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Journal of Analytical and Applied Pyrolysis 108 (2014) 26–34

Contents lists available at ScienceDirect

Journal of Analytical and Applied Pyrolysis

journa l h om epage: ww w.elsev ier .com/ locate / jaap

haracterization of an enriched biochar

hee H. Chiaa,∗, Bhupinder Pal Singhb, Stephen Josepha, Ellen R. Graberc, Paul Munroea

School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, AustraliaNSW Department of Primary Industries, Elizabeth Macarthur Agricultural Institute, Woodbridge Road, Menangle, NSW 2568, AustraliaInstitute of Soil, Water and Environmental Sciences, The Volcani Center, Agricultural Research Organization, POB 6, Bet Dagan 50250, Israel

r t i c l e i n f o

rticle history:eceived 5 September 2013ccepted 29 May 2014vailable online 5 June 2014

eywords:iochar

a b s t r a c t

Carbonized materials are responsible for maintaining a high level of fertility and soil organic matter in soilssuch as the Amazonian Dark Earths, also known as Terra Preta. It is hypothesized that an enriched biochar,which will have long term stability similar to Terra Preta, can be synthesized by mixing biochars withmanures, minerals and clays and heating the mixture at low temperatures. This treatment will promotebonding between the mineral and the organic phases, which may occur naturally after several years ofaging in soil. This paper describes the characterization of an enriched biochar by a range of analytical

haracterizationtabilityicrostructure

methods. Examination of the enriched biochar showed that it has high concentrations of exchangeablecations, available phosphorus and high acid neutralizing capacity. Structural analysis of the enrichedbiochar reveals a microstructure that suggests that bonding has indeed occurred between the biocharand mineral phases. Using natural 13C abundance and a two-pool exponential model, the half-life ofenriched biochar-C was estimated to be 104 years in a clayey soil.

. Introduction

Biochar is a carbon-rich solid material produced by heatingiomass in an oxygen-limited environment. It may be added tooils where, potentially, it can act as a means to sequester carbonC) and to maintain or improve soil and agronomic functions [1,2].iochar can form a highly stable pool of C, promote plant growthnd potentially mitigate greenhouse gas emissions from soil [3–5].esearch has shown that carbonized materials are responsible foraintaining a high level of soil organic matter (SOM) in ancient

oils such as Amazonian Dark Earths (ADE), which is likely due tots high stability in soil [6]. Recent studies using 11 different typesf biochars (Eucalyptus saligna wood and leaves, papermill sludge,oultry litter, cow manure, etc.) pyrolyzed at 2 different temper-tures has shown that the estimated mean residence time (MRT)f C in biochars varied between 90 and 1600 years [7]. However,ven though it is apparent that application of biochars to soil willncrease SOM over a long period of time, little is known about theeactions that takes place between biochars and soil.

Cheng and Lehmann [8] showed an increase in oxygenated func-

ional groups, acidity and negative charge at the surface of oakiochar particles aged for 12 months in a controlled aerobic incu-ation experiment. These findings are supported by results from

∗ Corresponding author. Tel.: +61 2 9385 4435; fax: +61 2 9385 6400.E-mail address: c.chia@unsw.edu.au (C.H. Chia).

ttp://dx.doi.org/10.1016/j.jaap.2014.05.021165-2370/© 2014 Elsevier B.V. All rights reserved.

© 2014 Elsevier B.V. All rights reserved.

Joseph et al. [9], who examined aged biochar particles extractedfrom field trials, and reported an increase in negative surface chargeand oxygenated functional groups as compared with freshly pro-duced biochar particles. It has been suggested that changes in thesurface properties of biochars may promote the aggregation oforgano-mineral complexes at biochar surfaces [9]. This is supportedby research into ADE, where it was deduced that slow oxidation atthe edges of the aromatic backbone of black C-generated carboxylicgroups, resulted in increased cation exchange capacity (CEC) andthe formation of organo-mineral complexes [10].

Studies of the structure and chemistry of ADE have revealedthat these soils are composed of micro-aggregates that may havebeen formed by the interaction of thermally treated organic mat-ter, charcoal and ash from fires, residual fired clay, and fragmentsof bones [11–13]. Liang et al. [14] found that ADE has a higherwater holding capacity (WHC), higher CEC and higher fertility com-pared to adjacent soils. Chia et al. [15] demonstrated, throughanalytical electron microscopy, that ADE particles consisted of amixture of black C (possibly arising from the breakdown of biochar),clays and other minerals, including titanium dioxide, manganeseoxides, iron hydroxides, calcium phosphate and calcium carbon-ate. Dünisch et al. [16] showed that by mixing charcoal with ashesor by impregnating wood residues with nutrients such as N, P,

and K, slow-release N- and K-fertilizers can be produced. It washypothesized that an organo-mineral complex that may have simi-lar properties to ADE could be produced by mixing manures/sludge,woody materials, clays, and other minerals, and heating these at

C.H. Chia et al. / Journal of Analytical and A

Table 1Raw materials used to synthesize the enriched biochar.

Component % dw

Air dried kaolinitic clay (sourced from Geraldton brick works,Western Australia)

36.0

Chicken manure 23.0A. saligna biochar (produced at 380 ◦C for 6 h and reacted with

phosphoric acid)30.0

Calcium carbonate, rock phosphate, ilmenite, manganese sulfate.Exact formulation is confidential

11.0

ltepgw

opeohfitttteo1issbwpfio

sfiouop

2

puAwrrtgsmac

Total 100

ow temperatures (up to 240 ◦C) [17]. It was further hypothesizedhat the reactions between the manure, woody materials and min-rals during heating would result in the formation of redox-activehases that would have variable charge, along with a range of oxy-enated labile organic C and N-containing compounds, some ofhich would be intercalated into the mineral phases [18].

In a further step, this study introduces a biochar-enrichedrgano-mineral complex (enriched biochar, EB). The EB wasroduced by coating biochar particles in clay, manure, and min-rals and then heating at 200–240 ◦C. Adding biochar to thergano-mineral complex, described in an earlier study [17], wasypothesized not only to increase the aromatic C content of thenal product, but also to increase the content of oxygenated func-ional groups and Lewis acid and base sites that may adsorb more ofhe organic matter from the breakdown of the manure. As a result,he physical and chemical structures of EB may be much closero that described for ADE by Liang et al. [14]. It was also hypoth-sized that the EB would have a longer half-life than that of thergano-mineral complex lacking biochar. A half-life greater than00 years is an important quality for biochar, and the high stabil-

ty of EB will ensure sustained translation of its beneficial effects tooil especially through the evolving cation retention property on EBurfaces. An earlier study performed by Lin et al. [19] showed thaty torrefying a Jarrah-based biochar (produced at 600 ◦C) togetherith chicken manure, clay and minerals, a higher concentration oflant available P was found with minimal N lost from the originaleedstock after torrefaction. The clay, minerals and manure werencorporated into the biochar structure and a higher concentrationf dissolved organic carbon was found [20].

The objectives of this paper are to present the methods used toynthesize EB, characterize the chemical, morphological and sur-ace properties of EB, and estimate its C stability in soil through anncubation experiment. This paper also explains the interaction thatccurs when reacting biochars with clay and minerals, which sim-lates the reaction that occurs between biochar and soils. Detailsf the results of field trials with this EB are reported in a separateaper [21].

. Materials and methods

The EB was manufactured by Anthroterra Pty Ltd using itsatented formulation (PCTI/AU2010/000534). The raw materialssed to produce the EB are listed in Table 1. Biochar produced fromcacia saligna wood at 380 ◦C in a batch reactor was pre-treatedith 10% phosphoric acid solution (at a 1:1 solution: biochar

atio), drained and then held for 24 h prior to mixing with theemaining raw materials. The purpose of the acid treatment waso oxidize the surface, to enhance the stability of the carbonylroups and to promote the loss of hydrogen from the biochar’s

urface [22]. Six kilograms of rainwater was added to a 20 kgixture of raw materials to ensure that the materials coagulate,

fter which the mixture was homogenized using a commercialement mixer. The resulting slurry was dried in 25 mm deep trays

pplied Pyrolysis 108 (2014) 26–34 27

at 80 ◦C for 24 h in a ventilated oven before being mechanicallybroken into <15 mm fragments with a garden mulcher. The mix-ture was then placed into a rotating torrefaction kiln and heatedat 5–7 ◦C min−1 up to 220 ◦C. The oxygen content in the torrefac-tion chamber was maintained at approximately 4% by purging withthe exhaust gas from a gas-powered electricity generator (Honda;5 kW). The materials were heated for 1hr at 220 ◦C and the finalproduct was cooled under atmospheric conditions to room tem-perature.

Ultimate, proximate, and ash analyses were performed byBureau Veritas International Trade Pty Ltd in Sydney, NSW,Australia according to the relevant Australian standards (AS1038.3,AS1038.6.1 and AS1038.6.2) respectively. Agronomic analyses(CEC, available phosphorus, extractable nitrate (NO3

−) andextractable ammonium (NH4

+) concentrations) were performedby New South Wales Industry and Investment, Wollongbar,NSW, Australia, using methods described by van Zwieten et al.[4]. X-ray photoelectron spectroscopy (XPS) analysis was per-formed on a Thermo Scientific ESCALAB250Xi using a 500micron diameter beam of monochromatic Al K� radiation (photonenergy = 1486.6 eV) at a pass energy of 20 eV. The core level bindingenergies (BEs) were aligned with respect to the C1s BE of 285.0 eV.

Low molecular weight organic molecules in the A. salignabiochar and EB were extracted by boiling 5 g of sample for1 h in 40 mL of solvent using an automated Soxhlet apparatus(VELP, Italy), rinsed for 1 h in the condensed solvent vapors, andthen reduced to 10 mL. The solvent system was dichloromethane(DCM):MeOH 95:5 (v/v), following Graber et al. [23]. A portion ofthe extract was evaporated to dryness and derivatized using 0.5 mLof N,O-Bis(trimethylsilyl)trifluoroacetamide (BSTFA) and 1 mL ace-tonitrile for 15 min at 60 ◦C. A blank (no biochar) was carriedthrough the whole process. Analysis was performed by GC/MS(Agilent, Santa Clara, USA; model number 6890N/5973N) in scanmode using temperature programming (oven 60 ◦C, initial hold5 min, ramp to 275 ◦C at 5 ◦C min−1, final hold 2 min; inlet 250 ◦C;split ratio 2; transfer line 280 ◦C) and a 30 m long capillary col-umn with a (5%-phenyl)-methylpolysiloxane phase, 0.25 mm innerdiameter, and 0.25 �m film thickness. Total ion chromatograms(TIC) were compared with the NIST08 mass spectral library (Fig. 1,Table 5).

An incubation experiment, where the total amount of addedEB-C that was mineralized and the mean residence time of theEB-C in a clayey soil (Vertisol) were estimated, was performedusing the methods described by Singh et al. [7]. Briefly, air-driedsoil (equivalent to 612 g, oven dry; <2 mm sieved), adjusted to∼67% of water holding capacity using a nutrient solution containingNH4NO3 (50 mg N kg−1 dry soil), and inoculated with a microbialinoculum, was placed in plastic jars. The microbial inoculum wasprepared by mixing soils collected from native eucalyptus forests,pine plantation, maize cropping and grazed pastures, to introducea diverse range of microbial communities to the soil [7,24]. Eachsoil jar was inoculated with ∼1 g of microbial inoculums [7]. Thesejars were then placed in sealed 5 L buckets and preincubated for 12days in the dark at 22 ± 1 ◦C. After pre-incubation, a nutrient solu-tion (6 mL) that contained (kg−1 soil) ca. 500 mg N, 80 mg P, 200 mgK, 30 mg S, and trace elements was uniformly mixed with the soil[6]. The EB (<2 mm sieved; ı13C −24.0‰; 29.1%C; pH 6.3) was mixedat 0.82% (wt/wt) with the soil containing C4 organic matter (ı13C−14.2‰; 0.42%C; pH 8.2) [7], and adjusted to the bulk density of1.2 g cm−3. A control soil (without the EB) was also included. TheEB-soil mixture and the control soil (n = 3) were then incubated at22 ◦C for 630 days. The CO2 evolved from the soil was trapped in

30 ml of 2 M NaOH. The trap was periodically removed at 3, 9, 20,126, 192, 287, 364, 462 and 630 days and analyzed for total CO2-Cand associated ı13C [7]. The proportion of added EB-C in the totalCO2-C evolved was determined by a two-pool 13C isotopic mixing

28 C.H. Chia et al. / Journal of Analytical and Applied Pyrolysis 108 (2014) 26–34

F blank

r this a

me

twAmAamto2dutwntpmt

3

3

itm

ig. 1. Overlaid TIC chromatograms of extracts from A. saligna (blue), EB (red), and

eferences to color in this figure legend, the reader is referred to the web version of

odel; the mean residence time was estimated using a two-poolxponential model [7].

Scanning electron microscope (SEM) analysis of the microstruc-ure of the EB was performed using a Hitachi S3400 SEM fittedith an X-ray energy dispersive spectrometry (EDS) detector.nalysis of approximately ten EB particles was carried out to deter-ine the range of particle types and their nominal composition.pproximately fifteen EB particles (based on size and physical char-cteristics) were then mounted in epoxy resin and polished usingethods described by Chia et al. [17]. Electron transparent sec-

ions (100 nm thick and ∼20 �m2) were prepared at the boundaryf a C-rich and a mineral-rich phase using an xT Nova NanoLab00 DualBeam focused ion beam (FIB) microscope using methodsescribed elsewhere [25]. The thin sections were then examinedsing a Philips CM 200 transmission electron microscope (TEM)o which an EDS detector was attached. More than ten particlesere examined using both SEM and TEM due to the heterogeneousature of the EB particles. As noted above, a large number of par-icles were examined. The observations made across the range ofarticles examined were found to be broadly consistent and theicrographs shown in this paper (Figs. 1–3) are representative of

he specimens examined.

. Results and discussion

.1. Main chemical features of EB

Proximate analysis of the EB showed that the volatile contents high, which is indicative of a large labile C component both inhe low temperature A. saligna biochar and the torrefied chicken

anure (Table 2). The relatively low C content of the A. saligna

(black). Putative peak identifications are given in Table 5. (For interpretation of therticle.)

biochar reflects the low temperature at which it was produced(∼380 ◦C) [26]. The A. saligna biochar reacted with phosphoric acidreduced the C content relative to the O content, which might sug-gest that either the surface of the A. saligna biochar was oxidized orthat some of the P precipitated to form phosphate particles. The Ncontent increased upon phosphoric acid treatment and this couldbe due to either the loss in C or due to local heterogeneity of thebiochar samples. The low C content of EB, as given by the ultimateanalysis, reflects the addition of minerals to both the biochar andthe torrefied manure.

The XPS analysis is in good agreement with the ultimate analy-sis, where the highest relative C concentration was found in the A.saligna biochar, followed by the oxidized biochar and EB (Table 3).As the XPS analysis provides a quantitative measure of the differentelements found on the surface, the presence of minerals, clays andphosphate particles on the surface will lower the total amount of Cfound on the surface of EB. The oxygen concentration was higher inEB, followed by the oxidized biochar and the A. saligna biochar. Thiscould be due to the mineral layer and the phosphate particles thatwere reacted and deposited on the surface. More oxygenated func-tional groups were also found on the surface of EB and the oxidizedbiochar.

The CEC of the fresh EB is 40.67 cmols(+) kg−1 (Table 4), whichis relatively high compared with that of wood biochar (approx-imately 10 cmols(+) kg−1) and high mineral ash biochars, suchas chicken manure (approximately 17–30 cmols(+) kg−1) [4,27].The CEC is similar to that reported for corn straw biochar bySilber et al. [28], but lower compared to enhanced biochar

(50–52 cmols(+) kg−1,torrefied from a Jarrah biochar produced at600 ◦C) [19]. Available P and extractable NH4

+ concentration aresignificantly higher compared to the greenwaste, biosolids andpaper sludge biochars [4].

C.H. Chia et al. / Journal of Analytical and Applied Pyrolysis 108 (2014) 26–34 29

F typics bioch

iwwEalafpaect

TP

ig. 2. Backscattered electron image acquired from a polished cross-section of aurrounding the biochar particle. (b) High magnification image of the clay/mineral–

Qualitative analysis of low molecular weight organic moleculesn both the biochar and EB showed that a wide range of chemicals

hich can be putatively identified to a reasonable level of certaintyere present in the organic extracts from both the A. saligna and

B (Fig. 1, Table 5). Substituted phenols, aromatic hydrocarbons,nd organic acids were detected in both extracts, with a mucharger proportion of medium and long chain n-alkanoic acids andlkanes found in the EB extract. Some of the chemicals extractedrom EB are identical to those found in burned leaf and needle sam-les of Pinus pinaster, Cistus monspelliensis, Arbutus unedo and Erica

rborea [29], and some are the same as found by Graber et al. [23] inxtracts of citrus wood biochar. Graber et al. [30] found that organicompounds in extracts of biochars can be redox active, and notedhat redox active organic molecules could play a role in abiotic

able 2roximate and ultimate analyses of EB, untreated A. saligna biochar and oxidized A. salign

Enriched biochar A. saligna bioch

Proximate analysis, % db% Ash 54.3 4.5

% Volatile matter 31.6 30.7

% Fixed Carbon 12.5 63.8

Ultimate analysis, % db% Carbon 26.9 75.0

% Hydrogen 2.3 3.6

% Nitrogen 1.2 0.7

% Oxygen 69.6 20.7

H/C 0.09 0.05

O/C 2.6 0.3

C/N 22.4 107.1

al EB particle. (a) Low magnification image showing a thin layer of clay/mineralar interface. EDS spectra of the mineral matrix (arrowed region) is shown in (c).

formation of humic structures in soil, in solubilizing Mn and Fe, inmicrobial electron shuttling between bacterial cells and Fe-bearingminerals, in scavenging radicals, and in contaminant immobiliza-tion. It was also suggested that such molecules could play a role inimproving germination, stimulating shifts in microbial populations,and in inducing systemic plant defenses against stress [23,31].Feng et al. [32] found that acetic acid, hexadecanoic acid and 2-methoxy-4-vinylphenol extracted from pine needles reduced thegrowth of pathogenic bacteria (e.g. Staphylococcus aureus, Bacilluscereus, Micrococcus luteus, Escherichia coli). Pizzeghello et al. [33]

have noted that phenolics in soil can act as signal molecules toeither facilitate or discourage interactions with other organismsand aliphatic acids can play a hormonal role. Furthermore, thesevolatiles, when added through EB to soil, could potentially enhance

a biochar (%db, dry basis) as provided by Bureau Veritas (NSW, Australia).

ar-untreated A. saligna biochar-reacted with phosphoric acid

4.022.173.9

68.42.61.8

27.20.040.4

38.0

30 C.H. Chia et al. / Journal of Analytical and Applied Pyrolysis 108 (2014) 26–34

Table 3XPS results for EB, untreated A. saligna biochar and oxidized A. saligna biochar, showing atomic% for all elements detected as well as the relative atomic % for the C1s peakcorresponding to different organic functional groups.

Transition Abundance (atom %) Relative Carbon 1s Distribution (atom %)

Enrichedbiochar

A. salignabiochar-untreated

A. saligna biochar-reactedwith phosphoric acid

Enrichedbiochar

A. salignabiochar-untreated

A. saligna biochar-reactedwith phosphoric acid

Al2pA

5.0 ± 0.5

C1sC C/C H 28.2 ± 2.8 74.7 ± 7.5 63.3 ± 6.3 70.5 84.1 78.2C O groups 7.8 ± 0.8 10.5 ± 1.1 11.7 ± 1.2 19.5 11.8 14.4C O groups 2.7 ± 1.3 2.2 ± 1.1 3.8 ± 1.9 6.8 2.5 4.7O C O groups 1.2 ± 0.6 1.4 ± 0.7 2.2 ± 1.1 3.1 1.6 2.7

Ca2s 0.9Fe2p3 0.5Mg1s 0.6 0.3N1s 2.4 ± 1.2 1.3 ± 0.6 0.8Na1s 0.4 0.4O1s 43.6 ± 4.4 9.1 ± 0.9 16.7 ± 1.7Si2p 10.8 ± 1.0 0.5Mn2p 0.4K2s 0.7S2p 0.4 0.1P2p 0.7 0.2 1.1

Table 4Chemical analysis of enriched biochar, untreated A. saligna biochar and oxidized A. saligna biochar.

Enriched biochar A. saligna biochar – untreated A. saligna biochar – reacted with phosphoric acid

pH (CaCl2) 7.4 8.81 2.45P (mg kg−1) 28,300 519 26,800NH4

+-N (mg L−1) 3.8 2.8 5.6NO3

−-N (mg L−1) 0.2 0.0 0.1EC (mS cm−1) 4.13 1.05 5.73Cation exchange capacity at pH 7cmols(+) kg−1 40.67 7.67 8.14

Table 5Putative identifications of compounds in the A. saligna and EB biochars.

Peak # RT (min) Name Saligna EB

1 9.244 Pyridine, 2,4,6-trimethyl- = =2 9.926 Trimethyl[4-(1,1,3,3,-tetramethylbutyl)]phenol = =3 11.422 2-hydroxypropanoic acid = =4 11.925 Hexanoic acid

√5 16.017 Dodecane

√6 16.732 2-Methoxyphenol

√7 17.271 Urea

√8 18.084 Glycerol

√9 19.348 2-Methoxy-5-methylphenol

√10 21.667 Tetradecane

√11 25.046 2,6-Ditert-butylphenol

√12 26.598 Hexadecane > <13 28.265 *Diisopropylnaphthalene isomer = =14 28.324 *Anhydro sugar

√15 28.433 *Diisopropylnaphthalene isomer = =16 28.562 Butylated hydroxytoluene > <17 28.865 Benzaldehyde, 3,5-dimethoxy-4-hydroxy-

√18 29.321 *Diisopropylnaphthalene isomer = =19 29.426 *Diisopropylnaphthalene isomer = =20 29.537 *Diisopropylnaphthalene isomer = =21 31.021 Octadecane > <22 31.708 2,4-Diphenyl-4-methyl-2(E)-pentene > <23 32.312 1,2-Benzenedicarboxylic acid, butyl octyl ester > <24 33.581 Hexadecanoic acid, methyl ester

√25 35.019 Eicosane > <26 35.837 Hexadecanoic acid

√27 39.362 Octadecanoic acid

√28 44.871 Hexadecanoic acid, 2,3-bishydroxypropyl ester > <29 47.652 Octadecanoic acid, 2,3-bishydroxypropyl ester > <30 49.545 Heptacosane

√

Symbol explanations: =, both extracts have approximately the same content of the compound;√

, only one of the extracts has the compound; > and <, both extracts have thecompound, but the extract labeled with > sign has substantially greater content than the extract with the < sign; *, indicates isomer.

C.H. Chia et al. / Journal of Analytical and A

Fa

map

3

ttMtpfrbtatsoaf

iiaoFtmo(tatwill

The combination of organic molecules and inorganic minerals

ig. 3. Bright field TEM image of an EB thin section milled at the interface between biochar particle and the surrounding clay-mineral aggregate.

ineralization of native soil organic via positive priming [34–36]nd consequently release nutrients such as mineral N for use bylants and microbes [37].

.2. Characterization of the structure of EB

A low magnification backscattered electron image revealed ahin layer (10–20 �m) of clay/mineral surrounding a biochar par-icle (Fig. 2a), which is similar to those reported by Lin et al. [19].

uch of the internal area of the biochar has pores with diame-ers in the range from 5 �m to 20 �m. These were similar to theores described by Ogawa et al. [38] as being the most suitableor microbial growth. These pores are also large enough for theoot hairs to penetrate [9]. A higher magnification image of theiochar–mineral interface is shown in Fig. 2b. The EDS spectraaken from the mineral matrix (Fig. 2c) showed a Si and O-richggregate, with a lower concentration of K, Al and Fe. This suggestshat the main components of the surrounding mineral matrix wereilica-based particles (labeled A in Fig. 2b), with a small amountf K-enriched clay and some Fe-rich particles scattered randomlycross the mineral matrix. The Cr peak observed at ∼5.5 keV arisesrom the conductive coating applied prior to analysis.

TEM analysis, coupled with EDS mapping, showed the complex-ty of this material at higher resolution. Fig. 3 shows a bright fieldmage of the EB showing the interface between a biochar particlend the surrounding clay aggregate. It should be noted that manyf the phases had dimensions of less than 100 nm and, in particular,e-rich phases had dimensions of less than 10 nm, consistent withhe findings of Lin et al. [19]. An amorphous C-rich phase (as deter-

ined by electron diffraction), which contains a low concentrationf K, is labeled A. The area analyzed contained both mesopores2–50 nm) and macropores (>50 nm) (labeled B in Fig. 3). Most ofhe macropores were found at the interface between the organicnd inorganic phases, whereas the mesopores were located withinhe amorphous C phase. The pore size in the amorphous C phaseas less than 30 nm. A mixed mineral phase, that was rich in Fe,

s labeled C and clay particles that have a pseudo-crystalline tubu-ar structure consistent with being crystalline (deduced from theattice fringes) are labeled D (Fig. 3).

pplied Pyrolysis 108 (2014) 26–34 31

A series of elemental X-ray maps of a biochar particle and itsadjacent phases acquired using scanning transmission electronmicroscopy (STEM) is shown in Fig. 4. These maps revealed thedistribution of mineral particles around a C-enriched region. Threedistinct mineral-rich phases were detected. The first phase (labeledA) was rich in Al, Si and O and tubular in structure, which suggeststhat it consisted mainly of clay particles [39]. The second min-eral phase (labeled B) was rich in Fe, P, and Mn. Part of this phasewas incorporated into the aluminosilicate phase, which indicatedthat this substitution was due to the metallic elements found inthe minerals used in the production of EB. The metal ions (espe-cially Fe) have replaced some of the Al or Si in the aluminosilicatelattice. There also appeared to be a thin layer rich in Fe and Mnfound at the interface between the organic and inorganic phases,and it is hypothesized that these elements played a major role inthe reactions that occurred between these two phases via redoxinteractions. Similar phase structures have been observed in ADEparticles [15].

3.3. An incubation study to estimate mean residence time of EBcarbon in soil

The rate of EB-C mineralization (expressed in mg CO2-C g−1 EB-C d−1) decreased rapidly from 2.26 on day 3, to 0.09 in 2 monthsand to 0.001–0.008 over 1–1.7 years (Fig. 5). This pattern and rateof mineralization are consistent with those of lower-temperaturemanure-based biochars by Singh et al. [7]. As the EB containedsome carbonate C (0.67%C), determined using a titrimetric method[40], some CO2-C would have been evolved through an abioticmechanism i.e. dissolution of carbonates in the EB [7,41]. As thecarbonate C contribution to the total EB derived CO2 was not dis-counted before estimating the MRT, this is likely to provide anunderestimated value of MRT of EB, which should be based onthermochemically altered forms of organic C in the EB and otherbiochars [42]. On a cumulative basis, 2.98% of the added EB-Cwas mineralized over 630 days, which is lower compared to ash-rich manure and paper mill sludge biochars produced at 400 ◦C,but greater compared to un-enhanced wood biochars produced at400 ◦C [7]. Correspondingly, the MRT of EB-C, which is estimatedto be 151 years (Table 6), is longer than the MRT of the manureand paper mill sludge biochars, but smaller compared to the MRTof wood biochars [7]. The above results indicate that although EB-C mineralizes, at first, rapidly, and may increase native organic Cmineralization via positive priming (data not shown) caused bylabile volatile organic matters and other organic substrates in theEB [23,34–36], the mineralization rate of EB-C stabilizes quickly aswell, possibly due to EB induced formation of organo-mineral asso-ciations in the soil [24,36,43,44]. Thus, EB may serve as a suitableamendment both for increased nutrient availability (directly, andpossibly indirectly via enhanced native organic matter mineraliza-tion) and through relatively long-term sequestration of organic Cin soil.

The EB had two distinct mineral phases along with the organicphase structures which could exhibit very similar internal bond-ing to organo-mineral complexes in soils, as described by Kleberet al. [45]. Lima and Marshall [46] found that the presence of acidicfunctional groups on the surface of biochars allows the absorptionof cations into the C lattice. Gaseous organic molecules formedduring the thermal degradation of manure could react with thevariably charged surfaces of the minerals, the functional groups onthe clay surfaces or with the functional groups on the surface of thephosphoric acid-treated biochar [46].

used to form EB micro-aggregates is similar to the microstructuresfound in aged biochars and ADE [15]. EB exhibited a range of poresizes at the interface of the organic and inorganic phases. The

32 C.H. Chia et al. / Journal of Analytical and Applied Pyrolysis 108 (2014) 26–34

F trix. TM

fptwt

TMa

ig. 4. Elemental EDS maps showing a C-rich particle adjacent to a mineral-rich man rich phase (B).

ormation of these pores probably occurred during the heating

hase when the manure started to break down during torrefac-ion to produce CO2, oxygenated organic molecules and H2O andhen redox reactions occurred between the mineral phases and

he C in the biochar. These additional pores increase the overall

able 6ean residence time and half-life of enriched biochar carbon in Vertisol estimated using

dded EB-C mineralized over 630 days.

Mean residence time

Labile (days) Resistant (yea

EB-C 11 151

wo distinct mineral phases could be detected: a Al/Si rich phase (A) and a Fe, P, and

soil surface area when EB is added as a soil amendment, which will

promote microbial activity and population, increase water hold-ing capacity in sandy soil or increase aeration in clayey soil. Thenano-scale organic and mineral phases within EB particles hadvery different chemical and structural properties that could lead to

a two-pool (labile and recalcitrant) exponential model fitted to the cumulative % of

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[31] Y. Elad, E. Cytryn, Y.M. Harel, B. Lew, E.R. Graber, The biochar effect: plant

ig. 5. Enriched biochar carbon mineralization rate (mg CO2-C g EB-C d ) andumulative % of added EB-C mineralized at 22 ◦C over 630 days in Vertisol. Errorars represent ± standard errors of the mean (n = 3).

omplex interactions between solid and liquid soil organic matter,icro-organisms, cations and anions in soil water [47]. Gilbert and

anfield [47] postulate that the free energy for reactions betweenoots/root hairs, minerals, soil organic matter and micro-organismss reduced at both the interfaces and on the surface of the nanopar-icles. This will result in increased rates of mineralization of theative soil organic matter which makes the mineral more easilyccessible and hence lead to greater nutrient uptake in plants.

The EB has a high P content and a high CEC compared to mostiochars. However, the P content of the EB is lower compared tohicken litter biochar [4]. Furthermore, the EB has a higher mineralontent and a lower mineralization rate than chicken litter biocharFig. 4) [7], which means that it can potentially remain longer in soilnd may continue to provide benefits to soil through long-term Cequestration, supplying nutrients, minerals and micro-aggregatetructures, as well as through the development of negative chargeuring aging of EB in soil. The higher mineral content also suggestshat the EB may be better at promoting plant growth compared toormal non-enriched biochars.

. Conclusions

An enriched biochar was synthesized by mixing biochars withanures, minerals and clays and heating the mixture at 220 ◦C.

hemical analysis of the enriched biochar showed that it has highoncentrations of exchangeable cations, available phosphorus andigh acid neutralizing capacity. Microstructural analysis reveals aorous biochar structure being surrounded by layers of mineral,orming organo-mineral complexes. Incubation studies shows thathe EB has a mean residence time of ∼150 years, and may serve as

suitable soil amendment both for increased nutrient availabilitynd through longer term sequestration of biomass C in soil.

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