Download reaction kinetics and mechanistic studies of nitric oxide

REACTION KINETICS AND MECHANISTIC STUDIES OF NITRIC OXIDE
REMOVAL BY COMBINED PERSULFATE AND FERROUS-EDTA SYSTEMS
Md A. Khan and Yusuf G. Adewuyi, Chemical, Biological and Bioengineering Department, North
Carolina A&T State University, Greensboro, NC
Biography
Md Arif Khan obtained his BS in chemical engineering from Bangladesh University of
Engineering and Technology (BUET) in 2008. Arif is currently a Chemical Engineering graduate
student at North Carolina A&T State University. He completed his coursework in 2012, and is
working on his thesis entitled: “Chemical Reactive Separations in Energy and Environmental
Processes”, under the supervision of Dr. Adewuyi. He plans to pursue a Ph.D upon graduation.
Dr. Yusuf G (Debo) Adewuyi is a Professor of Chemical, Biological and Bioengineering
Department at North Carolina A&T State University, where his research is in sustainable
production of energy and chemical products; synthesis of nanoscale materials for energy and
environmental applications; sonochemistry, cavitation and advanced oxidation processes
(AOPs) for pollution treatment; liquid-phase chemistry and kinetics of NOx, SO2 and Hg
removal; and ultrasound-assisted biofuels synthesis and fuels desulfurization.
Summary
The removal of nitric oxide (NO) from simulated flue gas in a bubble column reactor at
atmospheric pressure is investigated using combined aqueous persulfate (Na2S2O8) and
ferrous ethylenediamine tetraacetic acid (FeII-EDTA) systems. The results show significant
improvement in NO removal compared with thermally and Fe2+ activated persulfate systems.
Almost 100% NO conversion can be achieved at 70 0C compared to temperature and Fe2+activated persulfate systems, which could attain 100% conversions only at ≥ 90 0C, suggesting
the possibility of significant reduction in energy costs. The pH for optimal NO removal was
found to be near neutral (~6.5). A mathematical model, based on proposed reaction pathways
and film theory of mass transfer, has been developed, and will be used to correlate the
experimental data and evaluate the reaction kinetic parameters and mass transfer coefficients.
Introduction
Nitrogen oxides (NOx – mainly NO2 and NO) and sulfur oxides (SOx – mainly SO2) are
the most prominent acid gases emitted from the burning of fossil fuel, especially from the coal
fired power plants, and are responsible for widespread problems of air pollution, health
hazards, acid rains etc [1, 2]. Of these oxides, NO2 and SO2 are very soluble in water and can
be separated easily from the exhaust stream by simple scrubbing, but nitric oxide (NO) is
sparingly soluble in water and cannot be separated easily [3]. It is well known that the
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commonly practiced methods of removing NO such as selective catalytic and non-catalytic
processes have high capital costs and undesirable problems with high temperatures and
handling of harmful chemicals. Therefore, alternative cost-effective and environmentally
friendlier processes, such as scrubbing suitable oxidizing agents capable of significantly
increasing the solubility of NO in water, are of ardent interest [1-5]. Hence, the use of
aqueous hydrogen peroxide [6], sodium chlorite [7], aqueous NaClO2 solution, KMnO4
solutions [8-10] and various other reagents is widespread in the literature. In a previous
study, we used oxone or potassium hydrogen peroxymonosulfate (2KHSO5.KHSO4.K2SO4) in
the removal of NO in absence or presence of SO2 and reported detailed experimental and
kinetic aspects of this process, including feasibility, stoichiometry, reaction pathways and the
effects of various process parameters [4]. This was followed by work on NO absorption by
aqueous persulfate (Na2S2O8) activated by temperature. The use of persulfate was suggested
to reduce the process costs as persulfate is comparatively cheaper than oxone and also,
significant amount of NO removal could be achieved (up to 90% at 90 0C) [3]. In a recent
study involving the removal of NO in presence of SO2 by the aqueous persulfate systems, we
demonstrated that the absorption of NO was greatly enhanced in the presence of SO2 (7080% NO removal at 23-70 0C), while the SO2 itself was completely removed [2]. In our most
recent study on the removal of NO using persulfate systems simultaneously activated by
temperature and Fe2+ ion, we showed that NO removal further increased by almost 10% in
presence of Fe2+ at all temperatures compared to temperature-only activation [1]. In our
discussions of the results, we proposed that the thermal and Fe2+ ion activation of the
persulfate anion leads to the production of the sulfate radicals (SO4•−), which is responsible for
the production of hydroxyl radical (OH•−) that acts as the main oxidant for the conversion of
NO in the aqueous solution [1, 3]. The following major pathways were suggested.
(1)
(2)
(3)
(4)
(5)
(6)
(7)
It was shown that almost 100% NO removal is attainable by this system but only at very high
temperature (90 0C). Hence, the energy requirement for the higher temperature process could
be the main stumbling block that impedes further development. A number of reports have
suggested NO removal by iron chelating agent, especially ferrous ethylenediamine tetraacetic
acid (FeII-EDTA) with or without some other reagents (Na2SO3, MgSO3, Na2S2O4), where NO is
combined with FeII-EDTA by reversible binding and separated from the solution [11-16].
(8)
2
The main drawbacks of this process include removal of the FeII-EDTA(NO) complex, fast
oxidation of FeII-EDTA to the inert FeIII-EDTA and high cost of EDTA. The problems can be
somewhat overcome in BioDeNOx process where FeII-EDTA(NO) is broken down by
denitrifying bacteria and FeIII-EDTA is reduced by iron reducing bacteria and resultant FeIIEDTA is recycled back to the reactor [17-20]. But the process was impeded by the slow action
of the biological process, uncertainty involving bacteria culture and the additional cost of
installing another unit. Some researchers have suggested reducing FeIII-EDTA by the catalytic
activity of activated carbon [21, 22] or bio-film electrode reactor [23, 24] but the requirement
for additional unit still persists. It is widely reported in the literature that FeIII-EDTA can also
be reduced by S2O82-, SO4•−, and bisulfate and bisulfide ions, and the resulting FeII-EDTA
recycled for reuse [16, 25-27]. It has also been suggested that the FeIII-EDTA can also
activate persulfate anion [28, 29] , thus producing a complete synergistic relationship with the
persulfate in NO removal process. However, to the best of our knowledge the simultaneous
and synergistic application of S2O82- and FeII-EDTA for NO removal has never been studied.
Experimental Procedures
NO Absorption
The schematic diagram for the NO absorption, consisting of a thermally jacketed bubble
column reactor with flue gas blending system and analytical train of Fourier Transform
Infrared (FTIR) spectrometer, is shown in Figure 1, and is discussed in details elsewhere [1-3].
Flow Meter - NOx/N2
Jacket water out
Flow meter – SO2/N2
Cooling water out
pH Probe
Sample Syringe
Condenser
Cooling water in
NOx/N2 Cylinder
Bubble Column
Nitrogen in
SO2/N2 Cylinder
Nitrogen out
Jacket water in
Membrane dryer
Gas Sparger
VI
Dyna Blender (Flow Controller)
FTIR Gas cell
To Exhaust
Bypass Line
V2
FTIR
Figure 1. Schematic diagram of the experimental setup
Initially, the column was filed with 750 ml of de-ionized water and dry nitrogen gas was
passed through it for at least 15 min to purge it of all dissolved oxygen. Then, persulfate and
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FeII-EDTA solutions were added to the water and additional water was added to make the
total volume of solution 1 L before the bubbling of the gas through the solution was started.
Finally the outlet concentration in ppm level was analyzed in the FTIR and recorded using a
software program where gas concentration profiles were generated. The experiments were
performed at 23-70 0C. To maintain the solution, pH buffer solution was used for pH of 4.010.0, concentrated NaOH solution for pH 12.0 and concentrated H2SO4 solution for pH 2.0.
Spectrophotom etric Determ ination of I ron Species’
Iron species’ (Fe2+, Fe3+ and Fe-EDTA) were determined spectrophotometrically using
Beckman DU-7500 spectrophotometer. Fe2+ and (Fe2++Fe3+) ion concentration was
determined at 506-512 nm and 393-399 nm wavelength spectra by classical 1, 10phenanthroline colorimetric method and Fe3+ concentration was determined from the
difference. At low wavelength (270-330 nm) Fe-EDTA compound shows absorbance and can
be determined spectrophotometrically. At first, the calibration curves were drawn from the
absorption of known samples at 506-512 nm, 393-399 nm and 270-330 nm. The samples were
taken quickly from the reactor by the side syringe and the absorbance was found from the
spectrophometer. From these absorbance data using the calibration curves, the concentration
of Fe2+, Fe3+ and Fe-EDTA can be measured. For better accuracy the samples were diluted to
enable correct spectrophotometric determination and more than one wavelength were used
and final concentration was found by averaging the data.
Results and Discussion
Determ ination of Optim um Fe 2+ :EDTA R atio
The optimum ratio for Fe2+:EDTA has been investigated at different temperatures (40,
50 and 60 0C), and widely reported in the literature to be around 1:1 in the removal of organic
and inorganic pollutants. [13, 22, 30]. However, to the best of our knowledge, this is the first
time Fe2+-EDTA is used with persulfate system. Fe2+ and EDTA react with one another by the
following reversible reaction:
(9)
It was previously shown that the optimum Fe concentration needed for activation of 0.1M
NaS2O8 was 0.01 M Fe2+, as higher concentration of Fe2+ acted antagonistically with resultant
reduction of NO conversion [1]. Hence, these concentration levels were used to determine the
optimum Fe2+:EDTA ratios as shown in Table 1.
2+
Table 1. NO conversion for different EDTA concentration for 0.01 M Fe2+
NO conversion (%)
Temperature
0M
0.005 M
0.01 M
0.015 M
0.02 M
(0C)
EDTA
EDTA
EDTA
EDTA
EDTA
40
52.99
58.17
65.28
64.10
63.21
50
71.45
74.05
77.82
76.63
74.18
60
82.07
83.45
86.99
85.56
82.12
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and 0.1 M persulfate
0.025 M
EDTA
61.52
71.46
81.16
0.03 M
EDTA
60.62
67.45
78.13
Table 1 shows the NO conversion for the different concentrations of EDTA using 0.01 M Fe2+
and 0.1 M persulfate. It can be seen that the highest NO removal was obtained when
Fe2+:EDTA ratio was 1:1, thus confirming the prior literature results.
Effect of EDTA Addition and Tem perature on NO rem oval
Using the optimum initial Fe2+ and EDTA concentrations established with 0.1 M
persulfate solution (0.01M Fe2+ and EDTA), the effect of FeII-EDTA addition was compared to
the temperature-only activated persulfate system [3] and the Fe2+-persulfate systems.
100
(a)
% NO Conversion
NO conc. (ppm)
800
600
400
200
0
0
1000
2000
3000
Time (sec)
(b)
80
60
40
20
0
20
4000
30
40
50
Temperature ( C)
60
70
Figure 2. The effect of EDTA addition: (a) Comparative NO profiles at 30 0C, (b) NO
conversion at different temperature; -♦- persulfate only, -▲- persulfate with Fe2+, -*persulfate with both Fe2+ and EDTA
In Figure 2, NO absorption for combined persulfate and FeII-EDTA system is compared
with Fe2+ activated system and persulfate-only system [1, 3]. Figure (a) shows the
comparative NO absorption profiles for three systems, indicating higher removal for combined
persulfate and FeII-EDTA system. In Figure (b), NO conversion at 23-70 0C is compared for
the three systems. As shown in Figure (b) NO absorption is very high compared to Fe2+
activated system at lower temperature but at higher temperature the advantage appears to
diminish. This may be attributed to the reversible binding of NO with FeII-EDTA in reaction (8).
As the literature values suggest, the forward reaction constant, k8 (6 x 107 at 25 0C) is higher
than the lower temperature. At higher temperature k8 becomes lower (3.7 x 107 at 50 0C)
where the backward reaction constant, k-8 increases, resulting in a lower value of equilibrium
constant K8 [11, 30]. But the comparative advantage still holds (around 5% higher NO
conversion at 70 0C) due to the synergistic effect of FeII-EDTA on persulfate oxidation of NO.
Effect of pH on NO R em oval
The effect of pH on NO removal by combined persulfate and FeII-EDTA system was
investigated at 50 0C (a situation that mimics industrial condition). Khan and Adewuyi
observed the best NO removal for persulfate-only systems at near neutral pH [3]. Also,
Adewuyi and Sakyi reported that acidic pH (3.0 to 4.0) is best for Fe2+ activated persulfate
system as higher pH will result in Fe(OH)2 precipitate, which is severely detrimental to NO
absorption [1]. On the other hand, studies suggesting a neutral pH optimum for the FeII-EDTA
systems abound in the open literature [30, 31]. Therefore, a pH study for combined persulfate
and FeII-EDTA system was undertaken. Figure 3 shows NO conversion for pH 2.0-12.0 and
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comparison to the pH study of Khan and Adewuyi for persulfate system only [3]. As seen in
Figure 3 shows, whereas there is no significant difference in NO removal with pH for the
persulfate-only system, the combined persulfate and FeII-EDTA system shows optimal pH of
around 6.5 for maximum of NO removal.
100
NO conversion (%)
80
60
40
0.1M Persulfate with 0.01M Fe(II) and 0.01M EDTA
20
Only 0.1M Persulfate, from Khan and Adewuyi, 2010
0
2
4
6
8
10
12
pH
Figure 3. Comparative pH dependence of combined persulfate and FeII-EDTA system
Speciation and M aterial Balance of I ron
The iron species’ present in the solution (Fe2+, Fe3+ and Fe-EDTA) is determined
spectrophotometrically as discussed earlier. Total measured iron is the sum of Fe2+, Fe3+ and
Fe-EDTA and undetermined Fe is the difference between initial iron concentration (0.01M) and
total measured iron. Figure 4 shows the profiles of all the iron species at 50 0C. It can be seen
from Figure 4 that Fe2+ concentration remains close to zero throughout the experiment
suggesting near complete reaction with both persulfate and EDTA. The undetermined Fe
consists of non-labile iron hydroxides, FeII-EDTA(NO) and other non-labile iron species
presents in very small amounts. Since all other non-labile species except FeII-EDTA(NO)
remain constant throughout the experiment, the increase in the undetermined Fe suggests
increase in FeII-EDTA(NO) concentration, resulting reaction (8) progression.
Concentration (M)
0.01
0.008
Total
Measured Fe
Fe(III)
0.006
0.004
Fe(II)EDTA
0.002
Undetermined
Fe
Fe(II)
0
0
1000
2000
Time (s)
3000
4000
Figure 4. Concentration profiles and material balance for iron species’ at 50 0C
Conclusion
The chemistry and removal rates of NO by combined aqueous solution of persulfate and
Fe -EDTA was studied and reaction pathways proposed. The removal of NO by combined
II
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persulfate and FeII-EDTA system seems to be very promising as persulfate and FeII-EDTA act
simultaneously and synergistically to boost the NO conversion by almost 30% over Fe2+
activated persulfate system at lower temperature and by 5% at the highest temperature.
However, at very low or high pH the absorption of NO decreases as the optimum pH is near
neutral. Also, a very high concentration of FeII-EDTA is detrimental to NO removal due to the
antagonistic effects of Fe2+ and/or FeII-EDTA on persulfate activation and subsequent NO
absorption. We are currently working on a mathematical model based on proposed reaction
pathways to fit experimental results and obtain mass transfer and kinetic rate parameters.
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