Chemical Engineering Journal 197 (2012) 475–482
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Kinetic study of the simultaneous electrochemical removal of aqueous nitrogen
compounds using BDD electrodes
G. Pérez, R. Ibáñez, A.M. Urtiaga, I. Ortiz ⇑
Dpto. Ingeniería Química y QI. ETSIIyT, Universidad de Cantabria, Av. de los Castros s/n, 39005 Santander, Spain
h i g h l i g h t s
" Kinetic analysis of nitrate electro-reduction using BDD anode and cathode electrodes.
" Kinetic analysis of nitrite electro-oxidation using BDD anode and cathode electrodes.
" Kinetic analysis of the influence of chloride on nitrate and nitrite removal.
" Kinetic modeling of the electrochemical removal of aqueous nitrogen compounds.
a r t i c l e
i n f o
Article history:
Received 3 February 2012
Received in revised form 14 May 2012
Accepted 18 May 2012
Available online 27 May 2012
Keywords:
Nitrate electro-reduction
Nitrite electro-oxidation
Chloride oxidation
BDD electrodes
a b s t r a c t
This work investigates the kinetic behavior of aqueous nitrogen compounds in an electrochemical cell
provided with boron doped diamond (BDD) anode and cathode electrodes. Starting with initial solutions
of sodium nitrate or sodium nitrite and using NaCl as electrolyte in a concentration range from 0 to
28.2 mol/m3 the experiments were carried out working at constant current density of 400 A/m2 and
the change in the concentration of nitrite, nitrate, ammonia, chloramines and chlorinated ions was experimentally analyzed.
In the absence of chloride in the bulk solution oxidation reactions took place much faster than reduction reactions and the oxidation rate was further increased in the presence of chloride. Chloride exerted
the strongest influence on the oxidation rate of nitrite and ammonia.
With respect to the influence of chloride on the reduction rate of nitrate at first it appeared an apparent
negative influence that afterwards was explained through the coupled influence of the presence of nitrite
ions in the aqueous medium that were oxidized to nitrate almost instantaneously thus increasing the
concentration of the former ions. Previous works have reported either a positive or a negative influence
of chloride on the reduction of nitrate; we advance the former explanation by pointing out the coupled
influence of the dissolved nitrogen compounds on the individual removal kinetics. Finally a kinetic analysis of the reaction pathway has been proposed and the rates of the elementary reactions have been fitted
to pseudo-first order equations obtaining the values of the kinetic constants.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
In recent years, the removal of nitrogen compounds from water
resources and wastewaters has gained attention due to environmental problems. The large use of nitrogen compounds in agriculture and
in some industrial sectors, like nuclear industry, has caused a continuous increase of nitrate and nitrite concentration in many sources of
water such as groundwater, rivers, lakes and seas [1–3]. Nitrate, a
relatively non-toxic substance, represents a risk since it can be reduced to nitrite in the environment, in foods and in the digestive system causing serious human diseases. Ammonia also a common
⇑ Corresponding author. Tel.: +34 942 20 15 85; fax: +34 942 20 15 91.
E-mail address: [email protected] (I. Ortiz).
1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cej.2012.05.062
component of waste streams, can damage internal organ systems,
too [4–6]. For those reasons, intensive investigations have been carried out in order to understand the removal mechanism of these
nitrogen species from drinking water and wastewater.
Among current technologies applied to remove nitrogen compounds, membrane processes, ion exchange and biological processes
have been the most widely studied [7,8]. Ion exchange is a largely
used technique since it is a selective method which removes only
the required ion and no other useful ones. However, a large amount
of secondary wastes which must be treated later is produced in the
regeneration step and therefore the overall costs of the process are increased [9–11]. The same situation is found when membrane processes are applied, a concentrate stream is generated and an
additional treatment must be applied. Biological treatment at low
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G. Pérez et al. / Chemical Engineering Journal 197 (2012) 475–482
concentration levels (103 M) is considered as the most economical
technique for wastewater treatment, but the situation changes when
the concentration of the nitrate pollutants is high, because it is a slow
and incomplete treatment. Moreover, it requires a continuous monitoring, such as addition of a carbon source, pH control, temperature
maintenance and also requires the removal of by-products [12–14].
During last few years, electrochemical processes have emerged as
alternative technology for the removal of nitrates, nitrites and ammonia due to their advantages regarding environmental compatibility,
versatility, energy efficiency, safety, selectivity and cost effectiveness
[1,3,15–19]. The activity and the selectivity of the processes can be controlled by both the chemical composition of the electrode and several
process parameters like pH, temperature, electrolyte composition, applied potential or current and cell configuration [1,3]. Numerous efforts
have been made so far, concerning the electrochemical reduction of
nitrate on various electrode materials. Katsounaros et al. [20] studied
nitrate reduction using Sn and Bi cathodes, at different current density
values and observed that both cathodic materials were able to eliminate nitrate, obtaining higher reaction rates when higher current was
applied, but the formation of intermediate compounds in each case
was different. Graphite, iron, aluminum and titanium were selected
by Dash and Chaudari [15] who observed that after 5 h of electrochemical treatment 80% of nitrate reduction was achieved for all the cathode
materials except of graphite which after 9 h it was only reduced an 8%.
So the selection of the cathode materials plays an important role in the
process, mainly affecting the nitrate reduction percentage achieved
and the distribution of intermediate products.
Among the different electrode materials tested, for instance Cu,
Ni, Zn, Pb, Pt [20–24], boron doped diamond (BDD) electrodes have
appeared as new promising materials [25]. Diamond possesses
many outstanding properties such as wide working potential window, low and stable voltammetric background current, high overpotential for oxygen and hydrogen evolution in aqueous
electrolytes and stability. Moreover, diamond films were found to
exhibit remarkably reproducible behavior over prolonged periods
of time, even in the most corrosive electrolytes such as fluoride
solutions. These advantageous properties render diamond suitable
for many new applications, like electrochemical disinfection [26],
removal of emerging pollutants [17,27] and removal of other
refractory pollutants, (i.e. dyes, organic and inorganic toxic and
mutative compounds, etc.) [28–30]. In particular because of the
wide potential window of BDD electrodes, nitrates are expected
to be efficiently removed [31–37]. If thermodynamic potential, E0
of the nitrate reduction reactions which lead to the formation of nitrite and ammonia are calculated from the standard free energy
(DrG0) of the reactions, they show values of 0.39 V and 0.46 V,
respectively. Then both reactions are thernodynamically favored
since hydrogen evolution reaction using BDD cathodes takes place
at more negative potentials than nitrate reduction [32,36,37].
Due to the increasing applications of BDD electrodes to wastewater treatment processes they have been selected in this work
in order to study the removal mechanisms of nitrate and nitrite
by applying the electrochemical technology. Moreover, as it is
known that if chloride ion is present in the solutions chlorine is
generated and it reacts immediately with water to form hypochlorite which would react with the nitrogen species during the treatment, the influence of the NaCl concentration in the
electrochemical treatment has been also studied.
(Adamant Technologies, Switzerland). The undivided cell comprised two circular electrodes (100 mm diameter) with a surface
area of 70 cm2 each and with an electrode gap of 10 mm. Both electrodes consist of a boron-doped diamond (BDD) coating (2–3 mm
thick, 500–1000 ppm boron concentration) on a silicon plate
(1 mm thickness; 100 mX cm resistivity). The electrochemical cell
is connected to a power supplier (Vitecom 75-HY3005D, with maximum output of 5 A and 30 V). A storage tank and a recirculation
pump (Pan World Magnet Pump NH-100 PX, with a maximum
capacity of 20 L/min) complete the experimental system. A refrigeration fluid was circulated through the cooling jacket of the feed
tank to maintain the feed at a temperature of 293 K (Fig. 1). All
the experiments were performed with a feed volume of 1 L and
applying a current density value, J, of 400 A/m2. The experiments
were performed with model solution, which were prepared by
dissolving NaNO2 or NaNO3 in ultrapure water obtained from a
Milli-Q Plus apparatus; all the solutions contained 6.9 mol/m3
and different NaCl concentrations (0–28.2 mol/m3) in order to
evaluate its influence on the reactions kinetics. For the case of no
addition of NaCl, Na2SO4 (7 mol/m3) was added as electrolyte in
order to increase the conductivity of the model solution.
2.2. Analytical methods
At given time intervals, liquid samples were withdrawn from
the feed tank. The concentrations of nitrate, nitrite, chloride, chlorate and perchlorate were analyzed in a ICS-1100 (Dionex) ion
chromatograph provided with a AS9-HC column, using a solution
of Na2CO3 (9 mM) as eluent, with a flow-rate of 1 mL/min and a
pressure of around 2000 psi, based on Standard Methods 4110 B
[38]. Ammonia nitrogen concentration was determined by distillation and titration according to the Standard Methods 4500 NH3
[38]. Nitrogen gas species were not analyzed, although the concentration of nitrogen gas compounds were quantified by mass balance. Free chlorine and total chlorine were determined by DPD
Ferrous Titrimetric Method according to Standard Methods 4500Cl [38], while concentration of chloramines that were formed
during the electrochemical treatment was calculated from the
2. Experimental
2.1. Electrochemical experiments
Electrochemical experiments were performed in a recycling
mode at laboratory scale in a commercial cell DiaCell 110-PP
Fig. 1. Electrochemical set-up: (1) Electrochemical cell; (2) feed tank; (3) power
supply; (4) pump; and (5) heat exchanger.
G. Pérez et al. / Chemical Engineering Journal 197 (2012) 475–482
difference between total chlorine and free chlorine. pH and ORP
were measured by using a portable pH-meter.
3. Results
Two sets of experiments were performed in order to analyze the
kinetics of nitrate and nitrite removal. The first one was developed
without addition of NaCl in order to study the removal of both anions
in the absence of chloride, and the second one was carried out by
adding different NaCl concentrations in order to study the kinetics
of the indirect chloride mediated removal of nitrate and nitrite.
Experiments were replicated showing an experimental error of 3.2%.
3.1. Nitrate and nitrite removal in the absence of chloride ions
Fig. 2a–c reports the change of nitrate, nitrite and ammonia concentration with time when the feed solution contained only nitrate
initially. Along the experimental time, nitrate was reduced leading
to the formation of ammonia as the main by-product, and nitrite.
Whereas the concentration of ammonia increased continuously, nitrite concentration reached a maximum and then started decreasing at a slower rate mainly due to its further conversion to nitrate
and ammonia, reactions (1)–(3) [20,21,37,39]. This mechanism is
compatible with the sudden increase in the pH of the bulk solutions
that had an initial value of 6.5 as shown in Fig. 3.
NO3
þ H2 O þ 2e $
NO2
þ 2OH
NO3 þ 6H2 O þ 8e $ NH3 þ 9OH
ð1Þ
ð2Þ
477
NO2 þ 5H2 O þ 6e $ NH3 þ 7OH
ð3Þ
NO2 þ 2H2 O þ 3e $ 1=2N2 þ 4OH
ð4Þ
When sodium nitrite feed solutions were analyzed, first a fast
decrease on nitrite concentration was observed as depicted in
Fig. 4a, fact that is in accordance with other researches published
in the literature [5,40]. In the absence of NaCl after 1 h of treatment, the nitrite removal percentage was around 72% and then
the concentration continued decreasing at a slower rate. Nitrite
was rapidly oxidized to nitrate, Fig. 4b, according to reaction (1),
ammonia was formed through reactions (2) and (3) with a concentration that increased with time; again ammonia was found to be
the most recalcitrant compound to the electrochemical treatment
in the absence of NaCl; a small concentration of nitrogen,
N2 < 1 mol/m3, determined from the mass balance, was formed,
that after the analysis of the results shown in Figs. 2 and 4 was
attributed to reduction of nitrite ions according to reaction (4). A
fast increase of pH was also observed since the first moment –
Fig. 3. For both feed solutions, initial pH values were around 6.5
and then they increased rapidly achieving values as high as 11.8,
owing to the high OH production during nitrate and nitrite reduction, as it is indicated in reactions (1)–(4).
3.2. Influence of chloride concentration on the kinetics of nitrate and
nitrite removal
The kinetics of the indirect chloride mediated removal of nitrate
and nitrite were experimentally checked in specific experimental
Fig. 2. Change of the concentration of (a) nitrate, (b) nitrite and (c) ammonia with the experimental time for feed solutions containing nitrate (6.9 mol/m3) and initial chloride
concentrations of: 0 mol/m3; j 14.1 mol/m3; N 28.2 mol/m3, symbols represent experimental points and dotted lines represent predicted values of the concentrations
estimated by the proposed kinetic model for initial chloride concentration: ......... 0 mol/m3; ......... 14.1 mol/m3 and –– –– –– 28.2 mol/m3.
478
G. Pérez et al. / Chemical Engineering Journal 197 (2012) 475–482
Fig. 3. Change of pH with time in the experiments with:
nitrate feed solutions
and X nitrite feed solutions with initial chloride concentrations of: 0 mol/m3;
3
3
14.1 mol/m and 28.2 mol/m .
runs where nitrate and nitrite feed solutions were prepared with
different initial chloride concentrations. Fig. 2 shows that nitrate
was reduced during the experimental time for the two initial chloride concentrations, 14.1 mol/m3 and 28.2 mol/m3. Chloride concentration did not exert a significant influence on the kinetics of
nitrate removal during the process, although the rate was slightly
lower when a higher chloride concentration was used, fact that
could be attributed to the faster oxidation kinetics of the formed
nitrite to nitrate as the concentration of chloride ions increased,
according to reaction (10).
A higher influence of chloride concentration was noted on product distribution. Higher chloride concentrations led to a delay in
the appearance of ammonia and nitrite. The appearance of nitrite
depended on the presence of chloride in the reaction medium so
that when no chloride was added nitrite appeared in the first analyzed sample, whereas it only appeared after the second and fourth
hours when 14.1 mol/m3 and 28.2 mol/m3of chloride were added
respectively; this behavior is explained by the fast kinetics of the
indirect chloride mediated oxidation of nitrite. A similar behavior
has been detected with respect to ammonia formation, ammonia
was detected since the first analyzed sample in the absence of
NaCl, however it only appeared at the third and fourth hours, when
14.1 mol/m3 and 28.2 mol/m3 of chloride were added to the feed
solution. This delay in the nitrite and ammonia formation was
translated into a higher nitrogen gas concentration, because the
ammonia formed during nitrate reduction, can be further oxidized
to nitrogen by the indirect oxidation reaction with the hypochlorite ions that were formed from chloride oxidation, as it is indicated by reactions (5)–(9). Nitrite was detected when there was
no more chloride left indicating that in the presence of chloride, nitrite was oxidized almost instantaneously following reaction (10).
2Cl ! Cl2 þ 2e
ð5Þ
Fig. 4. Change of the concentration of (a) nitrite, (b) nitrate and (c) ammonia with the experimental time for feed solutions containing nitrite (6.9 mol/m3) and initial chloride
concentrations of: 0 mol/m3; j 14.1 mol/m3; N 28.2 mol/m3, symbols represent experimental points and dotted lines represent predicted values of the concentrations
estimated by the proposed kinetic model for initial chloride concentration: ........ 0 mol/m3; ........ 14.1 mol/m3 and –– –– –– 28.2 mol/m3.
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G. Pérez et al. / Chemical Engineering Journal 197 (2012) 475–482
Cl2 þ H2 O ! HClO þ Hþ þ Cl
ð6Þ
ð7Þ
ð8Þ
HOCl ! OCl þ Hþ ðpKa ¼ 7:5Þ
2NH3 þ 2OCl ! N2 þ 2HCl þ 2H2 O þ 2e
it was expected since reaction (11) takes place. Chlorate also
showed an intermediate compound behavior, achieving higher
maximun values when a higher inital chloride concentration was
used, too. This behavior can be justified because chlorate oxidation
to perchlorate occurred, according to reaction (12).
NH3 þ 4OCl !
NO3
þ
þ H2 O þ H þ 4Cl
ð9Þ
NO2 þ OCl ! NO3 þ Cl
ð10Þ
Previous works have reported on the influence of chloride concentration on the electro-reduction of nitrate using different cathode materials [1,2,41]. Li et al. [2] compared the performance of Cu,
Fe and Ti cathodes, using NaCl concentrations in the range 0–
500 mg/L, similar as those employed in this study. The nitrate
reduction efficiency was reported in the order Fe > Cu > Ti, and in
all cases the higher the chloride concentration, the lower the nitrate reduction percentage. In a different study the same authors
[1] concluded that nitrate could be efficiently eliminated using a
Cu/Zn cathode at 400 A/m2, in the presence of 500 mg/L of NaCl,
and that neutral initial pH contributed to the nitrate reduction
and basic pH favored ammonia production. On the other hand, a
positive influence of chloride concentrations on nitrate removal
was reported by Katsounaros and Kyriacou [21] working with Sn
cathodes and Lacasa et al. [25] working with two different anode
materials, DSA and BDD and a stainless steel cathode. So that
depending on electrode materials different behaviors have been reported so far [1,2,5,15,41].
Concerning the oxidation of nitrite, it took place almost instantenously in the presence of chloride ions. However when chloride
had been completely removed (Fig. 5), nitrite seemed to start
increasing again but at a very slow rate. Nitrate appeared as the
main product of nitrite oxidation, Fig. 4, that was formed according
to reaction (10), and as it was observed in the absence of chloride,
it reached a maximum concentration and then it started decreasing
at a slow removal rate forming nitrite, ammonia and nitrogen;
Fig. 4b shows a slight positive influence of the inital presence of
chloride on the kinetics of reduction of the formed nitrate, in
agreement with Lacasa et al. [25]; in order to search for a good
explanation of these results it must be considered that once the
maximum nitrate concentration is reached, Fig. 4b there is no more
nitrite left in the bulk solution so the contribution of reaction (10)
must be neglected and therefore a positive influence of chloride on
nitrate reduction is observed as it was previosly reported [21,25].
In the experiments with initial chloride, ammonia was only detected at times when chloride had been removed, appearing at
the second and fourth hours of treatment for 14.1 mol/m3 and
28.2 mol/m3 of chloride, respectively, thus indicating that in the
presence of chloride ammonia was oxidized to nitrogen at a very
fast rate according to reaction (8) [2,42,43]. As it occurred in the
experiments with sodium nitrate feed solutions, a higher chloride
concentration favored the formation of nitrogen gas as it has been
mentioned in other works using BDD anodes, Lacasa et al. [25] and
Pérez et al. [44]. Our results show that there is a coupled influence
of the presence of chloride and nitrite in the bulk solution on the
kinetics of nitrate removal being the presence of nitrite of high relevance and, therefore the distribution of products will be a result
of the relative kinetics of the involved reactions.
Chlorine species distribution are depicted in Fig. 5. According to
reactions (5)–(7), chloride concentration decreased with time, and
it was completely removed in all the studied cases. Chloride removal rate was higher when a lower chloride concentration was
used. Free chlorine appeared since the first moment, achieved a
maximum concentration that depended on the initial chloride concentration and then started decreasing. At the same time, while
chloride concentration decreased, chlorate was formed as well, as
6HOCl þ 3H2 O ! 2ClO3 þ 4Cl þ 12Hþ þ 3=2O2 þ 6e
ClO3 þ H2 O ! ClO4 þ 2Hþ þ 2e
ð11Þ
ð12Þ
The formation of chlorate and perchlorate during the electrolysis of chloride solutions using BDD anodes has been reported in literature; the situation has been explained in terms of occurrence of
hydroxyl radicals, which are formed in large quantities during the
electrolysis of aqueous solutions with BDD electrodes [25,44–47].
However the formation of those anions can be inhibited by the
presence of high concentrations of competitive ions, such as chloride; high chloride concentrations have shown to delay the formation of chlorate ions and to inhibit perchlorate formation due to
adsorption at the electrode surface, which blocks the oxidation of
chlorate to perchlorate [25,44,45]. However at low chloride concentrations the formation of those ions hinders some of the potential applications of the technology such as obtention of drinking
water – but it can be efficiently applied with different purposes
such as water reuse or treatment of industrial wastes [25].
Chloramines formation could occur attending to reactions (13)–
(15), due to the joint presence of ammonia and free chlorine. For
that reason, the samples taken during the experimental runs were
analyzed for chloramines concentration, but they were not detected in any sample. A possible explanation for such situation
could be related to the high pH value reached during the electrochemical treatment because it has been previously reported that
chloramines formation is insignificant at pH values higher than 8
[48,49].
NHþ4 þ HOCl ! NH2 Cl þ H2 O þ Hþ
ð13Þ
NH2 Cl þ HOCl ! NHCl2 þ H2 O
ð14Þ
NHCl2 þ HOCl ! NCl3 þ H2 O
ð15Þ
3.3. Reaction kinetics
Next, the kinetic analysis of the experimental results is performed; Fig. 6 depicts the reactions pathway including the reactions taking place in the presence of chloride.
Considering first order kinetics, Eqs. (16)–(19) are the kinetic
equations that express the rate of modification of the concentration
of nitrogen species in the absence of chloride, reactions (1)–(4).
d½NO3 =dt ¼ ðK 1 þ K 2 Þ½NO3 K 01 ½NO2 ð16Þ
d½NO2 =dt ¼ ðK 01 þ K 3 þ K 4 Þ½NO2 K 1 ½NO3 ð17Þ
d½NH3 =dt ¼ K 2 ½NO3 þ K 3 ½NO2 ð18Þ
d½N2 =dt ¼ K 4 ½NO2 ð19Þ
First, kinetic data obtained in the absence of NaCl were analyzed. The values of the corresponding kinetic constants, K1, K 01 ,
K2, K3 and K4 were estimated from the fitting of the experimental
data to the mathematical model developed in this work using
the minimum weighted standard deviation as optimization criterion; the obtained values are shown in Table 1. In the absence of
chloride ions, it was concluded that oxidation of nitrite to nitrate
was favored from a kinetic point of view, showing a kinetic
constant value of 3.5 104 s1. Next, at a slower rate nitrate
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G. Pérez et al. / Chemical Engineering Journal 197 (2012) 475–482
Fig. 5. Chlorine species distribution ( chloride; chlorate; perchlorate; free chlorine) along the experimental time for (a) nitrate feed solutions containing 14.1 mol/m3
of chloride; (b) nitrate feed solutions containing 28.2 mol/m3 of chloride; (c) nitrite feed solutions containing 14.1 mol/m3 of chloride and (d) nitrite feed solutions containing
28.2 mol/m3 of chloride. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1
Estimated kinetic constant values.
Ki (s1)
[Cl] (mol/m3)
Reaction
(1)
(10 )
(2)
(3)
(4)
(8)
(9)
(10)
Fig. 6. Reactions pathway in the electrochemical treatment of nitrogen species
using BDD electrodes.
reduction reactions occurred, with kinetic constant values of one
order of magnitude lower than those obtained for nitrite oxidation.
And finally, nitrite reduction reactions appeared as the slowest
ones. Simulated curves with the obtained constant values are
shown in Figs. 2 and 4.
Next, kinetic results obtained in the experiments performed
with 14.1 mol/m3 and 28.2 mol/m3 of chloride were analyzed. In
this case reactions (8)–(10) were also considered, and the kinetic
constants K8, K9 and K10 were estimated keeping constant the values of K1, K 01 , K2, K3 and K4, previosly obtained, as given in Table 1,
resulting in the following set of kinetic equations:
K1
K 01
K2
K3
K4
K8
K9
K10
NO
3 ! NO2
NO
2 ! NO3
NO
!
NH
3
3
NO
2 ! NH3
NO2 ! N2
NH3 + OCl ? N2
NH3 þ OCl ! NO
3
NO
þ
OCl
! NO
2
3
0
14.1
28.2
7.7 105
3.5 104
5.7 105
3.5 102
9.6 106
–
–
–
–
–
–
–
–
2.6 105
2.1 105
7.5 104
–
–
–
–
–
2.2 101
9.7 105
d½NO3 =dt ¼ ðK 1 þ K 2 Þ½NO3 ðK 01 þ K 10 Þ½NO2 K 9 ½NH3 ð20Þ
d½NO2 =dt ¼ ðK 01 þ K 3 þ K 4 þ K 10 Þ½NO2 K 1 ½NO3 ð21Þ
d½NH3 =dt ¼ K 2 ½NO3 K 3 ½NO2 þ ðK 8 þ K 9 Þ½NH3 ð22Þ
d½N2 =dt ¼ K 4 ½NO2 þ K 8 ½NH3 ð23Þ
The values of the kinetic constants K8 and K9 were made dependant on the concentration of initial chloride – Table 1 collects the
obtained values of the kinetic constants. Simulated kinetic curves
for nitrate, nitrite and ammonia are shown in Figs. 2 and 4 for
the experiments starting with sodium nitrate and sodium nitrite,
respectively. The values in Table 1 show that – K8 and K9 depend
clearly on the initial concentration of chloride showing a higher value when a higher initial chloride concentration was applied. However, the same value of K10 was estimated for the two inital
G. Pérez et al. / Chemical Engineering Journal 197 (2012) 475–482
Fig. 7. Parity graph of nitrate for simulated and experimental dimensionless
concentration values for all the treated solutions: (i) nitrate sodium solutions with
Cl: 0 mol/m3; j 14.1 mol/m3 and N 28.2 mol/m3 and (ii) nitrite sodium solutions
with Cl: } 0 mol/m3; h 14.1 mol/m3 and D 28.2 mol/m3.
chloride concentrations applied due to the lack of sensitivity of the
kinetic model to this parameter as the oxidation of nitrite to nitrate
in the presence of chloride occurred almost instantaneously. Again
the oxidation reactions were kinetically favored by the presence of
chloride in comparison to reduction reactions. Fig. 7 depicts a parity graph of the kinetic data of nitrate reduction showing 92% of the
simulated data (Csim) fall within the interval Cexp ±20% Cexp, thus
demonstrating the accuracy of the kinetic model and parameters.
4. Conclusions
This work reports the kinetic analysis of the removal of nitrogen
species, nitrate, nitrite and ammonia in aqueous solutions containing different initial chloride concentrations, by applying the electrochemical technology with BDD electrodes. In the absence of
chloride, oxidation reactions took place much faster than reduction
reactions, being the oxidation of nitrite the reaction that occurred
at the highest rate; the kinetic difference was further enhanced in
the presence of chloride. Chloride exerted a positive influence both
on oxidation reactions as well as on the reduction of nitrate, however the latter is hindered by the presence of nitrite in the aqueous
medium. A kinetic model constituted of pseudo-first order equations has been developed calculating the values of the kinetic
constants from the fitting of the experimental data to the mathematical model. Comparison between simulated and experimental
data showed the accuracy of the kinetic model and parameters.
Thus we report the kinetic parameters of great value to understand
the rate of the phenomena involved in the electrochemical treatment of aqueous nitrogen compounds. Furthermore, the kinetics
of the reactions involved offer an explanation to former literature
on the influence of operation variables, mainly chloride, on the
kinetics of nitrate reduction; thus the influence of chloride must
be analyzed together with the presence of nitrite.
Acknowledgments
Finantial support from Projects CTQ2008-0690 and CONSOLIDER CSD2006-44 is gratefully acknowledged.
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