TY - JOUR
T1 - Electrochemical characterization of natural organic matter by direct voltammetry in an aprotic solvent
AU - Pavitt, Ania S.
AU - Tratnyek, Paul G.
N1 - Funding Information:
The study was initiated for a presentation by P. Tratnyek for a symposium in honor of George Aiken at the 253rd ACS National Meeting in San Francisco, CA, 2–6 April 2017. Initially, a goal of the study was to compare results obtained on samples of NOM acquired over several decades from George Aiken, Donald Macalady, Yo-Ping Chin, and Baohua Gu. The scope was expanded to include new NOM samples from Paul Bloom (IHSS), Joel Coates, Gordon Getzinger, Andreas Kappler, Boris Koch, Joseph Needoba, and Brett Poulin. We thank all of these colleagues for their contributions to this work. Some preliminary electrochemical experiments were performed by undergraduates Eric Sauer, Nancy Nguyen, and Nick Slenning. Nguyen and Slenning were funded by a Murdock Trust grant to Paige Osberg Hall (Univ. of Portland). This material is based on the work supported by the National Science Foundation, Environmental Chemical Sciences Program under Grant # 1506744.
Publisher Copyright:
© 2019 The Royal Society of Chemistry.
PY - 2019/10
Y1 - 2019/10
N2 - The complex and indeterminant composition of NOM makes characterization of its redox properties challenging. Approaches that have been taken to address this challenge include chemical probe reactions, potentiometric titrations, chronocoulometry, and voltammetry. In this study, we revisit the use of direct voltammetric methods in aprotic solvents by applying an expanded and refined suite of methods to a large set of NOM samples and model compounds (54 NOM samples from 10 different sources, 7 NOM model compounds, and 2 fresh extracts of plant materials that are high in redox-active quinonoid model compounds dissolved in DMSO). Refinements in the methods of fitting the data obtained by staircase cyclic voltammetry (SCV) provided improved definition of peaks, and square wave voltammetry (SWV), performed under the same conditions as SCV, provided even more reliable identification and quantitation of peaks. Further evidence is provided that DMSO improves the electrode response by unfolding some of the tertiary structure of NOM polymers, thereby allowing greater contact between redox active functional groups and the electrode surface. We averaged experimental peak potentials for all NOM compounds and calculated potentials in water. Average values for Epa1, Epc1, and Ep1 in DMSO were -0.866 ± 0.069, -1.35 ± 0.071, and -0.831 ± 0.051 V vs. Ag/Ag+, and -0.128, -0.613, and -0.0930 V vs. SHE in water. In addition to peak potentials, the breadth of SCV peaks was quantified as a way to characterize the degree to which the redox activity of NOM is due to a continuum of contributing functional groups. The average breadth values were 1.63 ± 0.24, 1.28 ± 0.34, and 0.648 ± 0.15 V for Epa1, Epc1, and Ep1 respectively. Comparative analysis of the overall dataset - from SCV and SWV on all NOMs and model compounds - revealed that NOM redox properties vary over a narrower range than expected based on model compound properties. This lack of diversity in redox properties of NOM is similar to conclusions from other recent work on the molecular structure of NOM, all of which could be the result of selectivity in the common extraction methods used to obtain the materials.
AB - The complex and indeterminant composition of NOM makes characterization of its redox properties challenging. Approaches that have been taken to address this challenge include chemical probe reactions, potentiometric titrations, chronocoulometry, and voltammetry. In this study, we revisit the use of direct voltammetric methods in aprotic solvents by applying an expanded and refined suite of methods to a large set of NOM samples and model compounds (54 NOM samples from 10 different sources, 7 NOM model compounds, and 2 fresh extracts of plant materials that are high in redox-active quinonoid model compounds dissolved in DMSO). Refinements in the methods of fitting the data obtained by staircase cyclic voltammetry (SCV) provided improved definition of peaks, and square wave voltammetry (SWV), performed under the same conditions as SCV, provided even more reliable identification and quantitation of peaks. Further evidence is provided that DMSO improves the electrode response by unfolding some of the tertiary structure of NOM polymers, thereby allowing greater contact between redox active functional groups and the electrode surface. We averaged experimental peak potentials for all NOM compounds and calculated potentials in water. Average values for Epa1, Epc1, and Ep1 in DMSO were -0.866 ± 0.069, -1.35 ± 0.071, and -0.831 ± 0.051 V vs. Ag/Ag+, and -0.128, -0.613, and -0.0930 V vs. SHE in water. In addition to peak potentials, the breadth of SCV peaks was quantified as a way to characterize the degree to which the redox activity of NOM is due to a continuum of contributing functional groups. The average breadth values were 1.63 ± 0.24, 1.28 ± 0.34, and 0.648 ± 0.15 V for Epa1, Epc1, and Ep1 respectively. Comparative analysis of the overall dataset - from SCV and SWV on all NOMs and model compounds - revealed that NOM redox properties vary over a narrower range than expected based on model compound properties. This lack of diversity in redox properties of NOM is similar to conclusions from other recent work on the molecular structure of NOM, all of which could be the result of selectivity in the common extraction methods used to obtain the materials.
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U2 - 10.1039/c9em00313d
DO - 10.1039/c9em00313d
M3 - Article
C2 - 31576393
AN - SCOPUS:85073489218
VL - 21
SP - 1664
EP - 1683
JO - Journal of Environmental Monitoring
JF - Journal of Environmental Monitoring
SN - 2050-7887
IS - 10
ER -