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