Discovery and characterization of silver sulfide nanoparticles in final sewage sludge products

Nanosized silver sulfide (a-Ag₂S) particles were identified in the final stage sewage sludge materials of a full-scale municipal wastewater treatment plant using analytical high-resolution transmission electron microscopy. The Ag₂S nanocrystals are in the size range of 5-20 nm with ellipsoidal shape, and they form very small, loosely packed aggregates. Some of the Ag₂S nanoparticles (NPs) have excess S on the surface of the sulfide minerals under S-rich environments, resulting in a ratio of Ag to S close to 1. Considering the current extensive production of Ag NPs and their widespread use in consumer products, it is likely that they are entering wastewater streams and the treatment facilities that process this water. This study suggests that in a reduced, S-rich environment, such as the sedimentation processes during wastewater treatment, nanosized silver sulfides are being formed. This field-scale study provides for the first time nanoparticle-level information of the Ag₂S present in sewage sludge products, and further suggests the role of wastewater treatment processes on transformation of Ag nanoparticles and ionic Ag potentially released from them.