Proceedings of CUChE Alumni Symposium 2022
On “Circular Economy on Sustainable Basis: The Role of Chemical Engineers”
CUChEAA ISBN: 978-81-954649-1-3
December 2022 P a g e | 69 Volume 2, Issue 1
Waste remediation and hydrogen production by electrochemical splitting of ammonia/urea in
simulated wastewater
Abhra Shau*, P. De and P. Ray
Department of Chemical Engineering; University of Calcutta; 92, A. P. C. Road; Kolkata – 700 009; West Bengal; India
*Corresponding author: abhra.shau@gmail.com
Abstract
Electrolysis of simulated wastewater resembling effluents generated in fertilizer industries, ammonia/urea plant,
urinals were carried out in dilute acidic/alkaline medium using novel packed bed electrodes made of stainless
steel (SS 316) solid cylindrical packing. Rapid corrosion of electrodes was found to occur during electrolysis of
wastewater in acidic medium. The steady state current and energy efficiencies reached upto 99.1% and 48.3%,
respectively, while electrolyzing dilute aqueous ammonia solution with added 0.50% (w/v) KOH at applied
voltage of 3 V. During electrolysis of urea solutions corresponding efficiencies were found to reach 98.1% and
48.5%, respectively. Current and energy efficiencies attained in this study are higher than the values reported in
literature. Current efficiency remained less than 100% due to undesirable side reactions. Dissolution of Fe from
the surface of electrodes was confirmed by AAS (Atomic Absorption Spectroscopy) analysis of electrolyte before
and after electrolysis, which is one of the reasons for not attaining current efficiency to 100%. Formation of Fe
2
O
3
and Fe(OH)
3
at the surface of electrode consuming extra current were confirmed by the XRD (X-Ray Diffraction)
and FESEM (Field Emission Scanning Electrode Microscopy) analyses of the surfaces of fresh packing material
as well as materials used as anode and cathode during electrolysis of alkaline simulated wastewater containing
urea.
Keywords: Simulated wastewater; Urea; Ammonia; Packed bed electrode; Current efficiency; Energy efficiency.
1. Introduction
Electrolysis of water with 20−30% (w/v) KOH using flat
plate type stainless steel electrodes are being practiced
commercially for electrochemical production of
hydrogen, as electrical conductivity of aqueous KOH
solution in this range of concentration at operating
temperature of 20–40 °C exhibit maximum value. In this
process, KOH is normally added as supporting electrolyte
for increasing the conductivity of de-mineralized (DM) or
de-ionized (DI) water for preparing electrolytic solution.
Treatment for producing DM or DI water requires more
energy and operating cost.
Hydrogen can also be obtained by electrolysis of aqueous
solutions of hydrogen rich compounds requiring lower
activation energy. Many types of wastewater contain such
compounds – ammonia, urea for example are readily
available; and such wastewaters may be conveniently
used instead of purified water, as electrolyte for portable
or mobile or on-board production of hydrogen [1−11].
Actually, ammonia/urea gets effectively electrolyzed
during electrolysis of wastewater containing
ammonia/urea as activation energies required to split
ammonia/urea to hydrogen are lower than that of water.
Minimum theoretical cell potential required to electrolyze
water at standard conditions is 1.23 V. However,
ammonia and urea requires only 0.06 V [12] and 0.37 V
[10, 14], respectively which are 95% and 70% lower than
that of water. Electrolysis of wastewater containing
ammonia and/or urea has two-fold advantages: (i)
remediation of wastewater, and (ii) production of
hydrogen. The sources of this type of wastewater are
public urinals, urea plants, ammonia plants, fertilizer
plants etc. In industrial wastewater, urea concentration
varies from 0.3 to 1.5% (w/v) and ammonia content varies
from 2 to 9% (w/v) [7, 9]. It has been reported that
ammonia concentration in effluent from fertilizer plants
varies between 0.0017 to 1.7% (w/v) [12]. According to
The Environment (Protection) Rules, 1986 in India,
ammonia content in municipal or domestic wastewater
has permissible limit of 50 ppm, i.e. 0.005% (w/v) [15].
Current efficiency for production of hydrogen from
0.0017 to 1.7% (w/v) aqueous ammonia solution with
1.12 to 5.6% (w/v) KOH reported in literature is about
Proceedings of CUChE Alumni Symposium 2022
On “Circular Economy on Sustainable Basis: The Role of Chemical Engineers”
CUChEAA ISBN: 987-81-954649-1-3
December 2022 P a g e | 70 Volume 2, Issue 1
92% using Rh–Pt–Ir flat plate electrode [12] while energy
efficiency upto 45% could be reached using Pt rotating
disk electrodes during electrolysis of 0.17% (w/v)
ammonia with 5.6% (w/v) KOH [16]. Both the
efficiencies are considered as good enough in this field;
however, the materials of the electrodes are expensive
resulted in high capital cost for this process.
Conductivities of wastewater containing urea/ammonia
are not sufficient to carry out electrolysis at a reasonable
rate. The process therefore requires acid/alkali/salt to
increase conductivity of the electrolytic solution, thus the
rate of production of hydrogen.
Electrolytic production of hydrogen in dilute
acidic/alkaline aqueous solutions using novel packed bed
electrodes has been reported in our earlier work [17].
Dilute acidic medium has been found to be unfavorable
for production of hydrogen due to rapid corrosion of
stainless steel (SS 316) electrodes during electrolysis,
while current efficiencies for production of hydrogen in
dilute alkaline medium have been found to be comparable
with commercial alkaline water electrolytic cells operated
using high concentration of KOH.
Aim of the present work is therefore to study the
characteristics of electrolytic production of hydrogen
from simulated wastewater containing ammonia/urea
having resemblance with industrial and urinal wastewater
using novel packed bed electrode made of SS 316 solid
cylindrical packing.
2. Materials & Methods
2.1. Materials
Aqueous ammonia and urea solutions of different
concentrations in double distilled water were prepared in
the laboratory for carrying out the experiments.
Specifications of ammonia and urea used for preparing
electrolytes are presented below.
Specification of ammonia (NH
3
) (Manufacturer: Merck):
Specific gravity:
0.91
Assay (NH
3
):
≈ 25%
Chloride (Cl
–
):
≤ 0.005%
Sulphate (SO
4
2–
):
≤ 0.005%
Lead (Pb): ≤
0.0005%
Iron (Fe): ≤
0.001%
Non-volatile substances: ≤ 0.002%
Specification of urea (NH
2
CONH
2
) (Manufacturer: Nice
Chemicals):
Assay (Urea): ≥
99%
Sulphated
ash: ≤ 0.1%
Chloride (Cl
–
):
≤ 0.002%
Sulphate (SO
4
2–
):
≤ 0.05%
Lead (Pb): ≤ 0.002%
2.2. Methods
Experiments were performed in acidic and alkaline
medium using the H-type electrolytic cell with packed
bed electrodes made of SS 316 solid cylindrical packing
at ambient temperature, as discussed in earlier
communication [17], using either 0.25–1.00% (w/v)
aqueous ammonia solution or 0.75–2.00% (w/v) aqueous
urea solution. Experiments were performed in an H-type
electrolytic cell depicted in Figure 1 consisting of two
cylindrical limbs – anode and cathode chambers –
connected by a rectangular conduit without any
membrane in between. Electrolysis was carried out
applying constant DC voltage across the electrolytic cell.
Volume of the gases produced, current passing through
the electrolytic cell, temperature and pressure were
measured to calculate current and energy efficiencies.
Figure 1: Schematic of electrolytic cell (EC) filled with
electrolytic solution (ES). Cathode (E1) and anode (E2)
are connected to constant DC power source (PS).
Produced H
2
gas (G1) and produced N
2
gas (G2) collected
through their outlets.
3. Results and discussion
Experiments were carried out at ambient temperature of
25±4 °C and 34±3 °C using simulated wastewater
containing ammonia and urea, respectively, every run was
repeated at least thrice for checking reproducibility of the
data. The standard deviation for the data obtained using
wastewater containing ammonia and urea are ±1.99% and
±2.18%, respectively. These values are considered as
acceptable for temperature variation of 8 °C. In this study,
the current efficiency is defined as the ratio of theoretical
current required to the actual current consumed for
production of same amount of hydrogen. The energy
efficiency is defined as the ratio of higher heating value
of hydrogen produced to the energy consumed for
production of same amount of hydrogen. The current and
energy efficiencies reported here are cumulatively
calculated for hourly basis. The data obtained by carrying
out electrolysis in alkaline medium only are presented
here because the experiments using acidic wastewater
containing urea show signs of rapid corrosion. Ammonia
solution is already alkaline and hence was electrolyzed in
alkaline condition.
3.1. Aqueous Ammonia solution with KOH
The current and energy efficiencies calculated using the
expressions given earlier [17] for production of hydrogen
using 0.50% (w/v) KOH at an applied voltage of 3 V have
been plotted against time in Figures 2(a) and 2(b) with
concentration of ammonia as parameter. The current
Proceedings of CUChE Alumni Symposium 2022
On “Circular Economy on Sustainable Basis: The Role of Chemical Engineers”
CUChEAA ISBN: 987-81-954649-1-3
December 2022 P a g e | 71 Volume 2, Issue 1
efficiencies have been found to vary from 51.7 to 56.7%
after 1 hr of experiment and to reach steady state after
about 2 hr, the values lying within the range of 83.3 to
99.1%. The time required to reach steady state is
independent of concentration of ammonia for the
experimental conditions. It has been suggested that the
produced gas initially does not come to the collecting
chamber but dissolves into electrolytic solution. Primary
calculations using literature values for solubility of
hydrogen show that this unsteady behavior may be due to
dissolution of produced gas in the electrolytic solution
though it could not be experimentally verified due to
limitations of experimental set-up.
Figure 2: Effect of concentration of ammonia on (a)
current efficiency, and (b) energy efficiency for
production of H
2
using 0.50% (w/v) KOH solution at 3 V.
The steady state current efficiency has been found to be
lowest using simulated wastewater containing 1.00%
(w/v) ammonia. The steady state current efficiencies have
been found to vary with time from 96.5 to 99.6% using
aqueous 0.50% (w/v) KOH containing no ammonia, and
92.5–99.1% using 0.75% (w/v) ammonia in aqueous
0.50% (w/v) KOH. The report of AAS (Atomic
Absorption Spectroscopy) analysis of electrolyte before
and after electrolysis shows that the amount of Fe
increases from virtually zero to about 0.13±0.01 mg/l
(Table 1). The concentration of Fe in electrolyte after
experiment was independent of concentration of
electrolyte and voltage applied to the cell. Average value
of the concentration of Fe is presented here. Dissolution
of Fe from electrodes to electrolyte may therefore be
responsible for fall in current efficiency in some cases.
The current efficiency obtained in the present system
using dilute aqueous ammonia solution with dilute KOH
as supported electrolyte and packed bed electrodes is
higher than reported value in literature is about 92% using
Rh–Pt–Ir flat plate electrode [12] and about 97±2% [13].
Figure 2(b) shows that the concentration of ammonia has
minor effect on energy efficiency. The steady state energy
efficiencies for production of hydrogen have been found
to be varied from 45.6 to 48.3% using simulated
wastewater containing 0.75% (w/v) ammonia with 0.50%
(w/v) KOH which is higher than the value of 45% using
Pt rotating disk electrodes as reported in the literature
[16]. According to Zeng and Zhang [18], energy
efficiency of about 50% is considered as good for
production of hydrogen by electrolysis of 20–30% (w/v)
aqueous KOH solution. The maximum energy efficiency
achieved using this simulated wastewater is comparable
to that using 0.50% (w/v) aqueous KOH solution alone,
i.e. 49.2%.
Figure 3: Effect of applied voltage on (a) current
efficiency, and (b) energy efficiency for production of
H
2
using 0.50% (w/v) ammonia in 0.50% (w/v) KOH
solution as electrolyte.
To study the effect of applied voltage on current
efficiency for production of hydrogen by electrolysis of
simulated dilute ammonia wastewater, the current and
(a)
(b)
40
60
80
100
1 2 3 4 5 6
Current efficiency, %
Time, hr
(a)
3.0 V
3.5 V
4.0 V
5.0 V
20
30
40
50
1 2 3 4 5 6
Energy efficiency, %
Time, hr
(b)
3.0 V 3.5 V
4.0 V 5.0 V
Proceedings of CUChE Alumni Symposium 2022
On “Circular Economy on Sustainable Basis: The Role of Chemical Engineers”
CUChEAA ISBN: 987-81-954649-1-3
December 2022 P a g e | 72 Volume 2, Issue 1
energy efficiencies have been plotted against time at
different applied voltages in Figure 3(a) and 3(b),
respectively. Time required to attain steady state is higher
at lower voltages and the steady values of current
efficiency are close to 100%. The steady values of current
efficiencies are almost same at all applied voltages and
also independent of it. That value may be considered as a
“saturation” value of current efficiency using simulated
wastewater containing ammonia in 0.50% (w/v) KOH.
Figure 3(b) shows that the steady state energy efficiency
reaches the highest value, i.e. 48.3% at an applied voltage
of 3 V. The figure also shows that the energy efficiency
decreases with increase in applied voltage obeying the
relation − energy efficiency inversely proportional to the
applied voltage for constant current efficiency.
3.2. Aqueous Urea solution with KOH
The variations of current efficiency for production of
hydrogen in alkaline medium of 0.50% (w/v) KOH
with/without urea at an applied voltage of 3 V have been
depicted in Figure 4(a). At different concentrations of
urea solution, the current efficiencies for production of
hydrogen have been found to vary from 59.9 to 76.3% at
1 hr, and to reach steady state values of 81.4–98.1% in 2
hr.
Figure 4: Effect of concentration of urea on (a) current
efficiency, and (b) energy efficiency for production of H
2
in 0.50% (w/v) KOH solution at 3 V.
Time required to attain steady state is independent of
concentration of urea, but the steady state values of
efficiencies decrease with increase in urea concentration.
The steady state current efficiency for production of
hydrogen with 0.50% (w/v) KOH have been found to vary
from 92.0 to 98.1% using 0.75–2.00% (w/v) urea
solutions, while the same varies from 81.4 to 93.7% using
2.50% (w/v) urea solution. The current efficiencies have
always been found to be well below 100% using higher
concentrations of urea. AAS analysis report of
electrolytes for pre- and post electrolysis shows that the
amount of Fe to increase from almost zero to about
0.15±0.01 mg/l (Table 1). The reason for not attaining
100% current efficiency at steady state in these cases may
therefore be the dissolution of Fe from electrode(s) to
electrolyte.
Table 1: Concentration of Fe quantified by AAS analysis
in the electrolytes – simulated alkaline wastewater (SAW)
containing ammonia/urea – before and after electrolysis.
Current efficiency obtained during electrolysis of urea in
simulated wastewater is almost constant. It has been also
found that lower applied voltage is favorable for getting
higher energy efficiency for constant current efficiency.
Investigations have therefore been carried out using
simulated wastewater containing urea at applied voltage
of 3 V only.
Figure 4(b) shows that the steady state energy efficiencies
vary from 45.5 to 48.5% using 0.75–2.00% (w/v) alkaline
urea solutions; but at 2.50% (w/v) urea in KOH, it
decreases slightly to 39.9–46.2%. The energy efficiency
achieved in this study is comparable with that of
electrolysis of dilute aqueous KOH solution alone as well
as using alkaline simulated ammonia wastewater.
FESEM (Field Emission Scanning Electrode
Microscopy) analyses of the surfaces of fresh material as
well as materials used as anode and cathode during
electrolysis of alkaline simulated wastewater containing
urea are presented in Figure 5. Three rows of the figure
denoted by (a), (b), and (c) are for fresh material, material
used as anode and cathode, respectively. Five columns are
for different magnification levels, i.e. 5, 25, 80, 100 and
120 kx, respectively. Surface of the fresh material is free
of grains and flakes; however, small grains and flakes
were found to form on the surfaces of the anode and
cathode. Growth of grains on the surface of anode is
higher than that on cathode, while number of flakes on the
surface of cathode is higher compared to that on anode.
The grains are assumed to be oxides of Fe, i.e. Fe
2
O
3
and
flakes are considered to be as hydroxide of Fe, i.e.
Fe(OH)
3
. Iron, in form of Fe(OH)
3
, might be leached out
(a)
(b)
Electrolyte
Fe in electrolyte, mg/l
Before
After
SAW containing ammonia
0
0.13±0.01
SAW containing urea
0
0.15±0.01
Proceedings of CUChE Alumni Symposium 2022
On “Circular Economy on Sustainable Basis: The Role of Chemical Engineers”
CUChEAA ISBN: 987-81-954649-1-3
December 2022 P a g e | 73 Volume 2, Issue 1
to the electrolyte as confirmed by AAS analysis and
reported earlier. Formation of oxide of Fe at the surface
of the electrode after electrolysis of alkaline simulated
wastewater containing urea was confirmed by XRD (X-
Ray Diffraction) analysis as presented in Table 2.
Table 2: Name of the compounds identified in XRD
analysis of the surface material of the packing used as
material of electrodes for electrolysis of simulated
wastewater containing ammonia/urea.
Compound
Name
Chemical
formula
Compound
Name
Chemical
formula
Burnt, ochre
Fe
2
O
3
Quartz,
synthetic
SiO
2
Hematite
Fe
2
O
3
Silica
SiO
2
Green
cinnabar
Cr
2
O
3
Formation of oxide and hydroxide of Fe at the surface of
the packing material during electrolytic production of
hydrogen are responsible for changing the color of the
surfaces of packing materials of the electrodes before and
after electrolysis from grey to golden brown, respectively.
It appears that the similar types of electrochemical or
chemical reactions occur at surfaces of both the
electrodes, but to different extent. This undesired parallel
reaction(s) might be one of the reasons for not attaining
100% current efficiency in some cases. Similar changes
in color from grey to brown of the surface of the electrode
materials were observed when electrolysis of simulated
wastewater containing ammonia was performed. The
XRD analysis of packing material after electrochemical
splitting of ammonia shows formation of oxide of Fe –
similar to the case of urea. The FESEM analysis of the
samples obtained during electrolysis of simulated
wastewater containing ammonia was not therefore
conducted. Figure 5 also shows that the formation of
oxide and hydroxide of Fe are neither homogeneous nor
uniform. This may due to roughness of the surface of
electrodes which has not been studied or measured.
Figure 5: FESEM images of the surfaces of solid cylindrical SS 316 packing (a) before electrolysis, (b) after used as anode
during electrolysis, and (c) after used as cathode during electrolysis.
4. Conclusion
Experiments on electrolytic production of hydrogen have
been carried out using simulated wastewater containing
ammonia/urea in dilute acidic (in case of urea only) and
alkaline media in H-type electrolytic cell with novel
packed bed electrodes made of solid cylindrical SS 316
packing. In acidic medium, rapid corrosion of electrodes
has been observed. The maximum steady state current and
energy efficiencies attained using ammonia solutions are
99.1% and 48.3%, respectively; while using urea
solutions these values are 98.1% and 48.5%, respectively.
Efficiencies obtained in this study are higher than those
reported in literature. Dissolution of Fe from the electrode
confirmed by AAS analysis and oxidation of Fe present at
the surface of the packing material identified by XRD and
FESEM analyses are responsible for not attaining current
efficiency of 100%. Higher surface area of packed bed
electrodes compared to flat plate or rotating disk type
Proceedings of CUChE Alumni Symposium 2022
On “Circular Economy on Sustainable Basis: The Role of Chemical Engineers”
CUChEAA ISBN: 987-81-954649-1-3
December 2022 P a g e | 74 Volume 2, Issue 1
electrodes may be a plausible reason for better current and
energy efficiencies.
Acknowledgement
The experimental work as a part of Ph.D. dissertation of
the corresponding author was performed in the
Department of Chemical Engineering, University of
Calcutta, Kolkata, India. Technical support, Research
facilities and University Research Fellowship provided by
the University of Calcutta, Kolkata, India for carrying out
the work is gratefully acknowledged.
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