Remobilisation of silver and silver sulphide nanoparticles in soils

- Release of Ag retained in soil is limited under natural soil conditions.
- High levels of citrate and thiosulphate enhance release of Ag retained in soil.
- Majority of Ag released from soils or biosolids are in the colloidal phase.
- Intact nanoparticles (NPs) are not released in most biosolids samples.
- Environmental exposure to NPs as a result of release from soil or biosolids is low.

Manufactured nanoparticles (NPs) present in consumer products could enter soils through re-use of biosolids. Among these NPs are those based on silver (Ag), which are found sulphidised (e.g. silver sulphide, Ag2S) in biosolids. Herein, our aim was to examine the release of retained Ag0 and Ag2S NPs in soils and biosolids as facilitated by environmentally and agriculturally relevant ligands. Under natural soil conditions, exemplified by potassium nitrate and humic acid experiments, release of Ag retained in soil was limited. The highest total Ag release was facilitated by ligands that simulated root exudates (citrate) or fertilisers (thiosulphate). Released Ag was predominantly present in the colloidal phase (>3 kDa-< 0.45 μm); intact NPs only identified in Ag2S-NP extracts. For biosolids containing nanoparticulate-Ag-S, release was also enhanced by thiosulphate, though mostly as colloidal-Ag - not intact NPs. These results suggest that exposure to NPs as a result of its release from soils or biosolids will be low.