Profiles of Volatile Organic Compounds in Biochar: Insights into

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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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ACS Sustainable Chemistry & Engineering

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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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†

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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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‡

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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-

ACS Paragon Plus Environment

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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

280

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

281

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

284

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

292

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

293

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

295

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

296

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

297

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

298

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

299

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

300

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

301

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

305

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

306

conditions.

307

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

309

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

312

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

313

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

314

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

315

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

324

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

325

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

Profiles of Volatile Organic Compounds in Biochar: Insights into

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