JOURNAL OF COLLOID AND INTERFACE SCIENCE 177, 561–567 (1996) ARTICLE NO. 0070 Adsorption of Cu(II) on the (0001) Plane of Mica: A REFLEXAFS and XPS Study MORAG L. FARQUHAR,* JOHN M. CHARNOCK,* , ²K ATHARINE E. R. ENGLAND,* AND DAVID J. VAUGHAN * ,1 * Department of Earth Sciences, University of Manchester, Manchester M13 9PL; and ² Daresbury Laboratory, Warrington, Cheshire WA4 4AD, United Kingdom Received December 28, 1994; accepted June 15, 1995 tion and desorption reactions that control their aqueous mo- Using reflection extended X-ray absorption fine structure spec- bility (1, 2). To improve modeling of the mobility of heavy troscopy (REFLEXAFS) and X-ray photoelectron spectroscopy metals in aqueous environments, it is vital that the funda- (XPS) it has been established that Cu(II) in low concentration in mental adsorption and desorption reactions in which they aqueous media can be chemisorbed onto the (0001) surface of are involved are fully characterized. However, much of the muscovite mica. From the XPS studies it is suggested that the Cu available data provide only indirect evidence that cannot species is in a similar bonding environment to the copper in copper unambiguously distinguish reaction mechanisms, or surface hydroxide. A depth profile of the reacted mica surface was also species which may include chemisorbed or physisorbed mo- carried out using XPS and this suggests that there had been no diffusion of the Cu into the mica surface. REFLEXAFS studies nonuclear or multinuclear complexes and precipitates. By of the reacted mica surface provided information about shells of using spectroscopic techniques, particularly X-ray absorp- O, Al/Si, and Cu surrounding a central Cu absorber. The best-fit tion spectroscopy (XAS), the local environment of an ele- bond distances are as follows: O–Cu, 1.98 A ˚ ; Cu–Cu, 2.64 A ˚ ; Al / ment can be characterized in terms of the type, number, and Si–Cu, 3.09 A ˚ . This provides direct evidence forCu being bound length of chemical bonds to near-neighbors. For example, to the surface at aluminate or silicate groups and suggests that recent studies on first-series transition metals have estab- the Cu species adsorbed onto the surface are similarto the copper lished that cobalt(II) forms small multinuclear complexes species found in copper hydroxide, plancheite, and shattuckite. It on kaolinite, rutile, and alumina surfaces at low adsorption is proposed that Cu is chemisorbed at atomic imperfections, such densities ( 3, 4 ) . Studies involving chromium( III ) adsorption as steps and kinks, on the mica surface.  1996 Academic Press, Inc. on silica have shown that chromium hydroxide-like com- Key Words: adsorption; REFLEXAFS; XPS; copper(II); mica; surface species; chemisorption. plexes form at low surface coverage, well before the precipi- tation point is reached, and that the extent of nucleation varies depending on the surface coverage (5). INTRODUCTION Another first-series transition metal of interest is copper ; it is a widely occurring pollutant associated with base metal The cycling and recycling of the elements at or near the mining operations worldwide. A commonly occurring phyl- Earth’s surface is of great interest and concern to environ- losilicate mineral is muscovite mica, and this also serves as a mental scientists. The sources, dispersion, and distribution model for clay minerals. As muscovite mica and copper(II) of elements in the environment, particularly toxic elements, commonly occur together, it is important that the adsorption their pathways into soils, sediments, water supplies, plants, reactions between them are studied. Shown in Fig. 1 is the and animals are the subject of much research. One aspect layered structure and basal (0001) plane of muscovite. The of such research concentrates on reactions that take place at edge sites are reactive at moderate pH and can form chemi- the mineral/water interface. This is an important field of sorbed or physisorbed complexes with appropriate anions study because the crust of the Earth consists of trillions of and cations. The basal plane, however, is made up of silox- square kilometers of surface area, most of which is in contact ane rings, where oxygen bonding is fully satisfied by silicon, with water or water vapor. Elements in an aqueous environ- in SiO 0 2 units. Recently the proton-association constants for ment can generally travel much further and faster than those several important surface reactions involving various in the solid phase, so that information on how minerals react (hydr)oxides have been calculated (6). The log K values with elements in an aqueous phase is of great importance. that were calculated for some surface species are as follows: For many metals, especially the heavy metals, it is adsorp- Al 2 –O, 12.3; Al–OH, 10.0; Si–O, 11.9; Si–OH, 01.9; and Si 2 –O, 016.9. This means that above a pH of approximately 1 To whom correspondence should be addressed. 2.5 for SiO 0 2 groups there is no protonation and that the 561 0021-9797/96 $12.00 Copyright  1996 by Academic Press, Inc. All rights of reproduction in any form reserved.