The sulfidation and aging of silver nanoparticles (Ag-NPs) with natural organic matter (NOM) are major transformation processes along their pathway in wastewater treatment plants and surface waters. Although porous media, such as soils or riverbank filtration systems, appear to be a sink for disposed Ag-NPs, the impacts of biological interfaces, the presence of NOM in the aquatic phase, and of variable water saturation on the transport and retention of Ag-NPs are still not fully understood.

We have performed two laboratory studies to examine the mobility of Ag-NPs in porous media under different conditions. At first, we investigated the mobility of citrate-coated Ag-NPs in sand obtained from an artificial riverbank filtration system comparing pristine and pond-water aged sediments as well as different flow velocities. Second, we investigated how the sulfidation of Ag-NPs (S-Ag-NPs) and the presence of NOM in the aquatic phase can change the transport characteristics in saturated and unsaturated sand at different transport velocities. Flow experiments inside an X-ray microtomograph enabled to study the impact of phase distribution (solid, water, air) and their interfaces on the retention of Ag-NPs.

Our experimental results show that the mobility of Ag-NPs in porous media is affected by the presence of biological components, the sulfidation of particles and, when unsaturated, the additional air phase. In saturated riverbank filtration systems, naturally occurring biological aging processes on sediments enhanced the efficiency of the system to retain citrate-coated NPs. The sulfidation of Ag-NPs to S-Ag-NPs decreased the mobility in porous media while the NP-aging with NOM re-established mobility to some extent. In unsaturated sand, the retardation of NOM aged S-Ag-NPs was strongly increased by decreasing water content, i.e. the propagation of an air phase, and decreasing flow velocity.