Nanomaterial Probes in the Environment: Gold Nanoparticle Soil Retention and Environmental Stability as a Function of Surface Chemistry

Abstract

The increased prevalence of functionalized nanomaterials in a range of applications will inevitably lead to nanoparticle contamination of soil and groundwater. Here, we investigate how gold nanoparticles' (AuNPs) shape and surface chemistry influence their retention in soil columns and stability in simulated groundwater. When AuNPs are eluted from soil columns with simulated groundwater, spherical particles are more strongly retained in the soil than the rod-shaped AuNPs, regardless of the surface chemistry (as determined by ICP-OES). In a deionized water eluent, however, the same AuNPs showed a retention profile dependent upon surface chemistry (positively charged AuNPs are strongly retained by soil, while negatively charged particles are quickly eluted). This change in retention behavior suggests that the spherical AuNPs may undergo a physiochemical transformation (likely aggregation) during the elution process which reduces their mobility. AuNP stability against aggregation in simulated groundwater was investigated using absorbance spectroscopy and dynamic light scattering. We find that AuNP surface chemistry has a strong influence on AuNP stability against homoaggregation in simulated groundwater. The stability of the AuNPs depends primarily on the nature of the interaction between the AuNP surface and the capping agent (not simply the ligand charge). AuNPs protected with relatively labile capping agents are more susceptible to irreversible homoaggregation in groundwater than polyelectrolyte-coated AuNPs. However, in the presence of alginate, the AuNPs form heteroaggregates with the alginate polymers, regardless of the initial AuNP surface coating.