Exposure Pathways, Dissolution Kinetics, and Fate of Nano-Silver in the Environment

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AbstractProposal 1057547Objective:This project focuses on the exposure pathways, fate, and risk of nano-silver in the natural environment. The objective of this proposal is to address two hypothesis: (1) nano-silver can undergo complete reactive dissolution in the environment through kinetically limited oxidation processes. Nano-silver will be non persistent in particle form in environmental compartments containing dissolved oxygen, but persistent in anoxic compartments or in biological matrices containing reducing agents. (2) Reactive dissolution rates will show significant dependence on nAg particle size, surface state (ligands, coatings) and media composition, requiring detailed experimentation in complex media and biological tissue to make useful predictions for life-cycle risk assessment. The project will proceed in four tasks: (1) development of a first generation model of sources and materials flows of nano-silver in the environment accounting for dynamic dissolution behavior, (2) detailed kinetics and mechanisms of nanosilver dissolution, accounting for the effects of stabilizing ligands attached to the particle surfaces, coatings, composition of the near-particle environment, and surface oxidation state, (3) silver distribution and nanoparticle persistence in biological compartments, through experiments using the organism Xenopus laevis, an amphibian used as a model sentinel species in aquatic systems that is highly vulnerable to environmental pollutants and well suited to the study of nanosilver biodistribution and persistence, and (4) integrated model development and analysis to differential nano-risk. Intellectual merit: This proposal will find solutions to some of the toxicity issues as a result of nano-silver-containing products are washed, abraded, or discarded, silver can enter the environment either as nanoparticles, nanoparticle aggregates, or soluble ions. Silver is reported to be highly toxic to some aquatic organisms, and as an antibiotic may also damage or alter beneficial microbial communities in the environment. Silver is also known to bioaccumulate in phytoplankton and some marine invertebrates and has been shown to be highly toxic to zebrafish embryos. Broader Impacts This project addresses the potential environmental impacts of one of the highest production volume nanotechnology products on the current market: nano-silver. The main result of this project will be a conceptual model of nano-silver exposure pathways, allowing estimation of the location, forms, and concentrations of silver in a variety of environmental compartments. The work will provide policy-relevant information to help define the "differential nano-risk" defined as the extent to which nano-silver has the potential to pose environmental risks beyond those easily estimable from source terms and prior experience and data on conventional silver forms. This is a key regulatory issue that governs whether nAg requires special consideration as a nano-risk or can be treated as a conventional Ag ion release. Finally, insight into Ag+ release kinetics and mechanisms will and aid in the development of controlled release formulations that optimize the use of nAg and avoid material waste and eukaryotic toxicity as a side effect of antibacterial medical products.