Reduction of vanadium from alkaline solutions

Article abstract of DOI:10.1134/S1070427209070131

Effect of the potential sweep rate and temperature on the reduction kinetics of vanadate ions from alkaline solutions was studied. The nature of polarization in separate regions of the cathodic process was determined.

Full text of DOI:10.1134/S1070427209070131

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2009, Vol. 82, No. 7, pp. 1230−1233. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © E.G. Shirinov, Z.G. Gasanly, D.M. Ganbarov, 2009, published in Zhurnal Prikladnoi Khimii, 2009, Vol. 82, No. 7, pp. 1134−1137.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Reduction of Vanadium from Alkaline Solutions
E. G. Shirinov, Z. G. Gasanly, and D. M. Ganbarov
Nagiev Institute of Chemical Problems, National Academy of Sciences of Azerbaijan, Baku, Azerbaijan
Received December 3, 2008
Abstract—Effect of the potential sweep rate and temperature on the reduction kinetics of vanadate ions from
alkaline solutions was studied. The nature of polarization in separate regions of the cathodic process was
determined.
DOI: 10.1134/S1070427209070131
aluminate solutions from Gyandzha alumina plant,
produced by the Bayer method [5].
The available published data [1, 2] indicate that
vanadium(V) compounds in acid aqueous solutions
can be successively electrochemically reduced to
quadruple-, triple-, and double-charged states. All the
reduction products are soluble and remain in the course
of electrolysis in solution and impart to it blue, green, and
violet coloration. It is difficult to recover vanadium from
aqueous solutions because of the high negative potential
of the last stage of reaction [3].
The chemical composition of the solution under study,
as regards the main components of the industrial solution,
is as follows (g l–1): Na2O 186;Al2O3, SiO2 0.40; SO3 35,
V2O5 0.30; P2O5 0.30; and other impurities.
Polarization curves were measured in the potentio-
dynamic mode with a P-5827 M potentiostat and N 307/1
recorder.Aplatinum wire with a surface area of 0.22 cm2
served as the working electrode, and a platinum plate with
a surface area substantially exceeding that of the working
electrode, as the auxiliary electrode. A saturated silver
chloride electrode served as reference.
The reduction of vanadium in alkaline solutions is
rather poorly understood, with the exception of results of
[4], in which a low-concentration (0.05 M NaOH) alkali
solution was used. It should be noted that there have
been nearly no reports in the literature about studies of
the nature of polarization in electroreduction of vanadate
anions, although these studies are of indubitable interest
for development of a technology for recovery of metals
from industrial solutions.
As can be seen in Fig. 1 (curves 1–5), the cathodic
polarization curves show two portions of reduction of
vanadate anions. This means that, in the reduction of
vanadate ions, the process occurs in two stages. In portion
I of the polarization curves at potentials of 0...0.60 V,
vanadate ions are reduced to V(IV) (V2O4 ⋅ 2H2O). In
portion II of the polarization curves at potentials of
–0.750…–0.90 V, V2O4 ⋅ H2O is reduced to V(III) (V2O3⋅
3H2O).
Because of the importance of advances in this area, the
aim of the present study was to examine the electrolytic
reduction of vanadate ions from alkaline solutions.
EXPERIMENTAL
It is known that salts of vanadic acid are the most
stable vanadium compounds in alkaline media [6]. In
electrolysis, V(V) anions (VO43– and VO3–) are reduced
to V(IV) and V(III) compounds [7, 8].
The kinetics and mechanism of vanadium reduction
from alkaline solutions were studied using an electrolyte
containing 0.0034 M NH4VO3 and 6 M NaOH.
Cathode deposits were obtained from a solution of
6 M NaOH + 0.0034 M NH4VO3 by the potentiostatic
method at potentials of –0.60 and –0.75 V. The deposits
The concentrations of vanadium(V) and alkali in this
solution corresponded to the concentrations of industrial
1230

REDUCTION OF VANADIUM FROM ALKALINE SOLUTIONS
1231
−
√V, mV1/2 s–1/2
−
–E, V
Fig. 2. ip–√V dependence for portions I and II.
Fig. 1. Effect of the potential sweep rate on the electroreduction
of vanadate ions. Solution composition 6 M NaOH + 0.0034 M
NH4VO3. (ic) Current density and (E) potential; the same for
Fig. 3. Potential sweep rate (mV s–1): (1) 10, (2) 20, (3) 40, (4)
60, and (5) 80.
were subjected to an X-ray diffraction analysis and it
was found that their composition corresponds to the
compounds V2O4 ⋅ 2H2O and V2O3 ⋅ 3H2O.
The diffraction lines with interplanar spacings d 2.46,
6.42, 5.12 Å, found for the cathode slime obtained
under the given conditions, correspond to characteristic
reflections from hydrated vanadium oxide of composition
V2O4 ⋅ 2H2O [8].
–E(s.c.e.), V
Upon further shift of the cathode potential in the
negative direction, a steep rise in the polarization curves
is observed and hydrogen vigorously evolves by the
reaction
Fig. 3. Effect of temperature on the electroreduction rate of
vanadate anions in solution of 6 M NOH + 0.0034 M NH4VO3.
Potential sweep rate 10 mV s–1. Temperature (°C): (1) 30, (2)
40, (3) 50, (4) 60, and (5) 70.
2H2O + 2e = H2 + 2OH−.
More detailed evidence about the nature of the cathodic
polarization can also be obtained from data on how the
rate of the electrode reaction depends on temperature.
Therefore, the temperature dependence of the reduction
rate of vanadate anions was studied. To elucidate the
nature of the cathodic polarization in separate regions,
the temperature-kinetic method was used [10].
It can be seen in Fig. 1 that, as the potential sweep
rate increases from 10 to 80 mV s–1, the reduction process
becomes 2.5 times faster. In addition, the rate of the
cathodic process increases as the potential sweep rate
grows in both portions of the polarization curves.
The data in Fig. 1 were used to plot the dependence
of the peak current ip on the square root of the potential
sweep rate, √V. It is known [9] that such a dependence
A study of the temperature dependence of the rate
of the cathodic process of reduction of vanadate ions
demonstrated that this factor markedly accelerates
the electrochemical reaction, i.e., an increase in the
electrolyte temperature accelerates the electroreduction
of the vanadate to different extents at different cathode
potentials.As can be seen in Fig. 3, raising the temperature
−
makes it possible to determine the nature of polarization
in reduction of metals. As can be seen in Fig. 2, this
dependence is linear in both portions I and II, which
confirms the diffusion nature of the cathodic process in
the vicinity of the maximum in the curves.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 82 No. 7 2009

1232
SHIRINOV et al.
(a)
–E, V
Fig. 5. Effective activation energy Aeff vs. the polarization
potential E in a solution of 6 M NaOH + 0.0034 M NH4VO3
for portions I and II.
As can be seen in Fig. 4, the dependence of the rate of
the electrode process on the inverse temperature is rather
well expressed by straight lines with different slopes
dependent on the cathode potential.
(b)
The data in Fig. 4 were used to calculate the effective
activation energies Aeff of reduction of vanadate anions
at various potentials, with the results obtained presented
in Fig. 5.
It can be seen in Fig. 5 that, in portion I at potentials
in the range –0.02…–0.60 V, Aeff reaches a value
of 49.6 kJ mol–1. As the cathodic polarization increases,
A
eff rather sharply decreases to 11.2 kJ mol–1 at 0.40 V,
and in portion II of the polarization curves at potentials of
–0.75…–0.90 V, Aeff decreases from 14.2 to 4.45 kJ mol–1.
103/T, K–1
Fig. 4. log ic–1/T dependence at various potentials. Potential
(V): (a) (1) 0.02, (2) 0.03, (3) 0.05, (4) 0.10, (5) 0.15, (6) 0.20,
(7) 0.30, (8) 0.40, (9) 0.50, and (10) –0.60; (b) (1) 0.75, (2)
0.82, and (3) 0.90.
Comparison of the data in Fig. 5 with published data [1]
suggests that, in cathodic polarization, the process rate is
controlled by the electrochemical polarization in the range
–0.02…–0.10 V, and occurs under mixed kinetic control at
–0.10…–0.40 V. In the range –0.40…–0.60 V, the cathodic
process is limited by the concentration polarization. In
portion II at potentials of –0.75…–0.90 V, the process also
occurs in the concentration polarization region.
from 30 to 70°C leads to a 3.9-fold increase in the reaction
rate at a cathode potential of –0.10 V. Upon a shift of the
cathode potential in the negative direction, the influence
of temperature becomes weaker. For example, the process
becomes 1.22 times faster at a cathode potential of
–0.60 V, and approximately 1.434 times faster at –0.90 V.
Thus, the temperature-kinetic data confirm conclusions
based on results obtained in a study of the effect of the
potential sweep rate on the cathodic process.
The graphical data in Fig. 3 were used to find the
effective activation energy in regions I and II of the
cathodic process. This was done using the equation
CONCLUSIONS
(1) It was found that, in reduction of vanadate anions
from alkaline solutions, the process rate is controlled
by the electrochemical polarization at low potentials
of –0.02…–0.10 V, occurs under mixed kinetic control
at –0.10…–0.6 V, and is limited by the concentration
polarization at –0.75…–0.90 V.
(2) The experimental results obtained can be used to
recover vanadium by the electrochemical method from
industrial aluminate solutions.
Aeff
log ic = const −
,
2.303RT
where ic is the current density; Aeff, effective activation
energy; R, gas constant; and T, absolute temperature.
The activation energies were calculated from the slopes
of straight lines plotted in the logic–1/T coordinates.
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REDUCTION OF VANADIUM FROM ALKALINE SOLUTIONS
1233
6. Shalavin, E.L. and Ivanova, G.A., Trudy IMiO KazSSR,
REFERENCES
1966, vol. 17, pp. 20–26.
7. Goncharenko, A.S., Elektrokhimiya vanadiya i ego
soedinenii (Electrochemistry of Vanadium and Its
Compounds), Moscow: Metallurgiya. 1969.
1. Dzhafarov, F.Z. and Gorbachev, S.V., Zh. Fiz. Khim., 1964,
vol. 38, no. 5, pp. 1334–1337.
2. Dzhafarov, F.Z. and Gorbachev, S.V., Zh. Fiz. Khim., 1964,
8. Powder Diffraction File, Alphabetical Index Inorganic
Phases, USA. A. 1985.
vol. 38, no. 6, pp. 1672–1675.
3. Goncharenko, A.S. and Suvorova, O.A., Zh. Prikl. Khim.,
9. Galus , Z., Fundamentals of Electrochemical Analysis,
1960, vol. 35, no. 4, pp. 846–849.
Ellis Horwood, 1994.
4. Zazubin,A.I., Romanov, G.A., and Saltovskaya, L.A., Trudy
IMiO, Akad. Nauk KazSSR, 1965, vol. 12, pp. 41–48.
10. Gorbachev, S.V., Zh. Fiz. Khim., 1950, vol. 24, no. 7,
pp. 888–896.
5. Ivanova, R.V., Khimiya i tekhnologiya galliya (Chemistry
and Technology of Gallium), Moscow: Metallurgiya,
1973.
11. Spravochnik khimika (Chemist’s Reference Book),
Nikol’skii, B.P., Ed., Moscow: Khimiya, 1964, vol. 3.
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