Interactions of gold (III) chloride and elemental gold with peat-derived humic substances

Peat-derived humic and fulvic acids, isolated using widely accepted techniques, were investigated for their ability to reduce gold (III) chloride and dissolve elemental gold. Au(III)-chloride (1-20 mg 1^-1) was reduced to elemental colloidal Au by both humic and fulvic acids (1-20 mg 1^-1) under all conditions studied. The growth rate and mean size of the colloidal Au formed was monitored using UV-visible spectroscopy. Larger colloids were formed in the presence of humic (∼60-nm mean diameter) than fulvic acid (∼20-nm mean diameter). Otherwise, colloidal growth rates were similar for humic and fulvic acids. Colloidal Au formation was completed in ∼8 days at pH 4 and slowed to > 14 days at pH > 7. Cupric ion (2 · 10^-5-2 · 10^-3 M) accelerated colloidal Au development from 2 to 20 times while equal concentrations of Ca²⁺ had no effect. About 10 mg 1^-1 humic and fulvic acids were necessary to completely reduce 10 mg 1^-1 Au (III)-chloride which is equivalent to a reduction capacity of ∼15 meq e^-/g humic substance. IR spectroscopy and acid titration results suggest that humic substance oxidation included the addition of phenolic, alcohol and ketone groups which helps to account for the appreciable reduction capacity observed. Ten mg 1^-1 solutions of these same humic substances did not dissolve elemental Au to levels > 1 μg 1^-1 over periods up to 150 days. Consequently, it is concluded that humic and fulvic acids function primarily as reductants of oxidized Au species rather than as dissolution and complexation agents for elemental Au. This implies that other components of natural organic matter are primarily responsible for the dissolution and complexation of elemental Au in surficial environments.