BIOCHAR - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 101

While much of the surface acidity in the cellulose and pine materials were attributed to
strong acidic groups (e.g. carboxylic groups), moderately acid groups (i.e. phenols of low pK a
values) and lactones had a substantial contribution to total acidity of lignin derived biochar
(Rutherford et al., 2008).
Inasmuch as biochar CEC is affected by feedstock source and composition, CEC
decreases with increase in production temperature due to the effect of production temperature
on oxygen functional group and total acidity. Gaskin et al. (2008) showed significant decrease
in CEC of biochars made from chicken manure (61.1 to 38.3 cmol kg -1 ), peanut ( Arachis
hypogaea L. ) hull (14.2 to 4.63 cmol kg -1 ), and pine chips (7.27 to 5.03 cmol kg -1 ) as
production temperature increased from 400 to 500 °C, all respectively. Similar findings, i.e.,
reduction of biochar CEC and increase in biochar pH, surface area, and porosity were
reported for chicken manure biochar produced between 300 to 600 °C (Song & Guo, 2012)
and sewage sludge biochar produced at 400 and 600 °C (Mendez et al. 2013). During
pyrolysis of rice straw at different temperatures (300 to 700 °C) and duration (1 to 5 h), Wu et
al. (2012) found a decrease from 184 to 46 cmol kg -1 in biochar surface acidity and an
increase from 84 to 147 cmol kg -1 in surface alkalinity as pyrolysis temperature increased
from 300 and 700 °C, all respectively. A slight increase in biochar CEC produced at 300 and
400 °C (57 to 62 cmol kg -1 , respectively) was followed by a sharp decrease to 32 and 23 cmol
kg -1 at 500 and 700 °C, respectively; with pyrolysis duration (1 to 5 h) with no significant or
consistent effect (Wu et a., 2012). Overall, CEC, Olsen extractable P, extractable Ca, K, and
Mg, and total N content increased with temperature, reaching a maximum at 400 °C (Wu et
al., 2012).While steam activation is known to have marked effect on biochar surface area
(Lima et al., 2010), conducted at the same temperature as biochar production temperature,
steam activation reduced or had no significant effect on CEC of the pine chip derived biochar
(Gaskin et al., 2008).
Biochar CEC increases once biochar is applied to soil because of both biotic and abiotic
oxygenation processes that occur under natural soil conditions and are positively correlated
with annual mean temperature (Cheng et al., 2006). However, the oxygenation process is
slow and affects mostly the external biochar surface (Nguyen et al., 2009). As such, this
effect may be limited as the high surface area of biochar is attributed to the increase in inner
sphere porosity, which occurs with increase in pyrolysis temperature and/or upon (steam)
activation processes (Lima et al., 2010). In all, low production temperature (ca. 250 to 350
°C) promotes higher total acidity, and as a result a higher CEC, while high temperatures (ca.
>500 °C) tend to promote higher total surface area, with much of the increase resulting in
inner surface area of developed pores. Such increase in porosity, and subsequent increase in
surface area will be diminished at elevated temperatures (ca. >700 °C).
Feedstock nutrient recovery and availability for plant uptake tend to decrease with
increase in pyrolysis temperature. Much of the loss of nutrients as volatile compound (e.g. N
and S) occur at temperatures above 350 - 400 °C and is process- and feedstock-dependent
(Gaskin et al., 2008; Wang et al., 2010; Wu et al., 2012). The remaining nutrients are
redistributed into chemical forms less available for plant uptake. Wang et al. (2010) showed
N loss as ammonia (NH 3 ), hydrogen cyanide (HCN), isocyanic acid (HNCO), and acetonitrile
(CH 3 CN) from pyrolysis of wheat ( Triticum spp. ) straw starting at 250 °C, to sharply peaked
at 350 °C, and to tapered off with minimal changes at temperatures above 550 °C (Figure 3).
Increase in pyrolysis temperatures also results in decrease in hydrolysable organic N content
and increase in aromatic and condensed heterocyclic N structures in the biochar.