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Microscopic characterisation of synthetic Terra Preta.


Organic carbon in soil is reaching a critically low level due to an intensification of agricultural production required to support the increasing world population, and the demand for biofuels. The reduction in organic carbon in the soil leads to dramatic soil degradation and, as a result, soil erosion, increased salinity, lower cation exchange capacity, and lower water and nutrient retention abilities (Glaser et al. 2002; Lehmann and Joseph 2009).

Glaser et al. (2002) showed that carbonised materials, such as biochars, are responsible for maintaining high levels of soil organic matter (SOM) and available nutrients in anthropogenic soils such as Amazonian Dark Earth (or Terra Preta). Terra Preta is a unique, anthropogenic soil that can sustain higher fertility than adjacent soil due to its higher mineral and organic matter content, water holding capacity, pH, and cation exchange capacity (Sombroek 1966; McCann et al. 2001). Terra Preta also contains very high concentrations of elements such as P, Ca, Mn, Cu, K, Mg, Na, and Zn (Arroyo-Kalin et al. 2009). Studies of the structure and chemistry of Terra Preta have revealed that these soils are composed of micro-aggregates that may have formed by the interaction of a range of materials including thermally treated organic matter, charcoal and ash from fires, clay particles, microscopic fragments of bones, residual fired clay, sand, microorganisms, and decomposing/cooked food (Lima et al. 2002; Costa et al. 2004; Schaefer et al. 2004; Arroyo-Kalin et al. 2009). Observed black carbon concentrations in the SOM of up to 35% provide evidence that black carbon, with radiocarbon ages of up to 2000 years, is important for the SOM stability in Terra Preta soils (Glaser et al. 1998). It is assumed that the high carbon content of this soil is caused by significant deposits of charcoal during occupation by the Native Americans (Glaser et al. 1998). Steiner (2006) notes that Amazonian Indians manufacture a mixture of charcoal and ash from the fire, soil with high clay content, and decomposed biomass. This mixture is baked at low temperatures in an open fire and then applied to the soil. Lima et al. (2002), Liang et al. (2006), and Solomon et al. (2007) showed that increases in cation exchange capacity may be due to the presence of black carbon. The high Ca and P contents of Terra Preta are attributed to bone fragments and animal residues, with the majority of the P adsorbed onto clayey materials (Lehmann et al. 2004). However, Heckenberger et al. (1999) and Heckenberger (2005) have also shown that addition of charcoal and organic amendments alone may not lead to lasting improvements to the soil; that is, the mineral phases in Terra Preta, and the interactions which occur between the organic and inorganic phases, may be essential to achieve the desirable properties of this material.

Since Terra Preta, as an aggregated mixture of organic and inorganic phases, is effective in promoting soil fertility, a program has been initiated at the University of New South Wales, in collaboration with Comell University, Industry & Investment NSW, and Department of Agriculture and Food, Western Australia, to develop synthetic high organo-mineral micro-aggregates (termed synthetic Terra Preta, STP) using a process of 'biomass torrefaction'. Torrefaction is a process in which biomass is heated to 220-270[degrees]C in the absence of air. During torrefaction, native hemicellulose is partly depolymerised by hydrolysis and/or thermal chain scission to provide 'reacting hemicellulose' (Bourgois and Guyonnet 1988). This intermediate phase is then decomposed by acid and radical reactions to yield many substances (e.g. furfural), which recombine to form torrefied hemicellulose. Water and acids are also formed during these reactions and are released into the reaction environment (Bourgois et al. 1989). Some of this water may be reused to depolymerise hemicellulose or to release acids from the hemicellulose by hydrolysis of acetate groups. Acids can also be formed by radical reactions. The water and acids released by hemicellulose become available to depolymerise cellulose and lignin (Sharma et al. 2004). Microcrystalline cellulose can form 'disordered pyrolytic cellulose', i.e. a 3-dimensional polymer, at ~270[degrees]C (Pastorova et al. 1993). If the cellulose is exposed to acids and radicals that originate with hemicellulose, even more degradation of cellulose would be expected. This disordered cellulose is thermostable and contains furan, aliphatic, and ketone groups (Pastorova et al. 1994). Reacting biomass and minerals at temperatures below which pyrolysis occurs may assist in understanding the mechanisms responsible for the formation of the organomineral particles found in the Amazon. Moreover, this heat treatment would accelerate chemical interactions between the organic and inorganic components of the STP.

This research program was initiated to test the hypothesis that torrefaction of biomass will produce a carbon structure that will react with clay and other minerals to produce a material that has similar properties to the Terra Preta soils in terms of stability, cation exchange capacity, EC, pH, and adsorption of dissolved organic matter. The raw materials used to synthesise STP include a mixture of raw clay, crushed brick, lime, calcium phosphate, and waste biomass, which are broadly similar to the materials generally found in Terra Preta. This paper focuses on the experimental results of the microstructural characterisation of both STP and Terra Preta, using principally scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray photoelectron spectroscopic (XPS) analysis and nuclear magnetic resonance (NMR) were also used to help elucidate the microstructures observed.

Materials and methods

Preparation of materials

To simulate the complex structure of Terra Preta, a mixture of 1 : 1 and 2 : 1 clay was used. Crushed brick and cement kiln dust were used to simulate the pottery used in Terra Preta soils; 'blood and bone' was used to simulate fish bones and other food waste commonly found in Terra Preta. The biomass used consisted of a mixture of sawdust and chicken manure. Typical constituents of the manure include partially digested seeds, partially digested biomass fibres (lignin and cellulose), and micro- and macro-elements such as potassium, sodium, chromium, and manganese. The sawdust used was eucalyptus wood-based material supplied by Boral Ply Ltd. The total amount of raw material used to synthesise STP was: 500g sawdust, 400g chicken manure (containing carbon-rich phases and some mineral phase rich in Na, K, Mn), 100g blood and bone mixture (which has a high calcium content), 350 g bentonite [[(Na,Ca).sub.0.33][(Al,Mg).sub.2]([Si.sub.4][O.sub.10]) [(OH).sub.2.]n[H.sub.2]O , but the mineral and water content varies when substituted with other minerals], 250g kaolinite [[Al.sub.2][Si.sub.2] [O.sub.5][(OH).sub.4]], 300g crashed brick, and 100g cement kiln dust (mainly calcium carbonate).

The raw materials were initially mixed with sufficient water to form a paste, to coagulate all the raw materials together. The paste was then heated in an oven to 800C for 1 h to dry. The sample was then heated at either 240[degrees]C (material termed STP240) or 220[degrees]C (termed STP220) for 8 h in an oxidising environment.

Terra Preta samples from Hatahara (Liang et al. 2006) in the Manaus district of the Amazon were examined by TEM. This and Terra Preta soils sourced from other locations were also analysed by NMR.

Analysis of materials

Ultimate analysis, proximate analysis, and ash analysis were performed by Bureau Veritas International Trade Pty Ltd in Australia. X-ray diffraction, NMR, electron spin resonance, Fourier-transformed infrared spectroscopy, and XPS were also performed to examine the chemical structure of the surfaces (results of the spectroscopic techniques to be presented elsewhere).

The XPS was carried out using a Thermo Scientific ESCALAB220i-XL spectrometer. The electron source used in this spectrometer was mono-chromated Al K[alpha] for which the energy of the electron is 1486.6eV. The spot size used in this analysis was ~1 mm in diameter. XPS spectra were collected over an energy range 0-1100 eV.

The bulk STP samples were adhered to an SEM sample holder/stub using carbon tape. The samples were sputter coated with chromium and analysed using a Hitachi $3400 SEM.

Elemental analysis was carried out using an energy dispersive spectroscopy (EDS) detector interfaced to the SEM.

The bulk samples were also mounted in Spurr's resin (10 g vinylcyclohexane dioxide, 6 g diglycidyl ether of polypropylene glycol, 26g nonenyl succinic anhydride, and 0.3g diamethylamine ethanol). The resin mixture was placed in an oven at 60[degrees]C to cure for 2 days. Cross-sections of STP were prepared by grinding the mounted samples with a grade 800 grinding paper for 5 rain followed by a grade 1200 grinding paper for 5 min. The ground samples were then polished with a 3-[micro]m diamond polish followed by 1-[micro]m polishing. The polished samples were analysed using SEM. The polished mounts containing the cross-sectioned STP samples were adhered to the SEM sample holder using carbon tape. EDS analysis was also performed to determine the elemental distributions in STP.

The mounted and polished samples were placed into a focus ion beam (FIB) microscope. A layer of platinum (2 [micro]m thick) was deposited on the region of interest (ROI) to protect it from damage by the ion beam. The area around the ROI was then milled away using a 30kV ion beam. A high current beam (6500 pA) was used to mill away the outer region and the beam current was slowly decreased from 6500 pA to a final current of 70pA as the milled region become closer to the electron transparent section. The final TEM specimen was around 100-150nm in thickness. The TEM specimen was freed from the bulk sample and a micromanipulator was used to 'lift-out' the specimen from the trench created by milling. The final TEM specimen was placed on a carbon-coated copper mesh TEM grid (Giannuzzi and Stevie 1999) and placed into a Philips CM200 field emission gun TEM.

Solid-state NMR spectra were acquired using a Varian Inova-300 spectrometer operating at 299.97 MHz for [sup.1]H and 75.4 MHz for [sup.13]C, with Chemagnetics 4-mm double air-bearing cross-polarisation (CP) probes. Quantities of ~50mg of the mineral/char samples were packed into the 4-mm outside-diameter rotors, and subjected to 'magic-angle spinning' (MAS) at various speeds noted below. Spectra were recorded at 294K using the following conditions for each nucleus:

[sup.1]H: 3-13kHz MAS; 2ms, 45[degrees] single pulse; 5s relaxation delay;

[sup.13]C: 4kHz, CPMAS; 4.5ms, 90[degrees] pulse; 1.5-2ms contact time; 5 s relaxation delay.

Free induction decays were acquired and zero-filled to 8K prior to Fourier transformation; 1000-2000 scans were collected for sufficient signal/noise. The secondary references and samples used for Hartman-Hahn match were deuterated water for [sup.1]H ([sup.[delta]H 4.7 ppm) and hexamethylbenzene for [sup.13]C([sup.[delta]C 17.3 ppm for CH3 peak). NMR studies were also performed on 4 Terra Preta samples sourced from a range of sites.

Results and discussion

Chemical and spectroscopic analysis

Table 1 summarises ultimate and proximate analysis of the 2 STP materials, together with data for the raw materials. The total carbon and nitrogen contents for STP220 were nearly equal to those in the raw material, whereas there was a loss of ~28-40% for STP240.

A higher oxygen/carbon ratio was observed for both STP220 and STP240 than for the raw materials, with the STP240 showing the more substantial increase. The volatile content and the ratio of fixed carbon/volatile carbon (0.8) for STP220 were higher than STP240 (0.72). At 220[degrees]C there was very little liberation of volatiles (no visible emissions were observed during reaction), whereas at 240[degrees]C, liberation of a small amount of volatiles was observed during reaction. Concentrations of most of the mineral constituents of the ash of the raw and the heat-treated samples were similar, which indicates very little mineral loss during reaction at 220[degrees]C and 2400C. However, the ash analysis showed some discrepancy in Ca and Fe contents, which probably can be attributed to sampling or analytical error.

NMR analyses were performed on several Terra Preta samples (Lago Grande, Dona Stella, Hatahara) extracted from 3 archaeological sites in Brazil (Fig. 1). Preliminary NMR results showed that the spectra from the Term Preta samples and both STP samples exhibited a major peak at -130ppm arising from aromatic carbon, with weaker contributions from aliphatic carbon at ~30-40 ppm. STP220 showed a closer match to the Terra Preta spectra than did STP240. NMR was also performed on the STP mixture before torrefaction. This spectrum indicated greater contributions from the cellulosic components at 60-105ppm, but much weaker contributions from aromatic carbon at 130-150ppm.

Results of XPS showed that the total carbon content found on the STP240 surface was almost twice that on the surface of STP220 (see Table 2). As the total amount of carbon before heating was similar for both samples, there has likely been deposition/condensation of carbon compounds on the clay surfaces when volatilisation of the biomass occurred. XPS analysis also showed differences in the concentrations of the different functional groups at the surface of the particles. The concentrations of C-O, C=O, and C-C/C-H were significantly higher for STP240 than STP220, while the carboxylic concentration was lower. The concentration of nitrogen functional groups was also higher for STP240. This indicates greater reactivity between the organic compounds and mineral matter at the higher heat treatment temperature.


SEM analysis of STP220

The structure and composition of STP particles is complex and reflects the heterogeneous nature of the feedstock materials that are mixed together. Figure 2 shows an SEM image of a torrefied particle covered with mineral phases. A vascular structure is evident, which suggests that this was originally a sawdust particle. EDS analysis of the mineral phases, for example at point A, shows that the mineral is rich in O, Si, and A1, the principal elements of kaolinite and bentonite. EDS analysis from the torrefied material, at point B, shows that this particle is rich in carbon, which is not unexpected for an organic material. It is also evident from Fig. 2 that the distribution of the clay layer surrounding the torrefied material is non-uniform. Some areas are densely covered in mineral deposit, while other regions remain bare of these phases. This suggests either poor mixing between the various phases during preparation or, possibly, local differences in surface reactivity. The chromium peak observed in both spectra arises from the chromium coating applied before analysis.

Given the heterogeneous nature of these materials, SEM examination of the internal structure of STP220, via polished cross-sections, was performed for over 20 particles. This analysis showed that the interfaces between the organic and mineral phases were both heterogeneous and complex. Figures 3 and 4 are backscattered electron images where the strong compositional contrast reveals a range of different phases present. Figure 3 shows a region where there is a distinct interface between the sawdust particle (at the top of the image) and the clay phase (at the bottom of the image). The EDS spectrum for the sawdust phase (point A) depicts a carbonrich phase, while the spectrum for point B shows an Al/Si/O-rich phase, consistent with the clay phase. This clay phase appears to surround the wood particle without reacting with it.

In contrast, Fig. 4 illustrates a more complex microstructure between the torrefied biomass, on the left and right of the image, and mineral (clay) phase, in the centre of the image. The interfaces between these phases appear less distinct. Moreover, both EDS spectra (points A and B) are taken from the carbon-rich phase, but peaks from elements such as A1 and Si are evident in these spectra, which suggest chemical reactions between the organic and mineral phases and physical incorporation in the pore structure of the torrefied biomass.



SEM analysis of STP240

Figure 5 shows 2 secondary electron images of particles of STP240. The particle in Fig. 5a is identified as a torrefied sawdust particle, which exhibits a distinctive porous vascular structure, and EDS analysis (point C) shows a high carbon content, together with the presence of low concentrations of elements such as Na, Si, K, and Ca. In contrast, the particle shown in Fig. 5b can be identified as a chicken manure particle due to its lack of vascular structure. The particle has a high mineral content in the predominately carbon matrix (see the EDS spectrum, point D). The EDS spectrum for point E is of a clay particle (kaolinite or bentonite), which is consistent with its high Al, Si, and O content. Therefore, it can be concluded that Fig. 5b is a chicken manure particle covered with a layer of minerals.


SEM examination, via a polished cross-section, of the internal structure of STP240 compared with STP220, revealed more widespread reactions between the high carbon phases (irrespective of whether these are derived from the sawdust or chicken manure feedstocks) and the crushed brick, the calcium-rich material, and the 2 different types of clay. Figure 6 shows a backscattered electron image of a chicken manure-derived particle with several distinct phases as shown by the differing contrast in the image. Region A is rich in Al, Si, O, and K, which means that it is most probably a clay particle. Region B is rich in Ca, C, O, and P, which could be a mixture of calcium carbonate and calcium phosphate. Calcium carbonate is the main component of cement kiln dust, whereas calcium phosphate probably originated from either chicken manure or blood and bone. Region C is rich in C and O with traces of P and C, which suggests that it is an organic phase. Al, Si, and O were detected in region B, which suggests that it is a clay particle. The interfaces between these phases are often not distinct, which suggests interaction between them. Again, a large number of sections of the STP240 were examined, and in general, the phase interfaces observed were consistent with those shown in Fig. 6.


Figure 7 shows a series of elemental X-ray maps recorded from STP240. The scale bar towards the left of each elemental map depicts the relative concentration of each particular element in that map, with light the highest and dark the lowest concentration. Two distinct phases can be seen from this map grout--a carbon-rich phase and a mineral-rich phase. The results suggest that the carbon-rich phase, on the right, is a torrefied biomass particle due to its high carbon content. The pores which are still visible on the elemental maps suggest a vascular structure consistent with this being a sawdust particle prior to reaction. It can also be deduced that the traces of carbon visible on the left side of the map are due to the resin used to mount the sample, due to the difference in contrast between sawdust particle and resin. Calcium appears to be a dominant element that exists within the carbon phases at the interface and around the pores of the torrefied sawdust particle. There is also a random distribution of calcium phosphate micro-particles within the organo-clay outer coating of the reacted biomass. The high Al/Si/O content suggests that the particle on the left is a clay particle. It can be seen from the maps that reactions have occurred along the surface and pores of the reacting sawdust particle. The presence of calcium along the edges of the torrefied sawdust particle suggests that calcium might have reacted with the carbon surface or the volatiles given off during the higher temperature torrefaction. Some mineral phases (high in Ca and Si/Al/O complex) were found in the pores of the torrefied sawdust, suggesting both incorporation and then reaction with the carbon surface.


TEM analysis of STP

Analysis of STP240 was performed at higher resolution using TEM. The bright field TEM image shown in Fig. 8 shows a region of STP240 that contains 3 distinct, and separately labelled, phases (a carbon-rich phase, clay, and calcium phosphate). Figure 8 also shows EDS spectra from these 3 regions. The darkest region is the calcium phosphate particle (rich in Ca, P, and O), the bright region is the carbon-rich particle, and the grey region is the clay particle (rich in A1, Si, and O). The significant carbon content found in the EDS spectrum of the calcium phosphate particle suggests that the calcium phosphate particle might have originated from the chicken manure.

An EDS linescan (shown in Fig. 9) was performed along a region defined by the arrow shown on Fig. 8. The region on the left hand side of the linescan represents the bottom of the marked line on Fig. 8. Five distinct regions were detected along the arrowed line. These 5 regions are labelled A-E. The X-ray counts of Ca, O, and P are high in region A, indicating the presence of a calcium phosphate particle. The high X-ray counts of equal concentrations of A1 and Si in region B indicate the presence of kaolinite. Region C consists of a carbon-rich phase, which diffraction analysis in the TEM showed was amorphous in structure. The X-ray counts of region D are similar to region A, indicating the presence of a calcium phosphate particle, whereas the X-ray counts in region E, with double the concentration of Si to A1, indicate the presence of bentonite. The high X-ray counts of carbon in regions B and D indicate the formation of organomineral phases at the interface. The presence of calcium on the boundary between regions C and D suggests that calcium might play an important role in promoting the reaction between the organic and inorganic phase possibly through calcium bridging.

In contrast, TEM and EDS analyses of STP220 (images not shown) revealed minimal interaction between the organic and inorganic phases.

TEM analysis of Terra Preta

SEM has been used previously to study Terra Preta (Lima et al. 2002; Costa et al. 2004; Schaefer et al. 2004), and the structures observed elsewhere are similar to those observed for STP. However, to the authors' knowledge, TEM analysis has not been performed on Terra Preta. TEM was used to examine electron-transparent Terra Preta samples and compare with the STP samples. Three distinct regions were detected in the TEM images shown in Fig. 10. These images were taken at an interracial region between a carbon-rich particle (labelled A) and a mineral phase (labelled B). Several voids (labelled P), typically a few 10s of nm in size, can be seen at the interface between these phases.

The EDS spectrum taken in region A shows carbon-rich phases with calcium present in solution. The traces of Ga and Pt detected on the EDS spectrum are due to the FIB-based sample preparation method and can be ignored. The EDS spectrum taken from the mineral phase is rich in A1, Si, and O. The clay nanoparticles are formed in the shape of long hollow tubes. As nanotubes generally do not occur naturally, and presence of clay nanotubes in Term Preta has not previously been observed, more research is currently being undertaken to determine the composition and origin of these tubes.


EDS mapping of the Terre Preta particles (Fig. 11) shows several distinct phases. The carbon-rich phase is clearly rich in Ca, the mineral phase is rich in Si/A1/O. Locally, iron- and titanium-rich phases are also detected, scattered within the AI/Si/ O matrix. Iron might be present between the layers within the silicate structure as one of the interlayer transition metals (Laszlo 1987).



General discussion

As the biomass depolymerises during thermal treatment and forms a range of functional groups, complex reactions take place with the different mineral phases. The degree of interaction has been shown to be a function of temperature, with the STP240 produced at 240[degrees]C forming more complex organo-mineral phases than the STP220 produced at a lower temperature. This trend was observed after analysing a large number of particles for each treatment. It is evident that the phase boundaries between the organic and mineral phases for STP240 are no longer distinct and well defined, whereas a clear phase boundary was more commonly observed for STP220. One explanation for this trend is that the higher temperature might catalyse the breakdown of the biomass to form liquid and gas (Mok and Antal 1983), which recondenses on, or in, the interplanar spacing of the clay particles. This theory is also supported by the work of Lagaly (1984), who found that hydrogen bonds or strong dipole interaction to the silicate layers in the clay was formed through the dissolution and intercalation of organic compounds. Other studies (Fujita et al. 1991; Lehmann et al. 2002) have shown that biochar has the ability to readily adsorb ammonium, nitrate, and phosphate.


The higher rate of interaction between the organic and mineral phase for STP240 can also be explained by the higher amount of functional groups that are found on the surface of the STP240 (Table 2). The ubiquitous presence of the pores, coupled with the various types and number of functional groups that are found on the surface, gives the torrefied biomass the ability to adsorb and react with most minerals. The range of functional groups which exists on the surface includes carboxyls, carbonyls, and phenols (Amonette and Joseph 2009), similar to what was observed here for the STP samples. The presence of both Lewis acid and Lewis base on the surface of clay minerals allows torrefied biomass and biochar to absorb both cations (Lima and Marshall 2005) and anions (Boehm 1994) and it is hypothesised that this is one of the main interactions that is taking place between the organic and inorganic phases in STP to form organo-mineral complexes. It has been shown that the difference in electronegativity of the heteroatoms (oxygen, hydrogen, and nitrogen) relative to the C atom gives torrefied and pyrolysed biomass its surface chemical heterogeneity (Brennan et al. 2001).


The observations in this analysis are also consistent with the results of Lehmann et al. (2007). It was found that a complex was formed between the calcium, clay, and carbon phases in both the STP and Tera Preta sample. Lehmann et al. (2007) hypothesised that the organic phase will be sandwiched between the inorganic phases (clay particles), and the calcium-rich particles will act as a catalyst for the reaction to occur. Czimczik and Masiello (2007) also found that the calcium in soils interacts with functional groups on thermally treated biomass to form organo-metal complexes.

The microstructure of STP240, shown in Fig. 8, is also similar to the microstructure of Terra Preta shown in Fig. 10. Both the clay nanoparticles found in STP and those in Terra Preta exist in the form of long, thin tubes. However, the structures observed in STP were found to be more complex than in Terra Preta. This might be due to the different processing conditions under which STP and Terra Preta were formed. STP was thermally torrefied at high temperature, whereas Terra Preta was formed through a series of biotic and abiotic reactions in the soil. Even though the raw materials used for STP are broadly similar to the ones used to form Terra Preta, some of the compounds and elements might be lost in the STP through decaying or thermal degradation. Lehmann and Joseph (2009) found that biochar in soils will be broken into smaller pieces the longer they remain in the soil. As further reaction occurs between the biochar and soils, the biochar will be coated with more minerals and clays and eventually the high carbon biomass will be a minor constituent in a micro-agglomerate. This is similar to the theory proposed by Brodowski et al. (2005), where it was stated that the stability of biochar particles in soil might be due to the coating and physical entrapment of particles within the microaggregates formed between the biochar and the minerals.

In-tube field trials and pot trials using STP with wheat crops, carried out in sandy soils, have generated encouraging preliminary results (Chia et al. 2008). These trials undertaken by the Department of Agriculture and Food, Western Australia indicate that STP220 and STP240 applied at 10t/ha increased yields of wheat by >100% compared with a control without fertiliser treatment and by >30% with NPK fertiliser. Addition of a similar quantity of fresh wood-based biochar did not result in a significant change in yield (compared with the control). These yield increases are similar to those observed in the Terra Preta soils in the Manaus region of Brazil (Glaser et al. 2002), which suggests that STP may be interacting with soils in a manner similar to Terra Preta.


Biochar mineral complexes were processed at 2 temperatures, 220[degrees]C and 240[degrees]C. At the higher processing temperatures more significant interactions between the organic and inorganic source materials were observed. Significant interdiffusion of elements such as Ca into the organic-based phases was observed. Examination of Terra Preta samples at high resolution using TEM indicated significant similarity of the synthetically processes materials to authentic Terra Preta.


Manuscript received 5 January 2010, accepted 17 May 2010


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Chee Hung Chia (A), Paul Munroe (A,B), Stephen Joseph (A), and Yun Lin (A)

(A) School of Materials Science and Engineering, University of NSW, Sydney, NSW 2052, Australia.

(B) Corresponding author. Email:
Table 1. Proximate analysis, ultimate analysis, and ash analysis of
STP and raw materials (%db, dry basis)


                                   Proximate analysis, % db

Ash                                         63.19
Volatile                                    20.1
Fixed carbon                                16.0

                                   Ultimate analysis, % db

Carbon                                      22.3
Hydrogen                                     1.45
Nitrogen                                     1.86
Oxygen                                      74.39

                                        Ash analysis, %

Silicon as Si[O.sub.2]                      55.3
Aluminium as [Al.sub.2][O.sub.3]            20.9
Iron as [Fe.sub.2][0.sub.3]                  3.0
Calcium as CaO                               9.7
Magnesium as MgO                             1.3
Sodium as [Na.sub.2]0                        2.8
Potassium as [K.sub.2]0                      1.9
Titanium as Ti[O.sub.2]                      0.56
Manganese as [Mn.sub.3][O.sub.4]             0.07
Phosphorus as [P.sub.2][O.sub.5]             4.0
Sulfur as S[O.sub.3]                         0.74

                                      240[degrees]C   Raw materials

                                        Proximate analysis, % db

Ash                                       76.9            53.6
Volatile                                  13.4            39.2
Fixed carbon                               9.7             7.2

                                         Ultimate analysis, % db

Carbon                                    14.2            23.6
Hydrogen                                   0.86            2.97
Nitrogen                                   1.39            2.07
Oxygen                                    83.55           71.36

                                             Ash analysis, %

Silicon as Si[O.sub.2]                    57.5            58.0
Aluminium as [Al.sub.2][O.sub.3]          19.5            20.5
Iron as [Fe.sub.2][0.sub.3]                9.1             3.2
Calcium as CaO                             1.4             9.6
Magnesium as MgO                           1.4             1.4
Sodium as [Na.sub.2]0                      0.53            0.7
Potassium as [K.sub.2]0                    2.1             1.9
Titanium as Ti[O.sub.2]                    0.54            0.54
Manganese as [Mn.sub.3][O.sub.4]           0.08            0.08
Phosphorus as [P.sub.2][O.sub.5]           4.1             4.2
Sulfur as S[O.sub.3]                       0.78            0.62

Table 2. Summary of the XPS peak positions, structures, and
the total percentage various elements (at%, atomic %)

          Peak (eV)    Structure                  at%

Ols         532.26     Silicates                 46.82
Al 2p       74.35      Silicates                  8.68
C 1s        289.16     -COOH                      2.13
            287.96     C=O                        2.99
            286.46     C-O                        5.53
            284.96     C-O/C-H                   13.79
Si 2p       102.68     Silicates                 14.82
N 1s        400.43     O-C=N, pyridine            2.12
            399.09     >N pyridinic, Ar-N-Ar      0.61

          Peak (eV)    Structure                  at%

 O 1s       532.45     Silicates                 33.67
 Al 2p      74.36      Silicates                  5.58
 C 1s       289.69     -OOOH                      1.16
            288.25     C=O                        4.57
            286.46     GO                         9.46
            284.94     C-0/C-H                   28.90
 Si 2p      102.70     Silicates                 10.36
 N 1s       400.66     O-C=N, pyridine            2.70
            399.20     >N pyriddnnc, Ar-N-Ar      2.45
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Author:Chia, Chee Hung; Munroe, Paul; Joseph, Stephen; Lin, Yun
Publication:Australian Journal of Soil Research
Article Type:Report
Geographic Code:8AUST
Date:Sep 1, 2010
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