Retention and dissolution of engineered silver nanoparticles in natural soils

Abstract

Soils are likely to be increasingly exposed to nanoparticles due to growing consumer use of nanoparticles. This has necessitated an investigation into the fate and bioavailability of nanoparticles in natural soils. However, the effect of soil properties on these processes are unknown. To find the dominant properties that determine AgNP retention in natural soils, nonequilibrium retention (Kr) values of polyvinylpyrrolidone (PVP) coated silver nanoparticles (AgNP) were obtained in suspensions of 16 soils having a wide range of physical and chemical properties. The AgNP dissolution was investigated using ultrafiltration, but could only be detected in six soils, possibly due to strong partitioning of dissolved Ag (median Kd 1791 L kg–1); a process that increased predominantly with the organic matter content of the soils. When corrected for partitioning, dissolution of AgNP was higher (median 26% of total Ag added as AgNP) in these six soils compared to dissolution in artificial soil solutions. The homocoagulation kinetics of AgNP as a function of increasing NaClO4 concentrations were studied at pH 4 and pH 8, showing that homocoagulation of AgNP is unlikely in the studied soil suspensions. Moreover, Kr values (median value 589 L kg–1) only correlated with the soil granulometric clay content and not with parameters that increase the homocoagulation rate, a correlation that suggests that negatively charged AgNP were adsorbed preferentially at positively charged surface sites of clay-sized minerals. Adsorption of negatively charged engineered nanoparticles by Fe and Al oxides and mineral clay edges may thus be an important fate-determining reaction in soils, and possibly also in aquatic systems.