ALHO, C. F. B. V.

CROOS, A.

ESTRADA, R. A.

LELIS, R. C. C.

MAIA, C. M. B. de F.

NOVOTNY, E. H.

SOHI, S.

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340ABSTRACT: Although biochar is often regarded CU KPGTV KV ECPDGQZKFK\GF CPFƒPCNN[OKPGT-alized into carbon dioxide. However, it is un-clear how much of this material is rapidly oxi-dized, that is, how much remain in the soil for NQPIGT RGTKQFU 'ƑQTVU JCXG DGGP WPFGTVCMGPto develop methodologies to quantify the sta-ble fraction present in biochar, quickly, easi-ly reproducible, and that somehow represents TGCNƒGNFEQPFKVKQPU6JGTGHQTGVJGCKOQHVJKUUVWF[ YCU VQ GXCNWCVG VJG GƑGEV QH FKƑGTGPVR[TQN[UKUƒPCN VGORGTCVWTGUQP VJGUVCDKNKV[QHbiochar produced from Eucalyptus dunnii wood by analyzing biochar samples before and after a thermo-chemical oxidation using 13C NMR spec-troscopy. Stable-C fraction increased as higher R[TQN[UKUƒPCNVGORGTCVWTGYCU6JGTOQEJGO-ical oxidation was responsible for removing la-bile structures still present as well as aromatic structures less resistant to degradation. Ther-mo-chemical oxidation was also responsible for the functionalization of biochars, being this ef-HGEVOKNFGTCUJKIJGTR[TQN[UKUƒPCNVGORGTCVWTGwas. The balance between C storage (Stable-C HTCEVKQPCPFCITQPQOKEDGPGƒVUHWPEVKQPCNK\C-tion) of biochar should be carefully evaluated, as its characteristics can vary depending on the conditions by which it was produced.KEYWORDS: biochar, carbon sequestration, sta-bility, thermo-chemical oxidation, functionalizationINTRODUCTION: The term biochar was recently proposed after studies with regard to the qual-ity of Terra Preta de Índio’s (TPI) soil organic matter as a model for sustainable use of soils in agreement with climate changes and carbon (C) sequestration issues (LEHMANN & JOSEPH, 2009). The term is used for lingo-cellulosic ma-terials undergoing pyrolysis (environments with little or no oxygen supply), similar to the pro-cess for charcoal production. However, biochar distinguishes from charcoal and other similar materials, since it is produced to be applied to soils in order to improve their chemical, phys-ical and biological characteristics. Its distinct use presents some requirements and challenges /#ķ'-GVCN#NVJQWIJDKQEJCTKUQHVGPTGICTFGFCUKPGTVKVECPDGQZKFK\GFCPFƒPCN-ly mineralized into carbon dioxide (GOLDBERG, 1985). However, it is unclear how much of this material is rapidly oxidized, that is, how much remain in the soil for longer periods. Some authors have proposed methods using high temperatures aerobic oxidation (TOLES et al., 1999), reaction with strong oxidizing chemicals (MORENO-CASTILLA et al, 2000), or ozone ox-idation (SERGIDES et al, 1987; KAWAMOTO et al, 2005), which demonstrates that biochar can be abiotically oxidized in the short term. Other authors suggested that abiotic oxidation can occur through chemical adsorption of ox-ygen by aging processes (PURI, 1963, PURI, 1970; BILLINGE & EVANS, 1984; ADAMS et al, 1988). However, the short term oxidation under ambient temperature and atmosphere has PQVDGGPTGEQIPK\GFCPFQTUWƔEKGPVN[SWCPVK-ƒGFCPFVJGGZVGPVQHDQVJGƑGEVUDKQVKECPFCDKQVKE QZKFCVKQP KU UVKNN PQV UWƔEKGPVN[ ENGCT%*'0) GV CN  6JGTGHQTG GƑQTVU JCXGbeen undertaken to develop methodologies to quantify the stable fraction present in biochar, quickly, easily reproducible, and that somehow TGRTGUGPVU TGCNƒGNFEQPFKVKQPU6JGTGHQTG VJGCKOQHVJKUUVWF[YCUVQGXCNWCVGVJGGƑGEVQHFKƑGTGPVR[TQN[UKUƒPCNVGORGTCVWTGUQPVJGUVC-bility of biochar produced from Eucalyptus dun-nii wood by analyzing biochar samples before and after a thermo-chemical oxidation using 13C Nuclear Magnetic Ressonance (NMR) spectros-copy.MATERIAL AND METHODS: Eucalyptus dun-nii (DUN) wood samples were obtained from experimental plantations in Embrapa Florestas (Colombo, Brazil). Wood samples were ground-BIOCHAR STABLE FRACTION QUANTIFICATION BY THERMO-CHEMICAL OXIDATION AND ASSESSEMENT BY 13C NMR SPECTROCOPYCARLOS FRANCISCO BRAZÃO VIEIRA ALHO1, ANDREW CROOS2, SARAN SOHI2, RUBEN AUC-CAISE ESTRADA3, ETELVINO HENRIQUE NOVOTNY3, CLAUDIA MARIA BRANCO DE FREITAS MAIA4, ROBERTO CARLOS COSTA LELIS51Wageningen University, Wageningen, the Netherlands, alhojunior@gmail.com; 2United Kingdom Biochar Re-search Centre, Edinburgh, United Kingdom; 3Embrapa Solos, Rio de Janeiro, RJ; 4Embrapa Florestas, Colom-bo, PR; 5Universidade Federal Rural Rio de Janeiro, Seropédica, RJ341ed in a knife mill and then sorted for greater uniformity of particle size. The fraction retained between sieves of 0.5 and 2.00 mm was used. Initially, wood samples were dried at 103 °C for 48 h for moisture removal before pyroly-sis. Approximately 10 g of wood samples were weighed and placed in commercial aluminum foil then placed in metal tubes. Pyrolysis was RGTHQTOGFKPVTKRNKECVGKPCOWƕGHWTPCEG3WK-mis, model 319) with limited oxygen supply, equipped with heat indicator (Incon, model CNT 110) at a heating rate of 10 °C.min-1 (slow py-TQN[UKU VQ VJTGG ƒPCN VGORGTCVWTG  and 550 °C) maintained for 60 min. After this VKOGVJGOWƕGHWTPCEGYCUVWTPGFQƑHQTITCF-ual cooling of samples. Thermo-chemical oxidation was performed ac-cording to CROSS & SOHI (2012), as a pro-posed accelerated aging of biochar aiming to oxidize unstable fraction (labile) and to evaluate biochar stability by quantifying the remaining (stable) C content after oxidation. Initially bio-char samples were pulverized in a ball mill to increase particles physical exposure and then dried at 80 °C for 24 h. Approximately 0.2 g sample were weighed in a test tube, then 7 mL of 5% hydrogen peroxide (H2O2) solution was added under stirring. Samples were then kept in an oven at 80 °C for 48 h under periodical stirring (approximately every 2 h). After this pe-riod, samples were dried at 105 °C for 24h. Samples were then placed in a desiccator for approximately 60 min before weighing to obtain Stable-C yield gravimetrically. Elemental analy-sis was performed (C content) before and after thermo-chemical oxidation. Solid-state 13C NMR spectra were obtained in a 500 MHz spectrometer (Varian), using a 4 mm triple resonance probe (T3 NB HXY) to im-plement VACP-MAS (Variable Amplitude Cross Polarization - Magic Angle Spinning) experi-ment to detect the 13C nuclei of biochar sam-ples before and after oxidation. The speed of rotors was 15 kHz under compressed dry air at room temperature. The time of ʌ/2 pulses ECNKDTCVKQPHQT*PWENGKYCUzU%UKIPCNCE-quisition time was 16 ms, and repetition time was 500 ms. Initially, equipment calibration was proceeded using glycine sample as a ref-erence, C methyl at 43.7 ppm, and the speed of rotors of 5 kHz, under compressed dry air at room temperature. Subsequently, the data were processed in software VNMRJ © version 3.1 (Agilent).RESULTS AND DISCUSSION: It is observed VJCVCUJKIJGTR[TQN[UKUƒPCNVGORGTCVWTGYCUhigher was the Stable-C content, ranging from 73.86, 80.79 and 86.67% for 350, 450 and 550°C, respectively. For the lowest pyroly-UKU ƒPCN VGORGTCVWTG  u% KV KU QDUGTXGFthat thermo-chemical oxidation was responsible for removing part of the labile structures still present in biochars as well as remove aromatic structures less resistant to degradation, which may be evidenced by a decrease in the frequen-cy and narrowing of the region corresponding to aromatic structures, which explains the high-er mass loss of these samples after oxidation, which also result in lower levels of Stable-C fraction. Thus, there was no selective oxidation observed as both labile and non-labile (in this case, aromatic) underwent oxidation. For bio-chars produced at 450 and 550°C, there was a predominance of aromatic structures there-fore thermo-chemical oxidation acted mainly on these structures. However, in a much milder way, occurring only slight decrease in the fre-SWGPE[QHCTQOCVKEUVTWEVWTGUDGKPIVJKUGƑGEVeven lower when comparing biochars produced at 550°C to 450°C. Thermo-chemical oxida-tion was responsible for the functionalization of biochars, which can be evidenced by signal at 172 ppm, related to the carboxylic groups, giv-ing the similar structure of TPI’s humic acids as reported by NOVOTNY et al. (2007), as being structures that ensure the high fertility of TPI's as well as its sustainability, since these func-tional groups are bonded directly to the aromatic structure of these materials. However, this ef-fect is much less evident at temperatures of 450 VQu%(KIWTG5KOKNCTGƑGEVKUGZRGEVGFto occur in natural environments through biotic (microorganisms) and abiotic (weathering) oxi-dation acting on biochars once applied to soil. However, these results suggest that, although more stable, biochars produced at higher tem-peratures (above 400-450 °C), oxidation of pe-ripheral aromatic structures and appearance of carboxylic groups linked to aromatic matrix may take longer. In this sense, when thinking about RTQFWEKPIDKQEJCTCPFGURGEKCNN[VJGGƑGEVUQHR[TQN[UKUƒPCNVGORGTCVWTGQPVJGRTQRGTVKGUQHbiochar, the balance between C storage and ag-TQPQOKEDGPGƒVUOWUVDGYGNNWPFGTUVQQF+VKURQUUKDNGVJCVDKQEJCTUCTGRTQFWEGFYKVJFKƑGT-ent functions (or C sequestration or agronomic DGPGƒVUCPFDKQEJCTUEQODKPKPIVJGVYQEJCT-acteristics, which is preferable.342CONCLUSIONS: Thermo-chemical oxidation was responsible for removing labile structures still pres-ent as well as aromatic structures less resistant to degradation. Thermo-chemical oxidation was also responsible for the functionalization of biochars, DGKPI VJKU GƑGEVOKNFGT CUJKIJGTR[TQN[UKUƒPCNtemperature was. The balance between C storage 5VCDNG%HTCEVKQPCPFCITQPQOKEDGPGƒVUHWPE-tionalization) of biochar should be carefully evalu-ated, as its characteristics can vary depending on the conditions by it was produced.ACKNOWLEDGEMENTS: International Humic 5WDUVCPGEU5QEKGV[ HQTRTQXKFKPIƒPCPEKCN UWR-port through a Training Award to the author. Dr. Edson Alves de Lima and Dr. Ananda Virginia de Aguiar, Embrapa Forests, for providing the wood samples. BSc. Ann Mennim for the elemental analysis (C) results. Dr. Clare Peters, University of Edinburgh, for providing training in the use of laboratory assays. REFERENCESADAMS, L.B.; HALL, C.R.; HOLMES, R.J.; NEWTON, R.A. An examination of how expo-sure to humid air can result in changes in the adsorption properties of activated carbons. Car-bon v. 26, p. 451–459, 1988.BILLINGE, B.H.M., EVANS, M.G. The growth of surface oxygen complexes on the surface of activated carbon exposed to moist air and VJGKTGƑGEVQPOGVJ[N+QFKFGTGVGPVKQPJournal de Chimie Physique et de Physi-co-Chimie Biologique, v. 81, p. 779–784, 1984.CHENG, C.H.; LEHMANN, J.; THIES, J.E.; BUR-TON, S.D.; ENGELHARD, M.H. Oxidation of black carbon by biotic and abiotic processes. Organic Geochemistry, v. 37, p. 1477-1488, 2006.CROSS, A.; SOHI, S.P. A method for screening the relative long-term stability of biochar. GCB Bioenergy, v. 5, p. 215-220, 2012.GOLDBERG, E.D. Black Carbon in the Environ-ment: Properties and Distribution. Wiley, New York, 1985.KAWAMOTO, K.; ISHIMARU, K.; IMAMURA, Y. Reactivity of wood charcoal with ozone. Jour-nal of Wood Science, v. 51, p. 66–72, 2005.LEHMANN, J.; JOSEPH, S. Biochar for Environ-mental Management, p. 1-5, 2009.a) b)c) d)Figure 1. Solid state 13%0/4GURGEVTCHQTVJGFKƑGTGPVVGORGTCVWTGUCVQECPFVJGURGEVTCQHVJGFKƑGTGPEGUF343/#ķ'-1$419051462%4155#51*+5+PƓWGPEGQHRTQFWEVKQPEQPFKVKQPUQPthe yield and environmental stability of biochar. Fuel, in press, 2011.MORENO-CASTILLA, C.; LOPEZ-RAMONS, M.V.; CARRASCO-MARIN, F. Changes in sur-face chemistry of activated carbons by wet oxidation. Carbon, v. 38, p. 1995–2001, 2000.NOVOTNY, E.H.; DE AZEVEDO, E.R.; BONAG-AMBA, T.J.; CUNHA, T.J.F.; MADARI, B.E.; BENITES, V.M.; HAYES, M.H.B. Studies of the compositions of humic acids from Amazonian Dark Earth soils. Environmental and Science Technology, v. 41, p. 400-405, 2007.PURI, B.R. Chemistry and physics of carbon. Walker, P.L., Jr.Jr. (Ed.), Chemistry and Physics of Carbon, p. 191–282, 1970.PURI, B.R. Surface oxidation of charcoal at ordi-nary temperatures. In: Fifth Carbon Conference. Proceeedings. Symposium Publications Division, Pergamon Press, p. 165–170, 1963.SERGIDES, C.A.; JASSIM, J.A.; CHUGHTAI, A.R.; SMITH, D.M. 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Biochar stable fraction quantification by thermo-chemical oxidation and assessement by 13C NMR spectrocopy.

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Biochar stable fraction quantification by thermo-chemical oxidation and assessement by 13C NMR spectrocopy.

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