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

Print version Updated: 17 April 2013 Lecture #33 Toxics: Photolysis (Chapra, L42) David Reckhow CEE 577 #33 1

Overall decay rate  k 1 is a combination of  Hydrolysis (k h )  Photolysis (k p )  Biodegradation (k b )  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 𝑙 1 = 𝑙 ℎ + 𝑔 𝑞1 𝑙 𝑞 + 𝑙 𝑐 David Reckhow CEE 577 #33 2

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 David Reckhow CEE 577 #33 3

Solar Radiation David Reckhow CEE 577 #33 4

Attenuation of light through water Attenuation coefficient (cm -1 )  Beer-Lambert’s Law − α λ λ = λ ( ) z I ( z , ) I ( 0 , ) e D Light Intensity (einstein/cm 2 /s) at depth z, and wavelength, λ  and the attenuation coefficient is from:  λ  1 I ( 0 , ) α λ = ( ) ln   λ D   H I ( H , ) David Reckhow CEE 577 #33 5

Measured light intensity with depth  Lake San Vincente  Tyler & Smith, 1970 David Reckhow CEE 577 #33 6

 Average light absorption over depth H ( ) λ I ( 0 , ) − α λ = − ( ) H I 1 e D H einsteins/cm 3 /s David Reckhow CEE 577 #33 7

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Spectral overlap  (a) benzene; (b) naphthacene or tetracene David Reckhow CEE 577 #33 9

Absorption of Light Molar extinction coefficient (L/mole/cm)  Attenuation due to background and contaminant [ ]  − α λ + ε λ λ = λ  ( ) ( ) c I ( , ) I ( 0 , ) e  Integrating over the depth and isolating on the contaminant we get: − α λ − ε λ λ ( ) H ( ) I ( 0 , )( 1 e ) D λ = ( ) I c α λ a H ( ) David Reckhow CEE 577 #33 10

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 number of moles transform ed Φ r λ = ( ) λ total moles of photons of wavelengt h absorbed  And combining with absorption equation: − α λ ε λ λ − ( ) H ( ) I ( 0 , )( 1 e ) D λ = Φ λ λ = Φ λ ( ) ( ) ( ) ( ) k k α λ p a H ( ) David Reckhow CEE 577 #33 11

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 To next lecture David Reckhow CEE 577 #33 13