Transport of silver nanoparticles in saturated columns of natural soils

With industrialization and urbanization soils are increasingly exposed to engineered nanoparticles (ENP), yet knowledge regarding the transport of ENP in natural soils is lacking, a process that was examined further in the current study. Saturated columns of 11 natural soils with varying physical and chemical properties were spiked with two pore volumes of a solution containing 1.7 mg Ag L(-1) as polyvinylpyrrolidone (PVP)-coated silver nanoparticles (AgNP) (40 nm actual diameter) and eluted at a constant flow rate of 1 ml min(-1). Breakthrough of Ag was analyzed using filtration theory and a HYDRUS-1D transport model that incorporated two-site kinetic attachment-detachment. Separate kinetic batch studies suggested fast heteroaggregation between negatively charged AgNP and positively charged sites on the common soil colloids maghemite or montmorillonite. The concentration of such sites in the soil correlates positively with the oxalate-extractable aluminum concentration of the soils, a measure that correlated positively with collision efficiency. This correlation thus suggested favorable deposition of AgNP and/or enhanced straining following heteroaggregation of AgNP with mobile soils colloids. Occurrence of heteroaggregation was supported by the batch studies, enhanced size-exclusion in the soil with the highest porosity, and reversible attachment-detachment predicted from HYDRUS modeling, whereas straining and favorable deposition were suggested by irreversible attachment. Our study suggests that under similar experimental conditions, PVP-coated AgNP would rapidly interact with natural colloids in soils significantly reducing their mobility and hence potential risk from off-site transport.