Risk of Silver Transfer from Soil to the Food Chain Is Low after Long-Term (20 Years) Field Applications of Sewage Sludge

The increasingly widespread usage of silver (Ag) nanoparticles has raised concerns regarding their environmental risk. The behavior of Ag and its transfer risk to the food chain were investigated using a long-term field experiment that commenced in 1942 in which Ag-containing sewage sludge was repeatedly applied to the soil (25 applications during 20 years). The speciation of the Ag in both the sludge and the soils retrieved from the long-term experimental archive was examined using synchrotron-based X-ray absorption spectroscopy, and extractable Ag concentrations from soils were determined using 0.01 M Ca(NO3)2 and 0.005 M DTPA. The total Ag in the sludge during the time period of 1942–1961 ranged from 155 to 463 mg kg–1. These values are 1–2 orders of magnitude higher than those in currently produced sludge (ca. 0.5–20 mg kg–1). Long-term repeated applications of these sludges resulted in an increase of Ag in soils from 1.9 mg kg–1 in the control to up to 51 mg kg–1. The majority (>80%) of the Ag in both the sludge and the sludge-treated soils was present as insoluble Ag2S, thereby markedly reducing the bioavailability of this Ag. Concentrations of Ag in the archived crop samples were generally <0.70 mg kg–1 in edible tissues, much less than those in diets that may cause an adverse effects in animals and humans (>100 mg kg–1). These data indicate that the transfer of Ag (derived from both traditional Ag industry and current nano Ag industry) to the terrestrial food chain is limited.