Effects of natural organic matter and sulfidation on the flocculation and filtration of silver nanoparticles

Surface properties of engineered silver nanoparticles (AgNPs) are strongly affected by environmental transformation. The fate and transport of these transformed AgNPs is largely unknown and cannot be fully explained by the traditional Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The objective of this study was to investigate the changes in the composition and surface properties of polyvinylpyrrolidone (PVP) capped AgNPs after environmental transformation and their subsequent effects on the flocculation and filtration of these transformed AgNPs during water treatment processes. To study the aggregation and deposition behavior of the transformed particles, PVP-AgNPs exposed to humic acid (HA) and/or sulfidation were characterized, followed by separate flocculation, granular media filtration, and quartz crystal microgravimetry (QCM) experiments. X-ray photoelectron spectroscopy revealed that HA exposure modified the original PVP capping via adsorption and/or ligand exchange and that 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 but enhanced the mobility of AgNPs in granular media filtration and quartz collector deposition. Without unbound macromolecules in the background solution, polymers on the particle surface largely prevent self-aggregation but allow favorable clean bed deposition. This difference between the effects on self-aggregation and granular media filtration is in contrast to traditional DLVO theory. QCM yielded two types of results, the initial rate of deposition and the ultimate deposition, and both gave insights into expected filtration behavior.