Responses and mechanisms of perfluoroalkyl acid release to the cumulative effects of propeller jet force over time.

As the discharge of exogenous perfluoroalkyl acids (PFAAs) is gradually controlled, the secondary release of endogenous PFAAs is poised to become a major challenge. However, the effects of complex hydrodynamic conditions, such as disturbance from ship propeller jets, on PFAA release, are still poorly understood. In the present study, a propeller jet disturbance simulation experiment was carried out using an indoor flume to investigate the responses and mechanisms of PFAA release to the cumulative effects of jet force over time. Although an increase in jet action time (t) increased the total amount of dissolved PFAAs in the overlying water, PFAA dissolution intensity decreased. Conversely, increase in t induced a rise in both total PFAA amount adsorbed and adsorption intensity in suspended particulate matter (SPM). Moreover, differences in functional groups and carbon-fluorine chain lengths resulted in different types of PFAAs exhibiting different sensitivities to changes in t during dissolution and adsorption. During the jet disturbance, there were distinct shear zoning and pressure gradients in the flow field, contributing to the tendency of PFAAs to be distributed in the overlying water. However, after the disturbance, PFAAs tended to distribute in the SPM. Changes in t consistently affected the partition behavior of long-chain PFAAs (C ≥ 7), whereas the effect on the partition behavior of short-chain PFAAs (C < 7) was rather short-lived. In contrast, Perfluoroalkyl carboxylic acids were more readily released from the sediments and porewater than Perfluoroalkyl sulfonic acids; however, the difference decreased with increasing t. Both shear impulse and squeeze impulse showed a significant linear positive correlation with PFAA release (p < 0.05), but the effect of shear impulse was greater. The findings provide perspectives for further understanding of PFAA release under jet disturbance and could facilitate endogenous PFAA contamination control.