Temperature dependence of the point of zero charge of oxidized and non-oxidized activated carbons

Three activated carbons in the form of grains, fibers and pellets were oxidized with ammonium peroxidisulfate. The oxidized and non-oxidized activated carbons were characterized by: N2 adsorption at 77 K, temperature programmed desorption up to 1273 K, and potentiometric titrations in the temperature range between 288 and 318 K. The point of zero charge (PZC) of the oxidized activated carbons decreased with increasing temperature and this decrease was in the same direction as the change in 1/2pKw, the neutral point of water. The non-oxidized activated carbons behaved in a similar manner, but the PZC decreased by more than the corresponding change in 1/2pKw. Variation of surface charge of the three oxidized activated carbons was fitted to a single second-order function with respect to pH and temperature. However, a single equation could not be found for the three non-oxidized activated carbons. Standard thermodynamic functions at the PZC were obtained from potentiometric curves. Acidity constant distributions were obtained and showed an increase in the number of acidic groups when the temperature increased. The number of carboxylic and phenolic groups obtained from these distributions was compared with that obtained from the deconvolution of temperature programmed desorption spectra.