Thirty-nine biochars, produced from distinct feedstocks at different temperatures, were characterized to determine their properties. The specific surface area of biochars charred at 300 Â°C varied from 1.06 to 11.3 m2 g-1, much less than that of biochars charred at 600 Â°C and charcoal, which varied considerably from 1.93 to 243 m2 g-1. Biochars produced from poultry manure and mushroom-soil had the lowest C content, ranging from 22.8-60.1%, and the highest S content, ranging from 0.48-3.40%. Other biochars also contained elevated concentrations of a variety of sulfur species. FT-IR spectra showed all biochars included hydroxyl, aliphatic, and quinone groups. The concentrations of organic acids and dissolved organic carbon (DOC) released in aqueous leaching experiments by biochars prepared at 600 Â°C were consistently less than that of biochars prepared at 300 Â°C. The carbon in the organic acids accounted for 10-60% of DOC, which indicated other forms of carbon were present in the solution.
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Biochar, aqueous leaching, short-chain organic acids, dissolved organic carbon, sulfur
Biochars, waste organic matter pyrolyzed under low oxygen and temperature (<700 Â°C) conditions, are attracting great interest for wide application in many areas (Lehmann and Joseph, 2009). Biochar can be produced from many types of carbonaceous materials, such as plants, grasses, municipal sludge, agricultural residue, manures, food waste, paper mill waste, rubber, and many others (Cantrell et al., 2012; Hale et al., 2012; Lehmann and Joseph, 2009; Shen et al., 2012). The production of biochars can reduce organic waste and produce biofuel.
Biochars can be used as a soil amendment to improve soil quality and increase crop yield and as an adsorbent to stabilize organic and inorganic pollutants. The relatively stable nature of biochar material also can contribute to carbon sequestration (Lehmann and Joseph, 2009). These benefits depend on the properties of the biochars. The published literature is limited to the discussion of a small number of biochars, and little attention has been focused on the release of short-chain organic acids and dissolved organic carbon (DOC), both of which are important properties controlling the biogeochemical cycling of carbon, nitrogen, and phosphorus, and the transport of pollutants. Cantrell et al. (2012) discuss the physical, elemental, and FT-IR characteristics for five manure biochars prepared at 350 and 700 Â°C. Enders and Lehmann (2012) report the elemental composition of corn stover, oak wood, and poultry manure with sawdust biochars at 300 and 600 Â°C. The oxygen-containing functional groups and elemental composition for cottonseed hull and broiler litter biochars were described by Uchimiya et al. (2011a). Hale et al. (2012) quantify the bioavailable polycyclic aromatic hydrocarbons (0.17 ng g-1 to 10.0 ng g-1) and dioxins (<92 pg g-1) released by digested dairy manure, paper mill waste, food waste, corn stover, wheat straw, pine wood, switch grass, laurel oak, hardwood, rice husk, maize, empty fruit bunches, coconut shell and sawdust biochars . Lin et al. (2012) report on the water-extractable organic carbon in acacia saligna, sawdust, and jarrah biochars and observed that the water-extractable organic carbon is affected by both feedstock type and pyrolysis temperature. Although these studies have characterized many chemical and physical properties of biochars, the biochars were not produced under similar pyrolytic conditions. Characterization of a wider range of biochar types is needed to evaluate their potential versatility. Their potential to release organic acids and DOC needs to be assessed to provide basic data for future applications of biochars.
To compare biochar properties produced from different feedstocks at different pyrolysis temperatures, seven groups of biochars were selected for characterization - 39 biochars in total. The objectives of this study were (1) to characterize the biochars from a variety of feedstocks produced at different temperatures to gain a comprehensive understanding of their physical and chemical properties, (2) to examine their potential to release short-chain organic acids and DOC, and (3) to provide basic data for the assessment of biochar for future applications in the environment.
2.1 Biochars Employed
Cocoa and cotton husks, corn cobs and stover, four types of grasses, pine mulch and bark, three poultry manures and mushroom soils from different localities, and one type of cattle manure were dried at room temperature and then pyrolyzed at 300 and 600 Â°C for 2-3 h using a kiln under limited oxygen supply. The resulting biochars were allowed to cool to room temperature overnight. Three commercial charcoals pyrolized at 700 Â°C were purchased (Wicked Good Charcoal Co., US (CL1), Cowboy Charcoal Co., US (CL2), and Biochar Engineering Corp., US (CL5)). Two types of activated carbon were used as controls (Sigma-Aldrich Corp., St. Louis, USA). All the biochars and charcoal samples were ground in the same manner and sieved through 2 mm sieve for characterization.
2.2 Characterization of Biochar
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The bulk density of the biochars was determined gravimetrically. A known mass of biochar sample was placed into a cylinder; the cylinder was kept vibrating until a minimum volume was recorded following the method of Ahmadpour and Do (1997). The density was calculated according to, where d was density (g mL-1), m was sample mass (g), and V was sample volume (mL).
Particle size distribution was determined using a laser diffraction method (Analysette 22 NanoTec Plus, FRITSCH, Germany). Before analysis, biochar and charcoal samples were mixed with Alconox detergent (Alconox, Inc., USA) for 24 hours to enhance dispersion of the biochar particles in deionized water. The frequency, mode, and median particle diameters of samples were calculated using the program Analysette 22 NanoTec Plus.
The specific surface area (SSA) was analysed with N2(g) at 77 K using a Gemini VII 2390 Surface Area Analyzer (Micromeritics Co., Norcross, USA). All samples were out-gassed at 250 Â°C under vacuum conditions for 24 hours by using a Gemini 2375 ValPrep 061 (Micromeritics Co., Norcross, USA) before N2(g) adsorption. The multipoint Brunauer-Emmett-Teller (BET) method was employed to calculate specific surface areas (Brunauer et al., 1938).
Carbon and sulfur contents were measured by resistance furnace in an Eltra CS-2000 Infrared Carbon/Sulfur Apparatus (ELTRA GmbH, Germany). Samples were oven-dried at 105 Â°C for 24 hours before analysis. The standard materials included coal, graphite, and sucrose (Alpha Canada Inc., Canada). MgSO4 (BDH, VWR, Canada) was also used as a reference material for analysis of sulfate in the solids.
The surface functional groups were characterized using Fourier Transform Infrared spectroscopy (FT-IR). Samples were first dried for 24 h at 105 Â°C. Then samples were ground and mixed with dried KBr (Fisher Scientific Co., USA) at a ratio of 1:100 in an agate mortar. The mixture was prepared in the form of thin discs (about 1 mm) under 34-41 kPa (5-6 psi) using a hydraulic press (Chemplex Industries Inc., USA). Spectra were recorded at a resolution of 4 cm-1 from 500 to 4000 cm-1 with 20 scans per sample using a Tensor 27 FTIR spectrometer (Bruker Optics Inc., Germany).
Surface investigations of samples were performed using a field emission scanning electron microscope (FE-SEM) (LEO 1530, LEO, Germany) equipped with an energy dispersive X-ray detector (EDAX Pegasus 1200, EDAX Inc., USA). Samples were mounted on aluminium stubs with conductive carbon tape, and coated with gold to maintain conductivity. A beam energy of 20 kV was employed to obtain backscatter electron imaging and semi-quantitative EDS data.
2.3 Release of organic acids and DOC from biochar
Batch-style experiments were conducted to evaluate the potential for the biochars to release short-chain organic acids (OA) and DOC. Biochars were added at a 1:75 mass ratio to river water which had concentrations of OA and DOC below the analytical detection limit (0.01 and 0.5 mg L-1 respectively). The water and biochar were mixed thoroughly at first, and then allowed to settle for equilibration at room temperature (25 Â± 2 Â°C) for 2 days, and finally the water samples were filtered with Norm-Ject syringes (Henke Sass Wolf, Germany) through 0.45 Î¼m Acrodisc syringe filters (Pall Corp., UK) for OA and DOC analysis.
Concentrations of OA (lactate, acetate, propionate, and formate) were determined on unacidified samples using ion chromatography (Dionex DX-600, Dionex Corp., USA) with an IonPac AG9-HC 4x50mm column. The flow rate was 1 mL min-1 using 9 mM sodium carbonate as an eluent. Dissolved organic carbon was determined on samples acidified with ultra-pure H2SO4 to pH < 2 using an automated TOC analyzer (Aurora 1030W, OI Analytical, USA) following EPA Method 415.3.
2.4 Principal component analysis of FT-IR data
Principal component analysis (PCA) was employed to process the FT-IR data (Nieuwoudt et al., 2004); all 1094 wavenumbers were used for PCA. A tool was developed using Graphical User Interface (GUI) in MatLab 2012a (Mathworks Inc., USA) and applied to calculate the principal components.
3. Results and Discussion
3.1 Bulk density and particle size distribution parameters
The biochar bulk density varied among the different groups of biochar based on source material and pyrolysis temperatures (Table 1). The bulk density of the charcoal and activated carbon ranged from 0.19 to 0.63 g cm-3, whereas the bulk density of the seven biochar groups ranged from 0.06 to 0.72 g cm-3 (Table 1), which fell within the range of values for other biochars reported previously (Lehmann et al., 2011; Song and Guo, 2012).The bulk densities of manure and mushroom soil biochars were greater than those of charcoal and activated carbon, while the bulk densities of husk, corn, grass, and wood biochars were less than those of charcoal and activated carbon. The grass biochars had the lowest bulk density, and the densities of the cow manure were between those of grass and corn biochars, while poultry manure biochars showed the highest bulk density, which was in the same range as published poultry litter biochars (Song and Guo, 2012). The differences in bulk density among biochar groups indicated that the feedstock of the biochars played a key role. Analysis of the samples using SEM/EDX (Fig. 5) indicated the presence of mineral particles in the poultry and mushroom soil biochars which would account for the higher density observed for these biochar samples.
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The biochars pyrolysed at 600 Â°C had higher bulk densities than biochars pyrolysed at 300 Â°C. The samples with higher bulk densities also contained higher concentrations of S, K, Si, and Ca (Table 2; Fig. 5; Supplementary Fig. S3a-e). The lower densities observed for the high T biochars may be due to the slow release of volatiles which can cause the formation of large meso/macro networks which are not abundant at low pyrolysis temperatures (Bandosz, 2006). Typical soil bulk densities are around 1.4 g cm-3 (Tranter et al., 2007). Consequently, soil bulk density would decrease after amendment with the experimental biochars.
No clear patterns were observed among the mode and median diameters among different biochar groups, while the mode and median diameters of the biochars charred at 300 Â°C were greater than those of biochars charred at 600 Â°C with the exception of poultry manure and mushroom soil biochars. When the pyrolysis temperature was increased, the biochars became more fragile, consistent with previous findings (Uzun et al., 2007), whereas poultry manure and mushroom soil appeared to be more aggregated when pyrolysed at high temperature (Fig. 5; Supplementary Fig. S3a-e). All the biochar median diameters were less than the mode diameters, which indicated particle-size distributions were skewed toward finer particle size (Supplementary Fig. S1a-c). The histogram also showed all biochar particle size distributions were non-lognormal and asymmetric, even when bimodal or trimodal distributions existed (Supplementary Fig. S1a-c).
3.2 Specific surface area, carbon and sulfur content
The specific surface area (SSA) of activated carbon was around 575 m2 g-1, and the SSA of charcoal was between 52.8 m2 g-1 and 243 m2 g-1. The particle-size distribution is correlated to the SSA, which can be seen from coarse and dust charcoal (CL5). The SSAs of biochars charred at 300 Â°C varied from 1.06 m2 g-1 to 11.3 m2 g-1, and were much less than those of charcoal and activated carbon, while the SSA of biochars charred at 600 Â°C varied considerably from 1.93 to 229 m2 g-1. The feedstocks did not likely play a key role in determining the SSA value in this study, but the preparation temperatures did. The ratio of biochar SSAs prepared at 600 Â°C over that of biochars prepared at 300 Â°C ranged between 0.67 and 131, which indicated that high temperature pyrolysis could greatly increase the SSA. However, the SSAs of cotton seed husk, mixed grass, hop, poultry manure, and mushroom soil biochars charred at 600 Â°C were similar to those of corresponding biochars charred at 300 Â°C. Uchimiya et al. (2011b) showed the same trend with respect to cotton seed biochars. The low SSAs of biochars charred at 300 Â°C indicated that either the biochars were not micro-porous or that the pores within these biochars were small and dead-ended, preventing access to the adsorbing gas (Suhas et al., 2007). These structures can be observed in SEM images (Fig. 5). Shen et al. (2012) also observed the same trend with regard to coconut coir biochars. As pyrolysis temperature increased, more micro-pores and meso-pores were formed (Fig. 5), and these pores increased the SSA (Bandosz, 2006). However, this is not the case for poultry manure biochars; the SSA was observed to decrease as pyrolysis temperature increased. This decrease in SSA can be attributed to the formation of larger particles as pyrolysis temperature increased (Supplementary Fig. S1a-c; Fig. 5). Additionally, mineral grains were within the poultry manure biochar samples (Fig. 5). Mineral matter may occupy the pores of biochars or be exposed at the surface of the biochar particles and block the pores (Downie et al., 2009). Amendment of soil by biochars would increase the SSA and water-holding capacity of soil, which benefits microbes (Downie et al., 2009; Lehmann et al., 2011).
The carbon content of the charcoal and activated carbon samples were the highest ranging from 74.5-99.9%; the carbon contents of manure and mushroom soil biochars were the lowest ranging from 22.8-63.5%; and the carbon content of other biochars ranged from 63.6-99.7% (Table 2.), which was consistent with published values for non-herbaceous biochars (Lin et al., 2012). The sulfur content of the poultry manure biochars was the highest from 0.48-3.40 % (Gaskin et al., 2008), and the sulfur content of charcoal and activated carbon was the lowest.
The carbon and sulfur contents were dependent on the raw materials and pyrolysis temperatures. The carbon and sulfur contents of biochars prepared at 600 Â°C were greater than those of biochar charred at 300 Â°C, consistent with findings reported by Gaskin et al. (2008). As pyrolysis temperature is increased, light elements (like hydrogen) evolve, also leading to enrichment of carbon and sulfur (Bandosz, 2006). The feedstock played a key role in the carbon and sulfur contents of biochars. For example, the poultry manure and mushroom soils had a greater mass of minerals than those of other feedstocks, which would cause a relative decrease in carbon content (Fig. 5). A comparison of the sulfur-combustion spectra of MgSO4 and a coal standard containing organic or reduced sulfur showed that the organic and reduced sulfur peak appeared first and then the sulfate peaks appeared (Fig. 1). Compared with the standard spectra, the results showed that both sulfate and organic or reduced sulfur existed in the mushroom soil and poultry manure biochars, which indicated that these biochars had large amounts of sulfate; this finding was consistent with FT-IR results (Fig. 2). The results of sulfur-combustion spectra showed that organic or reduced sulfur were the major sulfur forms in woody or herbaceous biochars, which is consistent with the previous findings for peat (Chou, 2012). It is important to understand sulfur forms in biochars, because sulfur plays a critical role in the functional groups of biochar, for example promoting adsorption reactions with heavy metals, and because sulfur may be released as a result of aging after the biochar is placed in the environment (DeLuca et al., 2009).
3.3 Fourier Transform Infrared Spectra (FT-IR)
The results from the FT-IR spectroscopic study showed that all the biochar samples contained hydroxyl (-OH, 3449 cm-1), aliphatic (-CH, -CH2, or -CH3, 2920 and 2847 cm-1), and quinone (-(CO)-, 1630 cm-1) functional groups (Fig. 2;(Mayo et al., 2004)). The peak of the hydroxyl group may be a result of the overlap of surface chemisorbed water and hydroxyl groups (Biniak et al., 1997); the quinone groups may be a combination of carboxyl, ketone and aldehyde groups (Moreno-Castilla et al., 1998). Carbonate functional groups (870 cm-1) were observed in most biochars prepared at 600 Â°C (Mayo et al., 2004). The results also indicated that charcoal and activated carbon had the least different types of functional groups, whereas poultry manure and mushroom soil biochars had the most functional groups. Sulfate groups (1114, 615 cm-1) were observed in all poultry manure and mushroom soil biochars and phosphorus-containing functional groups (1039 cm-1) were observed in poultry manure and mushroom soil biochars prepared at 600 Â°C (Cantrell et al., 2012; Mayo et al., 2004). Both inorganic (orthophosphate and its oligomers) and organic phosphate (mono- and diesters) can contribute the intense band which was observed at around the 1039 cm-1 wavenumber (Jiang et al., 2004). The results were consistent with the high phosphorus and sulfur contents observed in the SEM/EDX results for the poultry manure and mushroom soil biochars (Fig. 5; Supplementary Fig. S3c-e). The thiol (2511 cm-1) and C-S (713 cm-1) functional groups were observed in mushroom soil biochars prepared at 600 Â°C (Mayo et al., 2004). These functional groups play a key role in organic and inorganic pollutant removal (Cao and Harris, 2010; Yao et al., 2011).
As noted, fewer peaks were observed, and peak intensity decreased when the pyrolysis temperature was increased from 300 Â°C to 600 Â°C, except for the phosphate, thiol and C-S functional groups. For the hydroxyl and quinone peaks, these reductions may be due to the desorption of chemisorbed water and degradation of quinone structures as pyrolysis temperature increased. As the pyrolysis temperature increases, the structure of the biochar becomes more like that of graphite which is much simpler than that of biochar feedstocks (Bandosz, 2006). Cantrell et al. (2012) and Keiluweit et al. (2010) also observed a similar trend for poultry manure, wood and grass chars.
Principal component analysis (PCA) is one of the most common multivariate analysis techniques. The purpose of PCA is to decompose the complex data matrices and concentrate the source of variability within the complex array of characteristics into the first several principal components (PCs), while losing as little information as possible. For example, principal component analysis, based on FT-IR spectra, has been used in grouping different materials, identifying outlier samples, and determining changes in carbohydrate chemistry of pine leaf litter during thermal decomposition (Lammers et al., 2009; Nieuwoudt et al., 2004). Here, the score of the first principal component (PC1) was plotted against that of the second principal component (PC2) (Fig. 3). The results showed a distinct clustering of samples with regard to pyrolysis temperature; data points in the dashed circle were from the scores of charcoal, activated carbon, and biochars charred at 600 Â°C. The results indicated that pyrolysis temperature played a key role in altering the FT-IR spectra, as pyrolysis temperature increased, the structure tended to become similar for biochars produced from different feedstocks (Bandosz, 2006).However, different biochar groups could not be separated very clearly, except for mushroom soil biochars.
3.4 Scanning Electron Microscopy and Energy-dispersive X-ray (SEM/ EDX)
The SEM/EDX examination was carried out on all biochars, charcoal, and activated carbon. Results for six representative samples are presented in Figure 5 and additional results are provided in supplementary material (Supplementary Fig. S2a-g). Corn cob biochar prepared at 300 Â°C had mesopores (1-10 Âµm) and small number of micropores (<1 Âµm), with some mesopores dead-ended, whereas biochar prepared at 600 Â°C had not only mesopores, but also extensive micropores inside the mesopores. Microporous samples with open pores are expected to have high surface areas, which is consistent with the high SSA observed for the biochars charred at 600 Â°C (Table 2). The corn cob biochars charred at 600 Â°C had mineral particles on the surface and higher percentages of P, Cl, and K as determined by EDX results (Fig.5). These mineral particles may represent detrital minerals originally present in the biochar source or phases that were newly formed during pyrolysis as the temperature increased. The surface structures and elemental composition of poultry manure and mushroom soil biochars were very different from other herbaceous and woody biochars (Fig.5; Supplementary Fig. S3a-f). A greater abundance of mineral particles and higher concentrations of Ca, K, Cl, S, Si, Al, and Mg were observed on the surfaces of poultry manure and mushroom soil biochars than that of other biochars. The presence of sulfur is consistent with the sulfur analysis from combustion (Table 2; Fig. 1). The mineral particles were aggregated with large particles on the surface of poultry manure and mushroom soil biochars prepared at 600 Â°C, which was consistent with the density and particle size distribution analysis (Table 1). These observations are consistent with previous observations of the development of biochar-mineral complexes in biochars prepared at 600 Â°C, which played a key role in the fertility of the biochar-amended soils (Chia et al., 2010).
3.5 Low molecular weight organic acids and dissolved organic carbon
A series of batch experiments was conducted to evaluate the potential of biochars for release of dissolved substances in a natural water sample. Concentrations of different types of low molecular weight organic acids (formate, acetate, lactate, and propionate, Fig. 4) were observed in the equilibrating water. In general, the concentrations of organic acids decreased in the following order: acetate > formate > propionate > lactate. Organic acid concentrations varied with the feedstocks and pyrolysis temperature. Higher concentrations of formate, acetate, and propionate were observed in the batch samples containing coco husk biochars. Relatively high concentrations of these four organic acids were observed in the batch samples containing mixed grass, corn stover2, mushroom soil1, and mushroom soil2 produced at 300 Â°C, while lower concentrations of these acids were observed in the samples containing mulch and pine bark biochars. Moreover, low concentrations of organic acids were released from charcoal and activated carbon. Large differences in concentrations of organic acids were observed between biochars charred at 300 Â°C and biochars charred at 600 Â°C. The concentrations of acetate, lactate, and propionate released by most biochars charred at 600 Â°C were below detection limit (0.01 mg L-1), and the formate concentrations of biochars charred at 600 Â°C were also far lower than those observed for biochars charred at 300 Â°C, with the exception of cocoa husk biochars. The trend was consistent with that published for saw dust biochars (Lin et al., 2012).
Dissolved organic carbon concentrations showed the same trend as the organic acid concentrations. Dissolved organic carbon concentrations ranged from less than the analytical detection limit (0.5 mg L-1) for hop, poultry manure3, mushroom soil1, and mushroom soil2 biochars charred at 600 Â°C, to 148 mg L-1 for corn stover1 charred at 300 Â°C. Dissolved organic carbon concentrations were affected by pyrolysis temperature and biomass sources. Cocoa husk biochars pyrolysed at 300 Â°C and 600 Â°C showed the highest average DOC concentration and charcoal and activated carbon showed the lowest concentration. With respect to pyrolysis temperature, in general, the higher pyrolysis temperatures resulted in the lower dissolved organic carbon concentrations. Mukherjee and Zimmerman (2013) observed higher concentrations of DOC were released by the oak grass biochars produced at 250 Â°C compared with biochars produced at 650 Â°C.
The low molecular weight organic acids accounted for 1-60 % of the carbon in the dissolved organic carbon, with the exception of mulch biochars charred at 600 Â°C. Although organic acids released by biochars charred at 600 Â°C accounted for less than those released by biochars charred at 300 Â°C, no other clear patterns were discernible among different biochar groups. This observation is in contrast to previous observations, which indicate the percentage of the total organic carbon composed of low molecular weight organic acids increases at higher temperatures (Lin et al., 2012). Lin et al. (2012) showed that dissolved organic carbon released from biochars may include biopolymers, humic substances, building blocks (oxidation products of humics), low molecular weight neutral compounds and low molecular weight acids. They also found that the concentrations of low molecular weight acids decreased as pyrolysis temperature increased, while the percentage that accounted for dissolved organic carbon increased from 3.2 % to 41.9% as a result of a much greater decrease of dissolved organic carbon (Lin et al., 2012). Other low molecular organic acids may also exist, such as phenolic acids. Dissolved organic carbon may also include polycyclic aromatic hydrocarbons formed during the pyrolysis process (Hale et al., 2012).
The lower concentrations of organic acids released by mulch and pine bark biochars compared with other biochars may be due to the composition of feedstocks. Mulch and pine bark have higher percentages of lignin, while other feedstocks have higher percentages of hemicellulose and cellulose (Tiwari and Mishra, 2011). The decomposition of hemicellulose occurs around 220 Â°C; cellulose decomposes between 315 Â°C and 400 Â°C; while lignin is difficult to pyrolyze (from 160 to 900 Â°C) (Yang et al., 2007). Polysaccharides (hemicellulose and cellulose) are broken down at temperatures above 300 Â°C, and the products contain low molecular weight sugar derivatives (Shafizadeh, 1985). As pyrolysis temperature is increased, the products formed earlier may undergo further rearrangement through decomposition, condensation, cyclization and polymerization in a series of consecutive reactions (secondary reactions) (Shafizadeh, 1985), leading to the formation of new compounds. The decreased concentrations of organic acids and dissolved organic carbon for the higher T biochars observed in this study may be due to similar secondary reactions. The four low molecular weight organic acids included in this study are labile-C compounds, which could be utilized by microbes in the soil amended with biochars, and the activity of the soil microbial community may be enhanced and the community structure may shift in response to the abundance of these organic acids (Anderson et al., 2011). Due to the high affinity between dissolved organic matter and heavy metals (Ravichandran, 2004), the mobility of heavy metals may be enhanced after amendment with biochars. Beesley et al. (2010) report there is a 30 fold increase of copper and arsenic concentrations in soil pore water after adding biochars.
The physical and chemical properties of biochars, and the potential to release short-chain organic acids and DOC, varied depending on feedstock type and pyrolysis temperature. Based on the feedstock type and biochar properties, the biochars characterized in this work could be sorted into three categories of herbaceous (husk, corn, grass, and cow manure), woody (wood and charcoal), and manure (mushroom soil and poultry manure) biochar. The herbaceous biochars had the lowest bulk density and relatively higher potential to release short-chain organic acids and DOC compared with other biochars. The woody biochars showed medium bulk density, higher carbon content, and higher specific surface area than other biochars. The manure biochars had the highest bulk density, sulfur content, mineral contents, and the most complex FT-IR spectra and surface composition due to the minerals.
The biochars produced from different feedstocks at 600 Â°C tended to have similar properties. They had higher bulk density, specific surface area, and carbon and sulfur content than those pyrolysed at 300 Â°C; they also had fewer peaks in the FTIR spectra and a lower potential to release short-chain organic acids and DOC. They had similar pore structures characterized by SEM/EDX.
In conclusion, feedstock types and pyrolysis temperature played a key role in the properties of biochars. These properties have the potential to influence the future environmental application of biochars. To determine the biochar for a specific application, feedstock types and pyrolysis temperature should be taken into consideration.
This research was funded by the Natural Science and Engineering Research Council of Canada (NSERC) and E. I. du Pont de Nemours and Company. We thank Howard Siu and Jacob Fisher for their assistance with FTIR data processing and Alana Ou Wang, Krista Desrochers, and Krista Paulson for analytical assistance and advice on the experimental set-up. We also thank James Dyer from E. I. du Pont de Nemours and Company for his advice on the results and discussion.
Figure 1. Sulfur signal curve during biochar combustion at 1500 oC
Figure 2. FTIR spectra of seven representative biochars from each biochar group and one activated carbon sample
Figure 3. Principal component analysis of 1094 wavenumbers of all biochars and activated carbon.
Figure 4. Concentrations of short-chain organic acids and dissolved organic carbon for river water equilibrated with biochar samples.
Figure 5. SEM/EDX images of representative biochar samplesReference
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