the band intensity at approximately
1700 cm−1 attributed to the stretching vibration of CO bond in carboxylic,
lactone and ester [34] of AC0 is stronger than that of AC1, AC2 and
AC3. The bands at approximately 3306 and 3271 cm−1 are assigned to
phenolic hydroxyl [35]. It can be seen that the intensity of phenolic hydroxyl
in all the three thermal modified AC samples are lower than that
of the raw sample. Thus, FTIR analysis also confirms that thermal modification
leads to the decrease in main oxygen-containing functional
groups.