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
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