rational design of advanced oxidation processes using...

Daisuke Minakata1, Ke Li2,

John C. Crittenden1, David Hand3

1 School of Civil and Environmental EngineeringGeorgia Institute of Technology2 Faculty of Engineering, University of Georgia3 Civil and Environmental EngineeringMichigan Technological University

AOT 16, November 15-18, 2010 San Diego, CA.

Rational Design of Advanced Oxidation Processes using Computational Chemistry

1

Modeling AOPs

1. Simplified Pseudo-Steady-State Model

2. AdoxTM – AOP simulation software

3. Conventional Approach for Kinetic Model

4. Computer-based Kinetic Model

2

3. H2O2 is added to a water containing O3, i.e. [O3]0 is known.

3

2. H2O2 and O3 are added simultaneously

3 3O O

ss, 09 2 10 2 2 11 3 12 130 0 00 0

HOHO H O HCO R NOM

LK a P H

k k k k k

UV/H2O2 H2O2/O3 Simplified pseudo-steady-state model

R 12 ss, 0HOk k

H O2 2pH p

1 2 2 30 resss, 0

11 3 12 130 00

2 H O 10 OHO

HCO R NOM

Kk

k k k

2 2 2 2

-A

H O 0 H O

ss,0

10 2 2 0 11 3 0 12 0 13 0

2 I f (1- e )[HO ]

k [H O ] k [HCO ] k [R] k [NOM]

g

1. UV/H2O2

4

Flow Models: Ideal flow at steady state (1st order reaction)

1. Plug Flow Reactor (PFR)

k

0C C e

2. Completely Mixed Flow Reactor (CMFR)

0CC

(1 k)

3. Tank-in-series (TIS)

0

n

CC

1 kn

1/ n

0C nQV 1

C k

0Q C CV

r

0CQV ln

k C

5

Flow Models: Non-ideal flow

4. Dispersed flow model closed system

A

2 2Ao

1 vL4a exp

2 EC

a vL a vLC (1 a exp (1 a exp) )2 E 2 E

a 1 4k (E / vL)

7

Known reaction rate constants

Compoundk HO•

M-1

s-1 [HO•]=10

-9 M [HO•]=10

-10 M [HO•]=10

-11 M

MtBE 1.6×109 0.01 0.1 1

Oxalic acid 1.4×106 8 83 825

Acetate ion 7×107 0.2 2 17

Trichloromethane 5.0×106 2 23 231

1,1,2-Trichloroethane 1.1×108 0.11 1 11

Chloroform 5×106 2 23 231

Chloroacetic cid 4.3×107 0.3 2.7 27

Glycolic acid 6×108 0.02 0.2 2

1,1,1-Trichloroethane 4×107 0.3 3 29

Benzene 7.8×109 0.001 0.01 0.1

Phenol 6.6×109 0.002 0.02 0.2

Halflife, min

kHCO3- = 8.5×106 M-1s-1

kCO3-- = 3.9×108 M-1s-1

H2CO3* ↔ H+ + HCO3-

(pKa=6.3)HCO3- ↔ H+ + CO3

2-

(pKa=10.3)

kNOM =(3.0 ~ 4.5)×108 M-1s-1

8

Hypothesis -GCM-

Essence is same as the GCM for the gaseous phase Observed experimental reaction rate constant for a

given organic compound is the combined rate of all elementary reactions involving HO•, which can be estimated using Arrhenius kinetic expression.

The Ea consists of two parts: (1) Base part from main reaction mechanisms (i.e.,

H-atom abstraction, HO• addition to alkenes and aromatic compounds and HO• interaction with S, N, or P-atom-containing compounds).

(2) Functional group contribution partially from neighboring and/or next nearest neighboring functional group.

= aE

RTk Ae

Minakata, Li, Westerhoff, Crittenden, 2009 ES&T 43, 6220-6227

9

• Over or under prediction for the oxygenated multifunctional group compounds due to solvation effects, and for the halogenated and carboxylic compounds due to larger steric hindrance

GCM Results -overall 2/2-

1.0E+05

1.0E+06

1.0E+07

1.0E+08

1.0E+09

1.0E+10

1.0E+11

1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09 1.0E+10 1.0E+11

kca

l (M

-1s-1

)

k exp (M-1s-1)

Ideal fit

kexp/kcal = 0.5 and 2.0

H-atom abstraction (Calibrated)

HO addition to alkene (Calibrated)

HO interaction with N,S,P-atom-

containing compounds (Calibrated)

HO addition to aromatic compounds

(Calibrated)

H-atom abstraction (Prediction)

HO interaction with N, S, P-atom-

containing compounds (Prediction)

HO addition to aromatic compounds

(Prediction)

Minakata, Li, Westerhoff, Crittenden, 2009 ES&T 43, 6220-6227

GCM is limited because:1. Many datasets are required to calibrate parameters2. Only deal with molecules for which all required group rate constants

and group contribution factors have been calibrated before.3. Limited number of literature-reported experimental rate constants

for other reaction mechanisms than HO•4. Assumed that a functional group has approximately the same

interaction properties under a given molecule, so that the GCM disregards the changes of the functional properties that can arise from the intramolecular environment by electronic push-pull effects, hydrogen bond formation, or by steric effects.

Application of computational chemistry (quantum mechanical calculations) for predicting reaction rate constants in aqueous phase

Looking at reaction energies (transition states)

10

Challenges still remains

Modeling AOPs





11

AdOxTM, AOPs Simulation Software• AdOxTM is a part of the Environmental Technologies Design

Option Tools (ETDOTTM). It contains mechanism-based models

that can be used to evaluate and design advanced oxidation

processes.

• The final version of AdOx will include the following models:

– H2O2/UV model (available in version 1.0)

– H2O2/O3 model

– UV/O3 model

• A comprehensive database is also included in AdOxTM to provide

powerful support to the above models as well as ease-of-use

features for the user. 12

Crittenden et al., 1999. Wat. Res. 33, 2315-2328

Features of Current Version AdOxTM 1.0 for UV/H2O2

H2O2 + hν → 2HO•

H2O2 = quantum yield of H2O2 (=0.5) I0 = incident light intensity, einstein cm-2 sec-1 A=2.303b(εH2O2CH2O2+εRCR + εSCS + εHO2-CHO2-) b=pathlength, cm fH2O2= 2.303 b (εH2O2CH2O2 + εHO2-CHO2-)/A

H2O2/HO2- + HO H2O/OH- + HO2 2.7×107, 7.5×109

H2O2 + HO2/O2- HO + H2O/OH- + O2 3.0, 0.13

HO + HO H2O2 5.5×109

HO + HO2/O2- H2O/OH- + O2 6.6×109, 7.0×109

HO2 + HO2/O2- H2O2/HO2

- + O2 8.3×105, 9.7×107

R + hv Products

R + HO Products kMtBE=1.6×109, ktBA=6.0×108

HO + CO32-/HCO3

- CO3- + OH-/H2O 3.9×108, 8.5×106

HO + NOM Products 2.4×104 (mgC/L)-1s-1

NOM + hv Products 14

• Reactor Options

– Completely mixed reactor (CMBR)

– Completely flow type reactor (CMFR) w/o TIS

– Plug flow reactor

– Non-ideal Mixing

• Main Process Operational Variables :

– Initial hydrogen peroxide concentration

– Incident UV-light intensity

• Water quality parameters:

– TOC concentration

– pH

– Concentrations of target compounds

– Byproducts can be included if they are known

Features of Current Version AdOxTM 1.0

15

Concern about residual H2O2

• H2O2 has low UV light absorption

• Need to increase H2O2 concentration to lower EE/O

• High residual H2O2 will require treatment to reduce H2O2 such as chlorine

16

Simulation Results (LPUV/H2O2) – NAIX + Dealkalization -

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

5 10 15 20

EE

O (k

Wh

/kg

al-

ord

er)

H2O

2re

sid

ua

l co

nc.

(m

g/L

)

H2O2 Dosage (mg/L)

Residual of H2O2 MtBE tBA

• 10 mg/L of H2O2 dosage would be the better choice although the optimum dosage of H2O2 is over 20 mg/L. This is because the cost required for over 20 mg/L of H2O2 dose overweigh the energy cost. • At 10 mg/H2O2 dosage, UV dose was calculated as 2,100 mJ/cm2.

17Li; Hokanson; Crittenden; Trussell; Minakata. Wat. Res. 2008, 43, 5045-5053

References that were used for model validation

Glaze, W. H.; Lay, Y.; Kang J. W. Advanced oxidation processes. A kinetic model for the oxidation of 1,2-dibromo-3 chloropropane in water by the combination of hydrogen peroxide and UV radiation. Ind. Eng. Chem. Res. 1995, 34, 2314-2323.

Li, Ke.; Stefan, M.I.; Crittenden, J.C. UV photolysis of trichloroethylene: Product study and kinetic modeling. Environ. Sci. & Technol. 2004, 38, 6685-6693.

Li, Ke.; Stefan, M.I.; Crittenden, J.C. Trichloroethylene degradation by UV/H2O2 advanced oxidation process: Product study and kinetic modeling. Environ. Sci. & Technol. 2007, 41(5), 1696-1703.

Li, Ke.; Hokanson, D.R.; Crittenden, J.C.; Trussell, R.R.; Minakata, D. Evaluating UV/H2O2 processes for methyl tert-butyl ether and tertiary butyl alcohol removal: Effect of pretreatment options and light sources

18

19

AdOxTM 2 Simulation Software for O3, O3/H2O2, and mitigation of bromate

and THM formation

Concern about bromate

• Formation of bromate (BrO3-) during ozonation

in the presence of bromide ion (Br-)

• A nationwide survey of Br- in drinking water sources: approximately 80 μg/L (Amy et al., 1994)

• Br- in costal area is expected higher

• 10 μg/L of BrO3- standard of MCL associated with

cancer risk, Stage 1 of the Disinfectant/Disinfection

By-Product (D/DBP) Rule (EPA, 1998)

• When ozone is applied to disinfection, tradeoffs

between inactivation of cryptosporidium and

bromate formation should be evaluated. 20

Mechanisms of bromate formation

Simplified reaction scheme for bromate formation during ozonation

Ozone involving pathway HO• involving pathway

O3 + Br - → OBr

- + O2 160

OBr- + O3 → 2O2 + Br - 330

OBr- + O3 → O2 + BrO2 - 100

HOBr + O3 → O2 + BrO2 - + H+ <0.013

BrO2 - + O3 → BrO3 - + O2 5.7×10

4

O3 + Br• → BrO• + O2 1.5×108

HO• + HOBr → BrO• + H2O 2.0×109

HO• + OBr- → BrO• + OH- 4.2×10

9

HO• + Br- → Br• + OH- 1.1×109

BrO• + BrO• + H2O → BrO2- + OBr- +2H+

5.0×109

BrO• + BrO2- → OBr- + BrO2• 4.0×108

At lower pH (pH<pKa=8.8), less BrO3- is produced via ozone

pathway since HOBr is dominant. As a result, O3 decay is slower.

pKa = 8.8

Disproportionation

k (M-1 s-1)

21

Haag and Hoigné, 1983von Gunten and Hoigné, 1994

Control of bromate formation 1 – pH depression-

• pH depression resulting in more HOBr due to pKa=8.8 of

HOBr reduces BrO3- formation. However, in drinking water

treatment at around neutral pH, HOBr is mainly oxidized by

HO•. Therefore, pH depression does not reduce BrO3-

formation drastically.

• At reduced pH, O3 decay is slower and the *HO•+/*O3] is

decreased during ozonation. As a result, reduction of BrO3-

at the steady state is distinctive. It is noted that it cannot

be expected the proportional relationship between

*HO•+/*O3] and BrO3- formation.

22

Model simulation - pH effect (O3 with NOM)-HO• + NOM → products k = 19000 (mgC/L)-1s-1 Westerhoff et al 2007

Br• + NOM → Br- + products k = 83000 (mgC/L)-1s-1 Pinkernell and von Gunten 2001

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 50 60

O3

(mg

/L)

Time (min)

pH=6.0

pH=6.5

pH=7.0

pH=7.5

pH=8.0

pH=8.5

0

20

40

60

80

100

120

140

160

0 10 20 30 40 50 60

BrO

3-

(ug/

L)

Time (min)

pH=6.0

pH=6.5

pH=7.0

pH=7.5

pH=8.0

pH=8.5

Reactor type CMBR

Init O3 (mg/L) 1.0

pH 6, 6.5, 7, 7.5, 8, 8.5

Br- (µg/L) 300

NOM (mgC/L) 1.0

* CMBR: completely mixed batch reactor 23

Control of bromate formation 2 - NH3 addition -

Simplified reaction scheme for controlling bromate formation

• The maximum effect is at high NH3 as HOBr becomes very small. However, it is observed that excess NH3 decreases the efficiency of bromate control. • Not efficient in water containing medium to high NH3

k (M-1

s-1

)

HOBr + NH3 → NH2Br + H2O 7.5×107

OBr- + NH3 → NH2Br + OH- 7.6×104

NH2Br + OH- → OBr- + NH3 7.5×106

25

Pinkernell and von Gunten, 2001

Model simulation - NH4+ addition (O3 with NOM)-

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 50 60

O3

(mg/

L)

Time (min)

NH4+ = 0 ug/L

NH4+ = 50 ug/L

NH4+ = 100 ug/L

NH4+ = 200 ug/L

0

20

40

60

80

100

120

140

0 10 20 30 40 50 60

BrO

3-

(ug/

L)

Time (min)

NH4+ = 0 ug/L

NH4+ = 50 ug/L

NH4+ = 100 ug/L

NH4+ = 200 ug/L

NH4+ = 400 ug/L

NH4+ = 800 ug/L

Reactor type CMBR

Init O3 (mg/L) 1.0 (= 21 µM)

pH 8.0

Br- (µg/L) 300 (= 3.8 µM)NOM (mgC/L) 1.0

NH4+ (µg/L)0, 50, 100, 200, 400, 800

(0, 2.9, 5.8, 11, 23, 47 µM of NH3 tot)

26

Control of bromate formation 3 – Cl2-NH3 process -

• HOCl hinders Br- oxidation to Br• by HO•

NH3 additionCl2 addition OzonationSource water

HOCl + Br- → HOBr + Cl- 1550

OCl- + Br- → OBr- + Cl- 0.001

HOCl ↔ OCl- + H+ pKa = 7.5

• NH3 reacts with both HOBr and HOCl

• Effective to hinder HO• during ozonation to reduce BrO3-• HOCl and NH2Cl oxidize specific moieties of NOM and reduces their reactivities toward O3, and also scavenge HO•.

* HOCl, NH2Br, and HOBr react with NOM to produce THMs and TOX

HOBr + NH3 → NH2Br + H2O 7.5×107

OBr- + NH3 → NH2Br + OH- 7.6×104

HOCl + NH3 → NH2Cl + H2O 4.2×106

27

Buffle et al., 2004

Model simulation – Cl2-NH3 process 2 -

0.0E+00

2.5E-07

5.0E-07

7.5E-07

0 0.5 1

HO

Br

(M)

Time (min.)

HOCl = 5 uM

HOCl = 10 uM

HOCl = 15 uM

• NH3 addition (1 min.):HOBr is significantly masked by NH3.

• O3 (60 min.): Pre-addition of HOCl followed by NH3 decreased BrO3- by factor of 2~7 as compared with the case of only NH3

addition.

0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60

O3

(mg

/L)

Time (min.)

HOCl = 0 uMHOCl = 5 uMHOCl = 10 uMHOCl = 15 uM

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 20 40 60

BrO

3-(

ug/

L)

Time (min.)

HOCl = 0 uM

HOCl = 5 uM

HOCl = 10 uM

HOCl = 15 uM

29

[TTHM] = [Cl2]{ATTHM(1-exp(-kt)}

ln(k) = 5.41 – 0.38 ln + 0.27 ln([NH3-N]) – 1.12 ln(Temp) + 0.05 ln([Br-]) – 0.854 ln(pH)

ln(ATTHM)=-2.11-0.87 ln -0.41 ln([NH3-N]) + 0.21 ln([Cl2]) + 1.98 ln(pH)

[TTHM]=predicted trihalomethane conc. in initial phase (~5h), µg/L

[Cl2]=applied chlorine dose, mg/L

[DOC] = dissolved organic carbon, mgC/L

[NH3-N] = ammonia-nitrogen conc., mg/L as N

[Br-]= bromide concentration, µg/L

Temp = temperature, °C

t = reaction time, h

Model simulation – Cl2-NH3 process 4 –

TTHM formation

Improved EPA 1998 empirical model (Sohn et al., 2004)

0

2

4

6

8

10

12

14

16

0 10 20 30 40 50 60

TTH

M (

ug/

L)

Time (min)

HOCl = 5 uM

HOCl = 10 uM

HOCl = 15 uM

HOCl (uM) 0, 5, 10, 15

Br- (ug/L) 300

TOC (mgC/L) 1.0

pH 8.0

phosphate (M) 0.001

NH4+ (ug/L) 400

Chlorination 5 min.

Ammonia addition 1 min.

31

O3/H2O2 processfor MtBE removal and bromate mitigation

• Advanced oxidation process: O3/H2O2 process:

– HO2- + O3 → HO• + O2•- + O2 k = 2.2×106 M-1s-1

– H2O2 + O3 → O2 + H2O k = 0.0065 M-1s-1

– produce more HO• at higher pH due to pKa = 11.6 of H2O2

– H2O2 also scavenges HO•

• H2O2 + HO• → HO2• + H2O k = 2.7×107 M-1s

• optimum ratio: [H2O2]/[O3] = 0.5 mole/mole

• H2O2 addition keeps the O3 concentration low.

• In the presence of Br-, H2O2/HO2- reacts with HOBr/OBr- to produce Br- resulting in BrO3- formation. In addition, there is significant contribution of the reaction of O3 with Br• in regard with BrO3- formation (von Gunten and Oliveras, 1998).

32

Simulation – O3 with H2O2 process–

Simulation parameters

• MtBE = 300 ug/L

• pH = 7.6

• NOM = 1.4 mg/L

• Alkalinity = 318 mg/L as CaCO3

• initial O3 = 3 mg/L

• molar ratio: [H2O2]/[O3] = 0.5 (optimum), 0.75, 1.0

• H2O2 = 1.06, 1.59, 2.12 mg/L

• Br- = 300 ug/L

• Completely Mixed Batch Reactor (CMBR)

33

Simulation results – O3 with H2O2 process–

0.0

0.5

1.0

1.5

2.0

2.5

3.0

[O3]

(mg

/L)

[H2O2]/[O3]=0.5

[H2O2]/[O3]=0.75

[H2O2]/[O3]=1.0

0.0

0.5

1.0

1.5

2.0

0 20 40 60 80 100 120

[H2O

2]

(mg

/L)

Time (sec)

0

5

10

15

20

25

30

0

50

100

150

200

250

300

[tB

A]

(ug/

L)

[MtB

E]

(ug

/L)

MtBE tBA

0

1

2

3

4

5

6

7

0 20 40 60 80 100 120

[BrO

3-]

(u

g/L

)

Time (sec)34

Summary • A kinetic model for bromate formation during

ozonation was formulated at non-steady state and non-constant pH with various parameters (pH, alkalinity, NOM, Br-).

• 3 main bromate mitigation techniques (pH depression, NH3 addition, Cl2-NH3 process) give quantitative insight of bromate formation control.

• A kinetic model for O3 with H2O2 was simulated for MtBE and tBA along with bromate formation

35

Modeling AOPs





36

Conventional Approach for Kinetic Model in AOP 1. One “hot” parent organic chemical contaminant2. Conduct batch experiments to measure byproducts using GC/MS etc. 3. Obtain rate constant/Measure rate constant (e.g., pulse radiolysis)4. Build kinetic model based on both experimental findings and

literature-reported rate constants. Unknown rate constants are usually obtained by fitting with experimental data.

Parent compoundMajor byproducts

(yield 10-30 mole%)

Minor byproducts

(yield 2-5 mole%)

Acetone

*Stefan and Bolton, 1999

acetic, pyruvic, and oxalic acids

pyruvaldehyde

formic and glyoxylic acids

hydroxyacetone, formaldehyde

MtBE

* Stefan et al., 2000

acetone, acetic acid, formaldehyde,

tert-butyl formate (TBF), pyruvic acid,

tert-butyl alcohol (TBA), 2-methoxy-2-

methyl propionaldehyde (MMP), formic,

methyl acetate

hydroxy-iso-butylaldehyde,

hydroxyacetone,

pyruvaldehyde and hydroxy-

iso-butyric, oxalic acid

Dioxane

*Stefan and Bolton, 1998

1,2-ethanediol diformate, formic acid,

oxalic acid, glycolic acid, formaldehyde,

1,2-ethanediol monoformate

methoxyacetic acid glyoxal

TCE

* Stefan and Bolton, 2002formic acid, oxalic acid

dichloroacetic acid,

mono-chloroacetic acid 37

A few studies examined entire reaction mechanisms

39

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 5 10 15 20 25 30 35

Time / min

[Ch

lori

de

Io

n]

/ m

M

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

[In

term

ed

iate

s]

/ m

M

Chloride

Oxalic Acid

DCA

Formic Acid

MCA

Conventional Approach for Kinetic Model in AOP

Byproducts in TCE Destruction based on experimental studies

• UV irradiation ranging from 200 nm to 300 nm inan interval of 1 nm

• Molar extinction coefficients of TCE, H2O2 and 4types of main byproducts are measured andsimulated in an interval of 1 nm from 200 to 300nm

• pH change of the system

• Dissociation of 8 organic/inorganic acids

• 22 literature reported rate constants are used in the model

The model is a kinetic model and considers:

TCE UV/H2O2 Modeling

40

Li, Stefan and Crittenden, 2004, ES&T 38, 6685-6693Li, Stefan and Crittenden, 2007, ES&T 41, 1696-1703

• Complex radical reactions between hydrogen peroxide, hydroxyl radical, superoxide radical and formyl radical

• TCE direct photolysis and reactions with OH • and Cl•

• Formation and destruction of main byproducts and intermediates, such as di-chloroacetic acid (DCA), mono-chloroacetic acid (MCA), oxalic acid and formic acid

• Intermediate byproducts those are not detected: glyoxylic acid and phosgene

• Scavenging of hydroxyl radical by bicarbonate species

The model contains 34 differential equations for 8 molecules and 6 radicals, including:


41

0.E+00

2.E-03

4.E-03

6.E-03

8.E-03

1.E-02

1.E-02

0 5 10 15 20 25 30 35

Time (min)

Co

nc

en

tra

tio

n (

mo

l/L

)

exp

mod

0.E+00

2.E-04

4.E-04

6.E-04

8.E-04

1.E-03

1.E-03

0 10 20 30 40

Time (min)

Co

nc

en

tra

tio

n (

mo

l/L

)

mod

exp

0.E+00

1.E-06

2.E-06

3.E-06

4.E-06

5.E-06

6.E-06

7.E-06

8.E-06

9.E-06

0 10 20 30 40

Time (min)

Co

nc

en

tra

tio

n (

mo

l/L

)

exp

mod

0.E+00

2.E-06

4.E-06

6.E-06

8.E-06

1.E-05

1.E-05

0 10 20 30 40

Time (min)

Co

nc

en

tra

tio

n (

mo

l/L

)

mod

exp

Comparison of Modeled and Experimental Concentration Profiles 1/2

TCE

MCADCA

H2O2


42

Comparison of Modeled and Experimental Concentration Profiles 2/2

0.E+00

1.E-05

2.E-05

3.E-05

4.E-05

5.E-05

6.E-05

7.E-05

8.E-05

9.E-05

1.E-04

0 5 10 15 20 25 30 35

Time (min)

Co

nc

en

tra

tio

n (

mo

l/L

)

exp

mod

Oxalic acid

0.E+00

5.E-05

1.E-04

2.E-04

2.E-04

3.E-04

3.E-04

4.E-04

4.E-04

5.E-04

0 10 20 30

Time (min)

Co

nc

en

tra

tio

n (

mo

l/L

)

exp

mod

Formic acid

0

1

2

3

4

5

6

7

0 10 20 30 40

Time (min)

pH

exp

mod

pH


43

Modeling AOPs





44

A Computer-Based Kinetic Model in AOPs

Why so important?

We now know the general reaction mechanisms that HO• initiates in the aqueous phase AOPs based on a number of experimental studies.

We have a large number of datasets for reaction rate constants in the aqueous phase AOPs.

Concern about emerging contaminants

Greater than 50 million chemical compounds registered in CAS and more than 40 million chemicals are available

Is it feasible?

We now have robust tool (e.g., computational chemistry) and computational resources

We can make it feasible

45

“All models are wrong but some are useful.”

-- George E.P. Box

“Let’s develop some useful ones and make good use of them.”

-- an optimistic modeler

46

Reaction Pathway Generator (Graph theory)

Reaction Rate Constant Predictor (Quantum mechanical calculations)

Ordinary Differential Equations (ODEs) Solver

47

Reaction Pathway Generator

Reaction Rate Constant Predictor

ODE Generator and Solver

ToxicityProfiles

Toxicity Estimator

ConcentrationProfiles

A Computer-Based Kinetic Model in AOPs

Reaction Pathway Generator

48

H-atom abstraction by HO•or HO• addition

O2 addition

Uni/Bi molecular decay

HydrolysisHO• reactions

Β-scission, 1,2-H shift

Peroxyl radical mechanisms

Parent compound

Carbon centered radical

Peroxyl Radical

Oxy radical

Intermediates (aldehydes, alcohols etc.)

Carbon centered radical

Carboxyl acid

CO2/Minerals

Li and Crittenden, 2009 ES&T 43, 2931-2837

General reaction mechanisms that HO• initiates based on past experimental studies

ClHC=CCl2

ClCH(OH)-CCl2

HO

hv *Cl

Cl2HC-CCl2

ClH2CCOOH

ClHC-COOH

HO

O2

HC(O)ClCO2

-

OHC-COOH

HCl +CO

O2

OHC-CCl2+HCl

O2

OHC-CCl2+HCl

OHC-CCl2+HCl C(O)Cl2

CHO HCOOH

CO2+HCl

HCOOHOHC-CClO+Cl

OHC-COOH

H2O

HO

CO2-+H++H2O

H2O2

HCOOHCO2

H2O

HO

HOOC-COOH

HO

CO2-+CO2+H+

Cl2HCCOOH

Cl2C-COOH

HO

O2

C(O)Cl2

CO2

HOOC-C(O)Cl

HOOC-COOHHOOC-COOH

Cl2CHCHO

HCCCl

ClCCCl

H2O

HO

HO

Cl2CHCOOH

CO2 + Cl-

CO2 + Cl-

Proposed Mechanism4949

Details of TCE pathway accounts for the formation and decay of formic, glyoxylic, oxalic acids and phosgene.

Cl2C=CHCl

Cl2CCH(OH)Cl Cl2C

CHCl2

Cl2C-CHO

-HCl

HO

Cl

OO-Cl2C-CHO

OCl2C-CHO

O2

Cl(O)C-CHO

+ Cl

HOOCCHOCO2 + HCl

H2OH2O

CHO +C(O)Cl2

H2O

HC(OH)2

O2

OOCH(OH)2

HCOOH + HO2

HCOOH

H2O2

O2

OOCCl2CHCl2

OCCl2CHCl2

O2

+OCHCl2

Cl2CHC(O)Cl C(O)Cl2 + CHCl2

- Cl

Cl2CHCOOH

O2

OOCHCl2

C(O)Cl2

+(OH)CHCl2

C(O)Cl2

+H2O2

C(OH)Cl2HC(O)Cl

CO + HCl OOC(OH)Cl2

CCl2COOH

OOCCl2COOH

C(O)Cl2 +HO2

OCCl2COOH

C(O)Cl2+ COOH ClC(O)COOH

HOOCCOOHO2

OOCOOH

CO2 + HO2

H2O

HOOCCOOH

HO

CO2 + CO2

HO

CO2 H2O2O2

CO2 + O2

CO2 + HO 50

Another Challenge:

To predict reaction rate constants

in aqueous phase

51

GCM is limited because:1. Many datasets are required to calibrate parameters2. Only deal with molecules for which all required group rate constants

and group contribution factors have been calibrated before.3. Limited number of literature-reported experimental rate constants

for other reaction mechanisms than HO•4. Assumed that a functional group has approximately the same

interaction properties under a given molecule, so that the GCM disregards the changes of the functional properties that can arise from the intramolecular environment by electronic push-pull effects, hydrogen bond formation, or by steric effects.

Application of computational chemistry (quantum mechanical calculations) for predicting reaction rate constants in aqueous phase

Looking at reaction energies (transition states)

54

Challenges still remains

Linear Free Energy Relationships (lnk =-a∆G≠+b)

1

2

3

45

6

7

9

10

11

12 12

141516

17

1922

23

2425

26

13

21

8

y = -0.222x + 9.356R² = 0.746

0

2

4

6

8

10

12

-4 -2 0 2 4 6 8 10 12

log

kH

∆ G≠rxn,aq, kcal/mol

Neutral (G3 + COSMO-RS)

Ioznized (G4 + SMD)

y = -0.542x + 11.768R² = 0.817

0

2

4

6

8

10

12

0.0 2.0 4.0 6.0 8.0 10.0

log

kE

XP

∆GactI , kcal/mol

Neutral

Ionized

55

LFER based on calculated ∆G≠LFER based on experimental ∆G≠

• LFER is established for H-atom abstraction from a C-H bond of saturated aliphatic compounds. • kexp and ∆Gact

I represents experimentally obtained overall rate constant and activation energy, while kH is equal to kexp/(# of C-H bond) and ∆G≠

rxn,aq represents the lowest free energy of activation.

Free energy profiles for HO• with CH4

56

*CH3 + H2O, -23.27 *OOCH3, -30.01

CH4 + HO*, 0

CP,

-0.69

TS, 5.89

CP, -20.69

*OCH3 + O, 23.67

CH3OOOOCH3, -58.40

H3COOCH3 + O2 , -80.66

2*OCH3+O2, -33.66

2HCHO + H2O2, -95.75

HCHO+CH3OH+O2, -122.22

TS, 21.96

TS, -21.44

*CH3 + O2, 0

-150

-100

-50

0

50

0 1 2 3 4 5 6

Delt

a G

aq

, k

ca

l/m

ol

Reaction cordinate

H-atom abstraction

O2 addition

Tetroxide uni-molecular decay

Formation of tetroxide

57

Free energy profile for HO• with CH4 (continued)

*OCH3, 0.00

TS, 21.97

*C(OH)H2, -13.43

-20

-15

-10

-5

0

5

10

15

20

25

0 0 .5 1 1 .5 2 2 .5 3 3 .5

Delt

a G

aq

, k

ca

l/m

ol

Reaction cordinate

H-shift (Rearrangement of alkoxyl radical)

*OCH3 + H2O, 0.00

*C(OH)H2, -13.43

TS *OCH3-H2O,

18.56

CP OCH3+H2O,

5.23

CP *C(OH)H2 + H2O,

-9.73

*OCH3, 0

TS , 21.97

-20

-15

-10

-5

0

5

10

15

20

25

0 0 .5 1 1 .5 2 2 .5 3 3 .5

Delt

a G

aq

, k

ca

l/m

ol

Reaction cordinate

HCHO + 2H2O, 0.00

TS, 36.63

HOCH2OH + H2O, -1.65

CP HCHO+2H2O, 12.45

CP, 10.06

HCHO + H2O, 0 complex, 5.34

TS, 40.17

-10

0

10

20

30

40

50

0 0 .5 1 1 .5 2 2 .5 3

Delt

a G

aq

, k

ca

l/m

ol

Reaction cordinate

H2O-assisted hydrolysis

Oxygen-transfer

We should be able to establish a library of reaction rate constant predictors once the LFER for each elementary reaction is established.

Summary • Four different approaches have been shown to model aqueous phase advanced oxidation processes.

• A simplified pseudo-steady-state model is precise enough to examine the feasibility of the process.

• AdoxTM (UV/H2O2) simulation software can be used to design AOPs not assuming constant pH, steady-state.

• Conventional approach to establish kinetic model in AOP is time consuming and does not consider all possible by-products.

• Our under-developing computer-based kinetic model is robust for rational design of AOPs but there are still some challenges remains.

58

Aquatic toxicity assessment of AOPs byproducts- Relative toxicity of TCE degradation byproducts -

59

• 96-hr Green Algae ChV: most sensitive indicator among the acute and chronic endpoints for fish, daphnia, and green algae (EPA 2005)• TQ(Toxicity Quotient): {Concentration ÷ 96-hr Green Algae ChV} ∑TQ = Relative toxicity of a mixtureTQ or ∑TQ ≥ 1 : Toxic and unacceptable (Belden et al. 2007)

Green Algae

96-hr ChV (mg/L)

TCE 2.9

Formic acid 83.0

Oxalic acid 1050.0

MCA 529.0

DCA 556.00

5

10

15

20

0 5 10 15 20 25 30

Rela

tive T

oxic

ity

(Co

ncen

trati

on

/Ch

V)

Time (min)

Composite of TCE, formic acid, oxalic acid, MCA, and DCA

0

100

200

300

400

500

600

700

0 5 10 15 20 25 30

Re

lati

ve t

oxi

city

(C

on

cen

trat

ion

/ D

WEL

)

Time (min)60

Non-carcinogenic effect assessment - TCE degradation byproducts -

• DWEL (Drinking Water Equivalent Level): A lifetime exposure concentration protective of adverse non-cancer health effects from drinking water

• Relative toxicity = ∑[concentration / DWEL]

*Oxalic acid and Formic acid are not included in the drinking water criteria.

Composite of TCE, MCA, and DCADWEL

(mg/L)

TCE 0.25

MCA 0.35

DCA 0.14

61

Carcinogenic effect assessment - TCE degradation byproducts -

• Cancer Classification

TCE : sufficient evidence in animals and inadequate or no evidence in human (B2)

MCA : inadequate information to assess carcinogenic potential (I)

DCA : likely to be carcinogenic to humans (L)

• 10-4 Cancer risk concentration:

The concentration of a chemical

in drinking water corresponding

to an excess estimated lifetime

cancer risk of 1 in 10,000. (EPA 2006)

Chemical10-4 Cancer Risk

(mg/L)

TCE 0.003

MCA

DCA 0.007

0

1

2

3

4

5

6

0 5 10 15 20 25 30

Rela

tiv

e t

oxic

ity

(Co

ncen

trati

on

/ c

an

cer

risk

)x

10

00

0

Time (min)

Acknowledgement

• National Science Foundation (NSF): 0607332 and 0854416

• Water Reuse Foundation: WRF-05-010• Georgia Tech College of Computing, Office of

Information Technology and CEE IT Services for high computational resources.

• High Tower Chair and Georgia Research Alliance at Georgia Tech

• Brook Byers Institute for Sustainable Systems• University of Notre Dame Radiation Center and

Department of Energy

62

63

References • Glaze and Kang, 1988. Advanced oxidation processes for treating groundwater contaminated with TCE and PCE:

Laboratory studies. J. AWWA., 81, 5, 57-63.

• Stefan and Bolton, 1999. Reinvestigation for the acetone degradation mechanism in dilute aqueous solution by the UV/H2O2 process. Environ. Sci. Technol., 33, 870-873.

• Stefan, M. I.; Mack, J.; Bolton, J. R. Degradation pathways during the treatment of methyl tert-butyl ether by the UV/H2O2 process. Environ. Sci. Technol. 2000, 34, 650-658.

• Stefan, M. I.; Bolton, J. R. Mechanism of the degradation of 1, 4-Dioxane in dilute aqueous solution using the UV/hydrogen peroxide process. Environ. Sci. Technol. 1998, 32, 1588-1595.

• Stefan and Bolton, 2002.

• Westerhoff, P.; Mezyk, S.P.; Cooper, W.J.; Minakata, D. Electron pulse radiolysis determination of hydroxyl radical rate constants with Suwannee river fulvic acid and other dissolved organic matter isolates. Environ. Sci. & Technol. 2007, 41, 4610-4646.

• Li, K.; Hokanson, D.R.; Crittenden, J.C.; Trussell, R.R.; Minakata, D. Evaluating UV/H2O2 processes for methyl tert-butyl ether and tertiary butyl alcohol removal: Effect of pretreatment options and light sources. Wat. Res. 2008, 42, 5045-5053.

• Crittenden, J. C.; Trussell, R. R.; Hand, D. W.; Howe, K. J.; Tchobanoglous, G., Eds. Water Treatment: Principals and Design, 2nd ed.; John Wiley: New York, 2005.

• Crittenden, J.C.; Hu, S.; Hand, W. D.; Green, A. S. A kinetic model for H2O2/UV process in a completely mixed match reactor.Wat. Res. 1999, 33(10), 2315-2328.

• Li, K.; Crittenden, J.C. Computerized pathway elucidation for hydroxyl radical induced chain reaction mechanisms in aqueous phase AOPs. ES&T. 2009, 43(8), 2831-2837.

• Minakata, D.; Li, K.; Westerhoff, P.; Crittenden, J. Development of a group contribution method to predict aqueous phase hydroxyl radical reaction rate constants. ES&T. 2009, 43, 6220-6227.

• Westerhoff, P.; Crittenden, J.C.; Moon, H.; Minakata, D. Oxidative treatment of organics in membrane concentrates. Final project report, Water Reuse Foundation, Alexandria, VA, 2008.

• Li, K.; Crittenden, J.C. Computerized pathway elucidation for hydroxyl radical induced chain reaction mechanisms in aqueous phase AOPs. ES&T. 2009, 43(8), 2831-2837.

• Atkinson, R. Kinetics and mechanisms of the gas-phase reactions of the hydroxyl radical with organic compounds under atmospheric condition. Chem. Rev. 1986, 86, 69-201

• A division of the American Chemical Society. CAS (Chemical Abstract Service). http://www.cas.org/expertise/cascontent/index.html (accessed June 12, 2009).

• Benson, S.W. “Thermochemical Kinetics”, 2nd ed. Wiley, Interscience, New York, 1976.

http://www.cas.org/expertise/cascontent/index.html

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