Effects of macromolecular capping on the fate and transport of engineered silver nanoparticles in granular media filtration
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The increasing use of silver nanoparticles (AgNPs) inevitably leads to their release into the environment and raises the importance of removing these emerging “pollutants” by the conventional drinking water treatment processes. Granular media filtration is one of the primary treatment processes to remove aqueous colloids and its performance is related to surface properties of AgNPs and aqueous chemistry. This research focused on the fate and transport of engineered AgNPs in granular media filtration after humic acid (HA) exposure and sulfidation. The Derjaguin-Landau-Verwey-Overbeek (DLVO) energy of interaction was updated to account for the effects of the surface capping layer. The DLVO energy of interaction was first updated to include the effect of “soft” surface capping layer. Ohshima’s soft particle theory was numerically applied to obtain the electric double layer (EDL) interaction. Pair-wise summation between two multi-layered half-spaces was employed to compute the van der Waals (vdW) attraction. The updated calculation revealed that, in addition to steric hindrance, stability of polymer-capped particles can also be enhanced by an increased EDL repulsion from mixing and compression of the charged polymer capping, and a reduced vdW attraction due to the attenuation by the polymer capping which has a lower Hamaker constant than the core material. The effects of sulfidation of polyvinylpyrrolidone (PVP) capped AgNPs under different conditions of HA exposure were studied. ζ-potential of AgNPs became more negative upon sulfidation. AgNPs sulfidized with HA showed surface potentials closer to that of HA. Sulfidation resulted in the formation of silver sulfide and both core-shell structure and heterogeneous sulfidation patterns were observed. The S/Ag ratio was found to be the dominant factor in the sulfidation process while different conditions of HA exposure barely influenced the extent of sulfidation. Finally, the self-aggregation and granular media deposition of PVP-AgNPs after HA exposure and sulfidation were investigated. It was found that HA exposure modified the original PVP capping via adsorption and/or ligand exchange and sulfidation stripped the PVP from the particle surface as a result of the formation of silver sulfide. Sulfidation thereby reduced the stability of PVP-AgNPs in self-aggregation and enhanced the mobility of AgNPs in granular media filtration. Without unbound macromolecules in the background solution, polymer on the particle surface largely prevented self-aggregation but allowed favorable clean bed deposition due to bridging if the polymer had high affinity to the collector surface. Our study shows that though environmental transformations (e.g., HA exposure and sulfidation) complicate the fate and transport of AgNPs in water, as long as the AgNPs are effectively destabilized, they either self-aggregate or deposit on to collector surfaces. The “old” process of granular media filtration can still control the fate of the emerging pollutant of AgNPs.