a chemical characterization of landfill leachate
A CHEMICAL CHARACTERIZATION OF LANDFILL LEACHATE: OPERATIONAL AND TREATMENT COMPOSITIONAL VARIATIONS FOR LEACHATE ORGANIC MATTER S. BOLYARD*, B. COTTRELL**, D. LOZINSKI°, D. REINHART°°, A. MOTLAGH° * Environmental Research & Education Foundation. 3301 Benson Drive, Suite 101, Raleigh, NC 27609. ** University of California Irvine. Department of Civil and Environmental Engineering. 5200 Engineering Hall, Irvine, CA 92697 °University of Central Florida Department of Civil, Environmental, and Construction Engineering. 12800 Pegasus Drive, Suite 211, Orlando, FL 32816. °°University of Central Florida Office of Research and Commercialization. 12201 Research Parkway, Suite 501, Orlando, FL 32826.
SUMMARY: Leachate organic matter (LOM) is problematic because it is highly colored, may interfere with UV disinfection or lead to disinfection byproducts, is resistant to conventional biological treatment, and is known to transport heavy metals and hydrophobic organic contaminants. This research examined the effects of landfill operation and treatment on the chemical nature of LOM using traditional biochemical and advanced spectroscopic techniques. Twenty-three leachate samples were collected in Florida, Minnesota, and California. Most of the leachate samples were deemed hydrophobic and aromatic. Approximately 75% of the fluorescence signature of LOM was on average comprised of humic/fulvic-like peaks (Peaks A and C) with the remaining attributed to marine-like and protein-like humic matter. A majority (50%) of the Nuclear Magnetic Resonance chemical shifts were characteristic of carboxyl-rich alicyclic molecules (CRAM; 2.0-3.24 ppm) and material derived from linear terpenoids (0.751.49 ppm). CRAM are hydrophobic and refractory and could contribute to the recalcitrant nature of LOM. BOD/COD, total UV254, dissolved organic carbon were positively correlated with the spectral ratio (a higher SR reflects lower molecular weight dissolved organic matter). Results from this study can be used to better understand the humification of landfilled waste, with LOM as an indicator, and elucidate the fate of LOM during biological treatment and upon discharge to aquatic systems (e.g., sunlight driven photolysis).
1. INTRODUCTION Organic components of landfilled waste undergo changes over time mediated by physical, chemical, and biological processes. Due to these changing conditions within the landfill, leachate characteristics will vary dramatically over time. In-situ processes remove readily degradable organic matter (OM) in the leachate; however, persistent and recalcitrant OM may necessitate management of leachate well beyond the closure of the landfill. Leachate organic matter (LOM) is problematic because it is highly colored, may interfere with UV disinfection or Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
lead to disinfection byproducts, is resistant to conventional biological treatment, and is known to transport heavy metals and hydrophobic organic contaminants. LOM transitions from dominance by aliphatic, low molecular weight (MW) compounds to primarily complex and recalcitrant organic matter. The leachate biochemical oxygen demand (BOD) to chemical oxygen demand (COD) ratio becomes very low as the landfill ages, suggesting LOM recalcitrance. The humification of OM is often used as an indicator of the extent of waste stabilization in landfills, compost, and wastewater sludge. Traditional metrics of solid waste stability include leachate BOD/COD and waste volatile solids, biochemical methane potential, and carbon to nitrogen ratio; however, these tend to be nonspecific and time consuming. The utilization of advanced analytical techniques has increased in an attempt to more specifically characterize OM, including UV-Vis absorption, solid-state 13C nuclear magnetic resonance (NMR), 1H NMR, and Fourier transform infrared spectroscopy. Given the advances in analytical approaches and the strength of coupling these techniques, a more complete picture of the changes in LOM can be captured. In general, OM characteristics are dependent on the source (i.e., terrestrial vs. aquatic). In addition, it is expected that LOM characteristics will vary for each landfill due to the waste heterogeneity, climate, landfill operation, and type of leachate treatment (where applicable). This research examined the effects of landfill operation and treatment on the chemical nature of LOM. Results from this study can be used to better understand the humification of landfilled waste, with LOM as an indicator, and elucidate the fate of LOM during biological treatment and upon discharge to aquatic systems (e.g., sunlight driven photolysis).
2. METHODOLOGY Twenty-three leachate samples were collected from 13 landfills in Florida, California, and Minnesota after in situ landfill treatment by leachate recirculation (two sites), prior to cotreatment at a WWTP (ten sites), and before and after on-site biological treatment (two sites). The leachate organic strength was also determined using more traditional metrics. Samples were analyzed for BOD5, COD, dissolved organic carbon (DOC), total Kjeldahl nitrogen, ammonia-nitrogen, nitrite, and nitrate according to Standard Methods. Organic nitrogen concentrations were determined by the difference between the dissolved Kjeldahl nitrogen and ammonia-N in samples filtered through a 0.45-µm filter. The collected samples were filtered (0.20-µm) prior to LOM solid phase extraction (Dittmar et al., (2003)). The fractionated LOM was characterized using advanced spectroscopic tools to determine the structural and biochemical properties. LOM samples were also characterized using 1H NMR (chemical structures), UV– Visible Spectroscopy (200 nm-600 nm; correlate aromaticity and MW), size-exclusion chromatography (SEC; apparent MW), and excitation-emission matrix analysis fluorescence spectroscopy (EEMs; humic/fulvic acid and protein fluorescence signatures).
3. RESULTS AND DISCUSSION Leachate samples were characterized and the average, minimum, maximum, and standard deviation of the results are presented in Table 1. Preliminary results yielded a wide variation of values for the leachate parameters. Some of the observed differences can be attributed to landfill operation (e.g., recirculation, slurry wall, or conventional), age of the landfill, waste heterogeneity, and leachate treatment, where applicable. No significant differences were found that could be attributed to temperature or precipitation.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 1. Summary of Leachate Characteristics
Average
Min
Max
Standard
1030
30.8
4750
1460
4080 0.147
550 0.0239
14100 0.337
3540
7.92
6.92
9.59
934 49.6
16.4 3.1
2300 252
Total TKN (mg/L) Total Nitrogen (mg/L)
1040
19.1
2440
58.3 820
1090
22.2
2500
818
DON (mg/L)
63.8 1280
2.66 110
226 4420
63.9
26.3
0.227
88.4
3.44 43.1
1.39 0.662
4.92 189
4.67
2.30
6.40
45.8 1.15
46.9
2.83
191
45.1
Total cBOD5 (mg/L) Total COD (mg/L) cBOD5/COD pH (S.U.) Total NH3-N (mg/L) Total NOx (mg/L)
DOC (mg/L) DOC/DON SUVA (L/mg-m) Dissolved UV254 E4/E6 Total UV254
0.0949 0.0618 823
1250 24.4 1.17
Specific UV absorbance (SUVA) is an indicator of the hydrophobic or hydrophilic nature of OM and is a proxy for aromaticity. A higher value represents greater aromaticity while values lower than 2 L mg/cm-m reflect hydrophilic OM. The leachate collected from California was the only sample that had a SUVA less than 2 L/mg-m. The remaining samples were deemed hydrophobic and aromatic. Biological treatment appeared to increase SUVA. Approximately 75% of the fluorescence signature of LOM was on average comprised of humic/fulvic-like peaks (Peaks A and C) with the remaining attributed to marine-like and protein-like humic matter (Figure 1). Biological leachate treatment decreased the overall fluorescence of leachate samples with the loss primarily attributed to the protein peak (T) with no significant changes in the three other characteristics peaks (A, C, and M) due to their recalcitrance.
Figure 1. Average Leachate Fluorescence Signatures
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 2. Relative Fluorescence Peak Area of Raw and Treated Leachate
SEC yielded significant variations in apparent MW distributions among leachate samples. These distributions were either unimodal or multimodal. The dominant peaks were observed between 15,700-29,800 apparent MW. Some leachate samples contained a late eluting peak representing low MW material. The majority of the samples containing this peak were collected during fall, spring, and summer months. One sample that contained this late eluting peak was collected during the winter months (i.e., January), but due to the mild ambient temperatures in Florida, we would not expect to see climate affects during this season. The annual amount of waste received, rainfall, or average temperature did not appear to affect the MW distribution. Biological treatment of leachate did result in a decrease in lower MW materials (7,070-15,700 MW) causing a shift in the peak area towards higher MW for leachate collected in central Florida. The treated leachate collected from south Florida did not have as much of a decrease in the low MW material. Spectral slopes (S275-295 and S350-400) and the spectral slope ratio (SR = S275-295/S350-400) used to characterize terrestrial and marine dissolved organic matter (DOM) (Helms et al., 2008) were applied to LOM. A higher SR reflects lower MW DOM and was used to compare LOM with characteristics of high chromophoric DOM terrestrial (SR ≈ 0.7) and autochthonous (microbial derived) estuary/marine DOM (≥ 1). A significant number of samples resembled autochthonous estuary/marine DOM. There were no significant differences among the average SR during fall, winter, spring, or summer months. The lowest SR (1.0) was observed in winter and increased slightly during summer (1.10). S275-295 and SR were found to be positively correlated but the overall correlation was not strong (R2 of 0.76; Figure 3).
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 3. Leachate Spectral Slope (S275-295) versus Spectral Ratio Biological treatment resulted in a decrease in SR and S275-295 corresponding to an increase in apparent molecular size, which agrees with the SEC distribution changes. 1H NMR provided insight on the chemical structure of the LOM. A majority (50%) of the resonance area (chemical shifts) were characteristic of carboxyl-rich alicyclic molecules (CRAM; 2.0-3.24 ppm) and material derived from linear terpenoids (0.75-1.49 ppm). CRAM and terpenoids have been cited to account for up to 75% of DOM (Simpson et al., 2011). CRAM are hydrophobic and refractory (Hertkorn et al., 2006) and could contribute to the recalcitrant nature of LOM. NMR spectra will be analyzed further to identify additional chemical structures. Correlations with traditional biochemical metrics and spectroscopic characteristics were explored. BOD/COD, total UV254, DOC were positively correlated with the SR (Figures 4-6). The BOD/COD trend suggests that as leachate biodegradability decreases there is an increase in MW and aromaticity of LOM likely due to an increase in the degree of humification. From total UV254 measurements it was observed that as the overall absorbance of the leachate decreases there is an increase in the MW of the LOM when correlated to the SR.
Figure 4. Leachate BOD/COD versus Spectral Ratio
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 5. Leachate DOC versus Spectral Ratio
Figure 6. Leachate Total UV254 versus Spectral Ratio 4. CONCLUSIONS Results support that most of the leachate samples were deemed hydrophobic and aromatic. The recalcitrance nature can be attributed to humic/fulvic-like organic matter. Looking further at the chemical structure of the organic matter a majority of functional groups are carboxyl-rich alicyclic molecules and material derived from linear terpenoids which are known to be hydrophobic and recalcitrant. Biological treatment was successful at reducing protein-like organic matter and increasing the apparent MW matter but the recalcitrant organic matter was resistant to degradation. Results suggest that UV has the potential to be a quick and easy method to look at changes in MW, hydrophobicity, and aromaticity to understand the changes in LOM characteristics. Results from this study can be used to better understand the humification of landfilled waste (e.g., decrease in SR as waste stabilizes), with LOM as an indicator, and elucidate the fate of LOM during biological treatment and upon discharge to aquatic systems (e.g., sunlight driven photolysis; potential changes LOM can undergo based on function groups).
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
ACKNOWLEDGEMENTS We would like to acknowledge the landfill operators who helped us collect leachate samples throughout Florida, Minnesota, and California. This material is based upon work supported by the National Science Foundation (#1311037) and Environmental Research and Education Foundation (graduate scholarship).
REFERENCES Dittmar, T.; Koch, B.; Hertkorn, N.; Kattner, G., A simple and efficient method for the solid-phase extraction of dissolved organic matter (SPE-DOM) from seawater. Limnology and Oceanography: Methods 2008, 6, 230-235. Helms, J. R.; Stubbins, A.; Ritchie, J. D.; Minor, E. C.; Kieber, D. J.; Mopper, K., Absorption Spectral Slopes and Slope Ratios as Indicators of Molecular Weight, Source, and Photobleaching of Chromophoric Dissolved Organic Matter. 2008, p 955. Hertkorn, N., Benner, R., Frommberger, M., Schmitt-Kopplin, P., Witt, M., Kaiser, K., Hedges, J. I. (2006). Characterization of a major refractory component of marine dissolved organic matter. Geochimica et Cosmochimica Acta, 70(12), 2990-3010. Simpson, A. J., McNally, D. J., & Simpson, M. J. (2011). NMR spectroscopy in environmental research: From molecular interactions to global processes. Progress in Nuclear Magnetic Resonance Spectroscopy, 58(3), 97-175.
SUMMARY: Leachate organic matter (LOM) is problematic because it is highly colored, may interfere with UV disinfection or lead to disinfection byproducts, is resistant to conventional biological treatment, and is known to transport heavy metals and hydrophobic organic contaminants. This research examined the effects of landfill operation and treatment on the chemical nature of LOM using traditional biochemical and advanced spectroscopic techniques. Twenty-three leachate samples were collected in Florida, Minnesota, and California. Most of the leachate samples were deemed hydrophobic and aromatic. Approximately 75% of the fluorescence signature of LOM was on average comprised of humic/fulvic-like peaks (Peaks A and C) with the remaining attributed to marine-like and protein-like humic matter. A majority (50%) of the Nuclear Magnetic Resonance chemical shifts were characteristic of carboxyl-rich alicyclic molecules (CRAM; 2.0-3.24 ppm) and material derived from linear terpenoids (0.751.49 ppm). CRAM are hydrophobic and refractory and could contribute to the recalcitrant nature of LOM. BOD/COD, total UV254, dissolved organic carbon were positively correlated with the spectral ratio (a higher SR reflects lower molecular weight dissolved organic matter). Results from this study can be used to better understand the humification of landfilled waste, with LOM as an indicator, and elucidate the fate of LOM during biological treatment and upon discharge to aquatic systems (e.g., sunlight driven photolysis).
1. INTRODUCTION Organic components of landfilled waste undergo changes over time mediated by physical, chemical, and biological processes. Due to these changing conditions within the landfill, leachate characteristics will vary dramatically over time. In-situ processes remove readily degradable organic matter (OM) in the leachate; however, persistent and recalcitrant OM may necessitate management of leachate well beyond the closure of the landfill. Leachate organic matter (LOM) is problematic because it is highly colored, may interfere with UV disinfection or Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
lead to disinfection byproducts, is resistant to conventional biological treatment, and is known to transport heavy metals and hydrophobic organic contaminants. LOM transitions from dominance by aliphatic, low molecular weight (MW) compounds to primarily complex and recalcitrant organic matter. The leachate biochemical oxygen demand (BOD) to chemical oxygen demand (COD) ratio becomes very low as the landfill ages, suggesting LOM recalcitrance. The humification of OM is often used as an indicator of the extent of waste stabilization in landfills, compost, and wastewater sludge. Traditional metrics of solid waste stability include leachate BOD/COD and waste volatile solids, biochemical methane potential, and carbon to nitrogen ratio; however, these tend to be nonspecific and time consuming. The utilization of advanced analytical techniques has increased in an attempt to more specifically characterize OM, including UV-Vis absorption, solid-state 13C nuclear magnetic resonance (NMR), 1H NMR, and Fourier transform infrared spectroscopy. Given the advances in analytical approaches and the strength of coupling these techniques, a more complete picture of the changes in LOM can be captured. In general, OM characteristics are dependent on the source (i.e., terrestrial vs. aquatic). In addition, it is expected that LOM characteristics will vary for each landfill due to the waste heterogeneity, climate, landfill operation, and type of leachate treatment (where applicable). This research examined the effects of landfill operation and treatment on the chemical nature of LOM. Results from this study can be used to better understand the humification of landfilled waste, with LOM as an indicator, and elucidate the fate of LOM during biological treatment and upon discharge to aquatic systems (e.g., sunlight driven photolysis).
2. METHODOLOGY Twenty-three leachate samples were collected from 13 landfills in Florida, California, and Minnesota after in situ landfill treatment by leachate recirculation (two sites), prior to cotreatment at a WWTP (ten sites), and before and after on-site biological treatment (two sites). The leachate organic strength was also determined using more traditional metrics. Samples were analyzed for BOD5, COD, dissolved organic carbon (DOC), total Kjeldahl nitrogen, ammonia-nitrogen, nitrite, and nitrate according to Standard Methods. Organic nitrogen concentrations were determined by the difference between the dissolved Kjeldahl nitrogen and ammonia-N in samples filtered through a 0.45-µm filter. The collected samples were filtered (0.20-µm) prior to LOM solid phase extraction (Dittmar et al., (2003)). The fractionated LOM was characterized using advanced spectroscopic tools to determine the structural and biochemical properties. LOM samples were also characterized using 1H NMR (chemical structures), UV– Visible Spectroscopy (200 nm-600 nm; correlate aromaticity and MW), size-exclusion chromatography (SEC; apparent MW), and excitation-emission matrix analysis fluorescence spectroscopy (EEMs; humic/fulvic acid and protein fluorescence signatures).
3. RESULTS AND DISCUSSION Leachate samples were characterized and the average, minimum, maximum, and standard deviation of the results are presented in Table 1. Preliminary results yielded a wide variation of values for the leachate parameters. Some of the observed differences can be attributed to landfill operation (e.g., recirculation, slurry wall, or conventional), age of the landfill, waste heterogeneity, and leachate treatment, where applicable. No significant differences were found that could be attributed to temperature or precipitation.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 1. Summary of Leachate Characteristics
Average
Min
Max
Standard
1030
30.8
4750
1460
4080 0.147
550 0.0239
14100 0.337
3540
7.92
6.92
9.59
934 49.6
16.4 3.1
2300 252
Total TKN (mg/L) Total Nitrogen (mg/L)
1040
19.1
2440
58.3 820
1090
22.2
2500
818
DON (mg/L)
63.8 1280
2.66 110
226 4420
63.9
26.3
0.227
88.4
3.44 43.1
1.39 0.662
4.92 189
4.67
2.30
6.40
45.8 1.15
46.9
2.83
191
45.1
Total cBOD5 (mg/L) Total COD (mg/L) cBOD5/COD pH (S.U.) Total NH3-N (mg/L) Total NOx (mg/L)
DOC (mg/L) DOC/DON SUVA (L/mg-m) Dissolved UV254 E4/E6 Total UV254
0.0949 0.0618 823
1250 24.4 1.17
Specific UV absorbance (SUVA) is an indicator of the hydrophobic or hydrophilic nature of OM and is a proxy for aromaticity. A higher value represents greater aromaticity while values lower than 2 L mg/cm-m reflect hydrophilic OM. The leachate collected from California was the only sample that had a SUVA less than 2 L/mg-m. The remaining samples were deemed hydrophobic and aromatic. Biological treatment appeared to increase SUVA. Approximately 75% of the fluorescence signature of LOM was on average comprised of humic/fulvic-like peaks (Peaks A and C) with the remaining attributed to marine-like and protein-like humic matter (Figure 1). Biological leachate treatment decreased the overall fluorescence of leachate samples with the loss primarily attributed to the protein peak (T) with no significant changes in the three other characteristics peaks (A, C, and M) due to their recalcitrance.
Figure 1. Average Leachate Fluorescence Signatures
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 2. Relative Fluorescence Peak Area of Raw and Treated Leachate
SEC yielded significant variations in apparent MW distributions among leachate samples. These distributions were either unimodal or multimodal. The dominant peaks were observed between 15,700-29,800 apparent MW. Some leachate samples contained a late eluting peak representing low MW material. The majority of the samples containing this peak were collected during fall, spring, and summer months. One sample that contained this late eluting peak was collected during the winter months (i.e., January), but due to the mild ambient temperatures in Florida, we would not expect to see climate affects during this season. The annual amount of waste received, rainfall, or average temperature did not appear to affect the MW distribution. Biological treatment of leachate did result in a decrease in lower MW materials (7,070-15,700 MW) causing a shift in the peak area towards higher MW for leachate collected in central Florida. The treated leachate collected from south Florida did not have as much of a decrease in the low MW material. Spectral slopes (S275-295 and S350-400) and the spectral slope ratio (SR = S275-295/S350-400) used to characterize terrestrial and marine dissolved organic matter (DOM) (Helms et al., 2008) were applied to LOM. A higher SR reflects lower MW DOM and was used to compare LOM with characteristics of high chromophoric DOM terrestrial (SR ≈ 0.7) and autochthonous (microbial derived) estuary/marine DOM (≥ 1). A significant number of samples resembled autochthonous estuary/marine DOM. There were no significant differences among the average SR during fall, winter, spring, or summer months. The lowest SR (1.0) was observed in winter and increased slightly during summer (1.10). S275-295 and SR were found to be positively correlated but the overall correlation was not strong (R2 of 0.76; Figure 3).
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 3. Leachate Spectral Slope (S275-295) versus Spectral Ratio Biological treatment resulted in a decrease in SR and S275-295 corresponding to an increase in apparent molecular size, which agrees with the SEC distribution changes. 1H NMR provided insight on the chemical structure of the LOM. A majority (50%) of the resonance area (chemical shifts) were characteristic of carboxyl-rich alicyclic molecules (CRAM; 2.0-3.24 ppm) and material derived from linear terpenoids (0.75-1.49 ppm). CRAM and terpenoids have been cited to account for up to 75% of DOM (Simpson et al., 2011). CRAM are hydrophobic and refractory (Hertkorn et al., 2006) and could contribute to the recalcitrant nature of LOM. NMR spectra will be analyzed further to identify additional chemical structures. Correlations with traditional biochemical metrics and spectroscopic characteristics were explored. BOD/COD, total UV254, DOC were positively correlated with the SR (Figures 4-6). The BOD/COD trend suggests that as leachate biodegradability decreases there is an increase in MW and aromaticity of LOM likely due to an increase in the degree of humification. From total UV254 measurements it was observed that as the overall absorbance of the leachate decreases there is an increase in the MW of the LOM when correlated to the SR.
Figure 4. Leachate BOD/COD versus Spectral Ratio
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 5. Leachate DOC versus Spectral Ratio
Figure 6. Leachate Total UV254 versus Spectral Ratio 4. CONCLUSIONS Results support that most of the leachate samples were deemed hydrophobic and aromatic. The recalcitrance nature can be attributed to humic/fulvic-like organic matter. Looking further at the chemical structure of the organic matter a majority of functional groups are carboxyl-rich alicyclic molecules and material derived from linear terpenoids which are known to be hydrophobic and recalcitrant. Biological treatment was successful at reducing protein-like organic matter and increasing the apparent MW matter but the recalcitrant organic matter was resistant to degradation. Results suggest that UV has the potential to be a quick and easy method to look at changes in MW, hydrophobicity, and aromaticity to understand the changes in LOM characteristics. Results from this study can be used to better understand the humification of landfilled waste (e.g., decrease in SR as waste stabilizes), with LOM as an indicator, and elucidate the fate of LOM during biological treatment and upon discharge to aquatic systems (e.g., sunlight driven photolysis; potential changes LOM can undergo based on function groups).
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
ACKNOWLEDGEMENTS We would like to acknowledge the landfill operators who helped us collect leachate samples throughout Florida, Minnesota, and California. This material is based upon work supported by the National Science Foundation (#1311037) and Environmental Research and Education Foundation (graduate scholarship).
REFERENCES Dittmar, T.; Koch, B.; Hertkorn, N.; Kattner, G., A simple and efficient method for the solid-phase extraction of dissolved organic matter (SPE-DOM) from seawater. Limnology and Oceanography: Methods 2008, 6, 230-235. Helms, J. R.; Stubbins, A.; Ritchie, J. D.; Minor, E. C.; Kieber, D. J.; Mopper, K., Absorption Spectral Slopes and Slope Ratios as Indicators of Molecular Weight, Source, and Photobleaching of Chromophoric Dissolved Organic Matter. 2008, p 955. Hertkorn, N., Benner, R., Frommberger, M., Schmitt-Kopplin, P., Witt, M., Kaiser, K., Hedges, J. I. (2006). Characterization of a major refractory component of marine dissolved organic matter. Geochimica et Cosmochimica Acta, 70(12), 2990-3010. Simpson, A. J., McNally, D. J., & Simpson, M. J. (2011). NMR spectroscopy in environmental research: From molecular interactions to global processes. Progress in Nuclear Magnetic Resonance Spectroscopy, 58(3), 97-175.
