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Environmental Measurements Methods
Simultaneous quantification of electron transfer by carbon matrices and functional groups in pyrogenic carbon Tianran Sun, Barnaby D. A. Levin, Michael P. Schmidt, Juan J.L. Guzman, Akio Enders, Carmen Enid Martinez, David A. Muller, Largus T. Angenent, and Johannes Lehmann Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02340 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 4, 2018
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Simultaneous quantification of electron transfer by carbon matrices and functional groups
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in pyrogenic carbon
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Tianran Sun1,2†*, Barnaby D.A. Levin3†, Michael P. Schmidt1, Juan J.L. Guzman4, Akio Enders1,
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Carmen Enid Martínez1, David A. Muller3,5, Largus T. Angenent2,4,6, Johannes Lehmann1,6
1
Soil and Crop Sciences, School of Integrative Plant Sciences, College of Agriculture and Life
Sciences, Cornell University, Ithaca NY, 14853, USA 2
Center for Applied Geosciences, University of Tübingen, Tübingen, 72074, Germany
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School of Applied and Engineering Physics, College of Engineering, Cornell University, Ithaca
NY, 14853, USA 4
Department of Biological and Environmental Engineering, College of Agriculture and Life
Sciences, Cornell University, Ithaca NY, 14853, USA 5
Kavli Institute for Nanoscale Science, Cornell University, Ithaca NY, 14853, USA
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Atkinson Center for a Sustainable Future, Cornell University, Ithaca, NY 14583, USA
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†
These authors contributed equally to this work.
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* Corresponding author; Email:
[email protected] 1 ACS Paragon Plus Environment
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Abstract
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Pyrogenic carbon contains redox-active functional groups and polyaromatic carbon matrices that
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are both capable of transferring electrons. Several techniques have been explored to characterize
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the individual electron transfer process of either functional groups or carbon matrices
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individually. However, simultaneous analysis of both processes remains challenging. Using an
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approach that employs a four-electrode configuration and dual-interface electron transfer
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detection, we distinguished the electron transfer by functional groups from the electron transfer
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by carbon matrices and simultaneously quantified their relative contribution to the total electron
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transfer to and from pyrogenic carbon. Results show that at low to intermediate pyrolysis
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temperatures (400-500°C), redox cycling of functional groups is the major mechanism with a
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contribution of 100-78% to the total electron transfer; whereas at high temperatures (650-800°C),
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direct electron transfer of carbon matrices dominates electron transfer with a contribution of 87-
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100%. Spectroscopic and diffraction analyses of pyrogenic carbon support the electrochemical
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measurements by showing a molecular-level structural transition from an enrichment in
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functional groups to an enrichment in nano-sized graphene domains with increasing pyrolysis
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temperatures. The method described in this study provides a new analytical approach to
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separately quantify the relative importance of different electron transfer pathways in natural
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pyrogenic carbon and has potential applications for engineered carbon materials such as
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graphene oxides.
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TOC Art Dual-interface electron transfer
Simultaneous quantification
Red Functional groups Ox PyC thin sheets
Contribution
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Pt Red
Carbon matrix
2 Ox
Pyrolysis temperature
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Introduction
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Pyrogenic carbon is the solid carbonaceous residue from natural and anthropogenic forest and
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grassland fires and is known to constitute a ubiquitous component of natural organic matter1,2.
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While pyrogenic carbon can represent more than 30% of total organic carbon in topsoils3,4,
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organic matter is also enriched with pyrogenic carbon in subsoils5, aquatic systems6,7, and
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sediments8. Recent studies have indicated that pyrogenic carbon is capable of transferring
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electrons and has an electron transfer capacity (hundred µmol of electrons per gram of solid
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depending on the original biomass type and pyrogenic temperatures) that is comparable to those
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reported for non-pyrogenic organic matter9-12. Reported biotic and abiotic reactions that invoke
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the electron transfer property of pyrogenic carbon include eliciting reductions in nitrous oxide
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emission and total denitrification13,14, mediating iron mineral reduction15,16, and enhancing
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organic contaminant transformation17-19.
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Surface functional groups and condensed polyaromatic carbon matrices are major components
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that are responsible for the electron transfer in pyrogenic carbon. The reversible redox cycling of
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mainly quinone and hydroquinone pairs was shown to dominate the electron accepting and
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donating behaviors of pyrogenic carbon produced within a low to intermediate pyrolysis
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temperature range9. With an increase in pyrolysis temperatures, the contribution of functional
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groups declines as a result of lower electron transfer capacities. The estimated content of quinoid
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moieties ranged from 3.1 to 1.2 mmol g-1 char from low to high pyrolysis temperatures9.
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Meanwhile, the direct electron transfer through carbon matrices (i.e., electrons transferred by the
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conductance of carbon matrices to the surrounding acceptors in solution) becomes dominant at
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higher pyrolysis temperatures due to an increased electrical conductivity20 and highly refined
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carbon surfaces that facilitate rapid electrochemical reactions21.
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Several techniques have been utilized to characterize the electron transfer of pyrogenic carbon.
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For example, for pyrogenic carbon mediated microbial Fe(III)15,16 and NO3- reduction22, the
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production rate and average concentrations of Fe(II) and NH4+ were attributed to both electron
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transfer kinetics and capacities of functional groups in pyrogenic carbon. However, in such
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assessments the redox cycling of functional groups are highly dependent on the overall chemistry
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(e.g., pH and reduction potential Eh) of the specific reduction experiment. So-called mediated
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electrochemical analysis, in which electron donation and accepting capacities of functional
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groups were quantified by the charge exchange between pyrogenic carbon and redox-active
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mediators during chronoamperometric9 and
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quantification of the redox properties of functional groups. In comparison, measurements of
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direct electron transfer by carbon matrices was possible by cyclic voltammetry that utilized
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pyrogenic carbon as a working electrode21. Potential scanning of pyrogenic carbon can then
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capture the electron flow from redox reactions and thus gives rise to Faradaic current peaks.
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Based on the height of peak current and the corresponding applied potential, the electron transfer
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kinetics of pyrogenic carbon matrices can be determined.
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Although widely used, the above methods can only focus on an individual electron transfer
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pathway of either functional groups or carbon matrices. None of these methods are able to
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distinguish the electron transfer of both pathways simultaneously. Knowing the different electron
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transfer behaviors of carbon matrices and functional groups is important because it facilitates a
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direct comparison between these two components and allows for consistent monitoring of
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changes caused by increased pyrolysis temperature or natural degradation. Therefore, the overall
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goal of this study was to distinguish the electron transfer behavior of carbon matrices and
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functional groups and to quantify their relative contributions to the total electron transfer to and
voltammetric scans23, allowed improved
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from pyrogenic carbon. We adapted a four-electrode electrochemical system from scanning
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electrochemical microscopy techniques24 and employed a novel operational design using two
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differently polarized working electrodes. We hypothesized that the differential electron behavior
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of either matrices or functional groups are reflected in the carbon structure and surface
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functionality of pyrogenic carbon.
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Materials and Methods
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Chemicals and Materials. All chemicals were used as received unless otherwise noted.
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(Dimethylaminomethyl)ferrocene (FcDMAM, 96%), diquat dibromide monohydrate (DQ,
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99.5%), 1,4-hydroquinone (HQ, min. 99%), 1,4-benzoquinone (BQ, 97%), 1-methyl-2-
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pyrrolidinone (NMP, min. 99%), Nafion perfluorinated resin solution (5 wt. %), and graphite
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powder (
