You are here:
Speciation and Lability of Ag-, AgCl- and Ag2S-Nanoparticles in Soil Determined by X-ray Absorption Spectroscopy and Diffusive Gradients in Thin Films
Citation:
Sekine, R., G. Brunetti, E. Donner, M. Khaksar, K. Vasilev, A. Jämting, K. Scheckel, P. Kappen, H. Zhang, AND E. Lombi. Speciation and Lability of Ag-, AgCl- and Ag2S-Nanoparticles in Soil Determined by X-ray Absorption Spectroscopy and Diffusive Gradients in Thin Films. David L. Sedlak (ed.), ENVIRONMENTAL SCIENCE & TECHNOLOGY. ACS Publications, Washington, DC, 49(2):897-905, (2015).
Impact/Purpose:
Silver nanoparticles (Ag-NPs) constitute a major group of engineered nanomaterials (ENMs) increasingly found in consumer products. A large proportion of the Ag used in consumer products is predicted to be released into the wastewater system and there is now a wealth of evidence indicating that the majority of Ag released via this pathway transforms into silver sulfide (α-Ag2S, acanthite) or other sulfur-bound forms of Ag. Once formed, Ag2S is highly insoluble (Ksp = 6 × 10-51) and is orders of magnitude less toxic than free Ag+ or Ag-NPs. This transformation can therefore reduce the ecological risks associated with the release of Ag+. However, over extended time scales, little is known about the stability or bioavailability of Ag2S (and particularly Ag2S-NPs) in environmental compartments. In this study, we investigated the long term transformation profiles of Ag2S-NPs in soils together with that of a soluble Ag salt (AgNO3) and two other Ag based nanomaterials: Ag-NPs with citrate coating and AgCl nanoparticles (AgCl-NPs). These three types of laboratory-synthesized NPs were added to the same soil at 3 different pH values and Ag speciation and lability were examined over an aging period of up to 7 months. Silver speciation in soils was determined using Ag K-edge X-ray absorption spectroscopy (XAS) and labile Ag was measured using a DGT (Diffusive Gradients in Thin films) device newly designed for measuring element lability in the presence of nanoparticles.
Description:
Long-term speciation and lability of silver (Ag-), silver chloride (AgCl-) and silver sulfide nanoparticles (Ag2S-NPs) in soil were studied by X-ray absorption spectroscopy (XAS), and newly developed "nano" Diffusive Gradients in Thin Films (DGT) devices. These nano-DGT devices were designed specifically to avoid confounding effects when measuring element lability in the presence of nanoparticles. The ageing profile and stabilities of the three nanoparticles and AgNO3 (ionic Ag) in soil were examined at three different soil pH values over a period of up to 7 months. Transformation of ionic Ag, Ag-NP and AgCl-NPs were dependent on pH. AgCl formation and persistence was observed under acidic conditions, whereas sulfur-bound forms of Ag dominated in neutral to alkaline soils. Ag2S-NPs were found to be very stable under all conditions tested and remained sulfur bound after 7 months of incubation. Ag lability was characteristically low in soils containing Ag2S-NPs. Other forms of Ag were linked to higher DGT-determined lability, and this varied as a function of aging and related speciation changes as determined by XAS. These results clearly indicate that Ag2S-NPs, which are the most environmentally relevant form of Ag that enter soils, are chemically stable and have profoundly low Ag lability over extended periods. This may minimize the long-term risks of Ag toxicity in the soil environment.
