Adsorption of phenanthrene and pyrene by biochar produced from the excess sludge: experimental studies and theoretical analysis

A biochar derived from the pyrolysis of excess sludge of sewage treatment plant at 700 A degrees C was used to remove phenanthrene and pyrene from aqueous solution. The adsorption equilibrium for phenanthrene and pyrene was achieved within 7.5 and 15 h, respectively. The adsorption kinetics of phenanthrene and pyrene were well fitted to pseudo-second-order model, and the isotherms were well fitted to Freundlich adsorption isotherm model (R (2) > 0.96). The effect of variation of pH on the adsorption of the biochar was observed to be insignificant. The hydrophobic action and pore filling might be the main mechanisms of the adsorption of phenanthrene and pyrene on the biochar. The density functional theory calculations confirmed that the adsorption depended mainly on the biochar structure and the molecular volumes of phenanthrene and pyrene. Phenanthrene molecule is smaller than the pyrene molecule and therefore showed better adsorption characteristics with the biochar than the pyrene molecule.