Inter-molecular interactions of phthalic acid esters and multi-stage sorption revealed by experimental investigations and computation simulations

Because of the benzene ring and carbon chain structures, inter-molecular interactions among phthalic acid esters (PAEs) molecules may be significant, but their role in controlling the sorption characteristics was not investigated. This work selected diethyl phthalate (DEP) and dibutyl phthalate (DBP) as the model pollutants and their sorption was investigated on biochars produced from chicken feathers (CFBCs). The planar structure of CFBCs facilitated a comparative study combining lab experiments and computation simulations. DEP sorption kinetics was well described by a second-order kinetics model. However, DBP showed a very fast and high initial sorption, followed by an obvious decrease and then re-equilibrated sorption. Molecular dynamics (MD) simulations and density functional theory (DFT)-based calculations revealed the lower binding energy and stronger interactions of DBP molecules than DEP molecules, thus, DBP may form more stable clusters in aqueous phase at low concentration. These DBP clusters may be adsorbed and desorbed as a unit. The sorption of DBP clusters on CFBCs may result in an apparently fast initial sorption and decrease of the aqueous phase concentration, resulted in the breakup of the cluster and a re-equilibration of the sorption. The inter-molecular interactions should be considered in investigating PAEs behavior and risks.