Overview of Advanced Oxidation Processes (AOPs)-Understanding ...

Overview of Advanced Oxidation Processes (AOPs) -Understanding and Improving Process PerformancePotable Reuse for Water Supply Sustainability Critical Today – Essential Tomorrow, Nov 16-19, 2008. Long Beach, CA

John C. Crittenden, Ke Li, Daisuke Minakata, Paul Westerhoff Department of Civil and Environmental Engineering, Arizona State University 1

Outline

1. Introduction to Advanced Oxidation Processes (AOPs) 2. Various AOPs Technologies

3. Factors affecting performance of AOPs 4. Simplified steady-state model for AOPs 5. By-products of AOPs

6. Case study 1: RO retentate by various AOPs 7. Case study 2:

Removal of MtBE and tBA by UV/H2O2

8. Summary

2

Introduction - what are AOPs? -

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Advanced Oxidation Processes (AOPs) that produce hydroxyl radicals (HO•radicals) at ambient temperature and atmospheric pressure are promising water treatment technology. HO• radicals are highly reactive electrophiles, that react rapidly and nonselectively with the electron-rich sites of compounds. HO• radicals are capable of mineralizing organic compounds into carbon dioxide CO2 and water.

3

Introduction – Reaction rules in AOPs -

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The most significant observed by-products are the carboxylic acids, due to the fact that the second order rate constants for these compounds are much lower than those for most organics. If adequate reaction time is provided, all by-products (>99% as measured by a TOC mass balance) are destroyed. General reaction rules in AOPs:

Uni/Bi molecular decay

H-atom abstraction by HO• O2 addition or HO• addition Carbon Parent centered Peroxyl compound radical Radical

Oxy radical

Β-scission, 1,2-H shift

Intermediates (aldehydes, alcohols etc.)

Carbon centered radical Hydrolysis HO• reactions

Carboxyl acid

Peroxyl radical mechanisms

CO2/ Minerals 4

Various AOPs Technologies 1/2 Proved Advantages AOPs • Long stability and can be preserved prior to use of H2O2/ H2O2. UV • H2O2+UV → 2HO•+3O2 • Suitability for waters with poor UV light transmission • Special reactors H2O2/ requirement for UV O3 illumination2 • 2O3+H2O2 → 2HO•+3O2

TiO2/ UV

Disadvantages • Poor UV absorption of H2O2 • Interface of UV with the water matrix • Special reactors required for UV illumination • Residuals of H2O2 • Stripped volatile organics • Expensive and inefficient to produce O3 • Residues of gaseous O3 • Difficulty of maintenance (O3/H2O2 dosages) • Low pH is detrimental

• Activated with near-UV light • Occurrence of fouling of catalyst • Greater light transmission • Recovery of TiO2 required upon the use • TiO2+UV → h++ecb as a slurry h++H2O → H++HO• 5

Various AOPs Technologies 2/2 Purifics (UV/TiO2) 0.5 MGD Photo-Cat Purification

Low Pressure UV system (UV/H2O2) HiPOx (O3/H2O2) Applied Process Technologies, Inc 6

Factors Affecting AOP Performance 1/4 • Destruction rate of targeted compound R by HO•

rR  kR CRCHO

Typical kR: 107-109 M-1s-1, CR: μM or nM, CHO•: 10-11 ~ 10-9 M

• Rate raw for HO• that reacts with an organic compound

HO+ R   byproducts rR   kR CHO CR • Half-life of an organic compound for CMBR

ln(2) tR  kR CHO 

Halflife, min Compound MtBE

k HO•

-9

[HO•]=10 M

-1 -1 M s

1.6×109 6

-10

[HO•]=10

M

-11

[HO•]=10

0.01

0.1

1

Oxalic acid

1.4×10

8

83

825

Acetate ion

7×107

0.2

2

17

6

Trichloromethane

5.0×10

2

23

231

1,1,2-Trichloroethane

1.1×108

0.11

1

11

6

Chloroform

5×10

2

23

231

Chloroacetic cid

4.3×107

0.3

2.7

27

0.02

0.2

2

8

Glycolic acid

6×10

1,1,1-Trichloroethane

4×107

0.3

3

29

Benzene

7.8×10

9

0.001

0.01

Phenol

6.6×109

0.002

0.02

0.1 7 0.2

M

Factors Affecting AOP Performance 2/4 • Impact of carbonate species

k R CR QR  kR CR  k HCO CHCO  kCO2 CCO2 3

3

3

3

2-

CT, CO3,

HCO3-,

CO3 ,

mM

mM

mM

Q TCE, %

pH

10.9

7.0

1

0.997

0.003

5.78

7.0

2

1.994

0.006

2.98

7.0

4

3.988

0.012

2

7.0

6

5.982

0.018

5.55

8.0

2

1.99

0.01

3.04

9.0

2

1.904

0.096

0.754

10.0

2

1.333

0.667

kHCO3- = 8.5×106 M-1s-1 kCO3-- = 3.9×108 M-1s-1 H2CO3* ↔ H+ + HCO3(pKa=6.3) HCO3- ↔ H+ + CO32(pKa=10.3) At high pH, alkalinity is detrimental

* TCE = 0.100 mg/L **source Glaze and Kang 1988 8

Factors Affecting AOP Performance 3/4

• Impact of pH: O3/H2O2,

Overall: 2O3 + H2O2 → 2HO• + 3O2 H2O2 ↔ HO2- + O2- (pKa = 11.6) Rate limiting-step: O3 + HO2- → O3•- + HO2• (k=2.8×106) Scavenges by H2O2: HO• + HO2- → HO2• + OH- (k=7.5×109) HO• + H2O2 → HO2• + H2O (k=2.7×107) Inefficient at low pH(