Molecular-scale insights into retention and aggregation of organophosphate esters (OPEs) with clay-organic complexes

Clay-organic complexes, as the major substrate in surface environment, play an essential role in the retention of organophosphate esters (OPEs). Due to the complexity and heterogeneity of clay-organic aggregates, the molecular-level understanding of the interactions between OPEs and clay-organics has not been elucidated. This work systematically investigates the dynamic retention and aggregation behavior of three types of OPEs (TBEP, TCEP and TCP) with kaolinite-organic complexes by molecular dynamic (MD) simulations and density function theory (DFT) calculations. Flexible (SRFA) and rigid (WFA) molecular structures of organic matters are considered. Results demonstrate that both SRFA and WFA can act as bridges to connect the hydroxyl surface of kaolinite and OPE aggregates. The flexible SRFAs tend to fold themselves, thereby providing hydrophobic sites for OPEs. The aggregates of WFA are divided into several parts and attach to outer surface of the OPE polymers, as the rigid WFA cannot undergo conformational changes. According to DFT results, OPE oligomerization is primarily driven by vdW interaction attributed to their typical hydrophobic nature, supplemented by weak electrostatic forces from minor hydrophilic moieties. This work reveals that the transport and bioavailability of OPEs are significantly reduced by forming OPE-NOM-kaolinite co-aggregates, and provides a quantitative understanding of the environmental processes of OPEs at the molecular and electronic scales.