Biochar is widely recognized as an efficient tool for carbon sequestration
and soil fertility. The understanding of its chemical and physical
properties, which are strongly related to the type of the initial material
used and pyrolysis conditions, is crucial to identify the most suitable
application of biochar in soil. A selection of organic wastes with different
characteristics (e.g., rice husk (RH), rice straw (RS), wood chips of apple
tree (
The interest in the application of biochar as a method for mitigating the global-warming effects is steadily increasing. In addition to the studies on the use of biochar for carbon sequestration, a number of reports have focused on alternative applications of biochar for the improvement of soil fertility, plant growth, and decontamination of pollutants such as pesticides, heavy metals, and hydrocarbons (Beesley et al., 2011; Cabrera et al., 2011). The diverse range of biochar applications depends on its physicochemical properties, which are governed by the pyrolysis conditions (heating temperature and duration) and the original feedstock (Enders et al., 2012). Thus, detailed information about the complete production process is a key factor in defining the most suitable application of biochars.
The biochar physicochemical properties can cause changes in the soil nutrient and C availability, and provide physical protection to microorganisms against predators and desiccation; this may alter the microbial diversity and taxonomy of the soil (Lehman et al., 2011). The biochar derived from relatively low-temperature pyrolysis is characterized by a high content of volatile matter that contains easily decomposable substrates, which can support plant growth (Robertson et al., 2012; Mukherjee and Zimmerman, 2013). In contrast, the structure of biochar derived from high-temperature pyrolysis is characterized by a large surface area and aromatic-carbon content, which may increase the adsorption capacity (a desirable property for bioremediation) as well as the recalcitrant character (for carbon sequestration) (Lehmann, 2007).
The type of feedstock material is another important factor that determines
the final application of the biochar and its effect in soil, because its
properties are affected by the nature of the original material. For instance,
the soil cation-exchange capacity of manure-based biochars is higher than
that of wood (
The aim of our study is to optimize the physicochemical characteristics of
biochar for its use in agriculture by investigating different pyrolysis
conditions and agricultural wastes used as feedstocks. To achieve this aim,
the thermochemical properties of the biochars obtained at different
temperatures (400–800
The biochars used in this work were obtained from two rice residues
(
After the pyrolysis process, all samples were ground and sieved to less than
0.5 mm in diameter. The biochar yield was calculated as the proportion of
the weight of pyrolysis product to the original material. The determination
of the volatile matter and ash content was conducted according to the
American Society for Testing and Materials (ASTM) D1752-84, which is
recommended by the International Biochar Initiative. The volatile matter was
thus determined by measuring the weight loss that follows the combustion of
about 1 g of charcoal in a crucible at 950
The elemental composition of C, H, and N was determined using an elemental analyzer (Thermo Finnigan EA-1112, Thermo Fisher Scientific Inc., MA, USA); the O content was determined by Vario El cube, Elementar Analysensysteme GmbH.
Physical and chemical characteristics of the biochars derived from different feedstocks: apple tree branch (AB), oak tree (OB), rice husk (RH), and rice straw (RS).
The thermal analysis of the biochars was performed by using an SDT-2960
simultaneous DSC-TGA thermal analyzer (TA instruments) under static-air
atmosphere with the following temperature ramp: (1) temperature equilibration
at 30
FT-IR spectroscopy was analysed on a Varian 670-IR (Agilent Technologies
Inc., CA) using the pellet technique by mixing 1 mg of dried biochar with
300 mg of pre-dried and pulverized spectroscopic-grade KBr (from Merck and
Co., Whitehouse Station, NJ). The following broad-band assignment was used
(Chen and Chen, 2009; Haslinawati et al., 2011; Novak et al., 2010; Peng et
al., 2011; Yuan et al., 2011; Wu et al., 2012; Guo and Chen, 2014): 3400 to
3410 cm
Van Krevelen diagram of the biochars derived from different
feedstocks: apple tree branch (AB), oak tree (OB), rice husk (RH), and rice
straw (RS). Each symbol indicates the pyrolysis temperature as follows:
black: 800
Cross-polarization magic angle spinning (CPMAS)
Elemental composition of the biochars derived from different feedstocks: apple tree branch (AB), oak tree (OB), rice husk (RH), and rice straw (RS).
Thermal analysis of the biochars obtained from
Fourier-transform infrared (FT-IR) spectra of the biochars obtained
from
Cross-polarization magic angle spinning (CPMAS)
The characteristics of the biochars derived from different agricultural
wastes are shown in Table 1. Low-temperature pyrolysis produced a higher
biochar yield and enriched volatile-matter composition than the
high-temperature pyrolysis. The biochar yields and volatile contents
gradually diminished as the pyrolysis temperature increased. Moreover, the
type of feedstock also affected the biochar yields and the volatile-matter
content. Among the different biochar types, woody biochars (AB and OB) showed
a larger change in the volatile content from 400 to 800
The pH value of biochars increased with temperature, probably as a
consequence of the relative concentration of non-pyrolyzed inorganic
elements, already present in the original feedstocks (Novak et al., 2009).
The porosity and surface area represent the most critical physical properties
of biochar for the improvement of soil properties such as soil adsorption
capacity and water retention ability (Kalderis et al., 2008). The application
of the RH biochar has been reported to enhance these properties (Kalderis et
al., 2008; Liu and Zhang, 2009; Lei and Zhang, 2013). As shown in Table 1, a
biochar production at higher temperatures generally leads to an increase in
the MB number, I
Analytical elements and both H
Because of the high temperature of the charring process, the H
A comparison of the feedstocks in the diagram (Fig. 1) indicates that the
H
Thermal analysis is a useful method to study the structure of biochar materials (Kalderis et al., 2014; Mimmo et al., 2014). In this work, all biochar samples showed a similar thermal-degradation profile (Fig. 2), with the weight loss proportionally increasing with the temperature of pyrolysis. In this respect, a clear difference among the feedstocks (wood vs. non-wood) was observed, i.e., the weight loss of AB and OB and that of RH and RK was 90 and 40–50 % of the total weight, respectively; this behavior reflects the higher mineral content in rice materials. In addition, the mineral component functions as a barrier that prevents the diffusion of heat and therefore the release of the volatile component during the charring process (Xu and Chen, 2013).
FT-IR spectroscopy is a great tool to observe the shift change of chemical
composition. The aliphatic loss process is represented by the band of FT-IR
with aliphatic C–H stretching (2950–2850 cm
The nature of the feedstock was reflected by the presence of bands around
460, 800, and 1040–1100 cm
The
The data presented in this work showed that both the pyrolysis temperature
and the type of feedstock strongly influence the physicochemical properties
of the biochars. In particular, an increase in the temperature improved the
adsorption properties such as surface area, porosity, and recalcitrant
chemical character in woody biochars (AB and OB). In contrast, rice-material
biochar (RH and RS) shows a higher yield during the pyrolysis process than
that of AB and OB. In addition, the properties of the rice-material biochar
products are different from woody biochars, i.e., the inorganic components
are combined with organic moieties as a consequence of the carbon
encapsulation by silicon presence. Finally, higher-heat production
(temperatures above 600
This work was partly supported by the bilateral project of the Japan Society for the Promotion of Science (JSPS) and the Spanish National Research Council (CSIC). Edited by: X. Wang