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Electrochemical oxidation of urea in aqueous solutions using a boron-doped thin-film diamond electrode
Diamond & Related Materials 44 (2014) 109–116
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Electrochemical oxidation of urea in aqueous solutions using a boron-doped thin-film diamond electrode M. Cataldo Hernández a, N. Russo a, M. Panizza b, P. Spinelli a, D. Fino a,⁎ a b
Dipartimento Scienza Applicata e Tecnologia, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy Dipartimento di Ingegneria Civile, Chimica e Ambientale, Università di Genova, Piazzale Kennedy 1, Genoa 16129, Italy
a r t i c l e
i n f o
Article history: Received 30 October 2013 Received in revised form 31 January 2014 Accepted 9 February 2014 Available online 15 February 2014 Keywords: Urea Anode Electrooxidation Cyclic voltammetry Bulk electrolysis
a b s t r a c t In the present paper, the electrocatalytic abatement of urea in aqueous solutions has been studied by means of cyclic voltammetry and galvanostatic electrolysis, using different anodes such as Pt, Ti–Ru oxide, boron-doped diamond (BDD) and antimony-doped tin oxide. HPLC analysis, total organic carbon (TOC) and ionic chromatography have been used to evaluate the oxidation and the mineralization of the treated aqueous solutions. The results of the cyclic voltammetries have shown that, in the case of Pt and Ti–Ru oxides a decrease in current density in the oxygen evolution region can be observed in the presence of urea, due to the blockage of the electrode active oxygen evolution sites as a consequence of the reversible adsorption of urea. Instead, a notable increase in the current density has been observed in the region of the oxygen evolution for the BDD and antimony-doped tin oxide electrodes, in the presence of urea, indicating that the oxidation of urea involves hydroxyl radicals. The bulk electrolysis tests have shown that the complete removal of urea and TOC can only be achieved using a boron-doped diamond and that Pt, the Ti–Ru oxide and antimony-doped tin oxide only permit a partial oxidation of urea. On the basis of the TOC evolution and the identification of the organic intermediates and inorganic ions released during the treatment, a total mineralization has been proposed. Finally, electrolysis has been performed in the presence of chloride ions, which act as oxidation mediator, and a comparison has been done between direct and mediated electro-oxidation. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Urea from sources such as human, animal and fertilizer wastes, would generate aquatic ecosystem degradation with adverse effects on ecosystem and human well-being. The presence of urea in human wastewaters is a serious problem because it can be hydrolyzed by the urease enzyme produced by microorganisms to ammonia NH2 CONH2 þ H2 O→2NH3 þ CO2 :
ð1Þ
The discharge of ammonia containing effluents is an environmental and health problem since they increase the pH of the soils and cause euthrofication of the waters, and free ammonia can also be released into the atmosphere contributing to the formation of ammonium sulphates and nitrates. Consequently, increasing attention has been paid to efficient urea and ammonium removal methods from wastewaters, and a wide range of processes has been proposed, including adsorption, oxidation, ⁎ Corresponding author. Tel.: +39 011 0904710. E-mail address: debora.fi[email protected] (D. Fino).
http://dx.doi.org/10.1016/j.diamond.2014.02.006 0925-9635/© 2014 Elsevier B.V. All rights reserved.
biological decomposition, chemical oxidation and enzymatic decomposition [1–3]. Some of these processes require a high energy input, or complicated equipment, thus their implementation is limited at an industrial level. Electrochemical oxidation has also been shown to be a promising method for ammonium and urea removal from synthetic and real wastewaters. Many papers have dealt with electrochemical ammonia degradation, and have shown that ammonia removal mainly takes place because of an indirect oxidation process with active chlorine electrochemically generated at the anode surface. Many anode materials, such as Pt, IrO2 , boron-doped diamond (BDD), PbO 2 , SnO2 , and RuO2 have been tested for this process, and it has resulted that ammonia is effectively removed from solutions through a mechanism known as break point chlorination [4–10]. Cabeza et al. [11] have shown that a small quantity of nitrates can also be formed, when A BDD anode is used. They also showed that an increasing chloride concentration resulted in an increase in ammonium removal efficiency and a decrease in the amount of N–NH+ 4 transformed to nitrates. The electrooxidation of urea has been studied much less than ammonia, and its mechanism is still not fully understood. For example,
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some papers have reported that direct oxidation of urea on Pt involves its adsorption on the anode surface. However, some authors have asserted that the adsorption of urea is reversible [12–15], while others have suggested that it is not reversible [15]. Recently, Boggs et al. [16] have compared the electrocatalytic activity of several noble metals (i.e. Pt, Pt–Ir, Rh and Ni) for the direct electrooxidation of urea in alkaline media. Ni was found to be very effective because it is anodically converted to NiOOH (Ni3+), which is the active material for urea oxidation. The stability and the activity of Ni catalyst can be also enhanced through a combination of Ni with other metals in multimetal catalyst such as Pt, Ru, Rh, and Co. For example, Rh–Ni electrodes have been shown to improve the current density by a factor of 200, compared to Ni catalysts [17,18]. Several research groups have reported on the electrochemical oxidation of urea using different materials, such as Ti/Pt, Ti/Pt–Ir, RuO2, and IrO2, in the presence of chlorides [3,19–21]. In these conditions, even though direct oxidation cannot be excluded, the main reaction pathway, as in the case of ammonia, is indirect oxidation with electrogenerated active chlorine, and non-toxic N2 and CO2 are the main decomposition products of urea. In this work, the catalytic activities of four anode materials (i.e. Pt, Ti–Ru oxide, BDD and antimony-doped tin oxide) on the direct electrochemical oxidation of urea have been investigated by means of potentiodynamic measurements and bulk electrolysis in order to evaluate electrochemical oxidation as a treatment technology. The organic intermediates and released inorganic ions have been detected and quantified by means of several chromatographic techniques and on the basis of the by-products. Finally, electrolysis has been performed in the presence of chlorides that act as oxidation mediators and a comparison has been made between direct and mediated electro-oxidation. 2. Experimental 2.1. Chemicals The solutions were prepared by dissolving different amounts of urea (CO(NH2)2, (Sigma Aldrich)) in 1 M NaClO4, which was chosen as the supporting electrolyte because it does not generate oxidizing species that can react with the organics, as instead occurs when a Cl− medium 2 (i.e. generation of Cl2) or a SO24 − medium (i.e. production of S2O− 8 ) is used. For the tests carried out to compare direct and mediated electrooxidation, different amounts of NaCl (Sigma Aldrich) were added to the electrolyte. 2.2. Electrode materials Antimony-doped tin oxide anodes, hereafter referred to as SnO2–Sb2O5, were prepared by coating titanium substrates (2 mm thick) through the thermal decomposition of a mixture of 10 g dm−3 SnCl4 ∗ 5H2 O and 0.1 g dm − 3 SbCl3 dissolved in isopropanol. The titanium sheets were subjected to a surface pre-treatment that consisted in the mechanical polishing of the sheets with WS-FLEX 18-C sandpaper, followed by degreasing with 40% NaOH at 80 °C for 120 min, etching in hydrochloric acid 11.5 M for 1 min, washing in distilled water and soaking in an ultrasonic bath at 60 °C for 60 min. The precursor solution was painted onto the titanium plates and the solvent was evaporated at 100 °C for 15 min. The sample was calcined at 550 °C in a 2 dm3 min− 1 oxygen (pure) flow, for 15 min; this procedure was repeated 16 times. Finally the electrodes were calcined at 550 °C for 3 h in 0.5 dm3/min oxygen [22–25]. An average thickness value of about 4 μm was obtained. The nominal composition of the mixed oxide was Sn0.918Sb0.109O2. The boron-doped thin-film diamond electrode was supplied by CSEM Centre Swiss d'Electronique et de Microtechnique, Neuchâtel, Switzerland. It was synthesised by the hot filament chemical vapour
deposition technique (HF CVD) on single crystal p-type Si wafers. The doping level of boron in the diamond layer, expressed as the B/C ratio, was about 3500 ppm. The obtained diamond film thickness was about 1 μm and it had a resistivity of 10–30 mΩ cm. In order to stabilise the electrode surface and to obtain reproducible results, the diamond electrode was pre-treated at 25 °C, by means of anodic polarization, in 1 M HClO4 at 10 mA cm−2 for 30 min using stainless steel as the counter electrode. This treatment made the surface hydrophilic. Before each cyclic voltammetry test, the Pt electrode (radiometer) was treated in a 30% nitric acid solution under anodic and cathodic polarization at 50 mA cm− 2 for about 20 min. Subsequently, 10 voltammetric cycles (100 mV s−1) were performed in 1 M sulphuric acid between −300 mV and 1600 mV vs. SCE. This procedure allowed the voltammogram reported by Angerstein-Kozlowska et al. [26] to be obtained as a reference for clean Pt electrodes in sulphuric acid solutions. The Ti–Ru oxide anode, hereafter referred to as TiRuO 2 , was a commercial DSA® anode consisting of a titanium substrate covered with a TiO2/RuO2 layer, which was purchased from De Nora (Italy). 2.3. Electrochemical system Cyclic voltammetries were carried out in a conventional threeelectrode cell using a computer controlled Voltalab 301 potentiostat. Pt, Ti–RuO2, BDD and SnO2–Sb2O5 were used as working electrodes, while saturated calomel (SCE) was used as a reference and a Pt plate was used as the counter electrode. The exposed apparent area of the working electrodes was 1 cm2. Cyclic voltammetry was performed at room temperature (T = 25 °C) with a 30 mV s−1 scan rate. Bulk oxidations were carried out in a one-compartment electrolytic flow cell. A single-compartment cell was adopted because, preliminary experiments evidenced that when using stainless steel, there is no production of hydrogen peroxide. The flow rate was 4.5 l/min which corresponds to a mass-transfer coefficient in the cell of 2.3 ∗ 10− 5 m s− 1. The experiments were performed under galvanostatic conditions using an AMEL 2055 potentiostat/galvanostat with a current density of 20 mA cm − 2 . Pt, Ti–RuO2, BDD and SnO2–Sb2O5 electrodes were used as anodes, and stainless steel was used as the cathode. All the electrodes were circular in shape with a 50 cm 2 geometrical area and a 0.5 cm inter-electrode gap. An electrolyte solution, consisting of 2 g dm − 3 NaClO 4 and 2 mg dm − 3 of urea, was introduced into a 0.5 dm3 thermoregulated glass tank and circulated through an electrochemical reactor using a centrifugal pump with a flow-rate of 300 dm3 h− 1. The solutions were periodically sampled and analyzed by means of a spectrophotometer (Cary 5000 Varian), a TOC–TIC/TN analyzer (Hach Lange IL 550), ion chromatography (Dionex ICS-5000) and an HPLC (Shimadzu LC 20AT) to determine the dissolved chemical species during the treatment. The urea concentration was determined by means of a spectrophotometric method, based on the addition of a p-dimethylaminobenzaldehyde and hydrochloric acid solution to the urea sample in order to obtain a yellowish-green colour which was caused by the complexation reaction [27]. 3. Results and discussion 3.1. Cyclic voltammetries Fig. 1A shows the cyclic voltammograms obtained with the Pt electrodes in 1 M NaClO4 with a scan rate of 30 mV s− 1 and for different urea concentrations. The presence of urea resulted in an increase in the current density in the potential regions between − 0.1 V and 0.2 V vs. SCE and between 0.6 and 1.1 V vs. SCE, thus indicating the direct urea oxidation path [13,14]. However, this increase in current density, which was also observed by other authors [22–25], does not
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Fig. 1. Cyclic voltammetries in the 1 M NaClO4 electrolyte with different urea concentrations. Scan rate: 30 mV s−1. (A) Pt electrode. (B) Ti–RuO2 electrode. (C) SnO2–Sb2O5 electrode. (D) BDD electrode.
appear to be influenced by the urea concentration because of urea adsorption, which blocks the active sites of the anode. The determination of the onset potential for oxygen evolution, based on a significant increase (1 to 3% of the full scale) of the current above the flat trend preceding the sharp increase in current density, provided a value of about 1.2 V and it did not seem to be influenced by the presence of urea. However, the current in the oxygen evolution region at 1.2 V decreases significantly in the presence of urea, due to the adsorption of urea by the electrode, which blocked the electroactive sites [19]. Moreover, the current density decreases in the presence of urea and the reduction peak shifted towards negative potentials, on the cathodic branch of the voltammogram, in the region corresponding to the Pt-oxide reduction, that is between 0 and − 0.6 V vs. SCE. Consecutive cyclic voltammograms on Pt, carried out with 0.01 M of urea, showed that the curve did not change when the number of cycles was increased (up to 10 cycles), this suggests that the adsorption– desorption processes are reversible. Fig. 1B shows the cyclic voltammograms of the Ti–RuO2 electrodes obtained in 1 M NaClO4 with a scan rate of 30 mV s−1, for different urea concentrations. The voltammogram in 1 M NaClO4 presents two broad and not well defined peaks at 0.6 and −0.2 V vs. SCE, which are related to the redox processes for the lower metal oxide/higher metal oxide transition.
No significant change was observed in the presence of urea in the potential water stability region between −0.4 and 1.0 V vs. SCE while just a slight shift towards lower current densities was noted in the presence of urea. The onset potential for the oxygen evolution did not seem to be affected by the presence of urea either. On the contrary, at 1.25 V, an increase in urea concentration in the oxygen evolution region caused a decrease in current density, similar to what was observed with the Pt electrode, attributed to the blockage of the electrode active sites for oxygen evolution due to urea adsorption. These results are consistent with those obtained by Simka et al. for a Ti–RuO2 electrode [19]. Fig. 1C shows cyclic voltammograms on a SnO2–Sb2O5 electrode in 1 M NaClO4 with a scan rate of 30 mV s− 1 for different urea concentrations. The voltammogram (continuous line) is almost featureless in the studied potential region in the presence of a supporting electrolyte. The oxygen evolution (at 2 mA cm− 2) is shifted towards more negative potentials in the presence of urea and there is a considerable increase in the current density, which is proportional to the urea concentration. This indicates that the urea oxidation pathway involves hydroxyl radicals that form during the oxygen evolution [28–30]. Consecutive cyclic voltammograms were performed on SnO2–Sb2O5 in 0.01 M urea. The 5th and 10th cycles were perfectly overlapped. This observation, and the fact that no specific peaks in the curve were found, as occurred for platinum, make it possible to infer that urea is not adsorbed on the electrode.
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− −3 Fig. 2. Evolution of (A) urea concentration, (B) TOC, (C) NH+ of urea using the BDD anode with different current 4 and (D) NO3 concentrations during the degradation of 2000 mg dm densities. Legend: 5 mA cm−2 (■), 10 mA cm−2 (●) and 20 mA cm−2 (▲).
Synthetic boron-doped diamond (BDD) is an electrode material that has received great attention recently. On BDD electrodes, a potential of oxygen evolution (about 2.2 V vs. SHE) is thermodynamically possible to generate hydroxyl radical (2.38 V vs. SHE). In effect, the experimental evidence was given by Marselli et al. [31], who detected the presence of hydroxyl radical on BDD electrode by means of electron spin resonance. Fig. 1D shows the cyclic voltammograms recorded on the BDD electrodes in the absence and presence of urea in the 1 M NaClO4 supporting electrolyte, with a scan rate of 30 mV s−1. The voltammograms, before oxygen evolution, displayed no significant change in the presence of urea with respect to the voltammogram of the supporting electrolyte. The only difference was a slight decrease in the onset potential of the oxygen evolution, which indicated an effect of the organic compound on the overpotential of the oxygen evolution. As reported by Kapalka et al. [28] in the presence of organic compound the hydroxyl radical surface concentration is correlated with the shift of current potential curves towards lower potentials caused by the organic reaction with the hydroxyl radical electrochemically generated. However, the current density at 1.5 V, in the oxygen evolution region, increased with the urea concentration. The current density in the case of the 0.1 M urea at 1.75 V vs. SCE was in fact three times the value of the current density in the absence of urea. This behaviour, which is similar to that obtained with SnO2 –Sb 2O 5 , indicates that the oxidation of urea involves hydroxyl radicals, which are available under oxygen evolution conditions.
Since it is known that the BDD electrode exhibits an inert surface with low adsorption properties, consecutive cyclic voltammograms with 0.01 M urea in the 1 M NaClO4 supporting electrolyte were performed in a similar way to the tests carried out with the other electrodes in order to investigate the effects of urea adsorption. As previously found for the SnO2–Sb2O5 electrode, it is possible to observe also in the case of the BDD electrode that the curves did not change with the subsequent cycling and they were completely overlapped. Considering the previous information and the fact that there were no changes in the current or peaks that could be attributed to the adsorption of urea, as in the case of Pt electrodes, it is possible to conclude that urea adsorption is negligible. From the voltammetries, in the absence of urea, reported in Fig. 1, it is possible to observe that the onset potential for oxygen evolution was 1.05 V, 1.2 V, 1.40 V and 1.65 V vs. SCE for Ti–RuO2, Pt, SnO2–Sb2O5 and BDD, respectively. These values confirm that Ti–RuO2 and Pt have a low overpotential for oxygen evolution, and therefore show an “active” behaviour, while SnO2–Sb2O5 and BDD have a high oxygen evolution overpotential and therefore have “non-active” behaviour. 3.2. Bulk electrolysis Electrochemical oxidation of 2 g dm−3 of urea in 2 g dm−3 Na2ClO4 has been performed with the different investigated anodes, Ti–RuO2, Pt, SnO2–Sb and BDD under galvanostatic conditions. The change in
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−3 − Fig. 3. Evolution of (A) urea concentration, (B) TOC, (C) NH+ of urea using the SnO 2–Sb anode with 4 and (D) NO 3 concentrations during the degradation of 2000 mg dm different current densities. Legend: 5 mA cm −2 (■), 10 mA cm−2 (●) and 20 mA cm− 2 (▲).
concentration of the different parameters has been plotted against the amount of charge passed through the cell in order to obtain comprehension of the influence of the current density. The initial pH in all the cases was 6.5, which was increasing during the test reaching final values between 7.8 using RuO2 and 9.1 using BDD. The effect of current density on the degradation rate of urea was examined using BDD at 5, 10 and 20 mA cm− 2. The obtained results (Fig. 2A) show that the effect of current density was not significant in terms of urea decomposition. In all cases, the degradation of urea was completed after 8 Ah dm− 3. In order to understand whether the decomposed urea has been completely mineralized or partly transformed to other organic compounds, the evolution of TOC versus passed charge was determined (Fig. 2B). The TOC versus charge curve shows the same trend as the urea concentration. The total removal of TOC was observed after 8 Ah dm−3 and a final TIC value of between 2 and 3.5 mg dm−3 indicates the complete oxidation of urea by means of the direct oxidation at the anode surface and through the reaction with •OH radicals electrogenerated at the BDD surface during the electrolysis at high potentials in the oxygen evolution region: •
þ
−
M þ H2 O→Mð OHÞ þ H þ e :
ð2Þ
The quantitative analyses of the inorganic by-products obtained from the electrooxidation of urea were performed by means of ion + chromatography and they indicated the release of NO− 3 and NH4 . − No other inorganic nitrogen, e.g. NO2 , was detected. The profile of + the NO− 3 and NH4 concentrations versus charge is plotted in Fig. 2C and D. The amount of NH+ 4 ion in the solution increased proportionally up to a passed charge of about 8 Ah dm−3 and then it began to decrease
reaching final values of 180 mg dm−3, 225 mg dm−3 and 270 mg dm−3 for 5, 10 and 20 mA cm−2, corresponding to 15%, 19% and 23% of the ini−3 tial nitrogen. NO− , which 3 reached maximum values at 7–8 Ah dm were about 100 mg dm−3, 500 mg dm−3 and 800 mg dm−3, for 5, 10 and 20 mA cm−2 respectively, corresponding to 2%, 10% and 16% of the initial nitrogen. The fact that only 17%, 29% and 39% of the initial nitrogen were converted to inorganic ions and almost all the urea is mineralised, suggests that the initial nitrogen was partly lost as a volatile N-compound, probably N2. Considering that urea is completely mineralized to CO2 and that nitrogen atoms are converted mainly to N2 by the reaction, þ
NH2 CONH2 þ H2 O→N2 þ 6H þ CO2 þ 6e−:
ð3Þ
The current efficiency was computed from TOC values using Chaâbane et al. equation [32]. When about 8 Ah dm− 3 charge was passed the current efficiency reached values of around 63%, 65% and 68% for 5, 10 and 20 mA. In order to verify the difference between urea oxidation using two non-active electrodes the previous tests using BDD were also carried using SnO2–Sb as well and the results are presented in Fig. 3. At a low current density the urea concentration decreased rapidly at the beginning of electrolysis until about 4 Ah dm−3 and then it was removed more slowly (Fig. 3A) until reaching final values of 1246 mg dm−3 and 880 mg dm−3 for 5 and 10 mA cm−2, respectively. On the contrary, at a higher current density (i.e. 20 mA cm−2) the urea concentration gradually decreased until the end of the test and the maximum removal was 90%. Only a slow TOC decrease was obtained at each applied current (Fig. 3B). The amount of TOC in the solution reached values of
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−3 − Fig. 4. Evolution of (A) urea concentration, (B) TOC, (C) NH+ of urea using the Pt anode with different 4 and (D) NO 3 concentrations during the degradation of 2000 mg dm current densities. Legend: 5 mA cm−2 (■), 10 mA cm−2 (●) and 20 mA cm−2 (▲).
209 mg dm − 3 at 20 mA cm− 2 , 240 mg dm − 3 at 10 mA cm− 2 and 260 mg dm− 3 at 5 mA cm − 2 , and a low generation of inorganic compounds was observed (TIC 2–3 mg dm− 3). The almost complete urea decomposition at 20 mA cm−2 and the relatively low TOC removal indicated that urea is only partially oxidized to intermediate compounds at the SnO2–Sb but it is not completely mineralised, as occurs on BDD. This can be explained by the fact that SnO2–Sb has a lower oxygen evolution overpotential than BDD and, consequently produces a lower quantity of hydroxyl radicals. In addition, a different reactivity of the electrogenerated hydroxyl radicals also occurs. In fact hydroxyl radicals are very weakly adsorbed on BDD, which is known to have weak adsorption properties due to its inert surface, and consequently they are very reactive towards the oxidation of organics [33]. On the contrary, hydroxyl radicals are more strongly adsorbed on the surface of SnO2–Sb and consequently less reactive [34]. + The release of inorganic NO − 3 and NH 4 during electrolysis is reported in Fig. 3C and B. It is possible to see that the amount of the NO− 3 ion in the solution increased linearly with the specific charge, reaching final values of about 410 mg dm− 3 (9% of the initial N), 260 mg dm− 3 (6% of the initial N), and 129 mg dm− 3 (3% of the initial N), at 20 mA cm− 2, 10 mA cm− 2 and 5 mA cm− 2, receptively. −3 NH+ and then 4 reached a maximum value at around 4 Ah dm gradually decreased. The performance of the Pt anode for the oxidation of urea was also investigated and the results are presented in Fig. 4. The decomposition rate of urea (Fig. 4A) depends to a great extent on the current density.
Increasing the current density from 5 to 20 mA cm− 2 produced an increasing rate of urea decomposition, namely 50% at 5 mA cm−2 and 71% at 20 mA cm−2. The change in TOC versus passed electric charge was also determined (Fig. 4B). A fast decrease in TOC can be observed up to 2.5 Ah dm− 3, and the amount of TOC in the solution, then reached an almost steady state at around 350, 290 and 250 mg dm− 3 for 5, 10 and 20 mA cm− 2. The low TOC removal indicated that with Pt anode urea was mainly oxidized to other organic intermediates. Even though the cyclic voltammetries have demonstrated that urea adsorption at a low potential can block the electroactive site of the Pt anode, however working at high current densities, in the oxygen evolution region, the Pt produces •OH radicals that prevent its deactivation. Pt anode is an “active” electrode and the electrogenerated •OH are chemisorbed on the surface where they form a higher oxide (PtOx). When Pt electrodes are used, the selective oxidation of organics mainly occurs by means of chemisorbed oxygen at electrogenerated active sites. However, their complete mineralization is not reached [35]. When RuO2 is used, •OH radicals can also be produced, but they are adsorbed more on Pt [36] and they are consequently less reactive. As in the case of SnO2–Sb, the amount of NO− 3 increases proportionally to the passed electric charge (Fig. 4D) and this behaviour may be caused by the direct oxidation of ammonia species on the anode surface at a high anodic potential. A maximum value for NH+ 4 concentration at about 4 Ah dm−3 can be observed in Fig. 4C then it decreases in agreement with the previous observation. Urea electrooxidation
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−2 − Fig. 5. Evolution of (A) urea concentrations, (B) TOC, (C) NH+ with NaCl in different concentrations. Legend: without NaCl (■), 1.5 g dm−3 4 and (D) NO3 using the Pt anode at 20 mA cm NaCl (●) 3.0 g dm−3 NaCl (▲) and 6.0 g dm−3 NaCl (▼).
was also performed using Ti–RuO2 at different current densities. The maximum removal values were 31% (1380 mg dm−3 of urea), 33% (1350 mg dm−3 of urea) and 35% (1300 mg dm−3 of urea) at 20, 10 and 5 mA cm− 2 respectively. The TOC values remained practically unchanged during the tests and a slight generation of inorganic compounds was found at the end of the tests (TIC between 1 and − 2.7 mg dm−3). In addition, negligible amounts of NH+ 4 and NO3 were generated when the Ti–RuO2 electrode was used. This small amount of inorganic by-products at the end of the treatment is consistent with the fact that urea is mainly oxidized to other organic compounds and it is only partially decomposed. Various authors [3,14,19] have reported that the addition of chloride ions to the electrolyte causes an increase in the removal efficiency, and that the complete degradation of urea can be obtained by using active chlorine, in the form of chlorine, hypochlorous acid and hypochlorite, formed on the anode surface. Some tests were performed to investigate the effect of chloride ions on the oxidation of urea using electrolysis with the addition of 1.5 g dm− 3, 3.0 g dm− 3 and 6.0 g dm− 3 of NaCl to the electrolyte. The results are shown in Fig. 5. When a Pt anode was used, the addition of NaCl significantly improved the oxidation of urea, and enabled the complete removal in the presence of 6 g dm− 3 of NaCl, mainly due to the mediation of the active chlorine. The presence of NaCl also favoured the TOC removal rate. The amount of TOC after the treatment was 88%, 95% and 99%, for 1.5, 3 and 6 g dm− 3 NaCl, respectively. The almost complete removal of urea and TOC at the end of the test indicates that urea is completely mineralised in the presence of chlorides. The results agree with experiments carried out for Simka and Piotrowski [3].
Even in the case of the tests carried out in the presence of NaCl, the final amount of ammonia and nitrate ions was checked. Fig. 5C shows that the amount of NH+ 4 at the beginning of the electrolysis was higher in the presence of NaCl, until about 4 Ah dm− 3 and then a sharp decrease occurred and a value close to 0 was reached at the end in the presence of NaCl (3 and 6 g dm−3), a value which cannot be attained in the absence of NaCl. This may be due to reactions between the ammonium ion and hypochlorous acid: þ
þ
−
2NH4 þ 3HClO→N2 þ 3H2 O þ 5H þ 3Cl :
ð4Þ
Nitrate generation was significantly reduced with the addition of NaCl, thus indicating the formation of molecular nitrogen N2.
4. Conclusions In this paper, the electrochemical oxidation of urea has been studied on different anode materials: Pt, Ti–Ru oxide, BDD and antimony-doped tin oxide. Cyclic voltammetry measurements have shown that when Pt and Ti–Ru oxides are used, the presence of urea causes a decrease in the current density in the oxygen evolution region. This behaviour can be attributed to the blockage of the electrode active oxygen evolution sites due to urea adsorption. It has also been shown that the adsorption–desorption of urea on a Pt anode is a reversible process. On the contrary, the presence of urea on the BDD and antimonydoped tin oxide resulted in an increase in the current density at a given potential in the oxygen evolution region, and this indicated that urea was oxidized by •OH electrogenerated during the oxygen
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evolution. No evidence of adsorption phenomena was found on BDD or on the antimony-doped tin oxide electrodes. Bulk electrolysis in the potential oxygen evolution region has shown that decomposition of the total urea was only reached for the BDD electrode. The Pt, the Ti–Ru oxide, and the antimony-doped tin oxide led to a removal of 75%, 60% and 85% of urea, respectively. When the BDD anode was used, the urea was completely mineralized. In addition to the high effectiveness in urea abatement, the test carried out using BDD showed that important differences in nitrogen formation have been shown from a comparison of the tests at different current densities. At high current the urea was oxidized to NO3- due to the excess of elecrons, while at lowerdensities was favoured the N2 and NH+ 4 generation. The presence of sodium chloride in the electrolyte has a positive impact on the abatement when the Pt anode is used, as it increases up to 40%. This behaviour can be attributed to a direct electrooxidation on the active surface of the electrodes, as well as to the indirect oxidation reactions that occur in the presence of chloride ions, which are also effective in decreasing the concentration of ammonium and nitrate at the end of the process. Prime novelty statement Bulk electrolysis in the potential oxygen evolution region has shown that decomposition of the total urea was only reached for the BDD electrode. In addition to the high effectiveness in urea abatement, the test carried out using BDD showed that important differences in nitrogen formation have been shown from a comparison of the tests at different current densities. References [1] W. Zaborska, B. Krajewska, M. Leszko, Z. Olech, Inhibition of urease by Ni2+ ions. Analysis of reaction progress curves, J. Mol. Catal. B: Enzym. 13 (2001) 103–108. [2] R. Hüttl, K. Bohmhammel, G. Wolf, R. Oehmgen, Calorimetric investigations into enzymatic urea hydrolysis, Thermochim. Acta 250 (1995) 1–12. [3] W. Simka, J. Piotrowski, Methods for removal of urea from aqueous solutions, Przem. Chem. 86 (2007) 841. [4] K. Van Hege, M. Verhaege, W. Verstraete, Indirect electrochemical oxidation of reverse osmosis membrane concentrates at boron-doped diamond electrodes, Electrochem. Commun. 4 (2002) 296–300. [5] K. Van Hege, M. Verhaege, W. Verstraete, Electro-oxidative abatement of lowsalinity reverse osmosis membrane concentrates, Water Res. 38 (2004) 1550–1558. [6] V. Amstutz, A. Katsaounis, A. Kapalka, C. Comninellis, K.M. Udert, Effects of carbonate on the electrolytic removal of ammonia and urea from urine with thermally prepared IrO2 electrodes, J. Appl. Electrochem. 42 (2012) 787–795. [7] C.G. Alfafara, T. Kawamori, N. Nomura, M. Kiuchi, M. Matsumura, Electrolytic removal of ammonia from brine wastewater: scale-up, operation and pilot-scale evaluation, J. Chem. Technol. Biotechnol. 79 (2004) 291–298. [8] L. Li, Y. Liu, Ammonia removal in electrochemical oxidation: mechanism and pseudo-kinetics, J. Hazard. Mater. 161 (2009) 1010–1016. [9] A. Kapaka, A. Cally, S. Neodo, C. Comninellis, M. Wachter, K.M. Udert, Electrochemical behavior of ammonia at Ni/Ni(OH)2 electrode, Electrochem. Commun. 12 (2010) 18–21. [10] A. Kapaka, S. Fierro, Z. Frontistis, A. Katsaounis, O. Frey, M. Koudelka, C. Comninellis, K.M. Udert, Electrochemical behaviour of ammonia (NH+ 4 /NH 3 ) on electrochemically grown anodic iridium oxide film (AIROF) electrode, Electrochem. Commun. 11 (2009) 1590–1592.
[11] A. Cabeza, A. Urtiaga, M.-J. Rivero, I. Ortiz, Ammonium removal from landfill leachate by anodic oxidation, J. Hazard. Mater. 144 (2007) 715–719. [12] V. Climent, A. Rodes, J.M. Orts, A. Aldaz, J.M. Feliu, Urea adsorption on Pt(111) electrodes, J. Electroanal. Chem. 461 (1999) 65–75. [13] O.A. Petrii, S.Y. Vassina, Adsorption of urea on platinum at low positive potentials: the time dependence and unexpectedly strong effect on the oxidation of unicarbon particles, J. Electroanal. Chem. 349 (1993) 197–209. [14] C. Carlesi Jara, S. Di Giulio, D. Fino, P. Spinelli, Combined direct and indirect electroxidation of urea containing water, J. Appl. Electrochem. 38 (2008) 915–922. [15] R.W. Keller Jr., S.J. Yao, J.M. Brown, S.K. Wolfson Jr., M.V. Zeller, Electrochemical removal of urea from physiological buffer as the basis for a regenerative dialysis system, J. Electroanal. Chem. 116 (1980) 469–485. [16] B. Boggs, R. King, G. Botte, Urea electrolysis: direct hydrogen production from urine, Chem. Commun. 32 (2009) 4859–4861. [17] R.L. King, G.G. Botte, Investigation of multi-metal catalysts for stable hydrogen production via urea electrolysis, J. Power Sources 196 (2011) 9579–9584. [18] W. Yan, D. Wang, G. Botte, Nickel and cobalt bimetallic hydroxide catalysts for urea electro-oxidation, Electrochim. Acta 61 (2012) 25–30. [19] W. Simka, J. Piotrowski, G. Nawrat, Influence of anode material on electrochemical decomposition of urea, Electrochim. Acta 52 (2007) 5696–5703. [20] W. Simka, J. Piotrowski, A. Robak, G. Nawrat, Electrochemical treatment of aqueous solutions containing urea, J. Appl. Electrochem. 39 (2009) 1137–1143. [21] B.J. Hernlem, Electrolytic destruction of urea in dilute chloride solution using DSA electrodes in a recycled batch cell, Water Res. 39 (2005) 2245–2252. [22] H. Ding, Y. Feng, J. Liu, Preparation and properties of Ti/SnO2–Sb2O5 electrodes by electrodeposition, Mater. Lett. 61 (2007) 4920–4923. [23] C.P. De Pauli, S. Trasatti, Electrochemical surface characterization of IrO2 + SnO2 mixed oxide electrocatalysts, J. Electroanal. Chem. 396 (1995) 161–168. [24] V.A. Alves, L.A. da Silva, J.F.C. Boodts, S. Trasatti, Kinetics and mechanism of oxygen evolution on IrO 2 -based electrodes containing Ti and Ce acidic solutions, Electrochim. Acta 39 (1994) 1585–1589. [25] A.I. Onuchukwu, S. Trasatti, Effect of substitution of SnO2 for TiO2 on the surface and electrocatalytic properties of RuO2 + TiO2 electrodes, J. Appl. Electrochem. 21 (1991) 858–862. [26] H. Angerstein-Kozlowska, B.E. Conway, W.B.A. Sharp, The real condition of electrochemically oxidized platinum surfaces: part I. Resolution of component processes, J. Electroanal. Chem. Interfacial Electrochem. 43 (1973) 9–36. [27] G.W.J. Chrisp, Spectrophotometric method for determination of urea, Anal. Chem. 26 (1954) 452–453. [28] A. Kapalka, G. Foti, C. Comninellis, The importance of electrode material in environmental electrochemistry. Formation and reactivity of free hydroxyl radicals on boron-doped diamond electrodes, Electrochim. Acta 54 (2009) 2018–2023. [29] A. Kapalka, G. Foti, C. Comninellis, Investigations of electrochemical oxygen transfer reaction on boron-doped diamond electrodes, Electrochim. Acta 53 (2007) 1954–1961. [30] A. Kapalka, G. Foti, C. Comninellis, Investigation of the anodic oxidation of acetic acid on boron-doped diamond electrodes, J. Electrochem. Soc. 155 (2008) E27–E32. [31] B. Marselli, J. Garcia-Gomez, P.-A. Michaud, M.A. Rodrigo, C. Comninellis, Electrogeneration of hydroxyl radicals on boron-doped diamond electrodes, J. Electrochem. Soc. 150 (2003) D79–D83. [32] S. Chaâbane, M. Elaoud, G. Panizza, T. Cerisola, Coumaric acid degradation by electro-Fenton process, J. Electroanal. Chem. 667 (2012) 19–23. [33] L. Gherardini, P.A. Michaud, M. Panizza, C. Comninellis, N. Vatistas, Electrochemical oxidation of 4-chlorophenol for wastewater treatment. Definition of normalized current efficiency, J. Electrochem. Soc. 148 (2001) D78–D82. [34] X. Chen, F. Gao, G. Chen, Comparison of Ti/BDD and Ti/SnO2–Sb2O5 electrodes for pollutant oxidation, J. Appl. Electrochem. 35 (2005) 185–191. [35] C. Comninellis, Electrocatalysis in the electrochemical conversion/combustion of organic pollutants for waste water treatment, Electrochim. Acta 39 (1994) 1857–1862. [36] A. Kapałka, G. Fóti, C. Comninellis, Kinetic modelling of the electrochemical mineralization of organic pollutants for wastewater treatment, J. Appl. Electrochem. 38 (2008) 7–16
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Electrochemical oxidation of urea in aqueous solutions using a boron-doped thin-film diamond electrode M. Cataldo Hernández a, N. Russo a, M. Panizza b, P. Spinelli a, D. Fino a,⁎ a b
Dipartimento Scienza Applicata e Tecnologia, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy Dipartimento di Ingegneria Civile, Chimica e Ambientale, Università di Genova, Piazzale Kennedy 1, Genoa 16129, Italy
a r t i c l e
i n f o
Article history: Received 30 October 2013 Received in revised form 31 January 2014 Accepted 9 February 2014 Available online 15 February 2014 Keywords: Urea Anode Electrooxidation Cyclic voltammetry Bulk electrolysis
a b s t r a c t In the present paper, the electrocatalytic abatement of urea in aqueous solutions has been studied by means of cyclic voltammetry and galvanostatic electrolysis, using different anodes such as Pt, Ti–Ru oxide, boron-doped diamond (BDD) and antimony-doped tin oxide. HPLC analysis, total organic carbon (TOC) and ionic chromatography have been used to evaluate the oxidation and the mineralization of the treated aqueous solutions. The results of the cyclic voltammetries have shown that, in the case of Pt and Ti–Ru oxides a decrease in current density in the oxygen evolution region can be observed in the presence of urea, due to the blockage of the electrode active oxygen evolution sites as a consequence of the reversible adsorption of urea. Instead, a notable increase in the current density has been observed in the region of the oxygen evolution for the BDD and antimony-doped tin oxide electrodes, in the presence of urea, indicating that the oxidation of urea involves hydroxyl radicals. The bulk electrolysis tests have shown that the complete removal of urea and TOC can only be achieved using a boron-doped diamond and that Pt, the Ti–Ru oxide and antimony-doped tin oxide only permit a partial oxidation of urea. On the basis of the TOC evolution and the identification of the organic intermediates and inorganic ions released during the treatment, a total mineralization has been proposed. Finally, electrolysis has been performed in the presence of chloride ions, which act as oxidation mediator, and a comparison has been done between direct and mediated electro-oxidation. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Urea from sources such as human, animal and fertilizer wastes, would generate aquatic ecosystem degradation with adverse effects on ecosystem and human well-being. The presence of urea in human wastewaters is a serious problem because it can be hydrolyzed by the urease enzyme produced by microorganisms to ammonia NH2 CONH2 þ H2 O→2NH3 þ CO2 :
ð1Þ
The discharge of ammonia containing effluents is an environmental and health problem since they increase the pH of the soils and cause euthrofication of the waters, and free ammonia can also be released into the atmosphere contributing to the formation of ammonium sulphates and nitrates. Consequently, increasing attention has been paid to efficient urea and ammonium removal methods from wastewaters, and a wide range of processes has been proposed, including adsorption, oxidation, ⁎ Corresponding author. Tel.: +39 011 0904710. E-mail address: debora.fi[email protected] (D. Fino).
http://dx.doi.org/10.1016/j.diamond.2014.02.006 0925-9635/© 2014 Elsevier B.V. All rights reserved.
biological decomposition, chemical oxidation and enzymatic decomposition [1–3]. Some of these processes require a high energy input, or complicated equipment, thus their implementation is limited at an industrial level. Electrochemical oxidation has also been shown to be a promising method for ammonium and urea removal from synthetic and real wastewaters. Many papers have dealt with electrochemical ammonia degradation, and have shown that ammonia removal mainly takes place because of an indirect oxidation process with active chlorine electrochemically generated at the anode surface. Many anode materials, such as Pt, IrO2 , boron-doped diamond (BDD), PbO 2 , SnO2 , and RuO2 have been tested for this process, and it has resulted that ammonia is effectively removed from solutions through a mechanism known as break point chlorination [4–10]. Cabeza et al. [11] have shown that a small quantity of nitrates can also be formed, when A BDD anode is used. They also showed that an increasing chloride concentration resulted in an increase in ammonium removal efficiency and a decrease in the amount of N–NH+ 4 transformed to nitrates. The electrooxidation of urea has been studied much less than ammonia, and its mechanism is still not fully understood. For example,
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some papers have reported that direct oxidation of urea on Pt involves its adsorption on the anode surface. However, some authors have asserted that the adsorption of urea is reversible [12–15], while others have suggested that it is not reversible [15]. Recently, Boggs et al. [16] have compared the electrocatalytic activity of several noble metals (i.e. Pt, Pt–Ir, Rh and Ni) for the direct electrooxidation of urea in alkaline media. Ni was found to be very effective because it is anodically converted to NiOOH (Ni3+), which is the active material for urea oxidation. The stability and the activity of Ni catalyst can be also enhanced through a combination of Ni with other metals in multimetal catalyst such as Pt, Ru, Rh, and Co. For example, Rh–Ni electrodes have been shown to improve the current density by a factor of 200, compared to Ni catalysts [17,18]. Several research groups have reported on the electrochemical oxidation of urea using different materials, such as Ti/Pt, Ti/Pt–Ir, RuO2, and IrO2, in the presence of chlorides [3,19–21]. In these conditions, even though direct oxidation cannot be excluded, the main reaction pathway, as in the case of ammonia, is indirect oxidation with electrogenerated active chlorine, and non-toxic N2 and CO2 are the main decomposition products of urea. In this work, the catalytic activities of four anode materials (i.e. Pt, Ti–Ru oxide, BDD and antimony-doped tin oxide) on the direct electrochemical oxidation of urea have been investigated by means of potentiodynamic measurements and bulk electrolysis in order to evaluate electrochemical oxidation as a treatment technology. The organic intermediates and released inorganic ions have been detected and quantified by means of several chromatographic techniques and on the basis of the by-products. Finally, electrolysis has been performed in the presence of chlorides that act as oxidation mediators and a comparison has been made between direct and mediated electro-oxidation. 2. Experimental 2.1. Chemicals The solutions were prepared by dissolving different amounts of urea (CO(NH2)2, (Sigma Aldrich)) in 1 M NaClO4, which was chosen as the supporting electrolyte because it does not generate oxidizing species that can react with the organics, as instead occurs when a Cl− medium 2 (i.e. generation of Cl2) or a SO24 − medium (i.e. production of S2O− 8 ) is used. For the tests carried out to compare direct and mediated electrooxidation, different amounts of NaCl (Sigma Aldrich) were added to the electrolyte. 2.2. Electrode materials Antimony-doped tin oxide anodes, hereafter referred to as SnO2–Sb2O5, were prepared by coating titanium substrates (2 mm thick) through the thermal decomposition of a mixture of 10 g dm−3 SnCl4 ∗ 5H2 O and 0.1 g dm − 3 SbCl3 dissolved in isopropanol. The titanium sheets were subjected to a surface pre-treatment that consisted in the mechanical polishing of the sheets with WS-FLEX 18-C sandpaper, followed by degreasing with 40% NaOH at 80 °C for 120 min, etching in hydrochloric acid 11.5 M for 1 min, washing in distilled water and soaking in an ultrasonic bath at 60 °C for 60 min. The precursor solution was painted onto the titanium plates and the solvent was evaporated at 100 °C for 15 min. The sample was calcined at 550 °C in a 2 dm3 min− 1 oxygen (pure) flow, for 15 min; this procedure was repeated 16 times. Finally the electrodes were calcined at 550 °C for 3 h in 0.5 dm3/min oxygen [22–25]. An average thickness value of about 4 μm was obtained. The nominal composition of the mixed oxide was Sn0.918Sb0.109O2. The boron-doped thin-film diamond electrode was supplied by CSEM Centre Swiss d'Electronique et de Microtechnique, Neuchâtel, Switzerland. It was synthesised by the hot filament chemical vapour
deposition technique (HF CVD) on single crystal p-type Si wafers. The doping level of boron in the diamond layer, expressed as the B/C ratio, was about 3500 ppm. The obtained diamond film thickness was about 1 μm and it had a resistivity of 10–30 mΩ cm. In order to stabilise the electrode surface and to obtain reproducible results, the diamond electrode was pre-treated at 25 °C, by means of anodic polarization, in 1 M HClO4 at 10 mA cm−2 for 30 min using stainless steel as the counter electrode. This treatment made the surface hydrophilic. Before each cyclic voltammetry test, the Pt electrode (radiometer) was treated in a 30% nitric acid solution under anodic and cathodic polarization at 50 mA cm− 2 for about 20 min. Subsequently, 10 voltammetric cycles (100 mV s−1) were performed in 1 M sulphuric acid between −300 mV and 1600 mV vs. SCE. This procedure allowed the voltammogram reported by Angerstein-Kozlowska et al. [26] to be obtained as a reference for clean Pt electrodes in sulphuric acid solutions. The Ti–Ru oxide anode, hereafter referred to as TiRuO 2 , was a commercial DSA® anode consisting of a titanium substrate covered with a TiO2/RuO2 layer, which was purchased from De Nora (Italy). 2.3. Electrochemical system Cyclic voltammetries were carried out in a conventional threeelectrode cell using a computer controlled Voltalab 301 potentiostat. Pt, Ti–RuO2, BDD and SnO2–Sb2O5 were used as working electrodes, while saturated calomel (SCE) was used as a reference and a Pt plate was used as the counter electrode. The exposed apparent area of the working electrodes was 1 cm2. Cyclic voltammetry was performed at room temperature (T = 25 °C) with a 30 mV s−1 scan rate. Bulk oxidations were carried out in a one-compartment electrolytic flow cell. A single-compartment cell was adopted because, preliminary experiments evidenced that when using stainless steel, there is no production of hydrogen peroxide. The flow rate was 4.5 l/min which corresponds to a mass-transfer coefficient in the cell of 2.3 ∗ 10− 5 m s− 1. The experiments were performed under galvanostatic conditions using an AMEL 2055 potentiostat/galvanostat with a current density of 20 mA cm − 2 . Pt, Ti–RuO2, BDD and SnO2–Sb2O5 electrodes were used as anodes, and stainless steel was used as the cathode. All the electrodes were circular in shape with a 50 cm 2 geometrical area and a 0.5 cm inter-electrode gap. An electrolyte solution, consisting of 2 g dm − 3 NaClO 4 and 2 mg dm − 3 of urea, was introduced into a 0.5 dm3 thermoregulated glass tank and circulated through an electrochemical reactor using a centrifugal pump with a flow-rate of 300 dm3 h− 1. The solutions were periodically sampled and analyzed by means of a spectrophotometer (Cary 5000 Varian), a TOC–TIC/TN analyzer (Hach Lange IL 550), ion chromatography (Dionex ICS-5000) and an HPLC (Shimadzu LC 20AT) to determine the dissolved chemical species during the treatment. The urea concentration was determined by means of a spectrophotometric method, based on the addition of a p-dimethylaminobenzaldehyde and hydrochloric acid solution to the urea sample in order to obtain a yellowish-green colour which was caused by the complexation reaction [27]. 3. Results and discussion 3.1. Cyclic voltammetries Fig. 1A shows the cyclic voltammograms obtained with the Pt electrodes in 1 M NaClO4 with a scan rate of 30 mV s− 1 and for different urea concentrations. The presence of urea resulted in an increase in the current density in the potential regions between − 0.1 V and 0.2 V vs. SCE and between 0.6 and 1.1 V vs. SCE, thus indicating the direct urea oxidation path [13,14]. However, this increase in current density, which was also observed by other authors [22–25], does not
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111
Fig. 1. Cyclic voltammetries in the 1 M NaClO4 electrolyte with different urea concentrations. Scan rate: 30 mV s−1. (A) Pt electrode. (B) Ti–RuO2 electrode. (C) SnO2–Sb2O5 electrode. (D) BDD electrode.
appear to be influenced by the urea concentration because of urea adsorption, which blocks the active sites of the anode. The determination of the onset potential for oxygen evolution, based on a significant increase (1 to 3% of the full scale) of the current above the flat trend preceding the sharp increase in current density, provided a value of about 1.2 V and it did not seem to be influenced by the presence of urea. However, the current in the oxygen evolution region at 1.2 V decreases significantly in the presence of urea, due to the adsorption of urea by the electrode, which blocked the electroactive sites [19]. Moreover, the current density decreases in the presence of urea and the reduction peak shifted towards negative potentials, on the cathodic branch of the voltammogram, in the region corresponding to the Pt-oxide reduction, that is between 0 and − 0.6 V vs. SCE. Consecutive cyclic voltammograms on Pt, carried out with 0.01 M of urea, showed that the curve did not change when the number of cycles was increased (up to 10 cycles), this suggests that the adsorption– desorption processes are reversible. Fig. 1B shows the cyclic voltammograms of the Ti–RuO2 electrodes obtained in 1 M NaClO4 with a scan rate of 30 mV s−1, for different urea concentrations. The voltammogram in 1 M NaClO4 presents two broad and not well defined peaks at 0.6 and −0.2 V vs. SCE, which are related to the redox processes for the lower metal oxide/higher metal oxide transition.
No significant change was observed in the presence of urea in the potential water stability region between −0.4 and 1.0 V vs. SCE while just a slight shift towards lower current densities was noted in the presence of urea. The onset potential for the oxygen evolution did not seem to be affected by the presence of urea either. On the contrary, at 1.25 V, an increase in urea concentration in the oxygen evolution region caused a decrease in current density, similar to what was observed with the Pt electrode, attributed to the blockage of the electrode active sites for oxygen evolution due to urea adsorption. These results are consistent with those obtained by Simka et al. for a Ti–RuO2 electrode [19]. Fig. 1C shows cyclic voltammograms on a SnO2–Sb2O5 electrode in 1 M NaClO4 with a scan rate of 30 mV s− 1 for different urea concentrations. The voltammogram (continuous line) is almost featureless in the studied potential region in the presence of a supporting electrolyte. The oxygen evolution (at 2 mA cm− 2) is shifted towards more negative potentials in the presence of urea and there is a considerable increase in the current density, which is proportional to the urea concentration. This indicates that the urea oxidation pathway involves hydroxyl radicals that form during the oxygen evolution [28–30]. Consecutive cyclic voltammograms were performed on SnO2–Sb2O5 in 0.01 M urea. The 5th and 10th cycles were perfectly overlapped. This observation, and the fact that no specific peaks in the curve were found, as occurred for platinum, make it possible to infer that urea is not adsorbed on the electrode.
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− −3 Fig. 2. Evolution of (A) urea concentration, (B) TOC, (C) NH+ of urea using the BDD anode with different current 4 and (D) NO3 concentrations during the degradation of 2000 mg dm densities. Legend: 5 mA cm−2 (■), 10 mA cm−2 (●) and 20 mA cm−2 (▲).
Synthetic boron-doped diamond (BDD) is an electrode material that has received great attention recently. On BDD electrodes, a potential of oxygen evolution (about 2.2 V vs. SHE) is thermodynamically possible to generate hydroxyl radical (2.38 V vs. SHE). In effect, the experimental evidence was given by Marselli et al. [31], who detected the presence of hydroxyl radical on BDD electrode by means of electron spin resonance. Fig. 1D shows the cyclic voltammograms recorded on the BDD electrodes in the absence and presence of urea in the 1 M NaClO4 supporting electrolyte, with a scan rate of 30 mV s−1. The voltammograms, before oxygen evolution, displayed no significant change in the presence of urea with respect to the voltammogram of the supporting electrolyte. The only difference was a slight decrease in the onset potential of the oxygen evolution, which indicated an effect of the organic compound on the overpotential of the oxygen evolution. As reported by Kapalka et al. [28] in the presence of organic compound the hydroxyl radical surface concentration is correlated with the shift of current potential curves towards lower potentials caused by the organic reaction with the hydroxyl radical electrochemically generated. However, the current density at 1.5 V, in the oxygen evolution region, increased with the urea concentration. The current density in the case of the 0.1 M urea at 1.75 V vs. SCE was in fact three times the value of the current density in the absence of urea. This behaviour, which is similar to that obtained with SnO2 –Sb 2O 5 , indicates that the oxidation of urea involves hydroxyl radicals, which are available under oxygen evolution conditions.
Since it is known that the BDD electrode exhibits an inert surface with low adsorption properties, consecutive cyclic voltammograms with 0.01 M urea in the 1 M NaClO4 supporting electrolyte were performed in a similar way to the tests carried out with the other electrodes in order to investigate the effects of urea adsorption. As previously found for the SnO2–Sb2O5 electrode, it is possible to observe also in the case of the BDD electrode that the curves did not change with the subsequent cycling and they were completely overlapped. Considering the previous information and the fact that there were no changes in the current or peaks that could be attributed to the adsorption of urea, as in the case of Pt electrodes, it is possible to conclude that urea adsorption is negligible. From the voltammetries, in the absence of urea, reported in Fig. 1, it is possible to observe that the onset potential for oxygen evolution was 1.05 V, 1.2 V, 1.40 V and 1.65 V vs. SCE for Ti–RuO2, Pt, SnO2–Sb2O5 and BDD, respectively. These values confirm that Ti–RuO2 and Pt have a low overpotential for oxygen evolution, and therefore show an “active” behaviour, while SnO2–Sb2O5 and BDD have a high oxygen evolution overpotential and therefore have “non-active” behaviour. 3.2. Bulk electrolysis Electrochemical oxidation of 2 g dm−3 of urea in 2 g dm−3 Na2ClO4 has been performed with the different investigated anodes, Ti–RuO2, Pt, SnO2–Sb and BDD under galvanostatic conditions. The change in
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−3 − Fig. 3. Evolution of (A) urea concentration, (B) TOC, (C) NH+ of urea using the SnO 2–Sb anode with 4 and (D) NO 3 concentrations during the degradation of 2000 mg dm different current densities. Legend: 5 mA cm −2 (■), 10 mA cm−2 (●) and 20 mA cm− 2 (▲).
concentration of the different parameters has been plotted against the amount of charge passed through the cell in order to obtain comprehension of the influence of the current density. The initial pH in all the cases was 6.5, which was increasing during the test reaching final values between 7.8 using RuO2 and 9.1 using BDD. The effect of current density on the degradation rate of urea was examined using BDD at 5, 10 and 20 mA cm− 2. The obtained results (Fig. 2A) show that the effect of current density was not significant in terms of urea decomposition. In all cases, the degradation of urea was completed after 8 Ah dm− 3. In order to understand whether the decomposed urea has been completely mineralized or partly transformed to other organic compounds, the evolution of TOC versus passed charge was determined (Fig. 2B). The TOC versus charge curve shows the same trend as the urea concentration. The total removal of TOC was observed after 8 Ah dm−3 and a final TIC value of between 2 and 3.5 mg dm−3 indicates the complete oxidation of urea by means of the direct oxidation at the anode surface and through the reaction with •OH radicals electrogenerated at the BDD surface during the electrolysis at high potentials in the oxygen evolution region: •
þ
−
M þ H2 O→Mð OHÞ þ H þ e :
ð2Þ
The quantitative analyses of the inorganic by-products obtained from the electrooxidation of urea were performed by means of ion + chromatography and they indicated the release of NO− 3 and NH4 . − No other inorganic nitrogen, e.g. NO2 , was detected. The profile of + the NO− 3 and NH4 concentrations versus charge is plotted in Fig. 2C and D. The amount of NH+ 4 ion in the solution increased proportionally up to a passed charge of about 8 Ah dm−3 and then it began to decrease
reaching final values of 180 mg dm−3, 225 mg dm−3 and 270 mg dm−3 for 5, 10 and 20 mA cm−2, corresponding to 15%, 19% and 23% of the ini−3 tial nitrogen. NO− , which 3 reached maximum values at 7–8 Ah dm were about 100 mg dm−3, 500 mg dm−3 and 800 mg dm−3, for 5, 10 and 20 mA cm−2 respectively, corresponding to 2%, 10% and 16% of the initial nitrogen. The fact that only 17%, 29% and 39% of the initial nitrogen were converted to inorganic ions and almost all the urea is mineralised, suggests that the initial nitrogen was partly lost as a volatile N-compound, probably N2. Considering that urea is completely mineralized to CO2 and that nitrogen atoms are converted mainly to N2 by the reaction, þ
NH2 CONH2 þ H2 O→N2 þ 6H þ CO2 þ 6e−:
ð3Þ
The current efficiency was computed from TOC values using Chaâbane et al. equation [32]. When about 8 Ah dm− 3 charge was passed the current efficiency reached values of around 63%, 65% and 68% for 5, 10 and 20 mA. In order to verify the difference between urea oxidation using two non-active electrodes the previous tests using BDD were also carried using SnO2–Sb as well and the results are presented in Fig. 3. At a low current density the urea concentration decreased rapidly at the beginning of electrolysis until about 4 Ah dm−3 and then it was removed more slowly (Fig. 3A) until reaching final values of 1246 mg dm−3 and 880 mg dm−3 for 5 and 10 mA cm−2, respectively. On the contrary, at a higher current density (i.e. 20 mA cm−2) the urea concentration gradually decreased until the end of the test and the maximum removal was 90%. Only a slow TOC decrease was obtained at each applied current (Fig. 3B). The amount of TOC in the solution reached values of
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−3 − Fig. 4. Evolution of (A) urea concentration, (B) TOC, (C) NH+ of urea using the Pt anode with different 4 and (D) NO 3 concentrations during the degradation of 2000 mg dm current densities. Legend: 5 mA cm−2 (■), 10 mA cm−2 (●) and 20 mA cm−2 (▲).
209 mg dm − 3 at 20 mA cm− 2 , 240 mg dm − 3 at 10 mA cm− 2 and 260 mg dm− 3 at 5 mA cm − 2 , and a low generation of inorganic compounds was observed (TIC 2–3 mg dm− 3). The almost complete urea decomposition at 20 mA cm−2 and the relatively low TOC removal indicated that urea is only partially oxidized to intermediate compounds at the SnO2–Sb but it is not completely mineralised, as occurs on BDD. This can be explained by the fact that SnO2–Sb has a lower oxygen evolution overpotential than BDD and, consequently produces a lower quantity of hydroxyl radicals. In addition, a different reactivity of the electrogenerated hydroxyl radicals also occurs. In fact hydroxyl radicals are very weakly adsorbed on BDD, which is known to have weak adsorption properties due to its inert surface, and consequently they are very reactive towards the oxidation of organics [33]. On the contrary, hydroxyl radicals are more strongly adsorbed on the surface of SnO2–Sb and consequently less reactive [34]. + The release of inorganic NO − 3 and NH 4 during electrolysis is reported in Fig. 3C and B. It is possible to see that the amount of the NO− 3 ion in the solution increased linearly with the specific charge, reaching final values of about 410 mg dm− 3 (9% of the initial N), 260 mg dm− 3 (6% of the initial N), and 129 mg dm− 3 (3% of the initial N), at 20 mA cm− 2, 10 mA cm− 2 and 5 mA cm− 2, receptively. −3 NH+ and then 4 reached a maximum value at around 4 Ah dm gradually decreased. The performance of the Pt anode for the oxidation of urea was also investigated and the results are presented in Fig. 4. The decomposition rate of urea (Fig. 4A) depends to a great extent on the current density.
Increasing the current density from 5 to 20 mA cm− 2 produced an increasing rate of urea decomposition, namely 50% at 5 mA cm−2 and 71% at 20 mA cm−2. The change in TOC versus passed electric charge was also determined (Fig. 4B). A fast decrease in TOC can be observed up to 2.5 Ah dm− 3, and the amount of TOC in the solution, then reached an almost steady state at around 350, 290 and 250 mg dm− 3 for 5, 10 and 20 mA cm− 2. The low TOC removal indicated that with Pt anode urea was mainly oxidized to other organic intermediates. Even though the cyclic voltammetries have demonstrated that urea adsorption at a low potential can block the electroactive site of the Pt anode, however working at high current densities, in the oxygen evolution region, the Pt produces •OH radicals that prevent its deactivation. Pt anode is an “active” electrode and the electrogenerated •OH are chemisorbed on the surface where they form a higher oxide (PtOx). When Pt electrodes are used, the selective oxidation of organics mainly occurs by means of chemisorbed oxygen at electrogenerated active sites. However, their complete mineralization is not reached [35]. When RuO2 is used, •OH radicals can also be produced, but they are adsorbed more on Pt [36] and they are consequently less reactive. As in the case of SnO2–Sb, the amount of NO− 3 increases proportionally to the passed electric charge (Fig. 4D) and this behaviour may be caused by the direct oxidation of ammonia species on the anode surface at a high anodic potential. A maximum value for NH+ 4 concentration at about 4 Ah dm−3 can be observed in Fig. 4C then it decreases in agreement with the previous observation. Urea electrooxidation
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−2 − Fig. 5. Evolution of (A) urea concentrations, (B) TOC, (C) NH+ with NaCl in different concentrations. Legend: without NaCl (■), 1.5 g dm−3 4 and (D) NO3 using the Pt anode at 20 mA cm NaCl (●) 3.0 g dm−3 NaCl (▲) and 6.0 g dm−3 NaCl (▼).
was also performed using Ti–RuO2 at different current densities. The maximum removal values were 31% (1380 mg dm−3 of urea), 33% (1350 mg dm−3 of urea) and 35% (1300 mg dm−3 of urea) at 20, 10 and 5 mA cm− 2 respectively. The TOC values remained practically unchanged during the tests and a slight generation of inorganic compounds was found at the end of the tests (TIC between 1 and − 2.7 mg dm−3). In addition, negligible amounts of NH+ 4 and NO3 were generated when the Ti–RuO2 electrode was used. This small amount of inorganic by-products at the end of the treatment is consistent with the fact that urea is mainly oxidized to other organic compounds and it is only partially decomposed. Various authors [3,14,19] have reported that the addition of chloride ions to the electrolyte causes an increase in the removal efficiency, and that the complete degradation of urea can be obtained by using active chlorine, in the form of chlorine, hypochlorous acid and hypochlorite, formed on the anode surface. Some tests were performed to investigate the effect of chloride ions on the oxidation of urea using electrolysis with the addition of 1.5 g dm− 3, 3.0 g dm− 3 and 6.0 g dm− 3 of NaCl to the electrolyte. The results are shown in Fig. 5. When a Pt anode was used, the addition of NaCl significantly improved the oxidation of urea, and enabled the complete removal in the presence of 6 g dm− 3 of NaCl, mainly due to the mediation of the active chlorine. The presence of NaCl also favoured the TOC removal rate. The amount of TOC after the treatment was 88%, 95% and 99%, for 1.5, 3 and 6 g dm− 3 NaCl, respectively. The almost complete removal of urea and TOC at the end of the test indicates that urea is completely mineralised in the presence of chlorides. The results agree with experiments carried out for Simka and Piotrowski [3].
Even in the case of the tests carried out in the presence of NaCl, the final amount of ammonia and nitrate ions was checked. Fig. 5C shows that the amount of NH+ 4 at the beginning of the electrolysis was higher in the presence of NaCl, until about 4 Ah dm− 3 and then a sharp decrease occurred and a value close to 0 was reached at the end in the presence of NaCl (3 and 6 g dm−3), a value which cannot be attained in the absence of NaCl. This may be due to reactions between the ammonium ion and hypochlorous acid: þ
þ
−
2NH4 þ 3HClO→N2 þ 3H2 O þ 5H þ 3Cl :
ð4Þ
Nitrate generation was significantly reduced with the addition of NaCl, thus indicating the formation of molecular nitrogen N2.
4. Conclusions In this paper, the electrochemical oxidation of urea has been studied on different anode materials: Pt, Ti–Ru oxide, BDD and antimony-doped tin oxide. Cyclic voltammetry measurements have shown that when Pt and Ti–Ru oxides are used, the presence of urea causes a decrease in the current density in the oxygen evolution region. This behaviour can be attributed to the blockage of the electrode active oxygen evolution sites due to urea adsorption. It has also been shown that the adsorption–desorption of urea on a Pt anode is a reversible process. On the contrary, the presence of urea on the BDD and antimonydoped tin oxide resulted in an increase in the current density at a given potential in the oxygen evolution region, and this indicated that urea was oxidized by •OH electrogenerated during the oxygen
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evolution. No evidence of adsorption phenomena was found on BDD or on the antimony-doped tin oxide electrodes. Bulk electrolysis in the potential oxygen evolution region has shown that decomposition of the total urea was only reached for the BDD electrode. The Pt, the Ti–Ru oxide, and the antimony-doped tin oxide led to a removal of 75%, 60% and 85% of urea, respectively. When the BDD anode was used, the urea was completely mineralized. In addition to the high effectiveness in urea abatement, the test carried out using BDD showed that important differences in nitrogen formation have been shown from a comparison of the tests at different current densities. At high current the urea was oxidized to NO3- due to the excess of elecrons, while at lowerdensities was favoured the N2 and NH+ 4 generation. The presence of sodium chloride in the electrolyte has a positive impact on the abatement when the Pt anode is used, as it increases up to 40%. This behaviour can be attributed to a direct electrooxidation on the active surface of the electrodes, as well as to the indirect oxidation reactions that occur in the presence of chloride ions, which are also effective in decreasing the concentration of ammonium and nitrate at the end of the process. Prime novelty statement Bulk electrolysis in the potential oxygen evolution region has shown that decomposition of the total urea was only reached for the BDD electrode. In addition to the high effectiveness in urea abatement, the test carried out using BDD showed that important differences in nitrogen formation have been shown from a comparison of the tests at different current densities. References [1] W. Zaborska, B. Krajewska, M. Leszko, Z. Olech, Inhibition of urease by Ni2+ ions. Analysis of reaction progress curves, J. Mol. Catal. B: Enzym. 13 (2001) 103–108. [2] R. Hüttl, K. Bohmhammel, G. Wolf, R. Oehmgen, Calorimetric investigations into enzymatic urea hydrolysis, Thermochim. Acta 250 (1995) 1–12. [3] W. Simka, J. Piotrowski, Methods for removal of urea from aqueous solutions, Przem. Chem. 86 (2007) 841. [4] K. Van Hege, M. Verhaege, W. Verstraete, Indirect electrochemical oxidation of reverse osmosis membrane concentrates at boron-doped diamond electrodes, Electrochem. Commun. 4 (2002) 296–300. [5] K. Van Hege, M. Verhaege, W. Verstraete, Electro-oxidative abatement of lowsalinity reverse osmosis membrane concentrates, Water Res. 38 (2004) 1550–1558. [6] V. Amstutz, A. Katsaounis, A. Kapalka, C. Comninellis, K.M. Udert, Effects of carbonate on the electrolytic removal of ammonia and urea from urine with thermally prepared IrO2 electrodes, J. Appl. Electrochem. 42 (2012) 787–795. [7] C.G. Alfafara, T. Kawamori, N. Nomura, M. Kiuchi, M. Matsumura, Electrolytic removal of ammonia from brine wastewater: scale-up, operation and pilot-scale evaluation, J. Chem. Technol. Biotechnol. 79 (2004) 291–298. [8] L. Li, Y. Liu, Ammonia removal in electrochemical oxidation: mechanism and pseudo-kinetics, J. Hazard. Mater. 161 (2009) 1010–1016. [9] A. Kapaka, A. Cally, S. Neodo, C. Comninellis, M. Wachter, K.M. Udert, Electrochemical behavior of ammonia at Ni/Ni(OH)2 electrode, Electrochem. Commun. 12 (2010) 18–21. [10] A. Kapaka, S. Fierro, Z. Frontistis, A. Katsaounis, O. Frey, M. Koudelka, C. Comninellis, K.M. Udert, Electrochemical behaviour of ammonia (NH+ 4 /NH 3 ) on electrochemically grown anodic iridium oxide film (AIROF) electrode, Electrochem. Commun. 11 (2009) 1590–1592.
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