Investigation on Direct and Indirect Electrochemical Oxidation of

Article pubs.acs.org/IECR

Investigation on Direct and Indirect Electrochemical Oxidation of Ammonia over Ru−Ir/TiO2 Anode Shilong He,*,†,‡ Qing Huang,† Yong Zhang,§ Lizhang Wang,† and Yulun Nie*,‡ †

School of Environment and Spatial Informatics, China University of Mining & Technology, Xuzhou 221116, China Graduate School of Environmental Studies, Tohoku University, 6-6-06 Aza-Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan § Research Center for Ecoenvironmental Sciences in Shanxi, Taiyuan 030009, China ‡

ABSTRACT: Commercial Ru−Ir/TiO2 was used as the anode for electrochemical oxidation of ammonia at neutral pH in a continuous electrochemical quadrangular reactor. The reactor was packed with PAC to avoid short circuit and enhance electric efficiency, which could efficiently enhance the direct eletrooxidation. The contribution of indirect oxidation caused by active free chlorine to ammonia removal increased with the increase of chloride concentration. Moreover, the effects of inlet velocity, salinity, and current density on ammonia removal were also investigated in detail. On the basis of the CV scan and COD removal results, both direct and indirect oxidations were involved in ammonia removal. The results indicated that ammonia removal by direct oxidation occurs at a slower rate than that of COD, while indirect oxidation prefers removal of ammonia than that of COD. Ammonia could be oxidized by either •OH radicals or active free chlorine, and the removal of COD only comes from the oxidation by •OH radicals. Eighty percent of ammonia could be removed and mainly transferred to N2 in a PAC packed bed reactor under optimum conditions (pH = 6.5, I = 0.9 A, 2% Na2SO4, Cl− = 1500 mg/L, and inlet velocity = 0.8 L/h). Hence, the Ru−Ir/TiO2-based PAC packed bed reactor provides an alternative for treatment of ammonia wastewater with high chloride concentration.

1. INTRODUCTION Ammonia from the animal agricultural operation and industrial production is present in variable concentrations in surface and groundwater.1 In marine environments, the safe level of ammonia is below 1 mg/L and over this high amount of ammonia would result in eutrophication and severe subsequent environmental effects, such as anoxia or reduction in the population of aquatic life.2−4 The presence of ammonia could reduce the efficiency of common water purification methods, such as chlorination and ozonation.2,5 Moreover, ammonia is also a major source of undesirable odor in sewage and wastewater.4 Conventional methods of ammonia removal from water include adsorption,6 ion exchange,7 stripping,8 breakpoint chlorination,9 chemical precipitation,10 biological denitrification, and other oxidation methods.11,12 However, each of these methods has its own limitation. For example, the stripping method may create additional air pollution when ammonia is converted from liquid to gas phase, and its efficiency is also greatly reduced at a lower ammonia concentration,13 while the biological denitrification process contains a series of reactors for nitrification, denitrification, BOD decomposition, and solid− liquid separation.14 Anode reaction

energy supply free of COx and environmental protection. In general, ammonia can be transformed to other nitrogen forms by direct anodic oxidation or indirect oxidation during the electrochemical process. The direct oxidation of ammonia (eqs 1−3) at a reasonable reaction rate,15 if possible, gives many advantages. However, the large-scale applications have been strongly hindered by the insufficient performance and high cost of the electrodes. In an indirect oxidation process, strong oxidants (HClO and •OH) are first produced in the bulk of solution via electrochemical reactions, and then these oxidants destroy the ammonia by oxidation reactions.16 The reported active components of electrodes include (1) pure noble metal, such as Pt, (2) Pt-based alloy with Ir, Ru, and Rh, and (3) Ptfree metal oxide, such as Ni/Ni(OH)2, IrO2, and RuO2.17 Despite the high catalytic activities of Pt and Pt−matrix binary alloys for electrochemical oxidation of ammonia, the nonaffordability of these electrodes limits their application at an industrial scale. Hence, the Pt-free electrodes provide an alternative for accelerating its application on a technological scale. In this manuscript, Ru−Ir/TiO2 was used as anode, and little is known about the electrochemical oxidation of ammonia over Ru−Ir/TiO2 under continuous flow conditions. It was reported that the electrochemical oxidation of ammonia was strongly pH dependent and proceeded mainly at a pH above 7.17 Hence, the Ru−Ir/TiO2 performance was evaluated in our study by ammonia removal via direct and indirect electrochemical

NH3(aq) + 6OH− → N2 + 6H 2O + 6e− (1)

Cathode reaction Overall reaction

6H 2O + 6e− → 3H 2 + 6OH− 2NH3(aq) → N2 + 3H 2

(2)

Received: Revised: Accepted: Published:

(3)

Electrochemical oxidation of ammonia has attracted much attention in recent years since it addresses both the clean © 2015 American Chemical Society

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September 28, 2014 December 17, 2014 January 22, 2015 January 22, 2015 DOI: 10.1021/ie503832t Ind. Eng. Chem. Res. 2015, 54, 1447−1451

Article

Industrial & Engineering Chemistry Research oxidation at neutral pH. The generation of NO2− and NO3− as a result of ammonia oxidation was also analyzed. Moreover, the effects of inlet velocity, current density, initial ammonia concentration, chloride concentration, and organics, etc., on the ammonia oxidation were discussed.

2. MATERIALS AND METHODS 2.1. Materials. (NH4)2SO4, Na2SO4, NaCl, and oxalic acid were purchased from Sinapharm Chemical Reagent Co., Ltd. and used without further purification. Powder activated carbon (PAC) was provided by Ningxia Taixi Carbon Corp., China. Prior to use, PAC was added into boiled water for 6 h and then rinsed for 6 h in 6 M H2SO4 solution, washed with water until the effluent was neutral, and dried at 105 °C for 24 h. Ru−Ir/ TiO2, a commercial anode, was used in this study, and titanium plate was used as cathode. An aqueous solution of ammonia was prepared by ultrapure water (18.2 MΩ·cm). The desired solution pH was adjusted with 1 M NaOH or 0.5 M H2SO4. 2.2. Experimental Setup. A continuous electrochemical quadrangular reactor as shown in Figure 1 was used in this

Figure 2. (A) Effects of PAC and TiO2 (0.3 g/L) on the direct electrooxidation of ammonia: (a) no PAC, (b) PAC, and (c) addition of TiO2 in PAC packed bed reactor. (B) Formation of nitrate and nitrite in PAC packed bed reactor. (Reaction conditions: [NH4+−N] = 100 mg/L, 2% Na2SO4, I = 0.9 A, inlet velocity = 0.8 L/h, pH = 6.5.)

2B). However, the ammonia oxidation was greatly inhibited due to the addition of TiO2 (curve c), which indicated that the amount of reactive oxygen species for ammonia oxidation was decreased. It was because as a typical semiconductor TiO2 could capture the electrons produced in the electrochemical process, leading to the decease of ammonia removal efficiency. The effects of inlet velocity and the current density and salinity on the removal efficiency of direct electrochemical oxidation were further investigated, and the results are shown in Figures 3 and 4. As shown in Figure 3 with a current density 0.6

Figure 1. Schematic diagram of the experimental apparatus for the electrooxidation of ammonia.

study with a total working volume of 500 mL. The outside of the glass cell was covered with a circulating water system in order to keep the temperature constant at 25 °C. The reactor was controlled by a dc power supply source KXN-1540D (Zhaoxin Instrument Corp. of Shenzhen). The size for the anode and cathode was 10 cm × 10 cm × 1 mm; hence, the surface of each electrode was 100 cm2. The ammonia solution without/with NaCl or oxalic acid was fed into the reactor continuously by the pump, and samples were taken from the reactor cell at given time intervals for analysis of NH4+, NO3−, NO2−, TN, and COD. 2.3. Analysis. The concentrations of residue ammonia and the formed NO2− and NO3− were analyzed spectrophotometrically according to standard methods.18 The COD was determined by closed reflux and colorimetric methods.19

Figure 3. Effect of inlet velocity on ammonia removal by direct oxidation with PAC: (a) 0.4, (b) 0.6, (c) 0.8, (d) 1.0, and (e) 1.2 L/h. (Reaction conditions: [NH4+−N] = 100 mg/L, 2% Na2SO4, I = 0.9 A, pH = 6.5.)

A and salinity of 1%, the ammonia removal rate increased with the increase of inflow rate from 0.4 to 0.8 L/h. However, when the inflow rate is larger than 0.8 L/h, such as 1.0 and 1.2 L/h, the removal efficiency was greatly inhibited. Hence, the optimal inflow rate of ammonia was 0.8 L/h. Figure 4A depicted the volt−ampere relation of Ru−Ir/TiO2 with different salinity. Obviously, the current increased almost linearly with the increase of voltage, and the electrical resistance (straight slope) of the packed bed reactor decreased with the increase of salinity, while the ammonia removal efficiency decreased significantly with the salinity and current density and reached a steady value (Figure 4B). The above results indicated that the increase of current density is not favorable to direct ammonia electrooxidation in high-salt medium. 3.2. Indirect Electrochemical Oxidation of Ammonia. Besides the contribution of direct electrooxidation to ammonia removal, indirect electrooxidation by active free chlorine

3. RESULTS AND DISCUSSION 3.1. Direct Electrochemical Oxidation of Ammonia. Figure 2A showed the effects of PAC and TiO2 on the direct electrooxidation of ammonia. Obviously, no significant ammonia was removed without the addition of PAC (curve a). A remarkable increase in the removal of ammonia (34.4%) was observed in the PAC packed bed reactor (curve b), and the contribution of PAC adsorption to ammonia removal could be ignored since there was almost no ammonia adsorption on PAC at neutral pHs. Moreover, the disappeared ammonia was mainly transferred to N2 and partially converted into various nitrogen compounds since the maximum concentration of formed nitrate and nitrite was only 1.43 and 0.31 mg/L (Figure 1448

DOI: 10.1021/ie503832t Ind. Eng. Chem. Res. 2015, 54, 1447−1451

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Industrial & Engineering Chemistry Research

Figure 4. Effects of current density and salinity on the ammonia removal efficiency by direct oxidation: (a) 1% and I = 0.6 A, (b) 2% and I = 0.9 A, (c) 3% and I = 1.2 A, (d) 4% and I = 1.5 A. (Reaction conditions: [NH4+−N] = 100 mg/L, inlet velocity = 0.8 L/h, pH = 6.5.)

generated from the chloride in solution should also be investigated. As shown in Figure 5, in comparison with direct

Figure 6. Effects of current density, chlorine concentration, and velocity on the ammonia oxidation efficiency by indirect electrooxidation. (Reaction conditions: [NH4+−N] = 100 mg/L, 2% Na2SO4, inlet velocity = 0.8 L/h, pH = 6.5.)

Figure 5. Ammonia removal and corresponding nitrate generation (inset) by indirect electrooxidation: (a) no PAC, (b) in PAC packed bed reactor with 500 mg/L Cl−, and (c) in PAC packed bed reactor with 1500 mg/L Cl−. (Reaction conditions: [NH4+−N] = 100 mg/L, 2% Na2SO4, I = 0.9 A, inlet velocity = 0.8 L/h, pH = 6.5.)

While the reaction rate constant was 0.311, 0.487, 0.684, and 0.884 mg/L·min, respectively when the current density was 5, 7.5, 10, and 12.5 mA/cm2, which indicated that the increase of current density could also result in the enhancement of ammonia removal. When the dosage of chloride increased from 100 to 700 mg/L, the corresponding reaction rate constant was 0.381, 0.606, 0.719, and 0.875 mg/L·min, respectively, and 60% of ammonia could be efficiently removed with the chloride concentration, 700 mg/L. The above results have proven that the electrochemical oxidation efficiency is limited under the given conditions (for example, at given chloride concentration), and more active free chlorine could be generated by increasing the current density and chloride concentration leading to more efficient removal of ammonia. 3.3. Discussion on Ammonia Removal Mechanism over Ru−Ir/TiO2. Figures 7 and 8 depict the evolution of COD (oxalic acid) and ammonia by direct and indirect electrooxidation in a packed bed reactor, respectively. Obviously, there was almost no influence on the COD removal in the presence of ammonia, even with variation of the chloride concentration (500−1500 mg/L). The results are consistent with those obtained by Cossu et al., who observed a negligible influence of the chloride concentration on the COD removal,20 while the ammonia oxidation was inhibited by 10% in direct electrooxidation due to the coexistence of COD (curve b in the inset of Figure 7). Moreover, the removal efficiency of ammonia was markedly enhanced by the increase of chloride

electrooxidation, the ammonia removal efficiency increased greatly by the addition of chloride whether PAC was present or not. Apparently, about 18.5% of ammonia could be oxidized without PAC, while the removal efficiency of ammonia was 46% in a PAC packed bed reactor. Moreover, the corresponding intermediate nitrate concentration also increased with reaction time (nitrite was not detected during the reaction), and the maximum concentration was 3.0 and 8.16 mg/L, respectively, which is also higher than that in the direct electrooxidation process. In view of the contribution of direct electrooxidation, the ammonia removal caused by active free chlorine was about 12−16% when the chloride concentration in solution was 500 mg/L. Nitrogen gas was still the main intermediate of indirect ammonia electrooxidation. The effects of initial ammonia concentration, current density, and chloride concentration on ammonia removal by indirect electrooxidation were further studied, and the results are shown in Figure 6. Complete removal could be achieved when the initial ammonia concentration was 50 mg/L, and the efficiency decreased greatly with the increase of initial ammonia concentration. The reaction rate constant was 0.782, 0.769, 0.7, and 0.594 mg/L·min, respectively when the initial ammonia concentration was 50, 100, 200, and 500 mg/L. 1449

DOI: 10.1021/ie503832t Ind. Eng. Chem. Res. 2015, 54, 1447−1451

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Industrial & Engineering Chemistry Research

Figure 9. Cyclic voltammetry scans at Ru−Ir/TiO2 electrode under various conditions: (a) direct oxidation without PAC, (b) direct oxidation in PAC packed bed reactor, (c) indirect oxidation without PAC, (d) indirect oxidation in PAC packed bed reactor. (Reaction conditions: [NH4+−N] = 100 mg/L, [Cl−] = 500 mg/L, 2% Na2SO4, I = 0.9 A, inlet velocity = 0.8 L/h, pH = 6.5.)

Figure 7. COD removal and ammonia oxidation (inset) by direct electrooxidation: (a) only ammonia and (b) ammonia and COD. (Reaction conditions: [NH4+−N] = 100 mg/L, [COD] = 700 mg/L, 2% Na2SO4, I = 0.9 A, inlet velocity = 0.8 L/h, pH = 6.5.)

oxygen evolution was then inhibited, and •OH radical’s generation was enhanced, which results in more efficient removal of ammonia. Compared with that in direct oxidation, the peak potential of oxygen evolution shifted to 1.4−1.6 V (curve c) after addition of NaCl could be attributed to oxidation of Cl− to Cl2 or active free chlorine. In particular, the peak for oxygen evolution almost disappeared in a PAC packed bed reactor (curve d), indicating that oxygen evolution was completely inhibited and PAC played an important role in the enhancement of electrochemical oxidation of ammonia.

4. CONCLUSIONS Ru−Ir/TiO2 used as anode was effective for ammonia removal by electrooxidation at neutral pH in a continuous electrochemical quadrangular reactor, and 80% of ammonia could be removed in a PAC packed bed reactor under optimum conditions (pH = 6.5, I = 0.9 A, 2% Na2SO4, Cl− = 1500 mg/L, and inlet velocity = 0.8 L/h). Moreover, the removal efficiency was significantly influenced by the inlet velocity, salinity, current density, and chloride concentration. Both direct and indirect oxidations were involved in ammonia removal. The results indicated that ammonia removal by direct oxidation occurs at a slower rate than that of COD, while indirect oxidation prefers removal of ammonia than that of COD. Ammonia could be oxidized by either •OH radicals or active free chlorine, and removal of COD only comes from oxidation by •OH radicals. A Ru−Ir/TiO2-based PAC packed bed reactor provides an alternative for treatment of ammonia wastewater with high chloride concentration.

Figure 8. COD removal and ammonia oxidation (inset) by indirect electrooxidation as a function of chloride concentration: (a) 0, (b) 500, (c) 1000, and (d) 1500 mg/L. (Reaction conditions: [NH4+−N] = 100 mg/L, [COD] = 700 mg/L, 2% Na2SO4, I = 0.9 A, inlet velocity = 0.8 L/h, pH = 6.5.)

concentration, and 80% of ammonia could be oxidized when the chloride concentration was 1500 mg/L. Since 46% of ammonia could be removed by direct electrooxidation in a PAC packed bed reactor, the contribution of indirect oxidation to ammonia removal was 34%. Moreover, ammonia removal occurs at a slower rate than that of COD in direct oxidation, while indirect oxidation prefers removal of ammonia than that of COD.19,21 It has been reported that the active free chlorine was the only reactive species in indirect oxidation,16 while •OH radicals generated from H2O2 decomposition were dominant species in direct oxidation.22 Hence, ammonia could be oxidized by either •OH radicals or active free chlorine, removal of COD only comes from the oxidation by •OH radicals, and the optimum conditions was as follows: pH = 6.5, I = 0.9 A, 2% Na2SO4, Cl− = 1500 mg/L, and inlet velocity = 0.8 L/h. As shown in CV scan curves of Figure 9, in direct oxidation, the peak at 0.6 V (curve a) could be assigned to the oxygen evolution from H2O2 decomposition23 and the peak at 0.9 V (curve b) may be attributed to the H2O2 decomposition into • OH radicals in a PAC packed bed reactor. It is because the PAC between the anode and the cathode could decrease the influence of short circuit and enhance electric efficiency. The

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-516-83591328. Fax.: +86-516-83591328. E-mail: [email protected]. *Tel.: +81-22-795-7205. Fax: +81-22-795-4856. E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest. 1450

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support by the Fundamental Research Funds for the Central Universities (2014QNA31).

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REFERENCES

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DOI: 10.1021/ie503832t Ind. Eng. Chem. Res. 2015, 54, 1447−1451