1908 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
The first studies of gold dissolution by amino acids
were conducted by Brown (1982) upon assessing the abil-
ity of amino acids to oxidatively dissolve gold. His work
revealed that gold can be dissolved by amino acids such as
cysteine, glutathione, penicillamine, alanine, and histidine
at different pH values. In addition to Brown’s work, Zhang
et al. (1996) also revealed that amino acids were responsible
for dissolving gold in geological ore bodies.
Building upon this historical evidence, Eksteen estab-
lished a high level of confidence in the use of amino acids in
gold leaching by patenting the process in 2014. With this
foundation, the work continued to intensely focus on the
smallest member of the alpha-amino acid group, glycine
(Eksteen &Oraby 2015 Eksteen &Oraby 2014 Oraby &
Eksteen, 2015a, 2015b). His work on glycine transitioned
from pure gold dissolution to gold ore and recyclable bat-
tery leaching, and this was governed by Equation 1 (Oraby
&Eksteen 2015), Equation 2 (Eksteen &Oraby 2015),
and Equation 3 (Oraby et al., 2020). Li et al. 2022 took the
research a bit deeper to investigate a clean way of using gly-
cine in leaching gold from the waste flotation tails. Recently,
Sarvar et al. (2023) expanded on this research by investigat-
ing various amino acids, including glycine, alanine, valine,
histidine, aspartic acid, asparagine and cysteine. Their focus
was on assessing the leaching capabilities of these amino
acids in dissolving pure gold.
4Au 4NH2 CH COOH 4NaOH O2
4Na6Au^NH CH COOh 6H O
2
2 2 2 2
"+++
+@
(1)
2Au 4NH CH COOH 2OH H O
2Au^NH CH COOh- 4H O
2 2 2 2
2 2 2 2
"+++
+
-
(2)
2Au 2MnO 4 NH CH COOHh- O
2Au(NH CH COO) 2MnO
2C O 2NH 2OH
4
2-
2 2 2
2 2 2 2
2 2
2-
3
"+++
+
+++
-
-
^
(3)
Despite considerable progress in understanding the
leaching kinetics, the in-depth chemistry or mechanisms
underlying the complexation of gold has remained unex-
plored. Existing studies that aimed to predict the complex-
ation chemistry of gold and amino acids relied strongly on
Density Function Theory (DFT) .This quantum mechani-
cal approach employs electron density to determine bond-
ing and chemical properties of gold amino acid complexes.
The early investigations into gold-amino acid interac-
tions focused on glycine and cysteine bonding with gold
nanoclusters through anchoring bonds (N-Au, O-Au,
S-Au) and nonconventional bonds (N-H----Au, O-H--
--Au) during complexation (Pakiari and Jamshidi 2007
Xie et al., 2012). Building on these foundations, Buglak
and Kononov (2020) extended DFT studies to include
all 18 alpha amino acids in bonding with gold clusters.
Their findings not only validated earlier results but also
showed larger Gibbs energy for deprotonated amino acid
complexes compared to neutral amino acid complexes
(Buglak and Kononov 2020). Moreover, Peng et al. (2023)
worked on the same amino acids with Au3 clusters under
the gas phase and water solvation, and their DFT calcu-
lation showed that the carboxylic and amide were prefer-
able binding sites, with the sulphur being the other site for
thiol-containing amino acids. Their work also showed the
complexation affinity changed with changes in the environ-
ment with high binding energies reported in the gas phase.
All this work by Peng et al. (2023), Pakiari and Jamshidi
(2007) Xie et al. (2012) primarily focused on the binding
of a single molecule of amino acid to either a lone gold
atom or a cluster of gold (Aun)
The reported DFT studies pave a way in investigat-
ing the chemistry of complexation of gold with amino
acid which is governed by a bi-ligand complex []AuL
2
x !
formed, which has never been investigated. Therefore, there
is a need to investigate this kind of complex through DFT
studies, however, this can be done after understanding the
bonding through Fourier Transform Infrared (FTIR), an
experimental approach that focuses on peak shift to pre-
dict bonding. This study adopts a threefold methodology,
combining FTIR, DFT, and dissolution studies. Through
this approach, we aim to establish a correlation between the
DFT’s predictive understandings and the FTIR’s experi-
mental observations, particularly concerning gold dissolu-
tion. For this work, alanine serves as the reference amino
acid, with the last part of the study focusing on a case study
in gold leaching.
MATERIALS AND METHODS
Fourier-Transform Infrared Spectroscopy
Alanine stock solution (100 mL, 10 mM) was prepared
using deionized water. Four (10 mL) samples were placed
in sample tubes with two samples adjusted to isoelectric
pH (pH 6) and the other two at the deprotonation pH
(pH 12) of alanine. One sample at pH 6 and another one
at pH 12 had 10 mg of powdered gold added to them. In
contrast, the remaining samples were kept like that without
gold added. The two samples with gold were allowed to
dissolve gold for 30 days with regular shaking to improve
the process. After dissolution, the solutions were filtered
despite the absence of visible gold particles, indicating that
most of the gold had likely dissolved.
The first studies of gold dissolution by amino acids
were conducted by Brown (1982) upon assessing the abil-
ity of amino acids to oxidatively dissolve gold. His work
revealed that gold can be dissolved by amino acids such as
cysteine, glutathione, penicillamine, alanine, and histidine
at different pH values. In addition to Brown’s work, Zhang
et al. (1996) also revealed that amino acids were responsible
for dissolving gold in geological ore bodies.
Building upon this historical evidence, Eksteen estab-
lished a high level of confidence in the use of amino acids in
gold leaching by patenting the process in 2014. With this
foundation, the work continued to intensely focus on the
smallest member of the alpha-amino acid group, glycine
(Eksteen &Oraby 2015 Eksteen &Oraby 2014 Oraby &
Eksteen, 2015a, 2015b). His work on glycine transitioned
from pure gold dissolution to gold ore and recyclable bat-
tery leaching, and this was governed by Equation 1 (Oraby
&Eksteen 2015), Equation 2 (Eksteen &Oraby 2015),
and Equation 3 (Oraby et al., 2020). Li et al. 2022 took the
research a bit deeper to investigate a clean way of using gly-
cine in leaching gold from the waste flotation tails. Recently,
Sarvar et al. (2023) expanded on this research by investigat-
ing various amino acids, including glycine, alanine, valine,
histidine, aspartic acid, asparagine and cysteine. Their focus
was on assessing the leaching capabilities of these amino
acids in dissolving pure gold.
4Au 4NH2 CH COOH 4NaOH O2
4Na6Au^NH CH COOh 6H O
2
2 2 2 2
"+++
+@
(1)
2Au 4NH CH COOH 2OH H O
2Au^NH CH COOh- 4H O
2 2 2 2
2 2 2 2
"+++
+
-
(2)
2Au 2MnO 4 NH CH COOHh- O
2Au(NH CH COO) 2MnO
2C O 2NH 2OH
4
2-
2 2 2
2 2 2 2
2 2
2-
3
"+++
+
+++
-
-
^
(3)
Despite considerable progress in understanding the
leaching kinetics, the in-depth chemistry or mechanisms
underlying the complexation of gold has remained unex-
plored. Existing studies that aimed to predict the complex-
ation chemistry of gold and amino acids relied strongly on
Density Function Theory (DFT) .This quantum mechani-
cal approach employs electron density to determine bond-
ing and chemical properties of gold amino acid complexes.
The early investigations into gold-amino acid interac-
tions focused on glycine and cysteine bonding with gold
nanoclusters through anchoring bonds (N-Au, O-Au,
S-Au) and nonconventional bonds (N-H----Au, O-H--
--Au) during complexation (Pakiari and Jamshidi 2007
Xie et al., 2012). Building on these foundations, Buglak
and Kononov (2020) extended DFT studies to include
all 18 alpha amino acids in bonding with gold clusters.
Their findings not only validated earlier results but also
showed larger Gibbs energy for deprotonated amino acid
complexes compared to neutral amino acid complexes
(Buglak and Kononov 2020). Moreover, Peng et al. (2023)
worked on the same amino acids with Au3 clusters under
the gas phase and water solvation, and their DFT calcu-
lation showed that the carboxylic and amide were prefer-
able binding sites, with the sulphur being the other site for
thiol-containing amino acids. Their work also showed the
complexation affinity changed with changes in the environ-
ment with high binding energies reported in the gas phase.
All this work by Peng et al. (2023), Pakiari and Jamshidi
(2007) Xie et al. (2012) primarily focused on the binding
of a single molecule of amino acid to either a lone gold
atom or a cluster of gold (Aun)
The reported DFT studies pave a way in investigat-
ing the chemistry of complexation of gold with amino
acid which is governed by a bi-ligand complex []AuL
2
x !
formed, which has never been investigated. Therefore, there
is a need to investigate this kind of complex through DFT
studies, however, this can be done after understanding the
bonding through Fourier Transform Infrared (FTIR), an
experimental approach that focuses on peak shift to pre-
dict bonding. This study adopts a threefold methodology,
combining FTIR, DFT, and dissolution studies. Through
this approach, we aim to establish a correlation between the
DFT’s predictive understandings and the FTIR’s experi-
mental observations, particularly concerning gold dissolu-
tion. For this work, alanine serves as the reference amino
acid, with the last part of the study focusing on a case study
in gold leaching.
MATERIALS AND METHODS
Fourier-Transform Infrared Spectroscopy
Alanine stock solution (100 mL, 10 mM) was prepared
using deionized water. Four (10 mL) samples were placed
in sample tubes with two samples adjusted to isoelectric
pH (pH 6) and the other two at the deprotonation pH
(pH 12) of alanine. One sample at pH 6 and another one
at pH 12 had 10 mg of powdered gold added to them. In
contrast, the remaining samples were kept like that without
gold added. The two samples with gold were allowed to
dissolve gold for 30 days with regular shaking to improve
the process. After dissolution, the solutions were filtered
despite the absence of visible gold particles, indicating that
most of the gold had likely dissolved.