Profiles of Volatile Organic Compounds in Biochar: Insights into

Oct 28, 2016 - CS350, 1.2, 2.1, 55, 0.43, 3.7, 1.2, 13, 0.80, 0.17, 56. CS400, 1.0, 2.9, 54, 1.1, 3.2, 1.2, 9.6, 0.71, 0.13, 61. CS450, 1.0, 2.2, 56, ...
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Profiles of volatile organic compounds in biochar: insights into process conditions and quality assessment Michele Ghidotti, Daniele Fabbri, and Andreas Hornung ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01869 • Publication Date (Web): 28 Oct 2016 Downloaded from http://pubs.acs.org on October 30, 2016

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Profiles of volatile organic compounds in biochar:

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insights into process conditions and quality

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assessment Michele Ghidotti *, †, Daniele Fabbri †, Andreas Hornung ‡

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University of Bologna, via S.Alberto 163, I-48123 Ravenna (Italy), [email protected],

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+39 0544 937344

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Branch Sulzbach-Rosenberg, An der Maxhütte 1, 92237 Sulzbach-Rosenberg (Germany)

Interdepartmental Centre for Industrial Research “Energy and Environment” and CIRSA,

Fraunhofer Institute for Environmental, Safety, and Energy Technology UMSICHT, Institute

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KEYWORDS

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char, biomass, volatilome, solid-phase microextraction, solventless, green analytical chemistry

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ABSTRACT

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Sustainable application of biochar to soil requires a careful consideration of sorbed mobile

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compounds that can be released into the environment and their relationship with bulk chemical

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properties. Seven biochars were produced by pyrolysis of corn stalk from 350 to 650 °C and

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residual volatile organic compounds (VOCs) were investigated by head space solid-phase

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microextraction and gas chromatography-mass spectrometry. Over 80 compounds were detected

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in poorly carbonised biochars, including thermal degradation products of lignin (2-

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methoxyphenols), cellulose and hemicellulose (1,4:3,6-dianhydro-β-D-glucopyranose, C1-C3

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furans and furanones, C1-C2 cyclopentenones), lipids (aromatic and C2-C8 aliphatic acids) and

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proteins (aromatic nitriles, amides). The presence of potentially harmful compounds such as

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benzene, toluene, ethylbenzene, xylenes, phenols, volatile fatty acids, polycyclic aromatic

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hydrocarbons, highlights the importance of controlling the biochar production process.

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Statistically significant decreasing trends emerged between the quantity of all VOC classes and

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the increasing carbonisation degree estimated by the hydrogen to carbon (H/C) ratios and volatile

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matter content. Biochars with H/C 400 °C) did not release VOCs

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at ambient temperatures (25, 50 °C). VOCs fingerprinting can contribute to the evaluation of

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biochar quality. The samples with the highest levels of VOCs did not inhibit the germination of

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cress seeds (40 gL-1 water suspensions) indicating that biochar contamination is not necessarily

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associated with adverse effects.

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INTRODUCTION

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Biochar is the solid carbonaceous material produced by the pyrolysis of biomass for a variety of

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applications in the agro/environmental field.1 The wide range of feedstocks that can be thermally

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converted, including animal (poultry litter, residues from anaerobic digestion) as well as

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vegetable based materials (agricultural residues, energy crops), and the different technologies

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currently under optimisation for biochar production highly influence its physical and chemical

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properties.2,3 The increasing interest in finding suitable biochar applications (e.g soil amendment,

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greenhouse gas mitigation, feed additive, new materials, remediation of polluted areas)4,5 led to a

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comprehensive investigation on its chemical characteristics to determine its quality prior to large

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scale use.6,7 Standardisation of analytical methods for biochar testing is under investigation with

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conventional and non-conventional techniques and standard parameters for its quality/safety

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were proposed.8–10 During pyrolysis condensable gases could be retained onto the biochar porous

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structure and these compounds could have a certain degree of mobility. Their release in air or

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water, could exert adverse effects on plants

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evidenced that biochar water soluble organic compounds resulted toxic to freshwater blue-green

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algae. Contaminated biochar showed phytotoxicity on cress seeds (germination tests) while non-

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contaminated biochar produced under the same conditions did not present any negative effects12.

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Biochar labile substances can have a priming effect on soil biota17 and the potential to influence

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the fate of soil organic matter18 and pyrogenic black carbon cycle.19 Moreover, if large amounts

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of biochar are produced for soil application, the presence of harmful compounds such as

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monoaromatic hydrocarbons (benzene, toluene, ethylbenzene, xylenes: BTEX), polyaromatic

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hydrocarbons (PAHs) and heavy metals, has to be monitored for health and safety concerns

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during its handling and storage and to prevent impacts on plants,11,15,20 or other organisms21.

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Spokas et al.22 qualitatively investigated volatile organic compounds (VOCs) released by 77

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biochars produced from different processes and different temperatures with traditional head

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space gas chromatography-mass spectrometry (HS)-GC-MS instrumentation. Differences in the

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amounts of VOCs (total ion current) from similar feedstocks (oak hardwood), pyrolysis units and

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temperatures were found as well as VOCs profiles of corn stover biochar produced at similar

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conditions but from different units. The top ten most frequently observed compounds were

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acetone, benzene, methylethyl ketone, toluene, methyl acetate, propanal, octanal, 2,3-butadiene,

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pentanal, and 3-methyl butanal.22 The overall trend associated with an increase in pyrolysis

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temperature within the same unit was a net decrease in total sorbed VOCs with an increasing

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proportion of aromatic compounds. Buss et al.12 studied softwood biochar produced at 550°C

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that were contaminated with pyrolysis gases during process operation. Even after 4 weeks

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and microorganisms.14–16 Smith et al.14

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storage, contaminated biochar released small quantity of vapours inhibiting germination, but the

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chemical nature of the contamination was only hypothesized. Further studies led Buss et al.20 to

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conclude that VOCs posed greater concern for plant growth than PAHs. EBC guidelines for

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biochar quality indicate that the VOCs are very important for the determination of biochar

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quality but no quantitative information or thresholds were given.8 The report cites the qualitative

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study conducted by Spokas et al.22 and concluded proposing the assessment of total VOCs by

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thermogravimetric analysis (TGA). IBI proposed the same determination of VOCs as advanced

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analysis optional for biochar quality.9 Solid phase microextraction (SPME) is a powerful

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solventless technique for the analysis of trace components in complex matrices by GC-MS, that

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integrates several analytical steps (sampling, extraction, pre-concentration and sample

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introduction for instrumental analysis).23 The potential of SPME for the analysis of mobile

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organic compounds was described in the characterisation of poultry litter biochar where the

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technique enabled the detection of alkyl phenols, small hydrocarbons, alkyl aromatics, small

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PAHs and nitrogen containing compounds.11 However, the effect of biochar characteristics on

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the qualitative and quantitative distribution of VOCs determined by HS-SPME has not been

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investigated yet. In this study, a method based on HS-SPME-GC-MS was developed in order to

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understand the pattern of VOCs retained or formed in biochar during pyrolysis of biomass and

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how they relate to process conditions and biochar bulk properties. The method was tested on

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seven biochars obtained from the pyrolysis of a typical lignocellulosic biomass, corn stalk, at

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increasing pyrolysis temperatures (350-650 °C). The purpose of this study was the detailed

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characterisation of the mobile organic compounds that could be volatilised from biochar,

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potentially affecting its environmental performance, and the correlation of the VOCs species

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with biochar carbonisation degree. Secondly, the potential of HS-SPME data for the evaluation

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of biochar quality for sustainable applications was discussed.

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MATERIALS AND METHODS

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Chemicals

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2-allyl-6-methoxy phenol (o-eugenol) and methanol were purchased from Sigma Aldrich. SPME

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fibre holder and Carboxen-polydimethylsiloxane (Car-PDMS) fibre were purchased by Supelco.

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Biochars

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Pellettised corn stalks were pyrolysed in a bench-scale horizontal quartz reactor24 at different

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temperatures (350, 400, 450, 500, 550, 600 and 650 °C) with fixed residence time of 20 minutes

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and 1000 ml/min nitrogen flow. During pyrolysis actual temperature in the reactor was measured

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with a thermocouple. For each pyrolysis batch about 15 grams of biomass were processed. The

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char produced was let to cool in the reactor under nitrogen flow, collected and after

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homogenisation, stored in the freezer. The syntheses of biochar were performed in triplicate,

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with relative standard deviation (RSD) of the yields for each pyrolysis condition less than 2%.

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Biochar samples obtained at a given pyrolysis temperature XXX °C were grouped, homogenised

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and stored in closed vials at – 25°C before analysis; the resulting sample was named as CSXXX.

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Biochar characterisation

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Elemental analysis of biochar samples was performed with a Thermo Scientific FLASH 2000

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Series CHNS/O Elemental Analyser. TGA of thermosequence biochar samples was performed

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with a Mettler Toledo TGA/DSC. Between 5 and 10 mg of biochar were placed in alumina pans.

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TGA analyses were performed with nitrogen purge gas (50ml/min) and the following

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temperature program: 25°C for 2 min, 25-110°C at 25°C/min, 110°C for 10min (segment A);

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110-900°C at 25°C/min, 900°C for 10min (segment B); 900°C for 20 min in air at 50ml/min

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(segment C). A blank curve of an empty pan was acquired for baseline correction during the

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evaluation of the biochar samples. Proximate analysis was conducted from the weight loss curve

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of each sample. Moisture content was calculated from the weight loss during segment A, while

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the volatile matter (VM) and fixed carbon (FC) from segments B and C, respectively. The ash

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content was calculated from the residual weight after fixed carbon oxidation (segment C).

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Analyses were performed in triplicate. Data were normalised by sample weight and expressed in

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% dry basis.

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HS-SPME procedure

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One gram of biochar was exactly weighed in 20 ml HS vials and spiked with 1 µl (weighed at ±

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0.01 mg) of methanol containing o-eugenol as surrogate pyrolysis product (1.00 mg/ml). Sealed

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vials were placed on a heating plate at 150°C where only the bottom part containing biochar was

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in contact with the heated surface. The Car-PDMS fibre was placed in the HS where the

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temperature was about 40°C. Temperatures were constantly measured by means of a PSC-MS

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Plus Portable Handheld infrared thermometer. The exposure time was fixed at 30 minutes

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according to Becker et al.25 Spokas et al.22 Preliminary experiments during method development

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showed that: Car-PDMS fibre gave better performance in terms of signal intensity compared to

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polyacrylate and polyethylene glycol SPME fibres; increasing exposure time more than 30

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minutes did not significantly increase GC signal. The HS-SPME procedure was also applied to

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calibration solutions and CS biochar samples heated at different temperatures (25, 50, 100,

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150°C) during sampling.

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GC-MS conditions

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After fibre exposure, the Car-PDMS was inserted into the split/splitless injector of an Agilent

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5977 gas chromatograph equipped with a straight SPME liner. Analytes were thermally desorbed

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at 250°C for 10 minutes and separation performed with a DB-FFAP polar column (30m length,

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0.25mm i.d, 0.25µm film thickness). Starting GC oven temperature was set to 36°C (5 minutes)

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and increased to 250°C (10°C/min). Detection was made with a quadrupole mass spectrometer

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Agilent 7820A operating under electron ionization at 70eV with acquisition at 1 scan/sec in the

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m/z 29 and 450 range. Mass spectra were acquired in full scan mode properly adjusting the

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electron multiplier voltage. Identification was based on library mass spectra matching (NIST)

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and literature data. Quantification was made from the peak area integrated by extracting

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characteristic ions from total ion current.

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Quantification and statistical analysis

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The quantity of each analyte released by biochar and captured by the SPME fibre in the HS of

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test vials (20 mL) was expressed in terms of normalised peak area NA. NA was calculated from

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the peak areas of the analyte (Aanalyte) and the internal standard (Ais), the quantity of added

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internal standard (µgis of methanol) and the analysed biochar using the following equation

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(Eq.1):

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Eq.1: ܰ‫= ܣ‬

஺ೌ೙ೌ೗೤೟೐

ஜ௚೔ೞ

஺೔ೞ

௚್೔೚೎೓ೌೝ

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This approach was considered adequate for the purposes of this study intended to compare the

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relative behaviour of different biochar samples produced from the same feedstock. The linearity

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of the HS-SPME method was tested on o-eugenol. Ordinary least squares regression computed

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on the peak area of o-eugenol in the calibration range of 108-0.108 ng (12 data points) resulted in

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the following linear model (‫ ݔܽ = ݕ‬+ ܾ): ‫ = ݕ‬4.43‫ܧ‬06‫ ݔ‬− 4.56‫ܧ‬06, where ‫ ݔ‬and ‫ ݕ‬are the

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amount of internal standard and its peak area respectively. 95% confidence intervals for a and b

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were [4.34‫ܧ‬06; 5.55‫ܧ‬06] and [9.42‫ܧ‬06; 8.38‫ܧ‬05] respectively, while the standard errors were

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5.43‫ܧ‬04 and 2.94‫ܧ‬06. R2 and Pearson correlation coefficient (r) were both 0.999. Average S/N

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values for 0.1 ng level was 50 (n=3, 4% RSD) indicating that under the selected conditions

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VOCs at the sub-ng level could be detected in the HS of biochar. All the analyses were

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conducted in triplicate; data were reported as mean values ± 1 standard deviation (s.d.) or %

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relative standard deviation (RSD). Blanks were performed to ascertain the absence of cross-

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contamination. The RSD of methanol peak area (Ais) from triplicate analyses of each biochar

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was < 10%. Ordinary least squares regression was computed with the software PAST

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(Paleontological Statistic vers. 2.16) while Pearson product moment correlation coefficients (r,

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with P level of significance) were calculated with the software SigmaPlot vers.12.0. Kruskal-

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Wallis and Jonckheere-Terpstra statistics were performed with the NSM3 package of R

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according to Hollander et al.26

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Germination tests

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Seed germination tests on cress (Lepidium sativum L.) were conducted with CS350 and CS400

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biochars in water suspensions at 40g/l (4 replicates) according to Rombolà et al.11

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RESULTS AND DISCUSSION

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Biochar characteristics

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The elemental composition of biochar samples produced in this study is reported in Table 1. The

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hydrogen and oxygen content decreased with increasing pyrolysis temperature and accordingly

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the values of H/C and O/C atomic molar ratios decreased (from 0.80 to 0.32 and from 0.17 to

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0.07 from 350 °C to 650 °C, respectively). The content of nitrogen also decreased with

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increasing pyrolysis temperature. Proximate analysis of biochar samples is reported in Table 2.

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The FC and ash content tended to increase with the pyrolysis temperature while the VM

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decreased. These trends were typical observed in thermosequence biochars due to the more

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severe thermal decomposition of biopolymers at higher temperature with enhanced elimination

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of volatile compounds and polycondensation, aromatisation and defunctionalisation of the

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carbonaceous matrix.27–30 H/C and O/C atomic molar ratios were proposed as an index of biochar

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carbonisation degree or its degree of aromaticity.31 H/C values were preferred in this study given

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that O/C values may not be adequate for biochars with high ash content.2 The trend of H/C

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demonstrated to be statistically significant with the Jonckheere-Terpstra statistics (P < 0.01). FC

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increased with the decreasing H/C of the biochars, with correlation coefficients of 0.83 (P
0.9, P < 0.05). The strong

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correlation with indexes of the degree of charring and aromaticity (H/C, VM) indicated that

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highly carbonised biochars should be produced in order to reduce the risk of VOCs emission.

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Notably, the molecular pattern of the volatilome of biochars produced at different temperatures

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was different. This was evidenced in Figure 3 showing the quantity of the different chemical

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families as a function of H/C values, for the most abundant (Figure 3A) and less abundant

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(Figure 3B) compounds. Aldehydes and ketones (mostly benzaldehydes and furaldehydes) were

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the most abundant family in weakly charred biochars (around 30%), but their quantity decreased

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sharply with decreasing H/C becoming negligible in biochars with H/C < 0.59. Aromatics

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(principally benzene, toluene and C2-benzenes) and phenols were the principal class of VOCs in

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all the biochars. Their contributions to total VOCs of slightly carbonised biochar were 20 and

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14%, respectively. In all other corn stalk biochars (H/C 0.71-0.32) aromatics were the most

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abundant compounds. Interestingly, aliphatic as well as aromatic (benzoic acid) acids

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represented a significant class of compounds featuring biochar volatilomes (25% for CS350).

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These compounds were detected principally as methyl ester derivatives probably from biochar-

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catalysed methylation. Traces of cyclic ketones like cyclopentanone and cyclohexanone were

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detected only in biochars with H/C of 0.49 and 0.44. In this range of temperature there could be a

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cyclic rearrangement of their smaller linear precursors. Aromatic aldehydes like furaldehyde and

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benzaldehyde were identified in all thermosequence biochars. Cyclopentenones, lactones, alkyl

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and aryl furans are representatives of thermal degradation of holocellulose.24,31 Their signal was

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strongly suppressed in biochars with H/C < 0.49. C1-C3 cyclopentenones were not found at H/C

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lower than 0.59. Volatile aldehydes and ketones could also originate from pyrolysis of

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holocellulose34. C3-C4 alkyl aldehydes and substituted derivatives of benzaldehyde were detected

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only in biochars with H/C < 0.59. Incomplete degradation of cellulose at low pyrolysis

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temperature (0.80 < H/C < 0.59) could be evidenced by the presence of 1,4:3,6-dianhydro-β-D-

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glucopyranose. Kruskal-Wallis test was applied to determine differences of VOCs concentrations

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in biochars with increasing carbonization degree. Results evidenced statistically significant

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differences (1% level of significance) on the medians of all VOCs compound classes except

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alkyl amides. The level of significance of alcohols was 5%. Jonckheere-Terpstra statistic was

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also performed to test the alternative hypothesis of an increasing trend in the medians of VOCs

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with the increasing H/C. The outcome evidenced statistically significant differences (1% level of

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significance) for all compound classes. These results were in accordance with those of Pearson

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product-moment correlation coefficients and confirmed the relevant effect of the carbonisation

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degree on the volatiles released by biochar. Quantitative analysis highlighted differences in

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VOCs relative abundance (Table S1 and S2), but all corn stalk biochars had toluene, acetic acid

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methyl ester and phenol among the most abundant compounds. The presence of aromatic

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compounds, volatile fatty acids and phenols in biochar could be therefore a crucial marker for its

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safety and quality for environmental applications. The %RSD calculated on total VOCs

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concentrations in triplicate analysis were 17, 18, 27, 23, 31, 34 and 14% for corn stalk biochars

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with H/C of 0.80, 0.71, 0.59, 0.49, 0.44, 0.36 and 0.32 respectively. Considering the complexity

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of the matrix and the number of compound identified, %RSD values were acceptable and

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demonstrated the suitability of the HS-SPME method for the analysis of VOCs in biochar. A

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rough estimation of the concentrations could be accomplished by utilising for all the compounds

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the response factor of the surrogate pyrolysis product o-eugenol. VOCs concentrations

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progressively decreased with the increasing carbonisation degree from about 6 to 0.09 µg/gbiochar.

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These values are within the range conservatively estimated in biochars (pg-µg/gbiochar) through

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direct HS-GC-MS analysis and could be sufficient to affect the soil microbiome22.

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VOCs emissions at ambient temperatures

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The effect of biochar temperature on the pattern of evolved compounds detectable by HS-SPME

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was investigated at 25, 50, 100 °C (the results at 150 °C were described in the previous section).

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The lowest value was chosen in order to verify if biochar could release volatile compounds at

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ambient temperature, while 50°C approximated a possible condition achieved in terrestrial soils

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during warm periods. The values of 100 °C was tested by Becker et al.25 As expected,

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temperature greatly affected the pattern and intensity of GC traces. As an example, the mass

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chromatograms relative to the ions m/z 122 in the elution region of interest indicative of alkyl

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aromatics, alkyl aldehydes and lignin phenols are shown in Figure 4 for different temperatures.

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GC peaks were barely detected at 25 and 50 °C, they could be revealed at 100 °C and became

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intense at 150°C. This latter temperature was selected in the procedure. Concerning the

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possibility of VOCs emission at ambient temperatures, quantitative results (Table S3) showed

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that VOCs could be detected only in CS350 and CS400 biochars with low carbonisation degree

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(H/C 0.80 and 0.71). The most abundant compounds were aldehydes like propanal, furfural,

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benzaldehyde, formic and acetic acids. The possible correlation of such compound classes,

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especially organic acids and phenols with the environmental performance of biochar were

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highlighted11,12and their presence could be a useful proxy to discriminate biochar quality.

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Biochars with H/C < 0.71 did not exhibit detectable compounds in HS at 25 and 50 °C. The

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threshold value of 0.70 for H/C was considered as a quality parameter for biochar.8,9 Similarly,

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the results of this study suggest that sufficiently carbonised biochars (H/C < 0.70) obtainable at

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relatively high pyrolysis temperatures (> 400 °C) are not prone to evolve VOCs at normal

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conditions.

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Evaluation of the HS-SPME-GC-MS method for biochar quality assessment

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The presence of trace VOCs in biochar was investigated with a green analytical method based on

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SPME. SPME application to biochar was seldom reported.32,33 Clough et al. applied the

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technique using DVB/CAR/PDMS fibre exposed at 40°C for 40 minutes but only α,β-pinene and

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acetaldehyde were identified.33 Higashikawa et al. selected DVB/CAR/PDMS fibre (20°C and 20

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minutes fibre exposure) to quantitatively detect model VOCs in complex environmental matrices

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and showed that the biochar-matrix reduced method performance due to high affinity for the

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spiked compounds.32 In the present study, several compound classes with different polarities and

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representative of biomass thermochemical degradation products were detected, corroborating the

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preliminary results obtained in a previous work.11 More recently, it was shown that the

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composition of pyrolysis vapours provided by SPME with Car-PDMS fibre was similar to that of

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the bio-oil analysed by direct GC-MS.24 This similarity confirms the suitability of the Car-PDMS

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fibre for the detection of biomass pyrolysis products. The ability of the methods to detect

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molecular markers of feedstock and carbonisation could allow the control of the occurrence of

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biochar contamination from vapour re-condensation during pyrolysis.14,37 The potential release

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of these compounds in air or pore water can affect biochar performance in soils and its

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sustainability. It was shown that VOCs and water soluble organic compounds in biochar can

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affect soil and aquatic micro-organism communities14,16,38. To the end of assessing the potential

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inhibitory effects of biochars with the highest levels of VOCs, germination tests on cress seeds

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were performed on samples CS350 and CS400. The average germination rate of CS350 and

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CS400 (96± 2% for both biochars) was similar to that of the control (97 ±1%). These results

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were consistent with a previous study on similar biochars11, indicating that a high VOC content

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is not necessarily associated with observable toxicity, at least on the germination of cress seeds.

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However, the importance of mobile organic compounds in biochar quality assessment emerges

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considering the similarity between some VOCs molecules presented in this study and volatile

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WSOC that showed toxicity on aquatic organisms14. The composition and quantity of biochar

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VOCs was dependent on chemical characteristics, as VOCs decreased with increasing

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carbonisation degrees, while the molecular pattern shifted from the prevalence of oxygenated

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compounds towards that of aromatic hydrocarbons. The risk of VOCs emissions can be reduced

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by producing biochar with a high carbonisation degree, represented by low values of H/C and

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volatile matter. The emission of VOCs at ambient conditions resulted minimal for cornstalk

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biochars with H/C