1. Introduction
1.1
Gold
1.1.1
Mining and Leaching
Gold is a heavy metal element that is highly sought-after due to its attractive nature and
economic value. Being a noble metal, it is one of the least reactive metals, having the ability to
resist corrosion and oxidation in air. Gold most commonly occurs in its elemental form and in
many cases as an alloy with other metals such as silver. As a precious metal, it most often used
in the jewellery industry, but also carries some value in electronics.
Gold is visible in ore when the grade is roughly 30 parts per million (ppm), or 30
milligrams of gold per kilogram of rock. In order for gold to be considered “economically
sensible,” it is necessary for the grade of the ore to be at least 0.5 ppm for mining. Depending on
the type of mine, the grades vary. For example, open-pit mines usually contain gold grades
ranging from 1 to 5 ppm, and underground mines may contain even higher grades. Therefore
gold is usually invisible to the naked eye during the mining process.
1.1.2
Extraction by Cyanide Leaching
In order to obtain gold from rock after it has been mined, a process known as leaching
must be done. Leaching is a method of extracting minerals from a solid by dissolving them in a
liquid. As mentioned before, gold is considerably non-reactive with many chemicals, but it will
dissolve in certain solvents. The current industrial process of gold extraction utilizes alkaline
(above pH 10) cyanide (CN-) solution, such as sodium cyanide (NaCN) as a leaching solvent.
The solution is sprayed onto the ore in large industrial vats, but also over large pits in some
countries. In this outdoor process, known as heap-leaching, cyanide solution is sprayed over top
1
a pit of finely-crushed ore sitting on an impermeable pad, and then collected underneath.1 The
reaction of the process, sometimes referred to as “cyanidation,” is as follows:
4 Au + 8 NaCN + O2 + 2 H2O  4 Na[Au(CN)2] + 4 NaOH
The cyanide ion has the ability to form a stable complex with gold, [Au(CN)2]-, by
reducing elemental gold to Au (I) in the presence of oxygen. This method allows for a
chemically efficient and inexpensive way for the extraction of gold.1 After the gold-cyanide
complex has been formed, a suitable recovery method is needed.
1.1.3
Recovery from Cyanide Complex
The current recovery process in the gold mining industry is known as the Carbon in Pulp
(CIP) technique. This method utilizes microporous activated carbon, meaning it contains
multiple pores of less than 2 nm in diameter, which acts as a sponge . The “pregnant” goldcyanide solution is loaded into the activated carbon, and the gold ions are adsorbed to the carbon
surface until equilibrium is reached. Thus, the gold has been fully extracted from the ore, and is
released from the carbon by elution at high temperature and pH.
1.1.4
Environmental Concerns
As useful as cyanide is to extract gold, it is very toxic, especially in an acidic
environment, when hydrogen cyanide (HCN) is formed. Workers in the industry must take
special precautions towards the cyanide, along with cyanide gas that is produced. Heap leaching
is just as, if not more dangerous, because the pits could become flooded, causing the cyanide to
be released into aquatic systems. Failure of equipment involved in the disposal and use of
cyanide has caused notable environmental problems. In addition to cyanide’s toxicity, metalcyanide by-products from cyanide leaching have also caused environmental distress.2 Due to its
dangerous nature, a number of countries, including the Czech Republic, Hungary, and U.S. State
2
of Montana have even banned cyanidation. It is clear that an alternative gold extraction method
is imperative for the sake of environmental protection. Alternatives that have been explored
include chloride, bromide, thiourea, thiosulfate, and others.
1.1.5
Chloride Leaching
The use of chloride solutions for gold leaching is a relatively older method of gold
extraction. As seen in Table 1.1, the gold chloride complex is a very stable one that can then be
loaded into activated carbon for recovery.2 The CIP method, though adequate, is not as efficient
as recovery from cyanide solutions. A more capable method of recovery may prove to be useful
and more economical.
1.1.6
Thiosulfate Leaching
Thiosulfate has been proposed as a non-toxic, suitable alternative to cyanide for the
leaching of gold. It has been shown to leach gold efficiently, and it also decreases the
interference by metal impurities in the leaching process. Copper is often found in the ore that
has proven to cause economically difficulties in cyanidation. Gold dissolves in a neutral to
weakly-basic solution of thiosulfate in the presence of an oxidant (oxygen).2 The reaction is
represented below:
4 Au + 8 S2O32- + 2 H2O + O2  4 Au(S2O3)23- + 4 OHIn this leaching process, the dissolution of gold in the thiosulfate solution is often
hindered due to the build-up of sulfur coatings on the surface of the gold. To accommodate for
this, copper (II) and ammonia (in the form of cupric ammine) are added to the system to complex
with the gold, and then substituted with thiosulfate to form the final gold-thiosulfate complex.
Gold-ammonical solutions in fact have a comparable stability constant to gold-thiosulfate (Table
3
1.1), but leaching with ammonia would require higher temperatures. Both copper and ammonia
act as oxidants to aid in the reaction:2
5 Au + 5 S2O32- Cu(NH3)42+  Au(S2O3)3- + 4 NH3 + Cu(S2O3)35The stability of the thiosulfate ion itself comes into question as well. Thiosulfate is a
meta-stable ion that can be decomposed or oxidized at certain pH levels and redox potentials
(fig. 1.1).3 Decomposition of thiosulfate could cause gold to precipitate out of a leaching
solution or hinder the leaching reaction. Based on this stability data of thiosulfate, and as
mentioned previously, thiosulfate leaching is accomplished at neutral to weakly-basic pH levels.
1.1.7
Recovery from Thiosulfate Complex
Recovery of Au (I) ions from the pregnant thiosulfate complex is a current problem in the
advancement of thiosulfate leaching. Cyanidation currently employs the CIP technique, in which
the gold ions adsorb onto the activated carbon from the cyanide solution. However, adsorption
of gold ions from the thiosulfate complex to activated carbon has proven to be inefficient. The
exact reason for this inefficiency is unknown. Some have suggested that the problem is the high
negative charge of the complex, causing it to be more hydrophilic. Others claim that the low
recovery levels are because of steric hindrance due to its molecular structure.2 Therefore, a
suitable recovery method is required in order for the continuation of the use of thiosulfate for
gold extraction.
1.2 Mesoporous Silica
In the past two decades, there has been increased interest in mesoporous silica-based
materials. These inorganic, amorphous mesostructured assemblies were first synthesized in
1992, with the synthesis of Mobil Composition of Matter No. 41, or MCM-41.4 This material, as
well as materials created since, exhibit high surface area, pore diameter, and thermal and
4
mechanical stability. They also have uniform pore distribution, controllable pore size, high
adsorption capacity, along with wide possibilities of functionalization.5 The synthesis of
mesoporous silica is a surfactant-directed sol-gel method of assembly.
1.2.1
Synthesis
Surfactants are large, complex compounds that form aggregate assemblies called micelles
when placed in aqueous media. Being amphiphilic, they have hydrophobic tails in the core of
the aggregates, and hydrophilic heads on the surface. The shape of these micelles can be tailored
by adjusting the concentration of the surfactant. Lower concentrations of surfactants will
produce spherical shaped micelles, whereas higher concentrations will result in rod shaped
micelles.
These micelles are used as a template, over which silica precursors, such as tetraethyl
orthosilicate (TEOS) are organized. TEOS is most commonly used in silica synthesis, and is
useful because it is often hydrolyzed under both acidic and basic conditions. Since MCM-41,
further advancements in the field have provided new types of mesoporous silica. The materials
are classified by conditions and reagents used in the sol-gel process, which forms the silica over
the micelle templates. The type of the surfactant that is used is a key factor in the assembly of
the silica. Some surfactants are used for an electrostatic assembly, and other surfactants are used
for non-electrostatic assembly. Figure 1.2 gives a simple illustration of the assembly of
mesoporous silica over a surfactant template.
Cationic surfactants are used in the direct electrostatic synthesis of mesoporous silica by
means of a basic hydrolysis. MCM-41 type materials are assembled using this mechanism,
which uses complex ammonium salts as surfactants.4 Anionic surfactants that also utilize
electrostatic interactions as a synthesis mechanism will form the large pore Santa Barbara
5
Amorphous No. 15 (SBA-15) type materials. This requires hydrolysis of the silica precursors
by an acid such as HCl. Electrostatic surfactants form ionic bonds with the inorganic silica
precursors during the hydrolysis and condensation reactions. After the material has been
assembled, calcination (>600o) or solvent extraction removes the surfactant template. The
resulting mesoporous silica exhibits a hexagonal arrangement of the mesopores.5
Uncharged surfactants are classified into two groups; neutral and non-ionic surfactants.
Neutral surfactants are involved in the synthesis of Hexagonal Mesoporous Silica (HMS) type
materials. Assembly is achieved by hydrolysis of the precursors under neutral conditions.6 Nonionic surfactants are used in Michigan State University (MSU) materials, which posses 3-D
wormhole motifs. Non-ionic surfactants micelles are considered to be less defined than other
types of surfactants, which explains the resulting silica structure. Non-electrostatic surfactants in
general, whether they are neutral or non-ionic, operate by a different mechanism by which the
reagents interact.8 The surfactant binds to the precursor by means of hydrogen bonding, and
since hydrogen bonding is weaker than ionic bonding, the silica structure is less crystalline.9
Again, the non-electrostatic surfactant template can be removed by calcinations or solvent
extraction.
1.2.2
Organic Functionalization
The ability to incorporate organic functional groups within the pores of mesoporous silica
is very important in the field of research. These organic-inorganic hybrids are attractive because
they posses both the physical properties of the silica framework along the chemical reactivity of
the organic groups inside.5 There are two methods of functionalization of mesoporous silica (see
fig. 1.3) including post-synthesis grafting and co-condensation.
Post-synthesis grafting of organic groups is accomplished after the mesoporous material
has been synthesized and the surfactant template has been removed. It is often done using other
6
silica precursors called organosilanes. Organosilanes are similar to precursors such as TEOS,
except they contain branched non-hydrolysable organic groups. An example of a commonly
used organosilane is (3-mercaptopropyl) trimethyoxysilane (MPTMS), which has a long
branched carbon chain with a terminal thiol group. Silanol groups within the pores of previouslysynthesized silica provide a site for the organosilanes to anchor themselves to the pores via
condensation.10 The grafting rates, along with the amount of resulting organic groups depend on
the number of silanol sites, reactivity of precursors, diffusion limitations, and steric factors.11
The other method of functionalization is done through a co-condensation reaction, which
is a one-pot synthesis, and process and exploits the versatility of the sol-gel synthesis.5
Organosilanes are once again used for this method, but in a way very similar to the TEOS. A
fixed ratio of the organosilane to TEOS determines the amount of organic groups to be
incorporated within the pores of the silica. The organosilanes and TEOS are both hydrolysed
and then undergo a condensation reaction to assemble around the surfactant template. The
resulting material usually exhibits more homogeneously distributed organic groups.5 The
surfactant is then removed by solvent extraction, because calcinations would burn the organic
groups off.
1.3 Periodic Mesoporous Organosilica
1.3.1
Synthesis
Bridged organosilane precursors have been investigated more recently and have been
used in an alternative strategy to the synthesis of mesoporous silica. This strategy relies on the
incorporation of these bis-silanes into the framework of the inorganic material. Such hybrids
been classified as periodic mesoporous organosilica (PMO). PMOs are useful because they
exhibit high surface area, large and tuneable pores, high degrees of order, and uniform pores.12
7
Figure 1.4 illustrates the synthesis of this material. Electrostatic surfactants have been the
preferred templating agents (similar to MCM-41), but amphiphilic block copolymers with the
addition of salt have been demonstrated to produce good PMOs as well.13
PMOs are easily synthesized in a one-pot process by using a ratio of TEOS to bridged
organosilane or by using 100% bis-silane. The advantage of using 100% bis-silane is that it
ensures a completely homogenous distribution of organic groups within the framework of the
PMO.13 The majority of the surface is attributed to the pores of the material instead of the
framework itself. Also, PMOs have displayed greater hydrothermal stability and mechanical
strength, increased hydrophobicity, less pore blockage, and higher adsorption capabilities of
organic compounds than general mesoporous silica such as MCM-41.14 The useful properties of
PMOs will be utilized in this research and will be the basis of the study.
1.4 Objective
This thesis paper will focus on the use of functionalized mesoporous silica, and in
particular, PMOs, for the adsorption of gold from thiosulfate leaching solution. As mentioned
previously, thiosulfate leaching does not complement the Carbon-in-Pulp (CIP) method used in
cyanidation. It has been previously shown that mesoporous silica functionalized with thiol or
amine groups could be useful in the uptake of heavy metal species in aqueous solution, such as
mercury ions (Hg2+).15 Gold is classified as a heavy metal as well, and would interact with thiol
or amine groups as mercury does (HSAB theory).
The question remaining is whether or not Au (I) ions will select the organic (thiol) groups
over the thiosulfate in solution. It is true that there will be a competition for bonding to the gold
between the two species. However, the functionalization of the pores could be adjusted to
achieve maximum adsorption of gold (or as much adsorption as possible). This research will
8
focus on achieving optimal functionalization of PMOs in order to compete with the thiosulfate
ion in the leaching solution. This study relies heavily on the special properties of PMOs in hopes
that those properties will aid in gold adsorption.
Branching organic groups within the pores of mesoporous silica are known to cause pore
blockage problems, which in fact inhibit the adsorptive efficiency of the material. With organic
groups incorporated into the framework of PMOs, blockage of the pore channels is greatly
reduced and often avoided.14 Certain functionalities will be tested in this study, but the sulfur
atom will be of considerable investigation, due to its ability to bond with gold ions. Both
chloride and thiosulfate solutions will be studied in the following experiments. Although gold
ions from chloride solutions do in fact adsorb to activated carbon, it is not as efficient as
adsorption from cyanide solutions. The ultimate uncertainty in both thiosulfate and chloride
leaching is the lack of a suitable recovery method. This project aims to find that method.
9
C. Xia. Department of Mining Engineering in Conformity – Queens University (2008) 1-246.
Figure 1.1 - The Influence of pH and Potential on Thiosulfate Stability
10
B. Hatton et al. Acc. Chem. Res, 38 (2005) 305-312
Figure 1.2 - General Synthesis of Mesoporous Silica with Amphiphilic Surfactant and
TEOS Silica Precursor
11
A. Walcarius, L. Mercier. J. Mater. Chem, 20 (2010) 4478-4511.
Figure 1.3 - Functionalization of Mesoporous Silica by (A) Post-Synthesis Grafting and (B)
Co-Condensation
12
Figure 1.4 - Synthesis of PMOs using Bridged Organosilane Precursors
13
Gold Species
Stability Log K
Au(CN)2-
38.3
Au(SCN)2-
16.98
Au(SCN)4-
10
AuCl4-
25.6
Au(NH3)2+
26
Au(S2O3)23-
28
M. G. Aylmore, D. M. Muir. Minerals Engineering, 14 (2001) 135-174.
Table 1.1 - Stability Constants of Various Gold Complexes
14
2. Experimental
2.1 Synthesis of Periodic Mesoporous Organosilica
2.1.1
Materials
Bis-(triethoxysilyl)ethylene (BTEE, Gelest, 95%) was used as the sole material precursor
in the synthesis. The surfactant that was used as a template was the non-ionic triblock
copolymer Poly(ethylene golycol-block-poly(propylene glycol)-block-poly(ethylene glycol)
(Pluronic P123, Aldrich). See figure 2.1 for precursor and surfactant structures. 10%
hydrochloric acid (HCl) was used to adjust the pH, and sodium chloride (NaCl) was used to
obtain highly ordered PMOs.
2.1.2
Synthesis
Following the synthesis procedure of PMOs by Qiao et. al.16, 1 g (recipe uses 2 g, see
discussion) of Pluronic P123 was dissolved in 79 mL of water. This part of the synthesis was
performed in a beaker in an incubator at 38 oC. The higher temperature allows for the
dissolution of P123 in water. After the surfactant had dissolved by stirring, and the solution had
become clear, 4.6 g of NaCl and 4.9 g of 10% HCl were then added to the mixture. Finally,
3.4736 g of BTEE was added to the solution, and the reaction was left to continue in the
incubator for 10 minutes. The final reactant molar composition was .0173 P123: 8 NaCl: 1.34
HCl: 444 H2O: 1 BTEE.16 The beaker was then removed from the incubator, covered, and
transferred to a water bath (38 oC) for 24 hours. After 24 h, the product was transferred to an
autoclave and heated at 100 oC for another 24 h. The precipitated white solid was filtered by
vacuum and washed with distilled water, then left to dry in air. Because ethylene groups were
incorporated in the framework of the material, the surfactant template was then removed by
15
Soxhlet extraction over ethanol for 24 to 48 h. The resulting powder (dried in oven at 60 oC) was
designated PMO-EE (Periodic Mesoporous Organosilica – Ethylene).
2.1.3
Characterization
PMO-EE was characterized using nitrogen adsorption isotherms and thermogravimetric
analysis. The nitrogen adsorption isotherms of nitrogen at 77K were measured on a
Micromeritics ASAP 2010 system. The sample was degassed for at least 6 h at 80 oC before
analysis. Analysis of the material led to calculation of surface area using the BET model. Pore
volume and pore size were also measured using the Broekhoff-de Boer model. This model is
considered to be the most widely accepted method for calculating pore size distributions in
mesoporous materials with cylindrical-shaped pores (only accurate for pore sizes over 20 Å).17
Thermogravimetric analysis (TGA) was also used to characterize the PMO. The
instrument used was a TA Instruments SDT Q600, which measures the change in weight of the
sample in relation to increasing temperature. This type of analysis was performed to confirm the
presence of organic groups in the mesoporous silica. Organic components will decompose to gas
at specific temperatures, and it will appear as a loss in weight of the entire material.
2.2 Bromination of PMO
Following the procedure by Stein et. al.18, the PMO-EE samples were placed in a small
beaker, and set inside a larger beaker. Several drops of bromine (BDH, 99%) were added to a
small test tube and also placed in the large beaker, avoiding direct contact with the material. The
beaker was then covered with Parafilm to contain the bromine (Br2) gas for 48 h. Exposing the
material to the Br2 caused it to turn an orange colour due to adsorbed bromine. By washing the
material in ethanol and water, the orange colour turned back to white. The resulting material was
a PMO with short bromine groups occupying its pores, in a reaction similar to the bromination of
16
alkenes. The brominated product was designated PMO-Br, and was also characterized by
nitrogen adsorption isotherms and TGA.
2.3 Organic Functionalization
With the brominated material, PMO-Br, the bromine groups are ideal for nucleophilic
substitution of other organic groups – specifically organic groups containing soft lewis bases
(thiol). The entire sample of PMO-Br was placed in 50 mL of solvent and a number of
nucleophilic compounds of were added (see Table 2.1). The compounds used were sodium
hydrosulfide hydrate (NaSH . xH2O, Aldrich), thiourea (Aldrich), and 1,2-ethanedithiol
(Aldrich). The amount of each nucleophile used was determined by multiplying the organosilane
amount by a factor of two (2:1 – Br2:ethylene). All three compounds contain a nucleophile
sulfur centre, which substitutes with bromine leaving groups in nucleophile substitution
reactions. The reaction with PMO-Br was performed under reflux for 48 – 72 h. The resulting
products were designated PMO-SH, PMO-TU, and PMO-EDT, respectively. Again, all three
materials were characterized by nitrogen adsorption isotherms and TGA.
2.4 Gold Adsorption
2.4.1
Recovery from Thiosulfate Complex
The adsorption of Au (I) ions from gold-thiosulfate solutions (labelled AuThioS) were
studied using these functionalized PMO materials. Using a procedure very similar to Mercier et.
al.19, various concentrations of AuThioS solutions (pH=7) were used in the experiment. In 25
mL of AuThioS, 5 mg of the silica material was suspended and stirred at room temperature for
12 h. It was imperative that the stirring was done in a mechanical shaker, because gold has the
tendency to adsorb to Teflon stirring beads. The solutions were then filtered by gravity and gold
ion concentration was measured by atomic absorption spectroscopy (AA).
17
2.4.2
The Effect of pH
The effect of pH was also studied on AuThioS adsorption by the PMO materials. pH
levels in the range of weakly acidic to weakly basic were used to monitor adsorption capabilities
of each PMO. Concentration of AuThioS was kept constant for each assay, and metal ion
concentration was again measured by AA. The pH levels were chosen in such a way to maintain
the preferred range that ensured thiosulfate stability, based on figure 1.1.
Figure 2.2 gives a simple illustration of the adsorption of gold ions to thiol groups within
the framework of the periodic mesoporous organosilica. The gold ions are once again competing
with the thiosulfate in solution and the organic functionalities of the material. PMOs with larger
pores and fewer blockages may help improve the adsorption of gold with influence on this
competition.
2.4.3
Recovery from Chloride Complex
Although the gold chloride complex is not as stable as the thiosulfate complex (see Table
1.1), it still is suitable for gold extraction. Gold chloride solutions of varying concentrations
were prepared at pH=2. To 25 mL of each solution, 5 mg of PMO material was added, as done
in the thiosulfate experiments. Again, the solutions were stirred for 12h at room temperature and
gold concentrations were measured by flame AA.
18
Figure 2.1 - The Structures of BTEE and Pluronic P123
19
Amount of
Nucleophile
Nucleophile
Designated Product
Solvent
Sodium Hydrosulfide
1.12 g (.02 mol)
PMO-SH
Ethanol
Thiourea
1.52 g (.02 mol)
PMO-TU
Ethanol
1,2-Ethanedithiol
1.88 g (.02 mol)
PMO-EDT
Acetonitrile
Table 2.1 - Nucleophiles for Substitution with PMO-Br
20
Figure 2.2 - The Adsorption of Gold to Thiol-Functionalized Periodic Mesoporous
Organosilica (PMO-SH) from Gold Thiosulfate Complex
21
3. Results and Discussion
3.1 Characterization of PMO-EE
3.1.1
Nitrogen Adsorption
The product designated PMO-EE existed in the form of a fine white powder. As noted in
the experimental section, half the amount of P123 was used during the synthesis. It was
determined that using 1 g instead of 2 g produced higher-ordered pores in the PMO. Nitrogen
adsorption data was indicative of high surface area and uniform pore structure throughout the
material. The adsorption curve (fig. 3.1) displayed a Type IV isotherm that is common for SBAlike materials such as this PMO. Type IV behaviour is characteristic of mesoporous solids in
which capillary condensation takes place at higher pressures of adsorbate, along with multilayer
adsorption at lower pressures.21
From this isotherm, a BET surface area was calculated to be 908 m2/g. This high surface
area was expected from the PMO, based on known information. Pore diameter was also
calculated using the Broekhoff de Boer method, and a pore distribution curve (fig. 3.2)
demonstrated uniform pore sizes between 60 and 70 Å, which lies within the mesoporous range.
The distribution curve also displayed high pore volume throughout the material (0.778 cm3/g).
The large pore nature of this PMO material is partially attributed to the high temperature
synthesis – a trend which is shown throughout all mesoporous silica materials.20
3.1.2
Thermogravimetric Analysis
The TGA data (fig. 3.3) of PMO-EE exhibited weight loss prior and up to 100 oC, which
is merely the loss of remaining moisture (water) in the sample. Significant (20%) weight loss at
300 oC was indicative of the decomposition of the ethylene groups in the framework of the
material. This was enough information to confirm the organic-inorganic hybrid nature of the
PMO, and continuing with the project, bromination was then possible.
22
3.2 Bromination of PMO-EE
3.2.1
Nitrogen Adsorption
The brominated material, PMO-Br, also displayed a Type IV isotherm, but there was a
noticeable depression in the curve (fig. 3.4). BET surface area was affected by the bromination,
as it dropped to 618 m2/g. Bromination of the PMO only caused bromination of the ethylene
groups in the framework of the material, and pores were only affected in their volumes.
Therefore, there was not a significant decrease in pore diameter of the PMO, but pore volume
had decreased from 0.778 to 0.558 cm3/g (fig. 3.5).
3.2.2
Thermogravimetric Analysis
The TGA data for PMO-Br (fig. 3.6) gave strong evidence of different organic
functionalization of the PMO hybrid. Neglecting the weight loss of the material for water, the
decomposition of organic components occurred around a temperature of 200 oC. The percent of
weight loss caused by this decomposition was roughly 33%. This information aided in the
confirmation of the bromination of the ethylene groups of PMO-EE. By treating the bromine
groups as leaving groups, nucleophilic substitution could then be investigated.
3.2.3
Calculation of Fraction Brominated
Both surface area and pore size decreased with the bromination of the PMO material.
This is likely caused by the increase in density of the material upon the addition of bromine
groups. The surface area of the material is calculated and reported in units per gram (i.e. g-1).
Logically, as the mass of the material is increased, the surface area decreases. Knowing this
relationship can aid in the calculation of the fraction of brominated ethylene groups. Using the
equation:
Fraction Brominated = MW PMO EE 1  x 
23
MWPMO BR x 
Where x = surface area of PMO-Br / surface area of PMO-EE, MWPMO EE is the molecular
weight of PMO-EE (65.1 g/mol), and MWPMO BR is the molecular weight of bromine (79.9
g/mol). Applying these values to the equation, x = (0.558 m2/g) / (0.778 m2/g) = 0.7172. Then:
Fraction Brominated = (65.1 g/mol)(1 – x ) / (79.9 g/mol)( x )
= 32.1%
Therefore, approximately 32% of the PMO-EE had been brominated.
3.3 Substitution of PMO-Br
3.3.1
Nitrogen Adsorption
All three samples, PMO-SH, PMO-TU, and PMO-EDT were characterized by nitrogen
adsorption isotherms (fig. 3.7). Each material once again displayed Type IV isotherms,
indicating that none of the samples were destroyed in the substitution reactions. Surface areas of
the three, in order, were 611, 597, and 600 m2/g. There were not significant changes in the pore
diameters from the PMO-Br, but pore volumes were altered with a decrease of 0.32, 0.15, and
0.08 cm3/g, respectively (fig. 3.8).
3.3.2
Thermogravimetric Analysis
TGA was necessary to confirm the substitution of the nucleophiles with the bromine
leaving groups in PMO-Br. At this point in time, this method is the only one used for this
purpose. As it will be discussed in the conclusion section of this paper, more accurate and useful
techniques will be used for confirmation of substitution in the future. Nonetheless, the weight
losses at certain temperatures corresponded to the decomposition temperatures of each organic
group at hand (fig. 3.9). Even though it was not completely certain of organic functionalization
(thiol, thiourea, and ethanedithiol), these materials could then be used for gold adsorption, and
even more confirmation of the presence of these organic groups would appear.
24
3.4 Gold Adsorption
3.4.1
Adsorption of Gold from Thiosulfate
The ability for these organic functionalized PMOs to adsorb gold ions from goldthiosulfate solutions is the main focus of this project. In neutral pH AuThioS solutions,
adsorption of gold was measured with units of millimoles of gold per gram of PMO (mmol/g).
Thiosulfate concentrations of 1 part per million (ppm) to 100 ppm were used for each material
(fig. 3.10).
PMO-SH proved to be extremely inefficient in the uptake of gold ions in solution. This
could be attributed to complications during the substitution reaction. IR data will have to be
obtained to give certainty in the problem. For all concentrations of AuThioS, the adsorption of
gold ions was negligible using PMO-SH. PMO-TU proved to be far more effective than PMOSH in AuThioS. The mechanism by which thiourea substitutes with bromine leaving groups is
through the nucleophilic sulfur atom in the compound, so it appears that the amide groups are
responsible for the gold adsorption. PMO-EDT also showed to provide little to no adsorption
capabilities of gold at the neutral pH.
In the case of PMO-TU, the material provided adsorption capabilities of gold from the
thiosulfate complex. The isotherm shown in fig. 3.10 indicates that the adsorption of gold ions
follows a type I character. The material can recover gold well at low concentrations, but
eventually reaches a saturation point in which no more gold can be recovered. This can be
attributed to the amount of adsorption sites available in the material. This adsorption trend is
beneficial for gold because it is often found at low concentrations in ore. It was concluded that
at neutral pH, PMO-TU was the most efficient material for gold adsorption from thiosulfate
25
solutions. Maximum adsorption capacity values of over 1 mmol of gold per gram of material
were determined at high concentrations of AuThioS.
3.4.2
The Effect of pH
Maintaining a constant AuThioS concentration of 10 ppm allows the pH to be varied for
adsorption studies. The pH levels of the solutions were adjusted from 5 to 9, which fits within
the range of stability of thiosulfate (as shown in fig. 1.1). Results of this experiment are found in
figure 3.11. It is easily observed that more gold adsorption from thiosulfate solutions occurs as
weakly basic pH levels. Even PMO-SH and PMO-EDT show indications of adsorption at pH=8
and pH=9. The results can also be seen in table form in Table 3.1.
This adsorption edge is expected out of the materials based on their functionalization. At
higher pH, the thiol and amine groups are deprotonated and possess negative charges, which
would attract Au (I) ions with a positive charge. These results further prove that the nucleophilic
substitution reaction was successful, and that all three of the materials could be used for gold
recovery. However the adsorption of gold from thiosulfate solutions is still limited for each of
the materials. Therefore, the best material out of the three for recovery of gold from thiosulfate
solutions was PMO-TU, which adsorbed roughly 0.15 mmol of gold per gram of material.
3.4.3
Adsorption of Gold from Chloride
Although the Carbon in Pulp method may be utilized for gold chloride solutions, the
method is not very efficient, as mentioned previously. The adsorption of gold from chloride
solutions was expected to be higher than that for thiosulfate solutions. That is, of course, if the
nucleophilic substitution reactions were successful. However, the pH test and TGA
measurements were evidence of this, so all three materials were expected to be sufficient.
26
Adsorption isotherms for the three PMOs can be found in fig. 3.12, for gold chloride
concentrations ranging from 1 to 200 ppm.
By viewing the isotherms, it is easily observed that each material behaves differently in
the adsorption of gold ions from chloride solution. PMO-TU and PMO-EDT were shown to be
efficient at low concentrations of gold chloride, eventually reaching saturation at 0.60 and 0.20
mmol Au/g PMO, respectively. These values were comparable to the gold chloride adsorbing to
activated carbon at 0.25 mmol/g. Therefore, PMO-EDT was proven to be an inefficient
alternative to activated carbon. PMO-TU, however, could be regarded as a useful alternative for
gold recovery from chloride solutions at low concentrations of gold. PMO-SH behaved
differently for gold chloride than the other two materials. The adsorption isotherm in fig. 3.12
shows little adsorption of gold at low concentration, but it gave a steep increase in adsorption at
higher concentrations. This may be attributed to the accessibility of the thiol groups within the
framework of the material. As more gold is added to the solution, it can reach these thiol groups
easier, providing high adsorption. The maximum adsorption capacity of PMO-SH was
determined to be much higher than that of PMO-TU and PMO-EDT. For gold chloride, a
maximum adsorption of over 2 mmol Au/g PMO could be achieved. It could then be concluded
that PMO-SH was very acceptable for gold adsorption from chloride leaching solutions.
27
Figure 3.1 – Nitrogen Adsorption Isotherm of PMO-EE
28
Figure 3.2 – Pore Distribution Curve of PMO-EE
29
Figure 3.3 – TGA Weight Loss Data for PMO-EE
30
Figure 3.4 – Nitrogen Adsorption Isotherm of PMO-Br
31
Figure 3.5 – Pore Distribution Curve of PMO-Br
32
Figure 3.6 – TGA Weight Loss Data for PMO-Br
33
Figure 3.7 – Nitrogen Adsorption Isotherms of PMO-SH, PMO-TU, and PMO-EDT
34
Figure 3.8 – Pore Distribution Curves of PMO-SH, PMO-TU, and PMO-EDT
35
Figure 3.9 – TGA Weight Loss Data for PMO-SH, PMO-TU, and PMO-EDT
36
Figure 3.10 – Gold Adsorption Isotherms with AuThioS concentrations 1-100 ppm using
PMO-SH, PMO-TU, and PMO-EDT
37
Figure 3.11 – The Effect of pH on Gold Adsorption with PMO-SH, PMO-TU, and PMOEDT
38
Amount of Au Adsorbed
pH
5
6
7
8
9
PMO
(mmol/g PMO)
PMO-SH
0.0002
PMO-TU
0.075
PMO-EDT
0
PMO-SH
0.002
PMO-TU
0.092
PMO-EDT
.0003
PMO-SH
0.009
PMO-TU
0.119
PMO-EDT
0.006
PMO-SH
0.044
PMO-TU
0.156
PMO-EDT
0.037
PMO-SH
0.082
PMO-TU
0.146
PMO-EDT
0.097
Table 3.1 – The Effect of pH on Gold Adsorption with PMO-SH, PMO-TU, and PMO-EDT
([Au] = 10 ppm)
39
Figure 3.12 – Gold Adsorption Isotherms with AuCl concentrations 1-200 ppm using PMOSH, PMO-TU, and PMO-EDT
40
4. Summary and Conclusions
In conclusion, periodic mesoporous organosilica (PMO) were synthesized using a nonionic surfactant assembly method. The PMO incorporated with ethylene groups within the
framework of the material showed high surface area along with uniform pore diameters. The
ethylene-incorporated PMO was able to be brominated using a simple bromination procedure for
alkenes. With the brominated material, and organic substitution achieved, thiol, thiourea, and
ethanedithiol were all used as nucleophiles in the substitution. Each reaction proved to not
damage the PMO. The advantage of organic functionalization of PMOs as opposed to regular
organic-inorganic mesoporous silica hybrids is the higher pore volume and less pore blockage.
The thiol-functionalized PMO (PMO-SH) showed extreme inefficiency in the adsorption
of Au (I) ions from gold-thiosulfate solutions, but was better in weakly basic conditions. The use
of a dithiol (PMO-EDT) also proved to be ineffective at gold adsorption from the thiosulfate
complex, with the same pH trends as PMO-SH. However, the thiourea-functionalized PMO
(PMO-TU) proved to be effective in adsorption of gold from thiosulfate, especially at weaklybasic pH levels. Gold chloride tests further investigated the gold recovery capabilities of the
three materials, and each showed better adsorption than their thiosulfate counterparts. PMO-SH
showed potential as an alternative gold recovery method with gold chloride. PMO-TU could
also be a useful alternative due to the high adsorption of gold from chloride solutions at low
concentrations. Combining this data with the thiosulfate data, PMO-TU should be considered
the best PMO that could be used for the recovery of gold from non-cyanide leaching solutions.
However, other nucleophiles could prove to be better than thiourea.
41
4.1 Future Plans
Certain characterization techniques could have proven to be useful to this project. X-ray
diffraction (XRD), infrared (IR) spectroscopy, and scanning electron microscopy (SEM) could
be used to improve results and understanding. Optimization of the PMO materials will be a goal
for the future, and more nucleophiles will be sought out to be used for gold adsorption. Further
investigations into heavy and precious metal systems will also be performed. Platinum and
rhodium seem to be potential candidates for recovery using PMOs, because adsorption will
operate by the same mechanism.
42
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