UV-Based Advanced Oxidation Technologies: Chemistry

Lecture 3

UV-Based Advanced Oxidation Technologies: Chemistry

Prof. James R. Bolton and Prof. Thomas Oppenlaender

CivE 729

Outline

• Photolysis in a collimated beam

• Photolysis of NDMA

• Treatment of MTBE by UV/H

O

• Solar degradation of 1,4-dioxane

N-Nitrosodimethylamine (NDMA)

• Widely distributed in the human environment.

• Found in outdoor air, surface waters, preserved meat, cosmetics, detergents and pesticides.

• Found in groundwater at levels as high as 10 ppb.

• Potent carcinogen and mutagen.

• On USEPA National Priorities List.

• Not currently easily removed.

• does not air strip

• does not biodegrade

• does not adsorb to activated carbon

AOT Treatment of NDMA

• UV-based Advanced Oxidation Technologies are very effective in treating NDMA.

• NDMA undergoes direct photolysis at wavelengths <350 nm.

• UV/H₂O₂treatment, which involves generation of

••••OH radicals, can also be used.

• High concentrations ratios of H₂O₂to NDMA are required.

• Stefan, M. I. and J. R. Bolton, 2002. “UV Direct Photolysis of N- Nitrosodimethylamine (NDMA): Kinetic and Product Study.”, Helv.

Chim. Acta, 85, 1416-1426.

Previous Work

• Several studies on UV direct photolysis of N- nitrosoamines were reported in the 1970-1980 literature, both in aqueous solution and in the gas phase.

• A strong pH dependence was found for the photolysis of NDMA.

• Dimethylamine and nitrite were found as major products.

• No intermediate time profiles, TOC or nitrogen balances were reported.

• Very little work on UV/H₂O₂treatment of NDMA.

0 2000 4000 6000 8000 10000

200 250 300 350 400

Wavelength / nm Molar Absorption Coefficient (M-1 cm-1)

UV Spectra

Nitrate NDMA

Nitrite

x 1000

x 400

x 50

H2O2x 10

0,0 0,2 0,4 0,6 0,8 1,0 1,2

0 40 80 120 160

time / min

concentration / mM

NDMA DMA nitrite nitrate HCHO HCOOH TOC(exp)/2 TOC(calc)/2

NDMA Photolysis - pH 7

0,0 0,2 0,4 0,6 0,8 1,0 1,2

0 25 50 75 100 125

time / min

concentration / mM

NDMA DMA nitrite nitrate HCHO HCOOH

UV/H

O

Treatment - pH 7

Methyl-tert-butyl Ether (MTBE)

• Fuel oxygenate used as an octane enhancer of reformulated gasoline; largely manufactured in the USA.

• High solubility in water; detected in ground and storm water as the second most frequent contaminant (after CHCl3).

• Carcinogen in animals; potential human carcinogen.

• Not currently regulated as a drinking water contaminant.

• Low odor (45 ppb) and taste (39 ppb) detection thresholds.

• Drinking water advisory limit of 20 - 40 ppb MTBE, recently issued by USEPA.

• Cater, S. R., M. I. Stefan, J. R. Bolton and A. Safarzadeh-Amiri, 2000

“UV/H2O2treatment of methyl tert-butyl ether in contaminated waters”

Environ. Sci. Technol. 34, 659-662.

MTBE Remediation

• Traditional Technologies:

• Air-stripping - can achieve 99% removal of MTBE from water if large air to water ratios are used but is only a mass transfer.

• Adsorption on granulated activated carbon - low affinity; effective at low concentrations, but a high cost of carbon replacement at high concentrations.

• Aerobic biodegradation - difficult to apply to large volumes of MTBE-contaminated water or to ppm-ppb levels.

• Advanced Oxidation Technologies:

• UV/H2O2, UV/O3and O3/H2O2processes.

Kinetic Model

• Assume rate can be estimated from initial conditions.

• Rate of removal of MTBE is Rate = k₂[•OH]_ss[MTBE]

• The steady-state •OH concentration is

where Gis the photon flux output from the UV lamp, χχχχis the fraction absorbed in the solution, ΦΦΦΦis the quantum yield of generation of •OH radicals from H2O2photolysis and V is the solution volume (L).

∑

∑

∑ ++++∑ ++++

Φ ΦΦ

==== Φ

••••

i k k

k

V G

o i S o 2 2 O H o MTBE

ss [MTBE] [HO ] [S]

] / OH [

i 2

2

χχχχ

0 1 2 3 4 5 6

0 100 200 300 400 500

[H2O2] / (mg L^-1) EEO/ (kWh/order/m3)

Exptl.

Model

E

vs. Concentration of H

O

0 2 4 6 8 10 12

0 2 4 6 8 10 12 14 16

[BTX] / (mg L^-1) EEO/ (kWh/order/m3) ^Exptl.

Model

E

vs. Concentration of BTX

0,0 0,2 0,4 0,6 0,8 1,0

0 10 20 30 40 50

time / min

concentration / mM

MTBE TBF TBA Acetone Methyl acetate

MTBE Degradation (1)

k1= 5.8 x 10^-3s^-1

0,0 0,1 0,2 0,3 0,4

0 10 20 30 40 50 60 70 80

time / min

concentration / mM

HCHO MMP Pyruvaldehyde HO-iso-Butyraldehyde iso-Butyraldehyde Hydroxyacetone

MTBE Degradation (2)

0,0 0,1 0,2 0,3 0,4

0 10 20 30 40 50 60 70 80

time / min

concentration / mM

Acetic Formic Pyruvic Oxalic HO-iso-Butyric

MTBE Degradation (3)

0 1 2 3 4 5

0 10 20 30 40 50 60 70 80

time / min

concentration / mM

TOC(calc) TOC(meas) H2O2/10

TOC Balance

Homogeneous Solar Photodegradation of Contaminants in Water

•

Based on the UV-Vis Fentons process with ferrioxalate as the absorber

•

Ferrioxalate absorbs out to 500 nm

• Bolton, J. R., M. Ravel, S. R. Cater and A. Safarzadeh-Amiri, 1996. “Homogeneous solar photodegradation of contaminants in water”, Proceedings of the ASME International Solar Energy Conference, San Antonio, TX, 31 March - 3 April, 1996, American Society of Mechanical Engineers, United Engineering Center, 345 East 47th St., New York, NY 10017, pp 53-60.

• Patented process developed by Calgon Carbon Corporation about 1994

• For Waters of high UV Absorbance, high COD or high pollutant concentration

• Involves addition of ferrioxalate Fe(C2O4)32-

which absorbs light over a wide range of wavelengths (including part of the visible) to generate hydroxyl radicals

Rayox

-A

0,1 1 10 100

0 80 160 240

Electrical Energy Dose (kWh per 1000 gal)

[BTEX] (ppm)

Treatment of BTEX

UV/H

O

Absorption Spectra

0 1 2 3 4 5

200 250 300 350 400 450 500

wavelength / nm molar absorption. coeff. / 1000 M-1cm-1

0 2 4 6

photon flux (rel)

UV Lamp Profile Hydrogen

peroxide

ferrioxalate

• Efficient use of lamp output due to absorption of ferrioxalate over the UV and visible range.

• High reactivity of complexed ferrous ion with hydrogen peroxide.

• High quantum yield of Fe(II) formation means a very high quantum yield for generation of hydroxyl radicals.

• Photolysis of Fe(III)-organic intermediate complexes enhances the treatment effectiveness.

Why is Rayox

-A So Efficient?

Solar Detoxification

• Most research in solar photocatalytic decontamination has dealt with

heterogeneous catalysts, such as titanium dioxide (TiO₂).

• TiO₂disadvantages include:

• low quantum yield for ^.OH production (ca.

5%)

• potential for fouling

• only absorbs 3% of the solar spectrum

• mass transfer limitation on rates

Calgon Carbon’s Solaqua

Sunlight Decontamination Process

• homogeneous process

• involves an absorber (ferrioxalate) that absorbs solar radiation out to 500 nm

• the reaction mechanism involves the generation of hydroxyl radicals with a quantum yield of about unity

• 18% of the solar spectrum is absorbed

0,001 0,01 0,1 1 10 100

0 10 20 30 40

Time / min

[TCE] / ppm

Solaqua ^® vs. TiO ₂

TiO

Solaqua

Solar Detoxification Collector

0 20 40 60 80 100

0 5 10 15 20 25 30 35

Time/min

[1,4-Dioxane]/ppm

1 10 100

[1,4-Dioxane]/ppm

Solaqua

Treatment of 1,4-Dioxane in a Test Solar Reactor

Photon Flux = 0.0057 ein m^-2min^-1 Flow rate = 58 L/min

[ferrioxalate] = 20 ppm

Q.Y. = 1.15

Hyperlinks >>>>

Chemical Structures

C H₃

N C H₃

N O

O CH₃ C H₃ C H₃

C H₃

O O

N-nitrosodimethylamine (NDMA): C₂H₆N₂O - rocket fuel contaminant

- formation in drinking water:

monochloramine is a precursor to NDMA formation during chlorination Methyl tert-butyl ether (MTBE): C₅H₁₂O

1,4-Dioxane: C₄H₈O₂

Chemical Structures > estimation of oxidation and mineralization products

C H₃

N C H₃

N O

O CH₃ C H₃ C H₃

C H₃

O O

What do you think are the products of mineralization?

CO₂, H₂O, NO₃^-, NO₂^-(NH₄⁺)

CO2, H2O

Chemical Structures > estimation of oxidation and mineralization products

C H₃

N C H₃

N O

O CH₃ C H₃ C H₃

C H₃

O O

Which intermediary oxidation products may be formed?

MTBE Intermediary Oxidation Products Mineralization

O CH

C

H

C H

C H

UV/H₂O₂