PD Dr. Rossitza Pentcheva
Ludwig-Maximilians-University of Munich
Fakultät für Geowissenschaften - Geo- und Umweltwissenschaft - Kristallographie

Prof. Dr. Gordon Brown jr.
Stanford University
School of Earth Sciences and Photon Science Dept.

Toxic heavy metals such as mercury and metalloids such as arsenic and selenium are causing widespread environmental problems because of their ubiquity in many environmental settings. A major technological challenge is the efficient capture and removal of these toxic elements (in various chemical forms) from flue gases, fly ash, mine wastes, and polluted sediments. A combined experimental and theoretical approach will be employed to gain understanding in the sorption interactions between selenium and arsenic species and mineral surfaces and nanoparticles dispersed in carbon nanotubes at the atomic scale. The study involves synchrotron-based x-ray absorption and photoemission spectroscopic studies of the chemical speciation of adsorbed mercury/arsenic on solid surfaces. Parallel density functional theory calculations on these systems shall provide information on reaction mechanisms, including reaction intermediates that are often difficult to observe spectroscopically but control reaction kinetics.

Final report:

A variety of technological and environmentally relevant processes on mineral surfaces occur at the interface to water. Therefore, a central topic of the collaborative project funded in the period 07/2010-12/2011 was the investigation of water adsorption at the Fe₃O₄(001) surface in a combined ambient-pressure X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) study. Good agreement was obtained between the experimental and calculated surface core-level shifts for different species (H₂O, OH groups), revealing a dominating contribution of final state effects. [J. Phys. Chem. C 117, 2719 (2013)]. A major result of this study was the evidence for a crossover from a dissociative adsorption of water at defect sites at low pressures to a mixed adsorption mode at higher surface coverages, indicating cooperative effects. We plan to extend this study to other adsorbates such as CO on iron oxide surfaces.

Another investigation that profited from the intensive discussions and scientific exchange between the project partners was a study of water and arsenate adsorption on the most prominent surfaces of the FeOOH polymorphs (funded by BMBF Geotechnologies). Here the relative stability of adsorbate configuration as a function of surface-site coordination was identified, shedding light on the mechanisms of binding of contaminants on mineral surfaces [Surf. Science 606, 1623 (2012), J. Phys. Chem. C 117, 15571 (2013)].

A further collaboration that emerged from this funding is with Prof. Wendy Mao and Dr. Arianna Gleason (Stanford), as well as Carmen E. Quiroga (PhD student, LMU). In a joint experimental and theoretical project we explored the role of hydrogen bond symmetrisation in the spin-transition in ε-FeOOH under pressure [Earth and Planetary Science Lett. 379, 49 (2013)].

T. Kendelewicz, S. Kaya, J. E. Newberg, H. Bluhm, N. Mulakaluri, W. Moritz, M. Scheffler, A. Nilsson, R. Pentcheva and G. E. Brown, Jr. Photoemission and DFT Study of the Reaction of Water Vapor with the Fe₃O₄(100)(v2xv2) R45° Surface at Near-Ambient Conditions, J. Phys. Chem. C 117, 2719 (2013).

A.E. Gleason, C.E. Quiroga, K. Otte, A. Sizuki, R. Pentcheva, W.L. Mao Symmetrization driven spin transition in ε-FeOOH at high pressures, Earth and Planetary Science Lett. 379, 49 (2013).

K. Otte, W. W. Schmahl, and R. Pentcheva, DFT+U Study of Arsenate Adsorption on FeOOH Surfaces: Evidence for Competing Binding Mechanism, J. Phys. Chem. C 117, 15571 (2013).

K. Otte, W. W. Schmahl, and R. Pentcheva Density functional theory study of water adsorption on FeOOH surfaces Surf. Science 606, 1623 (2012).