Environ. Sci. Technol. 2008, 42, 8070–8075
Removal of Ammonia by OH Radical in Aqueous Phase LI HUANG, LIANG LI,† WENBO DONG, YAN LIU,* AND HUIQI HOU Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China
Received March 23, 2008. Revised manuscript received May 29, 2008. Accepted July 21, 2008.
Many advanced oxidation technologies have been developed to remove ammonia in wastewater. All these technologies have one common characteristic, that is, the removal processes involve OH radical (•OH). In this research work, H2O2 was selected as •OH precursor. The removal of ammonia under 253.7 nm irradiation from low-pressure mercury lamp in the presence of H2O2 was studied to investigate the ammonia removal efficiency by •OH. Results show that the •OH, generated by H2O2 photolysis, could oxidize NH3 to NO2 and further to NO3 . Removal efficiencies of ammonia were low and were affected by initial pH value and ammonia concentration. Laser flash photolysis technique with transient absorption spectra of nanosecond was used to investigate the oxidation pathway and kinetics of ammonia oxidation by •OH. Results illustrate that •OH could oxidize NH3 to form •NH2 with a second-order rate constant of (1.0 ( 0.1) × 108 M-1 s-1 (20 °C). •NH2, the main product of •OH with NH3, would further react with H2O2 to yield •NHOH. Since •NHOH could not stay stable in solution, it would rapidly convert to NH2O2 and consequently NO2 and . The rate constants for these elementary reactions NO3 were also given. The low removal efficiency of ammonia by •OH was mainly due to the slow reaction rate constant.
1. Introduction Ammonia pollution has become a major environment issue these years. The removal of ammonia has attracted a great attention because it can cause eutrophication, give offensive odors, and hinder the disinfection of water supplies. Though several methods have been developed to remove ammonia from wastewater, including biological nitrification, stripping, ion exchange, break-point chlorination, and chemical precipitation (1, 2), each of these methods has disadvantages. Recently, with the development of advanced oxidation processes (AOPs), several researchers took the initiative in applying such techniques in ammonia removal. For example, direct photooxidation (3), ozone oxidation (4, 5), photocatalytic oxidation (6-9), and electrochemical oxidation (10, 11) have all been tried. Wang et al. (3) pointed out that NH3 could undergo direct oxidation under UV irradiation without additives in basic solution while its oxidation must occur in the presence of Fe/TiO2 in acid solution. The pH value, TiO2 amount and inorganic anion concentration were found to have remarkable influence on the removal efficiency of ammonia (9, 12). Since the year of 2000, some researchers * Corresponding author phone: 86-21-6564-3348; fax: 86-21-65643597; e-mail:
[email protected]. † Current address: Civil and Environmental Engineering, University of Utah, 122 South Central Campus Drive, Salt Lake City, Utah 84112. 8070
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have tried selective photocatalytic oxidation of NH3 to N2 in solution by using metallized TiO2 (e.g., Pt, Pd) (13, 14). Moreover, Liu et al. (15) have successfully realized coupled ammonia removal with coexisted nitrite/nitrate via Ag/TiO2 photocatalysis. Most of these work mainly focused on the removal efficiency and oxidation byproducts of ammonia by AOPs. As a matter of fact, one common characteristic is that all AOPs produce strong oxidative radical •OH. However, little study was reported to investigate the mechanism of ammonia oxidation by •OH. Even though some researchers tried to present the reaction mechanism, it was proposed without direct evidence. As a result, ammonia removal mechanism was still quite plausible. For instance, Nemoto et al. (16) proposed that photocatalytic removal of ammonia should be attributed to the hole (h+) in the valence band, whereas Lee et al. (14) speculated that •OH also contributed to the removal process. One possible reason for the poor understanding of ammonia oxidation mechanism was that the lifetime of reaction intermediates (generally on the time scale of 10-6 s) was too short to be detected by usual methods. Laser flash photolysis technique provides one of the most effective methods by which intermediates with ∼10-8 s lifetime could be observed directly (17, 18). Nadtochenko and Kiwi (19) and Pignatello et al. (20) have successfully explained the primary photochemical reactions in the photo-Fenton system by using this technique. We have already studied the oxidation mechanism of benzene and its derivates by •OH using laser flash photolysis technique (21, 22). Such technique might help for a better understanding of the removal pathway for ammonia. In addition, the rate constants for each elementary reaction could be derived at the same time. H2O2 was well-known as a precursor of •OH under UV illumination. Therefore, low pressure Hg lamp was used in this study to evaluate the possibility of ammonia removal by •OH. The removal efficiency was systematically determined as a function of irradiation time, pH value, and ammonia initial concentration. Meanwhile, the intermediates in the mixed solution of ammonia and H2O2 after irradiation was studied using laser flash photolysis technique to obtain the detailed removal pathway and reaction kinetics.
2. Experimental Section 2.1. Materials. In this study, ammonia solution was prepared by adding (NH4)2SO4 into tridistilled water. H2SO4 and NaOH were used to adjust the pH of the sample. All the chemicals were of analytical grade. 2.2. Photolysis Setup and Analysis Method. The photooxidation process of ammonia by •OH and the dependence of removal efficiency on reaction conditions were studied in a 1.0 L reaction cell. A low-pressure mercury lamp (Shanghai Rongbo Co., 20 W), emitting 253.7 nm irradiation, was employed as the light source. To avoid the volatilization of ammonia, the reaction cell was filled to the top to minimize headspace and tightly sealed by a Teflon lid. Figure S1 of the Supporting Information depicts the steady-state photolysis setup for the study of ammonia photooxidation by •OH. Control experiments confirmed that the small amount of headspace present in the reaction cell did not affect measured concentration of ammonia within the time scale studied. After the solution containing 1010 mg-N/L ammonia at pH 9.3 was placed in this reactor for 5 h in dark, its concentration would remain at its initial level (1008 ( 4 mg-N/L) (as shown in Figure S2 of the Supporting Information). During the irradiation, the solution was stirred with a magnetic stirrer at a constant speed. The temperature of the solution was controlled at (20 ( 1) °C by circling cooling water around the 10.1021/es8008216 CCC: $40.75
2008 American Chemical Society
Published on Web 09/27/2008
FIGURE 1. A sketch of the laser flash photolysis apparatus. (A) Laser generator; (B) Transient absorption spectra device; (C) Work station. (1) laser beam steering assembly; (2) spectrometor control unit; (3) xenon lamp; (4) cut-off; (5) lens; (6) sample cell; (7) monochromator; (8) photomultiplier; (9) digital oscilloscope. reactor. At regular time intervals, a 10 mL sample aliquot was withdrawn from the reactor for analysis. The concentration of residue ammonia, NO2 , and NO3 were analyzed spectrophotometrically according to Standard Methods (23, 24) (see Table S1 of the Supporting Information for a detailed description). The mechanism and kinetics for photooxidation of ammonia by •OH was investigated by nanosecond laser flash photolysis technique (see Figure 1). H2O2 was also used to produce •OH in this set of experiments. The apparatus for laser flash photolysis study has been described in detail previously (25). In brief, the output from a Quanta-Ray GCR150 Nd:YAG laser operating at its fourth harmonic mode (266 nm, laser energy 16.5 mJ/pulse, laser beam cross section 0.28 cm2, pulse duration 8∼10 ns) was used to initiate the photochemical reaction. After laser pulse, the emission from xenon lamp was increased by 100 times and entered the quartz cuvette at a vertical angle to the laser beam. Therefore, the kinetics curves at different wavelengths and the timeresolved UV-vis absorption spectra of the solution could be recorded. Relative rate technique was used to determine the second-order rate constant for reaction between NH3 and •OH. The reaction between •OH and carbonate was selected as reference reaction. CO•3 , the product of •OH with carbonate, was monitored at 600 nm. Other reaction rate constants were determined by Pro-Kineticist software (Applied Photophysics, UK).
3. Results and Discussion 3.1. The Photooxidation Efficiency of Ammonia by •OH. 3.1.1. Influence of pH. Table S2 of the Supporting Information details the influence of pH value on ammonia photooxidation. At initial pH of 2.0 (Runs 1, 2, and 3), no removal of ammonia was detected no matter its initial concentration or irradiation time. The concentration of NO2 and NO3 were less than 0.01 mg-N/L and 0.1 mg-N/L, respectively, all the time. The concentration of total nitrogen was always identical to its initial value. A fraction of ammonia (
