BIOCHAR - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 99

develops within the fused-ring matrix. Using cellulose, lignin, and woody material, these
researchers observed that total and microporosity developed at temperatures above 300 °C
and increased with temperature and heating time (Rutherford et al., 2004). The extent of
porosity development depends on material composition, with lignin developing porosity at
higher temperatures than cellulose. The increase in biochar surface area was correlated with
increase in microporosity (Rutherford et al., 2004), with cellulose surface area increasing
from 2.1 m 2 g -1 in the untreated cellulose to 147 m 2 g -1 at 400 °C, and lignin from <1 m 2 g -1 in
the untreated material and with no significant increase below 350 °C to only 2.0 m 2 g -1 at 400
°C and up to 162 m 2 g -1 during pyrolysis at 500 °C for 1h. Similar trends of increase in
porosity and surface area were reported for poplar (exact specie unknown) and ponderosa
pine ( Pinus ponderosa ) wood when pyrolyzed at 500 °C for 1h; surface area increased from
<2.0 m 2 g -1 for both woody materials to 354 and 501 m 2 g -1 , respectively. While lignin surface
area and porosity increased with increase in exposure time at each temperature level, surface
area and porosity of cellulose, poplar and pine wood, and pine bark decreased at exposure
times longer than 24 h (Rutherford et al., 2004).
Similarly, steam activation, i.e. physical activation of biochar using hot stream of water
vapor, also showed to markedly increase biochar surface area, attributed almost entirely to the
increase in inner sphere pores (Lima et al., 2010). Porosity and surface area of biochar of
different feedstock produced at 500 °C in a fast pyrolysis fluidized reactor with residence
time of 0.1-1.0 s was measured before and after steam activation at 800 °C for 45 min. While
all biochar had negligible surface area (<4.0 m 2 g -1 ) and non-detected porosity, steam
activation resulted in dramatic increases in biochar surface area (Lima et al., 2010). For
example, surface area of biochars derived from alfalfa ( Medicago sativa L. ) stems,
switchgrass, and corn stover increased from 2.3 to 204, 0.3 to 293, and 3.1 to 455 m 2 g -1 ,
respectively; with microporosity constituting 79, 85, and 75% of total surface area,
respectively (Lima et al., 2010).
Inasmuch as pyrolysis temperature affects biochar surface area and porosity, it also
affects its surface chemical properties (Boehm, 1994; Ramon et al., 1999; Brennan et al.,
2001; Chun et al., 2004). Aromatic regions with ˀ electron-rich areas (Lewis bases) at the
carbon plane contribute to biochar basicity while oxygen surface oxides tend to reduces
carbon plane electronic density, reducing its basicity and providing hydrophilic sites to the
otherwise hydrophobic surface associated with the aromatic ˀ-electrons (Boehm, 1994;
Ramon et al., 1999; Brennan et al., 2001). Increase in pyrolysis temperature increases the
content of aromatic structures in the biochar and the orientation and condensation of the
aromatic groups into amorphous graphene/graphite-like structure at higher temperatures.
Furthermore, increase in production temperature reduces biochar oxygen content while
shifting the proportion of remaining oxygen from strong acid functional groups, such as
carboxyl into weaker phenolic groups of low pK a or oxygen-containing groups, such as
pyrone-type structures, as well as from other impurities, such as amine groups that further
contribute to char basicity (Boehm, 1994). Increasing temperature from 300 to 700 °C of
biochar produced from wheat residue led to reduction in (1) total acidic surface functional
groups, from 2.83 to 0.30 mmol g -1 ; (2) functional groups surface density, from 15 to 1
groups nm -2 ; (3) carboxyl groups, from 0.74 to 0.17 mmol g -1 ; and (4) O/C ratio and biochar
hydrophilicity; while increasing (1) basic functional groups content, from 0.04 to 0.29 mmol
g -1 and (2) surface area, from 116 to 363 m 2 g -1 , all respectively (Chun et al., 2004). Indeed,
the point of zero charge (PZC), i.e. pH where the net surface charge is zero, of carbonized