Depending on what you want to do. If you want to remove positive ions from water, the AC needs to be functionalised with negative ions to work as cationic ion exchangers.
If you want to remove organic molecules, you may have it better in a passive state. Graphite is then better be hydrophobic to selectively adsorb the organic molecules and not the water.
There is an extensive literature about AC functionalization. The kind of functionalization indeed depends on what you want to adsorb. You can graft amine functions for adsorbing acidic compounds, thiols for adsorbing mercury, and so on. The list is almost endless. The surface can also be treated for being positively or negatively charged on the pH you want to work, making the adsorption of anions and cations easier respectively. Sulfonates can also be grafted to make AC act as a ions exchanger. Finally, doping AC with nanoparticles is also possible, e.g. with Pd for adorbing hydrogen, with iron oxide for adsorbing arsenic, etc.
Alain
carbonyl groups are important in case of removal of heavy metals as most of them are positively charged species in water
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