Aging and soil organic matter content affect the fate of silver nanoparticles in soil

Sewage sludge application on soils represents an important potential source of silver nanoparticles (Ag NPs) to terrestrial ecosystems, and it is thus important to understand the fate of Ag NPs once in contact with soil components. Our aim was to compare the behavior of three different forms of silver, namely silver nitrate, citrate stabilized Ag NPs (5nm) and uncoated Ag NPs (19nm), in two soils with contrasting organic matter content, and to follow changes in binding strength over time. Soil samples were spiked with silver and left to age for 2h, 2 days, 5 weeks or 10 weeks before they were submitted to sequential extraction. The ionic silver solution and the two Ag NP types were radiolabeled so that silver could be quantified by gamma spectrometry by measuring the (110m)Ag tracer in the different sequential extraction fractions. Different patterns of partitioning of silver were observed for the three forms of silver. All types of silver were more mobile in the mineral soil than in the soil rich in organic matter, although the fractionation patterns were very different for the three silver forms in both cases. Over 20% of citrate stabilized Ag NPs was extractible with water in both soils the first two days after spiking (compared to 1-3% for AgNO(3) and uncoated Ag NPs), but the fraction decreased to trace levels thereafter. Regarding the 19nm uncoated Ag NPs, 80% was not extractible at all, but contrary to AgNO(3) and citrate stabilized Ag NPs, the bioaccessible fraction increased over time, and by day 70 was between 8 and 9 times greater than that seen in the other two treatments. This new and unexpected finding demonstrates that some Ag NPs can act as a continuous source of bioaccessible Ag, while AgNO(3) is rapidly immobilized in soil.