Effect of modification with HNO₃ and NaOH on metal adsorption by pitch-based activated carbon fibers

Pitch-based activated carbon fibers (ACFs) were modified with nitric acid and sodium hydroxide. Their physicochemical properties were investigated in terms of N₂ adsorption, mass titration, FTIR and metal adsorption. The specific surface area of the ACFs decreased after oxidation with 1 M nitric acid, but the total acidity increased three times compared to the untreated ACFs, resulting in an improved ion-exchange capacity of the ACFs. The points of zero charge of the ACFs that affect the selectivity for the ionic species changed from pH 6 to pH 4 by 1 M nitric acid and to pH 10 by 1 M sodium hydroxide treatment. Upon oxidation of the ACFs with nitric acid, surface oxide groups were observed in the FTIR spectra by absorption peaks at 1750-1400 cm^-1. The carboxyl groups of ACFs decreased, while the lactone groups increased after the sodium hydroxide treatment. The adsorption capacity of copper and nickel ion is mainly influenced by the lactone groups on the carbon surface pH<pH_zpc, and by the total acidic groups at pH>pH_zpc.