Langmuir 1991, 7, 2153-2159
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X-ray Photoelectron Spectroscopic Study of the Adsorption Mechanism of Aurocyanide onto Activated Carbon C. Klauber CSIRO Division of Mineral Products, c/o Curtin University of Technology, GPO Box U1987, Perth, WA 6001,Australia Received September 11, 1990.I n Final Form: June 26,1991 The adsorption of aurocyanide anions, Au(CN)2-, from alkaline solutions onto two activated carbons over a range of coverages has been examined by using X-ray photoelectron spectroscopy. Stoichiometric analysisindicatedthat the Au(CN)z-anionadsorbed intact. An Au 4f7p binding energy shiftupon adsorption reveals a charge donation of 0.3-0.5 of an electronic charge to the central cationic gold atom. Symmetry of chemical environment for both cyanide ligands requires the complex to adsorb on and parallel to the graphitic planes in the activated carbons. The adsorption mechanism is explained in terms of a *-donor bond from the graphitic substrate to the gold atom of the complex. Aspects of the ion pair mechanism are discussed.
Introduction The mechanism of the adsorption of aurocyanide onto activated carbon has direct significance for the industrial process of carbon-in-pulp by which gold in very low concentration is selectively extracted from cyanide leach liquors. Although the phenomenon of aurocyanide adsorbing onto carbons has been known for over a century,l extensive investigation of the mechanism2-19 has only occurred with the recent utilization of the technology to treat gold-bearing ore bodies of low gold content. Most of these studies have centered around basic wet chemical approaches in order to ascertain the mechanism, although there have been some direct spectroscopic approaches utilizing X-ray photoelectron spectroscopy (XPS),2J3J6J8 MBssbauer spectroscopy,7J2J7Jg and Fourier transform infrared (FTIR) spectroscopy.14 The spectroscopic interpretations have not been in complete agreement for a variety of reasons. Overall the research has resulted in the proposal of a number of diverse adsorption mechanisms.I1 The most recent attempt to resolve the diverse (1)Davis, W. N. U.S. Patent 227,963,1880. (2)McDougall, G.J.; Hancock, R. D.; Nicol, M. J.; Willington, 0. L.; Copperthwaite, R. G. J. S. Afr. Inst. Min. Metall. 1980,80, 344. (3) Fleming, C. A.; Nicol, M. J. J. S. Afr. Inst. Min. Metall. 1984,84
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(4). 85.
(4) Abotei, G. M. K.; Oeeeo-Asare, K. Int. J. Min. Process. 1986,18,
217. (5) Teuchida, N.; Muir, D. M. Metall. Trans. B 1986,17B,523. (6)Teuchida, N.; Muir, D. M. Metall. Trans. E 1986,178,529. (7)Caehion, J. D.; Cookson, D. J.; Brown, L. J.; Howard, D. G. In Industrial Applications of the M6ssbauer Effect;Long, G. J., Stevens, Plenum ; Prws: New York, 1987;p 595. J. G., as. (8)Fuersbnau, M. C.;Nebo, C. 0.; Kelso, J. R.; Zaragoza,M. R. Miner. Metall. Process. 1987,4,177. (9)McDougall, G.J.; Adams, M. D.; Hancock, R. D. Hydrometallurgy 1987. 18. 125. (10) Adam, M. D.; McDougall, G.J.; Hancock, R. D. Hydrometallurgy 1987,18, 139. (11) Adame, M. D.; McDougall, G. J.; Hancock, R. D. Hydrometallurgy 1987,19,95. (12)Caehion, J. D.; McGrath, A. C.; Volz, P.; Hall, J. S. Trans. Inst. Min. Metall. 1988. 97. C129. (13)Klauber, C: Surf. Sci. 1988,203,118. (14) Van der Merwe, P. F.; Van Deventer, J. S. J. Chem. Eng. Commun. 1988,66,121. (15) Cook, R.; Crathorne, E. A.; Monhemius, A. J.; Perry, D. L. Hydrometallurgy 1989,22,171. (16)Adama, M.D.;Fleming, C. A. Metall. Trans. B 1989,2OB,315. (17)McGrath, A. C.; Hall, J. S.;Caehion, J. D. Hyp. Int. 1989,46,673. (18)Jones, W. G.;Klauber, C.; Linge, H. G. Nineteenth Biennial Conference on Carbon; Penn State: University Park, PA, June 1989;p 38.
(19)Kongolo, K.; Bahr,A.; Friedl, J.; Wagner, F. E. Metall. Trans. B 1990,2IB,239.
postulates favors an ion pair mechanism, the adsorbed species being a background cation in combination with the aurocyanide anion, i.e. Mn+[Au(CN)2-]fl.1s XPS and Mossbuaer spectroscopy can reveal complementary aspects of the aurocyanide-carbon adsorption system. XPS is able to follow the stoichiometry and chemical state of species other than gold, while Mossbauer spectroscopy can examine the system ”wet” (frozen aqueous) and in some circumstances reveal subtle aspects of the nature of the gold bonding environment. The Mossbauer investigations7J2J7J9have all been consistent with respect to the gold adsorbing primarily as Au(CN)2- from alkaline solutions with variations only on the finer points of analysis and consequently the proposed bonding scheme. The similarity of the isomer shift and quadrupole splittings of gold loaded on carbon to that of crystallined KAu(CN)2 lead Cashion et aL7J2not to differentiate between the adsorption possibilities of Au(CN)2-and/or KAu(CN)z (i.e. an ion pair). They further infer bonding to the activated carbon substrate via the cyanide ligand’s nitrogen atoms, possibly through an intervening oxygen atom. This is in line with the earlier proposed ion-exchange mechanism involving participation of the oxygen functionalities on the carbon.6*6 Subsequent Mossbauer work of McGrath17involvingcrystalline samples of the dicyanoaurates of Na+, K+, Ca2+/Na+, and Gd3+ and gold loaded onto carbon with K+, Na+, and Ca2+/Na+ counterions yielded a set of spectral parameters similar to frozen aqueous KAu(CN)2. This is interpreted as the cations and water molecules being well removed from the gold atom in all cases. As the gold atom is positively charged, this is not surprising behavior for the countercations. Such a view, however, is inconsistent with the formal ion pair Mfl+[Au(CN)2-]npostulate, unless the cations were to reside on the far side of the cyanide ligands.lg Unfortunately such a structure for the ion pair would not preserve the necessary ligand sy”etry.13 The cation and water molecule Mossbauer insensitivity is confirmed by Kongolo et al.19 who have comprehensively examined loaded carbons, wet and dry, over a pH range of 1 to 10,with and without coadsorbed Ca2+and Gd3+. Kongolo et al.lg find the ion-exchange mechanism of less appeal, preferring a bonding scheme involving the gold atom directly, both for Au(CN)2- and any adsorbed oligomer Au,(CN),+l-. The XPS and Mossbauer interpretations do diverge on the important point of charge transfer to the gold atom. Mossbauer requires a bonding mechanism which leaves the
0743-7463/91f 2401-2153$02.50/Q 0 1991 American Chemical Society
2154 Langmuir, Vol. 7, No. 10,1991 electron density at the gold nucleus essentiallyunchanged, whereas the XPS indicates a definite net electron transfer of 0.3-0.5 electron from the substrate to the gold.13 In this article the study of the adsorption of aurocyanide anions Au(CN)2- onto activated carbons under alkaline conditions, alluded to in an earlier paper,13 is presented in detail.
Klauber I
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C - Au(CN); N 1s
Results Illustrated in Figure 1are the N 1s photoelectron peaks and Au 4f doublets for the washed subsamples of the four different equilibrium loadings examined for GRC22 and the single equilibrium loading for R2520. A Shirley (20) Anthony, M. T.; Seah, M. P. Surf. Interface Anal. 1984,6, 95. (21) Van Attekum, P. M. Th.; Trooster, J. M. J. Chem. SOC.,Dalton Tram. 1980, 201.
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Experimental Section Prior to adsorption all carbon samples were ultrasonically washed in water until free of carbon fines. Samples of Calgon GRC22 (coconut based) activated carbon were loaded with aurocyanideto four different equilibriumlevels, nominally50,300, 800, and 2000 bmol gl of carbon, from borate buffered (pH 10) solutions of up to 0.1 mol dm-1 KAu(CN)zkept agitated for 24 h at 303 K. In addition, a sample of Norit R2520 (peat based) activated carbonwas loaded to about 800rmolg-l (usinga solution concentrationwhich would load GRC22 to about 2000 pmol gl). The extent of loadingwas determinedby atomic absorption (AA) of the residual gold in solution. After removal from the gold solution half of each sample was washed with gold-free borate buffer chilled to 273 K so as to remove the possibility of any entrained aurocyanide solution prior to drying. Unwashed sampleswere allowed to drain in air. Both washed and unwashed samples were dried (vacuum desiccation at room temperature), crushed,and pressed into indiumfoilfor analysis. All loose carbon fragments were removed from the pressed surface with compressed Freon. XP spectra were obtained by using a VG ESCALAB Mk I1 (base pressure
