Natural Organic Matter Aggregates Modulate Dibutyl Phthalate Desorption Hysteresis: Insights from Coarse-Grained Molecular Dynamics Simulations

Abstract

Sorption and desorption of organics by natural organic matter (NOM) govern their environmental fate and the associated risks. NOM can form aggregates due to its flexible framework and amphiphilic properties. However, the three-dimensional (3D) structural transformation of NOM and its role in controlling organic desorption has remained unexplored. In this work, sorption of dibutyl phthalate (DBP) on NOM was first investigated. Correlation analysis and density functional theory calculations revealed that nonpolar structures in NOM predominantly governed DBP sorption (309.4–387.8 mg/g). Subsequently, the DBP-NOM aggregates were prepared via solvent dialysis and freeze-drying, and DBP desorption was examined under various pH conditions. DBP desorption hysteresis increased with increasing pH, ranging from 26.02% (pH 9) to 63.91% (pH 4). At low pH, DBP desorption hysteresis was more pronounced for fulvic acids (FAs) than for humic acids (HAs). The 3D structure of aggregates was described using experimental characterizations and coarse-grained molecular dynamics simulations. The core–shell structure of NOM aggregates inhibited DBP desorption at high pH. The hydrophobic part of FAs was more likely to interact with DBP than HAs due to the higher degree of oxidation and polarity. This work innovatively highlights how 3D structures of NOM aggregates modulate the desorption hysteresis of HOCs.