In Situ Detection and Characterization of Engineered Ag Nanoparticles in Soil Pore Water.

Engineered silver nanoparticles (Ag NPs) have been used extensively in recent years in consumer products and electronics as antimicrobial agents and for use in electrically conductive inks.  Recent studies have shown that Ag NPs can be released into wastewater streams, where they will ultimately partition to sludge in wastewater treatment plants.   Sewage sludge is often applied to agricultural lands as biosolids for use as a soil amendment.  Several recent studies have demonstrated the toxicity of Ag NPs in a variety of soil organisms, yet few techniques are available for characterization of particle behavior in pore water.  We developed techniques using asymmetrical flow field flow fractionation (AF4) coupled to multi-angle light scattering (MALLS), ultraviolet-visibe spectroscopy (UV-Vis), and inductively coupled plasma mass spectrometry (ICP-MS)  to detect and characterize Ag NPs in soil pore water.  We investigated the effects of particle coating (citric acid or polyvinylpyrrolidone), sewage sludge amendment (0, 5 or 15%), and aging (up to six months) on partitioning and aggregation of Ag NPs in pore water in a natural sandy loam (Yeager series).  We observed profound effects of particle coating where the citric acid coated particles remained stable as unaggregated primary particles in the pore water to a much greater extent and for a longer period of time (up to six months) than the PVP coated particles.  Sewage sludge amendment played a more important role than particle coating, and differences in the colloidal distribution of Ag among particle coatings were only observed when sewage sludge was not present.  This was likely due to extensive sulfidation of the particles resulting in removal of the original manufactured coating.