In situ structural study of the surface complexation of lead(II) on the chemically mechanically polished hematite (11¯02) surface

Canrong Qiu; Frantisek Majs; Peter J Eng; Joanne E Stubbs; Thomas A Douglas; Moritz Schmidt; Thomas P Trainor

doi:10.1016/j.jcis.2018.04.005

In situ structural study of the surface complexation of lead(II) on the chemically mechanically polished hematite (11¯02) surface

J Colloid Interface Sci. 2018 Aug 15:524:65-75. doi: 10.1016/j.jcis.2018.04.005. Epub 2018 Apr 3.

Authors

Canrong Qiu¹, Frantisek Majs², Peter J Eng³, Joanne E Stubbs³, Thomas A Douglas⁴, Moritz Schmidt⁵, Thomas P Trainor²

Affiliations

¹ Department of Chemistry and Biochemistry, University of Alaska Fairbanks, Fairbanks, AK, USA. Electronic address: c.qiu@hzdr.de.
² Department of Chemistry and Biochemistry, University of Alaska Fairbanks, Fairbanks, AK, USA.
³ Center for Advanced Radiation Sources, University of Chicago, Chicago, IL, USA.
⁴ U.S. Army Cold Regions Research and Engineering Laboratory, Fort Wainwright, AK, USA.
⁵ Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Dresden, Germany.

PMID: 29631220
DOI: 10.1016/j.jcis.2018.04.005

Abstract

A structural study of the surface complexation of Pb(II) on the (11¯02) surface of hematite was undertaken using crystal truncation rod (CTR) X-ray diffraction measurements under in situ conditions. The sorbed Pb was found to form inner sphere (IS) complexes at two types of edge-sharing sites on the half layer termination of the hematite (11¯02) surface. The best fit model contains Pb in distorted trigonal pyramids with an average PbO bond length of 2.27(4) Å and two characteristic Pb-Fe distances of 3.19(1) Å and 3.59(1) Å. In addition, a site coverage model was developed to simulate coverage as a function of sorbate-sorbate distance. The simulation results suggest a plausible Pb-Pb distance of 5.42 Å, which is slightly larger than the diameter of Pb's first hydration shell. This relates the best fit surface coverage of 0.59(4) Pb per unit cell at monolayer saturation to steric constraints as well as electrostatic repulsion imposed by the hydrated Pb complex. Based on the structural results we propose a stoichiometry of the surface complexation reaction of Pb(II) on the hematite (11¯02) surface and use bond valence analysis to assign the protonation schemes of surface oxygens. Surface reaction stoichiometry suggests that the proton release in the course of surface complexation occurs from the Pb-bound surface O atoms at pH 5.5.

Keywords: Crystal truncation rod; Hematite; Pb(II).