Figs. 1–3 display the SEM images of the surface structure for
raw longan seed, carbonized material and LSAC at different
magnified times. There are significant differences in the surface
morphology of the samples. This phenomenon implies that
modification treatments of the carbon samples make apparent
changes in the surface morphology of the adsorbents. The raw
longan seed (Fig. 1a and b) has a smooth and featureless surface,and there were very little pores available on the surface of the raw
longan seed. The carbonized material (Fig. 2a and b) has a coarse
surface due to the pores formed during the thermal activation
process, while the LSAC (Fig. 3a and b) has a porous and
homogenous structure with a deep pore after chemical activation
by sodium hydroxide. This indicates that LSAC is an effective
adsorbent which has the maximum surface area and pore volume
compare to the raw longan seed and carbonized material.
Moreover, the N2 adsorption–desorption isotherms and the
pore size distribution of LSAC were presented in Figs. 4 and 5.
Adsorption data were obtained over a relative pressure range from
107 to 0.94. Surface area of the LSAC was determined by Brunauer–Emmett–Teller (BET) equation within the relative
pressure range from 0.006 to 0.10. The micropore area and
external surface area were determined by the T-plot method. The
micropore size distribution and micropore volume were obtained
by the Horvath–Kawazoe (HK) method. The total pore volume was
calculated from the amount of N2 adsorbed at a relative pressure.
Mesopore volume was calculated by subtracting the micropore
volume from the total pore volume.
From Fig. 4, LSAC exhibits an isotherm of type I, which indicates
that LSAC is microporous. The adsorption isotherm rises rapidly at
low relative pressure region where adsorption occurs within
micropore, the adsorption occurs at the outer surface in the flat region of the adsorption isotherm. The physical properties of LSAC
were summarized in Table 3. The BET surface area, total pore
volume, and average pore diameter of LSAC were determined to be
1511.8 m2/g, 0.7420 cm3/g and 0.48 nm, respectively, which show
that LSAC is reasonably good for adsorption. As can be seen from
Table 3 and Fig. 5, both micropore (