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Fluorescence Spectroscopy for Quantifying OH Radicals Produced by Electrical Discharge Plasmas 

Stephanie Hernandez1,2 and Selma Mededovic Thagard3

1ASSETs to Server Humanity REU Program, Clarkson University2Washington and Lee University, Department of Physics & Engineering

3Clarkson University, Department of Chemical and Biomolecular Engineering

 Symposium on Undergraduate Research Experiences (SURE), July 30, 2015This project was supported in part by the National Science Foundation under Grant No. EEC-

1359256.

Gas Discharge Plasma

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Plasma:A “soup” of electrons, ions, radicals and neutral molecules in an ionized gas which can be produced by electrical discharge in a liquid or gas.

Motivation and Objectives

Motivation:Plasma can be used for drinking and waste water treatment. As an Advanced Oxidation Process (AOP), its efficiency is proportional to the rate of ·OH radical production.

Objective: Development of a new method for quantifying ·OH radicals based on

fluorescence spectroscopy that uses sodium salicylate as a reagent. Comparison of ·OH radical production rates in three different plasma

reactors: liquid discharge, gas discharge, and gas discharge with bubbling.

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Fluorescence: The radiation emitted by certain substances when excited by a wavelength of light. The excitation and emission occur at different wavelengths.

Fluorescence Spectroscopy

Advantages: Sensitive Quantitative Quick Safe

Fluorescence Spectrophotometer

Ocean Optics Cuvettes

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·OH scavenger - good selectivity Very soluble in water Affordable compared to other fluorescent probes High quantum efficiency Linear response over a wide spectral region

Sodium Salicylate (NaSCL)

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Experimental: Plasma Reactor

Experimental ParametersVolume 100mLvoltage 20 kVRepetition Frequency 52 HzSalicylate 3.5mg/L  or 10 mg/LArgon Flow Rate 440 mL/min

Liquid Discharge Plasma Gas Discharge Plasma Gas Discharge Plasma with Bubbling

Experimental ParametersVolume 1500mLvoltage 20.6 kVRepetition Frequency 52 HzSalicylate 3.5mg/L  or 10 mg/LArgon Flow Rate 605 mL/min

Experimental ParametersVolume 1500mLvoltage 20.6 kVRepetition Frequency 52 HzSalicylate 3.5mg/L  or 10 mg/LArgon Flow Rate 605 mL/min

Methods & Data Collection Prepare a calibration curve. Run 3.5 mg/L and 10 mg/L

Solutions of NaSCL in plasma reactors.

Collect 3 mL samples every 10 min for fluorescence spectroscopy analysis.

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Experimental: Analytical Method

Salicylate Emission Spectra

Inte

nsity

(a.u

) 10min20min30min40min

0min

Wavelength (nm)

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Results: Fluorescence

0 5 10 15 20 25 30 35 40 45 500

100

200

300

400

500

600

700

800

Fluorescence Quenching of NaSCL in Liquid Discharge

Plasma Reactor

10mg/L NaSCL3.5 mg/L NaSCL

Time (min)

Inte

nsity

(a.u

)

0 5 10 15 20 25 30 35 40 450

100

200

300

400

500

600

700

800

Fluorescence Quenching of NaSCL in Gas Discharge

Plasma Reactor

3.5 mg/L Gas Discharge3.5 mg/L Gas Discharge with Bubbling 10 mg/L Gas DIscharge

Time (min)

Inte

nsity

(a.u

)

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Data Presentation/FindingsResults: Hydrogen Peroxide 2 2OH OH H OOH salycilate products

15 20 25 30 35 40 450.0

0.5

1.0

1.5

2.0

2.5

3.03.5

4.0

Hydrogen Peroxide Concentra-tion vs Time

0 mg/L NaSCL

3.5 mg/L Liquid Discharge

10mg/L Liquid Discharge

Time (min)C

once

ntra

tion

(mM

)0 10 20 30 40 50

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Hydrogen Peroxide Concentration vs Time

0 mg/L NaSCL Gas Discharge

3.5 mg/L Gas Discharge

3.5 mg/L Gas Discharge with Bubbling

10 mg/L Gas Discharge

Time (min)

Con

cent

ratio

n (m

M)

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OH

Cathode

Direction of e- movement Plasma interior

Interface

Bulk liquid

Anode

Direction of e- movement

e-aq

Small surfactant (PFBA)

Polar head

Nonpolar tail

Non-surfactant (Salicylate)

Large surfactant (PFOA, PFOS, Gemfibrozil)

Nonpolar tail

Polar head

OH

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Fluorescence spectroscopy appears to be a good technique for relative comparison of OH radicals produced by electrical discharge plasmas.

Salicylate is not hydrophobic enough (i.e., it does not lower the surface tension of the solution) so its concentration at the plasma-liquid interface is not high enough. Not all ·OH radicals can be scavenged.

Because salicylate is hydrophilic, not a significant difference between the three plasma reactors was observed.

Data Presentation/FindingsConclusions

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References

1. M.F. Al-Kuhaili, “A study of the Fluorescent Properties of spin-coated Sodium Salicylate Films,” Journal of Luminescence, vol. 117, iss.2, pp. 209-216, 2006.

2. Rahmi and H. Itagaki “Application of 2,5 Dihydroxybenzoic Acid as a fluorescent probe to the Clarification of Microevniorment in Hydrogels of Biopolymers,” Journal of Photopolymer Science and Technology vol. 24, no.5, pp.517-521,

2011.3. M. Karima, H.Leeb, Y. Kimb, H. Baeb, S. Lee “Analysis of salicylic acid based on the fluorescence enhancement of the

As(III)–salicylic acid system” vol. 576, iss 1, pp. 136-139, 2006.S. Kanazawa, T. Furuki, T. Nakaji, R. Ichiki, “Application of Chemical dosimetry to hydroxyl radical measurement during

underwater discharge.,” Journal of Physics: Conference Series, 2013.5. A. Gomes, E. Fernandes, J. Lima “Fluorescent Probes used for detection of reactive oxygen species,” Journal of

Biochemical and biophysical methods 2005 6. S. Kanazawa, T. Furuki, T. Nakaji, S. Akamine, R. Ichiki “Measurement of OH Radicals in Aqueous Solution Produced by

Atmospheric LF Plasma Jet,” 2005.7.R. Joshi, S.Mededovic Thagard, “Streamer-Like Electrical Discharges in Water: Part II Environmental Applications,” 2013.

References

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Data Presentation/FindingsAcknowledgments

MentorsSelma Mededovic Thagard

Special thanks to Gunnar Stratton

Joshua FranclemontFei Dai

Xiangru Fan