolo. Isotope Geossci. Section. 116, 305-315.

Rakovan J., Schoonen M.A.A., Reeder R.J. (1995). Epitaxial overgrowths of marcasite on pyrite from the Tunnel and Reservoir Project, Chicago, Illinois, USA: Implications for marcasite growth. Geochim. Cosmochim. Acta 59, 343-346.

Staudt W and Schoonen M.A.A. (1994) Sulfate in sedimentary carbonates. ACS Symp. Series Vol 612 Geochemical Transformations of Sedimentary Sulfur, Vairavamurthy and Schoonen (eds), Chap. 26, 8p.

Rickard D.T, Schoonen M.A.A., Luther (1995) Chemistry of iron sulfides in sedimentary environments. ACS Symp. Series Vol 612 Geochemical Transformations of Sedimentary Sulfur, Vairavamurthy and Schoonen (eds), Chap 8, 26 pp.

Xu Y. and Schoonen M.A.A. (1995) The stability of thiosulfate in the presence of pyrite in low-temperature aqueous solutions. Geochim. Cosmochim. Acta. 59, 4605-4622.

Dekkers M.J., Schoonen M.A.A. (1996) Magnetic properties of hydrothermally synthesized greigite (Fe₃ S₄). 1. Rock magnetic parameters at room temperature. Geophys. J. Int. 126, 360-368.

Chaturvedi S., Katz R., Guevremont J., Schoonen M.A.A., Strongin D.R. (1996) XPS and LEED study of a naturally occurring single crystal of pyrite. Am. Min. 81, 261-265