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Mineral-Biochar Composites: Molecular Structure and Porosity Aditya Rawal, Stephen D. Joseph, James M. Hook, Chee H. Chia, Paul R. Munroe, Scott W. Donne, Yun Lin, David Phelan, David Richard Graham Mitchell, Ben Pace, Joseph Horvat, and J. Beau W. Webber Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00685 • Publication Date (Web): 10 Jun 2016 Downloaded from http://pubs.acs.org on June 12, 2016

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Environmental Science & Technology

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Mineral-Biochar Composites: Molecular Structure and Porosity

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Aditya Rawal a*, Stephen D. Josephb*, James M. Hook,a Chee H. Chiab, Paul R. Munroe b,

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Scott Donnec, Yun Linc , David Phelan h, David R. G. Mitchell d, Ben Paceb, Joseph Horvat

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d

,

J. Beau W. Webber e,f.

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a

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Australia 2052

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b

Mark Wainwright Analytical Centre, University of New South Wales, Kensington, NSW,

School of Materials Science and Engineering, University of New South Wales,

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Kensington, NSW, Australia 2052

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c

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Chemistry, Callaghan, New South Wales 2308, Australia

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d

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Australia

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e

Lab-Tools Ltd., Canterbury Enterprise Hub, University of Kent, Canterbury, Kent. CT2 7NJ.

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f

Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh. EH14 4AS.

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h

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Wales 2308, Australia

University of Newcastle, School of Environmental and Life Sciences, Office C325,

Electron Microscopy Centre, AIIM, University of Wollongong, Wollongong NSW, 2522,

University of Newcastle, Electron Microscope and X-Ray Unit, Callaghan, New South

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* Corresponding authors. Tel.: +612 9385 4693; fax: +612 9385 4663.

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E-mail address: [email protected]; [email protected]

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Aditya Rawal and Stephen Joseph had equal intellectual and experimental inputs into this paper and are the principal leads in the work.

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Abstract

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Dramatic changes in molecular structure, degradation pathway and porosity of biochar are

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observed at pyrolysis temperatures ranging from 250–550 °C, when bamboo biomass is

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pretreated by iron-sulfate-clay slurries (iron-clay biochar), as compared to untreated

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bamboo biochar. Electron microscopy analysis of the biochar reveals the infusion of

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mineral species into the pores of the biochar and formation of mineral nano-structures.

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Quantitative

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C Nuclear Magnetic Resonance (NMR) spectroscopy shows that the

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presence of the iron-clay prevents degradation of the cellulosic fraction at pyrolysis

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temperatures of 250 °C, whereas at higher temperatures (350-550 °C), the clay promotes

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biomass degradation, resulting in an increase in both the concentrations of condensed

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aromatic, acidic and phenolic carbon species. The porosity of the biochar, as measured by

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NMR cryo-porosimetry, is altered by the iron-clay pretreatment. In presence of the clay, at

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lower pyrolysis temperatures, the biochar develops a higher pore volume while at higher

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temperature the presence of clay causes a reduction in the biochar pore volume. The most

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dramatic reduction in pore volume is observed in the Kaolinite infiltrated biochar at 550

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°C, which is attributed to the blocking of the mesopores (2-50 nm pore) by the non-porous

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metakaolinite formed from kaolinite.

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Keywords: biochar, clays, solid state NMR, TEM, cryoporometry.

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INTRODUCTION

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Biochar production is seen as a key part of a strategy to effectively recycle biomass carbon

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and nutrients1, improve plant growth and mycorrhizal colonization2, improve soils physical

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and chemical properties, provide a source of renewable energy and act as a means for

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carbon sequestration3-6. However, in China and other parts of South East Asia, an estimated

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100 million tonnes of biomass are burnt annually, either in the field or at processing

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factories. This results in loss of carbon and nutrients from the soil and a very large increase

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in harmful atmospheric carbon emissions7. The vast quantities of readily available biomass,

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which are currently treated as waste for disposal, could be readily, cost effectively and

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widely converted into valuable biochar4,

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matter to biomass before or after pyrolysis can result in increased crop yields and

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improvement of soil biome at application rates similar to those used for chemical

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fertilizers1-2, 11-15. For biochar production to be cost effective, however, its properties need

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to be tailored for specific applications, as has been the case of activated carbons for

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engineering applications7, 13, 16.

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. The addition of clay, minerals and organic

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Investigations into biochar residues collected from fields reveals that biochar is

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often coated in a small, but significant quantity of soil that typically contains a range of

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clays, organic matter, sand, salts, residual chemical fertilizers (e.g. urea) and minerals that

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include nano-phase iron-bearing compounds17. Joseph et. al,13 and Li et. al18 have

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suggested that these mineral-biochar composites lead to a favorable plant response due to a

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number of factors including: the enhanced redox activity19-20 and the ensuing biotic and

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abiotic reactions which assist in nutrient cycling and plant uptake of nutrients13, 17-18.

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Despite the significant advantages of such enriched biochar, there has been very

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little research on specifically how such surface coatings of soil on the biomass change the

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properties of the biochar. For example, previous work has demonstrated that biomass

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pyrolyzed with clay at 600 °C resulted in a biochar that had a greater surface exchange

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capacity and adsorption capacity for contaminants than the biochar produced without such

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amendments21. Elsewhere, it was found that addition of biochar (along with bamboo

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vinegar) to composting piles enhances nitrogen conservation and immobilizes heavy metal

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ions22. Recent work also shows that addition of clay and iron minerals to the biochar

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enhances their stability within the soil23. However, the specific biochar structure that leads

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to the observed properties is not readily understood, and although mineral-biochar

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composites are a subject of continuing research interest, much of the work has focused on

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their properties or applications. 21, 24-25 Within the biochar literature there is generally only

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a qualitative understanding of the molecular structure of mineral impregnated biochar 12, 26.

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Furthermore, standards for biochar stability and properties currently being developed by

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the International Biochar Initiative27 are primarily based on clean feedstock in stand-alone

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pyrolysis facilities. Biochar produced from feedstock that has been standing in fields for

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considerable periods of time or synthesized in simple ovens and kilns may nevertheless

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have very different properties.

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While the structure of biochar as a function of pyrolysis conditions has been

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investigated28-29, to the best of our knowledge there has been no systematic study of the

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more complex changes in the molecular and structural properties of biochar produced from

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biomass coated with mineral phases. The study herein bridges this gap in knowledge and is

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the first in a series that examines the interaction of biomass, clay and mineral species with

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respect to the molecular structure and the micro- and meso-porosity of the biochar

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produced at different temperatures. Our aim is to understand the effect of iron-clay

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pretreatment on the degree of carbon condensation, the concentration of functional groups

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in the biochar, the porosity of the biochar and the pyrolysis degradation pathway. The

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degree of carbon condensation is directly relevant to the stability of the biochar and its

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environmental implications relating to carbon sequestration.30-31 The concentrations of

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different functional groups, such as carboxylic acids, and the porosity of the biochar are

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pertinent to the agricultural applications of biochar, while the ability to control the

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degradation pathway is crucial for efficient production of the desired biochar structure. A

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fundamental understanding of the changes in the structure of biochar as a function of

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pyrolysis temperature and mineral additives is therefore crucial for the implementation of

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strategies to design biochar with superior properties, tailored to enhance performance. For

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this purpose, a series of biochars infiltrated with iron-clay minerals were synthesized and a

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detailed, quantitative analysis of their structure and porosity using NMR spectroscopy

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along with electron microscopy is reported herein.

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MATERIALS AND METHODS

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Synthesis of biochars. For the comparisons of molecular structure, it was important

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to use a biomass source which had a uniformity of structure and as such bamboo is an ideal

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option. Bamboo is a fast growing bio-resource, which yields extensive residue from the

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bamboo processing industry. To ensure minimal mineral and chemical variation of a

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naturally sourced lignocellulose feedstock, the biomass for the study was procured from a

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single 1 meter section of bamboo to synthesize all the biochars. The bamboo section was

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cut into cubes with a dimension of approximately 10 mm. Kaolinite (a 1:1 clay) and

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bentonite (a 2:1 clay) were chosen as clay minerals to observe the influence of different

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clay structures. Refined kaolinite and bentonite clays were sourced from: Keens Ceramics,

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Gindurra Rd., Sommersby, NSW, 2250, as detailed in the supplementary information under

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Technical Specifications. Iron is also seen to influence biochar properties, and

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FeSO4.7H2O, chosen as the iron source for mineral-biochar composites, was sourced from

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Sigma Aldrich. Two different iron-clay slurries were prepared to infiltrate the biomass with

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the minerals. The first was iron-kaolinite slurry comprising a 1:1 by weight ratio of

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FeSO4.7H2O and refined kaolinite clay. The second slurry was iron-bentonite slurry

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prepared from a 1:1 by weight ratio of FeSO4.7H2O and bentonite clay. Two sets of

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samples were prepared by soaking the bamboo cubes in the two iron-clay slurries at 80 °C

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for 3 hours, conditions optimized to yield maximum infiltration of the slurry into the

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biomass pores without drying out the slurry over the time period. A third set of bamboo

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cubes was left untreated to act as a control. These three sets of bamboo samples were

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prepared and dried at 110 °C for 24 hours prior to pyrolysis. The sample for pyrolysis was

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placed in a purpose built high nickel chromium (Sandvik 253MA stainless steel) reaction

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vessel with 1 mm hole to allow pyrolysis gas to escape but minimize any ingress of air, and

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this vessel then placed inside an electric kiln with an air-starved environment. An

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electronic temperature controller fitted to the kiln allowed controlled heating of the

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biomass. The temperature was ramped at a rate of 3 °C/minute until the desired pyrolysis

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temperature was attained, after which point it was held at its final pyrolysis temperature for

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30 minutes to form biochar. After this pyrolysis time was passed, cold water was sprayed

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onto the hot metal to create a steam environment and reduce the temperature to ~200 °C.

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The vessel was removed and quenched in iced water to lower the temperature to ~80 °C to

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prevent any chemisorption of air by the biochar. The biochar was then stored in air tight

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container for further characterization. This process was repeated for each sample at the

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final pyrolysis temperatures of 250 °C, 350 °C, 450 °C and 550 °C, making twelve samples

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in all. In the ensuing discussion, we label the biochar from untreated bamboo as BC,

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biochar from bamboo treated with iron-kaolinite as BKF and biochar from bamboo treated

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with iron-bentonite as BBF. Three different batches from each of the biochar samples were

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produced and these were then crushed and combined for analysis leading to more

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representative results. The sample codes and preparation conditions are summarized in

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Table 1.

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Elemental analysis. Analysis of the aggregated samples for ultimate and ash

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analysis was carried out four times (Table 1) as per ASTM D758232. The ash constituent

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analysis was carried out according to ASTM D432633 using x-ray fluorescence (XRF)

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spectroscopy.

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SEM and TEM. Scanning electron microscope (SEM) analysis was performed

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using a Zeiss Sigma SEM fitted with a Bruker X-ray energy dispersive spectrometry (EDS)

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detector on pieces of biochar that had been mounted on carbon tape and chromium coated.

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For transmission electron microscopy (TEM), the biochar samples were dispersed

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ultrasonically in ethanol and a drop of the suspension was placed on a carbon-coated

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copper grid with a pipette. The fine particles were examined using a JEOL 2100 TEM

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fitted with an Oxford EDS system.

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Solid-state NMR spectroscopy. The samples of biochar were finely ground and

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packed into 4 mm zirconia rotors fitted with Kel-F caps. The 13C NMR experiments were

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performed on Bruker AVANCE III 300 spectrometer with a 7 Tesla superconducting

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magnet operating at frequencies of 300 MHz and 75 MHz for the 1H and

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respectively. The ratios of the protonated and non-protonated carbon in the chars were

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determined according to the method of Brewer et al28. The quantitative

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Polarization with Magic Angle Spinning (13C DPMAS) NMR spectra of the material were

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acquired at 12 kHz MAS with a

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decoupling, and a Hahn-echo before signal detection to eliminate baseline distortion. 10 s

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recycle delays were used for the measurements, where greater than 95 % of the

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relaxation was observed as determined by

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Selective suppression of the protonated carbon species was achieved with dipolar

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dephasing (dephasing time: 68 µs) before acquisition, and yielded a quantitative signal of

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the condensed and non-protonated carbon species. The typical measurement time for each

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13

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solid state NMR spectroscopy are provided in the supplementary information Figure S5.

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C nuclei

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C Direct

C 90° pulse length of 4 µs, 80 kHz 1H SPINAL64

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13

C

C cross-polarization/T1 (CP/T1) filter34.

C DPMAS experiment was 11 hours. Details for the mineral analysis using 27Al and 29Si

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NMR Cryoporometry. The pore-size distribution and pore volumes of the stable

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carbon skeleton were assessed using a Lab-Tools Mk1 (1970s) solid-state NMR

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spectrometer with digital-switching transmitter. The magnet used was a low-field (0.5 T)

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Mullard permanent magnet with a 34 mm pole-gap, with a low homogeneity, often

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deliberately degraded to 100 µT over the 12 mm long and 2.4 mm diameter sample, in

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order to have a clear echo at 1 ms. The details of the method are given by Webber et al.35

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Repeats were carried out on two additional samples and indicated a variation of + 0.15

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ml/gm.

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RESULTS

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Infiltration of the mineral phase into the biochar scaffold. The pyrolysis of the iron-

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clay slurry treated bamboo biomass results in the incorporation of mineral phases into the

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biochar scaffold. The SEM images of the higher temperature mineral biochar composites,

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(BKF 450, BKF 550, BBF 450 and BBF 550), which are most relevant for potential

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applications, are shown in Figure 1 and illustrate the presence of the mineral in the

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biochars. Figure 1(a) shows the coating of clay and iron that covers the outside of all of the

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biochars at low magnification as evidenced by the rough particulate material, while Figure

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1(b) shows the macroporous structure of the biochar. SEM images of the lower temperature

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mineral biochar composites are provided in supplementary information (Figures S1 and

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S2). It is possible the surface roughening seen in Figure 1(a), may be from the texture of

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the biomass itself, therefore images were recorded at higher magnification. Figure 1(c) and

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(d), clearly show the mineral phases in the form of particles and agglomerates coating the

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surface of the biochar structure. The SEM was not sufficiently resolved to see the

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morphology of the infiltrated mineral phases, therefore TEM analysis was used to

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investigate the mineral structure on the internal pore surface of the biochar. Figure 1(e),

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demonstrates that the internal surface of a biochar pore is indeed coated by mineral phases

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(visualized as darker particles). The larger pore is probably derived from the carbonization

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of phloem or xylem36 and contains a range of amorphous and crystalline nano-phases

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composed of ferric and alumino-silicate species. Away from this macropore, we expect that

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minerals have impregnated existing mesopores as well. At higher magnification analysis,

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Figure 1(f) reveals that the mineral regions have a nano-structured dendritic morphology.

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The elemental analysis by energy dispersive spectroscopy (EDS) as shown in, Figure 1(g),

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indicates these dendritic nanostructures include an iron-bearing mineral phase. The SEM

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and TEM analyses both substantiate that soaking biomass in an iron-clay slurry followed

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by pyrolysis, result in the incorporation of nanostructured mineral phases within the porous

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structure of the biochar. It is important to note that while SEM and TEM analysis provide

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a detailed but local picture of the mineral coating to biochar, quantitative

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spectroscopy and porosimetry were essential to establish mineral infiltration within the

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bulk of the biochar and its influence on the molecular structure and porosity of the biochar.

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C NMR

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Elemental composition of the biochar. Table 1 summarizes the changes in the chemical

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composition of the biochar produced at different temperatures. With increasing pyrolysis

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temperature, the carbon content increases and the biochar yield decreases for the untreated

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BC series of biochar. This is expected as the oxygen-bearing and hydrocarbon moieties are

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volatilized at higher temperature. In the iron-clay biochar (BBF and BKF series), the

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volatilization process also results in increased concentration of the clay minerals (and a

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relative reduction of carbon content). It is important to note that the Al, Fe and S contents

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for the BKF and BBF samples were orders of magnitude greater than the BC samples

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(Table 1), indicating uptake of the minerals by the biomass. In particular, the Al content in

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Table 1 for the mineral infiltrated biochar varies from 0.01 to 0.1 wt. % (of the total

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biochar weight). Form the chemical composition of kaolinite (Al2 Si2O5(OH)4 where Al

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corresponds to ~20 % of the clay weight) and bentonite clay ((Na,Ca)0.3(Al,Mg)2Si4O10

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(OH)2•n(H2O) where Al is ~10% of the clay weight), we estimate that the slurry treated

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biochars contain between 0.1 to 1 wt% of clay mineral. Similar uptake of iron sulfate and

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either of the clay minerals by the bamboo biomass from the iron-clay slurries was also

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indicated by equivalent amounts of both Fe and Al for the BBF and BKF samples. The

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total N content was observed to be lower for the BBF450 and BBF550 samples compared

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with samples in the BC and BKF series. It is possible that the presence of the bentonite-

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iron minerals in the biochar promotes volatilization of the nitrogenous species from the

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biomass and further investigations are required to pinpoint the origin of this specific effect.

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As the pyrolysis temperature increased, the concentrations of most of the mineral species

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increased due to concomitant reduction in the biomass fraction due to loss of the oxygen

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and hydrogen moieties during degradation and volatilization. The sulfur content for the

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BBF and BKF samples reduced at 550 °C (after an initial increase at 250 °C due to

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reduction of the organic content) indicating a volatilization of sulfur, consistent with the

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degradation of iron sulfate at 550 °C37. We note that the variation in the carbon wt. % for

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the different biochars is not monotonic and is indicative of their differing molecular

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structures and ash content. Direct assessment of the differences in the carbon matrix of the

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biochars, required a comprehensive analysis using quantitative

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spectroscopy.

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C solid-state NMR

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Molecular structure of the biochar. The combination of the TEM and the elemental

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analysis confirm the uptake and dispersion of mineral phases by the biomass from the

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slurries. The molecular structure analysis of the biochar was conducted with quantitative

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solid-state 13C DPMAS NMR spectra, which are shown in Figure 2. Immediately evident

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in these spectra of the biochar are signals from cellulose (105 ppm, 90 ppm, 75 ppm and 65

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ppm), hemicellulose (with additional peaks at 20.5 and 172 ppm) and lignin (165ppm - 110

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ppm and 55 ppm). Also evident are distinct differences in the structure of the biochar as a

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function of the iron-clay pretreatment and the pyrolysis temperatures are observed. At the

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lower pyrolysis temperature of 250 °C (Figure 2 a-c), the influence of the iron-clay is most

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significant and is seen to retard the degradation of the cellulosic component. A comparison

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of the intensity of the peak at 75 ppm (representative of the cellulose) and the -OCH3 peak

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at 55 ppm (representing the lignin) shows that the BKF250 and BBF250 chars have a

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significantly higher fraction of cellulose present, compared to the BC250 char.

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Concomitantly, there is a strong signal of alkyl species in the region of 6-50 ppm for the

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BC250 char that corresponds to volatile cellulosic degradation products (such as high

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molecular weight alkenes). Although there is only about 1 wt. % of clay in the BKF and

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BBF series of biochar, as found with the ultimate (elemental) analysis, the clay exerts a

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strong effect in suppressing the degradation of the cellulose. The degradation of biomass is

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dependent on the rate of heat / heat transfer into the biomass particle, which is driven by

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conductive, convective and to a lesser extent by radiative mechanisms38-40. For large

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particles, such as the bamboo sections used in the current work, and the relatively lower

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temperature (250 °C), convective heat transfer is expected to be an important heat transfer

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mechanism, particularly since the biomass is a poor conductor of heat. We hypothesize that

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the coating of clay on the biomass reduces the gas permeability of the ligno-cellulosic

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biomass thereby suppressing the convective heat transfer into the biomass and suppressing

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degradation. The reduced gas permeability due to clays is particularly well known for the

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case of clay-polymer nanocomposites41, where the incorporation of clay nanoparticles into

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the polymer matrix suppresses gas diffusion through the polymer due to its very large

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surface area (~ 1000 m2/g) of clay.

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In contrast to the suppression of degradation at 250 °C, at elevated pyrolysis

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temperatures (350 °C to 550 °C), the mineral species appear to have an opposite effect. At

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350 °C (Figure 2 d-f) the structure of the untreated bamboo biochar (BC350) shows a

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strong signal for aromatic carbon species (160 ppm to 100 ppm) as well as aliphatic carbon

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species (50 ppm to 6 ppm). However, while the BKF350 and BBF350 biochar also show a

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strong aromatic signal, the signal intensity of the aliphatic species is reduced compared

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with that from the BC350 biochar. This difference is interpreted as an indicator that at the

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higher temperatures, the presence of clays promotes the formation of condensed carbon

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structure and enhances the cross-linking of the volatile aliphatic fraction into condensed

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aromatic species. This hypothesis is consistent with the documented ability of clay

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minerals to catalyze condensation and polymerization reactions from a variety of substrates

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such as aromatic ketones and aldehydydes.42-44 Investigations into low temperature (LT)

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biochar (350 °C) has shown that they are particularly beneficial to sandy soils,16, 45 and the

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ability to control the structure of the LT biochar may enable continued development of

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designer biochar in this temperature regime. Our result also opens up the possibility to

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study any possible downside46 to the presence low molecular degradation products in the

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biochar, which may compete with its beneficial effects in the long term. At pyrolysis

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temperatures of 450 °C - 550 °C, the

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being centered at ~ 130 pm as see in Figure 2 (g-l). The primary species produced at these

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temperatures have an aromatic carbon structure, in addition to some phenolic and

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carboxylic species observed at ~150-160 ppm and 165-180 ppm, respectively.

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The solid state

13

13

C NMR signal is very similar in all the materials

C DPMAS NMR spectra of the biochar samples enable the

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quantitative assessment of the changes in the biochar structure due to the presence of the

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mineral phases. The normalized

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temperatures (Supplementary information Figure S3), illustrating the quantities of specific

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carbon functionalities, viz: alkyl, HCOH, aromatic and acidic functionalities, present in the

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biochar. The data indicate that the BKF and BBF biochars generally have a higher fraction

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of non-protonated aromatic carbon as compared to the total carbon, with the BBF550

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biochar having the highest concentration. This implies that the BBF and BKF series of

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biochar have a greater degree of aromatic condensation that is correlated to greater stability

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of the biochar within the soil.30-31 Additionally the BBF and BKF series also have a higher

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concentration of phenolic species, particularly at 450 °C and 550 °C.

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13

C NMR intensities are plotted against the pyrolysis

For the amount of detailed information

13

C NMR spectroscopy provides, a proper

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quantification of carbon species is important to avoid errors in estimating the influence of

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mineral additives on biochar structure. As a case in point, we offer a specific study

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(Supplementary Information Figure S6), in which the mistaken application and evaluation

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of the NMR data has led to incorrect conclusions regarding the influence of the mineral

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additives on biochar structure.

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The overall effect of the pyrolysis temperature and the presence of clay on the

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biochar is better visualized by plotting the Van Kervelen diagram (the O:C vs. H:C atomic

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ratios along the x and y axes respectively), for each pyrolysis temperature, as shown in

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Figure 3. The data for the Van Kervelen diagram is extracted directly from the 13C NMR

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data (Supplementary Information Figure S3) which provides the ratios of the different C-H,

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C-O and quaternary C species in each biochar sample. For a given biochar, we can

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determine precisely which functionality is responsible for the observed hydrogen or oxygen

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content. Figure 3 indicates that the evolution of the biochar structure is very dependent on

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the presence of the iron-clay mineral during pyrolysis and that this effect is more

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pronounced at the lower temperatures. At lower temperatures, (250 °C), the BKF and BBF

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biochars have an increased amount of oxygen and a reduced amount of hydrogen as

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compared to the biochar without clay. At the intermediate temperatures, (350 °C to 450 °C)

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the compositions of the BKF and BBF biochars deviate the most from BC biochar.

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Specifically, the presence of mineral species suppresses the removal of the oxygen

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moieties, while enhancing the removal of the hydrogen moieties. The differences in the

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Van Kervelen diagram at 350 °C and 450 °C can be deduced from the data in Figure S3,

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and assigned to the reduced amounts of alkyl CH, CH2, CH3 species and increased amount

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of acidic and phenolic species in the BBF and BKF biochars as compared to the BC

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biochar. At the highest temperature (550 °C), the different biochars yield similar molecular

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structures. This similarity can be seen as a strong indicator that the clay-mineral exerts a

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catalytic influence on the structure of the evolving biochar by lowering the free energy for

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the condensation reactions (particularly at the lower temperatures). While individual

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reaction mechanisms during pyrolysis of biomass are exceedingly complex38,

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hypothesize that since reaction rates follow an exponential function of temperature, at the

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increased temperature (550 °C), there is sufficient energy to overcome the barriers to

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promoting carbon condensation, resulting in the similar biochar structures observed.

337

Further kinetic studies need to be undertaken to confirm this hypothesis. With respect to

338

the stability of the biochar, Figure 3 indicates that the BBF550 biochar yields the most

339

condensed and therefore most stable material,12,

340

biochar having comparatively less condensed aromatic carbon, would be expected to have

341

lower stability in the soil. The differences in the BBF550 and BKF550 biochar can be

342

related to the dramatic changes in the structure of the clay (vide infra) which occurs at the

Page 11 of 25

30-31, 48

47

, we

while the BC550 and BKF550



343

higher pyrolysis temperature49.

344 345

Inclusion and nature of the iron-bearing phases in the biochar. The intensity of the 13C

346

spinning sidebands (ssb) gives an insight into the structure of the iron-bearing phases

347

(Figure 2 and Supplementary Information, Figure S4). Generally, in organic solids, the

348

ssb’s arise as a result of the Magic Angle Spinning (MAS) averaging out the chemical shift

349

anisotropy (CSA), and occur at multiples of the spinning frequency from the center-band.

350

A faster MAS speed results in smaller ssb’s which are further away from the isotropic

351

peaks. The full spinning sideband envelope for the

352

supplementary information Figure S4. At the experimental conditions of 7 Tesla magnetic

353

field strength and 12 kHz MAS, we can expect the

354

intensity first order ssb’s and no higher order ssb’s. This is indeed the case for the all the

355

biochar produced at 250 °C as well as the BC series pyrolyzed at higher temperatures

356

(Supplementary information Figures S4 a-d, g and j respectively). In contrast, the BBF

357

biochars pyrolyzed at 450 °C and 550 °C have particularly high intensity ssb’s, with fourth

358

order ssb’s being visible (Supplementary information Figure S4, i and l). Such unusually

359

high intensity of the ssb’s can be attributed to a paramagnetic induced anisotropy arising

360

from the presence of highly paramagnetic phases formed from the adsorded iron salts, in

361

close (nanometer scale) proximity to the condensed carbon structure of the biochar. It is

362

well known that changes in the structure of iron oxides, such as Fe2O3, from the bulk scale

363

(micron-sized or larger) to nano-scale results in their transition from a ferromagnetic to a

364

super-paramagnetic state.50 Indeed in Figure 1, the TEM images corroborate that there are

365

nano-scale iron-bearing phases within the porous structure of the biochar. The intensity of

366

the ssb’s in the 13C solid-state NMR spectra, particularly of the BBF at 450 °C and 550 °C

367

therefore indicate that the nano-sized iron phases are distributed fairly uniformly

368

throughout the biochar. It is important to note that while both the bentonite and kaolinite

369

clay slurries were prepared with iron sulfate, the strongest influence from the paramagnetic

370

phases is observed with the biochar derived from the iron-bentonite slurry treated biomass.

371

This implies that the specific presence of the bentonite clay acts as a catalyst for the

372

formation of super-paramagnetic nano-Fe2O3 phases at temperatures above 450 °C. Kaolin

373

undergoes a structural transformation to metakaolinite in this temperature range, as

Page 12 of 25

13

C NMR spectra is plotted in the

13

C NMR spectrum to have very low


Page 12 of 25

Page 13 of 25


374

revealed by the solid state 27Al MAS NMR spectra (Supplementary Figure S5 (e) and (f))

375

and may not be able to exert a similar catalytic effect

376 377

Porosity of the biochar. The SEM and TEM (Figure 1) images show a complex porous

378

structure of the biochar with multiple length scales. NMR cryoporometry provides the total

379

pore volume of the biochar samples as shown in Table 2, as well as the distribution of pore

380

sizes as shown in Figure 4. NMR cryoporometry has the additional and significant

381

advantage of being applicable even when there are liquids and volatile components already

382

in the pores. The untreated bamboo biochar, BC (Table 2) follows the normal pattern of

383

increased pore volume with pyrolysis temperature from 250 °C to 350 °C, while above 350

384

°C, the pore volume is nearly constant.35 The lower pore volume at 250 °C in BC biochar is

385

attributable to hydrophobic volatiles formed from the biomass degradation, condensing into

386

the pores of the forming biochar.51-52 In comparison to the BC250 biochar, the BKF250

387

and BBF250 biochars have nearly double the pore volume which is attributable to the

388

reduced concentration of hydrophobic volatiles trapped in these chars as seen from the 13C

389

NMR (Figure 2 b and c). Within the error bar of the measurement the BKF and BBF

390

samples show pore-volume and pore-size distribution similar to the corresponding BC

391

biochar at 350 °C and 450 °C. The increase in pyrolysis temperature accelerates the

392

reaction between the iron, the clay and the volatiles that are liberated and the data indicate

393

that the continued aromatic condensation reactions at higher temperatures may lead to a

394

closing of the sub-micron pores or at least to an occlusion of the access to the pore space.

395

We also note that to some degree, the reduction may also be attributable to the enhanced

396

1

397

due to the increased aromaticity in the BBF and BKF series, as compared to the BC series,

398

which may yield lower than expected values for the porosity. However, in the particular

399

case of the BKF550 biochar, there is a large reduction in the pore volume as seen in Figure

400

4 (b) which cannot be assigned to any effect other that the collapse of the mesopore

401

volume. Considering that at 550 °C the molecular structure of the carbon matrix for both

402

BBF and BKF is quite similar (as seen in Figure 2), the change in porosity for the BKF550

403

must be due to another factor. Examination of the thermally treated neat iron-sulfate-clay

404

mixtures by solid state 29Si and 27Al NMR (Supplementary information Figure S5) shows

H-T2 relaxation of the water caused by the iron phases and reduced adsorption of water

Page 13 of 25



405

that while the iron-bentonite is stable to temperatures above 550 °C, the iron-kaolinite

406

undergoes a degradation-transformation into metakaolinite at 550 °C. Thermal

407

decomposition of kaolinite in this way, can explain the low porosity of the BKF550

408

biochar, as well as the sudden change in the Van Kervelen diagram (Figure 3). Below the

409

thermal decomposition temperature of the kaolinite (i.e. at 450 °C) both bentonite and

410

kaolinite have high surface areas that we deduce results in producing biochars with similar

411

structures. However, the metakaolinite formed at 550 °C, has a low surface area and lacks

412

the layered structure of kaolinite. This reduction in porosity, specifically in the mesoporous

413

regime, can be directly attributed to the metakaolinite blocking the mesoporous structure in

414

the biochar. Concomitantly, the reduced surface area of the metakaolinite, and, hence, the

415

reduced mineral-biochar interactions, significantly reduces the influence of the clay on the

416

biochar structure. Therefore, the BKF biochar shifts to a structure very similar to that of the

417

BC biochar. In contrast to the kaolinite, the bentonite clay does not undergo a structural

418

collapse at 550 °C as suggested by the

419

influence through the high interfacial contact with the biochar. This insight is crucial since

420

it can explain results observed in literature, such as the reduced surface adsorption capacity

421

of kaolinite-bamboo biochar (600 °C) seen by Yao et al.21

422 423

Implications for the synthesis of enhanced biochar. Previous work has demonstrated

424

that properties of the biochar, such as the concentration of acidic functionalities, the degree

425

of aromatic condensation and porosity of the biochar play key roles in determining its

426

stability within the soil, its ability to enhance soil fertility and its capability for pollutant

427

sequestration6, 17. Here, we provide quantitative

428

which demonstrate that the concentration of acidic functionalities (particularly the phenolic

429

species), the degree of aromatic condensation and the pore volume can be tailored to a high

430

degree by infiltrating the biomass with high surface area clay minerals and iron salts prior

431

to pyrolysis. Such control on the structure of the biochar will allow for selecting biochar

432

for specific applications. For example, as a means for carbon sequestration, the BBF550

433

biochar that has the highest degree of aromatic condensation, would be the most suitable,

434

as a higher degree of aromatic condensation is linked to enhanced biochar stability within

435

the soil3, 31. On the other hand, to enhance the soil fertility or immobilize soil contaminants,

Page 14 of 25

29

Si and

27

Al NMR, and continues to exert its

13

C NMR and porosimetry measurements


Page 14 of 25

Page 15 of 25


436

the BKF450 and BBF450 biochar with their higher concentration of phenolic and

437

carboxylic species as well as higher porosity would be well suited. Furthermore, infiltration

438

and formation of clay and metal oxide nanostructures within the pores of the biochar will

439

promote the redox reactions that can have a significant impact on nutrient availability and

440

uptake in plants, as well as increase beneficial micro-organisms which improve the health

441

of the rhizosphere18,

442

structure is enabled by use of materials which are inexpensive and readily available across

443

a wide geographical distribution. Detailed investigations of the composition, nanostructure

444

and magnetic properties of the mineral phases will soon be forthcoming elsewhere. The

445

next step will be to demonstrate the differences in the activity of these biochar materials

446

under field conditions and correlate them to the molecular structure measured herein.

53-58

. Finally, it is important to note that the control of the biochar

447 448

ASSOCIATED CONTENT

449

Supporting Information. Text figures and tables addressing the transformation of minera

450

impregnated biochar under changing pyrolysis conditions. SEM images of mineral on

451

biochar pores. Variation of biochar functionality as function of temperature. 13C NMR

452

detection of paramagnetic mineral infiltration into biochar pores. Effect of heating on 29Si

453

and

454

kaolinite from supplier. This information is available free of charge via the Internet at

455

http://pubs.acs.org/.

27

Al NMR of bentonite and kaolinite. Technical data/specifications of bentonite and

456 457

AUTHOR INFORMATION

458

Corresponding Author

459

*Phone:+61 02 93854616; e-,mail [email protected]

460

ACKNOWLEDGMENTS

461

We acknowledge the assistance of the Electron Microscope and X-ray unit of University of

462

Newcastle and the EMU and NMR Facility of the Mark Wainwright Analytical Centre,

463

University of NSW. This work was supported by ARC grant LP120200418, Renewed

464

Carbon Pty Ltd. and the DAFF Carbon Farming Futures Filling the Research Gap

465

(RG134978). We acknowledge extensive discussions with Dr. B. Njegic for insightful

466

comments regarding the manuscript.

Page 15 of 25



467 468

References

469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511

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Page 19 of 25



650 651 652 653 654

Page 20 of 25

Table 1. Composition (weight %) of the biochar materials by ASTM D758232 and ASTM D432633. Kao refers to kaolinite and Ben refers to bentonite. Sample

Pyrolysis temp.

Minerals

BC250

250 °C

-

78

BC350

350 °C

-

BC450

450 °C

BC550

550 °C

Yield % C %

H%

N%

S % Fe %

Al %

Ca % K % Ash %

51.9

5.45

0.87

0.08 0.014 0.0016 0.050 0.83 14.38

60

71.5

4.02

1.11

0.07 0.011 0.0022 0.075

1.4

28.08

-

35

75

3.42

1.38

0.12 0.023 0.0051 0.098

1.4

19.10

-

33

79.2

2.72

1.28

0.06 0.030 0.0050 0.078 0.85 26.34

BKF250 250 °C FeSO4 +Kao.

86

48.7

5.15

0.5

1.0

2.0

0.026 0.046 0.74 15.43


59

67.6

3.3

0.87

1.1

2.1

0.042 0.054

1.0

11.40


48

62.9

2.82

1.45

1.1

2.9

0.093 0.069

1.2

35.60


38

63.6

1.72

1.39

0.52

2.3

0.16

BBF250 250 °C FeSO4 + Ben.

76

48.7

5.15

0.5

0.73

1.2

0.010 0.053 0.35 15.91


63

57.2

3.51

0.8

1.4

2.5

0.029

0.12

0.82 24.14


42

61

2.79

0.84

1.3

2.5

0.034

0.12

0.63 56.87


38

70.2

2.49

0.79

0.63

1.7

0.10

0.10

0.37 31.64

0.049 0.64 14.61

655 656 657 658

Table 2. Pore Volume as a function of temperature and clay treatment from the NMR cryo-

659

porosimetry. Error bar for the measurements is + 0.15 ml/g

660 661 Pyrolysis BC series of BKF series of BBF series of Temperature biochar biochar biochar o -1 -1 C (Pore vol ml.g ) (Pore vol ml.g ) (Pore vol ml.g-1) 250

0.36

0.73

0.88

350

1.05

0.86

0.77

450

0.77

0.56

0.73

550

1.01

0.16

0.42

662 Page 20 of 25


Page 21 of 25


663 664 665

666 667 668

Figure 1. SEM images of BKF550 biochar (a), BKF450 biochar (b), BBC550 biochar (c)

669

and BBC450 biochar (d). The TEM image of a macropore of BKF550 (e) shows there are a

670

range of minerals (dark phases) both in the mesopores of the amorphous carbon lattice and

671

on the surface. TEM image of the nanostructured mineral decorating in the inner pore

672

surface (f). TEM-EDS spectra of the nanostructured mineral identifying it as a Fe-rich

Page 21 of 25



673

mineral (g).

674 675

Figure 2. Quantitative 13C DP MAS NMR spectra of the BC biochar series (a, d, g, and j),

676

the BKF biochar series (b, e, h, and k) and the BBF biochar series (c, f, i, and l). The

677

spectra of the chars are quantitatively measured by 13C DPMAS NMR. The spectra plotted

678

in bold lines represent the total 13C NMR signal while the spectra plotted in thin line were

679

acquired after 67 µs of recoupled dipolar dephasing and represent the signal of the non-

680

protonated carbon species. The difference between the bold and narrow spectra represents

681

the amount of protonated carbon species. * represents the spinning sidebands

682

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Page 23 of 25


683 684 685

Figure 3. Van Kervelen diagram (H:C vs. O:C atomic ratios) of the BC biochar series

686

(squares), the BKF biochar series (circles) and the BBF biochar series (triangles). The

687

atomic ratios are derived from the quantitative analysis of the functional groups in the

688

biochars plotted in Supplementary information Figure S2. The dashed lines are guides to

689

the eye.

690 691 692 693 694 695 696 697 698 699

Page 23 of 25



700 701 702

Figure 4. Porosity as a function of pore diameter for BC biochar (a), BKF biochar (b) and

703

BBF biochar (c) as measured by NMR cryo-porosimetry. The curves are for biochars

704

pyrolyzed at 250 °C (thin line), 350 °C thin dashed line, 450 °C bold line and 550 °C (bold

705

dashed line).

706 707

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708 709 710


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