A Study of the Photodegradation of Dissolved Organic Matter

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A Study of the Photo-degradation of Dissolved
Organic Matter
Brian L. Anderson Dr. Patricia Maurice Dr. Dave
Kreller August 3, 2005 Dept. of Civil
Engineering and Geological Sciences University of
Notre Dame
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Why study DOM photo-degradation?

• We need to know a lot about DOM
• It forms complexes with some pollutants,
  affecting pollutant mobility and reactions
• It actually helps in the degradation of some
  pollutants.
• Photo-degradation processes are important in
  changing the properties of DOM.

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Why study the photo-degradation of dissolved
organic matter (DOM)?

• DOM has a sun- blocking effect in aquatic
  ecosystems.
• As DOM is degraded,its absorptivity is decreased,
  which is of ecological concern.

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Much is known about DOM photo-degradation
processes

• Mineralization occurs
• DOM h? O2 ? DOMox HO2/O2- ? ? OH DIC
• Iron plays a catalytic role
• DOM-Fe(III) h? ? DOM Fe(II) ? DOMox
  Fe(III)

This goal of this study is to add to
understanding of the mechanism(s) of the
reactions.
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Questions asked in this study

• 1. What is the effect of decreasing the
  concentration of Fe(III) complexed by the DOM?
• 2. Is singlet oxygen involved?
• 3. Are free radicals involved?

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Experiment 1

• Q. What is the effect of decreasing Fe(III)?

DOM- Fe(III) h? ? DOM Fe(II) DIC Fe(II)
O2 ? Fe(III) O2-
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Method Experiment 1.
3. solar simulator
4. Analyses
1. Make Stock solns
2. Divide into 36 tubes
FeC 0.0147
pH 5.5
irradiate control (dark)
June 2002, NLC, MI DOM collected by RO
DOC UV-VIS HPSEC pH Fluorescence
FeC 0.0031 treated w/ Chelex-100
irradiate control (dark)
pH 5.5
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Results Fe experiment
Upon Irradiating
There was a greater decrease in pH in the natural
Fe DOM than the low Fe DOM
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Results Fe experiment
Upon Irradiating
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Effect of Fe on changes in Mw

• The weight average molecular weight (Mw) of the
  natural Fe DOM decreased at a greater rate in
  the first 12 hours than did the low Fe DOM.
• From 12 hrs up to the 72 hour time point (not
  shown) the two samples decreased in Mw at the
  same rate

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Experiment 2

• Q. Is singlet oxygen (1O2) involved?

1DOM h? ? 3DOM 3O2 ? 3DOM 1O2 1O2 DOM ?
DOMox N3- 1O2 ? N3- 3O2 heat
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Effect of adding sodium azide to solution

• The DOC of DOM with azide added decreased at a
  lesser rate than the DOC of the DOM without
  azide. The DOC of the natural Fe DOM
  decreased at a greater rate than the DOC of the
  low Fe DOM.
• These results suggest that 1O2 is involved in the
  photo-degradation.

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Experiment 3

• Q. Are free radicals (DOM or OH) involved?

DOM h? ? DOM (or OH) ? DOMox DOM (or OH)
CH3OH ? DOM (or H2O)
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Results methanol experiment

• The pH of DOM solution with methanol (1 v/v)
  decreased at a lower rate than DOM without
  methanol
• 280 nm absorbance of DOM solution containing
  methanol decreased at a lower rate than DOM
  without methanol

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Conclusions

• DOM-complexed Fe(III) speeds up the
  photo-degradation of DOM, removing Fe(III)
  reduces rate.
• The Fe(III) seems to exert its photo-catalytic
  properties within the first few hours of
  irradiation.
• The mechanism of DOM photodegradation is complex
• Singlet oxygen likely plays a role
• Free radicals likely play some role

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Further Research

• Use an in-line dissolved organic carbon (DOC)
  detector in the HPSEC analysis of the irradiated
  DOM this will show all DOM in solution (even UVA
  transparent, i.e. bleached molecules).
• Further investigate the involvement of singlet
  oxygen
• D2O experiment
• Study how the photo-degradation of DOM changes
  its ability to complex with pollutants.

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Acknowledgements

• National Science Foundation REU Program
• Dept. of Civil Engineering and Geological
  Sciences at the University of Notre Dame
• Dr. Maurice, Director of the Center for
  Environmental Sciences and Technology
• Dr. Dave Kreller
• Leilani Arthurs
• John Loftus