Polysaccharide coating on environmental collectors affects the affinity and deposition of nanoparticles
Résumé
The mobility and fate of engineered nanoparticles in aquatic environments drive the exposure of biota. This is affected by the naturally suspended particulate matter, porous media, or mineral substrate that constitutes the different environmental collectors to which nanoparticles may attach. Moreover, the bacterial biofilm colonization on these surfaces may act as a natural substrate to retain or repel the nanoparticles. Little is known of the effect of such organic coatings of the collector on the fate of nanoparticles. The objective of this work was to study the deposition of TiO2 nanoparticles on a model collector by exposing a bare SiO2 surface or a coating with bacterial polysaccharide to understand and compare the respective deposition mechanisms. The nanoparticle deposition was studied under favourable or unfavourable conditions, which were obtained by changing the electrostatic interactions between the nanoparticles and the collector from attractive to repulsive forces. The electrostatic interactions were tuned by modifying the nanoparticle coating (bare or PAA coatings), examining the pH vs. pH(IEP), and varying the salt concentration. Atomic Force Microscopy (AFM) was used to measure the deposition after dipping the substrate in a nanoparticle suspension, and Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) was conducted in flow mode to observe the deposition kinetics. Our results show that the physicochemical conditions strongly influence the mode of nanoparticle deposition. Under attractive interactions, the polysaccharide coating tends to decrease the deposition compared to the bare mineral substrate, due to lower surface charge density. In both cases, electrostatic repulsions between neighbouring nanoparticles were partially screened by the substrate with opposite charge during deposition. Along with a moderate salt concentration, this effect tended to favour high deposition density, although the nanoparticles experience dominant interparticle repulsion when they are suspended in the same electrolyte. The deposition rate was also dependent on the substrate capacity, and the rate displayed regimes that were either limited by the sticking reaction or by only transport, which are related to the ionic strength and the substrate type, respectively. Under repulsive electrostatic interactions between the nanoparticles and the substrate, while no deposition occurred on the silica substrate, a limited and partially reversible deposition occurred on the polysaccharide one, evidencing the existence of weak bonds with the nanoparticles. (C) 2017 Elsevier B.V. All rights reserved.
