High-salinity-enabled bilayer steric shielding overrides charge-reversal effects: Insights into enhanced nanoplastic mobility in marine sand by low-concentration disinfectants

The widespread accumulation of quaternary ammonium compound (QAC) disinfectants in post-pandemic coastal ecosystems introduces a critical, yet poorly understood, variable into the environmental fate of nanoplastics (NPs). However, how QACs govern NP mobility under high-salinity marine conditions remains largely unresolved. This study reveals that 35 PSU salinity profoundly lowers the critical micelle concentration (CMC) of long-chain QACs, which proportionally depresses their charge reversal points (CRPs). This severe CMC reduction enables disinfectants to trigger robust bilayer adsorption at concentrations of a few mg/L, governing NP transport in marine sand through two opposing effects: charge reversal and steric hindrance. Specifically, the CMCs of N-hexadecyltrimethylammonium chloride (CTAC) and N-hexadecyldimethylbenzylammonium chloride (HDBAC) dropped to 3.8 and 12 mg/L, respectively, bringing their CRPs down to merely 5.5 and 2.25 mg/L. While classical filtration theory predicts that the resulting positive surface charge would cause strong retention by negatively charged sand, extended DLVO (XDLVO) modeling revealed that the dense bilayers simultaneously generated a profound steric repulsion barrier (up to 57.9 kT). Crucially, this steric hindrance completely overrode the electrostatic attractive energy, thereby paradoxically enhancing NP mobility (M increased to 33.7% for CTAC and 41.1% for HDBAC). Furthermore, benzyl-substituted HDBAC induced stronger steric hindrance than linear CTAC because its bulky aromatic headgroup facilitated denser interfacial packing within the bilayer. In contrast, the CMC of short-chain N-decyltrimethylammonium chloride (DTAC) remained exceedingly high (3040 mg/L), completely failing to form bilayer adsorption and provide steric shielding, thereby yielding a baseline effective mass recovery (M) of only 28.2%. Control experiments in diluted seawater (3.5 PSU) confirmed that this steric-shielding mechanism is exclusively activated by high marine salinity. Ultimately, these findings reveal that high-salinity enables low concentration, long-chain disinfectants to act as chemical lubricants, unexpectedly facilitating the transport and dispersion of plastic debris within sandy sedimentary environments.