- Due to the wide application of engineered nanoparticles (ENP) in different industrial products, in the last years, the risk potential for their release in the environment is increased as never before. Mobility and toxicological behaviour of ENP in aquatic systems is strong depending on their stability and aggregation properties [1]. Furthermore, the changes in the chemical composition of the aqueous phase can influence the aggregation reversibility [2], which can lead to the remobilisation of previously aggregated and immobilised nanoparticles. Despite the increasing amount of studies on the environmental behaviour of ENP, there is a lack of information about reversibility of the aggregation. In this study, aggregation of citrate stabilised silver nanoparticles (Ag NP) was investigated in synthetic aqueous media and natural river water (Rhine). The influence of pH value, sodium and calcium cations and humic acid (HA) on the stability of Ag NP was characterised. The reversibility of the aggregation was studied after reducing of ionic strength (by centrifugation and washing) and applying of ultra sound at different energy levels. Ag NP showed high aggregation at high sodium and calcium concentrations (c(Na+) > 40 mmol/L, c(Ca2+) > 0.5mmol/L), at low pH values (pH <3) and in river water. HA stabilised Ag NP and the stabilisation effect was much higher for pH induced aggregation than for sodium and calcium cation induced aggregation. After reducing of ionic strength, the aggregation of Ag NP was reversible especially after applying of shear forces, but the disaggregation was not fully complete. After treatment, small aggregates remained in the dispersions, which were more stable, than the big aggregates. The results of this study show that the reduction of ionic strength and the presence of shear forces can lead to the partial disaggregation of previously aggregated nanoparticles. As a consequence, the probability of nanoparticle remobilisation can be increased. [1] Morones et al. (2005) Nanotechnology 16, 2346-2353 [2] Fabrega et al. (2009) Environ. Sci. Technol. 43, 7285-7290
