Environmental Transformations of Silver Nanoparticles: Impact on Stability and Toxicity

Abstract

Silver nanoparticles (Ag-NPs) readily transform in the environment, which modifies their properties and alters their transport, fate, and toxicity. It is essential to consider such transformations when assessing the potential environmental impact of Ag-NPs. This review discusses the major transformation processes of Ag-NPs in various aqueous environments, particularly transformations of the metallic Ag cores caused by reactions with (in)organic ligands, and the effects of such transformations on physical and chemical stability and toxicity. Thermodynamic arguments are used to predict what forms of oxidized silver will predominate in various environmental scenarios. Silver binds strongly to sulfur (both organic and inorganic) in natural systems (fresh and sea waters) as well as in wastewater treatment plants, where most Ag-NPs are expected to be concentrated and then released. Sulfidation of Ag-NPs results in a significant decrease in their toxicity due to the lower solubility of silver sulfide, potentially limiting their short-term environmental impact. This review also discusses some of the major unanswered questions about Ag-NPs, which, when answered, will improve predictions about their potential environmental impacts. Research needed to address these questions includes fundamental molecular-level studies of Ag-NPs and their transformation products, particularly Ag(2)S-NPs, in simplified model systems containing common (in)organic ligands, as well as under more realistic environmental conditions using microcosm/mesocosm-type experiments. Toxicology studies of Ag-NP transformation products, including different states of aggregation and sulfidation, are also required. In addition, there is the need to characterize the surface structures, compositions, and morphologies of Ag-NPs and Ag(2)S-NPs to the extent possible because they control properties such as solubility and reactivity.

Antibacterial activity of silver nanoparticles: A surface science insight

“…However, environmental systems present some specificities that will have an influence on the fate of nanoparticulates systems: (i) The presence of dissolved organic matter, constituted of a large variety of molecules and moieties that are not always well-defined, and that can alter the surface chemistry of nanoparticles [124][125][126]. (ii) Physical and chemical properties can show spatial and temporal disparities in different points of the studied system.…”

Particle Coating-Dependent Interaction of Molecular Weight Fractionated Natural Organic Matter: Impacts on the Aggregation of Silver Nanoparticles

“…Once entering aquatic environment, AgNPs could undergo profound physical and chemical transformations, including aggregation, sedimentation, adsorption, oxidative dissolution, sulfidation, and rereduction, which have great impacts on the fate, transport, and toxicity of AgNPs. 7 Natural organic matter (NOM), which is ubiquitous in aquatic environment, plays an important role in the physical and chemical transformation of AgNPs. NOM in natural waters could adsorb on the surface of AgNPs and displace the original capping agents of AgNPs such as polyvinylpyrrolidone (PVP) and citrate.…”

Photoreduction and Stabilization Capability of Molecular Weight Fractionated Natural Organic Matter in Transformation of Silver Ion to Metallic Nanoparticle

“…3−7 As demonstrated recently, AgNPs are toxic to microorganisms, algae, plants, invertebrates, vertebrates and human cell lines. 8 Thus, the widespread use and potential toxicity of AgNPs to various organisms and humans have led to concerns about their safety.…”

“…8 Unlike other persistent toxic substances, AgNPs are highly dynamic and prone to transform into other silver species in natural environments. 9−12 It is known that both metallic silver and silver ion (Ag + ) are not thermodynamically stable under environmental conditions.…”