Spectrophotometric titrations to characterize and monitor chromophoric dissolved organic matter (CDOM) in natural waters

Transcription

1 Spectrophotometric titrations to characterize and monitor chromophoric dissolved organic matter (CDOM) in natural waters Carmen Cartisano*, Rossana Del Vecchio, and Neil Blough* * Department of Chemistry & Biochemistry, University of Maryland, College Park, USA, Earth System Science Interdisciplinary Center, University of Maryland, College Park, USA, 1

2 Importance of CDOM Chromophoric dissolved organic matter: Natural component found ubiquitously throughout aquatic environments. Complex mixture of organic compounds from many sources. Can absorb light in the UV and visible wavelengths. Modified from: global-greenhouse-warming.com Wenk et al., 2013 Herzsprung et al., 2012 Kieber et al., 1989 Spencer et al., 2009 Arrigo and Brown, 1996 Nelson and Siegel, 2013 Coble, 2007 Boyle et al., 2009 Nelson and Siegel, 2002 Kirchman et al.,

3 Research Goals Method development: Spectrophotometric titrations of Natural water samples C18-OM vs. Natural water Implications of results: Examine the structural basis of the optical properties of CDOM Sodium borohydride reduction (carbonyl containing compounds) ph titration (phenols and carboxylic acids) Determine if marine and terrestrial CDOM are fundamentally different Comparison of open ocean samples to other regions and reference materials 3

4 Sampling locations Delaware Bay, and Middle Atlantic Bight (MAB) C18 extracts and Natural waters were collected onboard R/V Sharp in July 2016 Suwannee River Fulvic Acid (SRFA): from the Suwannee River provided by IHSS Google Earth 4

5 Sample collection and preparation Sample collection (in the field) Collect water Filter Acidify Extract (C18) Store Sample preparation (in the lab) Rinse Dry Elute (methanol) Rotovap Re-dissolve (water) 5

6 Extraction Efficiency (C18 columns) Absorbance (OD) Pre extraction water Post extraction water Preferential extraction of long wavelength absorbing material Extraction effects spectral slope Natural waters > C18-OM Extraction efficiency (%) DE. Bay and MAB samples Wavelength (nm) Spectral slope, S (nm -1 ) (C18-OM) linear reg. 1:1 UW 6: 2 m CTD 7: 2 m CTD 7: 1905 m CTD 6: 2 m CTD 6: 2002 m CTD 4: 132 m CTD 3: 841 m Extraction efficiency = 1 a CDOM post extraction water a CDOM pre extraction water 100 Spectral slope, S (nm -1 ) (Natural water) Andrew, A.A., Del Vecchio, R., Zhang, Y., Subramaniam, A., Blough, N. V., Are Extracted Materials Truly Representative of Original Samples? Impact of C18 Extraction on CDOM Optical and Chemical Properties. Front. Chem. 4, doi: /fchem

7 C18-OM reduction and titration method C18-OM sample Dilute Filter Divide (Untreated and Reduced) Untreated NaBH 4 reduction Adjust ph to ~10.0 Add NaBH 4 Reduce for 48 hrs Adjust ph to ~7.00 Re-oxidize for 72 hrs Untreated NaBH 4 Reduced ph titration Adjust ph to ~2.00 acquire abs. adjust to next ph Unt ph 2-12 Red ph

8 Natural water reduction and titration method Natural water sample Filter Divide (Untreated and Reduced) Untreated NaBH 4 reduction Add NaBH 4 (1mg : 1 ml) Reduce for 48 hrs Adjust ph to ~7.00 Filter Re-oxidize for 72 hrs Untreated NaBH 4 Reduced ph titration Create 5 aliquots Adjust ph individually (2-10) Acquire abs. using MPLCW Unt ph 2 Unt ph 4 Unt ph 6 Unt ph 8 Unt ph 10 Red ph 2 Red ph 4 Red ph 6 Red ph 8 Red ph 10 8

9 UltraPath MPLCW (WPI) protocol multi pathlength liquid capillary waveguide Clean Soap MeOH HCl H 2 O Set parameters Experiment specific parameters Reference and blank Dark Reference Blank Sample measurement Inject sample Scan Rinse 1. Miller, R.L., Belz, M., Del Castillo, C., Trzaska, R., Cont. Shelf Res. 22, doi: /s (02)

10 Chemical test: NaBH 4 Reduction Selectively reduces carbonyl containing compounds including ketones, aldehydes and quinones. Reduction of SRFA Reductions Presence of ketones and aldehydes Irreversible loss of absorbance Charge transfer interactions Preferential loss of long wavelength absorbance 1. Ma, J., Del Vecchio, R., Golanoski, K.S., Boyle, E.S., Blough, N. V., Environ. Sci. Technol. 44, doi: /es100880q 2. Schendorf, Tara; Effect of Borohydride Reduction and ph on the Optical Properties of Humic Substances; masters thesis, DRUM doi: /m2rw26 3. Schendorf, T.M., Del Vecchio, R., Koech, K., Blough, N. V., Limnol. Oceanogr. Methods 14, 2016, doi: /lom

11 C18-OM vs Natural water: Reduction Percent abs. loss (%) Percent absorbance loss following reduction CTD 7: 2 m UW 6: 2 m Natural water C18-OM CTD 7: 1905 m CTD 6: 2002 m Reduced Spectral Slope (nm -1 ) C18-OM vs. Natural water Similar Wavelength dependence Percent loss Change in Spectral Slope Natural water C18-OM Wavelength (nm) Untreated Spectral Slope (nm-1) -1 ) 11

12 Chemical test: ph titration Examines proton donating functional groups including phenols and carboxylic acids ph effect: Enhanced absorption with increasing ph at all λs ph ph 280 nm band: pka range of carboxylic acid 375 nm Band: pka range of phenols Carboxylic acids Phenols Loss of long wavelength ph dependence upon reduction 1. Schendorf, Tara; Effect of Borohydride Reduction and ph on the Optical Properties of Humic Substances; masters thesis, DRUM doi: /m2rw26 2. Janot, N., Reiller, P.E., Korshin, G. V., Benedetti, M.F., Environ. Sci. Technol. 44, doi: /es

13 C18-OM vs Natural water: ph titration Sharp UW 6: 2 meters 13

14 Moving along transect 14 Comparison of samples C18-OM vs. Natural water C18-OM Natural water C18-OM vs. Natural water: Similar response to ph Comparison along transect: Minor changes can be monitored

15 Moving offshore Comparison of samples - moving offshore SRFA DE River Untreated Reduced Unt-Red Transect comparison: DE river and Bay: Very comparable to SRFA (both untreated and reduced) MAB offshore: 280 nm band: shifted nm band: diminished > 400 nm absorption tail: present (untreated) and eliminated upon reduction DE Bay MAB St. 1 Google Earth 15

16 Comparison of Deep samples Untreated Reduced Unt-Red Open ocean comparison: EAO and NPO: same ph dependence SRFA EAO and NPO: similar ph dependence as SRFA however the ph dependent groups contribute less to the total absorbance EAO St. 51 EAO St. 73 Google Earth NPO Google Earth 16

17 Conclusions Changes in Natural waters can be monitored Reduction ph titration (MPLCW) Extracts are fairly representative of Natural waters High extraction efficiency Similar chemical response Similar responses to ph and borohydride reductions at all geographical locations indicate: Samples are structurally similar Presence of a terrestrial component in all samples including the open ocean 17

18 Acknowledgments Research Group Neil V. Blough Rossana Del Vecchio Marla Bianca Danielle Le Roux Sample collection Andrea Andrew Funding sources NSF Chemical Oceanography University of Maryland, College Park 18

19 Questions