Solvent Extraction Research and Development, Japan, Vol. 8, 172 - 185 (2001)

HIGH FUNCTIONAL LIQUID-LIQUID EXTRACTION SYSTEMUSING PHOTOCHEMICAL REDUCTION FOR METAL IONS
Syouhei NISHIHAMA, Takayuki HIRAI,* and Isao KOMASAWA
Department of Chemical Science and Engineering, Graduate School of Engineering
Science, and Research Center for Photoenergetics of Organic Materials, Osaka University,
Machikaneyama-cho 1-3, Toyonaka 560-8531, Japan

A high functional liquid-liquid extraction system using
photochemical reduction for metal ions has been investigated. The
distribution of the target metals between the aqueous and organic phasesin the extraction system can be controlled by the reduction of the metals,
and selective extraction or stripping progresses. Kinetic studies forsuch extraction systems revealed that the photochemical reduction of themetals is the rate-determining step. The photochemical reduction of
the metals mainly progressed at the interface and in the organic phase,
following photoabsorption at the absorption band for the metal extracted.
The technique can be applied to the cobalt-poisoning problem in thehydroxyoxime type extractant systems.

1. Introduction
Liquid-liquid extraction is a major method employed for separation of rare metals on
an industrial scale. Such extractive separation processes are generally based on
differences in the complex formation abilities between metals and their extractants.
Separation of some metals is, therefore, difficult when using a conventional extraction
system. For the development of more effective separation processes, introduction offunctional chemical reactions to the system, such as complexing reactions [1-3],synergistic reactions [4,5], and redox reactions [6-13], into the extraction system has been
investigated. In the case of an extraction system combined with a redox reaction,- 172 -

photochemical reaction has recently been investigated as the reaction source, since there isvery little discharge of waste. Several investigations of extraction systems combinedwith photochemical redox reactions have been reported [8,10-15]. There are, however,relatively few reports concerning the mechanism for such extraction systems.
In the present work, high functional liquid-liquid extraction systems combined withthe photochemical reduction of target metals have been investigated. The photochemical
reduction of Fe(III) and V(V) in separate aqueous or organic phases was first investigated.
The photochemical reduction was then applied to the extraction (stripping) system, and themechanism of extraction was investigated based on kinetic studies in Fe(III) [14] and V(V)[15] systems. The effect of the wavelength of the irradiated light on the reaction and thepossibility for the reuse of the organic phase were also investigated. The extraction
system was then applied to the cobalt-poisoning problem in hydroxyoxime extractant
systems [16]. The problem is caused by the oxidation of the cobalt species in the organic
phase, and the resultant complex cannot be stripped by conventional means.
2. Experimental
The commercial extractants, 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester(EHPNA, marketed as PC-88A by Daihachi Chemical Industry Co., Ltd.),bis(2-ethylhexyl)phosphoric acid (D2EHPA, marketed as DP-8R by Daihachi Chemical
Industry Co., Ltd.), 2-hydroxy-5-nonylacetophenone oxime (marketed as LIX-84I by
Henkel Hakusui Co., Ltd.), and 2-hydroxy-5-nonylbenzophenone oxime (marketed as
LIX-65N by Henkel Hakusui Co., Ltd.), were used as extractants, without further
purification. It has been shown by a vapor phase osmometric study that the majority oforganophosphorus acids are dimerized in nonpolar diluents [17]. The inorganic
chemicals, n-dodecane, and formic acid (90 %) were supplied by Wako Pure Chemical
Industries as analytical-grade reagents. In the cases of the Fe and Co systems, aqueous
feed solutions were prepared by dissolving each metal chloride in diluted HCl solutions.
In the case of the V system, the aqueous feed solution was prepared by dissolving either
NaVO3 or VOSO·nH2O into dilute H2SO4. The organic solutions were prepared bydiluting each extractant with n-dodecane.
The extraction equilibrium data were obtained by conventional procedures, at anorganic/aqueous (O/A) volume ratio of 1, at 298 K. The photochemical reaction of
metals in both single aqueous and organic solutions and photoreductive stripping or
extraction were carried out using a beaker-type glass bottle (32 mm in diameter). A 500
W xenon lamp (Ushio, UXL-500D-0) with an irradiation intensity of 580 kW/m2 was usedas the light source for the Fe and V systems, and a 500 W halogen lamp (Eikohsha Co.,
Ltd., EHC-500) with an irradiation intensity of 706 kW/m2 was used for the Co system.

- 173 -

The wavelength of irradiated light was adjusted by the Pyrex glass material of the reaction
bottle (greater than 300 nm) or by using appropriate cutoff filters. The concentration of
metals in the aqueous solutions were analyzed using an inductively coupled argon plasma
atomic emission spectrophotometer (Nippon Jarrell-Ash ICAP-575 Mark II). The
absorption spectra were measured using an UV-vis spectrophotometer (Hewlett-Packard
HP 8452A). The water concentration in the organic phase was determined by Karl
Fischer titration using a Kyoto Electronics MKS-1. The photointensity was measured bya thermopile (The Eppley Laboratory, Inc.).
3. Results and Discussion

102

D[-]

3.1 Extraction Equilibrium Formulations.
Figure 1 shows the effect of theequilibrium pH value on the distribution
ratios in Fe/EHPNA and V/D2EHPA
systems. Generally, the distribution ratios
for the metals having the larger valences arelarger than those of the metals having thesmaller valences [18]. For the Fe system,
the distribution ratio for Fe(II) is much
lower than that for Fe(III) over the pH range
investigated, thus indicating that selective
stripping of Fe may be expected to progressby reduction during the stripping process.
The slope analysis method revealed that theextraction equilibria for these metals could
be expressed as in Eqs. (1) and (2).
Fe 3+ + 3(RH ) 2 ↔ FeR 3 (RH ) 3 + 3H +100
key metal
Fe(III)Fe(II)
V(V)
V(IV)

10-2

0

1

2

pH [ - ]Figure 1. Effect of the aqueous pH on the
distribution ratio for Fe with EHPNA and V
with D2EHPA. [(RH) 2 ] feed = 0.5 mol/l and[M]feed = 0.01 mol/l.

Fe 2+ + 2(RH ) 2 ↔ FeR 2 (RH ) 2 + 2H +

(1)

(2)

For the V system, however, the distribution ratio for V(V) is smaller than that for V(IV),which is contrary to the valences, since, in an acidic solution with a pH value of 0-2, V(V)and V(IV) exist as VO2+ and VO2+ , respectively [19]. Selective extraction of V is thus
expected by reduction, especially at high pH. The slope analysis method revealed that theextraction equilibria for these metals could be expressed as in Eqs. (3) and (4).
+VO 2 + (RH) 2 ↔ VO 2 R (RH ) + H + ; Kex,V(V) = 1.12 x 10-1
(3)

VO 2+ + 2(RH ) 2 ↔ VOR 2 (RH ) 2 + 2H + ; Kex,V(IV) = 3.79 x 10-1

(4)

3.2 Photochemistry of the Metals in the Aqueous and Organic Solutions.
In the case of Fe(III), no photochemical reduction occurs without a radical scavenger- 174 -

in both aqueous and organic solutions. In aqueous solution, however, the reaction
progresses in the presence of a radical scavenger, such as formic acid. This is because thephotochemical reduction of Fe(III) in aqueous solution occurs by electron donation fromwater, and the OH radical produced, which promotes the reverse reaction, is scavenged byformic acid. The photochemical reduction of Fe(III) is expressed by Eqs. (5) and (6).
Fe 3+ + H 2 O → Fe 2+ + H + + ･OH(5)･OH + HCOOH → H 2 O + ･COOH(6)
In the case of V(V), photochemical reduction occurs in both aqueous and organic
solutions without a radical scavenger. The rate of photochemical reduction wasinvestigated following Ar bubbling in order to purge dissolved oxygen. A pseudo-firstorder kinetic plot for the variation in the concentration of V(V) revealed that the rate ofphotochemical reduction of V(V), in both aqueous and organic solutions, is first order withrespect to V(V) concentration and is independent of pH, sulfuric ion concentration, andextractant concentration, as shown by Eqs. (7) and (8).
d[V(V)](7)
rV ,a = −= k V ,a [V(V)] ; kV,a = 3.58 x 10-4 min-1
dt
d[V(V)]
rV ,o = −= k V ,o [V(V)] ; kV,o = 9.84 x 10-2 min-1(8)dt
In aqueous solution, the photochemical reduction of V(V) is likely to occur by electron
donation from water, as indicated in Eqs. (9)-(11).
+VO 2 (H 2 O) 4 + H 2 O → VO 2 (H 2 O) 4 + H + + ⋅OH(9)2+

VO 2 (H 2 O) 4 + 2H + → VO(H 2 O) 5
(10)1
1
⋅ OH → H 2 O + O 2(11)2
4
The overall reaction is thus shown by Eq. (12), which is identical with that found by
Jeliazkowa et al [20].
112++VO 2 (H 2 O) 4 + H 2 O + H + → VO(H 2 O) 5 + O 2(12)2
4
If the reduction of V(V) is the rate-determining step as shown by Eq. (9), then the overall
reaction rate may also be expressed by means of Eq. (7). For the organic solution, thephotochemical reduction of V(V) also seems to occur via electron donation from water
dissolved in the organic phase. Figure 2 shows the water concentration in the organic
solution versus the concentration of extracted V(V) or Fe(III). In the V(V)/D2EHPA
system, the quantity of water in the organic phase is much larger than that of V(V), andthus the photochemical reduction progresses in the separate organic solution. In the
Fe(III)/EHPNA system, however, the quantity of water is smaller than that in the V system.
This difference in the quantity of the water is likely to affect the ability of photochemical
reduction in the organic solution between the two metals.

- 175 -

0.01

key metal extractant
Fe(III) EHPNA
V(V) D2EHPA

0.1

0[ Fe ] [ mol/l ][ H2O ] [ mol/l ]0.2

key bubblingAr
without
O2

0.005

0
0

0.005

0.01

0

[ M ] [ mol/l ]

200

400

600

800

Irradiation time [ min ]Figure 2. Effect of the concentration of
Fe(III) and V(V) extracted on the concentrationof water in the organic phase. [(RH ) 2 ] feed =0.5 mol/l.

Figure 3. Effect of Ar or O2 bubbling on thephotoreductive
strippingof
Fe(III).
[(RH ) 2 ]feed = 0.24 mol/l, [Fe(III)] feed = 0.01
mol/l, and [HCl]feed = 0.24 mol/l.

3.3 Mechanism of the Extraction Systems Combined with Photochemical Reduction.
3.3.1 Photoreductive Stripping of Fe(III).
The organic solution following extraction of Fe(III) was contacted with a dilute HClsolution at O/A = 1, which was not expected to strip Fe(III), and then photoirradiated with
the Xe lamp with vigorous agitation by a magnetic stirrer at over 4000 rpm. Figure 3
shows the variation of the aqueous phase Fe concentration with time. Photoreductive
stripping of Fe(III) is seen to progress without a radical scavenger. For the case of zero
bubbling treatment, the reaction only started after an induction period. In the case of
bubbling with O2, this induction period was prolonged, whereas in the case of Ar bubbling,
the reaction was seen to start immediately. Donohue has reported that, for thephotochemical reduction of Yb or Sm in a methanol solution, no reaction was found tooccur until most or all of O2 was excluded [21]. In addition, following the induction
period, no difference was seen in the rate of stripping for all three cases studied here.
Therefore, during the induction period, the dissolved oxygen suppresses the reduction of
Fe(III) to Fe(II), and stripping only starts to progress when all of the oxygen in theextraction system has been consumed.
The investigation, at which phase the photochemical reduction of Fe(III) occurs, wascarried out with the rate of agitation being kept at 500 rpm to maintain a flat interface andeach phase being shielded from photoirradiation. Photoreductive stripping hardly
occurred in the case of irradiation of the aqueous phase alone. In the case of irradiationof the organic phase alone, however, photoreductive stripping was seen to occur, and thereaction progressed at a rate that was almost identical with that in the case of irradiation of- 176 -

Trans. Photo Intens.

[ Fe ] [ mol/l ][ kW/m2 ]50
both phases.
Because photochemical(b)
40
reduction is found not to occur in the30
organic solution, Fe(III) is likely to be20
photoexcited in the organic phase and to10
be reduced at the interface by electron
0.01(a)donation from water.
A
kinetic
study
of
the
photoreductive stripping of Fe(III) was
0.005
carried out. The pseudo-first-order plot,
calculation
for the variation in the concentration of
key [ HCl ]Fe(III) in the organic phase with respect
0.38 mol/l
0.24 mol/lto irradiation time, shows the rate of
0.12 mol/l
0.047 mol/l0photoreductive stripping is of first order
0
100
200
300
with respect to the Fe(III) concentration in
Irradiation time [ min ]the organic phase.
The rate of Figure 4. Effect of photoirradiation time on (a)photoreductive stripping can, therefore, be the concentration of Fe(III) in the aqueous phaseand (b) the transmitted photointensity.
expressed as shown in Eq. (13).
Comparison of observed data with prediction
d [Fe]
rFe = −
= k Fe '[Fe](13)
shown by solid lines. [(RH) 2 ] feed = 0.1 mol/ldt
and [Fe(III)] feed = 0.01 mol/l.
The effects of the initial hydrogen ion
concentration and the free dimeric EHPNA concentration, [(RH ) 2 ]feed − 3[Fe]feed , on theapparent rate constant k ' were then investigated and it was shown that the rate ofphotoreductive stripping of Fe(III) can be expressed as Eq. (14).
[Fe][H + ]d [Fe]= k FerFe = −
(14)dt[(RH) 2 ]0.5
where kFe = 0.0275 l0.5mol-0.5min-1.
Figure 4(a) shows the effect of photoirradiation time on the concentration of Fe in
the aqueous phase as compared with the calculated value based on Eq. (14). The
magnitude of the experimental values agrees well with the calculated values during theinitial part of the run but with increasing irradiation time these become greater than thecalculated value. This deviation becomes notable in the run with the solution of lower
HCl concentration. Figure 4(b) shows the variation with time on the photointensity oftransmitted light. In the case of low HCl concentration at 0.047 mol/l, the transmitted
light intensity increased as the photoreductive stripping of Fe(III) progressed, and thephotoreaction progressed more effectively than that predicted based on the initial
photoreductive stripping rate.
The overall reaction mechanism for the photoreductive stripping of Fe(III) can bedivided into two reaction stages, the photochemical reduction of extracted species of- 177 -

Fe(III) to Fe(II) at the interface, followed by the stripping of Fe(II) into the aqueous phase,as follows.
FeR 3 (RH ) 3 + H 2 O ↔ FeR 2 (RH ) 2 + (RH ) 2 + ･OH(15)
FeR 2 (RH) 2 + 2H + ↔ Fe 2+ + 2(RH) 2(16)

This reaction scheme is deduced on the assumptions that the rate-determining step is thephotochemical reduction of Fe(III), as shown by Eq. (15), and that the rate of distributionof the reduced Fe(II) is fast. The Fe(III)-extractant complex is found to be excited in theorganic phase and to be donated an electron from the water at the aqueous/organicinterface. Assuming the extractant to exist as a monomeric species at the interface, as
was found in the case of D2EHPA [22], the elementary reaction of Eq. (15) can be
expressed in more detail as in Eqs. (17)-(22).
(RH) 2 ↔ 2RH

; K1

(17)

FeR 3 (RH ) 3 ↔ FeR 3 (RH ) 3 *

; K2

(18)

; K3

(19)

; K4

(20)

; k1

(21)

FeR 3 (RH ) 3 * ↔ FeR 3 (RH) 3 *+FeR 3 (RH ) 3 * + H + ↔ FeR 2 (RH) 3 + RH+

FeR 2 (RH) 3 + H 2 O → FeR 2 (RH) 3 + H + + ･OH
FeR 2 (RH ) 3 → FeR 2 (RH ) 2 + RH

(22)

If the rate-determining step is the photochemical reduction of Fe(III) as shown by Eq. (21),the overall reaction rate equation can be expressed by Eq. (23).
+
k1 K 2 K 3 K 4 [FeR 3 (RH ) 3 ][H + ]d [Fe](23)r=−= k1 [FeR 2 (RH ) 3 ] =0.5
dt[(RH ) 2 ]0.5
K1This agrees with the result from the experimental kinetic study, as shown by Eq. (14),indicating that the mechanism of photoreductive stripping of Fe(III) is expressed by the
proposed scheme.
3.3.2 Photoreductive Extraction of V(V).
The extractant D2EHPA in n-dodecane was contacted with the aqueous solution,
containing NaVO3, at O/A = 1. The mixture was then photoirradiated following Ar
Figure 5 shows the effect of
bubbling with agitation by a magnetic stirrer.
photoirradiation time on the distribution ratio for V under conditions of vigorous stirring atover 4000 rpm. In the extraction system, the variation with time of the distribution ratio
can be predicted by assuming that the rate of the reaction is dominated by thephotochemical reduction of V in the aqueous and organic phases, and the distribution ofboth V(V) and V(IV) between the two phases is attained immediately. Such predictions,which are shown by dotted lines in Figure 5, however, lead to much lower values for the
distribution ratio than given by the experimental results. This difference may be causedby photochemical reduction of V at the interface between the aqueous and organic phases,- 178 -

as was also found in the case for the photoreductive stripping of Fe(III). The
photochemical reduction of V(V) at the interface may be thought to be expressed by themechanisms indicated in Eqs. (24)-(27).
VO 2 R (RH ) ↔ VO 2 R (RH )

; Ki

(24)

VO 2 R (RH ) + H 2 O → VO 2 R (RH ) − + H + + ⋅OH; k 'i(25)

VO 2 R (RH ) − + (RH ) 2 + H + → VOR 2 (RH ) 2 + H 2 O(26)

VOR 2 (RH ) 2 → VOR 2 (RH ) 2(27)

If the rate of photochemical reduction at the interface is first order with respect to theconcentration of V(V) at the interface, as in the case of reduction in aqueous and organic
solutions, then the interfacial surface rate equation can be expressed by Eq. (28),Q d[V(V)]= k V,i '[V(V)] = K V ,i k V ,i '[V(V)]
(28)
dt
A
where Q and A are the volume of the organic phase and the area of the interface,respectively. Assuming the interface area to be constant, the overall rate equation at theinterface can be expressed as Eq. (29).
A(29)
rV ,i = rV ,i ' = k V ,i [V(V)]Q
rV ,i ' = −The overall rate constant for photochemical reduction of V at the interface was determinedas ki = 1.75 x 10–1 min–1, by a nonlinear least-squares fit to the experimental results. The

101

D[-]

102

calculation
10-1
0.6

10

interface0

calculation
key

10

-1

10-2

0

20

Y[-]

D[-]

101

100

pHfeed
0.80
1.02
1.55

40

0.4

org

0.2
aq

60

0
0

Irradiation time [ min ]

20

40

60

Irradiation time [ min ]Figure 5. Effect of the photoirradiation time on
the distribution ratio for V.
Solid lines:
photochemical reduction at the interface only,
dotted lines: photochemical reduction only in the
aqueous and organic phases . [(RH ) 2 ] feed = 0.5
mol/l and [NaVO3]feed = 0.01 mol/l.

- 179 -

Figure 6. Effect of the photoirradiation timeon the distribution ratio and reduction yield ineach phase.
[(RH ) 2 ]feed = 0.5 mol/l,[NaVO3]feed = 0.01 mol/l, and pHfeed = 1.02.

change in the concentrations of V(V) and V(IV) in the aqueous and organic phases for atime interval Δt lying between time t and t + Δt can be calculated from a knowledge of thevalues of the rate constants, ka, ko, and ki, as indicated in Eqs. (30)-(33).
[V(V)]t[V(V)]t + Δt =(30)
e ka Δt[V(IV)]t + Δt = [V(IV)]t + {[ V(V)]t − [V(V)]t + Δt }
(31)
1
⎛ 1
⎞
[V(V)]t + Δt = ⎜ ko Δt + ki Δt − 1⎟[V(V)]te⎝e⎠[V(IV)]t + Δt = [V(IV)]t + {[ V(V)]t − [V(V)]t + Δt }

(32)
(33)

Assuming that an equilibrium distribution of both V(V) and V(IV) between the phases isattained immediately, the distribution ratio at the new time (t + Δt) can be calculated using
the extraction equilibrium formulations previously determined. Repeated calculation,
until the values of the distribution ratios and the proton concentration converge, enables
the apparent distribution ratio, D = ([V(V)] + [V(IV)]) /([V(V)] + [V(IV)]) , to be
determined. These predicted values according to the above method are shown in Figure 5
by the solid lines. The experimental data are seen to fall well on the prediction lines, thus
indicating that the photochemical reduction of V progresses in the aqueous phase, in theorganic phase, and at the interface. Figure 6 shows the variation with time in the
reduction yield for each phase, Y, defined as the ratio of the concentration of V(IV)reduced in the appropriate phase to that of the feed V(V) concentration. The
photoreductive extraction progresses mainly via photochemical reduction at the interface
and in the organic phase, with the reduction in the aqueous phase hardly contributing to theoverall photoreductive extraction.
3.4 Effect of the Wavelength of the Irradiated Light.
Figure 7 shows the effect of the wavelength of the irradiated light, when varied by
the use of appropriate cutoff filters, on the variation with time in the photoreductive
stripping of Fe(III) and photoreductive extraction of V(V). In the case of Fe, the reaction
hardly occurs when irradiated light of less than 400 nm is cut off. This may be caused by
the ability for photoabsorption of the extracted species. Figure 8(a) shows the absorption
spectrum for the organic solution with and without Fe(III) and indicates that the absorption
band, caused by Fe(III), appears at 200-350 nm. In the case of V, the absorption band,
caused by V(V), appears at 300-500 nm (Figure 8(b)), and the reaction hardly occurs whenirradiated light of less than 500 nm is cut off. These results thus indicate thephotochemical reduction of the metals in the organic phase and at the interface progresses
following photoabsorption at the absorption band for the metal extracted.

- 180 -

(a) Fe/EHPNA(b) V/D2EHPA
102

0.006

key wavelength> 300 nm
> 400 nm
> 450 nm
> 500 nm
> 550 nm

0.004

0.002

D[-]

[ Fe ] [ mol/l ]

0.008

101

0
0

100

200

100 0

Irradiation time [ min ]

20

40

60

Irradiation time [ min ]Figure 7. Effect of the wavelength of the light source on the rate of (a) photoreductive strippingof Fe(III) and (b) photoreductive extraction of V(V). (a) Fe/EHPNA system, [(RH ) 2 ] feed = 0.1

mol/l, [Fe] feed = 0.0088 mol/l, and [HCl]feed = 0.24 mol/l, and (b) V/D2EHPA system,[(RH) 2 ] feed = 0.5 mol/l, [NaVO3]feed = 0.01 mol/l, and pHfeed = 1.52.

Absorbance [ - ](a) Fe/EHPNA(b) V/D2EHPA

2[ Fe ] = 0.88 mmol/l[ Fe ] = 0 mol/l[ V ] = 0.32 mmol/l

1[ V ] = 0 mol/l

0
200

400

600

800

Wavelength [ nm ]

400

600

800

Wavelength [ nm ]Figure 8. Absorption spectrum for organic solutions containing (a) Fe(III) and (b) V(V).
[(RH) 2 ] feed = 0.1 mol/l. Reference: air

3.5 Reusability of the Organic Phase.
The reuse of an organic solution following photoirradiation was also investigated.
In the case of Fe, the aqueous feed solution and organic solution were contacted at O/A = 1.
The Fe loaded organic solution was then contacted with 0.36 mol/l of HCl solution at O/A= 1 and the phases photoirradiated for 300 min. In the case of V, the aqueous and organic
solutions were contacted at O/A = 1 and were photoirradiated for 60 min. The resultant
organic solution was then stripped using 6 mol/l of HCl at O/A = 0.5. These resultant
organic solutions were then used for repeated processing after washing with 3 mol/l of HCl
and then water. Figure 9 shows the concentration of Fe or V in the organic phase during
the repeated extraction-stripping processing. The results thus show that the organic phase- 181 -

(a) Fe/EHPNA(b) V/D2EHPA[ M ] [ mol/l ]

0.01

0.005

0
1st 1stExr. Str.

2nd 2nd
Exr. Str.

3rd
Etr.

3rd 1st 1stStr. Exr. Str.

2nd 2nd
Exr. Str.

3rd
Etr.

3rd
Str.

Figure 9. The concentration of (a) Fe and (b) V in the organic phase following therepeated extraction-stripping processing. [(RH ) 2 ]feed = (a) 0.1 mol/l and (b) 0.5 mol/l,[M]feed = 0.01 mol/l, and pHfeed = (a) 1.47 and (b) 0.86.

possesses a sufficient loading and stripping capacity for use in repeated processing andconfirm the possible reuse of the organic phase following photoirradiation.
3.6 Application for the Cobalt-Poisoning Problem.
It is well known that Co(II) when extracted via commercial chelating extractants,
such as hydroxyoxime type extractants, is difficult to recover by conventional stripping
procedures, owing to cobalt poisoning [23,24]. Figure 10 shows the effect of HClconcentration on the stripping efficiency at O/A = 0.5, without photoirradiation. The
extraction-stripping was carried out using both Ar or O2 bubbling and also without
bubbling. For both zero bubbling and bubbling with O2, the stripping efficiency obtainedwith 6 mol/l HCl was at most about 80 %, and thus quantitative stripping, by theconventional method, cannot be achieved using either extractant system. In the case of
Ar bubbling and thus the absence of oxygen molecules in the system during theextraction-stripping process, the stripping is substantially improved and quantitative
100

Stripping [ % ](a) LIX-84I(b) LIX-65N

50
key bubblingAr
without
O2
0

2

4

6

[ HCl ] [ mol/l ]

0

2

4

6

[ HCl ] [ mol/l ]Figure 10. Effect of HCl concentration on the stripping efficiency without photoirradiation in (a)LIX-84I and (b) LIX-65N systems, at O/A ratio of 0.5. [ RH]feed = 10 vol%, and [Co]feed = (a)6.76 mmol/l and (b) 7.13 mmol/l.

- 182 -

Stripping [ % ]100(a) LIX-84I(b) LIX-65N

50
key[ HCl ]
3.00 mol/l
1.75 mol/l
0.36 mol/l

0
0

1

2

3

0

Irradiation time [ h ]

1

2

3

Irradiation time [ h ]Figure 11. Effect of the HCl concentration on rate of photoreductive stripping in (a)LIX-84I and (b) LIX-65N systems. [RH ]feed = 10 vol%, and [Co] feed = (a) 6.53 mmol/land (b) 9.37 mmol/l.

stripping is achieved at 6 mol/l HCl. These results, thus, indicate that Co is oxidized tothe trivalent state by oxygen in the extraction system, and is then stabilized in the organic
phase, which suppresses quantitative stripping. Effective stripping is therefore expected
to be carried out, if the oxidized Co(III) complex can be photoreduced within the organic
phase.
The photoreductive stripping of Co was then carried out, by a similar procedure to
the photoreductive stripping of Fe, using a halogen lamp. Only visible light with awavelength greater than 550 nm was irradiated, since Co-LIX 84I has a large absorption
band at 400-800 nm. Figure 11 shows the effect of HCl concentration on thephotoreductive stripping of Co in LIX-84I and LIX-65N systems. Successive stripping
can be carried out in both systems using the photoreductive stripping technique. The
photoreductive stripping of cobalt is improved by increasing the concentration of HCl in
both systems, and a stripping efficiency of about 90 % can be achieved with 3 mol/l of HCl
following photoirradiation for 3 h. The organic phase in this process also possesses
sufficient loading and stripping capacity for use in repeated processing, as for the Fe and V
systems.
4. Conclusion

The high functional liquid-liquid extraction systems using photochemical reduction
for metal ions have been investigated, with the following results: (1) The distribution ratioof metals can be controlled by photochemical reduction of the metals. The mechanismsof the extraction system combined with photochemical reduction were elucidated based on
kinetic studies, in which the photochemical reduction of the metals is the rate-determining
step. (2) The photochemical reduction of the metals in the organic phase and at the- 183 -

interface takes place following photoabsorption at the absorption band for the extracted
metal. The organic solution following photoirradiation is reusable for repeatedextraction-stripping processing. (3) The photoreductive stripping technique can solve thecobalt-poisoning problem in hydroxyoxime extractant systems.
Acknowledgement

S.N. is grateful to the Research Fellowships of the Japan Society for Promotion of
Science for Young Scientists and to the Morishita Zintan Scholarship Foundation.
Nomenclature

A
D
kFe
kV
k Fe 'k V ,i '= interface area[dm2]
= distribution ratio
[-]
0.5
0.5
= rate constant for photoreductive stripping of Fe[l /mol min]= rate constant for photochemical reduction of V
[min-1]= apparent rate constant[min-1]= rate constant for photochemical reduction of V at the interface [dm/min]

Kex
M
Q
rFe
rV
rV '

= extraction equilibrium constant= Fe or V= volume of the organic phase= rate of photoreductive stripping of Fe= rate of photochemical reduction of V
= rate of photochemical reduction of V at the interface

RH
(RH)2
Y
[ ]
Subscript
a
feed
i
o
Superscripts= LIX-84I or LIX-65N= dimeric species of EHPNA or D2EHPA= reduction yield= concentration of the species in the bracket= aqueous phase= aqueous or organic feed solution= interface= organic phase= organic phase species= interface species- 184 -

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