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Biogenic coal-to-methane conversion efficiency decreases after repeated organic amendment Katherine Davis, Elliott P Barnhart, Matthew W. Fields, and Robin Gerlach Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03426 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 31, 2018
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Biogenic coal-to-methane conversion efficiency decreases after repeated organic amendment
Katherine J. Davisa,b*, Elliott P. Barnhartc,a, Matthew W. Fieldsa,d, Robin Gerlacha,b **
a
Center for Biofilm Engineering, Montana State University, Bozeman, MT, 59717 USA
b
Department of Chemical and Biological Engineering, Montana State University, Bozeman, MT, 59717, USA c
U.S. Geological Survey, Helena, MT, 59601, USA
d
Department of Microbiology and Immunology, Montana State University, Bozeman, MT, 59717 USA *
[email protected] **
[email protected] ACS Paragon Plus Environment
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Abstract
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Addition of organic amendments to coal-containing systems can increase the rate and extent of
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biogenic methane production for 60 to 80 days before production slows or stops. Understanding
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the effect of repeated amendment additions on the rate and extent of enhanced coal-dependent
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methane production is important if biological coal-to-methane conversion is to be enhanced on a
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commercial scale. Microalgal biomass was added at a concentration of 0.1 g/L to microcosms
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with and without coal on days 0, 76, and 117. Rates of methane production were enhanced after
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the initial amendment but coal-containing treatments produced successively decreasing amounts
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of methane with each amendment. During the first amendment period, 113% of carbon added as
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amendment was recovered as methane, whereas in the second and third amendment periods, 39%
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and 32% of carbon added as amendment was recovered as methane, respectively. Additionally,
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algae-amended coal treatments produced ~38% more methane than unamended coal treatments
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and ~180% more methane than amended coal-free treatments after one amendment. However, a
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second amendment addition resulted in only a ~25% increase in methane production for coal
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versus non-coal treatments and a third amendment addition resulted in similar methane
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production in both coal and non-coal treatments. Successive amendment additions appeared to
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result in a shift from coal-to-methane conversion to amendment-to-methane conversion. The
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reported results indicate that a better understanding is needed of the potential impacts and
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efficiencies of repeated stimulation for enhanced coal-to-methane conversion.
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1. Introduction
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Natural gas found in many of the world’s coal reserves is commonly referred to as
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coalbed methane (CBM). It can be formed by thermogenic or biogenic processes and is
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commercially produced in some regions. Unlike thermogenic CBM, formed by heat and pressure
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on geologic timescales, biogenic CBM is formed by microbial processes that convert coal to
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methane and occur on shorter timescales, thus providing an opportunity for enhancement
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strategies for increased production.1–4
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The Powder River Basin (PRB) in Montana and Wyoming is the largest known U.S. coal
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reserve and a site of commercial CBM collection. Previous studies have shown that the methane
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collected in the PRB is almost exclusively of biogenic origin.5–7 Commercial collection of
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biogenic CBM often exceeds the rate of microbial methane production; therefore most
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production wells have short lifespans (7-10 years).8,9 When wells no longer produce
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commercially-viable gas quantities to justify continued pumping, wells are often abandoned,
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leaving coal and valuable infrastructure in place. In the Montana portion of the PRB alone, there
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were 1046 shut-in or abandoned wells and an additional 597 with expired permits in 2015. Thus,
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the PRB is potentially a lucrative environment for application of strategies to increase the rates
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and amounts of microbially-produced methane by utilizing the already in place infrastructure for
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collection.
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Previous studies have tested methods for enhancing biogenic coal-to-methane conversion
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in the laboratory, demonstrating enhanced biogenic methane production from coal using a
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variety of inorganic and organic nutrient additions7,10,11 and coal oxidation pre-treatments.12–15
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However, most of these methods could be too costly to implement in situ due to the investment
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required for production, transport, and application of the amendment. The use of yeast extract or
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tryptic soy broth, which are commonly used nutrients for the cultivation of diverse
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microorganisms, have been implemented as methane-enhancing amendment strategies to reduce
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the necessity of determining the exact “recipe” of nutrients needed, making it a potentially less
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expensive addition.4,16–18 However, the application of even the lowest-cost commercial-grade
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yeast extract ($8.50 per kilogram) to in situ coalbeds could be prohibitively expensive compared
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to use in ex situ bioreactors, and less-expensive alternatives need to be investigated.16,19,20
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In an effort to address potential costs associated with enhanced CBM production by
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amendment addition, the use of algal extract as an alternative amendment for increasing
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microbial coal-to-methane conversion has been introduced.18 Microalgae can grow in CBM
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production water ponds, and a Neospongiococcum spp. isolated from a production water pond
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has been shown to accumulate lipids.21 In addition to lipids for potential biofuel production,
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microalgae have also been grown commercially for production of nutritional supplements, food
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additives, fertilizer, aquaculture feedstock, and other high-value chemicals.22 Potential
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amendment transportation costs could be reduced by growing algae on or near the CBM
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collection site, while providing alternative economic benefits in the form of biofuels or other
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value-added products.16,18 Thus, the costs associated with CBM enhancement could be reduced
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by using microalgal amendments. The methane enhancement effect of organic amendments
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(microalgae, cyanobacteria, and yeast cells and yeast extract) was investigated at two
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concentrations and demonstrated similar methane production with all amendments.17
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Most CBM enhancement studies have been performed using batch systems, and it has
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been observed that methane production rates and amounts can be increased for a period up to 80
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days before methane production slows down or ceases completely in nearly all studies.11,14,17,23,24
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The purpose of this study was to determine whether methane production can be increased
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repeatedly when systems are re-amended after enhanced methane production has slowed down.
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The studies presented here focus on microalgal amendment at a low amendment concentration
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(0.1 g/L) which was shown to better maintain the coal-to-methane converting microbial
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community structure compared to higher amendment concentration.17
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For a long-term application of enhanced CBM strategies in situ, it is necessary to
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ascertain the potential benefits and challenges arising from repeated amendments of coal systems
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and the potential economic impacts on methane enhancement strategies. The results of this study
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build on previous work17 and contribute towards a better understanding of the ability of complex
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organic amendments to increase microbial production of coalbed methane with repeated
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amendment addition.
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2. Materials and Methods
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2.1 Site and Sample Collection. The sampling site, located near Birney, Montana in the
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PRB, was thoroughly described previously.25 Water from the Flowers-Goodale (FG) coal bed
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was pumped and retrieved in May 2016 from the FGP-13 well. Two well volumes were pumped
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prior to formation water collection. Plastic jugs (6-gallon volume) were rinsed twice with
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formation water before being filled and stored at 4°C prior to microcosm set up. Formation water
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characterization for the FG coal bed has been previously reported.25 Coal cores were collected
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during the July 2013 drilling of the FG monitoring wells (FGM-13 and FGP-13). Twelve-inch
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sections of the 2-inch diameter cores were placed in polyvinyl chloride (PVC) tubes that were
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filled completely with formation water from the FG-11 well and sealed with flexible rubber caps.
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Microbes were collected from FGM-13 in September 2015 using microbial samplers similar to
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those previously described.26 Liquid from the FGM-13 sampler was added to 3 serum bottles
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prepared with 5 g of FG coal and 45 mL of anoxic FG formation water before being incubated at
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room temperature (21 ± 1ºC) in the dark. The three serum bottles were combined prior to use as
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inoculum for the described studies .
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2.2 Amendment Growth and Preparation. A Chlorella microalga species, strain SLA-
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04, was grown in photobioreactors for biomass accumulation as previously described.17 The
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biomass was lyophilized and stored at -20°C before being used in microcosm studies. The algal
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amendment was ground to a fine powder with a ceramic mortar and pestle. A 1g/L stock solution
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(dry w/v) was prepared in 0.2 µm filtered and degassed FG formation water and sealed in
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oxygen-free serum bottles using anoxic methods.
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2.3 Microcosm Set Up and Re-Amendment. Microcosms were set up anoxically in 26
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mL Balch tubes with butyl rubber stoppers and aluminum crimp seals. The FG coal core (depth
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374-376 feet [~114 meters]) was opened in an anaerobic glove bag, and the core material was
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dried, crushed, and sieved to an effective size range (0.85-2.0 mm). The prepared coal was stored
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in oxygen-free glass bottles until microcosms were established. Borosilicate glass beads (1 mm)
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were autoclaved and used in lieu of coal to provide a carbon-free solid substrate as appropriate
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controls. Each Balch tube received 1 g of prepared coal or glass beads (GB). The formation
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water was filtered with 0.2 µm bottle top filters and sparged for 5 hours with an oxygen-free gas
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mixture (5% CO2/95% N2). Sodium sulfide (1 mM as Na2S·9H2O) was used as an oxygen
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scavenger to ensure low redox conditions. All amended treatments received 1mL of the prepared
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amendment concentrate. The initial pH of the degassed and reduced formation water was 7.6,
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similar to what has been observed in the FG formation water.25 All inoculated treatments
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received 1 mL of inoculum from the combined serum bottles of the previously enriched FG
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microbial consortium. The initial total liquid volume of all microcosms was 10 mL. Inoculum
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slurry was frozen and lyophilized for carbon analysis (5 mL). All microcosms were incubated in
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the dark at room temperature (21 ± 1ºC), approximately 3ºC warmer than the in situ (~18ºC)
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temperature.25 Headspace gas was sampled and analyzed approximately every 2 weeks. coal – (n=9)
coal - (n=3)
coal - - (n=3)
coal - + (n=6)
coal - + (n=3) coal - + + (n=3)
GB + (n=9)
GB + (n=3)
GB + - (n=3)
GB + + (n=6)
GB + + (n=3) GB + + + (n=3)
GB – (n=3)
GB - (n=3)
GB - - (n=3)
coal + (n=9)
coal + (n=3)
coal + - (n=3)
coal + + (n=6)
coal + + (n=3) coal + + + (n=3)
0
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117
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Time (days)
Figure 1: Experimental treatments (FG coal and glass bead (GB)) for each amendment time period. Amendment addition is indicated for each amendment time as “+” (addition of algal amendment in FG formation water) or “-” (addition of FG formation water only). The number of replicates is indicated as “n= #”.
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After 76 and 117 days, appropriate treatments were re-amended as shown in Figure 1.
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The amendment was prepared identically to the initial microcosm set up and 1 mL of prepared
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amendment was added to each of the re-amended treatments. Treatments not re-amended with
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additional algae amendment received 1 mL of degassed FG formation water to account for
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volume changes and potential nutrient addition from the formation water added with the algal
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amendment.
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2.4
Gas Analysis. Methane and carbon dioxide were monitored using an SRI
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Instruments (Torrance, CA, USA) Model 8610C gas chromatograph (GC) equipped with a
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thermal conductivity detector (TCD) interfaced with PeakSimple Chromatography software. A
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Tupelo HayeSep-D packed stainless-steel column (6 feet x 1/8 inch O.D.) was used with ultra-
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high purity helium carrier gas for separation using the following conditions: manual injection,
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oven temperature 40°C, TCD temperature 150°C, and carrier gas pressure 8 psi. Gas (1 mL) was
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collected from the microcosm headspace for GC injection. To prevent creating a negative
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pressure in the tubes, 1 mL of anoxic 5% CO2/95% N2 gas was injected to replace the sample
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volume removed. After the initial 20-day incubation period, reactors were sampled
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approximately every 2 weeks for gas analysis for the duration of the 159-day study.
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2.5 Carbon Analysis. Dissolved inorganic carbon (DIC) and non-purgeable organic
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carbon (NPOC) were measured using a FormacsHT/TN instrument (Skalar, Inc., Buford, GA,
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USA). Samples were centrifuged for 10 minutes at 4700 rpm and 4°C and filtered through 0.7
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µm GD/X filters. NPOC samples were acidified with 3N HCl to decrease the pH below 2 and
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purged with oxygen gas for 180 seconds prior to analysis. DIC and NPOC measurements were
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made every 4 weeks and on each re-amendment day. Because these analyses required destructive
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sampling of reactors, only one reactor per treatment was sampled for each sampling date except
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at the end of the experiment on day 176 when DIC and NPOC measurements were taken by
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destructively sampling the three remaining samples of each treatment. Carbon, nitrogen,
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hydrogen, and sulfur analysis were performed on the dry algal biomass and lyophilized inoculum
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using a Thermo Scientific (Waltham, MA, USA) CE Elantech Flash 2000 CHNS-O Analyzer.
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2.6 Statistical Analysis. A two-way ANOVA was fit to the amount of methane produced
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and the maximum methane production rate for each amendment period with a factor for
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treatment (coal or GB) and a factor for the amendment condition over time (i.e. +, + -, + + -,
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etc.). The ANOVA and Tukey tests of the interaction were performed using statistical software
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R.27 A p-value < 0.05 determined statistical significance.
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3. Results
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3.1 Methane Production. All treatments, regardless of solid substrate (coal, glass beads
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[GB]) or algal amendment regimen, produced methane during the 159-day study (Figure 2a-c),
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and the largest amount of methane was produced during the first amendment period (days 0 to
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76). During the second and third amendment periods (76-117 days and 117-159 days,
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respectively), less methane was produced than during the initial amendment period, but more
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methane was always produced by treatments that were re-amended compared to treatments that
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were not re-amended (Figure 2d-f) (p
