Studies on the feasibility of electrochemical recovery of palladium from high-level liquid waste

Article abstract of DOI:10.1016/j.electacta.2008.08.034

The electrochemical behavior of palladium (II) in nitric acid medium has been studied at platinum and stainless steel electrodes by cyclic voltammetry. The cyclic voltammogram consisted of a surge in cathodic current occurring at platinum electrode at a p

Full text of DOI:10.1016/j.electacta.2008.08.034

Electrochimica Acta 54 (2009) 1083–1088
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Studies on the feasibility of electrochemical recovery of palladium from
high-level liquid waste
M. Jayakumar, K.A. Venkatesan, T.G. Srinivasan∗, P.R. Vasudeva Rao
Fuel Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 April 2008
Received in revised form 21 August 2008
Accepted 21 August 2008
Available online 28 August 2008
The electrochemical behavior of palladium (II) in nitric acid medium has been studied at platinum and
stainless steel electrodes by cyclic voltammetry. The cyclic voltammogram consisted of a surge in cathodic
current occurring at platinum electrode at a potential of −0.1 V (vs. Pd), which culminates in a peak at
−0.3 V was due to the reduction of Pd(II) to Pd. This was accompanied by a broad scant anodic peak (Ipa)
at 0.25 V during scan reversal. Reduction of Pd(II) was irreversible and the diffusion coefficient was found
to be 2.35 × 10−8 cm2/s at 298 K. At stainless steel electrode, a surge in the cathodic current occurring at
−0.4 V (vs. Pd) was due to palladium deposition, which was immediately followed by a steep increase in
cathodic current at −0.66 V due to H+ reduction. Electrolysis of palladium nitrate from 1 M to 4 M nitric
acid medium at stainless steel electrode resulted in complete recovery of palladium with reasonably
high Faradaic efficiency depending upon nitric acid concentration. However, the recovery and Faradaic
efficiency were significantly lowered (to 40%) in the case of electrolysis from simulated high-level liquid
waste due to other interfering competitive reactions.
Keywords:
Palladium
Fission product
High-level liquid waste
Electrodeposition
Voltammetry
© 2008 Elsevier Ltd. All rights reserved.
1. Introduction
demand the addition of external reagents in HLLW. Koizumi et al.
[10] reported the electrolytic extraction of fission platinoids from
PUREX process is being adopted for the separation of uranium
and plutonium from the spent nuclear fuel dissolver solution [1].
The nitric acid raffinate obtained after extraction is known as “high-
level liquid waste” (HLLW), which contains significant quantities
of valuable fission platinoids [2]. Most of the isotopes of platinum
group metals (Ru, Rh and Pd) formed from fission reaction are non-
radioactive or weakly radioactive. For example fission palladium is
composed of few stable isotopes (83%) and a radioactive 107Pd(17%),
which is a soft ␤− emitter with a half-life of 6.5 × 106 years. The
intrinsic radioactivity of 107Pd is very weak and can be tolerated
for several industrial applications. Therefore, there is a growing
interest in the recovery of palladium from HLLW [3].
Several separation methods such as solvent extraction [4], ion
exchange [5], non-aqueous methods [6], etc. have been reported
for the recovery of palladium. Excellent reviews in this regard
by Kolarik and Renard [3,7,8], and Pokhitonov and Romanovskii
[9], detail the methods reported to date for the separation of fis-
sion platinoids. The electrochemical method for the separation
and recovery of palladium is one of the easy and promising tech-
niques due to its simplicity, accessibility of reduction potential of
palladium in nitric acid medium. Besides, this method does not
nitric acid medium. A recovery of 90%, 23% and 10% was reported
respectively for the deposition of Pd, Rh and Ru, and the deposi-
tion rates were reported to decrease with increase of nitric acid
concentration. Ozawa et al. developed a new strategy for the back
end of nuclear fuel cycle, known as Adv.-ORIENT, to enhance the
separation of potentially useful fission products such as palladium
and transmutation of minor actinides [11,12]. Kirshin et al. [13]
studied the electrolytic recovery of palladium from nitric acid solu-
tions and reported the efficiency of the process in the presence of
HNO3, NaNO3, uranium and other admixtures. However, detailed
electrochemical behavior of palladium (II) in nitric acid medium,
speciation and recovery from simulated waste of fast reactor fuels
has not been reported so far. Therefore the objective of the present
paper is to report the results of the electrochemical behavior of
Pd2+ in nitric acid medium, investigated by cyclic voltammetry.
Electrodeposition of palladium in the presence of various interfer-
ing elements present in high-level liquid waste, effect of nitric acid
concentration, and recovery from simulated high-level waste, etc.
are also reported.
2. Experimental
All the chemicals used in the study were of analytical AR grade.
Palladium (II) nitrate in nitric acid solution (10%, w/v) was procured
from Arora Matthey, Kolkata, India.
∗
Corresponding author. Fax: +91 44 27480065.
E-mail address: tgs@igcar.gov.in (T.G. Srinivasan).
0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.08.034

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M. Jayakumar et al. / Electrochimica Acta 54 (2009) 1083–1088
2.1. Voltammetry of palladium (II) nitrate in nitric acid
A solution of palladium nitrate (20 mM) in nitric acid medium
was prepared by dissolving a required quantity of palladium nitrate
stock in appropriate concentration of nitric acid (25 mL). Elec-
trochemical studies and electrolysis were conducted at 298 K.
In voltammetric measurements, platinum wire was used as the
working (cylindrical, SA = 0.11 cm2) and counter electrodes, and
palladium wire was used as quasi-reference electrode. All the
voltammetric data were obtained after IR compensation. Elec-
trodeposition was conducted on a stainless steel (6 cm2) plate
with platinum plate as counter electrode and palladium wire as
quasi-reference electrode. The choice of stainless steel as working
electrode during electrolysis was due to its inexpensiveness and
its easy availability for scaling-up operations. After the deposition,
the plate was washed extensively with acetone and deionized water
before subjecting it for morphological examination.
2.2. Instrumentation
Fig. 2. Comparison of UV–vis absorption spectrum of Pd(II) in various concentra-
tions of nitric acid. [Pd] = 1 × 10−4 M.
Cyclic voltammograms of the solutions were recorded using
Autolab (PGSTAT- 030) equipped with an IF 030 interface. UV–Vis
absorption spectrum was recorded using Shimadzu UV–Vis spec-
trometer model 2500. A Philips field effect scanning electron
microscope (SEM), model XL 30, with energy-disperse spectrom-
eter (EDS) working at 30 kV was used to examine the surface
morphology and elemental composition of the deposit.
complexation. Purans et al. [14] studied the structure and speci-
ation of Pd2+ in nitric and perchloric acid medium. The stability
constants for the complexation of palladium (II) ion with nitrate
ions in nitric acid medium, derived from solvent extraction data,
were reported by Tarapcik [15]. Based on those stability constants
(ˇ1 = 3.28, ˇ2 = 2.13, ˇ3 = 0.223, ˇ4 = 0.004) speciation of palladium
in nitric acid medium was determined and is depicted in Fig. 3.
It is seen that in 0.1 M nitric acid medium, majority of palladium
exists as free ion [Pd(H2O)4]2+ and its concentration decreases
with increase in the concentration of HNO3. The concentration of
[Pd(NO3)(H2O)3]+ is maximum at 1 M HNO3, and the concentra-
tion of neutral species, [Pd(NO3)2(H2O)2], increases with increase
in concentration of nitric acid. Accordingly, the ꢀmax observed in the
visible absorption spectrum of palladium nitrate solution, shown in
Fig. 2, is also shifted bathochromically with increase in the concen-
tration of nitric acid. Thus, the charge of the electroactive species
decreases with increase in the concentration of nitric acid that
seems to reduce the diffusion of palladium species and lowers the
cathodic peak current (Ipc).
3. Results and discussions
Fig. 1 shows the cyclic voltammogram of palladium (II) in
nitric acid medium recorded at platinum electrode at the potential
sweeping rate of 100 mV/s. A surge in the cathodic peak current
occurring at a potential of −0.1 V (vs. Pd), which culminates in
a peak at −0.3 V, is due to the reduction of palladium. A broad
oxidation wave is observed at 0.25 V during scan reversal, indi-
cating that the reduction of palladium (II) at platinum electrode
is irreversible. Increasing the nitric acid concentration decreases
the cathodic peak current (Ipc) and broadens the cathodic wave.
This effect could be attributed to the conversion of free palladium
(II) ion in to various nitro-complexes of palladium. The absorp-
tion spectra of Pd(II), shown in Fig. 2, also exhibits a bathochromic
shift with increase in the concentration of nitric acid due to nitrate
Fig. 1. Comparison of cyclic voltammograms of Pd(II) in different concentrations of
HNO3 recorded at platinum electrode. Quasi-reference: palladium wire, T = 298 K,
Fig. 3. Speciation of various Pd(II) nitrate complexes in various concentrations of
nitric acid.
scan rate = 0.1 V s−1
.

M. Jayakumar et al. / Electrochimica Acta 54 (2009) 1083–1088
1085
tion of H+ is very high (4 M) as compared to palladium (II) ion
(in 20 mM), the reduction of palladium (II) at RDE is not appar-
ent in the voltammogram shown in Fig. 4. A similar behavior of
broadening of cathodic wave and shifting of cathodic peak poten-
tial (Epc ) to more negative values is also observed during the cyclic
voltammetric investigations (Fig. 1). Therefore the diffusion coeffi-
cient of palladium (II) could be determined only for the reduction
from 0.1 M nitric acid medium using Levich equation, shown in Eq.
(1) [16,17], that relates the limiting current (il) and angular velocity
(ω = 2ꢁr/60, r is revolutions per minute).
il = 0.62nFCAD2/3
Á
−1/6ω1/2
(1)
(2)
1
1
1
=
+
0.62nFCAD2/3 −1/6ω1/2
Á
i
iK
where Á is the kinematic viscosity (10−2 cm2/s). Fig. 5 shows the
cathodic sweep voltammogram of palladium (II) in 0.1 M nitric
acid at various rotation rates (300–1500 rpm) recorded at plat-
inum rotating disc electrode. It is observed that the limiting current
increases with increase of rotation speed. The limiting current
Fig. 4. Comparison of cathodic sweep voltammograms of Pd(II) present in var-
ious concentrations of nitric acid recorded at platinum rotating disc electrode
(r = 500 rpm). Counter electrode: platinum wire. Reference electrode: palladium
is measured at a potential of −0.40 V and plotted against ω1/2
.
The linear regression of the data showed that the intercept is
marginally deviating from the origin, as shown in Fig. 5. There-
fore, Koutecky–Levich equation (Eq. (2)) is used for determining
the diffusion coefficient [16,17], where i is the magnitude of cur-
rent at a potential of −0.4 V in the present case and iK is the current
in the absence of any mass transfer effects. A plot of inverse of
current (1/i,) against ω−1/2 (Koutecky–Levich plot) is also shown
in Fig. 5. From the slope of the straight line the diffusion coeffi-
cient of palladium (II) in 0.1 M nitric acid medium was found to be
2.35 × 10−8 cm2/s.
wire. T = 298 K, scan rate: 0.01 V s−1
.
The electrochemical behavior of palladium (II) was also investi-
gated at rotating disc electrode. Fig. 4 shows the cathodic sweep
voltammogram of palladium (II) present in nitric acid medium
recorded at platinum rotating disc electrode (radius = 1.5 mm) at
the scan rate of 10 mV/s. It is observed that the cathodic current
increases with increase of applied potential due to the reduction
of palladium (II) ion, which is subsequently followed by increase of
cathodic current arising from the reduction of H+ ion (H2 evolution).
A limiting current for the reductionofpalladium (II)is obtainedonly
in the case of reduction from 0.1 M nitric acid medium, where dis-
tinct curves for the reduction of palladium (II) as well as for H+ are
observed. However, at higher nitric acid concentrations the onset
of reduction of palladium is shifted to more negative potentials
and merges with H+ reduction. In addition, since the concentra-
4. Electrolysis experiments
4.1. Selection of applied potential
Electrodeposition of palladium from nitric acid medium was
studied at constant applied potential. The experiments involved
Fig. 5. Cathodic sweep voltammogram of Pd(II) present in 0.1 M nitric acid recorded at platinum rotating disc electrode at various rotations. Insert: Levich (il vs. ω1/2) and
Koutecky–Levich (1/i vs. ω−1/2) plots. The limiting current measured at −0.4 V.

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M. Jayakumar et al. / Electrochimica Acta 54 (2009) 1083–1088
Table 1
Electrolysis of palladium (II) nitrate (total Pd taken for electrolysis −55 mg) solution in 4.0 M nitric acid medium at various applied potentials
Applied potential (vs. Pd)
Charge passed in coulombs
Time, h
Recovery obtained
Expected deposit for the
charge passed (grams)
Faradaic efficiency (%)
grams
%
−0.4
−0.45
−0.5
−0.6
0.252
2
191
8.5
8.5
9.3
4.6
0
0
<5
100
100
0
0
<0.005
0.055
0.055
0.0028
0.1053
0.0848
36.3
52.2
64.8
154
electrolysis of a 20 mM palladium nitrate solution in nitric acid
medium with stainless steel (6 cm2) as cathode and platinum plate
as anode. The potential was varied from −0.4 V (onset of palladium
(II) reduction at stainless steel electrode) to −0.6 V (vs. Pd) to choose
the optimum potential required for electrolysis. The result of elec-
trolysis of palladium nitrate solution from 4 M nitric acid medium
is tabulated in Table 1. It is observed that the recovery of palla-
dium deposit increases with increase of applied negative potential
and at potentials more negative than −0.5 V (vs. Pd), the recovery
is quantitative. However, increasing the applied negative potential
may also favor other undesirable redox reaction and considering
this and duration of electrolysis the potential of −0.5 V is chosen
as the optimum potential for electrolysis using stainless steel as
cathode.
(<0.5 ppm) of a spectrophotometric method using Arsenazo III as
coloring agent. In all these experiments, significant evolution of
gases, hydrogen and NOx, was observed at the working electrode
only when the experimental solution was becoming colorless. The
recovery of palladium was quantitative at all nitric acid concentra-
tions investigated and the Faradaic efficiency (calculated after eight
hours of electrolysis) was −75% at 1.0 M nitric acid and was lowered
to −55% at 4.0 M nitric acid. This could be due to the electrochemical
reduction [18] of nitrate or H+ ion (discussed below). The deposit
obtained from the electrolysis was subjected to surface morpho-
logical examination by scanning electron microscopy (SEM). The
deposit, shown in Fig. 7, is uniform all over the surface of stainless
steel electrode with a dendrite growth observed in all cases. The
dendrites seem to increase with increase in the concentration of
nitric acid.
The interference of various cations and anions during the depo-
sition of palladium was investigated by cyclic voltammetry using
stainless steel as working electrode, platinum as counter electrode
and palladium as quasi-reference (same electrode system used for
electrolysis). The cyclic voltammogram of 0.1 M nitric acid solution
recorded at stainless steel electrode is shown in Fig. 8A and the
nitric acid is stable from −0.9 V to 0.17 V (vs. Pd). The reduction
of palladium (II) from nitric acid medium is compared with the
reduction of various ions such as Ag(I), UO22+, NO3−, Fe(II), etc.,
that are likely to be present in high level liquid waste and also
lower the Faradaic efficiency of palladium deposition. A surge in
the cathodic current occuring at −0.4 V (vs. Pd), is due to palla-
dium deposition (Fig. 8B), which is subsequently accompanied by
a steep increase in cathodic current at −0.66 V, is due to H+ reduc-
tion. Such behavior is not observed during the deposition of silver
(one of the fission products) on cathode (Fig. 8C). This clearly indi-
cates that the reduction of palladium (II) from nitric acid medium
results in a deposition of metallic palladium on stainless steel elec-
trode and due to the characteristic property of palladium metal,
the deposit seems to catalyze the underpotential reduction of H+
ion [18] at the electrode. It is also observed from Fig. 8 (voltammo-
grams C to F), that the onset of reduction for a solution of silver
nitrate, uranyl nitrate, ferric nitrate and sodium nitrate in nitric
acid medium at stainless steel electrode occurs at the potentials, of
−0.3 V, −0.71 V, −0.81 V,−0.65 V respectively. These potentials are
very close to the onset of reduction (−0.4 V) of palladium (II). How-
ever, when palladium is co-existing in the solution along with these
ions the reduction reactions are shifted to lower negative potentials
(i.e. underpotential reduction) as shown in Fig. 8 (voltammograms
G to J). This indicates that the initial deposition of palladium on
stainless steel electrode modifies the electrode surface to metallic
palladium, favors underpotential reduction and also substantially
increases the cathodic current in all cases. These factors tend to
lower the Faradaic efficiency of palladium deposition.
4.2. Variation of [HNO3]
The influence of nitric acid concentration on the electrodepo-
sition of palladium was studied at a constant applied potential
of −0.5 V. The concentration of nitric acid was varied from 1.0 M
to 4.0 M. Fig. 6 shows the number of coulombs passed as a func-
tion of time at various nitric acid concentrations. It is observed
that there is a gradual increase in the coulombic charge density
(number of coulombs passed per unit area) in the initial stages
of electrolysis and after three hours there is an abrupt increase in
the passage of coulombic charge. Significant deposition of palla-
dium was visually observed on cathode during this abrupt increase
in passage of coulombic charge density. This is accompanied by a
rapid change in the color of the solution from dark orange to color-
less. The concentration of palladium left in the solution after six
hours of electrolysis was found to be below the detection limit
It was also found that lowering the applied negative potential
(towards nobler) lowered the recovery palladium (as tabulated in
Table 1) and the presence of rare-earth elements, strontium and
cesium (equal to the amounts present in HLLW shown in Table 2)
did not affect the recovery of palladium to any significant extent.
Fig. 6. Charge–time curves for the electrodeposition of palladium on a stainless
steel plate from different nitric acid concentrations. A = 5.92 cm2. Time = 8 h. Poten-
tial = −0.5 V (vs. Pd). Counter electrode: platinum.

M. Jayakumar et al. / Electrochimica Acta 54 (2009) 1083–1088
1087
Fig. 7. SEM images of palladium obtained by electrolysis from different nitric acids and simulated high-level waste. (A) 1 M HNO3, (B) 2 M HNO3, (C) 3 M HNO3 and (D) fast
reactor high-level waste. Magnification 2000× for A, B and C images and 1000× for image D.
However, when the mole ratio of uranium to palladium exceeds
four and presence of sodium nitrate (>0.5 M) in 0.1 M HNO3 solu-
tion lowers the recovery (40% after 8 h at −0.5 V) and Faradaic
efficiency significantly. This could be due to other undesirable elec-
trochemical reactions as discussed above. A similar behavior was
also reported by others [10,13]. Krishin et al. [13] suggested that the
presence of U(VI) in HLLW promotes the reduction of nitric acid and
lowers the current efficiency.
Table 2
Elemental compositions of simulated high-level liquid waste (HLLW) showing pres-
surized heavy water reactor and fast breeder reactor spent fuel compositions [19]
Element
Elements present (g/L)
PHWR (6500 MWd/tonne)
FBR (80,000 MWd/tonne)
Sodium
Iron
Nickel
Selenium
Strontium
Zirconium
Cesium
Barium
Lanthanum*
Cerium
Praseodymium
Neodymium
Samarium
Europium
Gadolinium
Silver
Cadmium
Yttrium
Promethium
Terbium
Dysprosium
Uranium
Technetium
Molybdenum
Ruthenium
Rhodium
Palladium
Chromium
Rubidium
Tin
3.01
0.50
0.10
0.01
0.19
0.77
0.55
0.31
0.26
0.54
0.24
0.86
0.17
0.02
0.02
0.02
0.02
0.10
0.03
0.001
0.001
18.33
0.18
0.73
0.46
0.13
0.27
0.10
0.08
0.02
0.01
0.10
4.0
3.00
0.50
0.10
0.01
0.14
0.89
1.12
0.41
0.48
0.69
0.34
1.131
0.05
0.31
0.07
0.13
0.04
0.08
0.05
0.01
0.005
2.64
0.26
1.09
0.81
0.26
0.60
0.10
0.06
0.02
0.01
0.16
4.0
Fig. 8. Cyclic voltammograms of various interfering ions present in 0.1 M HNO3
medium recorded in the absence of palladium (C–F) and in the presence of palladium
(G–J), at stainless steel working electrode. Counter electrode: Pt wire. Reference elec-
trode: Pd wire. T = 298 K. (A) 0.1 M HNO3, (B) 50 mM Pd, (C) 2 mM AgNO3, (D) 0.3 M
NaNO3 (high concentration of NaNO3 required to obtain the reduction wave), (E)
6 mM Fe(NO3)3, (F) 30 mM UO2(NO3)2, G = 9 mM AgNO3, H = 0.3 M NaNO3, I = 6 mM
Fe(NO3)3, J = 30 mM UO2(NO3)2. The voltammograms G–J were obtained in the pres-
ence of 50 mM PdNO3.
Antimony
Tellurium
Acidity (M)
*La: added for Y, Pm, Te, Dy; Tc, Mo: not added; Co: added as Ni; Ru, Rh, Rb: added
as Pd; Sn, Sb, Te: not added due to poor solubility. Excess palladium taken to study
the feasibility of extraction.

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M. Jayakumar et al. / Electrochimica Acta 54 (2009) 1083–1088
4.3. Recovery of palladium from simulated high level waste
as silver, nitrate, iron, etc. during electrolysis. Due to these, the
recovery and Faradaic efficiency was below 40%. The study, there-
fore, suggests the requirement of other separation methods either
independent or coupled with electrochemical method for the quan-
titative recovery of palladium from high-level liquid waste.
Electrolysis of simulated waste solution was carried out to assess
the feasibility of separating palladium from high-level liquid waste
solution. The composition of simulated HLLW [19] is shown in
Table 2. Electrolysis was carried out at −0.5 V (vs. Pd) using stain-
less steel as cathode and platinum as anode for eight hours. The
results obtained from the electrolysis of palladium nitrate present
in simulated high-level waste of PHWR and FBR are identical. In
these experiments, gradual deposition of palladium on cathode was
observed during initial stages of electrolysis. However, copious evo-
lution of gases accompanied by rapid increase in coloumbic charge
density was observed after 4–5 h of electrolysis. Subsequently there
was no significant deposition of palladium even after eight hours.
Therefore, the electrolysis was discontinued after five hours and
the recovery was determined to be −40% (in some experiments
it was 25–30%) and Faradaic efficiency was 30%. The lower recov-
ery and Faradaic efficiency could be due to the presence of several
interfering ions like uranium (VI), nitrate, iron, silver, etc. in HLLW
whose redox reactions to lower valent state also occur near palla-
dium deposition potential in nitric acid medium (Fig. 8). In contrast,
electrodeposition of palladium was quantitative when electrolysis
was carried out in nitric acid medium alone as described earlier.
In those experiments, significant gas evolution was observed only
after 80% of palladium was deposited. However, in the present case,
the presence of elements such as iron, silver, nitrate in simulated
HLLW limits the deposition of palladium and facilitates the com-
petitive redox reactions and gas evolution (H2, NOx), that seems
to be responsible for lower recovery and Faradaic efficiency. The
SEM image of the palladium deposit obtained from the HLLW of
fast reactor is shown in Fig. 7, which also indicates the formation
of dendrites during deposition.
Acknowledgements
The authors are thankful to the reviewers for their construc-
tive suggestion. The authors also thank Ms. R. Sudha, Materials
Chemistry Division, IGCAR and Dr. R. Rangarajan, Water and Steam
Chemistry Division, BARC-F, Kalpakkam for providing SEM images
and RDE respectively.
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The neutral palladium (II) nitrate specie existing in 3–4 M nitric
acid medium undergoes an irreversible single step two-electron
transfer to metallic palladium at platinum electrode. While the pal-
ladiumnitratepresentin nitric acid medium could bequantitatively
recovered by electrolysis at −0.5 V (vs. Pd), several complications
aggravated in the presence of interfering elements of HLLW such