Open File Report 42
The sorption of gold and silver on soil minerals
The sorption of Au and Ag was investigated by reacting synthesised
Au and Ag complexes with a range of different soils. The systems
used were Au and Ag in humic, thiosulphate, iodide and chloride
solutions, and in a poorly complexed form.
Soils used were 4% Peat Moss - 96% Quartz, organic rich soil sample
1467, Fe oxide rich soil sample 1468, carbonate rich soil sample
1470, Fe oxide rich standard 7, Mn oxide rich Mount Keith Shaft,
44 m depth, and a Mn oxide rich segregation collected at Ora Banda.
The solutions and soils were equilibrated together and the Au and
Ag concentrations in the soil solutions were measured at two weeks
and again at three months.
Gold and Ag thiosulphates had a high initial solubility when contacting
most of the soils, with the exception of the Mn rich soils, which
quickly sorbed most of the Au and Ag. In general, both Au and Ag
had similar solubilities in thiosulphate solution. When finally
sorbed, Au did not redissolve.
In an acidic chloride-rich solution Au was more readily sorbed
than Ag, only having an appreciable solubility when in contact with
the most Mn rich material. This contrasts with the results for thiosulphate
complexes, demonstrating the critical importance of the Au complex
on the extent of sorption. Results were similar for Au iodide.
Humate complexes were found to maintain only small concentrations
of Au in solution, suggesting a weak interaction between Au and
humate. This is in contrast with work on Au humate interactions
by other workers which has suggested that humate can be very effective
at dissolving Au.
An important observation is the ability of the soils to redissolve
Au. When the uncomplexed Au and Ag were mixed with the various soils
they were quickly sorbed. After three months, however, a number
of the mixtures contained significant quantities of dissolved Au
(but not Ag). This is possibly due to soluble species with a high
affinity for Au being produced by biological activity. Very similar
effects were also observed for Au iodide. The humate mixtures also
showed appreciable redissolution of Au, though in a different manner
to the uncomplexed and the iodide mixtures, possibly due to the
different types and concentrations of organic matter in the humate
mixtures. These results suggest that Au is readily mobilized in
soils, and this metal should be considered to be mobile under such
Last updated: Thursday, January 06, 2000 08:33 AM