Gram-negative Escherichia coli promotes deposition of polymer-capped silver nanoparticles in saturated porous media

This study employed column tests to explore the co-transport of polyvinylpyrrolidone (PVP) polymer-capped silver nanoparticles (AgNPs) with Escherichia coli (E. coli) in model and natural porous medium systems, which respectively comprised artificial groundwater and two glass beads (uncoated and iron oxide-coated), and real groundwater and aquifer sand. The column test results suggest an insignificant effect of AgNPs on E. coli transport in all the systems. By contrast, the effect of E. coli on AgNP mobility differed between the model system and the natural system: E. coli promoted AgNP deposition in both glass beads despite their contrasting electrical properties, but had an insignificant effect on AgNP deposition in the natural system. Analyzing the filtrate of the AgNP–cell mixture in artificial groundwater suggests insignificant attachment of AgNPs to the cell while the cell-released polymers altered the surface property of the nanoparticles. It was speculated that the insignificant effect of AgNPs on E. coli mobility was due to the repulsive PVP–cell membrane interaction which prevented the nanoparticles from modifying the cell surface property; meanwhile, surface modification by the cell-released polymers for both the AgNPs and glass beads could lead to the promoting effect of E. coli on AgNP deposition. In the natural system, sulfide in groundwater inhibited the attachment of the cell-released polymers to AgNPs and consequently diminished the role of E. coli on AgNP mobility. This study highlighted the critical role of AgNP coatings and the outer membrane of bacteria in determining the eco-toxic impact of AgNPs in porous media. Additionally, it emphasizes that the characterization of field physicochemistry and the identification of the governing environmental factors are critical to better predict the mobility and impact of silver nanoparticles in the natural environment.