Retention and remobilization mechanisms of environmentally aged silver nanoparticles in an artificial riverbank filtration system

Highlights: • Outdoor, large-scale column experiments and laboratory batch studies • Strong retention of Ag NP, but highest mobility of soil-aged Ag NP • 50% of retained Ag is remobilizable by mechanical forces and hydrochemical changes. • NOM and ionic strength reduction enhances NP mobility, Ca reduces it. • Co-mobilization with natural colloids is an important remobilization mechanism. Riverbank filtration systems are important structures that ensure the cleaning of infiltrating surface water for drinking water production. In our study, we investigated the potential risk for a breakthrough of environmentally aged silver nanoparticles (Ag NP) through these systems. Additionally, we identified factors leading to the remobilization of Ag NP accumulated in surficial sediment layers in order to gain insights into remobilization mechanisms. We conducted column experiments with Ag NP in an outdoor pilot plant consisting of water-saturated sediment columns mimicking a riverbank filtration system. The NP had previously been aged in river water, soil extract, and ultrapure water, respectively. We investigated the depth-dependent breakthrough and retention of NP. In subsequent batch experiments, we studied the processes responsible for a remobilization of Ag NP retained in the upper 10 cm of the sediments, induced by ionic strength reduction, natural organic matter (NOM), and mechanical forces. We determined the amount of remobilized Ag by ICP-MS and differentiated between particulate and ionic Ag after remobilization using GFAAS. The presence of Ag-containing heteroaggregates was investigated by combining filtration with single-particle ICP-MS. Single and erratic Ag breakthrough events were mainly found in 30 cm depth and Ag NP were accumulated in the upper 20 cm of the columns. Soil-aged Ag NP showed the lowest retention of only 54%. Remobilization was induced by the reduction of ionic strength and the presence of NOM in combination with mechanical forces. The presence of calcium in the aging- as well as the remobilizing media reduced the remobilization potential. Silver NP were mainly remobilized as heteroaggregates with natural colloids, while dissolution played a minor role. Our study indicates that the breakthrough potential of Ag NP in riverbank filtration systems is generally low, but the aging in soil increases their mobility. Remobilization processes are associated to co-mobilization with natural colloids.