Water clusters contributed to molecular interactions of ionizable organic pollutants with aromatized biochar via π-PAHB: Sorption experiments and DFT calculations.

Molecular interactions between biochars and ionizable organic pollutants (IOPs) are of great concern in natural environments, however the role of water clusters on the biochar surface remain unclear. The pH-dependent adsorption of aniline, phenol, 2-chlorophenol, 3-chlorophenol, 4-chlorophenol, 4-methylphenol and 4-nitrophenol onto bamboo wood derived biochar (BW700) as a model was conducted to identify conventional and novel interaction mechanisms between aromatized surface and IOPs. The dissociation constant (pKa,surface ) of surface functional groups of BW700 was characterized by acid-base titration and Zeta potential measurements. The pH-dependent adsorption behavior depended on the pKa,IOP of IOPs and also related to the pKa,surface of biochar surface. An obvious peak of adsorption coefficients (Kd ) in the range of solution pH was shaped at pHpeak  = (pKa,IOP  + pKa,surface )/2, which cannot be well explained by the conventional mechanisms such as hydrophobic effects, π-π interaction, electrostatic attractions, and hydrogen-binding. The contribution of ice-like adlayer (water clusters) on aromatic surface as H-acceptors is proposed for the first time to the adsorption peak of IOP as H-donors at pHpeak . Density functional theory (DFT) calculations provided a possible structure of the complex combined with ice-like adlayer and aromatic substrate of BW700, and indicated that the adsorbing peak resulted from the multiple π-bond and polarization assisted H-bond (π-PAHB) interactions. Three distinct properties of π-PAHB were given, based on multiple π-bond, hydrophobicity-dependence and pH sensitivity. This novel mechanism extends the definition of H-bonds for better understanding the molecular interactions of IOP with carbonaceous materials and their environmental fate.