The phosphine oxides Cyanex 921 and Cyanex 923 as carriers for
facilitated transport of chromium (VI)-chloride aqueous solutions
Francisco José Alguacila, Aurora López-Delgadoa, Manuel Alonsoa, Ana
María Sastreb
a
Centro Nacional de Investigaciones Metalúrgicas (CSIC), Avda. Gregorio del Amo 8,
Ciudad Universitaria, 28040 Madrid, Spain. E-mail: fjalgua@cenim.csic.es
b
Departament d´Enginyeria Química, E.T.S.E.I.B., Universitat Politécnica de
Catalunya, Diagonal 647, E-08028 Barcelona, Spain
Abstract
The behaviour of the phosphine oxides Cyanex 921 and Cyanex 923 in the facilitated
transport of chromium (VI) from chloride solutions is described. Transport is studied as
a function of several variables such as stirring speeds of the aqueous phases, membrane
phase diluent, hydrochloric acid concentration in the source phase and chromium and
carrier concentrations. The separation of chromium (VI) from other metals presented in
the source phase as well as the behaviour of phosphine oxides with respect to other
neutral organophosphorous derivatives (tri-n-butylphosphate (TBP) and dibutyl
butylphosphonate (DBBP)) are also investigated. Moreover, by using hydrazine
sulphate in the receiving phase, Cr(VI) is immediately reduced to the less toxic Cr(III).
Keywords: Facilitated transport; Cyanex 921; Cyanex 923; Chromium (VI)
1.Introduction
Among the various metals considered as hazardous for living organisms, chromium
(VI) is recognized to be one of the most dangerous due to its effect on development of
cancer and noncancer diseases (Casadevall and Kortenkamp, 2002).
However, chromium (VI) is a common element in daily life, because it is used in
different modern industries (Nriagu, 1988), thus, large quantities of this element are
discharged into the environment, and being its removal of a primary importance in
industrialized countries.
A number of technologies (chemical precipitation, ion exchange, adsorption by
activated carbon, etc.) are available for the separation of this element from various
effluents (Beszedits, 1988).
Including in these separation technologies, membrane technologies has gained a
prominent importance in the last decades, competing with long established technologies
for different uses, e.g. water desalination, food processing and medical applications. On
the other hand, membrane technologies could add new solutions to new problems due to
their possibility of recovering valuable products as well as treating effluents and
minimizing environmental problems, which are normally claimed to be caused by the
chemical industry. At last but not least, membranes are also now playing a special role
in the field of alternative energy, as one of the fundamental parts of a fuel cell (Nunes
and Peinemann, 2003).
Membranes for the separation and concentration of solutes have received attention
throughout the past due to characteristics such as ease of operation, energy related
advantages, lack of side reactions, low cost operation factors, etc.
2
Membranes may be related as semipermeable barriers, however, when placed between
two aqueous phases, solute species can move through the membrane across its
concentration gradient (from a region of high solute concentration to a region of low
solute concentration) by means of a diffusional process. Moreover, species can also be
transported across the membrane against their own concentration gradient as a
consequence of an existing concentration gradient of a second species present in the
system: the namely coupled transport; if the transport process occurs in the presence of
an extractant or carrier contained within the membrane phase, the coupled facilitated
transport takes place.
If the membrane contains only liquid phases, the category is named non-supported
liquid membranes (NSLMs), but if a polymeric support is involved in addition to the
liquid phases, the category of supported liquid membranes (SLMs) appeared (Alguacil
and Villegas, 2002); thus, a typical SLM consists of a polymeric support, of a various
nature, impregnated or in contact with an organic phase (which usually contained the
carrier dissolved in an organic diluent) and two aqueous solutions. Common SLM
configurations include flat-sheet, hollow fiber and spiral wound, and if the former is
useful for first laboratory purposes, both the latters are more adequate for industrial uses
since they provide higher surface area to volume ratio. Furthermore, the use of SLMs
containing a carrier has been considered as an alternative to liquid-liquid extraction
processes, especially in the treatment of dilute solutions, because they combine the
solute extraction and stripping processes in a single step; the solute is transported from
the feed to the membrane phase (and from here to the stripping phase) without phase
dispersion, thus avoiding the formation of stable emulsions that may hinder the process.
SLMs also overcome other shortcomings of liquid-liquid extraction such as flooding
and loading limits, and phase entrainment.
The use of liquid membranes technologies on chromium (VI) has been considered
before, with quaternary ammonium salts being the most widely used carriers (Chiarizia,
1991¸Yun et al., 1993; Ni et al., 1994; Guha et al., 1994; Alonso et al., 1994; Yang et
al., 1996; Alonso and Pentelides, 1996; Ortiz et al., 1996; Wang et al., 1998; Alonso et
al., 1999; Alguacil et al., 2000; Youn et al., 2000; Vincent et al., 2000; Alguacil et al.,
2001; Bhowal and Datta, 2001; Harrington and Stevens, 2001; Castillo et al., 2002).
Few data are available in the literature (Castillo et al., 2002) in which the removal of
Cr(VI) using liquid membranes also includes a stripping step in which Cr(VI) is reduced
“in situ” to the less toxic Cr(III) form, and if so, the membrane technology has been
investigated only for analytical purposes.
Thus, the present investigation concerns the transport of Cr(VI)-HCl solutions, with the
commercially available solvating extractants Cyanex 921 and Cyanex 923, the transport
process includes a stripping step in which Cr(VI) is reduced “in situ” to Cr(III) by the
use of hydrazine sulphate as strippant.
2.Experimental
The phosphine oxides used in this work had the following specifications.
Cyanex 921 solid extractant is composed of tri-n-octylphosphine oxide (Dziwinski and
Szymanowski,1998) with a molecular weight of 386.
Cyanex 923 liquid extractant is composed of a mixture of four main trialkylphosphine
oxides (about 92 %) (Dziwinski and Szymanowski, 1998) with various other
trialkylphosphine oxides also detected. The average molecular weight is 348 and the
density (20 ºC) is 880 kg/m3. Cyanex 921 and Cyanex 923 extractants were supplied by
3
Cytec Ind., tri-n-butylphosphate (TBP) and dibutyl butylphosphonate (DBBP) were
supplied by Fluka and Albright and Wilson, respectively. All extractants were used as
supplied by the manufacturers as also did with Solvesso 100 diluent (ExxonMobil
Chem. Iberia, Spain) containing >99 % aromatics, boiling range 167-178 ºC and flash
point 48 ºC; all other chemicals were of AR grade.
The flat-sheet membrane used was Millipore Durapore GVHP4700, a 125-μm-thick
microporous polyvinylidenefluoride film with nominal porosity of 75 %, effective pore
size of 0.22 μm and tortuosity of 1.67. The flat-sheet supported liquid membrane
(FSSLM) was impregnated with the corresponding carrier solution by immersion for 24
h and then leaving it to drip for 10 s before being placed in the transport cell. Previous
tests demonstrated that longer immersion time do not influence metal transport.
The FSSLM transport experiments were carried out with a two-compartment
permeation cell consisting of a source phase (200 cm3) separated from the receiving
phase chamber (200 cm3) by the SLM having an effective membrane area of 11.3 cm2.
The source and receiving phases were mechanically stirred by using cylindrical Teflon
impellers having a diameter of 2.4 cm at the corresponding stirring speeds (see Sections
3.1 and 3.2.1) to avoid concentration polarization conditions at the membrane
interfaces.
The percentage of metal transported was determined by monitoring the metal
concentration by atomic absorption spectrophotometry in the source phase as a function
of time:
%T 
M0  Mt
M0
(1)
where [M]0 and [M]t are the respective metal concentrations at time zero and at elapsed
time. Under the above experimental conditions, results were found to be reproducible
within ±2 %.
In all the transport experiments, the receiving phase contained hydrazine sulphate. The
use of this reagent allowing for the stripping-reduction of chromium (VI) to chromium
(III), and thus the metal is recovered in the receiving phase as a less toxic form; in this
phase the probable reaction occurs:
4H 2 CrO 4  pL org  3N 2 H 4  H 2 SO 4 r  4Cr r3  3SO 24r  6OH r  3N 2  10 H 2 O  4pL org
(2)
where H2CrO4·pL refers to the species formed in the organic (membrane) phase (see
Section 3.2) and org and r refer to the organic (membrane) and receiving phases,
respectively.
3.Results and discussions
3.1.Transport by Cyanex 921
In all the transport experiments carried out with Cyanex 921 (see Section 3.2.1.),
stirring speeds of 1400 min-1, were used for both the source and receiving phases.
Previous experiments showed that the percentage of chromium transport becomes
virtually independent of the stirring speed in these ranges, which also indicates that a
minimum value of the thickness of the source phase boundary layer is reached.
3.1.1.Effect of the receiving phase composition
4
The influence of the addition of various quantities of the stripping agent to the receiving
solution on the transport of chromium by Cyanex 921 was evaluated. Organic solutions
of 0.52 M Cyanex 921 in Solvesso 100 were used. The source phase contained 40 mg/l
of chromium in 1 M HCl.
Table 1 shows the results obtained from these experiments. It can be seen that beyond 5
g/l hydrazine sulphate there is not any increase in the percentage of chromium transport,
though the variation in metal transport using 2.5 g/l or 5 g/l stripping reagent is
minimal.
3.1.2.Effect of HCl concentration (source phase)
Further transport experiments were carried out at various HCl concentrations, keeping
other variables constant. Thus, the single metal transport through the membrane of 25
mg/l Cr(VI) from an aqueous source phase of varying HCl concentrations was studied
using 5 g/l hydrazine sulphate as strippant and organic phases of 0.52 M Cyanex 921 in
Solvesso 100. Results shown that, after 2 h, the transport of chromium sligthly increases
as the HCl concentration is increased. However, the transport of chromium tends to
stabilize at near 98 % for HCl concentrations in the 1-2 M range.
3.1.3.Effect of carrier concentration
This variable was also investigated to study its influence on chromium transport. The
source phase contained 40 mg/l chromium and 1 M HCl, the corresponding organic
phases contained Cyanex 921 from 0.06 M to 0.52 M in Solvesso 100, respectively,
whereas the receiving phase was of 5 g/l hydrazine sulphate.
The results obtained showed that the effectiveness of Cyanex 921 for chromium
transport is practically constant, near 98 % after 2 h, if carrier concentrations beyond
0.13 M are used, though the difference respect to the lower carrier concentrations is of
less than 3% metal transport.
3.1.4.Effect of metal concentration
The influence of the initial chromium concentration in the percentage of transport of
this metal by Cyanex 921 was investigated. This study was carried out using source
phases that contained various chromium concentrations in 1 M HCl and organic phases
of 0.52 M Cyanex 921 in Solvesso 100.
The results are shown in Table 2 in which the metal flux defined as J=P[Cr(VI)] 0 is also
given. Permeability values (P) were calculated according to Alguacil and Navarro,
2001. The highest percentage of metal transport was obtained when the source phase
presented an initial concentration of 15 mg/l chromium. The decrease, though small, in
the percentage of metal transport at higher metal concentrations can be attributed to a
saturation of the membrane and lower effective membrane area when higher chromium
concentrations are used, and also to the build-up of a carrier layer on the membrane
interface that assists the retention of the constituents on the entry side of the membrane
and decreases the chromium transport. On the other hand, the metal flux is increased as
the initial metal concentration in the source phase is increases within the present
experimental conditions.
3.1.5.Separation of chromium (VI) from metal-chloride complexes
In order to investigate the effect of other metal-chloride compounds accompanying
chromium(VI), a study was made about their interference with the overall transport of
5
chromium (VI). Several aqueous source solutions were investigated at 1 M HCl, the
organic phase was 0.26 M Cyanex 921 in Solvesso 100 and the receiving phase
contained 5 g/l hydrazine sulphate.
The results obtained (Table 3) indicated negligible Cu(II) and Ni(II) transport, whereas
zinc (II) and iron (III) are co-transported with chromium. This can be attributable to that
both copper (II) and nickel (II) are present in the aqueous solution as Cu2+ and Ni2+,
respectively, and thus they were not transported by Cyanex 921 (solvation extractant).
This is not the case for zinc (II) and iron (III) which are present in the solution as the
corresponding chloro-complexes, and were transported by the extractant. On the other
hand, the percentage of chromium (VI) transport vary within each solution; if these
values are compared with the data shown in Table 2, it can be seen that although
chromium (VI) is transported preferably to other metals, the presence of these metals in
the source phase decreased the percentage of chromium transported, this probably being
due to a multi-ion competition or crowding effect (de Gyves and Rodriguez de San
Miguel, 1999).
3.2.Transport by Cyanex 923
Experimental work performed with Cyanex 923 was carried out at HCl concentrations
of 0.5 or 1 M in the source phase. Previous experiments had shown that in the case of
Cyanex 923, the percentage of chromium (VI) transport remained almost constant in the
HCl concentration range of 0.25-1 M. In this HCl concentration range, the extraction of
chromium (VI) by Cyanex 923 (and also Cyanex 921) should be represented by the
general equilibrium (Huang et al., 1997):

HCrO 
(3)
4 aq  H aq  pL org  H 2 CrO 4  pL org
where aq and org denote species in the aqueous and organic phases, respectively, and L
represents the active substance of the extractant (Cyanex 921 or Cyanex 923). The
reaction involves a number of extractant molecules which solvate the chromium species
and probably water molecules which participate in the solvation reaction.
3.2.1.Effect of the stirring speeds in the source and receiving phases
A number of experiments were performed to establish adequate hydrodynamic
conditions. The transport across the membrane was studied as a function of the stirring
speeds in the source and receiving solution sides. Results obtained are shown in Table
4, near constant percentage of chromium transport for stirring speeds (source phase)
higher than 1200 min-1 was obtained. Thus, the thickness of the aqueous diffusion layer
and the aqueous resistance to mass transfer were minimized, and the diffusion
contribution of the aqueous species to the mass transfer process is assumed to be
constant.
The variation of the stirring speed in the receiving phase has a negligible effect on the
variation of the percentage of chromium transport. Therefore, stirring speeds of 1600
and 1000 min-1 were used in the source and receiving phases, respectively.
3.2.2.Effect of the receiving phase composition
Experiments were carried out using solutions of 0.50 M Cyanex 923 in cumene and
aqueous solutions of 10 mg/l Cr(VI), which also contained 0.5 M HCl. Table 5 shows
the results obtained; it can be seen that up to 5 g/l hydrazine sulphate, the percentage of
6
chromium transport is increased, but beyond this concentration the transport is not
improved.
3.2.3.Effect of organic diluent
The influence of the diluent on both the stability and efficiency of SLMs for the
transport of metals has been reported in the literature (Molinari et al.,1989; Alguacil et
al.,2002; Fontas et al., 2003). Experiments were carried out with organic solutions of
0.50 M Cyanex 923 in each diluent and aqueous solutions of 20 mg/l Cr(VI) in 1 M
HCl, whereas the receiving phases were of 5 g/l hydrazine sulphate. After 2 h, the
percentage of metal transported were of 95.3 (cumene), 88.9 (xylene), 69.9 (toluene)
and 95.7 (Solvesso 100). The results obtained indicate that, under the present
experimental conditions, Cyanex 923 is a more effective carrier for chromium transport
in cumene or Solvesso 100 solutions than in toluene or xylene solutions, though the
performance of the system is difficult to relate to one of another property of the organic
phase (mainly in relationship with the diluent of the pahse).
3.2.4.Effect of carrier concentration
The variation in the percentage of chromium transported at different carrier
concentrations using organic solutions of Cyanex 923 (0.13-0.75 M) in cumene,
aqueous solutions of 10 mg/l Cr(VI) in 1 M HCl and receiving phases of 5 g/l hydrazine
sulphate, showed that upon the Cyanex 923 concentration range studied, no significant
change in the percentage of metal transported (99 %) is revealed.
3.2.5.Effect of initial chromium concentration
Fig.1 shows the variation in the percentage of chromium transport and the initial
chromium flux (see Section 3.1.3) against the concentration of chromium ranging 10 to
80 mg/l in the source phase side. It can be observed that within the present experimental
conditions, the percentage of metal transported reached a maximum at 10 mg/l
chromium, whereas the metal flux increased as the initial chromium concentration in the
phase increased. This behaviour is similar to that observed using Cyanex 921 as carrier
for chromium transport (see Section 3.1.3).
3.2.6.Chromium transport using neutral organophosphorous carriers
Chromium transport experiments were also carried out using TBP and DBBP as
carriers. Results obtained from these series of experiments are shown in Table 6. It can
be seen that lower chromium transport is achieved using TBP, whereas the percentage
of chromium transport is still small if compared with that obtained with Cyanex 923.
Thus, an apparent chromium transport order can be shown in the form: Cyanex 923 >
DBBP > TBP, which can be interpreted in terms of the greater electron-donor properties
of the active substance of the respective carrier: R3PO (Cyanex 923), R(RO)2PO
(DBBP) and (RO)3PO (TBP), R being various alkyl chains.
4.Conclusions
From the experimental data obtained from this investigation, it can be deduced that both
Cyanex 921 and Cyanex 923, commercially available phosphine oxides, can be used as
carriers for chromium (VI) transport from chloride solutions.
For both carriers, the transport of chromium is influenced by a number of variables (e.g.
metal concentration and membrane phase diluent). Both carriers seem to present the
7
same transport affinity for chromium (VI) although the liquid character of Cyanex 923
appears best suited for its use especially when using higher carrier concentrations in the
membrane phase. By using hydrazine sulphate solutions as stripping phases, the
reduction of Cr(VI) to the less-toxic form Cr(III) is immediate and effectively
performed.
Chromium is transported preferably against other metal ions presented in the source
solution side, whereas phosphine oxides (e.g. Cyanex 923) seem to transport chromium
to a greater extent with respect to other neutral organophosphorous derivatives (e.g.
TBP and DBBP), this being attributable to the higher electron-donor properties of the
phosphine oxides with respect to the other organic-phosphorous derivatives.
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Alguacil,F.J., Coedo,A.G., Dorado,M.T., Sastre,A.M., 2001. Uphill permeation of
chromium (VI) using Cyanex 921 as ionophore across an immobilized liquid
membrane. Hydrometallurgy 61, 13-19.
Alguacil,F.J., Navarro,P., 2001. Permeation of cadmium through a supported liquid
membrane impregnated with Cyanex 923. Hydrometallurgy 61, 137-142.
Alguacil,F.J., Villegas,M.A., 2002. Liquid membranes and the treatment of metalbearing wastewaters. Rev. Metal. MADRID 38, 45-55.
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media using Acorta M5640 extractant. Part II. Supported liquid membrane study.
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Alonso,A.I., Urtiaga,A.M., Irabien,A., Ortiz,I.M., 1994. Extraction of Cr(VI) with
Aliquat 336 hollow fiber contactors: mass transfer analysis and modeling. Chem. Eng.
Sci. 49, 901-909.
Alonso,A.I., Pentelides,C.C., 1996. Modelling and simulation of integrated membrane
processes for recovery of Cr(VI) with Aliquat 336. J. Membr. Sci. 110, 151-167.
Alonso,A.I., Galan,B., Gonzalez,M., Ortiz,I., 1999. Experimental and theoretical
analysis of a non-dispersive solvent extraction pilot plant for the removal of Cr(VI)
from a galvanic process wastewaters. Ind. Eng. Chem. Res. 38, 1666-1675.
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E.Nieboer (eds), John Wiley and Sons, New York.
Bhowal,A., Datta,S., 2001. Studies on transport mechanism of Cr(VI) extraction from
and acidic solution using liquid surfactant membranes, J. Membr. Sci. 188, 1-8.
Casadevall,M., Kortenkamp,A., 2002. Heavy Metals in the Environment. B.Sarkar (ed).
Marcel Dekker, New York.
Castillo,E., Granados,M., Cortina,J.L., 2002. Liquid-supported membranes in chromium
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Chiarizia,R., 1991. Application of supported liquid membranes for removal of nitrate,
technetium (VII) and chromium (VI). J. Membr. Sci. 55, 39-64.
Dziwinski,E., Szymanowski,J., 1998. Composition of CYANEX 923, CYANEX 925,
CYANEX 921 and TOPO. Solvent Extr. Ion Exch. 16, 1515-1525.
Fontas,C., Salvado,V., Hidalgo,M., 2003. Selective enrichment of palladium from spent
automotive catalysts by using a liquid membrane system. J. Membr.Sci. 223, 39-48.
Guha,K., Yun,C.H., Basu,R., Sirkar,K.K., 1994. Heavy metal removal and recovery by
contained liquid membrane permeator. AIChE Journal 40, 1223-1237.
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De Gyves,J., Rodriguez de San Miguel,E., 1999. Metal ion separations by supported
liquid membranes. Ind. Eng. Chem. Res. 38, 2182-2202.
Harrington,P.J., Stevens,G.W., 2001. Steady-state mass transfer and modelling in
hollow fibre liquid membranes. J. Membr. Sci. 192, 83-98.
Huang,T.-C., Huang,C.-C., Chen, D.-H., 1997. Thermodynamic equilibria of the
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Solvent Extr. Ion Exch. 15, 837-862.
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liquid membranes for Cr(III), Cr(VI) and Cd(II) recovery. Sep. Sci. Technol. 24, 10321040.
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microcapsules. J. Membr. Sci. 89, 1-8.
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E.Nieboer (eds), John Wiley and Sons, New York.
Nunes,S.P., Peinemann,K.-V., 2003. Membrane Technology in the Chemical Industry.
Wiley-VCH, Weinheim.
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recovery of Cr(VI) in hollow fiber extraction and back-extraction modules. J. Membr.
Sci. 118, 213-221.
Vincent,T., Gibal,E., 2000. Non-dispersive liquid extraction of Cr(VI) by TBP/Aliquat
336 using chitosan-made hollow fiber. Solvent Ext. Ion Exch. 18, 1241-1260.
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9
Table 1
Influence of strippant concentration on metal transport
Hydrazine sulphate (g/l)
2.5
5
10
a
After 2 h.
Percent transporta
97.1
97.8
97.7
10
Table 2
Influence of initial metal concentration on chromium transport by Cyanex 921
[Cr(VI)]0 (mg/l)
Percent transporta
Jx109 (mol/cm2s)
15
98.9
2.8
25
97.8
4.2
50
97.0
8.2
75
96.7
11.0
Receiving phase 5 g/l hydrazine sulphate. aAfter 2 h.
11
Table 3
Separation of chromium from metal-chloride mixtures
Mixture
Concentration (mg/l)
Cr(VI)
20
Fe(III)
25
a
After 2 h.
Percent transporta
74.3
8.4
Cr(VI)
Fe(III)
Zn(II)
Cu(II)
20
25
15
25
72.0
3.6
41.7
nil
Cr(VI)
Cu(II)
Ni(II)
20
25
20
86.5
nil
nil
12
Table 4
Influence of strirring speed on chromium (VI) transport by Cyanex 923
Stirring speed (min-1)
Percent transporta
1000
90.9
1200
97.0
1400
97.1
1600
97.3
Source phase 10 mg/l Cr(VI) and 0.5 M HCl. Membrane phase 0.50 M Cyanex 923 in cumene. Receiving
phase 5 g/l hydrazine sulphate. aAfter 2 h.
13
Table 5
Influence of hydrazine sulphate concentration (receiving phase) on chromium transport
by Cyanex 923
Hydrazine sulphate (g/l)
Percent transporta
1
90.0
2.5
94.2
5
97.3
7.5
97.0
10
97.2
a
After 2 h.
14
Table 6
Transport of chromium using various neutral organophosphorous derivatives
Carrier
Percent transporta
TBP
50.3
DBBP
55.1
Cyanex 923
99.0
Source phase 10 mg/l Cr(VI) and 1 M HCl. Organic phase 0.50 M carrier in cumene. Receiving phase 5
g/l hydrazine sulphate. aAfter 2 h.
15
Fig.1. Influence of initial chromium concentration on the transport (after 2 h) of
chromium and the initial flux (JCr). Source phase chromium and 1 M HCl. Organic
phase 0.50 M Cyanex 923 in cumene. Receiving phase 5 g/l hydrazine sulphate.
16
FIGURE 1