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Source and biological response of biochar organic compounds released into water. Relationships with bio-oil composition and carbonization degree Michele Ghidotti, Daniele Fabbri, Ond#ej Mašek, Colin Logan Mackay, Marco Montalti, and Andreas Hornung Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 26, 2017
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Environmental Science & Technology
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Source and biological response of biochar organic
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compounds released into water. Relationships with
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bio-oil composition and carbonization degree
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Michele Ghidotti,*,†, ¶ Daniele Fabbri,†, ¶ Ondřej Mašek,‡ Colin Logan Mackay,§ Marco
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Montalti,¶ Andreas Hornung,⊥
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†
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Chemistry “Giacomo Ciamician”, University of Bologna, via S. Alberto 163, I-48123 Ravenna,
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Italy,
[email protected], +39 0544 937388; ‡ UK Biochar Research Centre, School of
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GeoSciences, University of Edinburgh, Crew Building, Alexander Crum Brown Road,
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Edinburgh, United Kingdom; § SIRCAMS, School of Chemistry, University of Edinburgh,
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Joseph Black Building, King's Buildings, West Mains Road, Edinburgh, United Kingdom;
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¶
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⊥Fraunhofer
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Branch Sulzbach-Rosenberg, An der Maxhütte 1, 92237 Sulzbach-Rosenberg, Germany
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ABSTRACT
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Water-soluble organic compounds (WSOCs) were extracted from corn stalk biochar produced at
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increasing pyrolysis temperatures (350-650°C) and from the corresponding vapors, collected as
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bio-oil. WSOCs were characterized by gas chromatography (semi-volatile fraction), negative
Interdepartmental Centre for Industrial Research “Energy and Environment” and Department of
Department of Chemistry “G. Ciamician”, University of Bologna, Via Selmi 2, Bologna, Italy; Institute for Environmental, Safety, and Energy Technology UMSICHT, Institute
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electron spray ionization high resolution mass spectrometry (hydrophilic fraction) and
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fluorescence spectroscopy. The pattern of semi-volatile WSOCs in bio-oil was dominated by
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aromatic products from lignocellulose, while in biochar was featured by saturated carboxylic
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acids from hemi/cellulose and lipids with concentrations decreased with decreasing H/C ratios.
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Hydrophilic species in poorly carbonized biochar resembled those in bio-oil, but the increasing
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charring intensity caused a marked reduction in the molecular complexity and degree of
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aromaticity. Differences in the fluorescence spectra were attributed to the predominance of fulvic
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acid-like structures in biochar and lignin-like moieties in bio-oil. The divergence between
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pyrolysis vapors and biochar in the distribution of WSOCs with increasing carbonization was
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explained by the hydrophobic carbonaceous matrix acting like a filter favoring the release into
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water of carboxylic and fulvic acid-like components. The formation of these structures was
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confirmed in biochar produced by pilot plant pyrolysis units. Biochar affected differently shoot
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and root length of cress seedlings in germination tests highlighting its complex role on plant
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growth.
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INTRODUCTION
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Biochar (BC) research has made consistent progress since it was proposed as a sustainable
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strategy for the abatement of greenhouse gases in terrestrial ecosystems.1 However, the
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ameliorating effect of BC in soil applications is highly dependent on its physical and chemical
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properties, in turn affected by production technology and biomass feedstocks. The definition of
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BC quality is therefore fundamental, and different criteria were proposed for its classification,
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like carbon content, aromaticity, and the presence of harmful chemical species such as heavy
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metals or polyaromatic hydrocarbons (PAHs).2 Apart from priority contaminants, the role of BC
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mobile organic compounds is being evaluated as potentially affecting its performance in soil.
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Volatile organic compounds (VOCs) and water soluble organic compounds (WSOCs) were
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deemed responsible for the positive3 and negative4,5 effects on plants, microorganisms6 and
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aquatic organisms7,8. BC labile carbon structures could also affect the composition of soil
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derived dissolved organic matter, in turn influencing soil ecosystem processes. The presence of
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organic species leached from BC was confirmed in soil application. Riedel et al.9 evidenced a
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compositional change in the molecular fingerprint of the organic matter released from a soil
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mixed with BC compared to untreated soil in column experiments. The amendment caused a
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marked reduction of the organic matter mobilization from the soil, but a net increase in the
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intensities of black carbon-type and lignin-type compounds was observed. Uchimiya et al.10
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demonstrated the existence of polyaromatic moieties in the dissolved organic carbon (DOC)
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extracted from a BC-amended soil, attributed to unique structures of pyrogenic DOC. Vapors re-
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condensation and pyrolysis temperature were found to be of primary importance for the
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production of BC suitable for soil application5,8,11. The contact of the pyrolysis vapors with the
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carbonized biomass inside the reactor and their removal is critical to prevent BC
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contamination5,8. Temperature and residence time are crucial process parameters. At
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temperatures higher than 400°C a sharp decrease in VOCs adsorbed on BC was evidenced8,11.
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High nitrogen flow can reduce the content of PAHs12,13. The effect of process conditions on the
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composition of WSOCs has not been widely investigated, nonetheless WSOCs may play an
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important role in BC environmental impact due to their mobility in water. WSOCs were
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investigated by means of two dimensional GC8 and liquid chromatography3,14,15, while ultrahigh
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resolution mass spectrometry (Fourier Transform Ion Cyclotron Resonance Mass Spectrometry
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FT-ICR-MS) revealed the presence of thousands hydrophilic species, non-detectable with other
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techniques 8,9,16. Fluorescence spectroscopy with Parallel Factor Analysis (PARAFAC) was used
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as rapid and sensitive technique to investigate its aromatic fraction10,17–19. These studies have
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noticeably increased our understanding on the chemical composition of BC WSOCs, however,
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the relationship with production parameters and especially with the composition of the pyrolysis
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vapors are poorly known. The present study was primary focused on the comprehensive
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characterization of BC WSOCs with spectroscopic, chromatographic and mass spectrometry
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techniques in relation to the composition of the water-soluble fraction of the pyrolysis vapors,
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condensed (bio-oils) during the pyrolyses for BC production. The linkage between WSOC
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patterns, BC bulk properties and their implications on seeds germination, could eventually shed
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light on the role of BC mobile organic compounds in the determination of its quality for
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environmental applications, possibly leading to the proposal of threshold levels.
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MATERIALS AND METHODS
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Samples
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BC were produced from pelletized corn stalks at 350, 400, 450, 500, 550, 600, 650°C and
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characterized in a previous study.11 The vapors carried by the nitrogen flow at the outlet of the
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reactor presented temperatures ranging from 195°C (pyrolysis at 350°) to 310°C (pyrolysis at
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650°C), measured with a thermocouple. These values were considered sufficiently high to
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minimize vapors re-condensation in the part of the reactor where the biochar was synthesized.
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Vapors leaving the reactor were condensed in two ice/salt cold traps at -14°C to collect the
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pyrolysis liquids. The content of the two traps was merged to produce one bio-oil (OL) sample
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per temperature. In this study, the samples produced at the temperature XXX °C are named
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BCXXX and OLXXX.
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Lipid extraction from corn stalk biomass
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The total lipid fraction of the corn stalk biomass used in the pyrolysis experiments was
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determined by sequentially extracting the feedstock with CHCl3-MeOH 2:1 (v/v) at 50°C for 1.5
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hours (triplicate analysis). The profile of fatty acids was determined by GC-MS after
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methanolysis followed by the production of fatty acid methyl esters (FAME)20.
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Extraction of biochar WSOCs
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An amount of BC was weighed (1 g ± 0.01 mg) into 20 ml vials and 10 ml of deionized water
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(DW, HPLC grade) was added. The vials were sealed with aluminum crimp seals with
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PTFE/rubber septa. The sealed vials were then placed on a mechanical shaker (IKA KS 260)
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covered with an aluminum foil, and left shaking at 150 rpm for 72 hours at ambient temperature.
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The resulting solutions were centrifuged at 3800 rpm for 10 min (ALC4232 centrifuge) to
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separate the solid material and filtered with PTFE syringe filters 0.45µm (Sartorius Minisart
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SRP) thereafter.
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Analysis of BC WSOCs by direct immersion (DI)-SPME-GC-MS
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Each BC extract (1ml) was added with 0.5 ml of 2M phosphate buffer (KH2PO4/Na2HPO4) at pH
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5.7 in 1.5 ml vials. Carboxen-PDMS (Car-PDMS) SPME fiber was exposed to the solution under
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magnetic stirring for 30 minutes.4 The thermal desorption of the analytes and GC-MS analysis
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were performed with the method developed in a previous study11. The amounts of WSOCs were
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expressed as peak area counts normalized by the sample weight (NA). Blank analyses of
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phosphate buffer and DW were performed to check procedural contaminations. A calibration
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curve of volatile fatty acids (VFA) was performed with a standard VFA solution (0.1% Sigma-
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Aldrich) containing acetic acid, propanoic acid, methyl propanoic acid, butanoic acid, 3-methyl
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butanoic acid and pentanoic acid in DW. Serial dilutions were prepared at 10, 5, 1 and 0.1 mg/l
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in phosphate buffer, spiked with 2-ethyl butyric acid 5mg/l in DW (internal standard) and
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analyzed in triplicate. The concentration of each VFA was calculated using the response factors
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from the calibration curve and expressed as (µg/gBC).
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Analysis of BC WSOCs by ESI(-)FT-ICR-MS
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BC extracts were diluted 1:10 in methanol and analyzed by negative electrospray ionization
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(capillary voltage 4kV) on a Bruker solariX 12T FT-ICR-MS. 500 scans were acquired for each
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spectrum (syringe infusion, 200µL·h-1) using an 8 MW acquisition size (broadband). Ion
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accumulation time was set at 0.8sec. Samples were spiked with ES tuning mix (Agilent) and a
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starting calibration list was developed from single point correction on the m/z 301.998139. Mass
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spectra were analyzed using Data Analysis software (Bruker Daltonics). Peaks were assigned
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with a signal to noise threshold of 4 and absolute intensity threshold of 2·106. Calibration lists
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were developed over the m/z range 100-600 by Kendrick mass analysis. Mass spectra without
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the calibrant were recalibrated with a quadratic equation with a standard deviation < 100ppb (67
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points). Calibrated mass lists were processed with PetroOrg software. Peaks were assigned with
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a threshold of 100ppb and molecular formulas within the range: C1-100, H4-200, O1-20, N0-4.
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Analysis of BC WSOCs by fluorescence spectroscopy and PARAFAC
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BC extracts were diluted in DW until the absorbance in the UV-Vis wavelength range 200-800
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nm was 0.9, p
