Short-term Field Decomposition of Pineapple Stump Biochar in Tropical Peat Soil

Transcription

1 Malaysian Journal of Soil Science Vol. 17: (2013) ISSN: Malaysian Society of Soil Science Short-term Field Decomposition of Pineapple Stump Biochar in Tropical Peat Soil Cheah, P.M. 1, M.H.A. Husni 1*, A.W. Samsuri 1 and A. Luqman Chuah 2 1 Department of Land Management, Faculty of Agriculture, Universiti Putra Malaysia, Serdang, Selangor, Malaysia 2 Department of Chemical Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang, Selangor, Malaysia ABSTRACT The transformation of biochar on tropical peat is yet to be studied as all previous studies have been conducted on mineral or forest soils. The objectives of this study were to investigate the physical and chemical changes experienced by pineapple stump biochar (PSB) in tropical peat and to determine the short-term decomposition model of PSB in a C-rich environment. Elemental composition was determined using CHNS-O analyzer and surface area with Brunauer-Emmett-Teller (BET) method. Surface chemistry and structural study were conducted with Fourier Transform Infrared (FTIR) spectroscopy and 13 C solid state Nuclear Magnetic Resonance (NMR) spectroscopy, respectively. The PSB short-term decomposition was conducted with a litter bag study and best fitted into the hyperbolic decay model compared to exponential decay model because no significant mass loss was detected after 4 months. The stagnant phase was probably due to interaction with metals from peat. Redox reaction was prominent on the surface and structural chemistry. Surface oxidation of PSB produced more O-functionalities (hydroxyl, carboxylic and phenolic) and achieved chemical recalcitrance after 12 months. The carbon structure was reduced or saturated causing a decrease in electronegativity. Further PSB decomposition probably depends on biotic decomposition. Keywords: FTIR, hyperbolic decay model, litter bag study, NMR, organic C INTRODUCTION Biochar is widely regarded as a stable material with long resident time ranging from centuries to millenniums (Lehmann et al. 2007). However, biochar is still vulnerable to decomposition and diminishes over time due to abiotic and biotic reactions. The stability of biochar depends on the pyrolysis condition and type of feedstock. Hamer et al. (2004) reported 0.005%/day loss from the initial C, after 60 days of incubation, for oak biochar produced at 800 o C. Meanwhile, wheat straw biochar produced at 225 o C recorded 0.06%/day loss from the initial C (Bruun et al. 2009). Initial massive mass loss is usually reported for short-term decomposition studies, and mainly attributed to the early abiotic changes on the labile fraction of biochar such as carbohydrates and volatiles (Cheng et al. 2006). *Corresponding author : husni@upm.edu.my

2 Cheah, P.M., M.H.A. Husni, A.W. Samsuri and A. Luqman Chuah After incorporation in the soil, the outer surfaces of biochar are vulnerable to rapid surface oxidation (Lehmann 2007). The surface of aged biochar collected from historical charcoal blast furnaces in US was dominated by hydroxyl bonds, carboxylic acid groups and phenolic acids proving the increase in non-aromatic functionality over time (Cheng et al. 2008). The increase in non-aromatic functionality on the surface of biochar can affect the elemental composition of biochar. The increase in O and H content is associated with the acquisition of an acid functional group on the surface of biochar due to the oxidation process (Cheng et al. 2008; Hockaday et al. 2007). Moreover, the increase in acid functional groups of biochar also change the biochar into more hydrophilic in nature due to the evolution of surface positive charge compared to negatively charged functionality (Cheng et al. 2006). This phenomenon promotes the adsorption of readily available organic C in soil that is rich in functional groups on biochar and decrease the surface area over time (Carcaillet 2001; Cheng et al. 2006). Heavy molecular weight and complex organic matter like humic acid are responsible for obstructing pores smaller than 2nm on the external surfaces of powdered wood biochar (Pignatello et al. 2006). The blocked pores might deprive the microbial activities on biochar, inhibiting C mineralization and promoting biochar stability. Further, biocharmineral interaction might contribute to its stability by reducing the bioavailability of biochar. Higher levels of Fe, Al and Si have been detected within biochar structures after 10 years of application to soil (Nguyen et al. 2008). Aged biochar is more negatively charged (Cheng et al. 2008) and has higher affinity to form bonding with positive charges of metals or their oxides. Thus, the decomposition of biochar in a C-rich environment could be slower than expected. In this study, the biochar decomposition study was conducted on a tropical peat soil. Lawful open-burning is permitted within the Malaysian Environmental Quality Act (2009) limitation and is usually enacted to manage the pineapple waste on peat, thus producing biochar with the burning process. Approximately 0.78 Mg/ha biochar are produced from burning pineapple leaves per harvest (Leng et al. 2011). Biochar produced in situ from open-burning adds more C into peat organic C pool and accumulates at the top or surface of peat. Stable biochar could reduce C emission of peat over particular timescales. However, the stability of biochar in peat is relatively unknown because all the biochar decomposition studies up to date have been conducted on mineral soils or forest soils (Cheng et al. 2008; Wardle et al. 2008). Low ph and the presence of soluble organic matter in peat soil might affect the biochar resident time in peat. The presence of Bronsted acid such as humic acid in tropical peat contributes to high amounts of free H + that could saturate and keep biochar in reduced form. Besides, a high amount of dissolved organic matter is likely to encourage surface adsorption on aged biochar which could shield the biochar from further degradation. However, biochar behaves differently in acidic mineral soil. Thus, studying biochar transformation on peat can provide a lead in understanding the recalcitrance of biochar in a C-rich environment. The objectives of this study were 86 Malaysian Journal of Soil Science Vol. 17, 2013

3 Field Decomposition of Pineapple Stump Biochar to determine the decay model, physico-chemical and structural transformations of a short-term field study of pineapple stump biochar (PSB) in tropical peat. METHODOLOGY The PSB used in this study was laboratory-produced at 250 o C for 3 h with a Carbolite ELF 11/23 type furnace. Pineapple stump (PS) raw feedstock was collected from Peninsula Plantation, Simpang Renggam, Malaysia. The feedstock was air-dried and chipped into small pieces (3-5 cm) before pyrolysis. The PS char obtained was ground and sieved to 2 mm. Field decomposition study was based on 2 mm PS char. Chemical Composition and Surface Area The C, H, N and O content of PS at 0, 4, 8, 12 and 16 months (M) was analysed using CHNS-O analyser (LECO TruSpec CHN). Iron (Fe) content was determined using the dry ashing method (Mitra, 2003) and Atomic Absorption Spectrophotometry (Thermo Scientific S-Series). The surface area was determined using a surface area analyser (Quantachrome Autosorb-1) applying the Brunauer, Emmet and Teller method. Surface Chemistry and C Skeletal Structure The infrared spectral properties of PSB were determined by Perkin-Elmer Spectrum 100 Spectrometer with a Perkin-Elmer Universal Attenuated Total Reflectance (ATR) sampling accessory. Data collection and processing were conducted using Spectrum version software. The C skeletal structure of PSB was determined by a 13 C solid state NMR (Bruker Avance 400 MHz). Field Decomposition Study The litterbag method (Wardle et al. 2008) was used for this field decomposition study. Approximately 6 g of PS biochar were placed in a 6.5 x 10.5 cm nylon bags of 250 µm mesh. The weight of the nylon bags was taken before they were buried in peat soil. A total of 32 nylon bags were randomly placed at the Peninsula Plantation, Simpang Rengam, Malaysia after the plot was cleared after 1 month for replanting. The nylon bags were buried vertically at 0-10 cm depth and a total of 8 bags were recovered at intervals of four months till end of the decomposition study of 16 months. The harvested nylon bags were cleared of foreign materials and oven-dried at 60 C till constant weight to obtain the dry weight. The results of the remaining PSB mass from the initial mass over 16 months were plotted and best fitted into several decay models. Decomposition Model and Statistical Analysis The field decomposition data over 16 months were fitted into a hyperbolic decay model, single exponential decay model and double exponential decay model to select the best representation of short-term PSB decomposition in tropical peat. The decomposition model was generated by SigmaPlot version 11.0 using the Malaysian Journal of Soil Science Vol. 17,

4 Cheah, P.M., M.H.A. Husni, A.W. Samsuri and A. Luqman Chuah Marquardt-Levenberg algorithm to obtain the coefficients of the independent variables. The trend of PSB mass remaining over time was determined using regression analysis of SigmaPlot version 11.0 RESULTS AND DISCUSSION Elemental Composition and Surface Area The elemental composition and BET surface area of PSB over the course of 16 months are presented in Table 1. The N content of the PSB remained unchanged or concentrated over 16 months in tropical peat due to PSB mass loss. The C content decreased and H content showed no specific pattern throughout the 16 months. The drastic decrease in BET surface area (0-8 month) indicated the adsorption of organic matter on PSB. However, the decreasing C content conflicted with the adsorption of dissolved organic matter. The atomic O/C ratio increased over 16 months, indicating a higher degree of oxidation. Atomic H/C ratio increased initially (PS 0M-4M) from 0.98 to 1.60 and remained nearly idle afterwards. The increase in atomic H/C for the first four months could be due to the rapid loss of carbon. Since the PSB atomic H/C ratio remained unchanged after the first four months, this indicated no major structural reformation. The high atomic H/C ratio hinted at the aliphaticity nature of PSB. The atomic H/C ratio of peas biochar produced at 700 o C was 0.22 due to its high aromatic content (Braadbaart et al. 2004). Besides, Fe content in PSB accumulated over 16 months, indicating PSB adsorption of Fe from the peat. This biochar-fe interaction could protect PSB from degradation and explain the slow PSB weight loss (Fig. 1). Characteristics of tropical peat are presented in Table 2. The ph value was below 4 and the Fe content was high at 0.1%. Besides, the peat was drained for ongoing pineapple cultivation. Thus, top or surface peat was heavily oxidized due to exposure to the atmosphere. TABLE 1 Elemental composition and surface area of PSB Samples PS biochar month (M) 0 C (%) H (%) 4.27 N (%) 1.27 O (%) Fe (%) Atomic H/C 0.99 Atomic O/C 0.47 Surface Area (m 2 g -1 ) ±0.18 ±0.15 ±0.012 ±1.54 ± ±0.65 ±0.50 ±0.015 ±1.62 ± n.a* ±0.68 ±0.47 ±0.016 ±1.98 ± ±0.42 ±0.56 ±0.010 ±1.54 ± ± ± ± ± ± *The BET surface area for PS 8M was outside of the valid range. The surface area was too small. ±SE = Standard error of mean 88 Malaysian Journal of Soil Science Vol. 17, 2013

5 Field Decomposition of Pineapple Stump Biochar TABLE 2 Characteristics of tropical peat soil Parameters Readings Reference ph > 4 Lucas (1982) C (%) 58 Kanapathy (1976) N (%) Tie and Lim (1976) K (%) 0.04 Lucas (1982) Fe (%) 0.1 Kyuma (1991) Ca (%) 0.3 Lucas (1982) % Weight remaining (Wr) Month (t) Month vs % Weight remaining Month vs % Weight remaining 95% Confidence Band Fig. 1: Short-term decomposition model of PSB Surface Chemistry The FTIR spectrum of PSB over 16 months in tropical peat soil is shown in Fig. 2. The IR wavelength assignment was based on Lampman et al. (2010). The H bonded O-H detected at 3407 cm -1 indicated the polymeric nature of fresh PSB (PS 0M). This implied the orderly arrangement of the crystalline phase of PSB. Alkene C=C stretch group was detected at 1584 cm -1. Since no N-H group was detected, the presence of C-N stretch at 1369 cm -1 implied tertiary amines. The C-O stretch (1245 cm -1 ) combined with H bonded O-H suggested the presence of an alcohol group in PS 0M. After 4 months, PSB (PS 4M) showed signs of oxidation as carbonyl (C=O) group was detected at 1694 cm -1. The H bonded O-H was not detected and C-O Malaysian Journal of Soil Science Vol. 17,

6 Cheah, P.M., M.H.A. Husni, A.W. Samsuri and A. Luqman Chuah %T PS 0M PS 4M PS 8M PS 12M PS 16M cm-1 Fig. 2: FTIR spectrum of PS biochars (PSBs) over 16 months 90 Malaysian Journal of Soil Science Vol. 17, 2013

7 Field Decomposition of Pineapple Stump Biochar intensity decreased compared to PS 0M. Besides, without hydroxyl group, the C-O stretch (1216 cm -1 ) indicated an ether group. The absence of an alkyne group (C C) from PS 4M might be due to reaction with free acids of tropical peat converting the alkyne group into an alkene group (Vollhardt et al. 2007). The alkene group (C=C) was detected at 1593 cm -1. No major alterations in PSB were detected after 8 months (PS 8M) compared to PS 4M as carbonyl, alkene and ether groups were also detected. However, there was one difference as secondary amines were found in PS 8M instead of tertiary amines with the detection of N-H group (3199 cm -1 ). After 12 months, PSB (PS 12M) acquired oxygen functionalities like the hydroxyl, carboxylic and phenolic groups as reported by Cheng et al. (2008). The sorting of aromaticity (1594 cm -1 and 1433 cm -1 ) and carbonyl group (1645 cm -1 ) with hydroxyl group (3379 cm -1 ) indicated phenolic and carboxylic group, respectively. In general, PS 12M became negatively charged or more hydrophilic with the gain of the hydroxyl, carboxylic and phenolic groups. Similar findings were reported by Cheng et al. (2006) in an incubation study. The PSB (PS 16M) surface transformation halted at 16 months. The spectrum of PS 16M was similar to the PS 12M spectrum. Thus, PSB developed chemical recalcitrance after 12 months in a tropical peat soil. Further action of biochar decay might be mainly dependent on biotic reaction. Chemical Structure The 13 C NMR spectrum of fresh PSB (PS 0M) is presented in Fig. 3. All 13 C NMR chemical shift assignments were based on Lampman et al. (2010). Fresh PSB (PS 0M) was aliphatic in nature due to the absence of aryl C or aromaticity. Besides, the peak at ppm was relative to aliphatic sp 2 C-H(R 2 CH 2 ) or alkyl C. The O-alkyl C (C-O) peak at ppm implied the cellulosic origin of PS 0M. O-alkyl C is usually associated with carbohydrate degradation (Marin-Spiotta et al. 2008). This could be attributed to the thermal degradation of cellulose during the pyrolysis process. The peak at ppm was relative to alkyne C (C C). The 13 C NMR spectrum of PSB after 8 months (PS 8M) is shown in Fig. 4. Both alkyl C and O-alkyl C (C-O) were still present in PS 8M at ppm and ppm, respectively. Aromaticity was formed after 8 months as aryl C was found at ppm. The loss of alkyne C could be attributed to alkyne trimerisation, forming an aromatic ring in return (Agenet et al. 2007). After 16 months in tropical peat, PS 16M lost the aromaticity which hinted at the breakdown of the aromatic ring. (Fig. 5). The alkene (C=C) detected at ppm could be the product of aromaticity breakdown. The presence of a strong activating group like hydroxyl (O-H) and amine (N-H) on the surface of PSB (Fig. 2) or in the tropical peat could destabilize the aromatic ring by donating its electron density into the aromatic π system breaking the cyclic structure (Pocius 2002). Besides, a high amount of Bronsted acid such as humic acid could provide free H + to saturate aromaticity. Alkyl C (R 2 CH 2 ) and O-alkyl C (C-O) was detected at ppm and ppm, respectively. Malaysian Journal of Soil Science Vol. 17,

8 Cheah, P.M., M.H.A. Husni, A.W. Samsuri and A. Luqman Chuah Fig. 3: The 13 C NMR spectrum of PS 0M Fig. 4: The 13 C NMR spectrum of PS 8M Fig. 5: The 13 C NMR spectrum of PS 16M 92 Malaysian Journal of Soil Science Vol. 17, 2013

9 Short-term Decomposition Model The results of normality test, constant variance test, coefficient of determination (R 2 ) and mean square error (MSE) are presented in Table 3. PSB was best fitted into a hyperbolic decay with three parameters (recalcitrant fraction, labile fraction and decay rate) model after 16 months of residence in tropical peat soil (Fig. 1). It passed the normality test with the lowest MSE (6.28%2/t2), but failed the constant variance test, while the double exponential and single exponential decay model failed both the tests. The failure of the hyperbolic decay model to fit with the constant variance test could be attributed to the long period of unchanged mass. The recalcitrant fraction (W r ) or asymptotic value was 84% of weight remaining, and the decomposable fraction (W d ) was 15.84%. The decay constant (k) was According to the hyperbolic decay model, PSB short-term decomposition was divided into two phases; initial rapid loss of labile fraction (0-4 months) and the near-inert mass loss that reflects the recalcitrant fraction (4-16 months). TABLE 3 Comparison of 3 different decay models fitted with PSB field decomposition data Decay model Normality test Constant-variance test Hyperbolic decay with 3 parameters Double exponential decay with 4 parameters Single exponential decay with 3 parameters Field Decomposition of Pineapple Stump Biochar R 2 Mean Square Error (MSE) (% 2 t 2 ) Passed Failed Failed Failed Failed Failed The hyperbolic decay model differed from the double exponential decay model suggested by Lehmann et al. (2009). This could be largely attributed to the insensitiveness to detect significant mass loss in a short-term decomposition study. Thus the short-term PSB mass loss appeared stagnant while for the double exponential decay model, the mass loss progressed continuously at a very slow rate. This hinted the stability of PSB on peat but a longer decomposition study is necessary to further highlight the long-term stability of PSB. The hyperbolic decay equation generated by SigmaPlot version 11 is stated in Fig. 1. W at t = W r + W at t Wr Wd k = percentage of PSB weight at t (t in months) = percentage of PSB recalcitrance fraction = percentage of PSB decomposable fraction = decay constant Malaysian Journal of Soil Science Vol. 17,

10 Cheah, P.M., M.H.A. Husni, A.W. Samsuri and A. Luqman Chuah Transformation of PSB in Tropical Peat Short-term decomposition of PSB can be further explained by examining the changes on the physico-chemical and structural properties. The mass loss of the initial 4 months (PS 0M-PS 4M) was accompanied by a decrease in C content. The decreasing mass and C content could be attributed to the loss of labile C-like carbohydrate. Traces of degraded carbohydrates were picked up by the 13 C NMR spectrum of fresh PSB (PS 0M) (Fig. 3). The decrease in C-O intensity and elimination of O-H on the FTIR spectrum of PS 4M further proved the diminution of carbohydrates (Fig. 2). Increasing H/C ratio (Table 1) hinted saturation of C due to higher C-H bonds. This could be attributed to the free H + from Bronsted acid such as humic acid which is abundant in peat. After the end of the initial rapid loss of labile fraction, the near-inert stage that represents the recalcitrant fraction continued from 4th-16th months. Although there were no significant mass changes during this stage, the abiotic reactions were vigorous on the PSB surface until the 12th month when the PSB achieved chemical recalcitrance. The little mass change could be attributed to adsorption on the surface of PSB as indicated by the decreasing BET surface area. In a similar litterbag study of biochar decay in an organic horizon of a boreal forest, no loss of mass was detected after 10 years due to the adsorption of dissolved organic matter (Wardle et al. 2008). Abiotic reactions like oxidation and hydrolysis altered the surface charges of PSB. Oxidation preceded throughout the 16 months as shown by the increasing O/C ratio. As a result, more oxygen functionalities (hydroxyl, carboxylic and phenolic) were developed on the surface of PS 12M compared to fresh PSB (PS 0M) (Fig. 2) rendering PSB hydrophilic and leading to interactions with the peat constituents. However, adsorption of organic C onto PSB is unlikely or insignificant as C content of PSB decreased over 16 months. Instead, interactions of PSB with free metals and their oxides are more prominent. Iron (Fe) content was high at the experimental site (0.1%) and could interact with the negatively charged PSB by ligand exchange or cation bridging. The Fe content in PSB was concentrated and increased over time. This could be attributed to Fe adsorption and contributed to the low PSB weight loss. High amounts of ionic Fe and Al were discovered in the highly stable humic fractions of biochar (Nakamura et al. 2007). Thus, complexation between PSB surfaces and metal ions would reduce bioavailability of PSB and improve the stability or recalcitrance of biochar. According to Hockaday et al. (2006), aged biochar is less susceptible to enzymatic degradation than fresh deposited biochar and this could be due to biochar-mineral interactions. The high H/C ratio was generally unchanged after the first 4 months (PS 4M-PS 16M) and this showed PSB remained aliphatic or unaltered (Table 1). The chemical structures of PSB indicated PSB was saturated over time as shown by the 13 C NMR spectrum. The increase in aromaticity (PS 8M) is unexplainable. Alkynes detected in PS 0M could be further degraded to alkene (PS 16M). This was a sign of reduction reaction due to free H + in acidic peat. 94 Malaysian Journal of Soil Science Vol. 17, 2013

11 Field Decomposition of Pineapple Stump Biochar Electronegativity was reduced and this might affect the reactivity of PSB; slowing down further degradation. CONCLUSION The PSB displayed hyperbolic decay over 16 months in tropical peat. Oxidation increased the O- functionalities (hydroxyl, carboxylic and phenolic) on the surface and might have promoted biochar-mineral interaction that protected PSB from further degradation. The reactivity of PSB decreased over time as the C skeletal structure became saturated due to the low ph and reduced condition of the peat. ACKNOWLEDGEMENT This research was supported by the Fundamental Research Grant Scheme (FRGS) under Ministry of Higher Education in collaboration with Universiti Putra Malaysia. We wish to thank Ms Nor Azma Zaki for help rendered in the nonroutine sample analysis. REFERENCES Agenet, N., O. Buisine, F. Slowinski, V. Gandon, C. Aubert and M. Malacria Cotrimerisation of acetylenic compounds. Organic Reactions. 68: Braadbaart, F., J.J. Boon, H. Veld, P. David and P.F. Van Bergen Laboratory simulations of the transformation of peas as a result of heat treatment: Changes of the physical and chemical properties. Journal of Archaeological Science. 31: Bruun, S., T. El-Zahary and L. Jensen Carbon sequestration with biocharstability and effect on decomposition of soil organic matter. IOP Conference Series: Earth and Environmental Science.6: 1-2. Carcaillet, C Are Holocene wood-charcoal fragments stratified in alphine and subalphine soils? Evidence from the Alps based on AMS 14 C dates. The Holocene.11: Cheng, C.H., J. Lehmann, J.E Thies, S.D. Burton and M.H. Engelhard Oxidation of black carbon through biotic and abiotic processes. Organic Geochemistry. 37: Cheng, C.H., J. Lehmann and M.H. Engelhard Natural oxidation of black carbon in soils: Changes in molecular form and surface charge along a climosequence. Geochimica et Cosmochimica Acta.72: Environmental Quality Act and Regulations: Law of Malaysia International Law Book Services. Kuala Lumpur. Hamer, U., B. Marschner, S. Brodowski and W. Amelung Interactive priming of black carbon and glucose mineralisation. Organic Geochemistry. 35: Malaysian Journal of Soil Science Vol. 17,

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