Hatano Y., Katsumura Y., Mozumder A. (Eds.) Charged Particle and Photon Interactions with Matter - Recent Advances, Applications, and Interfaces

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920 Charged Particle and Photon Interactions with Matter

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923

Applications of Ionizing

Radiation

to Environmental

Conservation

Koichi Hirota

Japan Atomic Energy Agency

Takasaki,

Japan

32.1 introduCtion

New technologies and processes have promoted our economic growth and have been remarkably

changing our lifestyles. They have created a sophisticated society and we have been enjoying com-

fortable lives. However, technological innovation is not always welcome. It destroys the environ-

ment and puts us in a dangerous situation. The emission of inorganic pollutants such as nitrogen

oxides (NO

) and sulfur dioxide (SO

) has been remarkably reduced by state-of-the-art technolo-

gies during the past 50 years. However, industrial countries have been using a lot of oil and coal to

obtain energy for electricity, vehicles, chemical products, etc. The combustion of oil and coal pro-

duces large amounts of carbon dioxide, which causes the greenhouse effect. For this reason, energy

sources have been diversied from conventional oil and coal to atomic, solar, and biomass energies,

so as to reduce the emission of carbon dioxide and geopolitical risk against the energy supply. The

EU-15 emission trend for NO

and SO

shows emission reductions of 31% and 70%, respectively,

from 1990 to 2004 (European Environmental Agency, 2006). Although the developing countries

also try to increase the consumption of environment-friendly energies instead of fossil fuels, the

steep demand for energy due to the recent economic growth makes it difcult to reduce the usage

of fossil fuels. This causes a large amount of emission of NO

, SO

, as well as carbon dioxide to the

atmosphere (Streets et al., 2000; Kato and Akimoto, 2007; Gaffney and Marley, 2009; Ramanathan

and Feng, 2009). According to estimated long-term SO

emission trends for Asia (Carmichael et al.,

2002), even if various chemical processes emitting SO

are operated under current legislation, the

emission of SO

will increase. In particular, China is the major source of NO

and SO

emissions.

The emission of SO

in 1995 was about 25,200kt in China. It will increase by about 60,700kt,

if emission control is not implemented. Even with emission control, the SO

emission would be

Contents

32.1 Introduction ..........................................................................................................................923

32.2 Radiation

Treatment .............................................................................................................926

32.2.1

Electron

Beams.........................................................................................................926

32.2.2

Gamma

Rays..............................................................................................................929

32.3

and SO

in Flue Gases..................................................................................................929

32.4 VOCs

in Exhaust Gases........................................................................................................ 931

32.5

Dioxin

in Municipal Waste Incinerator Gases .....................................................................935

32.6

Organic

Pollutants in Wastewater......................................................................................... 937

References...................................................................................................................................... 938

924 Charged Particle and Photon Interactions with Matter

expected to be 30,600 kt (Streets and Waldhoff, 2000). The emission of NO

and SO

leads to acid

rain that is detrimental to forests, crops, and building structures (Gaffney et al., 1987; Irwin and

Williams, 1988; Xie et al., 2004). While the industrial countries have effected a sufcient reduction

and SO

emissions, it is still an urgent issue for the developing countries.

Volatile organic compounds (VOCs) are essential chemicals in various manufacturing processes

such as surface painting, petroleum rening, and metal cleaning (Theloke and Friedrich, 2007; Celebi

and Vardar, 2008; Jia et al., 2008). More than 300 VOCs were found at both indoor and outdoor loca-

tions in the ambient environment. Although the VOC concentration range in the indoor and outdoor

environments is quite low in the order of parts per billion (ppb) (Shah and Singh, 1988; Mohamed et

al., 2002), people spend a lot of time under indoor environments, where harmful VOCs such as form-

aldehyde, toluene, and chlorophenol exist. They cause heavy damage to the body and the environment.

Long exposure even at a low concentration of VOC may cause cancer. Headache, nausea, and stimula-

tion of the eyes and nose are acute toxic symptoms of VOCs. Volatile organic compounds also have an

inuence on the environment, where photochemical oxidant formation, stratospheric ozone depletion,

and troposphericozone formation takeplace. Photochemical oxidant formation, caused by tropospheric

ozone formation, is accelerated as hydrocarbon concentrations increase. Stratospheric ozone depletion

leads to excess radiation of UV rays on the earth, which induces skin cancer. Many OECD countries

have made legislations for the emission control of VOCs (Ministry of the Environment, 2009a), as

shown in Table 32.1. They established their own standards for target facilities in large scale, since

middle- and small-scale manufactures cannot afford to cope with the reduction of VOCs. In contrast,

there is no national legislation in China, where about 20.1Mt VOC were emitted in 2005. The major

sources of the VOC emissions are road transportation, industrial solvent use, and fuel combustion. The

VOC emission from gasoline and diesel vehicles used in road transportation accounts for about 25%

of the total emission (Wei et al., 2008). Also, the VOC emission in China will increase by 20.7Mt in

2030, which is about 11 times higher than that in Japan (Klimont et al., 2001). In response to the OECD

recommendation, the Government of Japan issued a new legislation of Pollutant Release and Transfer

trial corporations have to periodically report on the nature and quantity of pollutants emitted and the

table 32.1

oeCd legislation

for vo

mission

Control

nation legislation

number of

target

Chemicals

target

Facilities

beginning

period

United

States TRI

667 M.I.

1987

Canada NPRI

273 M.I.

1993

Australia NPI

90 M.I.

1998

United

Kingdom PI

170 M.I.

1991

Holland IEI

67 Facilities

1976

Japan PRTR 354 M.I.

2001

Source:

Hirota,

K. et al., Radiat. Phys. Chem., 45, 649, 1995.

With

permission.

Toxic release inventory.

National pollutant release inventory.

National pollutant inventory.

Pollution inventory.

Integrated emission inventory.

Manufacturing industries and others determined by the number of employee and

annual transaction volume.

Facilities required for the allowance under environmental management low.

Applications of Ionizing Radiation to Environmental Conservation 925

environmental media they are released into. The 2006 PRTR in Japan has reported that 0.46Mt/year of

registered chemicals were totally emitted from notied and un-notied facilities, mobile sources, and

households to the environment (Ministry of the Environment, 2009c). Figure 32.1 shows the emission

amounts of the 10 highest-ranked chemicals in Japan (Ministry of the Environment, 2009c). The total

emission of these chemicals, of which nine are VOCs, accounts for 86% of the total emission. The

emission control of VOCs is not sufcient to save our lives and the environment.

Dioxin is one of the most toxic man-made chemicals, involving polychlorinated dibenzo-p-

dioxins (PCDD) and polychlorinated dibenzofurans (PCDF). There are 210 PCDD/F isomers, the

toxicity of which depends on the number of substituent Cl and their positions on the rings. The

toxic equivalent (TEQ) is dened to evaluate the toxicity of the isomers because not all the iso-

mers show the same amount of risk. The total TEQ of a dioxin mixture is calculated using the

toxic equivalency factors (TEFs). The TEF of 2,3,7,8-tetra-chlorinated dibenzo-p-dioxin (TeCDD),

the most toxic dioxin of all, is dened as 1. Thus, the TEQ concentration of a dioxin mixture (ng

I(international)-TEQ)

can be calculated as follows:

I c

− =

∑

TEQ TEF (32.1)

where c

is the mass concentration of each isomer (ng). When co-planar polychlorinated biphenyls

(co-PCB) is included in dioxin, the TEQ concentration is expressed in a unit of ng WHO (World

Health

Organization)-TEQ.

Dioxins

have been anthropogenically emitted from many sources such as incineration facilities and

industrial sites (Carroll et al., 2001; Dyke and Amendola, 2007; Mininni et al., 2007; Choi etal., 2008).

The emission of dioxin from the major source, municipal solid waste incinerators and hospital waste

incinerators, has been drastically reduced worldwide. For instance, about 4000g I-TEQ of dioxin was

released to the environment from municipal solid waste incinerators in Europe in 1995. It decreased

by about 200g I-TEQ in 2005. In the case of hospital waste incinerators, about 90% reduction of

dioxin emission was achieved in 2005, compared with a emission of 2000g I-TEQ in 1985 (Quaβ

etal., 2004). Japan also had made many efforts to comply with a new law on the emission of PCDD/F

established in 1999 by the Government of Japan. The municipal solid waste incinerator is the largest

120

160

200

Toluene

Xylene

Ethylbenzene

Dichloromethane

Polyoxyethylenealkylether

p-Dichlorobenzene

n-Alkylbenzenesulfonic acid and its salt

Benzene

Formaldehyde

1,3-Dichloropropene

VOC emission/kt/year

Figure 32.1 Emission amount of the 10 highest-ranked chemicals in 2006.

926 Charged Particle and Photon Interactions with Matter

source for emitting dioxin, because about 80% of solid waste is incinerated in Japan (Kishimoto et al.,

2001). A successful reduction from about 3000g WHO-TEQ in 1999 to 290g WHO-TEQ in 2007 was

achieved by the replacement of electric precipitators with bag lters. The improvement of incineration

conditions such as turbulence, temperature, and time also contributed to reduction of PCDD/F emis-

sions (Ministry of the Environment, 2009d). Although dioxin emissions have progressively reduced

worldwide, the risk of dioxin to the body and the environment still remains (Li et al., 2008; Zhang

etal., 2008; Venier et al., 2009). Some of the local governments and the small islands in Japan cannot

afford to upgrade existing municipal solid waste incinerators or install new ones. They need to request

neighboring governments to incinerate waste, which is a big burden nancially.

It is well known that endocrine disrupting chemicals (EDCs) such as bisphenol A, nonylphenol,

and estradiol have been found in river water, which are detrimental to water creatures because

of their hormone-mimicking and hormone-antagonizing effects in these organisms (Jackson and

Sutton, 2008; Zhao et al., 2009). Even trace concentrations of EDCs have affected all kinds of

animals on the earth, through the food chain. EDCs have been released from industrial process

and residential waste. So far, bisphenol-A, a starting chemical for the synthesis of polycarbonate

and epoxy resins, was widely used in the manufacture of plastic dishes, baby bottles, etc. However,

many manufactures have replaced these with glass-made ones, because unreacted bisphenol-A is

dissolved out with hot foods and milk contained in the dishes and bottles. Some documents sound

the alarm about the accumulation of bisphenol-A in pregnant women consuming canned food and

drink. The intake of EDCs causes female reproductive disorders in mammals, which is accelerated

for infants (Crain et al., 2008; Mariscal-Arcas et al., 2009).

Analytical techniques advanced well in the twentieth century, and we can detect hazardous

chemicals such as dioxin, EDCs, and VOCs at quite low concentrations. Although some of the

existing technology can treat these chemicals, there still remain problems of cost, lower efciency,

and inuence on the environment. In this section, the application of ionizing radiation, one of the

advanced oxidation technology, is introduced to help the readers understand the importance of ion-

izing

radiation for environmental conservation.

32.2 radiation treatment

Among various sources of ionizing radiation, gamma-ray and electron beams can substantially treat

environmental pollutants. Any ion beam generated from accelerators cannot be applied to the treat-

ment from energetic and economic points of view. The shorter range of ions in polluted media such

as exhaust gas and waste water is also a disadvantage for the treatment of pollutants. This section

describes

the characteristics of electron beams and gamma-rays for environmental conservation.

32.2.1 electron beaMS

Electron beams are generated from an electron accelerator which can accelerate electrons with a

voltage of 10–10,000kV. The electron beam treatment is based on radical reactions. When electron

beams are incident in air containing environmental pollutants, more than 99% of the energy is

absorbed

in the main components of air, such as nitrogen, oxygen, water, and carbon dioxides. The

energy absorption to the pollutants is negligible because their concentrations are usually quite low

compared to the air components. There are two ways to lose energy as described below:

(a) Excitation and/or ionization by coulombic interaction

(b) Moderation

by coulomb eld of atomic core

case of (b), bremsstrahlung, equivalent to the energy loss is emitted. The energy losses (a) and

(b) are equal when the value of EZ is about 800, where E and Z denote electron energy (in MeV)

and atomic number, respectively. Thus, when electron beams are incident in air, electrons lose their

energies

by excitation and ionization.

Applications of Ionizing Radiation to Environmental Conservation 927

Electron-beam treatment is based on reactions with charged molecules and atoms. When air

containing gaseous pollutants is irradiated with electron beams, active species such as N, O,

, O

, e, OH, and O

are produced by the ionization and excitation of the basic components

of air, namely nitrogen, oxygen, and H

O (Mätzing, 1992). Among these species, gaseous pol-

lutants can be oxidized through reactions with hydroxyl oxides (OH). This is similar to those

observed in the atmosphere under the sunlight, where the fates of gaseous pollutants have been

determined by the rate constants for reactions with OH radicals (Finlayson-Pitss and Pitts,

1986; Atkinson, 1987). The important pathway for generating OH radicals under the irradiation

is as follows:

N H O N H O

2 2 2 2

+ +

+ → + (32.2)

N O N O

2 2 2 2

+ +

+ → + (32.3)

O H O O H O

2 2 2 2

+ +

+ → + (32.4)

H O H O H O OH

2 2 3

+ +

+ → + (32.5)

Charge-transfer reactions among the two charged molecules and water form hydroxyl radicals

(Willis et al., 1970; Willis and Boyd, 1976). Because the OH radicals are the predominant species

for the decomposition of organic pollutants, their OH rate constants are related to decomposition

G-values. Figure 32.2 shows correlation of rate constants for reactions with OH radicals and G val-

ues. Ninety chemicals in aromatics, aliphatics, and alicyclics were irradiated with electron beams

(Hirota et al., 2004). Interestingly, the data points obtained from the aromatics of benzene, toluene,

xylene, and ethylbenzene were tted on a straight line. The data points of aliphatics and alicyclics

also gave straight lines. We can estimate electron-beam decomposition G-values of chemicals from

their

chemical structures and OH rate constants.

Dose

rate (kGy/mA) is the one of the important parameters for treating air pollutants using

an electron beam. Although electron beams with a high dose rate can treat exhaust gas at a

high ow rate (∼10

/h), the high dose irradiation frequently leads to lower treatment per-

formance than lower dose irradiation. Figure 32.3 shows the decomposition prole for xylene

by an electron beam generated with 1.0 MV and 170 kV accelerators. Both the experiments

1 10 100 1000

(molecule s cm

)

G value

Figure 32.2 Correlation of OH rate constants and G values: (

◾

) aromatics, (

⚫

) alicyclics, (▴) aliphatics.

(From

Hirota, K. et al., Ind. Eng. Chem. Res., 43, 1185, 2004. With permission.)

928 Charged Particle and Photon Interactions with Matter

were conducted using the same irradiation reactor. It is obvious that irradiation by an 1 MV

accelerator caused a higher decomposition of xylene over the investigated doses. For instance,

xylene was completely oxidized at a dose of 6 kGy when treated with irradiation from a 1 MV

accelerator. On the other hand, electrons operated on 170 kV decomposed about 90% at the

same dose. The dose rate at a dose of 6 kGy was 26.1 kGy/s for the 170 kV accelerator, which

is about 10 times higher than that of the 1 MV accelerator. This indicates that electron energy

absorbed in the gases per unit time was different. The irradiation at the high dose rate produced

excess amount of active species for the xylene decomposition, leading to their recombination.

Irradiation with low dose rate is a key for reducing the recombination, namely effective treatment

of gaseous pollutants.

Although one of the better ways to obtain a low dose rate using the low energy electron

accelerator is to enlarge the irradiation reactor in the direction of the irradiation; much atten-

tion should be paid to the range of the electron beam. Figure 32.4 shows the dose depth curve

obtained for low energy electron accelerators operated at 170 and 300 kV. Supposing exhaust

gases at an ambient temperature (density ρ = 1.2 × 10

−3

g/cm

) are treated by the two electron

accelerators equipped with a Ti foil (ρ = 4.5 g/cm

) window in a thickness of 13μm, the range

of electrons generated from 175 and 300 kV are 18.5 and 53.5 cm, respectively. The depth of the

reactor in the irradiation direction must be less than these values to avoid non-irradiation space

in the reactor.

100

120

0 100 200 300 400 500 600 700 800

Range (g/m

)

Dose (% front surface)

300 kV

170 kV

Figure 32.4 Depth dose curve of low energy electron accelerators.

100

0 5 10 15

Absorbed dose (kGy)

Decomposition (%)

1.0 MV

170 kV

Figure 32.3 Decomposition efciency of xylene by electron-beam irradiation.

Applications of Ionizing Radiation to Environmental Conservation 929

32.2.2 gaMMa rayS

In the case of wastewater treatment, gamma-rays generated from

Co are applied to produce active

species for decomposing organic pollutants, because gamma-rays can penetrate deeply into water.

Cobalt-60 is an articial radioisotope that is produced in a nuclear reactor. It decays to nickel via beta-

radioactivity with the emission of gamma-rays at 1.17 and 1.33MeV. The thickness of water that reduces

the intensity of the gamma-rays by one-tenth is about 34cm. The range of electron beams at 2MeV is

about 0.9cm. When treating wastewater with electron beams, its thickness needs to be controlled.

When the energy of gamma-rays is absorbed in water, various ion and active species are pro-

duced

through ionization and excitation (Burton and Magee, 1976; Getoff, 1996):

H O e H O H OH H H O HO

2 aq 3 2 2 2 2

− +

, , , , , , (32.6)

Among these products, the hydrated electron (e

−

), the hydrogen atom (H), and the hydroxyl radical

(OH) contribute to the decomposition of organic pollutants in wastewater. For example, halogenated

compounds (RX) are subject to be de-halogenated through reactions with hydrated electrons that

are

produced with a G-value of 2.7:

RX e R X

+ → +

− −

(32.7)

Hydroxyl radicals with a G-value of 2.8 can decompose organic pollutants through addition or H

abstraction:

(32.8)

The reaction of hydrogen atom with alcohol proceeds through H abstraction from the alkoxy group:

H C H OH C H OH H

2 5 2 4 2

+ → +

•

(32.9)

The hydrogen atom is produced with a G-value of 0.6. In general, hydrated electrons and hydrogen

atoms undergo reactions with oxygen dissolved in wastewater and produce HO

radicals that have

lesser reactivity than OH radicals. Hydroxyl radicals play an important role in the treatment of

organic

pollutants in wastewater.

32.3 no

and so

in Flue gases

To meet the strict regulations established by the Japanese government and the local governments,

the wet lime scrubber method and the selective catalytic reduction method have been applied to

treat SO

and NO

. However, the wet lime scrubber method requires wastewater treatment, and the

catalysts have to be replaced periodically. New technology is expected for simple and simultaneous

treatment processing of the pollutants. The electron-beam irradiation process for ue gas purication

has

been proposed as an efcient method because it has the following advantages:

•

Has simultaneous denitrication and desulfurization possibilities

• Requires

no wastewater treatment due to dry process

•

Requires

no expensive catalysts

•

Has

simple process and operation

•

Produces

protable products

The

application of electron beams to treat ue gases, such as removing sulfur dioxide, was started at

Ebara Corporation in Japan. They demonstrated in 1971 that sulfur dioxide at an initial concentration

OH +

•