Sorption of norfloxacin by lotus stalk-based activated carbon and iron-doped activated alumina: Mechanisms, isotherms and kinetics

Two low-cost adsorbents, iron-doped activated alumina (Al2O3/Fe) and lotus stalk-based activated carbon (LAC) were employed to remove norfloxacin from aqueous solutions. Sorption of norfloxacin to both Al2O3/Fe and LAC showed a strong pH-dependent behavior. The maximum sorption capacity (21.58 μmol/g and 922.70 μmol/g) occurred at pH 6.5 and 5.5, respectively for Al2O3/Fe and LAC, which is near the pHpzc of the sorbent. While the equilibria adsorption isotherm data on LAC fit well to the Langmuir equation, both Langmuir and Freundlich models correlated the isotherm data on Al2O3/Fe quite well. The sorption kinetics of both sorbents followed the pseudo-second order model. Several possible mechanisms for the adsorption systems were proposed. For the sorption on Al2O3/Fe, surface complexation and cation bridging were dominant mechanisms responsible for norfloxacin removal, while hydrophobic interaction, cation exchange and π-electron-donor–acceptor interaction were likely important mechanisms for the sorption on LAC.