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Wood-Derived Black Carbon (Biochar) as a Microbial Electron Donor and Acceptor Jovita Saquing, YUHAN YU, and Pei Chiu Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.5b00354 • Publication Date (Web): 05 Jan 2016 Downloaded from http://pubs.acs.org on January 7, 2016
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Environmental Science & Technology Letters
Wood-Derived Black Carbon (Biochar) as a Microbial Electron Donor and Acceptor
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Jovita M. Saquing1, Yu-Han Yu1,2 and Pei C. Chiu1*
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Department of Civil and Environmental Engineering, University of Delaware, Newark, DE 19716, USA
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Department of Environmental Engineering, National Chung-Hsing University, Taichung 40227, Taiwan
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Corresponding Author Email:
[email protected] 7 8 9 10
Phone: +1 (302) 831-3104 Fax: +1 (302) 831-3640
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Manuscript contains 3020 words = 2620 (text) + 200 × 2 (figures)
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Abstract
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Research on the environmental impacts of black carbon has focused largely on sorption. Besides
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being a strong geosorbent, black carbon is redox-active and may facilitate abiotic and microbial
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transformation. Using a wood-derived black carbon (biochar) and the bacterium Geobacter
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metallireducens (GS-15), we showed that air-oxidized biochar served as an electron acceptor to
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enable acetate oxidation, and that chemically or biotically reduced biochar served as an electron
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donor for nitrate reduction. The bioavailable (to GS-15) electron storage capacity (ESC) of the
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biochar, estimated based on acetate oxidation and nitrate reduction, was 0.85 and 0.87 mmol e–/g,
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respectively, comparable to the ESCs of humic substances and other biochars measured
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electrochemically. We propose that black carbon should be regarded as a rechargeable reservoir
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of bioavailable electrons in anaerobic environments. The redox cycling of biochar in natural and
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engineered systems and its impact on microbial processes and contaminant fate merit further
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investigations.
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Introduction
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Black carbon and natural organic matter are the two major types of carbonaceous geosorbents
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that control the fate and transport of hydrophobic contaminants in the environment.1-3 Until
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recently, research on the impact of black carbon on contaminant fate has focused on adsorption,
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where black carbon was considered to be chemically inert toward adsorbates. This assumption is
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common in sediment remediation and risk assessment.4-6 However, studies in recent years have
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shown that black carbon, such as soot, activated carbon, graphite, char, carbon nanotubes, and
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graphene oxide, is not merely a passive adsorbent but a catalytically active material that can
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mediate the abiotic reduction of nitrogenous compounds.7-10 While the mechanisms are not fully
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understood, the catalytic ability of black carbon appears to stem from its graphitic structure
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and/or redox-active functions such as (hydro)quinones. First, through adsorption of reactants and
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conduction of electrons and hydrogen atoms, high-purity graphite can mediate the reduction of
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nitro and azo aromatic compounds, heterocyclic nitramines, and nitrate esters, even when these
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compounds are physically separated from the reductant.9,11,12 Second, similar to natural organic
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matter, black carbon that contains quinone groups may undergo reversible redox reactions, which
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explains the catalytic ability of less graphitic (more functionalized) black carbon such as soot and
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biochar.7,8,10 Indeed, the capacity of biochar to reversibly donate and accept electrons has been
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quantified by Klüpfel et al. in an electrochemical cell13 and was attributed to the redox-facile
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quinone groups in biochar. Depending on the source biomass and the pyrolysis temperature, the
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electron storage capacity (ESC) of plant-based biochar was up to 2 mmol e–/g,13 on a par with
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those of dissolved and particulate organic matter.14,15 Note that, in contrast to the first mechanism,
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which involves transfer of electrons through conductive (i.e., graphene) domains of black carbon,
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the second mechanism involves storage of electrons in biochar through charging and discharging
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(i.e., reduction and oxidation) of the quinone functions.
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In addition to being a geosorbent and mediator of abiotic redox reactions,14,16 natural organic
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matter such as humic acid is also an important electron acceptor17,18 and donor19 for anaerobic
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microbes. This suggests black carbon that has a significant quinone content (and thus ESC) may
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similarly support microbial transformation by serving as both an electron donor and acceptor. In
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this study, we tested this hypothesis using a wood-based biochar and the organism Geobacter
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metallireducens. We showed the biochar can be an electron acceptor to enable acetate oxidation,
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and an electron donor to support nitrate reduction to ammonium, by G. metallireducens. We also
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estimated the microbially accessible ESC of this biochar. The implications of our results for the
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potential roles of black carbon in biogeochemistry, pollutant fate, and engineering applications
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are discussed.
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Materials and Methods
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Biochar. Soil Reef biochar (The Biochar Company, PA) produced through pyrolysis of Southern
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Yellow hardwood chips at 550 oC was used. This commercial biochar has been used in pilot20
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and full-scale bioretention cells and infiltrations strips for stormwater treatment in Delaware and
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Virginia. The properties of the biochar are given in Table S1 in the Supporting Information (SI).
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Biochar was sieved and the 250-500 µm fraction was oxidized in continuously aerated deionized
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water13 (50 g/L) over 60 h. The oxidized biochar was filtered, vacuum-dried, autoclaved (121 oC,
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15 min), and purged with N2/CO2 (80:20) before use. Total organic carbon (TOC) analysis of the
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rinsates/filtrates suggests the washed/oxidized biochar contained little dissolved organic carbon
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(Table S2, SI).
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Microorganism. Geobacter metallireducens (GS-15) was chosen for this study because it can use
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humic acid as both an electron acceptor17 and donor19 but cannot use H2 as an electron donor,21
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which was present (5%) in the glove box and thus might exist in reactors. GS-15 (ATCC 53774)
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was grown on 5 mM each of acetate and nitrate in a modified ATCC 1768 medium. GS-15
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oxidizes acetate to CO2 and reduces nitrate dissimilatorily to ammonium through nitrite.22 After
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an 18-h incubation at 30 oC, culture was centrifuged at 1100g for 15 min, washed 4 times with an
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anoxic medium (N2/CO2-purged ATCC 1768 without electron donor, electron acceptor, or NH4+)
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and re-suspended to a density of 7.0(±1.6)×109 cells/mL, as measured by optical density at 600
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nm. Further details about cell density estimation are given in SI.
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Acetate Utilization Experiment. Serum bottles (125 mL) were set up in a glove box (N2/CO2/H2,
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75:20:5) in quintuplicates, each containing 104 mL of the anoxic medium (above) with known
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quantities of oxidized biochar (2 or 4 g) and cells (~2×108/mL). Cysteine (158 µmol,
