Electrochemical reduction of As(III) in acid media

Article abstract of DOI:10.1134/S0020168507010074

Measurements of the cathode potentials of different electrode materials in the galvanostatic electrolysis of As₂O₃ solutions in sulfuric acid indicate that the Pb cathode ensures the most stable negative potential, favorable for AsH₃ formation. Preparative electrolyses confirm stability of the arsine yield in a series of experiments. The current efficiency for arsine on the Pb cathode is 60-70%. The byproduct of this process is As⁰, with a current efficiency of about 2%. We have designed and tested an electrolyzer with improved hydrodynamics, which makes it possible to avoid the formation of dead zones and to prevent the cathode chamber from being clogged.

Full text of DOI:10.1134/S0020168507010074

ISSN 0020-1685, Inorganic Materials, 2007, Vol. 43, No. 1, pp. 25–29. © Pleiades Publishing, Inc., 2007.
Original Russian Text © M.K. Smirnov, V.V. Turygin, N.N. Shalashova, A.V. Khudenko, A.P. Tomilov, 2007, published in Neorganicheskie Materialy, 2007, Vol. 43, No. 1,
pp. 27−32.
Electrochemical Reduction of As(III) in Acid Media
M. K. Smirnov, V. V. Turygin, N. N. Shalashova, A. V. Khudenko, and A. P. Tomilov
State Research Institute of Organic Chemistry and Technology,
sh. Entuziastov 23, Moscow, 111024 Russia
Received April 17, 2006
Abstract—Measurements of the cathode potentials of different electrode materials in the galvanostatic elec-
trolysis of As2O3 solutions in sulfuric acid indicate that the Pb cathode ensures the most stable negative poten-
tial, favorable for AsH3 formation. Preparative electrolyses confirm stability of the arsine yield in a series of
experiments. The current efficiency for arsine on the Pb cathode is 60–70%. The byproduct of this process is
As0, with a current efficiency of about 2%. We have designed and tested an electrolyzer with improved hydro-
dynamics, which makes it possible to avoid the formation of dead zones and to prevent the cathode chamber
from being clogged.
DOI: 10.1134/S0020168507010074
INTRODUCTION
The electrodes thus prepared were immersed in a back-
ground electrolyte solution and cathode-polarized with
a current of 100 mA for 30 min (over this time, the cath-
ode potential stabilized somewhat). Electrolysis was
stopped, a cyclic voltammogram was recorded, an
appropriate amount of As(III) in the form of a concen-
trated NaAsO2 solution was added, and a cyclic volta-
mmogram was again recorded. Next, we conducted
electrolysis at a current of 100 mA for 30 min, while
monitoring the cathode potential, and then recorded yet
another voltammogram.
The synthesis of hydrogen arsenide (arsine) through
the electrochemical reduction of As(III) has been the
subject of much attention [1–5]. This approach was
proposed for the preparation of AsH3 in the laboratory.
At the same time, no attempts to scale-up this process
have been described in the literature. Given that electro-
chemical synthesis enables the preparation of purer
products in comparison with chemical processes, we
have focused our efforts on the application of this pro-
cess on a production scale:
Preparative electrolyses were carried out in a filter-
press electrolyzer with a working chamber 2.2 × 2.2 cm
in dimensions (cathode area of 4.84 cm2). In the elec-
trolyzer with improved hydrodynamics, the cathode
was 30 cm in length and ꢀ1.7 cm in width. The mem-
brane–cathode separation was 12 mm, and the working
surface area of the cathode was 50 cm2.
1. We studied the variation in the potential of various
cathode materials during galvanostatic electrolysis.
2. We compared the stability of various cathode
materials in a series of consecutive electrolyses without
cleaning the electrolyzer.
3. We examined the possibility of reducing the yield
of elemental arsenic, a reaction byproduct.
The was circulated by a centrifugal pump. The set
temperature was maintained by a remote heat
exchanger. The cathode gases were pyrolyzed at tem-
peratures from 650 to 700°C. The arsine yield was
determined from the weight gain of the silica tube in the
pyrolysis furnace.
The stability of various electrode materials was
assessed as follows: After 80 ml of 10% sulfuric acid
saturated with As2O3 (the As2O3 solubility in 10%
H2SO4 is ꢀ15 g/l) was introduced into the electrolyzer,
arsine electrosynthesis was conducted at a temperature
of 25–30°C and cathode current density of 0.2 A/cm2.
The duration of each run was 1 h. Then, the current was
turned off, and the electrolyzer was left filled with the
electrolyte for a day. Next, the spent catholyte was
4. We tested an electrolyzer with improved hydrody-
namics and measured its main discharge coefficients.
EXPERIMENTAL
Polarization measurements were performed with an
IPC-2000 programmed electronic potentiostat. The
potential was measured relative to a silver/silver chlo-
ride reference electrode in a three-electrode cell. As
working electrodes, we used sheet metal plates 0.5 cm2
in working area. The auxiliary electrode was of plati-
num. The measurements were made during galvano-
static electrolysis at a polarizing current of 100 mA
(current density of 0.2 A/cm2), while bubbling the solu-
tion with argon.
The cathode potential was measured as follows: The replaced with a fresh one, and the next experiment was
electrode was pretreated with sulfuric or hydrochloric conducted under the same conditions. The cathode was
acid and then thoroughly rinsed with distilled water. not treated within a given series of experiments.
25

26
SMIRNOV et al.
assumption, it is reasonable to measure the cathode
–700
–750
–800
–850
–900
–950
1
potential under galvanostatic electrolysis conditions.
We tested glassy carbon, titanium, zinc, lead, copper,
iron, and platinum cathodes.
The most stable potential in a sulfuric acid solution
over a period of 30 min in the absence of arsenic was
offered by the iron cathode (Fig. 1). After recording the
voltammogram in the presence of As(III), the cathode
potential shifts sharply to more negative values, which
can be accounted for by the deposition of an elemental
arsenic film onto the cathode surface. At the same time,
if arsenic fully shielded the cathode surface, the shift in
potential would be substantially greater (at a current
density of 0.2 A/cm2, the potential of arsenic cathodes
is –1.02 V [1]). Thus, we are led to conclude that
arsenic forms a porous coating. During further electrol-
ysis, the shift in potential decreases, indicating that no
dense arsenic coating is formed during electrolysis.
As(III) additions had a similar effect on the potential of
the platinum cathode (Fig. 2).
2
–1000
–1050
0
200 400 600 800 10001200140016001800
Time, s
Fig. 1. Variation in the potential of an iron cathode (steel-3)
during galvanostatic electrolysis (0.1-A current, 1.0 N
H SO solution with As(III) additions): (1) background
2
4
electrolyte, (2) background + 0.01 M As(III).
For cathode materials with a high hydrogen overpo-
tential, an opposite picture was observed after voltam-
metric measurements: the potential shifted to less neg-
ative values, approaching that of the arsenic electrode.
As an example, Fig. 3 shows the data for the lead cath-
ode. At the same time, during electrolysis the observed
depolarization value decreases. Our data on the varia-
tion in cathode potential over a period of 30 min are
presented in Table 1.
In experiments with continuous saturation of the
catholyte with arsenous acid (arsenic(III) oxide), the
solution was pumped through a tank in which the
arsenic oxide suspension was agitated by a mechanical
stirrer. To the bottom part of the tank, we introduced the
spent catholyte from the electrolyzer; from the top part,
we withdrew a saturated As2O3 solution free of solid
oxide particles.
Two general trends warrant attention. During volta-
mmetric measurements, depolarization was only
observed for the lead cathode. Arsenic deposition on
the other electrode materials shifted the cathode poten-
tial to more negative values. The shift was particularly
large for titanium and glassy carbon. This may be due
to the deposition of a low-conductivity form of arsenic.
Another general trend is that, during the first 30 min of
electrolysis, the cathode potential gradually shifts to
less negative values. The shift depends significantly on
RESULTS AND DISCUSSION
It is well known [1] that, during electrolysis of acid
As2O3 solutions, the cathode becomes covered with a
layer of elemental arsenic, and, during further electrol-
ysis, the cathodic process takes place at the arsenic
electrode. In light of this, Nilov [2] and Vorotyntsev
et al. [3] believe that the cathode material is of no sig-
nificance in the preparation of arsine. To check this
–1020
–600
2
–1040
1
2
–700
–800
–1060
–1080
–900
–1100
–1120
–1140
1
–1000
–1100
–1200
0
200 400 600 800 10001200140016001800
0
200 400 600 800 10001200140016001800
Time, s
Time, s
Fig. 2. Variation in the potential of a platinum cathode (the
same conditions and designations as in Fig. 1).
Fig. 3. Variation in the potential of a lead cathode (the same
conditions and designations as in Fig. 1).
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ELECTROCHEMICAL REDUCTION OF As(III) IN ACID MEDIA
27
Table 1. Variation in the cathode potential during electrolysis of 1.0 N H2SO4 in the presence of 0.01 M As(III) (current den-
sity, 0.2 A/cm2)
Cathode potential in the presence of As(III), mV
Cathode potential
in 1.0 N H2SO4, mV
Potential change
in 30 min, mV
Cathode material
at the beginning of electrolysis
after 30 min
Lead
1100
1140
815
1040
1140
860
1020
1120
790
20
20
Zinc
Copper
70
Platinum
Stainless steel
Iron
740
1180
1140
1020
1370
1350
1200
1010
880
80
850
130
140
190
210
725
Titanium
Glassy carbon
1200
1500
1180
1160
Table 2. Effect of current density on the current efficiency for arsine on various electrode materials (10% H2SO4, ꢀ2 g/l As2O3)
AsH3 yield, %
Current
density, A/cm2
cold-rolled
cadmium
lead
cast cadmium
stainless steel
copper
titanium
iron
0.05
0.3
28
75
26
65
–
26
54
20
52
22
50
21
45
52
the cathode material and seems to be due to structural the current density increases the arsine yield on all of
changes in the arsenic coating.
the cathode materials studied (Table 2). The highest
yield was reached on lead, and the lowest yield was
obtained on iron.
Testing various electrode materials, we found that
the activity of cadmium cathodes depended on the fab-
rication process (Fig. 4). The yield on an electrode pre-
pared by casting into a glass mold was equal to that on
the lead cathode. The arsine yield on cold-rolled cad-
mium was substantially lower.
An interesting trend was revealed in testing the lead
cathode. In a number of electrolyses, the cathode was
activated. Reducing the current density in each subse-
quent experiment with the same electrode reduced the
arsine yield more slowly than was observed during the
initial increase in current; that is, a significant hystere-
sis was observed (Fig. 5). It seems likely that the acti-
vation of the cathode is due to the modification of its
surface through arsenic incorporation into the lead
lattice.
Next, we assessed the stability of electrode materi-
als (lead, copper, and cadmium) in successive electrol-
yses. The results are displayed in Fig. 6. These experi-
mental data lend support to the conclusion drawn from
voltammetry data that a stable arsine yield can be
achieved on a lead cathode: the yield was about 70% in
the first run, then decreased to a level of 60%, and
Thus, voltammetry data indicate that arsenic is
deposited on all of the electrode materials studied, but
the deposit does not suppress the influence of the cath-
ode material. The behavior of arsenic coatings depends
significantly on the nature of the cathode material, and
an appropriate choice of the cathode material is of key
importance in developing the process for the prepara-
tion of arsine. It can be seen from Table 1 that, during
the first minutes of electrolysis, a high negative poten-
tial is observed on metals with a low hydrogen overpo-
tential, which drops rapidly over time. Lead and zinc,
metals with a high hydrogen overpotential, have a suf-
ficiently stable potential. These materials would be
expected to ensure a stable arsine yield over a long
time.
Taking into account voltammetry results, we contin-
ued experimental studies of electrochemical arsine syn-
thesis using different electrode materials with the aim
of optimizing the conditions for long-term electrolysis
without disassembling the electrolyzer.
As shown previously [5], the AsH3 yield on a lead
cathode increases significantly with increasing current
density. The maximum current efficiency for arsine,
about 75%, was achieved at a current density of 0.2–
0.25 A/cm2, whereas at 0.05 A/cm2 the current effi- remained unchanged in the next ten runs. The arsine
ciency was 28%. Our experiments show that increasing yield on the iron and copper cathodes was low even in
INORGANIC MATERIALS Vol. 43 No. 1 2007

28
SMIRNOV et al.
near unity. Given the low solubility and very weak dis-
80
60
40
20
sociation of arsenous acid, HAsO2, this ratio can be
obtained in 1% sulfuric acid solutions (Bouard et al.
[7], however, recommend a 1 M (ꢀ10%) H2SO4 solu-
tion). The current efficiency for arsine is then essen-
tially independent of current density: 25 l/(h m2) at
0.02 A/cm2 and 225 l/(h m2) at 0.15 A/cm2, which cor-
responds, according to our calculations, to current effi-
ciencies of 89.7 and 92.9%, respectively. Bouard et al.
[7] propose constantly saturating the solution with
arsenic(III) ions during electrolysis by pumping the
solution through a layer of arsenic oxide.
1
2
0.1
0.2
0.3
Current density, A/cm2
The drawback to the filter-press electrolyzer with a
working chamber 2.2 × 2.2 cm in dimensions is the
presence of dead zones in which elemental arsenic is
deposited. In connection with this, the data from [7]
were checked using an electrolyzer with improved
hydrodynamics, in which the height of the working
chamber was almost 20 times its width. This made it
possible to raise the linear velocity of the catholyte by
about one order of magnitude, without changing its vol-
umetric circulation rate. Preliminary testing of the elec-
trolyzer with 10% H2SO4 showed that the increased cir-
culation velocity had no effect on the arsine yield but
eliminated the dead zones in which elemental arsenic
accumulated.
Fig. 4. Effect of current density on the arsine yield on cad-
mium cathodes (10% H SO , 1.4% As O ): (1) cast cad-
2
4
2 3
mium, (2) cold-rolled sheet cadmium.
the first run and had a pronounced tendency to decrease
in subsequent runs.
For all of the cathode materials tested, arsine synthe-
sis with a low yield was always accompanied by the
formation of elemental arsenic. Its amount decreases
with increasing current density, and at 0.2 A/cm2 the
current efficiency for arsenic is about 2.5% [6].
Although the yield of arsenic is rather low, its deposi-
tion notably impedes maintenance of the electrolyzer
and makes periodic cleaning of the cathode chamber
necessary.
We conducted two types of experiments, saturating
the catholyte with arsenous acid (arsenic(III) oxide) in
a separate apparatus or utilizing an As2O3 suspension in
the catholyte. The results revealed no significant advan-
Since we failed to reduce the yield of elemental tage of the process described by Bouard et al. [7]. The
As3+ : H+ ratio recommended by them did not ensure a
high yield. Continuous saturation of the catholyte with
arsenous acid (see above) had low efficiency: we failed
arsenic to below 2.5% by varying the current density in
10% sulfuric acid, it was of interest to check the results
obtained by Bouard et al. [7], who reported that the
highest arsine yield was achieved at an As3+ : H+ ratio to achieve a stable arsine yield. The current efficiency
70
1
80
70
60
50
40
30
20
10
60
50
40
30
20
2
2
4
1
3
10
0
2
4
6
8
10
12
14
Run no.
0
0.05 0.10 0.15 0.20 0.25 0.30
Current density, A/cm2
Fig. 6. Variation in arsine yield in a series of successive
experiments (10% H SO , 1.4% As O , current density of
2
4
2 3
0.2 A/cm ) without cleaning the cathode or disassembling
the electrolyzer: (1) lead, (2) cast cadmium, (3) copper, and
(4) stainless steel cathodes.
Fig. 5. Activity of a lead cathode as a function of
(1) increasing and (2) decreasing current density (10%
2
H SO , 1.4% As O ).
2
4
2 3
INORGANIC MATERIALS Vol. 43 No. 1 2007

ELECTROCHEMICAL REDUCTION OF As(III) IN ACID MEDIA
29
tions, but the process parameters achieved are inferior
to those in the reduction of As(V) compounds, e.g.,
arsenic acid, H3AsO4 [8].
90
80
70
60
50
40
30
20
10
0
The use of a filter-press electrolyzer with forced
catholyte circulation makes it possible to achieve a sta-
ble current efficiency for arsine on the cathode of order
65%, with a selectivity of 97–98%. About 2% of the
power goes to arsenic release (the rest, to hydrogen
release). Varying the sulfuric acid concentration from 1
to 10% has an insignificant effect on the process param-
eters. The electrolyzer we tested can be regarded as a
prototype of an industrial apparatus.
1
2
3
4
5
6
7
8
9
10
REFERENCES
Run no.
1. Tomilov, A.P., Osadchenko, I.M., and Khomutov, N.E.,
Electrochemistry of Arsenic and Its Compounds, Itogi
Nauki Tekh., Ser.: Elektrokhim., 1979, vol. 14,
pp. 168−207.
2. Nilov, A.P., Optimization of Mass Transport and Phase
Formation Processes for the Electrochemical Reduction
of As(III), Sb(III), and Ge(IV) Compounds, Extended
Abstract of Doctoral (Chem.) Dissertation, Moscow,
2001.
3. Vorotyntsev, V.M., Balabanov, V.V., Abdrakhmanov, R.R.,
and Malygina, L.S., Electrochemical Synthesis of High-
Purity Arsine: I. Current Efficiency for Arsine in the
Cathodic Reduction ofAs2O3, Vysokochist. Veshchestva,
1993, no. 5, pp. 22–27.
4. Muratov, E.B., Bukhman, S.P., and Nosek, M.V., Reduc-
tion of As(III) at Mercury and Zinc–Amalgam Cathodes,
Izv. Akad. Nauk Kaz. SSR, 1963, no. 2, p. 15.
5. Smirnov, M.K., Smetanin, A.V., Turygin, V.V., et al.,
Electrochemical Reduction of As(III) in Acid Media,
Elektrokhimiya, 2001, vol. 37, no. 10, pp. 1214–1218.
6. Tomilov, A.P., Smetanin, A.V., Chernykh, I.N., and
Smirnov, M.K., Electrode Reactions Involving Arsenic
and Its Compounds, Elektrokhimiya, 2001, vol. 37,
no. 10, pp. 1157–1172.
Fig. 7. Current efficiency for arsine in a series of successive
experiments (lead cathode, 1% H SO , As O suspension,
2
4
2 3
2
current density of 0.1 A/cm ) in an electrolyzer with
improved hydrodynamics.
in those experiments varied from 35 to 75%. An As2O3
suspension in the catholyte ensured a more stable yield
(Fig. 7), which, however, had a tendency to decrease
with an increase in the voltage applied to the electro-
lyzer. The likely reason for this is the deposition of As
and As2O3 on the cathode. The current efficiency for
elemental arsenic was 1.8–2.1%.
With 1 and 10% H2SO4, we obtained comparable
data. Clearly, the sulfuric acid concentration must be
chosen in the implementation context.
The material and energy balances for experiments
with 1% H2SO4 and a current density of 0.1 A/cm2 indi-
cate that the preparation of 1 kg of arsine requires an
energy of about 20 kW h. Under these conditions, the
current efficiency for arsine is 64.6% and the substance
yield is 95.5%.
7. Bouard, P., Labrune, P., and Cocolios, P., US Patent
5425857, 1995.
8. Smetanin, A.V., Smirnov, M.K., Chernykh, I.N., et al.,
Electrochemical Preparation of Arsenic and Its Com-
pounds, Neorg. Mater., 2003, vol. 39, no. 1, pp. 27–42
[Inorg. Mater. (Engl. Transl.), vol. 39, no. 1, pp. 22–36].
CONCLUSIONS
The present experimental data demonstrate that ars-
ine can, in principle, be synthesized through the elec-
trochemical reduction of arsenous acid in sulfate solu-
INORGANIC MATERIALS Vol. 43 No. 1 2007