Mn3O4 is deposited on the surface of AC. Fig. 2 shows the SEM
images at 10,000 magnification to visualize the surface
morphology of pristine AC and Mn3O4/AC. Rough surface shown
on Mn3O4/AC is because of addedMn3O4 occupying the AC particle,
whereas AC has a relatively smooth surface. It should be noted that
the manganese oxide was more uniformly distributed than that of
previous studies in which the manganese oxide was coated using
chemical precipitation methods and formed clusters [3,16].
Therefore, the supercritical condition used in this study was
expected to bring better metal ion adsorption performance.
The elementary composition of the AC surface obtained from
the SEM–EDS analysis was mainly carbon (94–96%) and oxygen (4–
6%). A significant increase in oxygen content (31–32%) and
impregnation of manganese (4–5%) was observed by coating the
manganese oxide onto the AC (data not shown). The porosity and
surface area of the adsorbents are shown in Table 1. Surface area
and pore volume decreased by about 50% (from 1069.3 m2/g and
0.58 cm3/g to 457.6 m2/g and 0.30 cm3/g), probably due to
blocking of the AC micropores by manganese oxide, whereas pore
size increased slightly from 0.41 nm to 0.57 nm after the
manganese oxide coating.