formate-assisted photochemical denitrification
TRANSCRIPT
Formate-Assisted Photochemical Denitrification:
Synergistic Effect of Nitrate Photolysis with Highly
Reductive Formate Radicals
Gongde Chen
Advisor: Haizhou Liu
Department of Chemical and Environmental Engineering,
University of California, Riverside, CA 92521
Feb. 27, 20181
Figure 1. The main processes in the nitrogen cycle
Grand Challenges of Managing Nitrogen Cycle
Groundwater contamination
Average nitrogen uptake: 30-50%
Low retention in soil
NO3-
NO3-
NO3-
Nitrate
❖ High solubility and mobility
❖ Toxic and eutrophication effect
❖ MCL: 10 mg/L as nitrogen
2Lehnert N. FEEDING THE WORLD IN THE 21ST CENTURY: GRAND CHALLENGES IN THE NITROGEN CYCLE. 2015.
Figure 2. U.S. maps showing by state mean annual
number of systems in violation
Figure 3. Satellite image of algal blooms caused
by excessive nutrient loading
Proportion of PWSs violating the nitrate MCL (10mg-N/L) ↑: 95% from groundwater systems
Drinking Water Contamination & Eutrophication
Pennino M J, et al. Environmental science & technology, 2017, 51(22): 13450-13460. 3
Biological denitrification: enzymes Catalytic hydrogenation: In-Pd, Cu-Pd, etc
Photocatalytic reduction: Cu-Pd/TiO2Bioinspired catalyst: (N(afaCy)3) iron complexes
Denitrification Technologies
(Soares O, Chemical Engineering Journal, 2014, 251: 123-130) Ford C L, et al. Science, 2016, 354(6313): 741-743.
(Lehnert N. Grand challenges in the nitrogen cycle. 2015.) (Guo S, ACS Catalysis, 2017, 8(1): 503-515)
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Challenges of Denitrifying Technologies
Ford C L, Park Y J, Matson E M, et al. Science, 2016, 354(6313): 741-743.
Yoshioka T, Iwase K, Nakanishi S, et al. . J. Phys. Chem. B 2016, 120(29): 15729-15734.
Brown W A, King D A. J. Phys. Chem. B 2000, 104, 2578-2595. 2000.
Scheme1 Reaction pathways for heterogeneous and biological denitrification
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❖ Low binding affinity & weak complexation of nitrate
❖ High formation tendency to ammonia and N2O
rate-limiting step
Key intermediates to selectivity
Slow kinetics & poor selectivity
Recent research interests:
▪ Heterogeneous Catalysts with active and selective redox sites
▪ Bio-inspired catalysts mimicking enzymatic denitrification process
Rationale of Homogeneous Photochemical Denitrification
𝑁𝑂3− → 𝑁𝑂2
∙ + 𝑂∙− (1)
𝑂∙− + 𝐻+ → .𝑂𝐻 (𝑝𝐾𝑎 = 11.9)
𝑁𝑂3− → 𝑂𝑁𝑂𝑂− (2)
𝑂𝑁𝑂𝑂− + 𝐻+ → 𝑂𝑁𝑂𝑂𝐻 (𝑝𝐾𝑎 = 6.5~6.8)
𝑂𝑁𝑂𝑂𝐻 → 𝑁𝑂2∙ + .𝑂𝐻
❖ Nitrate photochemistry
.𝑂𝐻 & 𝑁𝑂2∙
❖ Inspired from hole-scavenging process in heterogeneous photocatalysis
ℎ+ + 𝐻𝐶𝑂𝑂− → 𝐻+ + 𝐶𝑂2·− (𝐸(𝐶𝑂2/𝐶𝑂2·−)
𝑜 = −1.9 𝑉) (𝐸(𝑁𝑂3−/·𝑁𝑂32−)𝑜 = −0.89 𝑉)
π→π*
n→π*
.𝑂𝐻 + 𝐻𝐶𝑂𝑂− → 𝐻2𝑂 + 𝐶𝑂2·− (3.2 × 109 M-1 s-1)
𝑁𝑂2∙ + 𝐻𝐶𝑂𝑂− → 𝑁𝑂2
− + 𝐶𝑂2·− + 𝐻+
𝑁𝑂3−ℎν⋯⋯
𝐶𝑂2·−
𝑁2
Figure 4 UV absorption spectrum of nitrate and nitrite
Mack J, Bolton J R. Journal of Photochemistry and Photobiology A: Chemistry, 1999, 128(1-3): 1-13.
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Photochemical Experiment & Analytical Methods
Figure 5 Experimental set-up of photocatalytic system, UV-vis absorption spectra of
nitrate, nitrite and formate, and output spectrum of medium pressure UV lamp.
Experimental parameters
▪ [NO3-]= 2 mM
▪ [HCOO-]= 0-20 mM
▪ 20 mM phosphate buffer (pH=7.2 )
Analytical Methods
• Ion chromatography:
Nitrate, nitrite, and formate
• Phenate method: ammonia
• TOC analyzer with TNB module:
TOC & TNB
• Gas chromatography: N2
• EPR: radicals
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Figure 6. Electron paramagnetic spectra of DMPO-radical adducts formed after 20 minutes of irradiation with
medium-pressure UV lamp. [Nitrate]= 100 mM, [Formate]= 300 mM, [DMPO]= 100 mM, and pH=7.2 with 200
mM phosphate buffer.
DMPO-CO2·- adduct
DMPO-HO· adduct
Characterization of Radical Species
HO
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Figure 7. Nitrate photolysis in the presence of formate. [Nitrate]= 2.0 mM, [Formate]= 6.2
mM, and pH=7.2 with 20 mM phosphate buffer.
Nitrate Photolysis in the Presence of Formate
0 30 60 90 120 150 1800.0
0.8
1.6
2.4
3.2
4.0
Dissolved Nitrogen
Nitrate
Nitrite
Ammonia
Time (Minutes)
Nit
rog
en S
peci
es
(mM
)
0.0
1.3
2.6
3.9
5.2
6.5 Formate
Fo
rma
te (
mM
)
▪ Simultaneous removal of dissolved nitrogen and formate
▪ Nitrate was transformed to gaseous nitrogen
▪ Negligible formation of nitrite and ammonia
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Kinetic Modeling, Sensitivity & Principal Component analysis
Parameter optimization
• Kinetic reaction models: 137 reactions
• Optimization algorithm: Powell method
• Comparison operator: Standard least square
• Optimization tolerance: 1×10-5
• Uncertainty analysis: 20% standard deviation on fitted rate constants
• Computer program: Kintecus V6.0.1
Sensitivity and Principal component analysis
• Purpose: reaction mechanism reduction
• Rationale: eigenvalue-eigenvector analysis of matrix based normalized sensitivity coefficient (NSC)
(NSC)𝑚,𝑖=
𝜕[𝑆𝑝𝑒𝑐𝑖𝑒𝑠]𝑚[𝑆𝑝𝑒𝑐𝑖𝑒𝑠]𝑚
𝜕𝑘𝑖𝑘𝑖 𝑘𝑗≠𝑖
=𝜕[𝑆𝑝𝑒𝑐𝑖𝑒𝑠]𝑚
𝜕𝑙𝑛𝑘𝑖 𝑘𝑗≠𝑖
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• Computer program: Kintecus V6.01 & Atropos V 1.00
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Figure 8. Nitrate photolysis in the presence of formate. [Nitrate]= 2.0 mM,
[Formate]= 6.2 mM, and pH=7.2 with 20 mM phosphate buffer.
▪ Point symbols: experimental data
▪ Lines with shaded bands:
predicted average concentrations
with 95% confidence intervals.
Kinetic Modelling & Optimization
▪ Kinetic modelling well fits
experimental observation
[Formate]/[Nitrate] = 3.1
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No. Reactions Rate constants
1 𝑁𝑂3− → 𝑂𝑁𝑂𝑂− 4.6× 10-4 s-1
2 𝑁𝑂3− → 𝑁𝑂2
∙ + 𝑂∙− 1.3 × 10-4 s-1
3 𝑂∙− + 𝐻+ → .𝑂𝐻 5.0 × 1010 M-1 s-1
4 𝑂𝑁𝑂𝑂− + 𝐶𝑂2 → 𝑂𝑁𝑂𝑂𝐶𝑂2− 3.0 × 104 M-1 s-1
5 𝑂𝑁𝑂𝑂𝐶𝑂2− → 𝑁𝑂3
− + 𝐶𝑂2 6.7 × 105 s-1
6 𝑂𝑁𝑂𝑂𝐶𝑂2− → 𝑁𝑂2
∙ + 𝐶𝑂3∙− 3.3 × 105 s-1
7 .𝑂𝐻 + 𝐻𝐶𝑂𝑂− → 𝐶𝑂2∙− + 𝐻2𝑂 3.2 × 109 M-1 s-1
8 𝐶𝑂3∙− + 𝐻𝐶𝑂𝑂− → 𝐶𝑂2
∙− + 𝐻𝐶𝑂3− 1.1 × 105 M-1 s-1
9 𝑁𝑂2∙ + 𝐻𝐶𝑂𝑂− → 𝑁𝑂2
− + 𝐶𝑂2∙− + 𝐻+ 2.1 × 105 M-1 s-1
10 𝑁𝑂2∙ + 𝐶𝑂2
∙− → 𝑁𝑂2− + 𝐶𝑂2 6.0 × 109 M-1 s-1
11 𝐶𝑂2∙− + 𝑂2 → 𝐶𝑂2 + 𝑂2
∙− 2.4 × 109 M-1 s-1
12 𝐶𝑂2∙− + 𝐶𝑂2
∙− → 𝐶2𝑂4− 6.5 × 108 M-1 s-1
NO. Reactions Rate constants
13 𝑁𝑂2− → 𝑁𝑂∙ + 𝑂∙− 7.9× 10-4 s-1
14 𝑁𝑂2− + 𝑂2
∙− → 𝑁𝑂22− + 𝑂2 5.0 × 106 M-1 s-1
15 𝑁𝑂22− + 𝐻2𝑂 → 𝑁𝑂∙ + 2𝑂𝐻− 4.3 × 104 s-1
16 𝑁𝑂. + 𝑂2∙− → 𝑂𝑁𝑂𝑂− 4.3 × 109 M-1 s-1
17 𝑁𝑂∙ + 𝐶𝑂2∙− → 𝑁𝑂𝐶𝑂2
− 2.9 × 109 M-1 s-1
18 𝑁𝑂∙ + 𝑁𝑂𝐶𝑂2− → 𝑁2𝑂2
− + 𝐶𝑂2 6.8 × 106 M-1 s-1
19 𝑁2𝑂2− → 𝑁𝑂∙ + 𝑁𝑂− 6.6 × 104 s-1
20 𝑁𝑂− + 𝐻+ → 𝐻𝑁𝑂 5.0 × 1010 M-1 s-1
21 𝐻𝑁𝑂 + 𝐻𝑁𝑂 → 𝑁2𝑂 + 𝐻2𝑂 8.0 × 106 M-1 s-1
22 𝐻𝑁𝑂 + 𝐶𝑂2∙− + 𝐻2𝑂 → 𝐻2𝑁𝑂
∙ + 𝑂𝐻− + 𝐶𝑂2 1.3 × 107 M-1 s-1
23 𝐻2𝑁𝑂∙ + 𝐻2𝑁𝑂
∙ → 𝑁2 + 2𝐻2𝑂 2.8 × 108 M-1 s-1
24 𝑁2𝑂 + 𝐶𝑂2∙− + 𝐻2𝑂 → 𝑁2 +
∙𝑂𝐻 + 𝑂𝐻− + 𝐶𝑂2 1.6 × 103 M-1 s-1
Table 1. Major reactions and rate constants. [Nitrate]= 2.0 mM, [Formate]= 6.2 mM, and pH=7.2 with 20 mM
phosphate buffer.
Major Reaction Mechanism
137 reactions → 24 major reactions
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ONOO-
NO3-
ONOOCO2-
NO2. NO2
-
NO.
NO22-
NOCO2- N2O2
-
HNO
N2O
H2NO .
N2
hυ
hυ
HO .
O2.-
CO2
CO3.-
CO2.-
H+
HO .
CO2.- NO.
CO2
CO2.-
hυ
H+R2
R3
R5
R1 R11
R13
R14R15 R16
R17
R19
R21
R8
O2.-H+
CO3.-
C2O42-
R6
CO2.-
CO2.-HCOO- HO .
R7 R10
O2CO2
R9
Scheme 1. Major reaction pathways of photochemical denitrification process. [Nitrate]= 2.0
mM, [Formate]= 6.2 mM, and pH=7.2 with 20 mM phosphate buffer.
N2: 30% N2O: 70%
NO2·: 49% HO· : 39% CO3·-: 12%
NO·: 42% CO2·-: 29% O2: 12% HNO: 3.9% N2O: 1.8%
[Formate]/[Nitrate]
Nitrate Removal
(%)
Nitrite Formation
(%)
Dissolved Nitrogen
Removal (%)
FormateConsumption
(%)
ExperimentalΔ[Formate]/Δ[Nitrate ]
ModelledΔ[Formate]/Δ[Nitrate ]
0 53.6 52.0 0 - - -1.1 54.9 52.6 0 100 - -1.7 69.6 51.8 17.4 100 - -3.1 97.9 0.53 97.1 93.5 3.1 3.15.8 100 0 99.0 50.9 3.0 3.2
11.3 100 0 99.6 29.2 3.3 3.2
Average Stoichiometry of Formate to Nitrate 3.1 ± 0.2 3.2 ± 0.1
Table 2. Impact of formate-to-nitrate molar ratio on denitrification and reaction stoichiometry between formate
and nitrate.
Reaction Stoichiometry of Formate to Nitrate
Increasing formate-to-nitrate molar ratio:
denitrification ↑
organic carbon residual↑
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Conclusions & Future works
Conclusions
1) Nitrate photolysis generated reactive radicals HO·, NO2·, and CO3·- ;
2) Highly reductive CO2·- was generated through partial oxidation of formate by HO·, NO2·, and CO3·
-;
3) The contribution of CO2·- to denitrification mainly resulted from its reduction on NO·;
4) The stoichiometry of formate to nitrate was 3.1±0.2
Future works
1) Minimize the formation of N2O
2) Explore co-treatment of the contaminants coexisting with nitrate (e.g., chromium(VI), vanadium(V),
and uranium(VI)) )
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Acknowledgments
Haizhou Liu
Sergei Hanukovich
Phillip Christopher
Michelle Chebeir
Yibo Jiang
Kun Li
Questions:
Gongde Chen
Department of Chemical & Environmental Engineering
University of California, Riverside
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