Thermal desorption of gallic acid from activated carbon surfaces
The thermal regeneration processes of two spect activated carbons from olive stones loaded with gallic acid (3,4,5-trihydroxybenzoic acid) have been studied by differential thermogravimetry (DTG) and by mass spectrometric temperature-programmed desorption (TPD-MS). The samples were heated to 1250 K in an inert gas flow both dry and wet. The percentage of gallic acid removed with regard to the amount adsorbed is between 70 and 80% and it depends on the activation degree of the carbon sample which affects the activated carbon–gallic acid interactions. At low temperatures a fraction of physisorbed gallic acid is released as heavy compounds and the rest is transformed to chemisorbed. Between 400 and 800 K the process is dominated by the decomposition of the chemisorbed gallic acid into CO2, H2O and CO. At temperatures higher than 800 K, ring condensation reactions take place. The presence of water in the regeneration atmosphere enhances the transformation of physisorbed chemisorbed gallic acid, degrades a fraction of the heavy compounds leaving the carbon surface to lighter compounds and gasifies the activated carbon up to 0.5% burn-off.