Thermal desorption of gallic acid from activated carbon surfaces

Abstract

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.