Several conflicting theories of the adsorption of
aurocyanide onto activated carbon presently exist. To
resolve the mechanism, adsorption and elution of
aurocyanide are examined by several techniques, including
Mossoauer spectroscopy, X-ray photoelectron spectroscopy,
X-ray diffractometry, Fourier Transform Infrared
spectrophotometry, ultraviolet-visible spectrophotometry
and scanning electron microscopy.
The evidence gathered indicates that, under normal plant
conditions, aurocyanide is extracted onto activated carbon
in the form of an ion pair M n+ [Au(CN) 2 ] n, and eluted by
hydroxide or cyanide. The hydroxide or cyanide ions react
with the carbon surface, rendering it relatively
hydrophilic with a decreased affinity for neutral species.
Additional adsorption mechanisms are shown to operate
under other conditions of ionic strength, pH, and
temperature. The poor agreement in the literature
regarding the mechanism of adsorption of aurocyanide onto
activated carbon is shown to be due to the fact that
different mechanisms operate under different experimental
conditions. The AuCN produced on the carbon surface by acid
treatment is shown to react with hydroxide ion via the
reduction of AuCN to metallic gold with formation of
Au (CN) 2 , and the oxidation of cyanide to cyanate. Other
species, such as An(CN)5 and Ag(CN)g adsorb onto
activated carbon by a similar mechanism to that postulated
for Au(CN)2 .
Ion association of MAu(CN) 2 salts in aqueous solution is
demonstrated by means of potentiometric titration and
conductivity measurements, and various associated
species of KAu(CN), salts are shown to occur in organic
solvents by means of infrared spectrophoteaietric and
distribution measurements.
A kinetic model was developed for elution of aurocyanide
from activated carbon and was found to predict gold elution
performance successfully using the Zadra procedure.
The influence of the surface chemistry and structure of
activated carbon on adsorption of aurocyanide was
investigated by characterization of activated carbons that
were synthesized or oxidized under various conditions.
Synthetic polymeric adsorbents with characteristics
similar to activated carbons were also studied. The
evidence suggests that a large micropore volume is
important in providing suitable active sites for
adsorption. Another important factor is the presence of
basic functional groups within the micropore, which act as
solvating agents for the ion pair.
The aim is to provide a self-consistent adsorption
mechanism that accounts for all observations presented in
the literature. Interpretation of results in terms of
preconceived ideas, and neglect of observations of other
authors has greatly contributed to current disagreement in
the literature.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/15251 |
| Date | 26 August 2014 |
| Creators | Adams, Michael David |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf, application/pdf |
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