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
β High solubility and mobility
β Toxic and eutrophication effect
β 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) β: 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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β Low binding affinity & weak complexation of nitrate
β High formation tendency to ammonia and N2O
rate-limiting step
Key intermediates to selectivity
Slow kinetics & poor selectivity
Recent research interests:
βͺ Heterogeneous Catalysts with active and selective redox sites
βͺ Bio-inspired catalysts mimicking enzymatic denitrification process
Rationale of Homogeneous Photochemical Denitrification
ππ3β β ππ2
β + πββ (1)
πββ + π»+ β .ππ» (ππΎπ = 11.9)
ππ3β β ππππβ (2)
ππππβ + π»+ β πππππ» (ππΎπ = 6.5~6.8)
πππππ» β ππ2β + .ππ»
β Nitrate photochemistry
.ππ» & ππ2β
β Inspired from hole-scavenging process in heterogeneous photocatalysis
β+ + π»πΆππβ β π»+ + πΆπ2Β·β (πΈ(πΆπ2/πΆπ2Β·β)
π = β1.9 π) (πΈ(ππ3β/Β·ππ32β)π = β0.89 π)
ΟβΟ*
nβΟ*
.ππ» + π»πΆππβ β π»2π + πΆπ2Β·β (3.2 Γ 109 M-1 s-1)
ππ2β + π»πΆππβ β ππ2
β + πΆπ2Β·β + π»+
ππ3ββΞ½β―β―
πΆπ2Β·β
π2
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
βͺ [NO3-]= 2 mM
βͺ [HCOO-]= 0-20 mM
βͺ 20 mM phosphate buffer (pH=7.2 )
Analytical Methods
β’ Ion chromatography:
Nitrate, nitrite, and formate
β’ Phenate method: ammonia
β’ TOC analyzer with TNB module:
TOC & TNB
β’ Gas chromatography: N2
β’ 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-CO2Β·- adduct
DMPO-HOΒ· 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
)
βͺ Simultaneous removal of dissolved nitrogen and formate
βͺ Nitrate was transformed to gaseous nitrogen
βͺ Negligible formation of nitrite and ammonia
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Kinetic Modeling, Sensitivity & Principal Component analysis
Parameter optimization
β’ Kinetic reaction models: 137 reactions
β’ Optimization algorithm: Powell method
β’ Comparison operator: Standard least square
β’ Optimization tolerance: 1Γ10-5
β’ Uncertainty analysis: 20% standard deviation on fitted rate constants
β’ Computer program: Kintecus V6.0.1
Sensitivity and Principal component analysis
β’ Purpose: reaction mechanism reduction
β’ Rationale: eigenvalue-eigenvector analysis of matrix based normalized sensitivity coefficient (NSC)
(NSC)π,π=
π[πππππππ ]π[πππππππ ]π
πππππ ππβ π
=π[πππππππ ]π
πππππ ππβ π
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β’ 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.
βͺ Point symbols: experimental data
βͺ Lines with shaded bands:
predicted average concentrations
with 95% confidence intervals.
Kinetic Modelling & Optimization
βͺ Kinetic modelling well fits
experimental observation
[Formate]/[Nitrate] = 3.1
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No. Reactions Rate constants
1 ππ3β β ππππβ 4.6Γ 10-4 s-1
2 ππ3β β ππ2
β + πββ 1.3 Γ 10-4 s-1
3 πββ + π»+ β .ππ» 5.0 Γ 1010 M-1 s-1
4 ππππβ + πΆπ2 β πππππΆπ2β 3.0 Γ 104 M-1 s-1
5 πππππΆπ2β β ππ3
β + πΆπ2 6.7 Γ 105 s-1
6 πππππΆπ2β β ππ2
β + πΆπ3ββ 3.3 Γ 105 s-1
7 .ππ» + π»πΆππβ β πΆπ2ββ + π»2π 3.2 Γ 109 M-1 s-1
8 πΆπ3ββ + π»πΆππβ β πΆπ2
ββ + π»πΆπ3β 1.1 Γ 105 M-1 s-1
9 ππ2β + π»πΆππβ β ππ2
β + πΆπ2ββ + π»+ 2.1 Γ 105 M-1 s-1
10 ππ2β + πΆπ2
ββ β ππ2β + πΆπ2 6.0 Γ 109 M-1 s-1
11 πΆπ2ββ + π2 β πΆπ2 + π2
ββ 2.4 Γ 109 M-1 s-1
12 πΆπ2ββ + πΆπ2
ββ β πΆ2π4β 6.5 Γ 108 M-1 s-1
NO. Reactions Rate constants
13 ππ2β β ππβ + πββ 7.9Γ 10-4 s-1
14 ππ2β + π2
ββ β ππ22β + π2 5.0 Γ 106 M-1 s-1
15 ππ22β + π»2π β ππβ + 2ππ»β 4.3 Γ 104 s-1
16 ππ. + π2ββ β ππππβ 4.3 Γ 109 M-1 s-1
17 ππβ + πΆπ2ββ β πππΆπ2
β 2.9 Γ 109 M-1 s-1
18 ππβ + πππΆπ2β β π2π2
β + πΆπ2 6.8 Γ 106 M-1 s-1
19 π2π2β β ππβ + ππβ 6.6 Γ 104 s-1
20 ππβ + π»+ β π»ππ 5.0 Γ 1010 M-1 s-1
21 π»ππ + π»ππ β π2π + π»2π 8.0 Γ 106 M-1 s-1
22 π»ππ + πΆπ2ββ + π»2π β π»2ππ
β + ππ»β + πΆπ2 1.3 Γ 107 M-1 s-1
23 π»2ππβ + π»2ππ
β β π2 + 2π»2π 2.8 Γ 108 M-1 s-1
24 π2π + πΆπ2ββ + π»2π β π2 +
βππ» + ππ»β + πΆπ2 1.6 Γ 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 β 24 major reactions
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ONOO-
NO3-
ONOOCO2-
NO2. NO2
-
NO.
NO22-
NOCO2- N2O2
-
HNO
N2O
H2NO .
N2
hΟ
hΟ
HO .
O2.-
CO2
CO3.-
CO2.-
H+
HO .
CO2.- NO.
CO2
CO2.-
hΟ
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%
NO2Β·: 49% HOΒ· : 39% CO3Β·-: 12%
NOΒ·: 42% CO2Β·-: 29% O2: 12% HNO: 3.9% N2O: 1.8%
[Formate]/[Nitrate]
Nitrate Removal
(%)
Nitrite Formation
(%)
Dissolved Nitrogen
Removal (%)
FormateConsumption
(%)
ExperimentalΞ[Formate]/Ξ[Nitrate ]
ModelledΞ[Formate]/Ξ[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 Β± 0.2 3.2 Β± 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 β
organic carbon residualβ
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Conclusions & Future works
Conclusions
1) Nitrate photolysis generated reactive radicals HOΒ·, NO2Β·, and CO3Β·- ;
2) Highly reductive CO2Β·- was generated through partial oxidation of formate by HOΒ·, NO2Β·, and CO3Β·
-;
3) The contribution of CO2Β·- to denitrification mainly resulted from its reduction on NOΒ·;
4) The stoichiometry of formate to nitrate was 3.1Β±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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