1. Introduction

2. Materials and Methods

3. Results and discussion

3.2. Supported liquid membrane experiments

In these membrane systems, the extraction (transport) of a given metal-species depends not only of the equilibrium parameters but also the kinetics parameters.

The influence of the variation (600-1600 min^-1) of the stirring speed of the feed phase on gold transport was investigated. The results from these experiments were shown in Table 6, and it can be observed that the overall mass transfer coefficient value (K_O) increased with the increase of the stirring speed from 600 to 1000 min^-1, and beyond that not increase in gold transport was observed. In fact, there was a decrease of the transport from 1400 min^-1 due to membrane instability probably attributable to the displacement of the organic phase from the membrane pores caused by the turbulence due to these higher stirring speeds.

In supported liquid membranes experimentation, two types of diffusional resistances are usually found: i) one due to the feed phase boundary layer, and ii) another in relation with the membrane support. Many times the magnitude of the values of both resistances competed between them [30]. These experimental results show that, in the 1000-1200 min^-1 range, the feed phase boundary layer reached a minimum and the aqueous resistance to mass transfer are minimized, thus, the diffusion contribution of the aqueous species to the mass transfer phenomena is constant [31].

The influence of the variation (800-1200 min^-1) of the stirring speed of the receiving phase on gold transport was also investigated using the very same experimental conditions as in Table 4, but using a stirring speed of 1000min^-1 in the feed phase. The results from these experiments showed that the variation of the stirring speed applied to the receiving phase had a negligible influence on gold transport. In the case of the receiving phase, and if the stirrer in the receiving compartment was close to the support, the thickness of the boundary layer was minimized and the resistance in the receiving side was neglected [32]. Thus, subsequent experiments were performed using stirring speeds of 1000 min^-1 for both feed and receiving phases.

Another variable investigated was the variation (0.5-6 M) of the HCl concentration in the feed phase on gold transport, keeping the carrier concentration in the membrane support constant. The results were shown in Table 7. It can be seen that gold permeation increased when the acid concentration in the feed solution was increased up to 1 M, being metal permeation independent of the HCl concentration in the 3-6 M range due to that the equilibrium:

$${\mathrm{HAuCl}}_4\leftrightarrow {\mathrm{AuCl}}_4^{-}+{\mathrm{H}}^{+}$$

(7)

was shifted completely to the left, and HAuCl₄ was the predominant species in the feed solution.

The influence of the variation of the carrier concentration on gold transport was next investigated. In these experiments, the feed phase was of 0.01 g/l Au(III) in 6 M HCl, whereas the organic phase contained 10-75% v/v 2-ethylhexanol in toluene or undiluted extractant. Table 8 showed the variation of the overall mass transfer coefficient with the variation of the carrier concentration in the membrane phase.

These results demonstrated that gold transport increased from 10 to 50% v/v carrier concentration in the membrane phase, then levels off and after decreased at the highest carrier concentrations. These phenomena can be attributable to that at low carrier concentrations, metal transport was dominated by membrane diffusion, whereas in the 50% v/v range the contribution of membrane diffusion was negligible and gold transport was controlled by diffusion in the stagnant film of the feed phase. At this maximum K_O value [33]:

$${\mathrm{K}}_{\mathrm{O}}=\frac{{\mathrm{D}}_{\mathrm{aq}}}{{\mathrm{d}}_{\mathrm{aq}}}$$

(8)

where D_aq represented the average aqueous diffusion coefficient (10^-5 cm²/s) of metal species in the feed phase [34], and d_aq was the minimum thickness of the feed phase boundary layer, thus, d_aq for the present system was estimated as 1.8·10^-3 cm. The decrease of gold transport at the highest carrier concentration was due to the increase of the organic phase viscosity, which decreased the Au(III)-2-ethylhexanol complexes diffusion coefficients values [35].

Investigation about the effect of the initial concentration (0.01-0.1 g/l) of gold, in the feed phase, on metal transport (Table 9), it was observed that the increase of the initial gold concentration was accompanied by a continuous decrease of metal transport, whereas the initial metal flux (J) defined as:

$$\mathrm{J}={\mathrm{K}}_{\mathrm{O}}{[\mathrm{Au}]}_0$$

(9)

initially increased from 0.005-0.04 g/L initial gold concentrations, and beyond this concentrations range, J became almost independent of the initial gold concentration in the feed phase.

According with these results, at low metal concentrations (0.01-0.04 g/L), the transport process was controlled by diffusion of gold species, whereas in the highest metal concentrations range, the near constant metal flux values were due to a change in the rate- determining step for the transport process. At these higher gold concentrations, membrane became saturate by metal-carrier species on the feed-membrane interface, resulting in a lower effective membrane area, which led to a near constant initial flux value.

Following the same considerations described in the literature [36], it can be concluded that the gold transport rate was determined by the rate of diffusion of the HAuCl₄ species across the feed phase diffusion layer and the rate of diffusion of the gold species eq. () and () through the liquid membrane. Thus, taking the same assumptions that in [37,38], a final expression for the overall mass transfer coefficient can be derived as:

$${\mathrm{K}}_{\mathrm{O}}=\frac{{\mathrm{K}}_{11}\left[\mathrm{ROH}{]}_{\mathrm{org}}+{\mathrm{K}}_{12}\right[\mathrm{ROH}{]}_{\mathrm{org}}^2}{{\mathsf{\Delta }}_{\mathrm{org}}+{\mathsf{\Delta }}_{\mathrm{f}}\left({\mathrm{K}}_{11}\left[\mathrm{ROH}{]}_{\mathrm{org}}+{\mathrm{K}}_{12}\right[\mathrm{ROH}{]}_{\mathrm{org}}^2\right)}$$

(10)

where ∆_org and ∆_f were the transport resistances due to diffusion across the membrane and the aqueous feed boundary layer, respectively. The above equation combined in one expression both equilibrium and diffusional parameters involved in the gold(III) transport process from the feed phase, across supported liquid membranes using 2-ethylhexanol as carrier.

To estimate the values of the resistance to the mass transfer, the above equation was linearized, resulting in:

$$\frac{1}{{\mathrm{K}}_{\mathrm{O}}}={\mathsf{\Delta }}_{\mathrm{f}}+{\mathsf{\Delta }}_{\mathrm{org}}\frac{1}{{\mathrm{K}}_{11}\left[\mathrm{ROH}{]}_{\mathrm{org}}+{\mathrm{K}}_{12}\right[\mathrm{ROH}{]}_{\mathrm{org}}^2}$$

(11)

and a plot of 1/K_O versus 1/$({\mathrm{K}}_{11}{[\mathrm{ROH}]}_{\mathrm{org}}+{\mathrm{K}}_{12}\left[\mathrm{ROH}{]}_{\mathrm{org}}^2\right)$ might result in a straight line with slope ∆_org and intercept ∆_f. From the plot, it resulted that ∆_org and ∆_f were found to be 2580 s/cm and 147 s/cm, respectively, with r²= 0.9895. The utility of eq.(11) to describe gold transport across the supported liquid membrane by 2-ethylhexanol was shown in Figure 1, where the experimental and calculated values using this equation have been represented versus the initial carrier concentration.

Whereas the first term (1/K_O) of eq. (11) represented the value of the total resistance (R_T), and being this resistance the sum of the mass transfer resistances due to the feed and the membrane phase, eq. (11) can be expressed as:

$${\mathrm{R}}_{\mathrm{T}}={\mathrm{R}}_{\mathrm{f}}+{\mathrm{R}}_{\mathrm{m}}$$

(12)

The total resistance calculated from experiments in Table 6 presented values in the 175-1052 s/cm range, in comparison the total resistance calculated by the model is 206 s/cm, which indicated that the resistance due to the membrane is dominant at low stirring speeds.

The contribution of the fractional resistances due to each step of the total transport process, R_f^o and R_m^o can be expressed by equations:

$${\%\mathrm{R}}_{\mathrm{f}}^{\mathrm{o}}=\frac{{\mathrm{R}}_{\mathrm{f}}}{{\mathrm{R}}_{\mathrm{T}}}·100$$

(13)

$${\%\mathrm{R}}_{\mathrm{m}}^{\mathrm{o}}=\frac{{\mathrm{R}}_{\mathrm{T}}-{\mathrm{R}}_{\mathrm{f}}}{{\mathrm{R}}_{\mathrm{T}}}·100$$

(14)

Under various experimental conditions, the values of %R_f^o and %R_m^o are summarized in Table 10.

Taking into account that:

$${\mathrm{D}}_{\mathrm{org}}=\frac{{\mathrm{d}}_{\mathrm{m}}}{{\mathsf{\Delta }}_{\mathrm{org}}}$$

(15)

where D_org represented the diffusion coefficient of the gold-containing species in the organic phase immobilized on the solid support and d_m was the membrane thickness, then, D_org was found to be 4.8·10^-6 cm²/s. The diffusion coefficient of the gold transported species in the bulk organic phase can be estimated by the next expression [39]:

$${\mathrm{D}}_{\mathrm{org},\mathrm{b}}=\frac{{\mathrm{D}}_{\mathrm{org}}{\mathsf{\tau }}^2}{ϵ}$$

(16)

where τ and ε were the membrane tortuosity and porosity values, respectively. The above resulted in D_org,b calculated as 1.8·10^-5 cm²/s. Comparison of D_org and D_org,b values showed that D_org was lower than D_org,b, which can be attributed to the diffusional resistance caused by the solid support separating the feed and receiving phases.

The selectivity of the present system against the presence of other metals in the feed phase was investigated. In this case, the feed phase contained 0.01 g/L each of Au(III), Ni(II), Cu(II), Fe(III) and PGMs (Ir, Os, Pd, Pt, Re, Rh, Ru) in 6 M HCl medium, being the organic phase of 50% v/v 2-ethylhexanol in toluene immobilized on Durapore GVHP4700 support. Like all the previous experimentation, water was used as receiving phase. The results indicated that PGMs were not transported under the present experimental conditions, whereas the transport of gold and the base metals followed the Au(III)>Fe(III)>Cu(II)=Ni(III) order, with separation factors Au/Fe, Au/Ni and Au/Cu of 1.3, 4.5 and 4,7, respectively. In these transport experiments the separation factors were calculated as:

$$\mathrm{SF}=\frac{{\mathrm{K}}_{\mathrm{O},\mathrm{Au}}}{{\mathrm{K}}_{\mathrm{O},\mathrm{M}}}$$

(17)

However, the presence of all these elements in the feed phase produced a decrease of the gold overall mass transfer coefficient from 5.7·10^-3 cm/s to 2.3·10^-3 cm/s. This behavior is not rare in supported liquid membrane operations and it is attributable to the multi-ion competition or crowding effect [40].

Once gold was transported from the feed solution to the membrane phase and finally to the receiving phase, it can be recovered from this last solution by precipitation as zero valent gold nanoparticles by the use of sodium borohydride. This procedure was used by one of us from years ago [41], otherwise the importance about the recovery of this precious metal as nanoparticles was also demonstrated [42], and uses of these nanoparticles in different fields (catalysis, medicine, sensors, etc,) regularly appeared in the literature [43-46]. In the present work, the receiving solution containing 0.009 g/L was precipitated by the direct action of solid sodium borohydride, after the reaction stopped, the dark solid was filtered. From this dry solid the next images were obtained.

Figure 2. Gold nanoparticles under magnifier.

Figure 2. Gold nanoparticles under magnifier.

Figure 3. TEM image of gold nanoparticles. Some degree of agglomeration is observed.

Figure 3. TEM image of gold nanoparticles. Some degree of agglomeration is observed.

Conclusions

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