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Open File Report 43

Chemistry of gold-humic interactions

Gray, D.J., Lintern, M.J. and Longman, G.D.

This report describes experiments on the interaction of ionic Au with various sources of soluble humic acid. The concentrations of soluble (<0.45 µm) Au in the presence of humic phases were dependent on a number of factors, including Au concentration, humic concentration, humic source, and the presence or absence of light. Such variations in solubility can readily explain the wide divergence of opinion on the effect of soluble humic phases on Au solubility, as detailed in the report.

Results obtained here indicated that the Au formed a very fine, highly coloured sol, in agreement with other work on the interaction of Au with humic phases (Ong and Swanson, 1969; Fedoseyeva et al., 1986) and with other organic molecules (Fabrikanos et al., 1963). Formation of the sol is activated by light, in agreement with previous work (Fabrikanos et al., 1963).

The Au sol is effectively decolourised by the addition of ligands with strong (CN-) or moderate (I-, S2O32-, SCN-) affinities for Au, or by ligands with weak (Cl-) affinities for Au when in high concentration. This suggests that the Au sol will only be stable in the absence of such ligands. As the unpurified humate preparations readily converted Au to the sol, rather than complexing it, these preparations do not contain significant concentrations of such Au ligands.

It is postulated that the colour of the Au sol, rather than just indicating the size of the Au particle, may be due to specific chemical factors. Thus, a further understanding of the mechanism of the colour of this phase could give further important information on the chemistry of Au in the presence of humic material.

Last updated: Thursday, January 06, 2000 08:36 AM


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