Preparation of granular activated carbons from grape seeds by cycles of liquid phase oxidation and thermal desorption

Activation of grape seed char upon successive cycles of liquid phase oxidation followed by high temperature desorption permits a tailored development of porosity. In this work three different oxidants (HNO3, H2O2, and (NH4)2S2O8), have been tested and the desorption temperature has been varied within 850–950 °C upon 10 activation cycles. A high increase of BET surface area was observed in the first five cycles with HNO3 as oxidizing agent giving rise to values higher than 1200 m2 g− 1 at around 50% burn-off. Activation with H2O2 and (NH4)2S2O8 led to a significantly lower development of surface area, with 600 and 800 m2 g− 1 respectively at that burn-off. The analysis of the pore size distribution showed that porosity was generated through the creation of new micropores and widening of existing ones upon activation with HNO3 and (NH4)2S2O8, whereas H2O2 mostly led to the widening of the narrow micropores already existing in the starting char. The activated carbons obtained are essentially microporous, with some small contribution of mesoporosity in the HNO3 series (Vmicro = 0.69 cm3 g− 1; Vmeso = 0.07 cm3 g− 1). SEM images showed that the activated carbons maintained the granular morphology of the seeds after 10 cycles showing a hollow core structure with a wall thickness of about 200 μm.