Ion Release Kinetics and Particle Persistence in Aqueous Nano-Silver Colloids

Many important aspects of nanosilver behavior are influenced by the ionic activity associated with the particle suspension, including antibacterial potency, eukaryotic toxicity, environmental release, and particle persistence. The present study synthesizes pure, ion-free, citrate-stabilized nanosilver (nAg) colloids as model systems, and measures their time-dependent release of dissolved silver using centrifugal ultrafiltration and atomic absorption spectroscopy. Ion release is shown to be a cooperative oxidation process requiring both dissolved dioxygen and protons. It produces peroxide intermediates, and proceeds to complete reactive dissolution under some conditions. Ion release rates increase with temperature in the range 0-37 degrees C, and decrease with increasing pH or addition of humic or fulvic acids. Sea salts have only a minor effect on dissolved silver release. Silver nanoparticle surfaces can adsorb Ag+, so even simple colloids contain three forms of silver: Ag-0 solids, free Ag+ or its complexes, and surface-adsorbed Ag+. Both thermodynamic analysis and kinetic measurements indicate that Ag-0 nanciparticles will not be persistent in realistic environmental compartments containing dissolved oxygen. An empirical kinetic law is proposed that reproduces the observed effects of dissolution time, pH, humic/fulvic acid content, and temperature observed here in the low range of nanosilver concentration most relevant for the environment.