Post on 02-Feb-2021
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−