89 CHAPTER 7
Gold(III) extraction and copper(II)-gold(III) separation by micellar enhanced ultrafiltration
Micellar enhanced ultrafiltration (MEUF) is a surfactant based separation technique that could be used to remove metal ions and/or dissolved organics from waste streams. Previous studies have demostrated that it is a promising method to extract metal ions with high yields, up to 99.8% (Klepac et al., 1991).
The separation of the micellar pseudo-phase from the aqueous pseudo-phase is achieved by ultrafiltration using membranes with pore diameter smaller than that of the involved micelles. While aqueous-phase passes through the membrane, the micellar phase will be retained above the membrane. In this way, it is possible to block the metal ions within the micellar pseudo-phase. As described in the first chapter, the interior of the micelle contains the hydrocarbon chain which forms a hydrophobic enviroment. Therefore, in the case of organic substances, these are retained in the micellar interior because of the hydrophobic interactions with the surfactant head group. In the case of metal ions, the attraction between metal ion and micelle is generally based on electrostatic interaction. With anionic surfactants, any cation present in solution will tend to adsorb or bind preferentially on the micelle surface due to electrostatic attraction. By converse, with cationic surfactant, anionic species in solution will bind to the micelle instead. In the non-ionic surfactant case, metal ions can still be blocked on the (neutral) micelle surface if the surfactant heads can act like complexing ligands.
In spite of MEUF’s advantages (high yield extraction, less energy consumption and softer environmental impact) over classical methods as distillation and evaporation, simple MEUF application has proven to lack of selectivity, since it
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has been found that positively charged metal ions Ca2+, Cu2+, Ni2+ and Zn2+ are extracted by negatively charged surfactant systems with almost the same yields (Christian S.D, et. al, 1989) .
On the other hand, to enhance selectivity of metal ion extraction Ligand Modified Micellar Enhanced Ultrafiltration method has been developed (LM- MEUF). In this way, by the addition complexing ligand which is able to dissolve in micelle, by hydrophobic effect, it is possible to increase selectivity concerning complexation reaction between metal and ligand. Moreover, the ligand modified extraction is useful to increase extraction yield where metal ion and surfactant system has same ionic charge and there is repulsion between metal ion and same charged micelle, as in the gold(III)-PADA system in SDS here studied.
In this part of the thesis, first the extraction of gold has been investigated in the presence of positively and negatively charged surfactants performing ultrafiltration process. Then, in order to explore the possibility of enhancing the extraction yield and selectivity, also LM-MEUF has been studied by using the complexing ligand PADA. Finally, the copper(II) extraction has been studied and the Cu2+/AuCl4−
separation has been investigated.
The metal amounts extracted have been evaluated by atomic absorption spectrophotometry, as the difference between the metal levels before and after ultrafiltration. The results are expressed in terms of the metal retention in micelle pseudo phase by equation 7.1 (Ghezzi et. al 1997; Ghezzi et. al, 1998)
M 100 1 M tention Re
%
i p×
−
= (7.1)
where Mp is the metal amount in the permeate (expressed in grams) and Mi is the metal amount in the initial solution (expressed in grams).
91 7.1. Gold(III) extraction with different charged surfactant systems by ultrafiltration method.
A previous study on gold(III) extraction with micelle has been done using a non-ionic surfactant system which behaves like a ligand (Akita et. al, 1997). In this thesis, gold and copper extraction and recovery has been investigated using micelles (SDS or DTAC) with charge opposite with respect to the metals to be extracted. First, the micelle extraction process was performed in the absence of the complexing ligand, i.e. simply relaying on the electrostatic interaction. Then, in order to increase the extraction yield, the ligand enhanced (LM-MEUF) process has been applied.
7.1.1 Gold(III) extraction by sodium dodecyl sulphate (SDS)
Micellar enhanced ultrafiltration of gold in SDS Firstly, the extraction of gold ([HAuCl4.3H2O] = 1×10-4 M) has been investigated in the absence of PADA at different concentrations of SDS (pH = 3.5; NaCl = 0 M). The percentage of retention is scarcely effected by the SDS concentration as shown in Figure 7.1 and Table 7.1.
0.00 0.02 0.04 0.06
0 20 40 60 80 100
% Retention
[SDS] (M)
Figure 7.1 Percentage of gold(III) retention vs [SDS] in the absence of PADA; pH = 3.5, [NaCl] = 0 M, T = 25 °C
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Table 7.1 Dependence of the relative amount of gold(III) retained on the micelle surface on the [SDS] concentration. CAu[initial] = 1×10-4 M, pH = 3.5, [NaCl] = 0 M, T = 25°C.
The results show that gold extraction by SDS using MEUF gives rather poor yields, as expected because of the repulsion between AuCl4−
and SDS which is also negatively charged. The fact that the retention fraction, although small, is not zero, could be explained assuming that the –SO3−
residues located at the surface of the SDS micelle could compete with the Cl− for the coordination shell of Au(III). In other words SDS can act as a ligand being able to bind the gold(III) complexes, with displacement of chloride ions.
LM-MEUF of gold in SDS In order to investigate if addition of a hydrophobic ligand could improve the extent of metal retention, the extraction of gold by SDS has been done using PADA as a complexing agent. The ultrafiltration treatment has been repeated changing the SDS concentration. The studies on the gold (1×10-4 M)-PADA (5×10-4 M) system at different SDS concentrations have been done at [NaCl] = 0.1 M and 0 M. The obtained results are summarised in Figure 7.2 and Table 7.2.
[SDS] (M) % Retention
0.00 0.00
0.01 34.5
0.02 20.0
0.03 31.0
0.04 24.4
0.06 23.0
93
0.01 0.02 0.03 0.04 0.05
0 20 40 60 80 100
0 M NaCl 0.1 M NaCl
% Retention
[SDS] (M)
Figure 7.2 Percentage of gold(III) retention vs [SDS] in the presence of PADA at different NaCl concentrations; pH = 3.5, T = 25°C.
Table 7.2 Dependence of the relative amount of gold(III) retained on the micelle surface on the [SDS] concentration in the presence of PADA, pH = 3.5, T = 25°C.
In the presence of PADA the percent of retention displays a dramatic increase compared to simple MEUF under all the employed conditions. One can, thus, conclude that the best way to extract gold in the presence of Cl- ions is to use
[SDS] (M) [NaCl] (M) % Retention
0.016 0 92
0.024 0 99
0.032 0 100
0.047 0 79
0.016 0.1 100
0.024 0.1 99
0.032 0.1 100
0.040 0.1 92
0.047 0.1 86
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LM-MEUF. The LM-MEUF process has been applied at constant surfactant concentration (0.04 M SDS) at different pH values. The results are shown in Figure 7.3 and Table 7.3 which show that the medium acidity does not influences the extraction yield.
2 3 4 5 6 7
0 20 40 60 80 100
% Retention
pH
Figure 7.3 Percentage of gold(III) retention vs pH in the presence of PADA; [NaCl] = 0 M, [SDS] = 0.04 M, T = 25 °C.
Table 7.3 Dependence of the relative amount of the gold(III) retained on the micelle surface on pH in the presence of PADA. [SDS] = 0.04 M, [NaCl] = 0 M, T = 25°C.
In conclusion, the comparison between the yields of metal extraction in the absence (MEUF) and in the presence (LM-MEUF) of PADA shows that the introduction of the ligand induces an enormous increase of the extraction level.
This effect is explained assuming that the ligand, once bound to the metal ion, is
pH % Retention
2.5 100
3.5 100
4.5 91.0
5.5 93.4
6.5 91.9
95 able to impart to the resulting complex not only its hydrophobic characteristics, but also a positive charge to the resulting chelate, as shown in Chapter 6. The result is that the gold(III)-PADA complex will be highly retained on SDS by virtue of the combined action of hydrophobic forces and electrostatic attraction.
Note that excellent yields are already obtained at very low (0.01 M) surfactant concentrations (Figure 7.2) and under very mild medium conditions (Figure 7.3).
7.1.2 Gold(III) extraction by dodecyl trimethyl ammonium chloride (DTAC) Micellar enhanced ulftrafiltration of gold in DTAC Micellar enhanced ultrafiltration (MEUF) experiments have been performed for gold solutions (1×10-4 M) changing the DTAC concentrations, both in the presence of added salt (0.1 M NaCl) and without adding salt (0 M NaCl). Note that the DTAC micelles are positively charged. The results obtained are shown in Figure 7.4 and Table 7.4.
0.00 0.02 0.04 0.06
0 20 40 60 80 100
0.1 M NaCl 0 M NaCl
% Retention
[DTAC] (M)
Figure 7.4 Percentage of gold(III) retention vs [DTAC] in the absence of PADA at different NaCl concentrations; T = 25°C.
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Table 7.4 Dependence of the relative amount of gold(III) retained on the [DTAC]
concentration; T = 25 °C.
Note that extremely good yields are obtained also for low DTAC concentrations and that the addition of salt does not affect the extent of retention.
The effect of pH on the percentage of retention of gold by DTAC is shown Figure 7.5 and Table 7. 5. The retention remains close to 99%, independent of the pH changes.
Figure 7.5 Percentage of retention of gold(III) dependence on pH; [DTAC] = 0.04 M, [NaCl] = 0 M, T = 25 °C.
[DTAC] (M) [NaCl] (M) % Retention
0.00 0 0
0.01 0 100
0.02 0 100
0.03 0 89
0.04 0 90
0.06 0 87
0.01 0.1 100
0.02 0.1 99
0.03 0.1 96
0.04 0.1 97
0.06 0.1 96
2 3 4 5 6
0 20 40 60 80 100
% Retention
pH
97 Table 7.5 Dependence of retention of gold(III) on pH; [DTAC] = 0.04 M, [NaCl] = 0 M, T = 25 °C.
LM-MEUF of gold in DTAC The experiments described above have show that AuCl4− could be extracted by DTAC with good yields also in the absence of ligand, owing to the positive charge of the DTAC surface. However, some ultrafiltration studies have been performed in the presence of PADA. The results obtained are reported in Table 7.6.
Table 7.6 Dependence of retention of gold(III) on pH in the presence of PADA;
CAu(initial) = 1×10-4 M, CPADA = 5×10-4 M; [DTAC] = 0.04 M, [NaCl] = 0 M, pH = 5.0, T = 25°C.
7.2. Copper(II) extraction by SDS and DTAC using MEUF
Although this part of the thesis is focused on the extraction (and recovery) of gold it is deemed to be interesting and useful to investigate the possibility of extracting gold when mixed to another metal. Since copper is present in most gold containing alloys, the behaviour, with respect to extraction, of copper
pH % Retention
2.5 98.5
3.0 99
4.0 98
5.0 96
6.0 99
[DTAC] (M) % Retention
0.00 0
0.01 100
0.02 100
98
alone and in presence of Au(III) has been studied by MEUF. The ultrafiltration procedure was applied to solutions containing DTAC and, in turn, SDS.
7.2.1 Copper (II) extraction by dodecyl trimethyl ammonium chloride (DTAC) Micellar enhanced ultrafiltration (MEUF) experiments have been performed for Cu(ClO4)2 solutions (1×10-4 M) in the presence of different DTAC concentrations without adding salt. The results obtained are shown in Figure 7.6 and Table 7.7. Being copper in the form of Cu2+ ion, this is totally repelled by the positive DTAC micelles and the extraction yield is zero.
0.00 0.01 0.02 0.03 0.04 0.05
0 20 40 60 80 100
%Retention
[DTAC] (M)
Figure 7.6 Percentage of retention of copper(II) dependence on [DTAC]; [NaCl] = 0 M, pH = 3.5, T = 25°C.
Table 7.7 Percentage of retention of copper(II) dependence on [DTAC], [NaCl] = 0 M, pH = 3.5, T = 25°C.
[DTAC] (M) % Retention
0.01 3
0.02 0
0.03 0
0.04 0
0.05 0
99 7.2.2 Copper(II) extraction by sodium dodecyl sulphate (SDS)
Also copper extraction studies have been done in the presence of SDS using the micellar enhanced ultrafiltration (MEUF) method. The yields obtained are shown in Figure 7.7 and Table 7.8
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0
20 40 60 80 100
%Retention
[SDS] (M)
Figure 7.7 Percentage of retention of copper(II) dependence on [SDS];
[Cu(ClO4)2] = 1×10-4 M, [NaCl] = 0 M, pH = 3.5, T = 25°C.
Table 7.8 Percentage of retention of copper(II) dependence on [SDS], [Cu(ClO4)2] = 1×10-4 M, [NaCl] = 0 M, pH = 3.5, T = 25°C.
In this case, as expected, the negative SDS micelles do attract Cu2+ ions with a yield that reaches 100% for [SDS] = 0.04 M.
[SDS] (M) % Retention
0.01 97
0.02 98
0.04 100
0.06 100
100
7.3 Gold(III)/Copper(II) separation by MEUF 7.3.1 Gold(III)/Copper(II) separation in DTAC
Firstly, the MEUF process has been applied to a mixture of AuCl4−
and Cu2+ in the presence of 0.04 M DTAC at different pH values. The results have shown that, in the presence of the positively charged surfactant, while positively charged copper ions pass through ultrafiltration membrane in the aqueous pseudo-phase because of the electrostatic repulsion, the negatively charged gold ions are retained on the micellar pseudo-phase, thanks to the electrostatic attraction. The results are shown in Figure 7.8 and Table 7.9.
3.0 3.5 4.0 4.5 5.0
0 20 40 60 80 100
Au Cu
% Retention
pH
Figure 7.8 Percentage of retentions of copper(II) and gold(III) vs pH in 0.04 M DTAC system; [NaCl] = 0 M, pH =3.5, T = 25°C.
Table 7.9 Percentage of retentions of copper(II) and gold(III) vs pH in 0.04 M DTAC;
[NaCl] = 0 M, pH = 3.5, T = 25°C.
pH Au(% Cu(%Retention)
3.0 95 7
3.5 95 4
4.0 96 0
4.5 96 0
5.0 96 3
101 7.3.2 Gold(III)/Copper(II) Separation in SDS
Further, the separation study of AuCl4−/Cu2+ mixture also has been performed by MEUF using negatively charged SDS micelles. Oppositely to the DTAC case, the negatively charged gold ions passed through the ultrafiltration membrane while the Cu2+ ions remain blocked in the micellar pseudo-phase above the membrane being blocked on SDS. Figure 7.9 and Table 7.10, show that the separation of the two metal is very effective independently of the medium acidity.
3.0 3.5 4.0 4.5 5.0
0 20 40 60 80 100
Au Cu
% Recovery
pH
Figure 7.9 Percentage of retention vs pH for the gold(III)/copper(II) mixture in 0.04 M SDS, [NaCl] = 0 M, pH = 3.5, T = 25°C.
Table 7.10 Dependence on pH of metal ion retentions by micellar enhanced ultrafiltration method; [SDS] = 0.04 M, [NaCl] = 0 M, pH = 3.5, T = 25°C.
pH Au(% Cu(%Retention)
3.0 4 100
3.5 2 100
4.0 4 100
4.5 0 100
5.0 0 99
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In conclusion, gold(III) and copper(II) separation by MEUF using respectively SDS and DTAC has been successfully performed. However, this method is suitable just for separate opposite charged metal ions. Otherwise, in the case of metal ions with charges of the same sign, extraction yield is approximately the same (Scamehorn et. al, 1994).
7.4 Gold recovery from micellar pseudo-phase
Once the metal has been extracted and concentrated on the micelle it is necessary to recover it adopting procedures of metal removal. Procedures for gold(III) recovery have been worked out in SDS and DTAC. The procedures used in this thesis to remove gold from the retentate phase are based on addition of salts which reduce the surface potential of the micelles, or addition of NH3 in order to convert the negatively charged AuCl4−
into the positively charged Au(NH3)43+ complex which, in the case of metal absorption on DTAC, is pushed from the micellar to the aqueous phase by the electrostatic repulsion.
Gold recovery from the DTAC-gold(III) system. The recovery procedure was applied to the gold extracted by DTAC. Table 7.11 shows the results at different concentrations of NaCl and at a single concentration of Na2SO4.
Table 7.11 Percentage of recovery of gold(III) from DTAC micellar system by adding NaCl and Na2SO4. In the case of NaCl two consecutive recovery steps (% Rec 1 and % Rec 2) have been done both with NaCl solutions; CAu = 1×10-4 M, [DTAC] = 2×10-2 M, T = 25 °C.
[NaCl] (M) [Na2SO4] (M) % Rec 1 % Rec 2 Total
0 0.5 17 17
0.3 0 2 3 5
0.5 0 3 21 24
0.5 0 15 5 20
0.75 0 2 3 5
1 0 20 11 31
1 0 5 3 8
1.5 0 3 2 5
2 0 1 1 2
103 The total percentage of recovery even after two consecutive extractions (i.e., after addition to the retentate of the salt solution twice) is rather low. Hence a set of recovery experiments has been carried out using a NH3 solution as the recovery agent (Table 7.12). The recovery after two extractions is also poor, although better compared with the data of Table 7.11.
Table 7.12 Percentage of recovery of gold(III) from DTAC by adding NH3 both in the first (%Rec 1) and second (%Rec 2) recovery step. CAu = 1×10-4 M, [DTAC] = 2×10-2 M, T = 25 °C.
[NH3] (M) %Rec 1 %Rec 2 Total
0.1 26 6 32
0.2 18 - 18
0.2 16 - 16
0.5 15 2 17
1 28 8 36
Table 7.13 shows the results in the presence of mixture of NH3 and NaCl. The recovery yield is quite good, revealing that the presence of NaCl, which reduces the surface potential combined with electrostatic repulsion, is important. Table 7.14 shows the results obtained by adding subsequently NaCl and HCl.
Table 7.13 Percentage of recovery of gold(III) from DTAC by adding in sequence NaCl (%Rec 1 and %Rec 2) and NH3 (%Rec 3); CAu = 1×10-4 M, [DTAC] = 2×10-2 M, T = 25 °C.
[NaCl] (M) [NH3] (M) % Rec 1 % Rec 2 % Rec 3 Total
0.5 0.2 21 - 60 81
0.75 0.2 2 3 50.4 56
1 0.2 20 11 55 86
2 0.2 1 1 83 85
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Table 7.14 Percentage of recovery of gold(III) from the DTAC by adding in sequence NaCl and HCl (%Rec 1 concerns first recovery by NaCl solutions, %Rec 2 concerns the subsequent recovery by addition of the HCl solution); CAu = 1×10-4 M, [DTAC] = 2×10-2 M, T = 25°C.
[NaCl] (M) [HCl] (M) % Rec 1 %Rec 2 Total
0.5 0.1 5 15 20
0.5 0.2 20 13 33
0.5 0.3 18 13 31
The results suggest that the addition of HCl does not improve the yield with substantially.
Gold Recovery from the SDS-gold-PADA system An attempt has been made to recovery gold from SDS-Au-PADA micelles by adding to the retentate different concentrations of NaCl and HCl. The obtained results demonstrate that in this case the gold recovery is very poor. The results are reported in Table 7.15.
Table 7.15 Percentage of gold(III) recovery from SDS in the presence of PADA by addition stripping agents as respectively NaCl, HCl and NH3; CAu = 5×10-5 M, CPADA
= 1×10-4 M, [SDS] = 2×10-2 M, pH = 3.5, T = 25 °C.
[NaCl] (M) [HCl] (M) [NH3] (M) % Rec 1 % Rec 2 Total
0.10 0 0 0 0 0
0.20 0 0 0.2 0.5 0.80
0.50 0 0 1.9 0 1.90
0.75 0 0 0 0 0
0 0.010 0 1.8 0.1 1.90
0 0.020 0 0.1 0 0.10
0 0.050 0 0 0 0
0 0.075 0 0 - 0
0 0.1 0 0.01 - 0.01
105 In conclusion, the MEUF procedure in the presence of DTAC provides excellent extraction yields and, under particular conditions (see Table 7.13), good recovery yields. On the other hand LM-MEUF procedure with SDS, while providing excellent gold extraction thank to the properties of PADA, provides, in contrast, a poor yield of recovery. An interpretation of this behaviour could reside on the fact that the Au-PADA chelates are too strongly adsorbed on the SDS surface owing to the combined action of hydrophobic forces and charge that act both in the direction of increasing the retention of the Au-PADA complex on the micelle.
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