comparing uv/chlorine advanced oxidation efficiency to uv ... · presence of elevated total...

Comparing UV/chlorine advanced oxidation efficiency to UV/H2O2

when using monochromatic UV light

by

Te Fang

A thesis submitted in conformity with the requirements

for the degree of Masters of Applied Science

Graduate Department of Civil Engineering

University of Toronto

© Copyright by Te Fang 2016

Te Fang ii

Department of Civil Engineering, University of Toronto 2016

Comparing UV/chlorine advanced oxidation efficiency to UV/H2O2 when using monochromatic

UV light

Te Fang

Department of Civil Engineering, University of Toronto

Degree of Masters of Applied Science

Convocation 2016

ABSTRACT

This thesis compares the efficiency of the UV/chlorine and UV/H2O2 advanced oxidation processes for

contaminant removal using monochromatic UV light. Previous work reported that UV/chlorine is more

efficient than UV/H2O2 for trichloroethylene removal in pure water at approximately pH 5 and below

using medium pressure (MP) lamps (Wang et al., 2012), and that it may even be more competitive in the

presence of elevated total inorganic carbon (TIC) and total organic carbon (TOC).

In this work, a LP kinetics model was adapted from the MP model developed by Wang et al. (2012).

The modelled results of sucralose decay were then validated by bench-scale experiments with a collimated

beam apparatus.

The adapted LP models successfully predicted sucralose decay under most of the experimental

conditions, but not at pH 10 for the UV/chlorine process. The reason for the inaccuracy of the UV/chlorine

model at pH 10 is not clear, and requires more work.

Te Fang iii


ACKNOWLEDGMENTS

I need to thank for a lot of people who give their hands to help me smoothly and successfully

accomplish my works. I’m grateful to my supervisor Prof. Hofmann, code supervisor Prof. Susan A.

Andrews, and Dr. Jim Bolton, to provide opportunity and their insight and patience to conduct this

research. Mr. Jim Wang assists me a lot for my experiments and method development. Previous Ph.D.

Ding Wang guided me to understand basic theories of this research, and Ph. D. Jacque-Ann also provided

many suggestions for my experiments. My office mates and colleagues are always willing to give me

support when I felt exhausted to my work, which motivates me to move forward. I have impressive and

unforgettable experience to be with these lovely people.

This research was funded by the Natural Sciences and Engineering Research Council of Canada

(NSERC), Stantec, and Calgon CarbonCorporation.

Te Fang iv


TABLE OF CONTENTS

ABSTRACT ................................................................................................................................................ ii

ACKNOWLEDGMENTS ......................................................................................................................... iii

LIST OF TABLES .................................................................................................................................... vii

LIST OF FIGURES ................................................................................................................................. viii

GLOSSARY ............................................................................................................................................... x

CHAPTER 1: INTRODUCTION AND RESEARCH OBJECTIVES ....................................................... 1

1.1 Introduction .................................................................................................................................. 1

1.2 Research Objectives ..................................................................................................................... 2

1.3 References .................................................................................................................................... 2

CHAPTER 2: ERRORS IN AN ENERGY-BASED APPROACH TO QUANTUM YIELD

DETERMINATION: THE IMPORTANCE OF A PHOTON-BASED APPROACH........ 3

2.1 Introduction .................................................................................................................................. 3

2.2 Objective ...................................................................................................................................... 5

2.3 Materials and Method................................................................................................................... 6

2.3.1 Reagents and Materials ......................................................................................................... 6

2.3.2 UV exposure and irradiance measurements .......................................................................... 6

2.3.3 Analytical methods ............................................................................................................... 8

2.4 Results and Discussion ................................................................................................................. 8

2.5 Summary and Conclusions ........................................................................................................... 9

2.6 References .................................................................................................................................. 10

CHAPTER 3: KINETIC MODEL OF THE UV/CHLORINE ADVANCED OXIDATION

PROCESS FOR THE DESTRUCTION OF TRICHLOROETHYLENE

USING LOW PRESSURE UV LAMPS ........................................................................... 12

3.1 Introduction ................................................................................................................................ 12

3.2 Inability to perform TCE experiments to confirm the model..................................................... 13

3.3 Low pressure kinetic model ....................................................................................................... 14

3.3.1 Kinetic parameters and reaction equations ......................................................................... 14

3.3.2 Discrepancies in the reported quantum yields of OH production ....................................... 16

3.3.4 Model equations .................................................................................................................. 18

3.4 Results and discussion ................................................................................................................ 19

3.4.1 Low pressure modelling results of TCE decay in pure water ............................................. 19

3.4.2 Effect of LP vs. MP lamps .................................................................................................. 20

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3.4.3 Effect of natural organic matter (NOM) ............................................................................. 21

3.4.4 Effect of total inorganic carbon (TIC) ................................................................................ 24

3.4.5 Effect of concentration of active chlorine ........................................................................... 26

3.5 Summary and Conclusions ......................................................................................................... 29

3.6 References .................................................................................................................................. 29

CHAPTER 4: COMPARING UV/CHLORINE TO UV/H2O2 EFFICIENCY USING LOW

PRESSURE LAMPS, WITH SUCRALOSE AS A MODEL CONTAMINANT ............ 33

4.1 Introduction ................................................................................................................................ 33

4.2 Objectives ................................................................................................................................... 34

4.3 Materials and methods ............................................................................................................... 35

4.3.1 Reagents and materials ....................................................................................................... 35

4.3.2 UV exposure and irradiance measurement ......................................................................... 35

4.3.3 Variable TIC ....................................................................................................................... 36

4.3.4 Analytical methods ............................................................................................................. 38

4.4 Results and discussions .............................................................................................................. 38

4.4.1 LP emission spectrum and molar absorption coefficients of sucralose at different pH ...... 38

4.4.2 Kinetic parameters and reactions ........................................................................................ 39

4.4.3 Experimental and modelling results of sucralose destruction by UV/chlorine

and UV/H2O2 in pure water ................................................................................................ 41

4.5 Hypotheses for discrepancy at higher pH for UV/chlorine process ........................................... 44

4.5.1 Reaction of sucralose with chlorine radical ....................................................................... 44

4.5.2 Effect of OCl- chain reaction and ozone production ........................................................... 46

4.5.3 Effects of chain reactions from reaction of TOC with OH and chlorine radical ................ 47

4.5.4 Effect of direct photolysis of sucralose at higher pH .......................................................... 47

4.5.5 Effect of nitrate and nitrite .................................................................................................. 48

4.6 Summary and recommendation .................................................................................................. 51

4.7 Reference .................................................................................................................................... 51

CHAPTER 5: COMMENTS ON A METHOD TO MEASURE SUCRALOSE USING UV

PHOTODEGRADATION FOLLOWED BY UV SPECTROPHOTOMETRY .............. 57

5.1 Introduction ................................................................................................................................ 57

5.2 Experimental .............................................................................................................................. 58

5.2.1 Reagents and materials ....................................................................................................... 58

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5.2.2 Apparatus ............................................................................................................................ 59

5.2.3 Irradiance measurements .................................................................................................... 59

5.2.4 Preparation of solutions ...................................................................................................... 59

5.2.5 Analytical Procedure and Methods ..................................................................................... 59

5.3 Results and discussion ................................................................................................................ 60

5.3.1 Results of data reproduction ............................................................................................... 60

5.3.2 Stability of the UV-active product ...................................................................................... 64

5.4 Conclusions ................................................................................................................................ 65

5.5 References .................................................................................................................................. 66

CHAPTER 6: SUMMARY, CONCLUSIONS AND RECOMMENDATIONS ..................................... 67

6.1 Summary and Conclusion .......................................................................................................... 67

6.2 Recommendations For Future Work .......................................................................................... 67

APPENDICES .......................................................................................................................................... 68

APPENDIX A DERIVATION OF CALCULATIONS FOR LP TCE DIRECT

PHOTOLYSIS & TCE DECAY BY UV/CHLORINE AND UV/H2O2

IN OTHERWISE PURE WATER .................................................................................. 69

APPENDIX B MATERIALS AND METHODS .................................................................................... 78

APPENDIX C QUALITY ASSURANCE AND QUALITY CONTROL (QA/QC) .............................. 86

APPENDIX D MATLAB CODES .......................................................................................................... 91

APPENDIX E STANDARD OPERATION PROCEDURES ............................................................... 109

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LIST OF TABLES

Table 3.1 Reaction parameters used in model .......................................................................................... 15

Table 3.2 Chain reactions associated with OH and Cl radicals in the presence of organic

scavengers ................................................................................................................................. 16

Table 3.3 Published quantum yield of OH production due to free chlorine photolysis............................ 16

Table 3.4 Modelled results of TCE photon fluence-based decay rate constants (Einstein-1 cm2)

based on absorbed fluence by LP UV alone, LP UV/chlorine, and LP UV/H2O2…………….20

Table 3.5 Predicted TCE first order decay rate constants (s-1 × 10-4) by UV/chlorine

and UV/H2O2 using LP and MP lamps ..................................................................................... 20

Table 4.1 TIC concentration in water in equilibrium with atmospheric CO2, and associated

scavenging potential at pH 5, 7.5, and 10 ................................................................................. 37

Table 4.2 Net increase in TIC concentration for various exposure times at pH 10 .................................. 37

Table 4.3 Additions to the reaction kinetic scheme reported in Table 3.1. .............................................. 40

Table 4.4 Sucralose decay rate constant using various quantum yield values .......................................... 42

Table 4.5 Sucralose concentration before and after various exposure times in the presence

of chlorine radicals .................................................................................................................... 46

Table 4.6 Chlorine concentration with light off and on for different exposure times .............................. 46

Table 4.7 Formation of nitrite via photolysis of ONOO- .......................................................................... 49

Table 4.8 Pathways of photolysis of nitrite .............................................................................................. 50

Table 5.1 Slopes of absorbance (270 nm) as a function of sucralose concentration under

different applied fluences……………………………………………………………………...61

Table 5.2 Final concentrations of seven replicates of sucralose solutions at an initial

concentration of 0.325 g/L for 1h UV irradiation ..................................................................... 65

Table 5.3 Concentrations of sucralose samples taken at different time intervals after

1h UV irradiation ...................................................................................................................... 65

Table C.1 Method detection limits............................................................................................................ 88

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LIST OF FIGURES

Figure 2.1 Picture of the lab-scale UV collimated beam apparatus ............................................................ 7

Figure 2.2 Relative spectral emittance of the MP lamp .............................................................................. 7

Figure 2.3 Molar absorption coefficient of atrazine from 200 to 400 nm .................................................. 9

Figure 3.1 Molar absorption coefficients of HOCl, OCl-, H2O2, and HO2- (Wang et al., 2012) .............. 13

Figure 3.2 TCE first order decay rate constant as a function of TOC in UV/chlorine

at three pHs ............................................................................................................................. 22

Figure 3.3 TCE first order decay rate constants as a function of TOC in UV/H2O2

at three pHs ............................................................................................................................. 22

Figure 3.4 Solution pH at which the UV/chlorine and the UV/H2O2 AOPs with LP and MP

lamps are equally efficient as functions of TOC concentration .............................................. 23

Figure 3.5 Effect of increasing reaction rate of TOC with OH radical on pH of equal efficiency

between UV/chlorine and UV/H2O2 ....................................................................................... 24

Figure 3.6 Contribution of inorganic carbon to total scavenging potential including chlorine:

UV/chlorine system ................................................................................................................ 25

Figure 3.7 TCE first order decay rate constant of UV/Cl2 as a function of alkalinity

at three pHs ............................................................................................................................. 25

Figure 3.8 TCE fist order decay rate constant of UV/H2O2 as a function of alkalinity

at three pHs ............................................................................................................................. 26

Figure 3.9 Percentage of inorganic carbon to total scavenging potential including H2O2

in the function of alkalinity. .................................................................................................... 26

Figure 3.10 Net formation rate of OH radicals as a function of chlorine concentration (a) at pH;

(b) at pH 7.5 and 10 ............................................................................................................... 28

Figure 4.1 TIC concentration as a function of fluence at pH 10 .............................................................. 38

Figure 4.2 LP spectrum emittance ............................................................................................................ 39

Figure 4.3 Absorption spectrum of sucralose ........................................................................................... 39

Figure 4.4 UV/chlorine: experimental versus model results of sucralose decay rate at pH 5

and 7.5 as a function of applied fluence ................................................................................. 41

Figure 4.5 UV/chlorine: experiment versus model results of sucralose decay rate at pH 10

as a function of applied fluence .............................................................................................. 42

Figure 4.6 UV/H2O2: experiment versus model results of sucralose decay as a function

of applied fluence .................................................................................................................... 43

Figure 4.7 Effect of various intial TIC concentrations on modelled results of sucralose

decay rate ................................................................................................................................ 43

Figure 4.8 (a) concentration of produced Cl radical versus OH radical as a function of applied

fluence at pH 5; (b) sucralose decay rate against applied fluence at pH 5 with

or without considering reactions from chloride ion ................................................................ 45

Figure 4.9 (a) concentration of produce Cl radical versus OH radical as a function of applied

fluence at pH 10; (b) sucralose decay rate against applied fluence at pH 10 with

or without considering reactions from chloride ion ................................................................ 45

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Figure 4.10 Predicted sucralose decay rate with or without involving reactions from ozone .................. 47

Figure 5.1 Absorbance of sucralose solution at different initial concentrations after

60 minutes of UV irradiation ................................................................................................... 60

Figure 5.2 Absorbance at 270 nm as a function of sucralose concentration for different

exposure fluences of 948, 2688, and 5688 mJ cm-2 ................................................................ 61

Figure 5.3 Graph of Equation 1 for large and small fluence rates relative to chromophore

decay rate. ............................................................................................................................... 63

Figure 5.4 Absorbance at 274 nm as a function of sucralose concentration ............................................ 64

Figure C.1 Calibration curve of atrazine ................................................................................................... 87

Figure C.2 Calibration curve of TCE ........................................................................................................ 88

Figure C.3 Calibration curve of sucralose ................................................................................................ 88

Figure C.4 Quality control chart of sucralose ........................................................................................... 89

Figure C.5 Quality control chart of atrazine ............................................................................................. 90

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GLOSSARY

AOPs Advanced oxidation processes

GC-ECD Gas chromatograph with electron capture detector

HOCl Hypochlorous acid

H2O2 Hydrogen peroxide

HO2- Hydroperoxide anion

LP Low pressure

LC-MS Liquid chromatography-mass spectrometry

MP Medium pressure

NO3- Nitrate

NO2- Nitrite

∙OH Hydroxyl radical

OCl- Hypochlorite ion

TCE Trichloroethylene

THMs Trihalomethanes

TOC Total organic carbon

TIC Total inorganic carbon

UV Ultraviolet

USEPA United States Environmental Protection Agency

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CHAPTER 1: INTRODUCTION AND RESEARCH OBJECTIVES

1.1 INTRODUCTION

Advanced oxidation processes (AOPs) have been used to remove contaminants in water treatment

applications for over thirty years, including groundwater remediation designed to remove, for example,

trichloroethylene (TCE) and tetrachloroethylne (PCE); the removal of pharmaceutical compounds and

pesticides in drinking water, especially those that cannot be completely removed by conventional water

treatment processes; and the removal of pollutants from industrial wastewater (Suty et al., 2004). These

processes are achieved by the generation of highly reactive and oxidative intermediates, the most common

one is the hydroxyl radical (∙OH) (Jin et al., 2011), strong, nonselective chemical oxidant capable of

destroying most of the contaminants encountered in drinking water and wastewater. The production of

OH can be initiated by many processes, but UV-based AOPs are being widely used.

Typically, hydrogen peroxide (H2O2) is added prior to UV irradiation to form ∙OH via its photolysis;

however, its use carries many operating challenges. For instance, the H2O2 remaining after treatment needs

to be quenched prior to water distribution by using either granular activated carbon or chemical reduction.

The former incurs high operating costs, and the latter can be operated (Pantin, 2010).

Chlorine has been suggested as a promising alternative to H2O2 because it is more cost-effective,

absorbs UV light more efficiently, and can be operationally simpler. However, much still remains

unknown about the UV/chlorine process. Wang et al. (2012) used a medium pressure (MP) UV lamp to

investigate the removal efficiency for TCE by UV/chlorine compared to that for the UV/H2O2 process.

They reported that the UV/chlorine process has comparable efficiency for removing TCE at lower pH,

and may even be more competitive in the presence of elevated total inorganic carbon (TIC) and total

organic carbon (TOC). However, the efficiency of UV/chlorine relative to UV/H2O2 has not been fully

explored with low pressure (LP) UV lamps (monochromatic at 254 nm). This research is an expansion of

the previous work and adds to our knowledge of the efficiency of the UV/chlorine process compared to

that of UV/H2O2 with the use of monochromatic light sources.

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1.2 RESEARCH OBJECTIVES

In general, the following objectives were fulfilled in this research.

1. To modify previous MP kinetics models, developed by Wang et al. (2012), to include reaction

mechanisms using LP UV light.

2. To compare the performance of the UV/chlorine to UV/H2O2 process for contaminant

decomposition using LP UV lamps via modelling, which was then confirmed by bench-scale

experiments.

3. To identify the effect of major water quality parameters, including pH, TIC, and TOC

concentrations, on the efficiency of UV/chlorine relative to that of UV/H2O2.

This thesis is written as a paper format, so that detailed background information is provided in each

chapter.

1.3 REFERENCES

Suty, H., Traversay, C. D. and Cost, M., 2004. Application of advanced oxidation processes: present and

future. Water Science and Technology. 49 (4), 227-233.

Pantin, S., 2010. Impacts of UV-H2O2 treatment for taste and odour control on secondary disinfection

(Master’s thesis, University of Toronto, Toronto, Canada). [Online] Available:

https://tspace.library.utoronto.ca/bitstream/1807/18973/1/Pantin_Sophie_200911_MASc_thesis.pdf

(assessed August, 2016)

Jin, J., Mohamed, G. E. and Bolton, J. R., 2011. Assessment of the UV/Chlorine process as an advanced

oxidation process. Water Research. 45 (4), 1890-1896.

https://tspace.library.utoronto.ca/bitstream/1807/18973/1/Pantin_Sophie_200911_MASc_thesis.pdf

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CHAPTER 2: ERRORS IN AN ENERGY-BASED APPROACH TO

QUANTUM YIELD DETERMINATION: THE IMPORTANCE OF A

PHOTON-BASED APPROACH

ABSTRACT

Bolton et al. (2015) proposed that a photon-based approach should be used

to determine photochemical parameters instead of an energy-based

approach if using polychromatic light sources. The reason stems from a

mathematical error in a weighting factor that is introduced in the reported

conventional equation (energy-based) when using spectral photon flux

instead of spectral irradiance. To illustrate the magnitude of the error, the

quantum yield of atrazine photolysis was determined by measuring its

decomposition rate under a medium pressure lamp. The results showed that

the use of the incorrect weighting factor introduced a 27% error in quantum

yield determination. Furthermore, simplification of the equation via

expanding the fraction of light absorbed as a Taylor series introduced a 3%

error. Analysis revealed that the error when using the incorrect weighting

factor would be more significant when the photolyzed compound has a

broader absorption spectrum.

2.1 INTRODUCTION

Stefan and Bolton (2005) developed an expression for determining quantum yield that is based on the

observed fluence-based decay rate constant of a photolyzed compound obtained from the slope of a plot

of the logarithm of compound concentration against the fluence (mJ cm-2). Recently, however, Bolton et

al. (2015) reported that photochemical parameters, such as quantum yields and decay rate constants,

should be determined by using photon fluence-based units instead of energy fluence-based units if using

polychromatic light sources, and that the earlier approach reported by Stefan and Bolton (2005) leads to

errors. The photon fluence-based approach is based on the observed initial decay rate of the photolyzed

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compound. In this chapter, a comparison of results of calculated quantum yield using both the photon

fluence-based approach and the energy fluence-based approach is presented, using atrazine as a case study.

The equation to calculate atrazine quantum yield using a collimated beam apparatus by using the

photon fluence-based equation with a weighted average photon fluence, as reported by Bolton et al.

(2015), is as follows:

∅𝑎𝑡𝑟𝑎𝑧𝑖𝑛𝑒 = [− 𝑑[𝑎𝑡𝑟𝑎𝑧𝑖𝑛𝑒]

𝑑𝑡]𝑡=0

𝑉

𝐴𝑚𝑎𝑠𝑘(𝑃𝐹) ∫ 𝐸𝑝0[𝑅𝐹(𝜆)]𝑥𝐵(𝜆)𝑑(𝜆)

𝜆2𝜆1

Eq. 2.1

where ∅𝑎𝑡𝑟𝑎𝑧𝑖𝑛𝑒 is the quantum yield of atrazine, V is the volume of the irradiated sample, 𝐴𝑚𝑎𝑠𝑘 is the

exposed area, PF is the petri factor, 𝐸𝑝0 is the photon irradiance at the center of the Petri dish, RF is the

reflection factor, and 𝑥𝐵(λ) is the fraction of light absorbed at wavelength λ. 𝑥𝐵(λ) can be calculated as

follows:

𝑥𝐵(λ) = (𝛼𝐵𝑘𝑔𝑑 (λ)

𝛼𝐵𝑘𝑔𝑑 (λ)+ 𝛼𝐵 (λ)) (1-10−[𝛼𝐵𝑘𝑔𝑑 (λ)+𝛼𝐵 (λ)]𝑧 ) Eq. 2.2

where 𝛼𝐵𝑘𝑔𝑑 (λ) is the absorption from the background water matrix, and 𝛼𝐵 (λ) is the absorption from

the compound whose quantum yield is being determined (i.e., atrazine in this work).

The previously-reported energy-based approach (Eq. 2.3) is identical to the approach in Eq. 2.1, but is

a simplified version, which is based on the measured fluence-based atrazine decay rate constant that can

be obtained by plotting the logarithm of atrazine concentration against applied fluence.

∅𝑎𝑡𝑟𝑎𝑧𝑖𝑛𝑒 = 10𝑘𝑎𝑡𝑟𝑎𝑧𝑖𝑛𝑒

ln (10) ∫𝑁𝜆 𝜖𝑎𝑡𝑟𝑎𝑧𝑖𝑛𝑒 (𝜆)

𝑈(𝜆)

𝜆2𝜆1

Eq. 2.3

where katrazine is the experimental fluence-based rate constant of atrazine (mJ-1 cm2), Nλ is the ratio of

photon flow at wavelength λ to the total photon flow over the wavelength band from 200 to 400 nm,

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𝜖𝑎𝑡𝑟𝑎𝑧𝑖𝑛𝑒 is the molar absorption coefficient of atrazine at wavelength λ (M-1 cm-1), and U(λ) is the photon

energy at wavelength λ (J/Einstein).

There is an error contained in Eq. 2.3. The term inside the integral is a “weighting factor” that is used

to (erroneously) tally how much energy is available for photolysis by atrazine at each wavelength in its

adsorption spectrum. The sum total of energy absorbed is then assumed to be proportional to the amount

of atrazine that undergoes photolysis. This is incorrect. As noted by Bolton et al., 2015, the rate of a

photochemical reaction is proportional to the rate of photon absorption by atrazine, and not the rate of

energy absorption. In other words, the rate of atrazine decay would be the same whether absorbing a

constant number of photons per second if the photons where at 250 nm or 300 nm. The photons at 250 nm

would deliver more energy per second to the atrazine, but this is not important (provided that the quantum

yield at 250 nm and 300 nm is the same: an assumption that is correct in this case). Eq. 2.3 erroneously

suggests that absorption of photons of higher energy would cause more atrazine photolysis. Note,

however, that if using a monochromatic light source where all photons carry the same energy, Eq. 2.3

would provide a correct calculation of quantum yield.

There is a second inaccuracy in Eq. 2.3 stemming from the simplification of the calculation of the

fraction of light absorbed, 𝑥𝐵(λ), shown in Eq. 2.2, by using a Taylor series if background solution

absorption approaches zero (a clean water matrix) and 𝛼𝐵 (λ)𝑧 is less than 0.02 (e.g. a dilute atrazine

solution). The details for the Taylor series expansion are shown in Appendix A. The simplification is

believed to be a result of textbook theory developed prior to the common availability of computer

spreadsheets or software that eliminates the need for such simplifications.

2.2 OBJECTIVE

To illustrate the magnitude of the error that might be caused by the incorrect weighting factor and the

Taylor series expansion, a lab-scale experiment was conducted to calculate the quantum yield of atrazine

using both the (correct) photon-based approach (Eq. 2.1) and the (incorrect) energy-based approach (Eq

2.3). The general methodology was as follows:

1. Expose atrazine to UV light to measure the first order rate of photolysis. A lab-scale UV collimated

beam equipped with a 1 kW medium pressure (MP) mercury lamp was used.

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2. Calculate the quantum yield of atrazine by using both mathematical methods.

3. Perform a theoretical analysis to identify where the error would be more/less significant in other

situations.

2.3 MATERIALS AND METHOD

2.3.1 Reagents and Materials

Atrazine (≥99.9% pure, Sigma-Aldrich) was dissolved in Milli-Q water containing (v/v) 50%

methanol (Pestanal grade, Sigma-Aldrich) to prepare working solutions at an atrazine concentration of

approximately 324 μg/L (1.50 × 10-6 M). The methanol was required due to the low water solubility of

atrazine (water solubility = 33 mg/L (Mandelbaum et al., 1993); methanol solubility = 18,000 mg/L

(Tomlin, 1997).

The molar absorption coefficients of atrazine from 200-400 nm were determined by diluting the

atrazine standard stock solution (100 µg/ml in methanol) dissolved in methanol in water (Pestanal grade,

Sigma-Aldrich) to 1.195 mg/L (5.56× 10-6 M).

2.3.2 UV exposure and irradiance measurements

A lab-scale UV collimated beam apparatus (Model: PS1-1-120, Calgon Carbon Corporation) (Figure

2.1) equipped with a 1 kW MP mercury UV lamp (Heraeus Noblelight GmbH, Germany) was used to

irradiate 15 ml atrazine solutions contained in Pyrex Petri dishes having a diameter of 4.9 cm. A circular

screen with a 4.5 cm inner diameter opening was placed on the top of the Petri dish to reduce reflection

from the dish walls. Several exposure times were applied to deliver the desired photon fluence into each

sample.

Ferrioxalate actinometry was used to determine the incident photon irradiance (1.13 × 10-8 Einstein

cm-2 s-1) from 200 to 345 nm as calculated from the difference in the number of moles of Fe2+ produced

with or without placing a 345 nm long-pass filter on the top of a screen with a diameter of 1.5 cm,

according to the procedures described by Bolton et al. (2009) and Sharpless and Linden (2003). Since

quantum yield of Fe2+ is not consistent and linear from 200 to 400 nm, it was interpolated for a specific

region of wavelength according to measurements reported by Goldstein and Fabani (2008). The photon

irradiance (2.04 × 10-8 Einstein cm-2 s-1) at each wavelength from 200 to 400 nm was calculated after

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obtaining the relative emission spectrum of the MP lamp that was determined by normalizing the photon

irradiance at each wavelength relative to the total photon irradiance from 200 to 400 nm (Figure 2.2). A

reflection factor, determined according to equations reported by Edlen (1966) and Quan et al. (1995), was

the only correction factor that was applied to determine the average incident photon irradiance, while the

Petri factor, divergence factor, and water factor were not applied due to the use of the screens and

otherwise clean and pure water, respectively.

Figure 2.1 Picture of the lab-scale UV collimated beam apparatus

Figure 2.2 Relative spectral emittance of the MP lamp

0.00

0.02

0.04

0.06

0.08

0.10

200 220 240 260 280 300 320 340 360 380 400

Re

lati

ve

la

mp

em

iss

ion

Wavelength (nm)

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2.3.3 Analytical methods

Atrazine concentrations were measured by a gas chromatograph with electron capture detector (GC-

ECD) (Agilent Technologies 7890B), following USEPA Method 505 (USEPA, 1989). The method

detection limit was 6 μg/L. The relative emission spectrum of the MP lamp was measured using a

calibrated spectroradiometer (USB4000-UV-VIS, Ocean Optics) with a fiber-optic cable (QP200-2-SR-

BX, Ocean Optics) and a cosine corrector (CC-3-UV, Ocean Optics). The Petri factor was determined

using both the calibrated spectroradiometer and an IL-1700 radiometer that was calibrated using KI/KIO3

(iodide/iodate) actimometry (Bolton et al., 2009) at 254 nm. In addition, a Cecil UV/vis spectrophotometer

(CE3055, Cecil Instruments) was used to measure the absorbance of atrazine at a fixed wavelength and in

a wavelength band from 200 to 400 nm.

2.4 RESULTS AND DISCUSSION

2.4.1 Molar absorption coefficient of atrazine

Quantum yield determination requires measurement of the molar absorption coefficient of atrazine at

wavelengths ranging from 200 to 400 nm (Figure 2.3). The maximum molar absorption coefficient

(377,723 M-1 cm-1) appeared at 222 nm, and decreased to near zero beyond 310 nm. The molar absorption

coefficient at 254 nm was determined to be 3,456 M-1 cm-1 which is close to that (3,683 M-1 cm-1) reported

by Bolton et al. (2002). The absorbance of pure methanol, which was present as an aid to help to dissolve

the atrazine at a molar concentration of 1.82% relative to atrazine, was only 2% of atrazine absorbance

from 200 to 300 nm, and was therefore ignored.

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Figure 2.3 Molar absorption coefficient of atrazine from 200 to 400 nm

2.4.2 Results of quantum yield calculation

The quantum yield of atrazine photolysis determined using the MP collimated beam and when

calculated using a (correct) photon-fluence approach (Eq. 2.1) is 0.033. This is the same as that reported

by Stefan and Bolton (2008) when using a LP lamp. When using the previously-reported (incorrect)

energy-fluence approach along with its Taylor series expansion (Eq. 2.3), the calculated quantum yield is

0.044. This is a 27% error. The majority of this error (24%) is from the incorrect weighting factor, while

the Taylor series expansion approximation introduces only a 3% error.

2.4.3 Discussion

If a monochromatic light source is applied, the error in the “weighting factor” in the denominator of

Eq. 2.3 has no effect and the only inaccuracy is the small one associated with the Taylor series

simplification. If a polychromatic light source is used, the magnitude of the error in the weighting factor

will increase with the amount of photons being absorbed by a compound over a wider range of

wavelengths (i.e. a broader emission/absorption spectrum). As such, the error is likely to be largest when

applying polychromatic light with a compound that has a broad absorption spectrum.

2.5 SUMMARY AND CONCLUSIONS

This study demonstrated that the use of the previously-published method to calculate quantum yield

(Eq. 2.3) introduced a 27% error when using a medium pressure UV lamp, using atrazine as the case study.

222, coeff. = 37723

254, coeff. = 3456

0

10000

20000

30000

40000

200 220 240 260 280 300 320 340 360 380 400

Mo

lar

ab

so

rpti

on

co

eff

icie

nt (M

-1c

m-1

)

Wavelength (nm)

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The error would be larger for compounds that absorb light over a broader spectrum, and smaller for cases

where light is absorbed over a narrower spectrum (e.g. a monochromatic source, or a compound with a

fundamentally narrow absorption spectrum). The majority of the error associated with Eq. 2.3 is due to

the weighting factor, whereas the Taylor series expansion contributed to only 3% of the overall 27% error.

While this is small, there is no reason to apply the Taylor series simplification with modern spreadsheets.

2.6 REFERENCES

Bolton, J. R., and Stefan, M. I., 2002. Fundamental photochemical approach to the concepts of fluence

(UV dose) and electrical energy efficiency in photochemical degradation reactions. Research on

Chemical Intermediates. 28, 857-870.

Bolton, J. R., Stefan, M. I., Shaw, P.-S. and Lykke, K. R., 2009. Determination of the quantum yield of

the ferrioxalate and KI/KIO3 actinometers and a method for the calibration of radiometer detectors.

In: CDROM Proceedings 5th UV World Congress, Amsterdam, The Netherlands.

Bolton J. R., Mayor-Smith, I. and Linden, K. G., 2015. Rethinking the concepts of fluence (UV dose)

and fluence rate: The importance of photon-based units – A systemic review. Photochemistry

Photobiolology. 91 (6), 1252-1262.

Edlen, B., 1966. The refraction of air. Metrologia. 2, 71-80.

Goldstein, S., Aschengrau, D., Diamant, Y. and Rabani, J., 2007. Photolysis of aqueous H2O2: quantum

yield and applications for polychromatic UV actinometry in photoreactions. Environmental Science

& Technology. 41 (21), 7486-7490.

Quan, X. and Fry, E. S., 1995. Empirical equation for the index of refraction of seawater. Applied

Optics. 34, 3477-3480.

Sharpless, C. M. and Linden, K. G., 2003. Experimental and model comparisons of low- and medium-

pressure Hg lamps for the direct and H2O2 assisted UV photodegradatio of N-nitrosodimethylamine

in simulated drinking water. Environmental Science & Technology. 37 (9), 1933-1940.

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Stefan, M. I. and Bolton, J. R., 2005. Fundamental approach to the fluence-based kinetic and electrical

energy efficiency parameters in photochemical degradation reactions: polychromatic light. Journal

of Environmental Engineering and Science. 4, S13-S18.

Tomlin CDS., 1997. The pesticide manual – world compendium: 11th ed. Surrey, England: British Crop

Protection Council, 55.

United States Environmental Protection Agency (USEPA), 1989. Method 505. Analysis of organohalide

pesticides and commercial polychlorinated biphenyl (PCB) products in water by microextraction and

gas chromatography, Revision 2.0. [Online] Available: http://www.caslab.com/EPA-Method-505/

(assessed July, 2015)

Te Fang 12


CHAPTER 3: KINETIC MODEL OF THE UV/CHLORINE

ADVANCED OXIDATION PROCESS FOR THE DESTRUCTION

OF TRICHLOROETHYLENE

USING LOW PRESSURE UV LAMPS

ABSTRACT

A low pressure (LP) kinetic model was adapted from a previous medium

pressure (MP) model developed by Wang et al. (2012) to predict the decay

rate of trichloroethylene (TCE) when using LP lamps. Preliminary tests

demonstrated that TCE decay cannot be tested experimentally due to

volatility losses from the experimental apparatus during the long UV

exposure times. For this reason, this chapter only describes modelled TCE

decay rates with no experimental validation. The results are reported for

UV/alone, UV/chlorine, and UV/H2O2 under different water conditions.

The LP modelling results are consistent with the previously-reported MP

results, showing that UV/chlorine is more efficient than UV/H2O2 at pH 5

if using pure water, while it would be more competitive at higher pH in the

presence of organic and inorganic scavengers.

3.1 INTRODUCTION

Wang et al. (2012) developed a mathematical model that predicted TCE destruction due to the medium

pressure (MP) UV/chlorine and UV/H2O2 advanced oxidation processes (AOPs) under different water

quality conditions, with the modelling results then confirmed by bench-scale experimentation. In that

work, both the modelling and experimental results demonstrated that the decay of TCE in otherwise pure

water was more efficient when using UV/chlorine compared to UV/H2O2 at approximately pH 5 and

below. However, as hydroxyl radical scavenger concentrations increased, using TOC as a surrogate, the

pH at which UV/chlorine was competitive was predicted to increase such that both UV/chlorine and

UV/H2O2 led to equal modelled rates of TCE decay at pH 7 when the TOC was approximately 5 mg-C/L.

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In theory, UV/chlorine may be more cost competitive relative to UV/H2O2 when using MP UV lamps

than LP due to the high absorbance of chlorine and corresponding lack of absorbance of H2O2 at

wavelengths ranging from 270 to 350 nm (Figure 3.1) (Wang et al., 2012). In this work, low pressure

reaction kinetic models of UV/Cl2 and UV/H2O2 were developed similar to the previous medium pressure

models to investigate the TCE decay rate by UV/Cl2, UV/H2O2, and UV alone, as well as the effects of

pH, TOC, and total inorganic carbon (TIC) on the efficiency of TCE removal. The models were also used

to compare the difference in UV/chlorine treatment efficiency between LP and MP UV lamps.

Figure 3.1 Molar absorption coefficients of HOCl, OCl-, H2O2, and HO2- (Wang et al., 2012)

3.2 INABILITY TO PERFORM TCE EXPERIMENTS TO CONFIRM THE MODEL

In medium pressure UV-AOP tests conducted by Wang et al. (2012) to monitor the decay of TCE, it

was reported that 25 ± 0.51% of the initial TCE was lost due to evaporation within 5 min of exposure to

air without the addition of either free chlorine or H2O2 (the UV collimated beam test is open to the

atmosphere). This loss was accounted for in the model. The exposure period for the LP lamp is much

longer than that of MP lamp—30 minutes or more—and it was suspected that an LP experiment might not

be a feasible means of confirming the kinetic model predictions, since volatility would be the dominant

mechanism of TCE loss by a large margin. To verify this, evaporation tests were performed which

simulated the LP UV collimated beam test conditions to observe TCE losses.

HOCl

OCl-

H2O2

HO2-

0

100

200

300

400

500

600

200 240 280 320 360 400

Mo

lar

ab

so

rpti

on

co

eff

icie

nts

of

HO

Cl,

O

Cl-, H

2O

2, a

nd

HO

2-(M

-1cm

-1)

Wavelength (nm)

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Two evaporation trials were performed to determine an approximate percent loss of TCE due to

evaporation after 30 minutes of exposure to air in a 50 ml beaker. TCE working solutions were prepared

by diluting an appropriate volume of TCE (ClHC=CCl2, molecular weight: 131.39 g/mole, ≥99.5%,

A.C.S. grade, Sigma-Aldrich) in Milli-Q water at a final concentration of 22.6 μg/L and 31.7 μg/L for the

first and second evaporation tests, respectively. A 15 ml TCE sample was placed in a 50 ml beaker with a

gentle stir. TCE concentrations were measured by gas chromatography-electron capture detector (GC-

ECD) (Agilent Technologies 7890B), following USEPA Method 551.1 (USEPA, 2008). Based on 10

replicates, approximately 48.4 % (first test) and 65.5% (second test) of the initial TCE concentration was

lost (57.0% on average). While the containers used in the tests were not identical to those in the UV

collimated beam exposure experiment, which uses a Petri dish, it is expected that there will be a similarly

high loss of TCE due to volatility. For this reason, a low pressure UV collimated beam experiment to

determine TCE decay could not be conducted to confirm the results from the reaction kinetics model.

3.3 LOW PRESSURE KINETIC MODEL

3.3.1 Kinetic parameters and reaction equations

A reaction kinetic model was used to predict TCE decay by UV alone, UV/chlorine, and UV/H2O2

AOPs using LP UV lamps. The majority of the kinetic parameters used in the low pressure model were

the same as those used by Wang et al. (2012). Reactions used in the model are summarized in Table 3.1.

The quantum yield of TCE photolysis is taken as the sum of the quantum yields of each chain reaction

(Reactions 1 to 4). The quantum yield of HO2- photolysis and the quantum yield of OH radical (∙OH)

production from HO2- photolysis were assumed to be one, given a lack of published literature values. HO2

-

concentrations are likely only to become important as the pH approaches the pKa of H2O2, which is 11.6

(Song, 1996). Reactions 12-16 are the dominant reactions occurring in the UV/H2O2 process.

There are several chain reactions that may occur due to OH or Cl radicals reacting with organics (Table

3.2), which can result in extra consumption of HOCl, but these chain reactions were not included in the

model due to the lack of related rate constants.

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Table 3.1 Reaction parameters used in model

Reaction

Number Reaction Rate constants Reference

UV alone

1 TCE + hv → ClHC=C∙Cl + Cl∙ 𝜙TCE,1 = 0.13 Li et al., 2004 2 TCE(H2O) + hv →

ClHC(OH)CHCl2

𝜙TCE,2 = 0.1 Li et al., 2004

3 TCE + h𝜈 → HC≡CCl + Cl2 𝜙TCE,3 = 0.032 Li et al., 2004 4 TCE + hv → ClC≡CCl + HCl 𝜙TCE,4 = 0.092 Li et al., 2004 5 TCE + Cl∙ → Cl2HC-C∙Cl2 4.88×1010 M-1 s-1 Li et al., 2004

UV/chlorine

AOP

6 OCl- + H2O → HOCl + OH- kforward = 1.8×103 s-1 Fogelman et al. 1989 kbackward = 3.0×109 M-1 s-1 7 HOCl + hv → ∙OH + Cl∙ 𝜙HOCl = 1.0 Feng et al. 2007 𝜙HOCl,OH = 0.46 Jing et al. 2011 8 OCl- + hv → ∙OH + other products 𝜙OCl- = 0.9 Feng et al., 2007 𝜙OCl-, OH = 0.7 Chan et al. 2012 9 HOCl + ∙OH → H2O + ClO∙ 8.46×104 M-1 s-1 Watts and Linden, 2007

10 OCl-+ ∙OH → ClO∙ + OH- 8.8×109 M-1 s-1 Buxton and Subhani, 1972b 11 TCE + ∙OH → ClCH(OH)-C∙Cl2 2.4×109 M-1 s-1 Li et al., 2007

UV/H2O2

AOP

12 H2O2 → H+ + HO2- kforward = 0.126 s-1 Song, 1996 kbackward = 5×1010 M-1 s-1

13 H2O2 + hv → 2∙OH 𝜙H2O2 = 1.0 Stefan et al., 1996 𝜙H2O2, OH = 1.11 Goldstein et al., 2007

14 HO2- + hv → 2∙OH 𝜙HO2- = 1.0 (assumed) 𝜙HO2-, OH = 1.0 (assumed)

15 H2O2 + ∙OH → HO2∙ + H2O 3.2×107 M-1 s-1 Yu, 2004 16 HO2- + ∙OH → ∙O2- + H2O 7.5×109 M-1 s-1 Stefan et al., 1996 17 ∙OH + ∙OH → H2O2 1.1×1010 M-1 s-1 Buxton et al., 1988 18 HO2∙ + HO2∙ → H2O2 + O2 8.3×105 M-1 s-1 Bielski et al., 1985 19 HO2∙ + H2O2 → H2O + O2 + ∙OH 3.7 M-1 s-1 Farhataziz and Ross, 1977 20 HO2∙ + ∙OH → H2O + O2 6×109 M-1 s-1 Buxton et al., 1988

Scavengers

21 TOC + ∙OH → products 3.0×108 M-1 s-1 Westerhoff et al., 1999 22 HCO3- + ∙OH → products 8.5×106 M-1 s-1 Isil, et al., 2004 23 CO32- + ∙OH → products 3.9×108 M-1 s-1 Boxton et al., 1988 24 HCO3- ↔ CO32- + H+ kforward = 2.345 s-1 Zeebe, et al., 2001 kbackward = 5×1010 M-1 s-1

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Table 3.2 Chain reactions associated with OH and Cl radicals in the presence of organic scavengers

OH chain reactions ∙OH + RH → ∙R + H2O Oliver and Carey (1977) ∙R + HOCl → RCl + ∙OH Cl chain reactions ∙Cl + RH → ∙R + HCl Oliver and Carey (1977) ∙R + HOCl → ROH + ∙Cl

3.3.2 Discrepancies in the reported quantum yields of OH production

Buxton and Subhani (1972) defined a yield factor to describe the amount of OH radical produced by

chlorine photolysis, which is expressed as:

Ƞ = ∆ [∙𝑂𝐻]

∆[𝑎𝑐𝑡𝑖𝑣𝑒 𝐶𝑙] Eq. 3.1

This yield factor suggests that the amount of OH formed is a simple function of the amount of chlorine

photolyzed. In practice, however, it appears that this yield factor is not constant, and may vary due to

complexities in chlorine photolysis. This is summarized in Table 3.3, which shows the published quantum

yields of OH radical production from photolysis of HOCl and OCl-.

Table 3.3 Published quantum yield of OH production due to free chlorine photolysis

Quantum yield of OH formation Reference

HOCl OCl-

0.79±0.01 1.18±0.12 Wang et al. (2012) 0.46±0.09 0.70±0.02 Jin et al. (2011); Chan et al. (2012)

0.85 0.12 Nowell and Hoigne (1992b)

1.4 0.28 Watts and Linden (2007); Watts et al., 2007

1.0 1.2 Feng et al. (2007)

The methods that were used to determine the quantum yields of OH production reported in Table 3.3

varied significantly, which might partially account for the differences. Jin et al. (2011) used methanol as

a scavenger to determine the amount of OH radicals generated in the UV/chlorine process by measuring

production of formaldehyde upon methanol reaction with OH at pH 5 with 300 s UV irradiation using a

low pressure high output lamp with an irradiance of 0.38 mW cm-2. The quantum yield of OH radical

formation due to HOCl decomposition was reported as 0.46±0.09 with an initial chlorine concentration

Te Fang 17


of 50 mg-Cl/L (1.41 mM). Feng et al. (2007) reported the quantum yield from HOCl as 1.0 at pH 5, using

the same method as Jin et al. but with a much higher chlorine concentration (213 mg-Cl/L; 6 mM). Nowell

and Hoigne (1992b) reported a quantum yield from HOCl of 0.85 at pH 5. In their work, the rate of OH

radical production was determined by directly measuring the degradation rate of chlorobutane and

nitrobenzene in the absence of scavengers with a chlorine concentration of 0.11 mM using a LP lamp

which emits 95% of its total energy at 255 nm with a measured photon irradiance of 3.7 µEinstein l-1 s-1.

The quantum yield due to OCl- photolysis measured by Chan et al. (2012) was 0.70±0.02 using the

same method as Jin et al. (2011) with a OCl- concentration of 1.13 mM, under a MP lamp with a 303 nm

filter. This value is much higher than that reported by Nowell and Hoigne (1992b) (0.12 at pH 10). The

quantum yields determined by Wang et al. (2012) were derived from an observed first order fluence-based

decay rate of TCE in pure water with a chlorine concentration of 0.155 mM using a MP lamp with an

incident irradiance of 6.33 mW cm-2 from 200 to 400 nm.

Compared to the chlorine concentrations mentioned above, the concentration used in this study was

much lower, at 0.14 mM (10 mg-Cl2/L), along with a low pressure lamp. As such, the model used values

of 0.46 (Jin et al., 2011) and 0.7 (Chan et al., 2012) for the quantum yield of OH formation due to HOCl

and OCl- photolysis, respectively, since both of these studies used relatively small chlorine concentrations

and monochromatic light sources. However, the formation rate of OH radicals is nearly proportional to

the quantum yield of either HOCl or OCl- photolysis, so that the use of an improper published value may

lead to error.

3.3.3 Modelling conditions

The model was used to predict TCE degradation with an initial concentration of 1.1× 10-6 M at pH

5, 7.5, and 10, with the following water quality conditions:

1. Pure water, assuming that TCE is the only species present except for 0.155 mg-C/L TOC and

0.71 mg-CaCO3/L TIC in the solution (this is the normal amount expected in Milli-Q water).

2. Natural water, containing various concentrations of natural organic matter (NOM) using TOC as

a surrogate at concentrations from 0 to 10 mg-C/L.

3. Pure water with inorganic carbon ranging from 0 to 500 mg/L as CaCO3.

4. Pure water containing different initial chlorine concentrations (0 to 1000 mg/L as Cl2).

Te Fang 18


The model was also used to compare the rate of TCE decay in a Petri dish due to UV/chlorine

relative to UV/H2O2, using MP and LP lamps in a collimated beam apparatus that applied an equal

incident photon irradiance (1.81× 10-8 Einstein cm-2 s-1), which is the reported incident irradiance of the

MP collimated beam determined by Wang et al. (2012).

3.3.4 Model equations

The equations used in the model are based on the steady-state assumption, that is, the concentration of

chlorine or hydrogen peroxide will not change significantly during oxidation of TCE. For this reason, the

concentration of produced OH radical is essentially constant.

The overall kinetics of TCE decay in either the UV/chlorine or the UV/H2O2 system can be

summarized as the sum of direct photolysis by UV exposure and indirect photolysis due to reaction with

the OH radical. The kinetic equation is written as:

dTCE

dt = -(kuv + kTCE,OH[∙ OH]ss) [TCE] = -ktotal [TCE] Eq. 3.2

where [∙ OH]ss is the steady state concentration (M) of ∙OH, kTCE,OH is the second-order rate constant (M-

1 s-1) between TCE and ∙OH, and ktotal is the apparent rate constant comprised of both direct photolysis

and ∙OH reaction mechanisms.

The steady state concentration (M) of ∙OH can be calculated by the following equation, assuming that

the quantum yield of ∙OH formation is independent of wavelength:

[∙OH]ss = ΦB,OH×

Ep(254) ×ε254×[1−10−α254z]×1000

α254z×[B]

[B]×KB,OH + [DOC]×KDOC,OH + [TCE]×KTCE,OH+[S]×ks,oH Eq. 3.3

where ΦB,OH is the quantum yield of ∙OH formation by either chlorine or hydrogen peroxide photolysis

(Table 3.1), Ep(254) is the incident photon irradiance (Einstein cm-2 s-1) at 254 nm, 𝜀254 is the molar

absorption coefficient (M-1 cm-1) of the chlorine or hydrogen peroxide species at 254 nm, α254is the

decadic absorption coefficient (cm-1) of the solution at 254 nm, z is the depth of solution (cm), 1000 is the

Te Fang 19


conversion factor from cm3 to L, [B] is the concentration (M) of oxidant, i.e., chlorine or hydrogen

peroxide species, kB,OH is the second-order reaction rate constant (M-1 s-1) between oxidants and ∙OH,

KDOC,OH and KTCE,OH are the second-order reaction rate constants (M-1 s-1) between DOC or TCE and ∙OH,

[DOC] and [TCE] are expressed in (M), ks,oH represents the second-order reaction rate constants (M-1 s-

1) between other scavenger (except for those mentioned above) and ∙OH, such as carbonate, bicarbonate,

nitrite, and nitrate, and [S] is the concentration (M) of the corresponding scavenger S. Additional

derivation of the modelling equations is given in Appendix A.

3.4 RESULTS AND DISCUSSION

3.4.1 Low pressure modelling results of TCE decay in pure water

The rate of TCE decay in pure water that contains only TCE and spiked oxidants (i.e., free chlorine or

H2O2) was modelled for direct LP UV photolysis alone, reaction in a LP UV/chlorine system, and reaction

with LP UV/H2O2. In all cases the decay followed pseudo first-order kinetics. Equal incident photon

irradiances (3.03× 10-10 Einstein cm-2 s-1) were modelled for both UV/chlorine and UV/H2O2 systems,

although the actual absorbed photon fluence would be different due to the differing light absorption

properties of chlorine and hydrogen peroxide. The resulting decay constants (i.e. ktotal in Eq. 3.2) are shown

in Table 3.4.

The data demonstrated that TCE decay by direct photolysis is less than 1% of the decay due to

advanced oxidation. UV/chlorine led to 19% faster decay of TCE at pH 5 compared to UV/H2O2, while

UV/H2O2 resulted in roughly two magnitudes faster TCE decay than UV/chlorine at pH 7.5 and 10. The

significant pH dependence of UV/chlorine is due to chlorine’s pKa of 7.5, with OCl- predominating above

pH 7.5 and having a five-times higher OH radical scavenging rate compared to HOCl, which slows the

TCE decay. In contrast, H2O2 has a much higher pKa of 11.6, making UV/H2O2 efficiency largely

independent of pH in the range of 5-8. Only as pH increases towards 10, where the formation of HO2-

starts to become significant and its scavenging potential begins to be exerted, does the predicted TCE

decay rate decrease.

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Table 3.4 Modelled results of TCE photon fluence-based decay rate constants (Einstein-1 cm2) based on

absorbed fluence by LP UV alone, LP UV/chlorine, and LP UV/H2O2

pH 5 pH 7.5 pH 10

UV alone 9.58 9.58 9.58

UV/chlorine 2.29×106 4.05×104 2.49×104 UV/H2O2 1.85×106 1.83×106 5.86×105

3.4.2 Effect of LP vs. MP lamps

The model can be used to compare the TCE decay rate by UV/chlorine and UV/H2O2 with LP

(monochromatic) versus MP (polychromatic) lamps, when applying equal incident photon irradiance

(1.81× 10-8 Einstein cm-2 s-1). This is a way to compare the efficiencies of LP versus MP systems when

normalized to the same amount of photons. The predicted decay rates are again pseudo first-order, with

the rate constants shown in Table 3.5.

Table 3.5 Predicted TCE first order decay rate constants (s-1 × 10-4) by UV/chlorine and UV/H2O2 using LP and MP lamps

pH 5 pH 7.5 pH 10

UV/chlorine LP 438 7 4

MP 338 52 49

UV/H2O2 LP 347 340 105

MP 237 234 88

In general, the pH trend for MP is more or less the same as mentioned earlier for LP in that the

efficiency of UV/chlorine for TCE destruction is highest at pH 5, but significantly reduces as pH increases

to 7.5 and higher, while UV/H2O2 is more resistant to pH change.

For UV/chlorine, LP is more efficient than MP at pH 5 when emitting the same total number of

photons, but the reverse is true at pH 7.5 and 10. For a MP lamp, the majority of the photon irradiance—

approximately 80%—is applied at wavelengths above 260 nm (1.448×10-8 Einstein cm-2 s-1 out of the

total photon irradiance 1.81×10-8 Einstein cm-2 s-1 from 200 to 400 nm, as shown in Figure 2.2) where

HOCl that is predominant at pH 5 has a relative small absorption ability compared to that below 260 nm.

In contrast, if the irradiance is completely emitted at 254 nm (i.e., using a LP lamp) which corresponds to

a strong absorbance by HOCl, then the efficiency will be higher. However, as pH increases to 7.5 and 10,

OCl- becomes the predominant form of chlorine which has a strong absorbance from 250 to 350 nm, which

Te Fang 21


is in the region where the majority of MP light is emitted, making UV/chlorine more competitive relative

to UV/H2O2 at the higher pH values when using MP lamps compared to LP.

For UV/H2O2, LP is always more efficient than MP if applying an equal incident photon irradiance

since only a small portion (35%) of the total photon irradiance of MP lamps is present from 200 to 300

nm (i.e., 6.34×10-9 Einstein cm-2 s-1) where H2O2 and HO2- have a strong UV absorption ability. The

remaining MP photons at wavelengths above 300 nm are therefore wasted.

3.4.3 Effect of natural organic matter (NOM)

The effect of NOM on the rate of TCE decay during UV/chlorine and UV/H2O2 treatments was

modelled with NOM concentrations, as measured by TOC, ranging from 0 to 10 mg-C/L. NOM reduces

the TCE decay rate by scavenging the OH radical. For UV/chlorine, an increase in TOC concentration

had a significant effect on the first-order decay rate coefficient of TCE at pH 5, with a 97.3% reduction as

TOC concentration increased from 0 to 10 mg-C/L. The effect, however, was relatively small at pH 7.5

(26.7% decrease) and 10 (15.5% decrease) (Figure 3.2). The reason for the impact of pH on the importance

of TOC is that at high pH, the predominant OH scavenger in the system is OCl- and so changes to TOC

concentration are relatively unimportant. At pH 5, since HOCl is a weaker scavenger, the impact of TOC

on scavenging is much greater. In contrast, the TCE decay rate in the UV/H2O2 process was always

sensitive to an increase in TOC concentration, with reductions in the decay coefficient always greater than

90% as TOC increased to 10 mg-C/L regardless of pH (Figure 3.3). This is because the scavenging of

H2O2 is relatively consistent across the pH range modelled so the relative contribution of TOC to the

overall scavenging also remains consistent with pH.

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Figure 3.2 TCE first order decay rate constant as a function of TOC in UV/chlorine at three pHs, along

with percent reduction of the rate with increasing TOC concentration

Figure 3.3 TCE first order decay rate constants as a function of TOC in UV/H2O2 at three pHs, along

with percent reduction of the rate with increasing TOC concentration

Figure 3.4 shows the predicted solution pH at which the UV/chlorine and UV/H2O2 process are equally

efficient, i.e., producing the same steady state concentration of OH radicals given the same molar

concentrations of chlorine and H2O2 under the same incident photon irradiance. It can be observed that

the pH at which UV/chlorine becomes competitive relative to UV/H2O2 increases as the TOC

concentration increases. In the absence of TOC, the pH of “equal efficiency” was approximately 5.3, and

it increased to 7.1 when the TOC was 10 mg-C/L when using LP lamps. When using MP lamps, the TOC

concentration of “equal efficiency” tends to be lower. For example, UV/chlorine and UV/H2O2 are equally

pH 597.3%

pH 7.526.7%

pH 1015.5%

0.0

0.6

1.2

1.8

2.4

0

40

80

120

160

200

0 2 4 6 8 10

TC

E d

ecay r

ate

co

nsta

nt (s

-1)

pH

7.5

& 1

0

TC

E d

ecay r

ate

co

nsta

nt (s

-1)

pH

5

TOC (mg-C/L)

×10-5 ×10-5

pH 5 & 7.597.3%

pH 1087.6%

0

40

80

120

160

200

0 2 4 6 8 10

TC

E d

ecay r

ate

co

nsta

nt (s

-1)

TOC (mg-C/L)

×10-5

Te Fang 23


efficient at pH 7.1 when only 4.2 mg-C/L TOC is present when using MP lamps, compared to 10 mg-C/L

for LP lamps. The reason for these effects can be explained by considering that in general, the main

weakness of UV/chlorine relative to UV/H2O2 is the strong OH scavenging by OCl-. Any condition that

minimizes the adverse impact of OCl- will make UV/chlorine more competitive. When there is a greater

concentration of other scavengers present (e.g. TOC), the relative adverse impact of OCl- becomes smaller,

and so UV/chlorine treatment becomes more competitive relative to UV/H2O2 as shown by the rising

curves in Figure 3.4 for both LP and MP systems. Similarly, the main benefit to MP lamps versus LP for

UV/chlorine is the ability of MP lamps to photolyse OCl- to produce OH radicals. With higher TOC

concentrations tending to mask the adverse effect of OCl-, MP lamps become more competitive relative

to LP lamps at the higher pH values where UV/OCl- photolysis begins to occur.

Figure 3.4 Solution pH at which the UV/chlorine and the UV/H2O2 AOPs with LP and MP lamps are

equally efficient as functions of TOC concentration

It is important to note that the modelled impact of TOC on the advanced oxidation processes assumed

a literature value for the rate of scavenging of OH by TOC. Arakaki et al. (2013) reported that the mean

value of the rate constant is 3.0±2.2 ×108 M-1 s-1 when using organic substances extracted from

atmospheric water samples, 5.4±3.6 ×108 M-1 s-1 from terrestrial waters, and 3.5±2.0 ×108 M-1 s-1 by

directly using aquatic humic substances. The percent difference between these highest and lowest average

values is about 44%. This variability affects the predicted pH values of “equal efficiency” between

UV/chlorine and UV/H2O2, as illustrated in Figure 3.5 whereby the value for the reaction rate between

TOC and OH is arbitrarily increased by factors of 2 and 3 over the value used in the model (3×108 M-1s-

5.0

5.5

6.0

6.5

7.0

7.5

8.0

0 2 4 6 8 10

Eq

ua

lly

eff

icie

nt p

H

TOC concentration (mg-C/L)

MP

LPUV/H2O2more efficient

UV/chlorinemore efficient

Te Fang 24


1). When the TOC was assumed to be more reactive with OH than originally modelled, UV/chlorine

becomes more efficient than UV/H2O2 at pH values about 0.5-1 units higher.

Figure 3.5 Effect of increasing reaction rate of TOC with OH radical on pH of equal efficiency between

UV/chlorine and UV/H2O2

3.4.4 Effect of total inorganic carbon (TIC)

To illustrate the effect of TIC on the decay rate of TCE in the UV/chlorine system, the ratio of

scavenging potential from HCO3- and CO3

2- to total scavenging potential including chlorine was estimated

by the model (Figure 3.6). The TIC contribution to overall scavenging is observed to be lowest around pH

7.5. The pKa of both bicarbonate (HCO3-CO3

2-) and hypochlorous acid (HClOClO-) are around pH

7.5. At pH 5, HOCl is a weak scavenger so bicarbonate concentration has a relatively large impact on

overall scavenging as shown in Figure 3.6. At pH 10, carbonate is a very strong scavenger so again, TIC

dominates the scavenging relative to chlorine. At pH 7.5, however, there is enough OCl- present and a

lesser amount of carbonate such that chlorine tends to dominate the overall scavenging relative to TIC.

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

0 1 2 3 4 5 6 7 8 9 10

Eq

uall

y e

ffic

ien

t p

H

TOC concentration (mg-C/L)

3 times model value

2 times model value

Model value

Te Fang 25


Figure 3.6 Contribution of inorganic carbon to total scavenging potential including chlorine:

UV/chlorine system

The predicted TCE decay rate constant for the UV/chlorine LP system as affected by TIC reflects

these scavenging trends, as shown in Figure 3.7. The TCE decay rate is least affected by TIC around pH

7.5, whereas it is much more sensitive at pH 5 or 10. In contrast, the effect of TIC on UV/H2O2

performance is more straightforward, largely reflecting the amount of carbonate relative to bicarbonate

since it is the much stronger scavenger (Figure 3.8 and Figure 3.9). The impact of TIC on UV/2O2 is

similar at pH 5 and 7.5 where carbonate concentrations are relatively small, but increases at pH 10 due to

carbonate’s dominance.

Figure 3.7 TCE first order decay rate constant of UV/Cl2 as a function of alkalinity at three pHs

66.3

90.8

1.36.38.6

31.9

0

10

20

30

40

50

60

70

80

90

100

100 500

% o

f in

org

an

ic c

arb

on

s to

to

tal

sc

av

en

gin

g p

ote

nti

al i

nc

lud

ing

ch

lori

ne

Alkalinity (mg-CaCO3/L)

pH 5

pH 7.5

pH 10

pH 1055%

pH 586.2%

pH 7.56.4%

0.0

0.4

0.8

1.2

1.6

0

40

80

120

0 100 200 300 400 500

TC

E d

ecay r

ate

co

nsta

nt (s

-1)

pH

7.5

& 1

0

TC

E d

ecay r

ate

co

nsta

nt (s

-1)

pH

5


×10-5 ×10-5

Te Fang 26


Figure 3.8 TCE fist order decay rate constant of UV/H2O2 as a function of alkalinity at three pHs

Figure 3.9 Percentage of inorganic carbon to total scavenging potential including H2O2 in the function of

alkalinity.

3.4.5 Effect of concentration of active chlorine

The concentration of active chlorine directly determines the amount of photon absorbance and

subsequent OH production. The initial formation rate of OH radicals can be expressed as:

OH formation rate (M s-1) = ΦCl2,OH ×Ep(254) ×ε254×[1−10

−α254z]×1000

α254z× [chlorine] Eq. 3.4

pH 5 & 7.586.3 & 87%

pH 1095.4%

0

40

80

120

0 100 200 300 400 500

TC

E d

ec

ay

ra

te c

on

sta

nt (s

-1)


×10-5

67.0

91.0

67.9

91.4

79.6

95.1

0

10

20

30

40

50

60

70

80

90

100

100 500

%o

f in

org

an

ic c

arb

on

s to

to

tal

sc

av

en

gin

g p

ote

nti

al i

nc

lud

ing

H2O

2


pH 5

pH 7.5

pH 10

Te Fang 27


where ΦCl2,OH is the quantum yield of ∙OH formation by chlorine photolysis (Table 3.1), Ep(254) is the

incident photon irradiance (Einstein cm-2 s-1) at 254 nm, 𝜀254 is the molar absorption coefficient (M-1 cm-

1) of chlorine at 254 nm, α254is the decadic absorption coefficient (cm-1) of the solution at 254 nm, z is

the depth of solution (cm), 1000 is the conversion factor from cm3 to L, and [chlorine] is the chlorine

concentration (M).

At the same time, chlorine scavenges the produced OH radicals with a rate that can be expressed as:

OH scavenging rate (M s-1) = kCl2,OH × [chlorine] × [OH] + ks,oH × [S] × [OH] Eq. 3.5

where kCl2,OH is the second-order reaction rate constant (M-1 s-1) between chlorine and ∙OH, ks,oH

represents the second-order reaction rate constants (M-1 s-1) between other scavengers and ∙OH, such as

carbonate, bicarbonate, nitrite, and nitrate, [OH] is the concentration (M) of OH radicals, and [S] is the

concentration (M) of the corresponding scavenger S. In this section, it is assumed that chlorine is the only

present scavenger of OH radicals.

In the case where chlorine is the only scavenger, then the net formation rate of OH radicals can be

calculated as follows:

OH formation rate - OH scavenging rate by chlorine

ΦCl2,OH ×Ep(254) × ε254 × [1 − 10

−ε254[chlorine]z] × 1000

ε254[chlorine]z× [chlorine] - kCl2,OH × [chlorine] × [OH]

ΦCl2,OH ×Ep(254) × [1 − 10

−ε254[chlorine]z] × 1000

z

- kCl2,OH × [chlorine] × [OH] Eq. 3.6

A B

Eq. 3.6 is the final equation showing the net formation rate of OH radicals. At the early stage, both

formation and scavenging rates would show a significant increase with increasing chlorine concentration,

and A is much higher than B; however, the OH formation rate would eventually reach a plateau since the

value in the bracket in the numerator of A would be extremely close to one. In contrast, the scavenging

Te Fang 28


rate would keep increasing, and consequently the gap between formation and scavenging rate would be

smaller.

Figure 3.10 shows the resulting net formation rate of OH radicals with increasing initial chlorine

concentration at three pHs. As expected, it grows fast at the beginning (where A>> B), but reduces a lot

with further increasing chlorine concentration where A starts to reach a plateau and B keeps growing.

The solution pH affects the optimum chlorine concentration. The optimum chlorine is 127 mg-Cl2/L

at pH 5 (Figure 3.10 (a)), but is only 10 and 7 mg-Cl2/L at pH 7.5 and 10 (Figure 3.10 (b)), respectively.

This trend is because because OCl- plays a major role in scavenging OH radicals, more so than HOCl

(pr

comparing uv/chlorine advanced oxidation efficiency to uv ... · presence of elevated total...

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