Ligand-selective complexation of natural organic matter with Mg2+ modulates nanoplastic transport in seawater-saturated porous media

Understanding how the intricate components of marine environments govern the transport of nanoplastics (NPs) is crucial, as this directly influences their distribution and ecosystem exposure risk. While extensive research has been conducted on natural organic matter (NOM)-regulated micro-interfacial processes of NPs in marine environments, the synergistic effect of divalent cations, particularly magnesium ion (Mg), remains underappreciated. This study elucidates how ligand-specific NOM-Mg complexation significantly influences the transport behavior of NPs in seawater. In 35 PSU seawater, humic acid (HA) and fulvic acid (FA) increased NP surface charge, achieving ζ-potentials of -25.46 mV and -19.58 mV, as compared to -18.00 mV for pristine particles. These macromolecules maintained colloidal stability with hydrodynamic diameters (d) around 600 nm and enhanced mobility, elevating the mass percentages of effluent (M) from 26.5% to 43.9% and 35.4%, respectively. In contrast, tannic acid (TA) reduced ζ-potential to -10.33 mV, triggering severe aggregation with a d of 919 nm and diminished mobility, reducing M of 7.28%. Removing Mg mitigated HA/FA-mediated mobility enhancements, decreasing M to 29.7% and 35.6%, and restored TA-induced mobility suppression, bringing M back to 28.0%, confirming the significance role of NOM-Mg interactions. Concentration-dependent experiments indicated that HA/FA-enhanced mobility correlated with Mg and NOM levels, whereas TA-induced suppression was solely dependent on Mg levels, emphasizing the more pronounced impact of TA-Mg complexes. Mechanistically, weak binding of Mg to high-molecular-weight HA (112 kDa) and FA (79 kDa), with constants K = 0.289 and 0.697, contributed to partial charge neutralization and steric hindrance. Conversely, low-molecular-weight TA (1.9 kDa) formed strong catechol-Mg bridges (K = 3.746), inducing charge-neutral aggregates. Two-dimensional correlation Fourier-transform infrared spectroscopy (2D-COS-FTIR) and solid-state C nuclear magnetic resonance (NMR) analyses identified carboxyl/phenolic groups in HA/FA and ortho-polyphenols in TA as primary Mg binding sites, indicating that structural differences among various NOM results in distinct ligand selectivity in their interactions with Mg. Molecular dynamics simulations illustrated TA-Mg bridging dynamically induced large NP aggregates within 20 nanoseconds. These findings highlight the importance of ligand-selective NOM-Mg complexation as a critical regulator of NP fate in marine ecosystems.