Interaction between biochar and soil minerals --

revealed by electron microscopy

and X-ray photoelectron spectrometry

Yun Lin*, Paul Munroe, Stephen Joseph

School of Material Science and Engineering, University of NSW, NSW 2052 Australia

Lukas van Zweiten , Stephen Kimber

Wollongbar Primary Industries Institute, Industry and Investment NSW,

1243 Bruxner Highway, Wollongbar NSW 2477

The University of New South Wales

Outline

Introduction

Materials and Experimental Methods

Results and Discussion

�XPS examination of biochar surface

�SEM observation and EDS analysis on biochar surface

�Cross-sectional SEM and TEM observation at the interface

between biochar and mineral phases

Conclusion

Acknowledgements

Application of biochar as soil amendment:

Detailed studies of ancient Amazonian Terra Petra soils have revealed that anthropogenic original

black carbon with a high aromatic content has been stabilized, in part, due to interactions with

minerals, micro-organisms and soil organic matter (Brodowski et al. 2005; Liang et al. 2010).

The adsorption of soil materials to black carbon protects black carbon from oxidation and

decomposition (Nguyen et al. 2008). This implies the mineral attachment to biochar is important in the

stabilization process. Interactions and reactions happened immediately on application of biochar to

soil, especially at its surface.

The biochar surface properties play an important role in these interactions and reactions, and the

fresh biochar surface properties are controlled by the pyrolysis conditions applied and feed stocks

used.

Investigations on the mechanisms of mineral incorporation with biochar would therefore be useful to

reveal the interactions / aggregation of biochar and soil materials, in particular factors of soil affecting

biochar stability.

Introduction

Brodowskia S, Amelung W, Haumaier L, Abetz C, Zech W (2005) Morphological and chemical properties of black carbon in physical soil fractions as revealed by

scanning electron microscopy and energy-dispersive X-ray spectroscopy. Geoderma 128: 116-129

Liang B, Lehmann J, Sohi SP, Thies JE, O’Niell B, Trujillo L, Gaunt J, Solomon D, Grossman J, Neves EG, Luizão FJ (2010) Black carbon affects the cycling of non-

black carbon in soil. Organic Geochemistry 41: 206-213

Nguyen BT, Lehmann J, Kinyangi J, Smernik R, Riha SJ, Engelhard MH (2008) Long-term black carbon dynamics in cultivated soil, Biogeochemistry 89:295–308

Introduction

Techniques used:

X-ray photoelectron spectroscopy (XPS) test is an effective technique to provide the quality and

quantity information of surface chemical composition due to its capability to detect the

elements on surface less than 10nm depth , importantly for analysis of carbon-rich materials

such as carbon bonding states on biochar surface.

Scanning electron microscopy (SEM) provides data on surface morphology and, together with

energy dispersive spectrometry (EDS), can be used to determine the chemical composition of

any mineral particles attached.

Cross-sectional transmission electron microscopy (TEM) is able to visualize the structure of soil

aggregates and provides a means to analyze the mineral phases present at a nano-scale. Dual

beam Focus Ion Beam (FIB) was used to prepare TEM specimen.

Aim of the study:

To investigate both the biochar surface oxidation and the degree of interaction between the

biochar and clay/minerals, either in agronomic environment or under elevated temperature

Materials and Experimental Methods

The chicken litter (CL) biochar was pyrolysed with a highest temperature of treatment (HTT) of 450°C and held at this

temperature for 30 min using BEST Energies continuous slow pyrolysis process in the near absence of air. A small amount of

air comes into the reactor with feedstocks, which also included 20-30 wt% of woody material (sawdust). The CL biochar was

trialed for 9 months in a ferrosol soil in NSW where sweet corn was grown. The application rate in soil was 10

tonnes/hectare. After harvest, biochar particles were separated and tested.

Biochar-mineral complex (BMC): A biochar from an acacia saligna wood pyrolysed at approximately 400°C for 4hrs, named

as saligna biochar, was activated with 1M phosphoric acid (without separation after reaction), and then mixed with clay from

a local brickworks (high in calcium sands, apatite and illite), minerals (a high loading of manganese dioxide and waste

illemnite) and local chicken litter (with sawdust). Water was added to the mixture so that it had a moisture content of about

30%. This mixture was stirred at the temperature of 80°C for 2 hrs and then placed into a batch reactor. The material was

brought up to ~220°C in approximately 1 hr and then held at this temperature for 3hrs.

All sample pieces were dried at 60°C before use and mounted in resin and polished. Samples were coated with Cr to

improve the surface conductivity, and examined using scanning electron microscopy (SEM) with energy X-ray dispersive

spectrometry (EDS) facilities attached.

An electron transparent section at the interface between the biochar and mineral phases was prepared using a focused ion

beam (FIB) microscope. Samples were examined by transmission electron microscopy (TEM). Scanning TEM mode (STEM)

was used to perform EDS mapping , line scans (points taken at 1 nm intervals) and points analysis in region of interest.

The atomic concentrations of elements, as well as carbon chemical bonding state at biochar surfaces, were investigated by

XPS (ESCALAB-220i-XL manufactured by VG Scientific UK) with a monochromatic Al Kα X-ray source ( hν = 1486.6eV )

induced by 10kV, 15mA Al Kα radiation. Photoelectrons were collected at the take-off angle of 90°.

Cross-section TEM specimen preparation

Figure 1. A systematic diagram showing the layout of dual beam FIB system

The transfer of the specimen from the sample holder to a carbon-coated TEM grid was made ex situ using

the electrostatic force of a glass needle attached to a piezoelectric device and monitored by an optical

microscope. The novelty of this method is that it allows the preparation of cross-sectional TEM specimens

from localized regions without destruction.

Figure 2. Localizing an area of interest and producing a cross-section foil for TEM analysis

Experimental Methods (continue)

Results and discussion --- X-ray Photoelectron Spectrometry (XPS) - wide scan

Fresh

CL biochar

Aged

CL bicoar

Fresh

Saligna biochar

BMC

C 55.5 40.0 73.4 33.5

N 2.8 4.1 0.9 2.4

O 28.4 42.4 21.6 43.6

Na 0.5 - 0.3 0.4

Mg - - - 0.6

Al 0.3 7.0 - 5.0

Si 0.9 5.3 - 10.8

P 3.1 - - 0.7

S - 0.1 - 0.4

Cl 0.7 - - -

K 3.3 - 0.7 0.7

Ca 4.5 0.6 1.3 1.0

Mn - - - 0.4

Fe - 0.4 1.8 0.5

Table 1 Chemical composition (at%±1%) of the biochar surfaces as determined by XPS

After the field trial, on the aged CL biochar surface, the elemental content of O, N, Al/Si, and Fe increased. Meanwhile, Na, P, Cl,

K, and Ca decreased or could not be detected.

For Saligna biochar, after mixed with clay and minerals and thermal treatment, clay and minerals attached to the saligna biochar

surface. It’s noted that the surface content of Ca and Fe was decreased in respect to fresh biochar, which possibly resulted from

burying by clay and other minerals .

Results and discussion --- XPS C1s region scan and peak deconvolution

Fresh CL biochar

Aged CL biocharBMC

Fresh Saligna biochar

Binding Energy

(eV)

Structure Fresh

CL biochar

Aged

CL biochar

Saligna biochar BMC

C 1s 285.0±0.1 C-C/C-H 64.3 43.5 88.7 34.9

286.5±0.1 C-O 18.0 36.5 8.0 44.9

288.1±0.2 C=O 7.9 15.3 3.3 13.2

289.2±0.1 Carboxylic 3.6 4.7 - 7.0

290.5±0.2 Carbonate 6.2 - - -

Table 2 C 1s bonding state and its relative atomic percentage on the fresh and aged biochar surfaces as determined by XPS

Oxygen-contained functional group increased, especially C-O and C=O, indicating either biochar surface oxidation and adsorption of soil organic matter

Results and discussion --- SEM secondary electron images

The surface of aged CL biochar

The surface of biochar in BMC

Minerals attachment occurred on both the surfaces of aged biochar and BMC

Results and discussion --- Cross-sectional SEM backscattering electron

images and EDS mapping

BSE image C O P Al Si

Ca Mg Fe Ti Mn Al Si Ti Mn Fe

BSE image C O P Al Si

K Ca Mg Fe Ti Mn

BMC

Aged CL biochar

Except Al and Si, P, K, Ca, Mg and Fe are present in the mineral phases next to the biochar surface

Results and discussion --- cross-sectional TEM bright field images

Aged CL biochar BMC

minerals

mineralsminerals

biochar

minerals

minerals

minerals

minerals

minerals

minerals

biochar

minerals

minerals

mineralsbiochar

Minerals attachment occurred at the biochar surface.

The microstructure and composition of surrounding mineral phases are different from each other.

Results and discussion --- STEM-EDS Line scan

Aged CL biochar

interface500nm

Aluminum rich interface was discerned, possibly

due to Al-rich in Ferrosol (highly weathered soil)

Results and discussion --- STEM-EDS points analysis

BMC

EDS microanalysis

indicated that:

The interface between

biochar and mineral

phases (Point2) is rich

in P, Mg, K, Ca and Mn,

which contributed to

the incorporation of

minerals to the biocahr.

Except carboxylic and

phenols groups,

phosphate group

would provide another

reaction site produced

by H3PO4 treatment.

Conclusion

Analysis of CL biochar, recovered from the soil, showed that soil mineral

incorporation on to the biochar surfaces happened in the first year. The attachment

was localized, which indicated incorporation/adsorption of soil minerals onto

biochar surfaces was at some specific sites. Relatively high concentrations of Al

were found at the interface between biochar and mineral phases, indicating the

binding role of Al, which possibly because of the formation of carboxylic and

phenolic function groups on aged CL biochar surface by oxidation reactions.

On the other hand, H3PO4 treatment following with thermal treatment of biochar

with clay and minerals also prompt the incorporation of biochar with clay and

minerals. Multiple valence cation plays an important role in the complex process.

It is also implied that the complex is a competitive process of various cations with

activated sites, such as carboxylic group, phenolic group, and phosphate group.

Acknowledgement

Best Energies for the supply of the biochar

The Australian Microscopy and Microscopy Research Facility (AMMRF)

for access to electron microscopy and dual beam FIB.

Thanks for your attention