singlet oxygen kinetics for trace organic transformation ... · photosensitizers in wastewater...
TRANSCRIPT
Singlet Oxygen Kinetics for Trace Organic
Transformation in Wastewater Effluent
Eduardo Saez, David Quanrud, Robert Arnold, Minkyu Park,
Itzel Marquez, Doorae Lee, Violeta Chichique, Natalia Rojas,
Carlos Weiler
Department of Chemical and Environmental Engineering
The University of Arizona
2019 WEST Center Industry Meeting and Research Symposium
November 18, 2019
• Trace organic compounds (TOrCs) with potential health and ecological
effects (e.g. pharmaceuticals, endocrine disruptors) must be removed for
reuse of treated wastewater
• Preliminary studies in the Santa Cruz River showed that some important
TOrCs are attenuated with distance form treatment plant outfall
• Research shows that the mechanism responsible is reaction with singlet
oxygen produced when photosensitizers in the wastewater effluent react
with sunlight
Rationale
The Santa Cruz River in Tucson, Arizona
(wastewater-dependent stream)
Tucson
Fluoxetine (Prozac): Antidepressant
Tonalide: Fragrance
Attenuation of TOrCs in the Santa Cruz River
Distance (km)
Attenuation of Estrogenic Activity in the SCR
Possible mechanisms: Sediment sorption, biodegradation, photolysis
Solar Photolysis
Indirect solar photolysis plays a role in attenuation of TOrCs
(estrogenic compounds in particular).
Model system to investigate mechanism:
Alkylphenols
p-Cresol (PC)Nonylphenol (NP)
kNP + *OH = 1.30x1010 M-1s-1 kPC + *OH = 1.20x1010 M-1s-1
Widespread endocrine disrupter Model compound
Solar indirect photolysis of p-cresol
Initial p-c concentration: 50 mM
UVA (300-400 nm) indirect photolysis of p-cresol
0 50 100 150 200 2500.70
0.75
0.80
0.85
0.90
0.95
1.00
[p-C
]/[p
-Co
]
Milli-Q water control
Dark control
p-cresol
(a)
0 50 100 150 200 2500.70
0.75
0.80
0.85
0.90
0.95
1.00
(b)
p-cresol
p-cresol + IPOH
Deoxygenated
p-cresol + NaN3
Time (min) Time (min)
Changes in the Wastewater Matrix During Solar Photolysis
Fluorescence intensity (EEM) plots of wastewater samples before (A) and
after (B) 4 hours of sunlight without p-cresol, and before (C) and after (D) 4
hours of sunlight with [p-cresol]o = 50 μM.
Summary of Observations on Solar Light Attenuation of TOrCs
1. Requires: Solar light, wastewater and dissolved oxygen
2. It is not caused by direct photolysis
3. It is not caused by formation of hydroxyl radicals
Hypothesis:
Compounds in effluent organic matter (EfOM) are excited to a
triplet state by solar light and the excited molecules react with
dissolved oxygen to form singlet oxygen, a powerful oxidizer
𝐸𝑓𝑂𝑀 → 1𝐸𝑓𝑂𝑀∗ → 3𝐸𝑓𝑂𝑀∗ + 𝑂2 → 1𝑂2
1𝑂2 + 𝑇𝑂𝑟𝐶 → 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠
Mechanism
EfOM: Photosensitizer in wastewater effluent
Validation of the mechanism using a model
photosensitizer: methylene blue (MB)
200 300 400 500 600 700 8000.0
0.1
0.2
0.3
0.4
0.5
Abso
rban
ce (
cm-1)
Wavelenght (nm)
Targets (TOrCs): Furfuryl alcohol (FFA), p-cresol (p-c)
UVA/sunlight Light Emitting Diodes (LEDs)
300 400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
1.2
Irra
dia
nce
(W
/m2/n
m)
Wavelengths (nm)
UVA- Spectral Irradiance
Global spectral Irradiance
300 350 400 450 500 550 600 650 700 750
0.0
0.5
1.0
Realtiv
e s
pectr
al pow
er
Wavelength (nm)
Light Sources Used in Experiments
A predictive kinetic model was developed to simulate target
destruction in a batch reactor by singlet oxygen produced from
the light excitation of MB
Photo-degradation of FFA by 1O2 using MB as photosensitizer
𝑀𝐵 + hv → 𝑀𝐵•→
3𝑀𝐵• fMB* = wavelength dependent
3𝑀𝐵• + O2(aq) → 1O2 + 𝑀𝐵 k1 = 2.0×109 M-1s-1
1O2 + FFA→ product k2 = 1.2×108 M-1s-1
1O2 → O2(aq) k3 = 2.54×104 s-1
𝜙 =𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 𝑡ℎ𝑎𝑡 𝑟𝑒𝑎𝑐𝑡
𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑏𝑦 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 𝑎𝑡 𝑓𝑖𝑥𝑒𝑑 𝑤𝑎𝑣𝑒𝑙𝑒𝑛𝑔𝑡ℎ
(quantum yield)
0 10 20 30 40 50 60
0.0
5.0x10-6
1.0x10-5
1.5x10-5
2.0x10-5
365 nm
460 nm
610 nm
650 nm
[FF
A]
(M)
Time (min)
Degradation of FFA by Singlet Oxygen Produced from MB
Using LEDs
Solid lines: Model predictions, adjustable parameter: MB quantum yield
Quantum Yield of MB for Production of Singlet Oxygen
Quantum yield: Moles of MB transformed to triplet state per mole of photon
absorbed
If the model is correct
It should predict degradation of any compound
with a known rate constant with singlet oxygen
Model extended to p-cresol under sunlight
*Sunlight irradiance
*p-cresol rate constants
Photo-degradation of p-cresol by 1O2 with MB as sensitizer
p-C + 1O2 → products k5 = 1.2×107 M-1s-1
p-C_ion + 1O2 → products k6 = 3.7×108 M-1s-1
p-C → p-C_ion + H+ pKa = 10.26 (at 25 °C)
Photo-degradation of p-cresol with MB (5 mM) under sunlight
18
0 100 200
0.0
5.0x10-5
1.0x10-4
1.5x10-4
2.0x10-4
2.5x10-4
30 mM p-C
100 mM p-C
220 mM p-C
[p-C
] (M
)
Time (min)
UF ROPermeate
RO Concentrate
After UF
Purple lineSecondary
Effluent w/
chlorination
Before UF
At Aqua Nueva WRF At WEST CenterReclaimed water
Reverse osmosis concentrate from wastewater effluent was
selected to increase photosensitizing activity
Experiments with wastewater effluent
Fractionation of RO concentrate by molecular weight
Degradation of FFA by fractionated RO concentrate
Conclusions
Photosensitizers in wastewater effluent produce singlet oxygen
when exposed to solar light, thus attenuating trace organics
Production of singlet oxygen from photosensitizers can be
represented by a simple kinetic model based on the quantum
yield for excitation to the triplet state
Preliminary experiments indicate:
1. Photosensitizers in wastewater effluent have molecular
weight lower than 1 kDa
2. Photosensitizers are hydrophilic
3. Wavelengths in the range 450-550 induce significant
photosensitizing activity