## 3 Results and Discussion
### Practical Applications  
On account of the relatively high surface area and oxidized surface of APC, which favors strong binding of various substances to the surface, this section mainly focuses on some practical aspects of this sample. In a quite common demonstration, APC was added to an aqueous solution of methylene blue, shaken for few seconds and passed through a filter to give a colorless filtrate due to complete retention of the dye by the activated carbon [39,40]. The high surface area of APC together with the strong electrostatic interactions between the positively charged methylene blue and the negatively charged sites of the porous solid (e.g., ionizable oxygen-containing surface groups, such as phenolic and carboxylic) leads to strong and quantitative binding of the dye onto the carbon matrix. This simple process can minimize water pollution from dye industry, where the breakdown of the released dye by UV light or bacteria may form potentially cancer-causing compounds in water, such as amino-naphthol and anisidine [41]. Another interesting application refers to pocket warmers (or hand warmers) and the generation of useful heat by mechanically mixing APC (1 g) with iron powder (5.3 g) and NaCl (1 g) in the presence of a small amount of water (3 g). For this application it is essential to use high surface area carbon. Since APC entraps O\({}_{2}\) from air into its pores, the obtained mixture resembles a galvanic cell composed of an iron anode (\(-\)) (Fe\({}^{3+}\) + 3 e \(\rightarrow\) Fe, E\({}^{o}\) = \(-\)0.04 v), an oxygen cathode (\(+\)) (O\({}_{2}\) + 2 H\({}_{2}\)O + 4 e \(\rightarrow\) 4 OH\({}^{-}\), E\({}^{o}\) = +0.40 v) and a salt electrolyte (NaCl-H\({}_{2}\)O). As the electrons move from the anode to the cathode, the following exothermic redox reactions occurs: 4 Fe + 3 O\({}_{2}\) + 6 H\({}_{2}\)O \(\rightarrow\) 4 Fe(OH)\({}_{3}\), thus producing enough heat to raise the temperature of the mixture up to 80 \({}^{\circ}\)C within 5-10 min of mechanical stirring. Notably, the temperature of the mixture slowly drops from 80 to 40 \({}^{\circ}\)C within an hour, thus retaining its warmness long enough for practical applications. This effect can be accredited to the thermal insulating properties of the porous carbon matrix used in the mixture. At this stage, the further addition of water (2 mL) and stirring again raises the temperature of the mixture, this time up to 65-70 \({}^{\circ}\)C. Alternative to pocket  
Figure 9: (**a**) XPS survey of PC material; (**b**) high resolution C1s spectra of PC material; (**c**) XPS survey of APC material; and (**d**) high resolution C1s spectra of APC material. warmers, APC can be easily mixed with KNO\({}_{3}\) and S to make energetic gunpowder as well (75% KNO\({}_{3}\)-15% APC-10% S). Another practical application of APC pertains to hexavalent chromium Cr(VI) removal from water. Hexavalent chromium Cr(VI) is used in several important industrial processes that produce wastewaters with high levels of this toxic and carcinogenic species [42] (at least one to two orders of magnitude higher than the recommended levels). Hence, the removal of Cr(VI) from industrial wastewaters before releasing them into the environment becomes a great necessity. Among several existing materials and methods used for Cr(VI) removal [43,44], carbon absorbents appear quite appealing due to their good removal efficiency, inexpensiveness and low toxicity [20,21,45,46,47]. Characteristically, APC can effectively remove Cr(VI) in the form of Cr\({}_{2}\)O\({}_{7}\)\({}^{-2}\) at pH = 3. This acidic pH is common for industrial wastewaters and also helps to inverse the surface charge of the carbon support by protonation of the -OH groups into -OH\({}_{2}\)\({}^{+}\), thus favoring the electrostatic binding of the dichromate ions. Interestingly, no removal was observed at neutral pH, possibly due to electrostatic repulsions between the dichromate anions and the negatively charged phenolate and carboxylate groups on the carbon surface.