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Kinetic study of the simultaneous electrochemical removal of aqueous nitrogen compounds using BDD electrodes
Chemical Engineering Journal 197 (2012) 475–482
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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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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.
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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.
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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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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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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. References [1] M. Li, C. Feng, Z. Zhang, X. Lei, R. Chen, Y. Yang, N. Sugiura, Simultaneous reduction of nitrate and oxidation of by-products using electrochemical method, J. Hazard. Mater. 171 (2009) 724–730.
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Cañizares, M.A. Rodrigo, Electrochemical denitrification with chlorides using DSA and BDD anodes, Chem. Eng. J. 184 (2012) 66–71. [26] G. Pérez, P. Gómez, R. Ibañez, A.M. Urtiaga, I. Ortiz, Electrochemical disinfection of secondary wastewater treatment plant (WWTP) effluent, Water Sci. Technol. 62 (4) (2010) 892–897. [27] E. Bringas, J. Saiz, I. Ortiz, Kinetics of ultrasound-enhanced electrochemical oxidation of diuron on boron-doped diamond electrodes, Chem. Eng. J. 172 (2011) 1016–1022. [28] D. Montanaro, E. Petrucci, Electrochemical treatment of Remazol brilliant blue on a boron doped diamond electrode, Chem. Eng. J. 153 (2009) 138–144. [29] C. Zhang, J. Wang, H. Zhou, D. Fu, Z. Gu, Anodic treatment of acrylic fiber manufacturing wastewater with boron-doped diamond electrodes: a statistical approach, Chem. Eng. J. 161 (2010) 93–98. [30] A. Bedoui, M.F. Ahmadi, N. Bensalah, A. Gadri, Comparative study of eriochrome black T treatment by BDD-anodic oxidation and Fenton process, Chem. Eng. J. 146 (2009) 98–104. [31] C. Reuben, E. Galun, H. Cohen, R. Tenne, R. Kalish, Y. Muraki, K. Hashimoto, A. Fujishima, J.M. Butler, C. Lévy-Clément, Efficient reduction of nitrite and nitrate to ammonia using thin-film B-doped diamond electrodes, J. Electroanal. Chem. 396 (1995) 233–239. [32] A.N. Ndao, F. Zenia, A. Deneuville, M. Bernard, C. Lévy-Clément, Effect of boron concentration on the electrochemical reduction of nitrates on polycrystalline diamond electrodes, Diamond Relat. Mater. 9 (2000) 1175–1180. [33] F. Bouamrane, A. Tadjeddine, J.E. Butler, R. Tenne, C. Lévy-Clement, Electrochemical study of diamond thin films in neutral and basic solutions of nitrate, J. Electroanal. Chem. 405 (1996) 95–99.
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[34] C.M. Welch, M.E. Hyde, C.E. Banks, R.G. Compton, The detection of nitrate using in situ copper nanoparticle deposition at a boron doped diamond electrode, Anal. Sci. 21 (2005) 1421–1430. [35] S. Ward-Jones, C.E. Banks, A.O. Simm, L. Jiang, R.G. Compton, An in situ copper plated boron-doped diamond microelectrode array for the sensitive electrochemical detection of nitrate, Electroanalysis 17 (20) (2005) 1806– 1815. [36] J.T. Matsushima, W.M. Silva, A.F. Azevedo, M.R. Baldan, N.G. Ferreira, The influence of boron content on electroanalytical detection of nitrate using BDD electrodes, Appl. Surface Sci. 256 (2009) 757–762. [37] C. Lévy-Clément, N.A. Ndao, A. Katty, M. Bernard, A. Deneuville, C. Comninellis, A. Fujishima, Boron doped diamond electrodes for nitrate elimination in concentrated wastewater, Diamond Relat. Mater. 12 (2003) 606–612. [38] Standard Methods for Examination of Water and Wastewater, 20th ed., APHA, AWWA, WEF, American Public Health Association/Water Environment Federation, Washington, DC, USA, 1998. [39] I. Katsounaros, G. Kyriacou, Influence of nitrate concentration on its electrochemical reduction on tin cathode: identification of reaction intermediates, Electrochim. Acta 53 (2008) 5477–5484. [40] H.Y. Hu, N. Goto, K. Fujie, Effect of pH on the reduction of nitrite in water by metallic iron, Water Res. 25 (11) (2001) 2789–2793. [41] M. Li, C. Feng, Z. Zhang, Z. Shen, N. Sugiura, Electrochemical reduction of nitrate using various anodes and a Cu/Zn cathode, Electrochem. Commun. 11 (2009) 1853–1856.
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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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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.
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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.
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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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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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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. References [1] M. Li, C. Feng, Z. Zhang, X. Lei, R. Chen, Y. Yang, N. Sugiura, Simultaneous reduction of nitrate and oxidation of by-products using electrochemical method, J. Hazard. Mater. 171 (2009) 724–730.
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