Apart from the textural characteristics of activated carbon, it is well known that the applicability of activated carbon is determined by its surface chemistry. The surface chemistry of the activated carbon plays an important role in determining how efficient the activated carbon can be in the adsorption of a specific compound. Since the surface chemistry influences certain features of activated carbon such as wettability, hydrophobicity-hydrophilicity, adsorptive, catalytic, acid/base and redox properties, the modification of its surface can therefore enable the control of these features [9].
For example, chemical species removal by activated carbon adsorption is due predominantly to the surface complex formation between the species and the surface functional groups. This is especially significant in the case of removing inorganics and metals from aqueous solutions where activated carbons are generally less effective as compared to removing organic compounds [91, 92]. This is because metals often exist in solution either as ions or as hydrous ionic complexes [91]. Also, it is a known fact that surface oxygen functional groups decrease the adsorption of organic compounds in aqueous solution, while their absence favors adsorption, independently of the compounds polarity. This implies that while the acidic functional groups on the activated carbon surface augment their metal adsorptive capacities, the presence of these functional groups do not favor adsorption of organic compounds like phenolic compounds [91].
Surface modification of the activated carbon simply talks about the treatment processes carried out to modify the chemical characteristics of the activated carbon surface in order to enhance or improve
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its adsorption capacity for a specific compound. For example, CO2 adsorption capacity of activated
carbon can be increased by the introduction of basic nitrogen functionalities into the carbon surface. This is based on the premise that, due to the acidic role of CO2 (weak Lewis acid), the introduction of
Lewis bases onto the activated carbon surfaces will favor the CO2 capture performance [76]. Also,
acidic treatment of activated carbon has been reported to enhance its uptake of metal contaminants from aqueous solution [91].
The modification can be carried out by formation of different types of surface functional groups. For instance, surface oxygen functional groups are formed by oxidation of the carbon surface with oxidizing gases or solutions; carbon-hydrogen surface groups by treatment with hydrogen gas at elevated temperatures; carbon-nitrogen surface groups by treatment with ammonia. In addition, degassing and impregnation of the surfaces of the activated carbons are other methods by which the carbon surfaces can be modified [15].
2.9.1 Effect of surface modification on pore volume
Stravropoulos et al. [74] reported the modification of a commercial activated carbon using four methods, namely thermal partial oxidation by oxygen, liquid phase oxidation by nitric acid solution, thermal treatment of urea pre-impregnated samples and thermal treatment under a urea saturated helium flow. The authors reported the initial surface area, total pore volume and micropore volume of the activated carbon to be 1003 m2/g, 0.374 cm3/g and 0.338 cm3/g respectively. After the modification, it
was found that there was a smaller change in the pore structures of the activated carbon modified via thermal partial oxidation by oxygen as compared to the other methods. For the urea modified activated carbon, the nitrogen enrichment led to increase in the microporous volume of the activated carbon. The significant reduction in the pore structure development was observed during liquid phase nitric acid treatment. The surface area, total pore volume and micropore volume of the modified activated carbon reached values as low as 260 m2/g, 0.116 cm3/g and 0.104 cm3/g respectively.
According to El-Hendawy [75], the loss of the surface area and the reduction in the pore volume can be attributed to the incorporation of oxygen functionalities in pore walls and the erosive effect of nitric acid on the carbon structure. The incorporated oxygen functional groups will increase the weight of the modified activated carbon and the extent of oxygen-carbon formation will naturally affect the accessibility of the adsorbate to the modified activated carbon.
El-Hendawy [75] also reported a similar finding for the modification of a corncob-based activated carbon using nitric acid. The author concluded that liquid phase oxidation leads to fixation of
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large amounts of oxygen functionalities on the carbon surface, with simultaneous partial destruction or degradation of the porous structure of the activated carbon. Meanwhile Jaramillo et al. [93] estimated the percentage of microporosity loss of a cherry stone-derived activated carbon modified by nitric acid solution to be 43.3% and 6.7% for activated carbon modified by ozone gas. The microporosity loss is explained to be the widening and transformation of the micropores into large size mesopores. Hence, there was a significant mesoporosity development but no significant difference in the macroporosity of the activated carbon. Therefore, the inference concerning the effect of the oxidizing agents during surface modification of activated carbon on its porous structure is that it leads to widening of narrower pores, causing microporosity loss and mesoporosity development [93]
2.9.2 Effect of surface modification on surface chemistry
The surface modification of activated carbon results in the activated carbon surface becoming either more acidic or more basic in nature. The incorporation of oxygen functionalities will render the activated carbon more acidic and hydrophilic, decrease the pH of their point of zero charge and increase the surface charge density [94]. The removal of the oxygen functionalities from the activated carbon through heat treatment will result in the activated carbon surface becoming more basic and hydrophobic in nature [71]. The surface oxygen functionalities generally decompose upon heating by releasing CO and CO2 at different temperatures.
Figueiredo et al. [95] investigated the modification of a NORIT activated carbon using gas phase oxidation (O2 and N2O) and liquid phase oxidation (hydrogen peroxide and nitric acid solution).
The authors reported that there was an increase in the oxygen functional groups of the modified activated carbon. Specifically, the authors noted that the gas phase oxidation led to an increase in the concentration of hydroxyl and carbonyl surface groups while the liquid phase oxidation increased the concentration of the carboxylic acids.
Jaramillo et al. [93] asserted that the main factor in surface chemistry change during surface modification is the oxidizing agents and not whether the process was carried out in the dry phase or liquid phase. In their own investigation on the modification of cherry stone-derived activated carbon with nitric acid (HNO3), hydrogen peroxide (H2O2), ozone (O3) and oxygen (O2), Jaramillo et al. [93]
reported that O3 and HNO3 treatments were the most effective in forming acidic oxygen surface groups.
For the O2 modified activated carbon, the presence of phenolic and carbonylic groups were detected;
carboxylic acid groups and lactone groups were found in HNO3 modified activated carbon while O3
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