Retention of sterically and electrosterically stabilized silver nanoparticles by soil minerals

Abstract

Engineered silver nanoparticles (Ag ENP) enter the environment and lead to concerns about their environmental effects. Clay‐sized particles are assumed to govern the environmental mobility and bioavailability of ENP, although little is known about their interaction with pedogenic minerals. The interactions of sterically (AgNM–300k, OECD standard) and electrosterically (AgCN30) stabilized Ag ENP with iron and clay minerals were investigated at a range of pH values from 4 to 7. The Ag ENP were separated from mineral suspensions by filtration at <7 and <0.45 µm and by ultracentrifugation at <0.003 µm. The colloidal stable Ag ENP were analysed by dynamic light scattering (DLS). The retention of AgNM–300k (pH adjusted to 6, Ag content ≈ 8 mg l−1) was greater than 85% for all minerals except silica and showed the tendency to increase in the following order: ferrihydrite, smectite, illite, kaolinite, goethite and allophane. The retention of AgCN30 (Ag content ≈ 8 mg l−1) was less than for AgNM–300k, except for silica at pH 6, and showed the tendency to increase in the following order: goethite, smectite, allophane, kaolinite, illite and silica. The zeta potential in the fraction <0.45 µm was not sufficient to explain the difference in stability of colloidal Ag ENP in the Ag ENP–mineral suspensions. However, the retention increased for Ag ENP with decreasing pH, most probably because of homo‐ and hetero‐aggregation, but it was not necessarily related to dissolution of the Ag ENP. In general, iron and clay minerals showed clearly the potential to retain Ag ENP under relevant environmental conditions even though the retention was incomplete.HighlightsInteraction of different Ag ENP with soil minerals was tested in short‐term batch experiments. Ag ENP showed hetero‐aggregation with minerals, but the retention was incomplete. Hetero‐aggregation between minerals and Ag ENP was favoured by acidic conditions (pH ≤ 5). Iron and clay minerals showed the potential to retain Ag ENP in the environment.