Changha Lee

School of Chemical and Biological Engineering

Seoul National University

http://artlab.re.kr

고도산화환원환경공학연구실

Advanced Redox Technology (ART) Lab

Water & Wastewater Treatment-2- Chemical Oxidation

(from MWH's Water Treatment Principles and Design by Crittenden et al.)

Definition of Reduction and Oxidation

Definition of reduction & oxidation reactions (i.e., redox reactions)

- Oxidation: loss of e− or H, gain of O, increase of oxidation number

- Reduction: gain of e− or H, loss of O, decrease of oxidation number

e.g., C + H2 → CH4

C + O2 → CO2

Fe0 → Fe2+ + 2e−

e.g., O2 + H2 → H2O2

O2 + 2e− + 2H+ → H2O2

e.g., O2 + 2H2 → 2H2O

O2 + 4e− + 4H+ → 2H2O

→

Oxidation number

Half

Reactions

2e−

Reductant Oxidant

Oxidant and Reductant

Oxidation Processes for Water Treatment

1. Chlorine

2. Ozone

3. Chlorine dioxide

4. Permanganate

5. Hydrogen peroxide

6. Organic contaminants

TASTE AND ODOR CONTROL

- In surface water: originated from algal bloome.g., Geosmin, 2-Methylisoborneol (2-MIB)

- Threshold odor concentration: 4 ng/L for geosmin, 9 ng/L for 2-MIB

- In groundwater: sulfides (hydrogen sulfide and organic sulfides)Production of polysulfides (oxidation byproducts, usually S8, generates turbidity) is a problem when H2S > 1 ppm

COLOR REMOVAL

OXIDATION AS A COAGULATION AID

IRON AND MANGANESE REMOVAL

OXIDATION OF SELECTED TRACE ORGANIC CONSTITUENTS

2D Graph 3

DEA

Eili

min

ation (

%)

0

20

40

60

80

100

120

Col 1 vs Col 2 - Col 2

Col 1 vs Col 3 - Col 3

Col 1 vs Col 4 - Col 4

Col 1 vs Col 5 - Col 5

Iopromide

Diazepam

Atrazine

DIA

Ibuprofen

Clofibric acid

Cyanazine

Atenolol

Bezafibrate

Caffeine

Propanolol

Naproxen

Trimethoprim

Carbamazepine

17-Estradiol Diclofenac

17-ethinylestradiolSulfamethoxazole

Predicted oxidation of grouped organic pollutants in 1 ppm of ozone dose

Fundamentals of Oxidation and Reduction

Definitions of oxidation and reduction

Half Reactions

Balancing Redox Reactions

Define final products (make an unbalanced equation with them)

Balance atoms and charge (Use H+, OH− or H2O if necessary)

Standard Redox Potential

“Reduction (redox) potential” is not for a compound, but for a reaction

(or a redox couple)!

The reduction potential of A3+ is ??? VSHE X

e.g.,

Because A3+ may go through different redox reactions with

different reduction potentials.

A3+ + e− → A2+

A3+ + 3e− → A0

The reduction potential of A3+ + e− → A2+ is ??? VSHE O

The reduction potential of A3+/A2+ is ??? VSHE O

Eo[A3+/A2+] = ??? VSHE O

Expression of Redox Potential

Standard Redox Potentials for Several Oxidants

Source: Lee et al., 2019 (Membr. Wat. Treat.)

We can tell if a certain redox reaction is thermodynamically favored or not

by reduction (redox) potentials of its half reactions!

Ni0 + Fe2+ → Ni2+ + Fe0 (favored or unfavored) ?

e.g.,

Fe2+ + 2e− → Fe0 Eo = −0.44 VSHE

Ni2+ + 2e− → Ni0 Eo = −0.20 VSHE

Fe2+ + 2e− → Fe0 Eo = −0.44 VSHE

H+ + 2e− → H2 Eo = 0 VSHE

Redox Potential and Thermodynamics

Fe0 + 2H+ → Fe2+ + H2 (favored or unfavored) ?

Redox Potential and Thermodynamics

Ni0 + Fe2+ → Ni2+ + Fe0 (favored or unfavored) ?

e.g.,

Fe2+ + 2e− → Fe0 Eo(1) = −0.44 VSHE

Ni2+ + 2e− → Ni0 Eo(2) = −0.20 VSHE

Fe0 + 2H+ → Fe2+ + H2 (favored or unfavored) ?

Fe2+ + 2e− → Fe0 Eo(2) = −0.44 VSHE

H+ + 2e− → H2 Eo(1) = 0 VSHE

Ni0 → Ni2+ + 2e− Eo(2) = +0.20 VSHE

E◦Rxn = Eo(1) + Eo(2) = −0.24

Fe0 → Fe2+ + 2e− Eo(2) = +0.44 VSHE

E◦Rxn = Eo(1) + Eo(2) = +0.44

E◦Rxn > 0 Thermodynamically favored

E◦Rxn < 0 Thermodynamically unfavored

G◦ = −nFE◦

Redox Potential and Thermodynamics

Redox Potentials of Combined Reactions

Eo(3) = 2Eo(1) + Eo(2) / (2 + 1) = (2x0.4 + 0.2)/3 = 0.33 VSHE

A3+ + 2e− → A+ Eo(1) = +0.4 VSHE

A+ + e− → A0 Eo(2) = +0.2 VSHE

e.g.,

A3+ + 3e− → A0 Eo(3) = ?

pH-Dependence of Redox Potential

m

??? + ne- + mH+ → ???

m

m

∆

Ered (at specific pH) − Eredo (pH =0)

Gr = Gro + RT lnQr

−nFEred = − nFEredo + RT lnQr

Ered − Eredo = − RT/nF lnQr

Ered − Eredo = − 2.303RT/nF log(1/[H+]m)

OXYGEN

HYDROGEN

2H+ + 2e− H2(g) Eo = 0 V

(at pH 0)

(at pH 14)

*pe = −log[e−] = F/(2.3RT)Eh

CHLORINE SPECIES

(1)

(2)

(3)

pKa = 7.5

(4)

(1)

(2) (3)

(4)

*pe = −log[e−] = F/(2.3RT)Eh

Kinetics of Redox Reactions

Homework

What would the Eh-pH diagram for chlorine species look like

when the total chlorine concentration changes?

Think about it !

Common Chemical Oxidants for Water Treatment Applications

Oxidation rate

Redox potential

Applications

- Oxidation of iron, manganese,and sulfides

Redox potential

Applications

- Oxidation of iron, manganese, sulfides, taste and odor compounds (but not applicable to Geosmin & 2-MIB), ammonia

- Oxidation byproducts (chlorinated organic compounds) are problematic

Redox potential

Decomposition

Applications

- Oxidation of iron, manganese, sulfides, taste and odor compounds (little applicable to Geosmin & 2-MIB), and some of organic compounds

ClO2 + e− → ClO2

− Eored = 0.95~1 V

Redox potential

Applications

- Oxidation of iron, sulfides

- A Reagent for advanced oxidation technology (a precursor of OH radical)e.g., UV/H2O2, O3/H2O2, Fe(II)/H2O2

Redox potential

Applications

- Oxidation of iron, manganese, sulfides, taste and odor compounds, micropollutamnts, removal of color, control of DBP precursors (oxidation of NOM)

- A Reagent for advanced oxidation technology (a precursor of OH radical)e.g., UV/O3, O3/H2O2, Fe(II)/O3

pH

5 6 7 8 9

log

(k

O3, a

pp (M

-1 s

-1))

-1

0

1

2

3

4

5

6

7

8

9

10

log

(

(s))

-5

-4

-3

-2

-1

0

1

2

3

4

5D M F A

D M A

T M A

D M A B

D M E A

D M D C

D M A ID M A P

pH

5 6 7 8 9

log

(k

ClO

2, a

pp (M

-1 s

-1))

-3

-2

-1

0

1

2

3

4

5

6

7

8

9

log

(

(s))

-4

-3

-2

-1

0

1

2

3

4

5

6

7

D M F A

D M A

T M A

D M A B

D M E A

D M D C

D M A I

D M A P

O3 ClO2

pH

5 6 7 8 9

log

(k O

H (M

-1 s

-1))

7

8

9

10

11

log

(

(s))

1

2

3

4

D M F A

D M A

T M A

D M A B

D M E A.

D M D C

D M A I

D M A P

•OH

Second-order rate constants for reactions of NDMA precursors with oxidants

Source: Lee et al., 2007 (Environ. Sci. Technol.)

Redox potential

Applications

- Oxidation of iron, manganese, taste and odor compounds

- Pink color: needs dosage control

MnO4−

MnO4−