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Ind. Eng. Chem. Res. 2008, 47, 8550–8554
Reduction of Nitrite by Ultrasound-Dispersed Nanoscale Zero-Valent Iron Particles Feng Liang,† Jing Fan,*,† Yanhui Guo,† Maohong Fan,‡ Jianji Wang,† and Hongqun Yang*,§ School of Chemical and EnVironmental Sciences, Henan Key Laboratory for EnVironmental Pollution Control, Henan Normal UniVersity, Xinxiang, Henan 453007, P. R. China, School of Materials Science and Technology, Georgia Institute of Technology, Atlanta, Georgia 30332, and Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta T6G 2G6, Canada
This research focuses on the removal of nitrite by ultrasound-dispersed nanoscale zerovalent iron (NZVI) particles. The factors affecting the removal of nitrite, namely, the length of ultrasonication time, the dosage of NZVI, the initial nitrite concentration, the temperature, and the solution pH, were investigated. Kinetics studies revealed that the denitrification process is a pseudo-first-order reaction with respect to the concentration of nitrite under the given experimental conditions. The derived activation energy of NZVI-based nitrite reduction is 31.44 kJ · mol-1. 1. Introduction Nitrite is a serious contaminant in bodies of water. As an intermediate compound in nitrogen circulation, nitrite usually comes from the oxidation of NH3 or the reduction of NO3- in water. It has long been reported that nitrite in bodies of water can impose direct or indirect threats to human, animals, and plants. Nitrite can combine with hemoglobin in the blood to form methemoglobin leading to methemoglobinemia (blue baby syndrome).1 It can cause cancer;2 for instance, the nitrosamines formed through the reaction between nitrite and secondary and tertiary amines may result in stomach cancer.3 The regulated maximum contaminant levels (MCLs) of nitrite in the world are strict, although they vary from one country to another. The current MCL of nitrite for drinking water is 1 mg · L-1 in the U.S. and 0.5 mg · L-1 in the European Union.4,5 Meeting the nitrite standard requirements for drinking water is a challenging issue in many countries and districts because of the rapid development of their industries. Therefore, the development of cost-effective nitrite removal technologies has become increasingly important. Zero-valent iron (ZVI) is an effective reductant and catalyst for various applications in environmental remediation. Especially in recent years, people have become increasingly interested in using nanoscale ZVI (NZVI) for the removal of different contaminants in water such as halogenated organic compounds,6-11 heavy metals,12-16 and oxo anions17-20 because of the drawbacks of using microscale ZVI (MZVI), including low pH requirements and low nitrate removal efficiency. However, studies on the removal of nitrite using NZVI have rarely been reported, even though some research has been done on the reduction of nitrite by MZVI.21-24 Therefore, this study was focused on the removal of nitrite with NZVI. Furthermore, ultrasonication was employed to improve the heterogeneous reaction between nitrite and NZVI. Both the factors affecting the removal of nitrite and the reaction kinetics of the denitrification process were studied, in an effort to provide the basis * To whom correspondence should be addressed. Tel.: +86-3733326335(J.F.), +1-780-492-8481(H.Y.). Fax: +86-373-3326445 (J.F.), +1-780-492-2881 (H.Y.). E-mail:
[email protected] (J.F.),
[email protected] (H.Y.) † Henan Normal University. ‡ Georgia Institute of Technology. § University of Alberta.
for the design of a larger-scale demonstration of the ultrasounddispersed NZVI-based nitrite removal technology. 2. Materials and Methods 2.1. Chemicals and Solutions. Sodium borohydride (NaBH4, >98%) and sodium nitrite (NaNO2, >99%) were purchased from Sinopharm Chemical Reagent Co. Ultrapure argon was ordered from Praxair Inc. N-(1-Naphthyl) ethylenediamine hydrochloride (C12H16Cl2N2, >98.5%) and ferric chloride (FeCl3 · 6H2O, >97%) were supplied by Shanghai Chemical Co. All other chemicals used in this work were analytical reagent grade and were used as received. Freshly deionized water was used to prepare all solutions and conduct all of the tests. Stock nitrite solutions (250 mg · L-1 N-NO2-) were prepared by first dissolving 1.232 g of sodium nitrite in 1 L of deoxygenated deionized (DODI) water and then diluting the resulting solution with DODI water. 2.2. Preparation and Characterization of NZVI Particles. The NZVI particles were synthesized by using the well-known liquid-phase reduction method 25,26 based on the following reaction Fe(H2O)63+ + 3BH4- + 3H2O f Fe0V + 3B(OH)3 +
21 H 2 2 (R1)
All tests were carried out under argon atmosphere, and all solvents were degassed and saturated with argon before use. The first step in preparing NZVI was the addition of 1.6 M NaBH4 solution to an equal volume of 1 M FeCl3 · 6H2O solution, followed by vigorous stirring of the resulting solution at ambient temperature. Then, Fe3+ was reduced by BH4- to form black NZVI particles. Stirring was continued for 20 min after the addition of NaBH4 solution. Then, the freshly prepared NZVI particles were separated by vacuum pump and then washed sequentially with 1 M HCl solution, degassed and deionized water, and then 1:1 (v/v) ethanol/acetone. The resulting NZVI particles were dried under a vacuum at 60 °C for 2 h and then sealed in a brown bottle full of argon for the subsequent nitrite reduction tests. The size distribution of the prepared NZVI particles was analyzed using a JEM 100CX-II transmission electron microscope with ethanol as the dispersant. The crystal structure of
10.1021/ie8003946 CCC: $40.75 2008 American Chemical Society Published on Web 10/17/2008
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Figure 1. TEM image of the prepared NZVI particles (20 000× magnification).
Figure 3. Effect of ultrasonication time on the reduction of nitrite (initial nitrite concentration, 10 mg N · L-1; initial dosage of NZVI, 0.6 g · L-1; temperature, 25 °C; pH, 6.0; ultrasonication power, 80 W).
Figure 2. XRD pattern of NZVI (current, 20 mA; voltage, 40 kV).
the iron particles was determined using a Bruker D8-Advance X-ray diffractometer. A Cu KR radiation source was used in the X-ray tests. 2.3. Reduction of Nitrite. All nitrite reduction tests were carried out in glass serum bottles. To start the nitrite reduction, 250 mL of prepared aqueous nitrite solution purged with argon was first placed in a clean glass serum bottle, and then a predetermined amount of freshly prepared NZVI was added. Then, the bottle was sealed immediately and ultrasonicated with a Shumei KQ2200B ultrasonicator for 5 min unless otherwise specified, after which it was placed in a Shanhai Yaojin THZ82A rotary shaker with a rotating rate of 200 rpm as the reduction of nitrite proceeded. Samples were removed within an argon environment using a glass syringe after a predetermined period of reaction and filtered with a 0.45-µm-pore-size Millipore filter. Then, the collected filtrates were immediately analyzed for their concentrations of nitrite and ammonium27,28 with a Lengguang 752S UV-vis spectrophotometer. The pH of the reaction system during nitrite reduction was monitored with a Dongxing PHS-3C pH meter. In addition, temperature control of the nitrite reduction was realized using a Chongqing model 501 thermostat bath. 3. Results and Discussion 3.1. Characteristics of the Synthesized NZVI Particles. A transmission electron microscopy (TEM) image of the particles is presented in Figure 1. The results showed that the synthesized NZVI particles were nearly spherical with a size range of 20-80 nm. An X-ray diffraction (XRD) spectrum of the NZVI particles is shown in Figure 2. The peak distribution indicates that the prepared NZVI particles had a poor crystal structure. 3.2. Factors Affecting the Reduction of Nitrite. 3.2.1. Ultrasonication Time. The reductions of nitrite were conducted with and without NZVI within the given ultrasonication environment for different periods of time, and the results
are shown in Figure 3. Standard errors for each set of data are also plotted. Figure 3 indicates that ultrasonication itself could not cause any reduction of nitrite because no nitrite was reduced after 60 min of ultrasonication when NZVI was not present. The longer the duration of ultrasonication of the reaction system under otherwise constant reaction conditions, the higher the reduction efficiency of nitrite, which resulted from the better dispersion effect of longer ultrasonication on the otherwise easily aggregating NZVI. For instance, the nitrite removal efficiency achieved with a reaction system subjected to 15 min of ultrasonication was at least 3.5 times higher than that without any ultrasonication in the first 10 min of reaction. The acoustic cavitation generated by ultrasound through the formation, growth, and implosive collapse of bubbles in liquids played an important role in the improvement of nitrite reduction by NZVI. The implosive collapse of bubbles creates localized shortlifetime hot spots with high pressure through adiabatic compression or shock-wave formation within the gas phase of the collapsing bubbles. These transient, localized hot spots can significantly improve the mixing and dispersion of NZVI and, thus, enormously accelerate the heterogeneous reaction between NZVI and nitrite, as they do for other reported reactions and processes.29,30 Based on the aforementioned test results and the actual design of the study, 5 min of ultrasonication was chosen for conducting the tests of this work. 3.2.2. Initial Concentration of Nitrite. The reduction of nitrite with different initial nitrite concentrations (namely, 1.0, 5.0, 10, and 25 mg N · L-1) by 1 g · L-1 NZVI was investigated, and the results are shown in Figure 4, in which the removal efficiency of nitrite is defined as removal efficiency )
C0 - C × 100% C0
(1)
where C0 and C represent the concentrations of nitrite at reaction times 0 and t, respectively. Our preliminary tests indicate that the reduction of NZVI is unsatisfactorily slow from the perspective of its potential application in actual industrial wastewater treatment when the NZVI dosage is lower than 1 g · L-1. On the other hand, the reaction is too fast to be measured accurately for this study if the NZVI dosage is larger than 1 g · L-1. Therefore, 1 g · L-1
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Figure 4. Effect of initial nitrite concentration on the removal efficiency (dosage of NZVI, 1.0 g · L-1; temperature, 25 °C; pH, 6.0; ultrasonication time, 5 min).
Figure 6. Linear relationship between ln kobs and NZVI dosage (initial nitrite concentration, 10 mg N · L-1; initial dosage of NZVI, 1.0 g · L-1; temperature, 25 °C; pH, 6.0; ultrasonication time, 5 min).
Figure 5. Effect of different NZVI dosages on the reduction of nitrite (initial nitrite concentration, 10 mg N · L-1; temperature, 25 °C; pH, 6.0; ultrasonication time, 5 min).
Figure 7. Relationship between ln C0/C and reaction time at initial different pHs (initial nitrite concentration, 10 mg N · L-1; initial dosage of NZVI, 1.0 g · L-1; temperature, 25 °C; ultrasonication time, 5 min).
was chosen to be the NZVI dosage for the study of the effects of different initial nitrite concentrations on the reduction of nitrite. Standard error bars are included in Figure 4. It can be seen that the removal efficiency of nitrite decreased with increasing initial nitrite concentration for otherwise constant reaction conditions. This can be easily understood because increasing the initial nitrite concentration at the same dosage of NZVI is equivalent to decreasing the molar ratio of NZVI to nitrite in the reaction system or the contact of NZVI per unit of nitrite, thus decreasing the efficiency of nitrite removal. 3.2.3. Dosage of NZVI. The changes of nitrite concentration with reaction time under the five different NZVI dosages of 0.5, 0.7, 1.0, 1.3, and 1.5 g · L-1 are presented in Figure 5. Standard error bars are included. Figure 5 indicates that good linear relationships between ln(C0/C) and reaction time t (where C0 and C are the concentrations of nitrite at times 0 and t, respectively) exist because all of the derived correlation coefficients are above 0.95. The slopes in Figure 5 for NZVI dosages of 0.5, 0.7, 1.0, 1.3, and 1.5 g · L-1 were 0.0399, 0.0461, 0.0878,
0.1206, and 0.1349 min-1, respectively. The change of slope, s, with the dosage of NZVI reflects that the increase of the initial concentration of NZVI enhances the removal of nitrite, which can be attributed to the increase of the surface area of NZVI accessible per unit of nitrite. Further analyses are shown in Figure 6 along with standard error bars. Figure 6 indicates that the correlation between the values of ln s and dosages of NZVI, X, can be expressed with the linear equation ln s ) 1.3194X - 3.8894 (2) 3.2.4. pH. Nitrite solutions with different initial pH values of 5, 6, and 7 were used to investigate the effect of the initial solution pH on nitrite reduction by NZVI. The test results are plotted in Figure 7. The good linear relationship between ln(C0/ C) and t shown in Figure 7 indicates that the nitrite removal by NZVI in the given pH range is a pseudo-first-order reaction with respect to nitrite. Yang et al.31 reported that nitrate was more easily reduced by NZVI at lower pH than at higher pH because the reduction of nitrate by NZVI is driven by H+ ions in solution. In Figure 7, standard error bars are included.
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Figure 8. Effect of temperature on reduction rate of nitrite by NZVI (initial nitrite concentration, 10 mg N · L-1; initial dosage of NZVI, 1.0 g · L-1; pH, 6.0; ultrasonication time, 5 min).
Figure 10. Nitrogen mass balance of the sum of nitrite and ammonia/ ammonium (initial nitrite concentration, 10 mg N · L-1; initial dosage of NZVI, 1.0 g · L-1; temperature, 25 °C; pH, 6.0; ultrasonication time, 5 min).
where Ea is the activation energy (kJ · mol-1) for the reaction, which can be obtained from the slope of the linear plot between ln(kobs) and 1/T. Based on the regression equation obtained from Figure 9 ln kobs ) -3782.1/T + 10.178
(4)
the calculated Ea of nitrite reduction by NZVI is 31.44 kJ · mol-1. 3.3. Reduction Mechanism. Analyses demonstrated that ammonium was the major product of nitrite reduction. Nammonium accounted for more than 90% of total converted N-nitrite, as indicated in Figure 10. Standard error bars are also included. The conversion of NO2- to NH4+ can be expressed as 3Fe0 + NO2- + 8H+ f 3Fe2+ + NH4+ + 2H2O Figure 9. Relationship between ln kobs and 1/T (initial nitrite concentration, 10 mg N · L-1; initial dosage of NZVI, 1.0 g · L-1; pH, 6.0; ultrasonication time, 5 min).
Although the results are statistically different, initial solution pH does not appear to have a strong effect on the nitrite reduction rate. The reduction rate is satisfying at neutral conditions. The implication of this result is that the application of this technique does not require a harsh low-pH environment and allows a wider range of pH. 3.2.5. Temperature. Batch experiments at 0, 10, 20, and 30 °C were conducted to study the effect of temperature on the reduction of nitrite by NZVI at an initial nitrite concentration of 10 mg N · L-1 and an NZVI dosage of 1 g · L-1. The obtained results are presented in Figure 8; standard error bars are also included. The data points for 10 °C at times 20, 30, and 40 min show larger uncertainty. This could be attributed to some unexpected experimental disturbances. The relationship of ln(C0/ C) to t at different temperatures appears fit the pseudo-firstorder kinetics model, and the reaction rate constants increase with increasing temperature. The linear relationship between ln(kobs) and 1/T is shown in Figure 9. Standard error bars are also included. The temperature dependency of kobs indicated in Figure 9 is consistent with the Arrhenius equation kobs ) A exp(-Ea/RT)
(3)
(R2)
The slight difference between the initial concentration of N-nitrite and the sum of the concentrations of remaining N-nitrite and N-ammonium shown in Figure 10 might be due to the generation of a small unmeasured amount of nitrogen gas through the reaction 3Fe0 + 2NO2- + 8H+ f 3Fe2+ + N2 + 4H2O
(R3)
and the adsorption of unmeasured nitrite on the surface of the Fe(OH)2 resulting from the reaction 3Fe0 + NO2- + 6H2O f 3Fe(OH)2 + NH4+ + 2OH- (R4) 4. Conclusions NZVI particles prepared with a liquid-phase reduction method are nearly spherical with a size range of 20-80 nm and exhibit poor crystal structures. Reduction test results indicate that ultrasonication can accelerate the reduction of nitrite with NZVI. NZVI is much more efficient than microscale ZVI for nitrite reduction, and its denitrification process follows a pseudo-firstorder kinetic model with respect to nitrite concentration. The efficiencies of nitrite reduction with NZVI are affected by the initial nitrite concentration, the NZVI dosage, the reaction solution temperature, and the pH. Ammonia was found to be the primary product of the nitrite reactions. However, other
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ReceiVed for reView March 10, 2008 ReVised manuscript receiVed August 12, 2008 Accepted September 2, 2008 IE8003946
