Simultaneous dispersion-dissolution behavior of concentrated silver nanoparticle suspensions in the presence of model organic solutes.

The premise of the nanotechnology revolution is based on the increased surface reactivity of nanometer-sized particles. Thus, these newly realized applications of noble metal nanoparticles introduce new concerns about the environmental fate of these materials if released during use or product disposal. In this paper, the focus is on silver nanoparticles, a known biocidal agent. In particular, this work explores the effect of model solutes chosen for their simple chemical structure yet their ability to simulate chemical attributes common to soil humic material: a chelating molecule, EDTA; a nonionic surfactant, Brij 35; and a large polysaccharide, alginic acid. Batch systems containing concentrated (1600 mg L(-1)) silver nanoparticle (nAg) suspensions were equilibrated with varying additions of EDTA, Brij 35, or alginic acid to solutions containing 1 or 100 mM NaNO3 background electrolyte. In general, both EDTA and alginate were shown to exhibit poor control over nAg dispersion stability, while Brij 35 served as a good dispersant of nAg particles, showing little difference in particle size with respect to electrolyte concentration. The data also show that loading of the model organic compounds resulted in the supersaturation of dissolved Ag for most of the systems. Mechanisms by which these occurred are discussed in more detail. The evidence suggests that regardless of the effect of humics on the stability of nAg dispersions in aqueous systems, polymer loading may enhance the dissolution and release of dissolved Ag into the environment.

Surfactive stabilization of multi-walled carbon nanotube dispersions with dissolved humic substances.

Soil humic substances (HS) stabilize carbon nanotube (CNT) dispersions, a mechanism we hypothesized arose from the surfactive nature of HS. Experiments dispersing multi-walled CNT in solutions of dissolved Aldrich humic acid (HA) or water-extractable Catlin soil HS demonstrated enhanced stability at 150 and 300 mg L(-1) added Aldrich HA and Catlin HS, respectively, corresponding with decreased CNT mean particle diameter (MPD) and polydispersivity (PD) of 250 nm and 0.3 for Aldrich HA and 450 nm and 0.35 for Catlin HS. Analogous trends in MPD and PD were observed with addition of the surfactants Brij 35, Triton X-405, and SDS, corresponding to surfactant sorption maximum. NEXAFS characterization showed that Aldrich HA contained highly surfactive domains while Catlin soil possessed a mostly carbohydrate-based structure. This work demonstrates that the chemical structure of humic materials in natural waters is directly linked to their surfactive ability to disperse CNT released into the environment.