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Non-dispersive Extraction of Chromium(VI) by Cyphos IL102/Solvesso 100 Using the Pseudo-Emulsion Based Strip Dispersion Membrane Operation

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Abstract
The removal of chromium(VI) from acidic (HCl) medium through non-dispersive extraction with strip dispersion (NDXSD) was investigated using a microporous PVDF membrane support in a permeation cell. The ionic liquid Cyphos IL102 (phosphonium salt) in Solvesso 100 was used as organic phase. In NDXSD the stripping phase (NaOH) is dispersed in the organic phase in the cell side with an impeller stirrer adequate to form a strip dispersion. This pseudo-emulsion phase (organic + strip solutions) provides a constant supply of the Cyphos IL102/Solvesso 100 to the membrane phase. Various hydrodynamic and chemical parameters, such as variation in the feed and pseudo-emulsion stirring speeds, HCl and Cr(VI) concentrations in the feed phase, and carrier concentration, were investigated. From the experimental data, several mass transfer coefficients were estimated, as well as it was investigated the performance of the present system against other ionic liquids and the presence of base metals in the feed phase.
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Subject: Environmental and Earth Sciences  -   Pollution

1. Introduction

Ionic liquids (ILs) are a group of chemicals, which are receiving considerable attention in research and application due to their recognition as green solvents. Due to its specific properties, they are considered as alternatives to the use of some organic or inorganic chemicals, but also the possibility of customization (task-specific ILs) accordingly to special applications, it expanded its usefulness to various fields of interest as protein dissolution, stabilization, extraction, and purification, etc. [1,2,3,4,5].
Chromium and its related compounds are of a wide use in different industries, resulting in the discharge of the element in different environments. This element (as many others) is an essential micronutrient in humans and animals, but it is also a known carcinogenic (especially in the VI oxidation state) when its ingesta occurs at high concentrations, thus its removal from generated effluents is of a general concern.
Thus, different technologies had been proposed to remove Cr(VI) from aqueous environments. In solvent extraction studies, Deep Eutectic Solvents combining tetra-n-octyl ammonium bromide and carboxylic acids [6] or amide C14H29NO [7] are used to remove this element. Impregnating resins or polymer materials with DEHPA or ionic liquids (Cyphos IL 101, Cyphos IL 104 [8]) is another technology investigated in the removal of chromium(VI), whereas adsorption using nanocomposites [9], biochar [10] or activated carbon [11] are recent examples of various adsorbents used in this role.
In the case of solvent extraction, some drawbacks related to this technology, especially about to the treatment of diluted metal solutions, lead to the development of alternatives to its use, being supported liquid membrane (SLM) separation technology of interest because it combines the kinetics and selectivity of solvent extraction and the simplicity of the membrane diffusion processes. SLMs belong to the advanced variation of extraction operation [12]. In conventional SLM technology (either in flat-sheet, spiral wound or hollow fiber modules), the extraction and stripping processes are carried out simultaneously. In SLM separations, the feed and stripping or receiving aqueous solutions are separated by a hydrophobic membrane, which has been impregnated with the organic phase (normally comprised by the extractant or carrier and the diluent). This configuration facilitated the transport of the solute from the feed to stripping phase, of further interest is that this operational mode allowed the transport of a solute against its concentration gradient. However, some difficulties resulting from stability or long-term performance of these SLMs led to the development of a more advanced SLM operational mode as being the pseudo-emulsion-based strip dispersion, both in hollow fiber or flat-sheet operational modes [13,14,15,16,17]. Under this operation (see Section 2 for details) a pseudo-emulsion (organic and stripping phases) is formed by their mixing in a vessel, and due to that the membrane support is hydrophobic, the organic phase is immobilized into the membrane pores.
The present work investigates the removal of hazardous chromium(VI) from solutions using the ionic liquid Cyphos IlL02 dissolved in Solvesso 100 as carrier phase. An advanced membrane operation such as non-dispersive extraction with strip dispersion is used to investigate such removal. In this membrane technology, the strip phase is dispersed into the organic phase to form a pseudo-emulsion phase, which disengage at the end of the operation to yield an organic phase and a strip solution containing the transported chromium(VI). Several hydrodynamic and chemical parameters influencing the transport of chromium8VI) are investigated, as well as the use of other ionic liquids and the presence of common base metals in the feed phase. From the strip phase, Cr(VI) was reduced to Cr(III) using hydrazine sulphate to render a less toxic solution with a certain degree of profitability (possible formation of pigments). Some mass transfer parameters are estimated from experimental results.

2. Materials and Methods

2.1. Materials

The ionic liquid Cyphos IL102 was obtained from Cytec Ind. (Canada) now Solvay (France), being its active group trihexyl(tetradecyl)phosphonium bromide and it was used without further purification. Cyphos IL101 (trihexyl(tetradecyl)phosphonium chloride) and Aliquat 336 (trioctyl(methyl)ammonium chloride) were obtained from Cytec Ind, and Fluka (Switzerland), respectively. In the experimentation, the ionic liquids were dissolved in Solvesso 100 (99% aromatics, Exxon Chem. Iberia, Spain). Dissolution of the ionic liquid is of interest due to: i) it decreases the viscosity of the organic phase, this is of general importance in separation processes, i.e. solvent extraction, to facilitate phases disengagement, and in the case of membrane operations also to facilitate the transport of a given solute, since the diffusion coefficient of the extractant-solute species in the membrane support is inversely proportional to the viscosity of the organic phase [18], ii) to adequate the range of extractant concentrations to a given system, and iii) in connection to ii), to avoid the use of unnecessary, and thus, unusable extractant in the global inventory of the system.
All the other chemicals in the present work were of AR grade.
The membrane support was Durapore GVHP4700 (Millipore, USA). This PVDF (poly(vinylidenefluoride) support has a diameter of 47 cm diameter, with 0.22 μm nominal pore size, 125·10-4 cm thickness and porosity of 75%.

2.2. Methods

The liquid membrane operation was performed with a flat sheet membrane reactor that comprised two cells (200mL each) provided of impeller stirrers with 2.5 cm diameter. The liquid membrane phase was prepared by soaking the membrane support overnight in a mixture of the ionic liquid and Solvesso 100, and leave to drip during twenty seconds before putting in the cell. In the cell side corresponding to the organic and stripping phases, the impeller was located into the strip phase to provide dispersion of this phase into the continuous organic phase (Figure 1).
When the experiment runs, a pseudo-emulsion phase was immediately formed, and the metal was transported from the feed phase to the membrane phase and to the strip phase (Figure 2), mixing of the phases favored the stripping process.
At the end of the experiment, and after the stirring was stopped the phases disengage in a very few minutes (Figure 3), and the system reached the initial disposition.
Membrane permeabilities were determined by monitoring chromium or metal concentration by atomic absorption spectrometry (Perkin Elmer 1100 B spectrophotometer, UK) in the feed phase (or the stripping phase at the endo of the experiment) as a function of time. The chromium analysis was found to be reproducible within ±2 %. From the slope of the straight line obtained by plotting the left hand side of the next equation versus time (t), the overall permeation coefficient (P) was estimated:
l n C r f , t [ C r ] f , 0 = − A P V · t
where A was the effective membrane area (11.3 cm2), V was the volume (200 cm3) of the feed solution, [Cr]f,t and [Cr]f,0 were the chromium concentrations in the feed solution at an elapsed time t and time zero, respectively.
The percentage of chromium recovered in the strip phase was determined using the next relationship:
% R = [ C r ] s , t V f V s ( C r f , 0 − [ C r ] f , t · 100
where [Cr]s,t represented the metal concentration in the strip solution at an elapsed time, [Cr]f,0 and [Cr]f,t have the same meaning as in Eq. (1), and Vf and Vs are the volumes of the feed and stripping solutions, respectively.

3. Results and Discussion

3.1. Influence of the Stirring Speed in the Feed and Pseudo-Emulsion Phases on Cr(VI) Transport

The influence of this variable was investigated to optimize uniform mixing of the feed solution and to minimize thickness of aqueous feed boundary layer with feed and pseudo-emulsion conditions maintained as 0.01 g/LCr(VI) in 0.1 M HCl and 10% v/v Cyphos IL102 in Solvesso 100 + 1 M NaOH, respectively. Results derived from these experiments were shown in Figure 4.
The permeation coefficient increased from 200 to 1000 min-1indicating a progressive decrease in the boundary layer thickness, and then become independent of the stirring speed above 1000 min-1, thus, a minimum value of thickness is reached in this range of stirring speeds. The appearance of this plateau in the 1000-1200 min-1 region does not imply the complete elimination of the aqueous diffusion layer, but the minimization of the resistance due to it [19], resulting in a constant contribution of the diffusion of the chromium(VI) species to the mass transfer phenomena [20]. The stirring speed of 1000 min-1 in the feed phase was kept constant throughout the experimentation.
Using the same experimental conditions as above, the influence of the stirring speed in the pseudo-emulsion phase was also investigated. In this case, the speeds were varied in the 400-800 min-1range, and the results indicated that this variation did not influence the metals transport. In consequence, the stirring speed applied on the pseudo-emulsion phase was fixed at 600 min-1 in all the experiments.

3.2. Influence of the Stripping Phase Composition on Cr(VI) Transport

It was described elsewhere [21] that NaOH solutions were effective to strip Cr(VI) from loaded Cr(VI)-Cyphos IL102 organic phases. In the strip process, Cr(VI) released to the stripping solution as chromate species, regenerating the ionic liquid. Thus, within the present membrane methodology NaOH solutions were also used as stripping phases. The results of these experiments were summarized in Table 1. The permeation coefficients obtained using the different NaOH concentrations became almost independent of this variable, however, the recovery rate in the strip phase increases with the increase of the alkali concentration though this variation was negligible in the 0.5-1 M NaOH concentrations range. It can be also observed that the chromium concentration in the stripping phase averaging a concentration factor of 1.7 with respect the initial chromium concentration in the feed phase. As result of these experiments, 0.5 M NaOH was used as the stripping phase.

3.3. Influence of the HCl Concentration on Cr(VI) Transport

To asses the significance of the role of HCl concentration in the feed phase during the permeation of chromium(VI), HCl concentration variation investigations in the range 0.1-10 M were carried out in the presence of 0.01 g/l Cr(VI) in the feed phase (200 cm3), and pseudo-emulsion phase (200 cm3) of 10% v/v Cyphos IL102 in Solvesso 100 (100 cm3) + 0.5 M NaOH (100 cm3). As seen from Figure 5 and Table 2, permeation of Cr(VI) decreased with the increase in the HCl concentration in the feed phase, and thus, with the increase of the aqueous ionic strength which negatively influenced the metal transport within the present system.

3.4. Influence of the Carrier Concentration in the Organic Phase on Cr(VI) Transport

In all the liquid membranes technologies the carrier or extractant plays a key-role in making the transport operation efficient, in fact, the presence of this carrier facilitates the transport of the solute from the feed phase to the stripping phase by forming a specific solute-carrier complex, which also assisted to the selectivity of the process. Moreover, a supported liquid membrane having no carrier immobilized within its pores results in a negligible solute transport. Thus, it is of the utmost importance the evaluation of the influence of Cyphos IL102 concentration in the organic phase on the transport of chromium(VI). The influence of Cyphos IL102 concentration on the permeation of chromium(VI) was studied in the 0.6-10 % v/v (0.01-0,17 M) concentrations range.
As it can be seen from Figure 4, the transport of Cr(VI) increases with the increase of the carrier concentration at 2.5% v/v, with no further increase up to 10% v/v. From these data, the permeation coefficients for chromium(VI) transport at the different carrier concentrations were given in Table 3.
These results show that the permeation coefficient reached a maximum or limiting value (5.7·10-3M) at the carrier concentration of 2.5 % v/v and levels off. This limiting value also represented the value of the mass transfer coefficient in the feed phase (∆f-1) and:
P l i m = D f d f
where Df is the metal diffusion coefficient in the feed phase (averaging 10-5 cm2/s [22], and df is the minimum thickness of the aqueous boundary layer. Thus, this df for the present system is estimated as 1.8·10-3 cm. The above results indicated that at low carrier concentrations in the membrane phase, diffusion of the Cr(VI)-Cyphos IL102 complex across the liquid membrane is the rate-determining step, whereas in the 2.5-10% v/v concentrations range diffusion of the metal species across the aqueous boundary layer governed the transport process.
Assuming that the carrier concentration in the membrane phase is constant [23], the next equation allowed to estimate the value of the diffusion coefficient of the chromium-carrier species in the membrane phase [24,25]:
D m = J d m [ C y p h o s I L 102 ]
where dm is the membrane thickness (125·10-4 cm), and J is the metal flux, calculated as:
J = P [ C r ] f , 0
Thus, using a carrier concentration of 10% v/v (0.17 M), an initial chromium(VI) concentration in the feed phase of 0.01 g/L, and taking the correspondent value of the permeation coefficient (Table 3), the value of the flux is calculated as 1.1·10-9 mol/cm2s. Substituting this value in eq.(4), the value of Dm is estimated as 8.2·10-8 cm2/s.

3.5. Influence of the Chromium(VI) Concentration in the Feed Phase on Metal Transport

Figure 7 shows the variation in chromium(VI) transport with the variation of the initial metal concentration in the feed phase ranging from 0.01 to 0.075 g/L. Within this range of metal concentrations, the transport decreases with the increase of the metal concentration. Results in Figure 4 also indicated that an induction period was not observed which makes eq.(19 valid during all the experiments.
Accordingly with the above, the permeation coefficients decreased with the increase of the chromium(VI) concentration in the feed phase (Table 4).
In this same Table 4 it can observed that the metal flux (J), calculated as in eq.(5) increases with the increase of the initial chromium(VI) concentration, thus, within the present experimental conditions the transport of chromium(VI) is controlled by diffusion of metal species (HCrO4- accordingly with the range of metal concentrations used in this work [21]). Metal recoveries in the stripping phase decreased with the increase of the initial metal concentration. After 2 hours of reaction time, metal concentrated in the stripping phase except in the case of the feed solution containing 0.075 g/L Cr(VI), being this probably attributed to the low recovery rate derived with the use of this metal concentration.
It is worth to note that chromium(VI) was transported against its concentration gradient, however, the time to occur these phenomena is dependent on the initial metal concentration in the feed phase (Table 5). As it is seen, this period time increased with the initial chromium(VI) concentration.

3.6. Diffusional Parameters and Contribution of Mass Transfer Resistances to the Overall Chromium(VI) Transport Process

As it was mentioned in subsection 3.4., the mass transfer coefficient in the feed phase is estimated as 5.7·10-3 cm/s. The membrane mass transfer coefficient was estimated as [26]:
k m = D m ε τ d m
where the tortuosity (τ) is 1.67, the porosity (ε) 0.75, and dm of 125×10−4 cm. Thus, taking the value of 8.2×10−8 cm2/s for the membrane diffusion coefficient, see Eq. (4), for the chromium-carrier species in the organic solution, an estimated value of 2.9×10−6 cm/s is obtained for the present membrane system.
Furthermore, an effective membrane diffusion coefficient (Deff,m) of the chromium-ionic liquid species flowing across the membrane can be defined as [27]:
D e f f , m d m τ
and Deff,m is estimated as 6.1×10−8 cm2/s, which is of the same magnitude order than the value of the membrane diffusion coefficient.
The diffusion coefficient of the chromium(VI) species in the bulk membrane phase (Dm,b) can be also estimated as [28]:
D m , b = D m τ 2 ε
thus, the value of Dm,b is 3.1x10-7 cm2/s. The comparison of Dm and Dm,b values for the present system shows that Dm value is lower than that of Dm,b, this being attributable to the diffusional resistance caused by the membrane.
It was described in the literature [29], that the equilibrium and diffusional parameters involved in the transport process, can be combined in an equation of the form:
1 P = ∆ f + ∆ m C
where C is a parameter, involved in the extraction or transport process, which considered the extraction equilibrium constant or constants and the concentration of the species which participated within the process. 1/P was the overall resistance and Δf and Δm were the transport resistances in relation to diffusion by the feed boundary layer and the membrane, respectively.
In a transport process, the overall mass transfer resistance was the sum of the different resistances participating in the process, and thus, eq. (9) was rewritten as:
R = R f + R m
Table 6 showed the contribution, %Rf0 and %Rm0, of these various resistances to the overall resistance.
It was concluded that the diffusion by the aqueous feed boundary layer contributed in a major extend to the overall transport process. A mixed contribution of both aqueous and membrane diffusion under certain experimental conditions it was also found.

3.7. Treatment of the Cr(VI)-Bearing Strip Phase

The recovery of chromium(VI) from the strip phase was investigated by treatment of this phase with solid hydrazine sulphate in order to reduce Cr(VI) to the Cr(III) oxidation state [30]. The redox reaction can be described as:
4 C r O 4 2 − + 7 H 2 O + 3 2 N 2 H 6 2 − → 4 C r ( O H ) 4 − + 7 O H − + 3 2 N 2
This reduction is an instant reaction, and allows obtaining a potential pigment.

3.8. Comparison of the Performance of Cyphos IL102 against Other Ionic Liquids (Cyphos IL101 and Aliquat 336)

This investigation was performed using feed solutions of 0.01 g/L Cr(VI) in 0.1 M HCl and pseudo-emulsion phases containing 0.17 M of the single ionic liquid in Solvesso 100 + 0.5 M NaOH. The results of these experiments were shown in Table 7.
Results indicated that the performance of the three ionic liquids can be considered in similar terms, since if the permeability coefficient is somewhat greater in the case of Aliquat 336, the recovery of chromium(VI) in the strip phase is better in the case of Cyphos IL102.

3.9. Transport of Chromium(VI) in the Presence of Base Metals

It was investigated the transport of chromium(VI) in the presence of various base metals (Cu(II), Fe(III), Zn(II), Co(II)). These experiments were carried out using binary solutions of Cr(VI) and each base metal at initial metal concentrations (each) of 1.9·10-4 M (roughly 0.01 g/L) in 0.1 M HCl. The pseudo-emulsion phase contained 10% v/v Cyphos Il102 in Solvesso 100 + 0.5 M NaOH. The results indicated that none of the investigated base metals permeates, thus, chromium(VI) can be selectively separate from these elements. However, it was found that the value of the chromium(VI) permeation coefficient decreased from 5.7·10-3 cm/s, using single Cr(VI) solutions, to an average value of 4.2·10-3 cm/s for the binary systems. This decrease can be attributable to the crowding or population effect due to the presence of these ions in the feed solution [31].

4. Conclusions

The transport of chromium(VI) from HCl medium through a supported liquid membrane containing a Solvesso 100 solution of Cyphos IL102 was investigated. A microporous PVDF film, Durapore GVHP4700, was used as a solid support, whereas the pseudo-emulsion with strip dispersion membrane technology was used as advanced membrane operational mode. Chromium(VI) transport depends on a series of hydrodynamic and chemical variables. The aqueous boundary layer minimizes (daq= 1.8·10-3 cm) at stirring speeds in the 1000-1200 min-1 range, and carrier concentration in the membrane phase in the 2.5-10% v/v (4.3·10-2-1.7·10-1 M) range. An increase of the initial metal concentration in the feed phase produced adecrease of the permeation coefficient, but the metal flux increased, indicating an aqueous diffusion controlled transport. The mass transfer coefficient in the aqueous film, the diffusivity of the Cr(VI)-Cyphos IL102 complex in the bulk organic membrane solution, and in the membrane are also determined. The present system compares well with the use of other ionic liquids, being Cr(VI) transported selectively from a series of base metals. From the strip solution and by the use of hydrazine sulphate, chromium(VI) was effectively reduced to the lesser toxic Cr(III) state, opening the possibility of obtaining a valuable pigment.

Funding

This research received no external funding and the APC was funded by Membranes Editorial Office.

Acknowledgments

To the CSIC (Spain) for support. To Martin Ian Maher for English supervision.

Conflicts of Interest

The author declares no conflicts of interest.

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Figure 1. Non-dispersive extraction with strip dispersion. Location of the phases at t= 0.
Figure 1. Non-dispersive extraction with strip dispersion. Location of the phases at t= 0.
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Figure 2. Non-dispersive extraction with strip dispersion. Experiment running at an elapsed time.
Figure 2. Non-dispersive extraction with strip dispersion. Experiment running at an elapsed time.
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Figure 3. Non-dispersive extraction with strip dispersion. Phases at the end of the operation. It can be observed the transport of the metal (blue color) from the feed to the strip phase.
Figure 3. Non-dispersive extraction with strip dispersion. Phases at the end of the operation. It can be observed the transport of the metal (blue color) from the feed to the strip phase.
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Figure 4. Variation of the permeation coefficient with the stirring speed of the feed phase. Feed phase (200 cm3). Pseudo-emulsion phase (200 cm3): 100 cm3 organic phase + 100 cm3 stripping phase. Stirring speed pseudo-emulsion phase 400 cm3. Temperature 20°C.
Figure 4. Variation of the permeation coefficient with the stirring speed of the feed phase. Feed phase (200 cm3). Pseudo-emulsion phase (200 cm3): 100 cm3 organic phase + 100 cm3 stripping phase. Stirring speed pseudo-emulsion phase 400 cm3. Temperature 20°C.
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Figure 5. Chromium(VI) transport at various HCl concentrations in the aqueous phase. Temperature: 20°C.
Figure 5. Chromium(VI) transport at various HCl concentrations in the aqueous phase. Temperature: 20°C.
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Figure 6. Chromium(VI) transport at various carrier concentrations in the organic phase. Feed phase (200 cm3): 0.01 g/L Cr(VI) in 0.1 M HCl. Pseudo-emulsion phase (200 cm3): 100 cm3 carrier in Solvesso 100 + 100 cm3 0.5 M NaOH. Temperature: 20°C.
Figure 6. Chromium(VI) transport at various carrier concentrations in the organic phase. Feed phase (200 cm3): 0.01 g/L Cr(VI) in 0.1 M HCl. Pseudo-emulsion phase (200 cm3): 100 cm3 carrier in Solvesso 100 + 100 cm3 0.5 M NaOH. Temperature: 20°C.
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Figure 7. Chromium(VI) transport at various initial metal concentrations in the feed phase. Feed phase (200cm3): Cr(VI) in 0.1 M HCl. Pseudo-emulsion phase (200 cm3): 100 cm3 10% v/v Cyphos IL102 in Solvesso 100 + 100cm3 0.5 M NaOH. Temperature: 20°C.
Figure 7. Chromium(VI) transport at various initial metal concentrations in the feed phase. Feed phase (200cm3): Cr(VI) in 0.1 M HCl. Pseudo-emulsion phase (200 cm3): 100 cm3 10% v/v Cyphos IL102 in Solvesso 100 + 100cm3 0.5 M NaOH. Temperature: 20°C.
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Table 1. Influence of the stripping phase composition on Cr(VI) recovery in the strip phase.
Table 1. Influence of the stripping phase composition on Cr(VI) recovery in the strip phase.
Stripping phase ª% Cr(VI) recovery ª[Cr]s,t , g/L
0.1 M
0.5 M
1 M		 90
96
98		 0.016
0.017
0.018
ªAfter 2 hours. Experimental conditions as in Section 3.1.
Table 2. Chromium(VI) permeation coefficients at various HCl concentrations in the feed phase.
Table 2. Chromium(VI) permeation coefficients at various HCl concentrations in the feed phase.
[HCl], M P·103, cm/s
0.1
0.5
1
2.5
5
10		 5.7
5.5
5.0
4.7
2.1
0.83
From Figure 5.
Table 3. Permeation coefficients for Cr(VI) at the different Cyphos IL102 concentrations in the organic phase.
Table 3. Permeation coefficients for Cr(VI) at the different Cyphos IL102 concentrations in the organic phase.
[Cyphos IL 102], % v/v P·103, cm/s
0.6
1.3
2.5
5
10		 4.6
5.0
5.7
5.7
5.7
From. Figure 6.
Table 4. Cr(VI) permeation coefficients, fluxes and recoveries in the strip phase at various initial metal concentrations in the feed phase.
Table 4. Cr(VI) permeation coefficients, fluxes and recoveries in the strip phase at various initial metal concentrations in the feed phase.
[Cr]f,0, g/L P·103, cm/s J·109, mol/cm2s ª% Cr recovery ([Cr]s,t, mg/L ) Concentration factor
0.01
0.025
0.05
0.075		 5.7
5.1
4.4
3.7		 1.1
2.4
4.2
5.2		 96 (17)
91(40)
70(58)
50(60)		 1.7
1.6
1.2
0.8
From Figure 7. ªAt time 2 hours.
Table 5. Estimated time from which Cr(VI) is transported against its concentration gradient.
Table 5. Estimated time from which Cr(VI) is transported against its concentration gradient.
[Cr]f,0, g/L Time, min
0.01
0.025
0.05
0.075		 22
24
33
51
From Figure 7.
Table 6. Contribution of Rf and Rm to the overall chromium(VI) transport process.
Table 6. Contribution of Rf and Rm to the overall chromium(VI) transport process.
Experimental condition R, s/cm Rf, min/cm %Rf0 %Rm0
Stirring speed
ªHCl concentration
Cr(VI) concentration
Carrier concentration		 175-833
1204-175
175-217
175-270		 175
175
175
175		 100-21
15-100
100-81
100-65		 0-79
85-0
0-71
0-35
ªThe influence of the aqueous ionic strength should also be considered (Section ).
Table 7. Chromium(VI) transport using different ionic liquids.
Table 7. Chromium(VI) transport using different ionic liquids.
Ionic liquid P·103, cm/s ª%Cr recovery
Cyphos IL101
Cyphos IL102
Aliquat 336		 5.8
5.7
6.0		 80
96
77
ªAt time 2 hours. Temperature 20°C.
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