Hierarchical tannin-derived carbons as efficient tetracycline adsorbents

Mimosa tannin has been used to produce two different types of carbons materials: hydrothermal carbon (HTC) on the one hand, and ordered mesoporous carbon (OMC) on the other hand. Both were obtained after pyrolysis at 1173 K, resulting in materials with similar BET areas, close to 580 m(2)/g, but very different pore size distributions (PSDs). HTC was essentially microporous while OMC had a large fraction of mesopores. Tetracycline (TC) adsorption on these carbons was studied at equilibrium but kinetics experiments were also carried out at two different temperatures. The TC adsorption data were fitted by different mathematical, classical models, and new kinetic models were introduced to take into account their very different PSDs. While the two materials presented similar BET areas (ABET), the OMC had a TC adsorption capacity six times higher than that of the HTC. Adsorption kinetics were also strongly affected by the PSD. The importance of having high accessible pore volumes and not only high surface areas in order to have good performances in TC adsorption has been evidenced not only with our materials but also in the light of the TC adsorption data reported in the open literature. We proved that ABET, while easy to calculate and always reported, is not the best tool to assess the potential suitability of an adsorbent to the removal of aqueous pollutants.