Pyrolysis for Biochar Purposes - ACS Publications - American

Critical Review pubs.acs.org/est

Pyrolysis for Biochar Purposes: A Review to Establish Current Knowledge Gaps and Research Needs Joan J. Manyà Thermo-chemical Processes Group (GPT), Aragón Institute of Engineering Research (I3A), University of Zaragoza, Technological College of Huesca, crta. Cuarte s/n, E-22071 Spain ABSTRACT: According to the International Biochar Initiative (IBI), biochar is a charcoal which can be applied to soil for both agricultural and environmental gains. Biochar technology seems to have a very promising future. Nevertheless, the further development of this technology requires continuing research. The present paper provides an updated review on two subjects: the available alternatives to produce biochar from a biomass feedstock and the effect of biochar addition to agricultural soils on soil properties and fertility. A high number of previous studies have highlighted the benefit of using biochar in terms of mitigating global warning (through carbon sequestration) and as a strategy to manage soil processes and functions. Nevertheless, the relationship between biochar properties (mainly physical properties and chemical functionalities on surface) and its applicability as a soil amendment is still unclear and does not allow the establishment of the appropriate process conditions to produce a biochar with desired characteristics. For this reason, the need of enhancing the collaboration among researchers working in different fields of study is highlighted: production and characterization of biochar on one hand, and on the other measurement of both environmental and agronomical benefits linked to the addition of biochar to agricultural soils. In this sense, when experimental results concerning the effect of the addition of biochar to a given soil on crop yields and/or soil properties are published, details regarding the properties of the used biochar should be well reported. The inclusion of this valuable information seems to be essential in order to establish the appropriate process conditions to produce a biochar with more suitable characteristics. carbon in stable biochar.8 Biomass pyrolysis and gasification are well-known technologies for the production of biofuels and syngas. However, commercial exploitation of biochar as a soil amendment is still in its infancy.2 Pyrolysis process and its parameters (principally final temperature, heating rate, pressure, and residence time at the final temperature) greatly condition the biochar production and quality. In addition to this, the intrinsic nature of the biomass feedstock also interacts with the rest of the variables in determining the properties of the produced biochar.9,10 The relationship between biochar properties and its potential to enhance agricultural soils is still unclear and does not allow the establishment of the appropriate process conditions in order to produce a biochar with desired characteristics.11 Several recent studies have been focused on providing a characterization methodology of biochars.11−14 These studies represent an initial step, but further efforts are needed to perform soil tests in order to establish an appropriate formulation of desired biochar properties.

1. INTRODUCTION Concerns about climate change and food productivity have recently generated interest in biochar, a form of charred organic matter which is applied to soil in a deliberate manner as a means of potentially improving soil productivity and carbon sequestration.1 The idea of adding charcoal to soil in order to increase its fertility is to be inspired by the ancient agricultural practices, by means of which terra preta soils were created.2 These soils, which may occupy up to 10% of Amazonia,3 are characterized by high levels of soil fertility compared to other soils where no organic carbon addition occurred. Besides the potential of biochar to enhance the fertility of agricultural soils, its apparent ability to increase the capacity of soil to retain water makes biochar a very promising alternative in the current context of climate uncertainty. A high number of recent studies have highlighted the benefit of using biochar in terms of mitigating global warming and as a strategy to manage soil health and productivity.4−8 In most cases, these studies are constrained by limited experimental data and are geographically limited. This fact can be considered as expected because of the complexity of the experimental tasks. Biochar can be produced by several thermochemical processes: conventional carbonization or slow pyrolysis, fast pyrolysis, flash carbonization, and gasification. Slow pyrolysis has the advantage that can retain up to 50% of the feedstock © 2012 American Chemical Society

Received: Revised: Accepted: Published: 7939

March July 4, July 9, July 9,

16, 2012 2012 2012 2012

dx.doi.org/10.1021/es301029g | Environ. Sci. Technol. 2012, 46, 7939−7954

Environmental Science & Technology

Critical Review

a result of many relatively recent studies focused on increasing charcoal yields,9,10,19−25 several variables and factors that play a critical role during the pyrolysis process have been identified: among these are peak temperature, pressure, vapor residence time, and moisture content.19 The peak temperature is the highest temperature reached during the process.19 As a general rule, the charcoal yield decreases as temperature increases. However, an increase of the peak temperature results in an increase of the fixed-carbon content in biochar.19,26,27 This increase is especially pronounced in the temperature range from 300 to 500 °C. In addition, the peak temperature has influence on surface area and pore size distribution (both properties generally related to specific adsorptive properties) of charcoals. Khalil28 reported very low surface areas for charcoals (from a wide variety of biomass feedstocks) pyrolyzed at temperatures near 550 °C. However, setting peak temperatures higher than 700 °C does not seem appropriate to generate charcoals with potentially better adsorptive properties.29−31 Pyrolysis or carbonization at elevated pressure (1.0−3.0 MPa) seems to improve the charcoal yield as a consequence of the increase of the vapor residence time within the solid particle. This effect, which results in a substantial increase of the secondary charcoal production (as a consequence of the decomposition of vapors onto the solid carbonaceous matrix), is magnified when the gas flow through the particle bed is small.19 Furthermore, it should be kept in mind that the energy demand of the pyrolysis process is closely related to the production of charcoal by primary (endothermic) and secondary (exothermic) reactions.20,32 In line with this, an increase of the charcoal produced by secondary reactions can significantly reduce the amount of energy required to sustain the process. Pyrolysis pressure also produces an effect on the porosity of produced charcoals. Cetin and co-workers33 reported a slight decrease of the total surface area by increasing pressure during the pyrolysis of several biomass feedstocks (radiata pine, eucalyptus wood, and sugar cane bagasse). However, in a recent study conducted by Melligan and co-workers,34 a dramatic decrease of the BET (Brunauer, Emmett, and Teller) surface area of charcoals obtained by slow pyrolysis (at 13 K min−1 and at a peak temperature of 550 °C) of miscanthus is reported (from 161.7 m2 g−1 at 0.1 MPa to 0.137 m2 g−1 at 2.6 MPa). The authors attributed this result to a clogging of the pores by tar deposits as a consequence of the high pressure. In addition to this, Melligan and co-workers also reported that chars formed at high pressure had more extended fused aromatic structures, reflected also in the higher carbon contents, than those obtained at atmospheric pressure. Regarding the moisture content of the biomass feedstock, results obtained in previous studies20−35 indicated that high moisture contents (in the range of 42−62%) can improve the yield of charcoal at elevated pressures. This finding makes certain agricultural residues which are characterized by high moisture contents particularly attractive for biochar purposes. In addition to the moisture effect, it must also be taken into account that the charcoal yield from a given biomass feedstock is influenced by its inherent composition (holocellulose, lignin, extractives, and inorganic matter). In this sense, pyrolysis of biomass species with high lignin contents can produce higher charcoal yields.19,36 An increase of the charcoal production was also observed by Di Blasi and co-workers37 when they pyrolyzed extractive-rich woods (e.g., chestnut) instead of other wood varieties with lower extractives contents (e.g., beech). Special attention has been focused on discussing the influence of the inorganic matter on pyrolysis product

The specific aim of the present study is to review and analyze the available published studies related to biochar production, characterization, and its addition into agricultural soils. As a result of this review process, the objective of the author is to highlight the research needs for this exciting field of study. Among other potential research gaps, this paper focuses on the interaction between biochar production and its potential applicability to agricultural soils. In this sense, the knowledge of the effect of the operating conditions governing the pyrolysis process on the properties of the resulting biochar (degree of aromaticity, cation exchange capacity, etc) for a given biomass feedstock, seems to be necessary to facilitate future research on this topic.

2. BIOCHAR CONCEPT Biochar is a carbon-rich, fine-grained, porous substance, which is produced by thermal decomposition of biomass under oxygen-limited conditions and at relatively low temperatures (100 Mg ha−1) seemed to inhibit plant growth. On the other hand, a later study conducted by Steiner and co-workers132 showed that biochar application (at a rate of 11 Mg ha−1) significantly improved plant growth for a highly weathered Central Amazonian upland soil fertilized with NPK (in comparison to the effect of the same rate of NPK-fertilizer without biochar). Table 4 presents examples of experimental studies focused on investigating the response of crops to biochar application. As can be deduced from the data reported in Table 4, the effect of biochar depends on several factors including the soil type, the addition rate, and the kind of crop. Moreover, an interaction between biochar and fertilizer addition is generally observed. In this sense, and as argued above, the fertility of tropical and subtropical soils (such as acidic ferralsols and nitisols) seems to substantially improve by biochar treatment,5,96,125,132,133 especially when biochar was applied together to inorganic fertilizers.96,134,135 However, Van Zwieten and co-workers96 reported significant decreases in wheat and radish biomass production for a high pH calcisol. Negative impacts on crop yield were also observed by Haefele and co-workers133 for rice growth in a gleysol (which had a high CEC and base saturation and high N, P, and K availability). A mechanism to explain the negative effect of biochar for these soil types was proposed by Lehmann and co-workers:126 the available nutrients applied with biochar in this type of soils are not limiting, the CEC is very high already, and water stress does not occur; nevertheless, the high C/N ratio of biomass probably limits N availability (from both soil and inorganic fertilizer), causing a decrease of grain yield. From Table 4, it is also important to highlight the promising results reported by Vaccari and co-workers136 regarding the yield in durum wheat for a silt loam soil (with a pH of 5.2) under the Mediterranean climate conditions. These preliminary results, which should be confirmed in further studies, could indicate that the positive effect of biochar addition on soil production is also possible for other soils than ferralsols in tropical environments. Unfortunately, very little information is available in the literature with respect to the influence of both biochar properties and pyrolysis conditions on plant growth. Nevertheless, a recent study conducted by Peng and co-workers135 reveals some interesting findings. These authors analyzed the effect of both the pyrolysis peak temperature and the soaking time at this temperature for rice straw-derived biochar on soil properties and production function. Regarding the biochar

that the O/C ratio of a given biochar is a potential indicator of both its hydrophilicity and polarity, an increase of the pyrolysis peak temperature probably causes a decrease in polar surface groups, and consequently, a reduction of the biochar affinity for water molecules. As a consequence of both the increase of pore surface area and the decrease of water affinity, the sorption capacity of biochars is expected to increase with the pyrolysis final temperature, as observed by Chun and co-workers118 and Wang and co-workers.116 In contrast to these results, Kinney and co-workers119 observed that biochars obtained from several feedstocks (magnolia leaves, apple wood chips, and corn stover) by slow pyrolysis (at a peak temperature ranging from 400 to 600 °C) exhibited a very low hydrophobicity. Kinney and co-workers also reported a statistically significant effect of the presence of surface alkyl functionalities (C−H), which were detected when biochars were pyrolyzed at a peak temperature below 400 °C, on the hydrophobicity of the analyzed biochars. These apparently contradictory results suggest that further studies focused on analyzing the hydrophobicity of biochars are required. Other Sources of Soil Contamination. This section is focused on the potential for soil contamination linked to some component of biochar, such as heavy metals and PAHs. Despite the fact that this type of contamination can lead to severe public health problems, relatively little attention has been focused on this issue.90 Biochar produced from pyrolysis of some organic wastes, such as sewage sludge and tannery residue, generally retains high levels of heavy metals (e.g., chromium, cooper, nickel, and zinc).97,120 However, McHenry17 suggested that high levels of biochar addition (>250 t ha−1) are needed to potentially contaminate soil, surface water, and crops. Obviously, this topic needs further assessment in future studies. Otherwise, it seems clear that biomass pyrolysis at peak temperatures above 700 °C could generate heavily condensed PAHs.121,122 Brown and co-workers123 reported that several biochar products, which were obtained by slow pyrolysis at different peak temperatures (ranging from 450 to 1000 °C) from pitch pine wood, exhibited PAHs concentrations ranged from 3 to 16 μg g−1 (with the highest value at the highest peak temperature). Brown and co-workers also analyzed the PAHs content of a natural biochar (charred pine from a prescribed burn area), showing that this value (28 μg g−1) was slightly higher than that measured for synthetic biochars. This preliminary finding could suggest that PAHs levels in biochar can be often comparable to (or even lower than) those found in some soils.90 Soil Productivity. An increase in soil fertility is the most frequently reported benefit linked to adding biochar to soils. Most of the published studies to date have been conducted for tropical soils.90 In tropical or subtropical environments, soil fertility tends to be poor due to rapid mineralization of soil organic matter, the low cation exchange capacity (CEC) of the tropical soils (which is usually due to their clay content and mineralogy), and the low nutrient contents.5 Moreover, the use of inorganic fertilizers in these types of soils has certain drawbacks, the most important of which are the high cost of continuous applications of fertilizers and their low efficiency in highly weathered soils.124,125 Some previous studies reported that biochar addition in several tropical soils resulted in an increase of soil nutrient availability.5,126−129 In the short term, the direct nutrient additions with the added biochar (e.g., K, P, and Ca, which are present in the inorganic fraction of biochar) 7947



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Table 4. Summary of Studies Assessing the Impact of Biochar on Crop Yield reference

soil type

biochar

crop

response

Glaser et al. (2002)5

A weathered xanthic ferralsol from Central Amazonia (Brazil)

From secondary forestry wood. Addition rate: 67.2 and 135.2 Mg ha−1.

Rice and cowpea

Steiner et al. (2007)132

A weathered xanthic ferralsol from Central Amazonia (Brazil) A ferralsol (acidic soil) from New South Wales (Australia)

From secondary forestry wood. Addition rate: 11.0 Mg ha−1.

Rice and sorghum

From mixtures of paper mill wastes and wood chips pyrolyzed at 550 °C. Addition rate: 10.0 Mg ha−1.

Wheat, soybean and radish

Van Zwieten at al. (2010)96

A carcisol (alkaline soil) from Victoria (Australia)

From mixtures of paper mill wastes and wood chips pyrolyzed at 550 °C. Addition rate: 10.0 Mg ha−1.

Wheat, soybean, and radish

Major et al. (2010)125

A clay-loam ferralsol from the oriental savanna of Colombia

Commercial wood charcoal. Addition rate: 20.0 Mg ha−1.

Maize

Hossain et al. (2010)134

A luvisol from Sydney (Australia)

From sewage sludge pyrolyzed at 550 °C. Addition rate: 10.0 Mg ha−1.

Cherry tomato

Haefele et al. (2011)133

An anthraquic gleysol from Laguna (Philippines)

From rice husk partially burned in a combustion chamber Addition rate: 0.413 Mg ha−1.

Rice


A humic nitisol from Siniloan (Philippines)


Rice


A gleyic acrisol from Ubon (Thailand)


Rice

Peng et al. (2011)135

A typical ultisol from southern China

From rice straw pyrolyzed at low heating rate and at peak temperatures below 450 °C. Addition rate: 240 Mg ha−1.

Maize

Vaccari et al. (2011)136

A silt loam soil (with a subacid pH of 5.2) from the region of Tuscany (Italy)

A commercial charcoal obtained from coppiced woodlands (beech, hazel, oak, and birch). Addition rate: 30−60 Mg ha−1.

Durum wheat

Van Zwieten at al. (2010)96

At a rate of 67.2 Mg ha−1 biomass increased by 20% (rice) and 50% (cowpea) compared to control (no biochar). At 135.2 Mg ha−1 biomass cowpea increased by 100%. Stover and grain yields increased by 29% and 73%, respectively; compared to control treatment (only inorganic fertilizer). Wheat: biochar improved yield by a factor of 1.3. When biochar was applied together to inorganic fertilizer, yields increased by a factor of 2.4 compared to using fertilizer alone. Soybean: biochar improved yield by a factor of 1.4 in the presence of fertilizer. Radish: dry biomass production was significantly increased by a factor of 1.5−2 both in the presence and absence of fertilizer. Biochar significantly increased both pH and CEC and reduced Al availability. Wheat: yields were reduced by a factor of 2 both in the presence and absence of inorganic fertilizer. Soybean: biochar improved yield by a factor of 1.3 in the presence of fertilizer. Radish: biochar addition in the absence of fertilizer increased biomass production by a factor of 1.5. However, in the presence of fertilizer, yields were reduced by a factor of 2. Effect for 4 years (2003−2006). Maize grain yield did not significantly increase in the first year, but increases over the control were 28, 30 and 140% for 2004, 2005 and 2006, respectively. Application of biochar improves the production of cherry tomatoes by 64% above the control soil conditions. The yield of production was found to be at its maximum when biochar was applied in combination with an inorganic fertilizer. Effect for 4 years (2005−2008). Application of carbonized rice husks increased total organic carbon, total soil N, the C/N ratio, and available P and K. Biochar application decreased rice yields, especially in the first few seasons after application. For the entire period evaluated, rice yield decreased by 2.5% relative to control conditions. Effect for 4 years (2005−2008). Application of carbonized rice husks increased total organic carbon, total soil N, the C/N ratio, and available P and K. For the entire period evaluated, rice yield increased by 8.9%. Effect for 4 years (2005−2008). Application of carbonized rice husks increased total organic carbon, total soil N, the C/N ratio, and available P and K. For the entire period evaluated, rice yield increased by 8.7%. The effected of biochar amendment application on the 40-day maize dry matter production was marked: in the absence of NPK fertilizer, biomass production increased by 64%. In the presence of fertilizer, biomass production increased by a factor of 3 compared to using fertilizer alone. Effect for 2 years (2009−2010). Biochar addition significantly increased grain production with respect to the control. On average, the grain yield increase ranged from 28% to 39%. No significant differences were observed between biochar rate treatments of 30 and 60 Mg ha−1.

stability and its liming effect increased with pyrolysis peak temperature and residence time. Nevertheless, and interestingly, no significant effects of pyrolysis conditions on the CEC of the tested soil (a highly weathered ultisol from southern China) and the maize yield were observed by Peng and co-workers. In another interesting study, Deenik and co-workers at the University of Hawaii137 showed that partially carbonized

characteristics, Peng and co-workers observed that increasing both peak temperature (from 250 to 450 °C) and soaking time (from 2 to 8 h) obviously decreased the biochar yield and volatile matter content but increased the C, K, and P contents. In addition, volatile matter, O, H, and aliphatic functional groups decreased at the expense of aromatic C as peak temperature and soaking time increased. As a result, the biochar 7948



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using analytical devices, the operation of which is based on the complete combustion with a pure oxygen atmosphere.139 Inorganic Fraction Characterization. Two techniques are generally applied to isolate the inorganic fraction of carbonaceous materials:139 low-temperature ashing (LTA) in an oxygen plasma at 100−150 °C and medium-temperature ashing (MTA) in air a 600 °C. Suárez-Garciá and co-workers140 suggested the use of both isolation techniques to securely identify the inorganic constituents of a given sample. Once the inorganic fraction has been isolated, several analytical techniques can be applied to characterize the inorganic species: inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray fluorescence (XRF), and X-ray diffraction (XRD). ICP-AES is able to determine the absolute concentration of inorganic elements (Al, Ca, Fe, K, P, Mg, Si, etc.).141 XRF spectrometry is useful to determine the ash compositions in terms of weight fraction of oxides140 and XRD can be used to identify the crystalline minerals in ash.141 Both exchangeable K and P are important parameters that can partially establish the capability of biochar to supply nutrients to soil on a short-term basis. These contents (exchangeable K and exchangeable P) in biochar were found to range widely as a function of the feedstock, with values of 1.0−58.0 and 2.7−480 g kg−1, respectively.142 These ranges are somewhat wider than those reported in the literature for typical organic fertilizers.90 Nevertheless and according to Joseph and co-workers,143 the role of high-ash biochars is still unknown and experimental data are needed in order to determine the effect of the ash on soil properties on the medium- and longterm basis. Textural Characterization and Morphology. As has already been mentioned in the earlier sections, both the specific surface area and pore size distribution depend mainly on two factors: the nature of the biomass feedstock and the pyrolysis operating conditions (especially peak temperature). To experimentally determine the textural parameters of a biochar sample, adsorption of N2 at 77 K and adsorption of CO2 at 273 K are typically used. From the results corresponding to the N2 adsorption isotherms, the specific surface area based on the equation of Brunauer, Emmett, and Teller (SBET) can be determined.88 From the same adsorption isotherms and adopting the Dubinin−Radushkevich method, the micropore volume (V0) can be calculated.88 Furthermore, the volume of mesopores (Vme) can be estimated from the isotherm as the difference between the volume of N2 adsorbed at a relatively pressure of 0.95 and the value of V0.144 On the other hand, the narrow micropore volume (W0; pore width below 0.7 nm) can be estimated from the CO2 adsorption isotherms assuming the Dubinin−Radushkevich method.145 Regarding the morphological characterization, scanning electron microscopy (SEM) is commonly used to analyze the char particle structure and surface topography.2,11 Surface Functionality. Surface functionality can be investigated by means of Fourier transform infrared (FTIR) spectroscopy. The FTIR spectra of both biomass feedstock and biochars obtained at different pyrolysis peak temperatures are useful to analyze the gradual loss of lignocellulosic functional groups (change in the O−H stretch peak around 3400 cm−1, which dominates the feedstock’s spectrum).11 Assignment of other spectral peaks of interest for biochar samples, including the aliphatic C−H stretch at 3000−2860 cm−1, the aromatic C−H stretch around 3060 cm−1, and the various aromatic ring modes at 1590 and 1515 cm−1, was proposed by Sharma and

biochar containing a relatively high volatile matter (VM) content produced lower yielding plants in biochar-amended soil compared with soil not treated with biochar. The poor yield in the high-VM biochar amended soils could be due to an inhibition of N availability (the authors attributed this effect to the presence of phenolic compounds in the volatile matter, which stimulated microbial activity leading to a reduction of inorganic N). In contrast, more fully carbonized biochar with low-VM content did not produce a negative effect on plant growth, and when it was combined with N fertilizer, there was a significant improvement in crop yield compared with the fertilized control. Both biochars were obtained from macadamia nut shells by means of a flash carbonization process at different peak temperatures: 430 °C for the high (225 g kg−1) VM biochar and 650 °C for the low (63 g kg−1) VM biochar. Deenik and co-workers,137 who conducted a series of shortterm (4−6 weeks) greenhouse experiments and laboratory incubations, observed the above-mentioned effects for two types of Hawaiian soils: an andosol (a volcanic soil) and an uncultivated, highly weathered and extremely acid ultisol. The results obtained for the tested ultisol are clearly in disagreement with many other studies,5,96,125,132,135 in which positive effects on plant growth of biochar addition to acid tropical soils are reported. Nevertheless, it should be noted that not all biochars will exhibit the same effects for a given soil type. In other words, the negative results reported by Deenik and coworkers137 only suggest that the quality of biochar is at least as important as the soil type.

5. BIOCHAR CHARACTERIZATION REQUIREMENTS Taking into account that the form of carbon (aromatic or nonaromatic C) present in biochar is believed to be related to the stability of this material on soil, a key aspect of determining the potential of a given charcoal for biochar purpose may be the ability to characterize its surface chemistry.11 However, additional properties should be considered in order to preliminarily evaluate the potential of a given biochar. These properties can be physical (e.g., specific surface area and morphology) or chemical (such as proximate and elemental analysis and mineral content). Recently, the International Biochar Initiative has published guidelines138 to provide standardized information regarding the characterization of biochar materials and to assist in achieving more consistent levels of product quality. These Biochar Guidelines identify three categories of tests for biochar: test A for basic utility properties, test B for toxicant assessment, and test C for advanced analysis and soil enhancement properties. In the next sections, information concerning the analytical methods used to measure biochar properties is given. Proximate and Elemental Analysis. The proximate analysis yields the weight fractions of moisture, volatile matter (VM), ash, and fixed carbon (FC). There are standardized methods for performing a proximate analysis (ASTM, ISO, DIN, and BS). These standards are very similar in nature except for slight differences in the operating conditions (temperature and soaking time) used to quantify the volatile matter content. As was mentioned in previous sections, the volatile matter content is negatively correlated with peak temperature and, according to earlier studies,137 a high value of this parameter could indicate a low potential of a given biochar for soil amelioration purposes. Regarding the elemental analysis, the weight percentages of carbon, hydrogen, nitrogen, and sulfur are usually determined 7949



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co-workers.146 The peaks characteristic of the carbonyl groups should appear in the range 1660−1725 cm−1. The exact position of the peaks depends on whether the carbonyl groups are in conjunction with the aromatic ring (position below 1700 cm−1) or not (position above 1700 cm−1).146 X-ray photoelectron spectroscopy (XPS) can also be used for surface analysis.16,101,147 The XPS wide-scan spectrum usually shows the presence of two main peaks in C (C1s) at around 285 eV and O (O1s) at around 530 eV. The spectra of high resolution XPS of C1s and O1s are used to quantify the carbon and oxygen forms on the biochar surface. For the C1s spectrum, different binding energies are assigned to C−C, CC, C−H, C−O, CO, and COO stretches; whereas for the O1s spectrum, signal peaks at different binding energies can be attributed to OC and O−C stretches.101 Aromatic Character. Solid-state 13C magic angle spinning (MAS) nuclear magnetic resonance (NMR) is commonly used for making quantitative comparisons without recurring to the procedure of taking peak ratios. Rather, each resonance peak can be quantified in relation to the total resonance intensity, giving therefore the relative abundance of individual molecular groups.146 As mentioned before, the aromatic character of the produced biochar seems to be directly correlated to the value of the pyrolysis peak temperature: as peak temperature increases it is expected to show a higher aromatic structure. Freitas and coworkers148 reported 13C cross-polarization (CP) MAS NMR spectra for biochars obtained by pyrolysis of rice hulls at different peak temperatures. For the biochars obtained at a peak temperature of 300 °C, these authors observed two main resonance lines, around 130 ppm (broader) and 148 ppm, associated with nonoxygenated and oxygenated aromatic carbons, respectively. Simultaneously and for the same biochar samples, Freitas and co-workers147 observed broad resonance around 31 ppm, probably associated with aliphatic chains, and the development of a small signal near 208 ppm, ascribed to ketone groups. Regarding the biochars produced at higher peak temperatures (390−605 °C), the authors showed a progressive development of a well-defined aromatic resonance, centered at 125 ppm, which occurs simultaneously with the attenuation of the signals corresponding to oxygenated aromatic carbons (around 150 ppm) and aliphatic groups (broad line around 30 ppm). Recently, McBeath and co-workers149 analyzed both crosspolarization (CP) and direct polarization (DP) spectra for chestnut wood-derived biochars pyrolyzed at different peak temperatures. Results from the work of McBeath and coworkers indicated that aromaticity of biochar rapidly increases when peak temperature is above 400 °C. In addition, the authors also reported that proportion of aromatic C detected was similar for both CP and DP techniques for all charcoals.

and lignin contents, and mineral matter characterization), the process chosen for the biochar production and the detailed operating conditions of which (peak temperature, soaking time, heating rate, etc.), and information concerning the properties of the used biochar (ultimate and proximate analysis, specific surface area, pore size distribution, organic character, etc.). The inclusion of this valuable information seems to be essential in order to establish the appropriate process conditions to produce a biochar with more suitable characteristics. In addition to the general consideration outlined above, several research gaps and issues have been identified through this literature review. These research priorities are listed below: • Among the operating conditions of the slow pyrolysis process, the peak temperature seems to be the most important parameter affecting the characteristics of biochar product. An increase of peak temperature seems to lead to the generation of biochars with higher aromatic character and fixed carbon and higher porosity. At the current state of the art, this fact seems to be positive regarding the stability of the carbon in the biochar and the enhancement of nutrient retention of a given biochar-amended soil. Further studies analyzing the effect of pyrolysis peak temperature on both biochar stability and nutrient retention (CEC) are required to confirm this preliminary trend. • Although slow pyrolysis or carbonization is the process commonly used to produce biochar, because of the high charcoal yields obtained, other technologies cannot be underestimated. In this sense, in situ catalytic fast pyrolysis can be an interesting option to simultaneously produce a bio-oil with enhanced properties and a biochar at an acceptable yield. On the other hand, developing innovative processes, such as the flash carbonization process, would be a key priority for the research community in order to improve both the productivity and the quality (fixed carbon yield) of the produced biochar. • The specific surface area and the micropore volume of a given biochar obtained after pyrolysis can be substantially enhanced through an activation step. This secondary activation process can be a gasification step (physical activation by using an oxidizing agent at a final temperature of 700−850 °C) or an additional carbonization step (under an inert atmosphere at a temperature of 850−1000 °C). In both cases (but especially in the physical activation process), the benefit of improving biochar porosity (and, consequently, the potential of the biochar to improve the soil−water retention and soil aeration) is accompanied by a loss of carbon retention and sequestration capacity. For this reason, further investigations would be required to reach a compromise between the desired textural properties and the carbon sequestration potential for a biochar obtained from a given biomass feedstock. • Despite the fact that the form of carbon (aromatic or nonaromatic C) present in biochar is believed to be related to the stability of this material on soil, the influence of additional properties (physical and chemical) on the stability of the biochar placed in soil remains still unclear, and further studies, in which the effect of environmental conditions (i.e., water regime) on biochar stability can be measured, are needed. As mentioned

6. CONCLUSIONS The present review highlights the need for greater collaboration among researchers working in different fields of study: production and characterization of biochar on one hand, and on the other, measurement of both environmental and agronomical benefits linked to the addition of biochar to agricultural soils. In this sense, when experimental results concerning the effect of the addition of biochar to a given soil on crop yields and/or soil properties are published, details about the properties of the used biochar should be well reported. These details include the biomass feedstock and its composition (elemental and proximate analysis, holocellulose 7950



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before, the properties of the tested biochars must be reported in these studies. • Very little information is now available regarding the influence of both biochar properties and pyrolysis conditions on plant yield. Consequently, further research studies, at the field scale, focused on analyzing the effect of a given biochar, obtained under a given set of operating conditions, on the biomass yield of a given plant in a given type of soil will be crucial to gain knowledge on this topic.

AUTHOR INFORMATION

Corresponding Author

E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS I thank Prof. Clara Marti ́ for useful remarks and comments in the field of soil science. I also thank reviewer no. 2 for his detailed comments and helpful suggestions aimed at improving the paper.

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