Escherichia coli and phosphate mediated the distinct retention of small-sized nano-plastic particles in seawater-saturated porous sands

Small nano-plastics (NPs, < 30 nm) with a high accumulation in biological organisms in coastal areas might react with widely presented bacteria and phosphate, which remains unclear. Therefore, the mechanisms governing the transport of two-sized NPs with Escherichia coli (E. coli) and phosphate were investigated in hyper-saline water-saturated sand porous media. The results showed that 20 nm NPs exhibited more hetero-aggregation with E. coli than 80 nm NPs, associated with lower k_1d/k₁ values (0.268 vs. 0.412) and more substantially suppressed depth of φ_max (17.83 K_BT vs. 23.44 K_BT), based on two-site kinetic attachment retention model fitting and extended-Derjaguin-Landau-Verwey-Overbeek theory. Accordingly, even though the mass recovery percentage of both sized NPs alone was similar, the irreversible deposition of 20 nm NPs doubled by E. coli, increasing the coastal environmental risks. In contrast, 80 nm NPs reversibly attached to the sands with less effect by E. coli, causing secondary pollution. The copresence of phosphate pronouncedly enhanced the transportability of two-sized NPs with E. coli, especially increasing 20 nm NP mobility from 17.7% to 39.2% in 200 mM NaCl by preferentially adsorbing onto E. coli to avoid agglomeration with NPs. This study highlights the potential risk of small NPs in complicated coastal ecosystems.