The charcoals were activated at 850 C with CO2 over different
time intervals of 0.5e4.0 h. The evolution of burn-off grade and
activated carbon yield as a function of activation time is shown in
Fig. 3. An increasing activation time progressively increases the
burn-off grade and reduces the yield of activated carbon; this was
expected, since an increase in burn-off represents an increase in the
weight losses due to the activation process, thereby resulting in a
decreasing yield. It was observed that the relationship between
burn-off and activation time was linear.
Pendyal et al. [14] verified that an activation time of 6.0 h was
necessary for the production of granular activated carbon from
sugarcane bagasse/molasses (1:0.5 ratio) with a burn-off grade of
26%; however, the researchers used a 13% CO2 and 87% N2 gas
mixture for pyrolyzed carbon activation. In our case, the carbon
materials were activated with pure CO2, with a burn-off of 25%
obtained in 2.0 h.
The N2 adsorption isotherms of the activated carbon samples
prepared over different activation times without treatment and
washed with HCl are shown in Fig. 4(a) and (b), respectively. It can
be seen that the shape of the activated carbon adsorption isotherms
are approximately Type I isotherms, according to the International
Union of Pure and Applied Chemistry (IUPAC); these isotherms are
typical of microporous materials, in that micropore filling occurs
significantly at low relative pressures (P/P0 < 0.1) [27]. Samples CA-
0.0A, which represents the pyrolyzed carbon, presented a low
volume of adsorbed N2; however, the adsorption capacity isotherms
were found to significantly increase as the activation time