Agriculture Reference
In-Depth Information
material was inversely related to its oxygen content, i.e. PZC increases with decrease in
oxygen content (Boehm, 1994; Ramon et al., 1999).
The pattern described above, i.e. increase in aromatic structures and reduction in biochar
oxygen content and strongly acidic functional groups with increase in pyrolysis temperatures,
leads to an increase in biochar pH and hydrophobicity, resulting in decrease of biochar cation
exchange capacity (CEC). This decrease, however, leads to increase in biochar's (1) anion
exchange capacity, (2) affinity to polar organic molecules, and (3) specific sorption ability
and affinity towards heavy metals (Chun et al., 2004; Lima & Marshall, 2005; Lima et al.,
2010; Uchimiya et al., 2010) as the π electron-rich areas (Lewis bases) act as ligands forming
coordinated covalent bonding with transition metals. Moreover, the increase in temperature
increases biochar surface area, which compensates to some extent for the loss of strong acid
functional groups density.
Using Boehm titration method to assess biochar acidic functional groups, Rutherford et
al. (2008) evaluated the effect of feedstock type, pyrolysis peak temperature, and reaction
time on development of biochar surface acidic functional groups. Pyrolysis resulted in initial
increase in biochar total acidity compared to the original feedstock (cellulose, lignin, pine
wood, and pine bark). While marked increase in total acidity with a sharp peak at 250 °C was
noted in cellulose derived biochar, a much moderated development of total acidity was noted
in lignin derived biochar (Figure 2). Furthermore, total acidity of lignin increased throughout
the temperature range (200 to 400 °C), while that of cellulose and pine bark and wood
decreased at pyrolysis temperatures above 250 °C (Figure 2).
(U.S. Geological Survey/ figure by Rutherford et al., 2008).
Figure 2. Total acidity of cellulose, lignin, pine wood, and pine bark after 72 hours of charring at
various temperatures.
Search WWH ::
Custom Search