Natural water-borne nanoparticles are ubiquitous. Their very small size, ranging from 1 to 100 nanometers means they are both highly mobile and chemically reactive. Nanoparticles are central in buffering environmental systems, serving the dual role of limiting potentially toxic metal concentrations, while at the same time providing a supply of metals at levels that enables biochemical reactions to take place.

The term nanoparticle describes a subset of the colloidal range between 1 and 100 nm (Hochella 2002)(Box 1). The distinction is justified partly on their very high specific surface area (Lead & Wilkinson 2006) and partly on their potentially different behavior at this small scale, due to the spatial constraint of electronic properties (in an analogous manner to engineered nanoparticles (Madden et al. 2006))(Figure 1). As particles transition to smaller and smaller sizes, they become effectively all surface with minimal internal volume, giving rise to their enhanced reactivity (Figure 1).