The humic acid-mediated mechanism of 6PPD-Q during transport process in porous media: Insights from column experiments and molecular dynamics simulations.

The environmental risks and fate associated with N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q) have aroused increasing concern. This study investigated the transport of 6PPD-Q under various hydrochemical conditions (including pH and electrolyte conditions) and the influence of humic acid (HA) on 6PPD-Q transport via column experiments and molecular dynamics (MD) simulations. MD simulations revealed the interaction mechanism between 6PPD-Q and HA, including van der Waals (vdWs), electrostatic interactions and hydrogen bond. Generally, the mobility of 6PPD-Q decreased with lower pH and higher ionic strength. In the absence of electrolyte, HA inhibited the migration of 6PPD-Q through providing additional deposition sites and increasing its hydrophobicity. HA facilitated 6PPD-Q transport under neutral condition with CaCl2, while HA played negligible effect on 6PPD-Q transport under neutral condition with NaCl. Despite exhibiting stronger binding affinity, HA failed to remobilize 6PPD-Q which had been priorly retained on quartz sand. This study provides novel mechanistic insights into the environmental behavior of 6PPD-Q and quantifies its co-transport risks mediated by natural organic matter, contributing to the prediction of 6PPD-Q fate in subsurface environments.