1. Optical spectroscopy to gain further insights on the chemical structure of humic substances (HS) and chromophoric dissolved organic matter (CDOM). Rossana Del Vecchio1,2,*,Tara M Schendorf1, Kevin Koech 1, Marla Bianca1 and Neil V Blough1 (1) Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20740, USA (2) Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20740, USA (*correspondence :

2. Why CDOM……
It is found ubiquitously in all aquatic environments
It is a complex mixture of unknown structure
It absorbs light across UV and visible wavelengths
It contributes to the optical properties of most surface waters
It can alter the Global Carbon Cycle
Goals……
Employ optical spectroscopy coupled with chemical reduction/reoxidation and pH titrations to characterize and monitor HS/CDOM in natural waters
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3. HS/CDOM: optical properties
Longer wavelength excitation emitting species - subset of shorter wavelength excitation
Individual chromophores cannot be resolved within our 5 nm resolution
Not a simple linear superposition of the spectra of numerous independent chromophores
Near-continuum of emitting species or states.
Short wavelength (<350 nm)
absorption: local donor/acceptor states
fluorescence: local donor states
Long wavelength (>350 nm)
absorption/fluorescence tail: optical charge-transfer transitions
Optical charge-transfer transitions can arise via a ground-state association between electron donors (D) (phenols and carboxylics) and electron acceptors (A) (ketones/aldehydes and/or quinones) to form a D-A charge-transfer complex. These D-A complexes exhibit new broad absorption bands that are not shown by either the D or A molecules alone.
Hernes and Benner (2003), JGR, 108(C9), 3291
Del Vecchio and Blough (2004), EST, 38,
Boyle et al. (2009), EST, 43,
Furman and Lonsky (1986). JWC
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4. Optical properties and structure
Donors
Acceptors
CT complexes
Hernes and Benner (2003), JGR, 108(C9), 3291 Del Vecchio and Blough (2004), EST, 38, Boyle et al. (2009), EST, 43, Furman and Lonsky (1986). JWC
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5. Functional groups in HS/CDOM
Aromatic ketones have been proposed to comprise the majority of these carbonyl-groups content (Leenheer, 1987).
Quinones from the degradation and biological reworking of plant tissue are also believed to be significant constituents of HS (Thorn EST 1992).
Phenolic & carboxyl moieties (Ritchie and Perdue, GCA, 2003)
~5-10 and ~10-20% of C, respectively;
ratio of 21:79 of total acidity, respectively;
FA~HA (Phenolic);TFA>AFA>NOM>AHA>THA (carboxyl);
However, the relative contribution of each group to the HS optical properties is still ill-defined
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6. Selective manipulation of specific functional groups
Chemical reduction
pH titration
NaBH4
Na2S2O4
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7. Schendorf et al., L&O Methods, 2016.
Chemical reduction of
HS/CDOM (NaBH4)
Schendorf et al., L&O Methods, 2016.
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8. Schendorf et al., L&O Methods, 2016.
Chemical reduction of
HS /CDOM (NaBH4)
Schendorf et al., L&O Methods, 2016.
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9. NaBH4 reduction preferential loss of long-wavelength absorption
Del Vecchio et al., submitted
Schendorf et al., L&O Methods, 2016.
Ma et al., EST, 44, , 2010.
preferential loss of long-wavelength absorption
3 components
mostly irreversible
minor reversibility
enhanced, blue-shifted emission
primarily due to aromatic ketones/aldehydes
minor contribution from quinones
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10. (Del Vecchio et al., submitted)
Na2S2O4 reduction
(Del Vecchio et al., submitted)
minor loss of VIS absorption
minor enhanced, blue-shifted emission
consistent with previous work (Maurer et al EST 2010; Fimmen et al EST 2007; Aeschbacher et al EST 2010)
reversible
quinones
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11. Na2S2O4 vs NaBH4 Del Vecchio et al., submitted
Schendorf et al., L&O Methods, 2016.
Ma et al., EST, 44, , 2010.
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12. Selective manipulation of specific functional groups
Chemical reduction
pH titration
NaBH4
Na2S2O4
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13. pH dependence of optical properties
Dryer, D. J.; Korshin, G. V.; Fabbricino, M., Environ. Sci. Technol
Korshin, G. V.; Benjamin, M. M.; Li, C.-W., Water Sci. Technol
absorbance increased with increasing pH
pH dependence of absorption changes indicates occurrence of specific functional groups
carboxylic acids at low pH
phenols at high pH
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14. pH dependence of optical properties
Increasing pH
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15. pH dependence of optical properties
ApH(x)/ApH(2)
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16. pH dependence of asborption difference
Dryer, D. J.; Korshin, G. V.; Fabbricino, M., Environ. Sci. Technol
ΔA = A[pH (x)] – A[pH (2)]
pH<7: peak at 280 nm
pH>7: broad peak at nm
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17. pH dependence of absorption difference:
carboxylic acids (pKa=3-5) vs phenols (pKa >8)
Dryer, D. J.; Korshin, G. V.; Fabbricino, M., Environ. Sci. Technol
𝐀(𝐩𝐇 𝐗 )−𝐀(𝐩𝐇 𝟐 )
𝐀(𝐩𝐇 𝐗 )−𝐀(𝐩𝐇 𝟖 )
Low pH (carboxylic acid)
A peak at 280 nm
Extends into the long wavelengths
Consistent among HS
High pH (phenol groups)
A broad band at nm
Extends into the long wavelengths
Wide variation among HS
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18. pH dependence of optical properties Untreated vs NaBH4 reduced
Increasing pH
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19. pH dependence: untreated vs reduced HS
Peak at 280 is unaffected
Peak at 330 nm is significantly affected
Peak at ~375 nm is substantially decreased
Peak ratio is inverted
A at >400 nm is diminished
Trend varies among HS
ΔA = A(pH (x)) – A(pH (2))
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20. pH dependence of absorption for reduced HS
Summary
𝐀(𝐩𝐇 𝐗 )−𝐀(𝐩𝐇 𝟐 )
𝐀(𝐩𝐇 𝐗 )−𝐀(𝐩𝐇 𝟖 )
Peak at 280 is unaffected
A at >330 nm is eliminated
Trend ~ consistent among HS
A at 280 is unaffected
A at ~330/400 nm is lowered
A at >450 nm is lowered
Trend varies among HS
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21. Conclusions
Selective manipulation of functional groups has allowed us getting further insights on HS/CDOM chemical structure in relation to their optical/photochemical properties
NaBH4/Na2S2O4 reduction of HS allowed to conclude that
aromatic ketones/aldehydes are
significant contributors to absorption
quinones are
minor contributors to absorption
pH titrations………
absorption
both carboxylic and phenolic groups contribute to the HS absorption with the latter contributing more
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22. Conclusions …… continued
NaBH4 reduction + pH titrations
both carboxylic and phenolic groups are coupled with carbonyls to form long-wavelength CT transitions
……………consistent with the CT model
absorption changes are more variable among different HS
……………a new method for discriminating among HS
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23. Thank you for your attention!
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