(Chapra, L42) David Reckhow CEE 577 #33 1 Overall decay rate k - - PowerPoint PPT Presentation

Lecture #33 Toxics: Photolysis

(Chapra, L42)

David Reckhow CEE 577 #33 1

Updated: 17 April 2013

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Overall decay rate

 k1 is a combination of

 Hydrolysis (kh)  Photolysis (kp)  Biodegradation (kb)  Any other loss process not otherwise included in the

model

 Any of these processes may be affected by sorption

 Photolysis is most commonly attributed to dissolve

forms only

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𝑙1 = 𝑙ℎ + 𝑔

𝑞1𝑙𝑞 + 𝑙𝑐

Photolysis

 Chemical breakdown initiated by light energy  two types

 direct photolysis  sensitized (or indirect) photolysis

 Several steps

 some solar light reaches water surface  some of this light penetrates to the solute  some of this is absorbed by the solute  some of absorbed light is capable of causing a

reaction

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Solar Radiation

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Attenuation of light through water

 Beer-Lambert’s Law  and the attenuation coefficient is from:

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z

D

e I z I

) (

) , ( ) , (

λ α

λ λ

−

=

Light Intensity (einstein/cm2/s) at depth z, and wavelength, λ Attenuation coefficient (cm-1)

      = ) , ( ) , ( ln 1 ) ( λ λ λ α H I I H

D

Measured light intensity with depth

 Lake San Vincente  Tyler & Smith, 1970

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 Average light absorption over depth H

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( )

H

D

e H I I

) (

1 ) , (

λ α

λ

−

− =

einsteins/cm3/s

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Spectral overlap

 (a) benzene; (b) naphthacene or tetracene

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Absorption of Light

 Attenuation due to background and

contaminant

 Integrating over the depth and

isolating on the contaminant we get:

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[ ]



c

e I I

) ( ) (

) , ( ) , (

λ ε λ α

λ λ

+ −

=

Molar extinction coefficient (L/mole/cm)

c H e I I

H a

D

) ( ) 1 )( , ( ) ( ) (

) (

λ α λ λ ε λ

λ α −

− =

Quantum Yield

 Once absorption occurs, energy is dissipated by:

 emission of heat  emission of light: luminescence  contact with another molecule: photosensitization  decomposition: direct photolysis

 Fraction following last pathway is quantum yield  And combining with absorption equation:

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absorbed h wavelengt

• f

photons

• f

moles total ed transform moles

• f

number ) ( λ λ = Φr

) ( ) 1 )( , ( ) ( ) ( ) ( ) ( ) (

) (

λ α λ λ ε λ λ λ λ

λ α

H e I k k

H a p

D

−

− Φ = Φ =

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 To next lecture

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