Effects of ultraviolet light on silver nanoparticle mobility and dissolution

Abstract

Nanomaterials are subject to various physical, chemical, and biological transformations, necessitating a better understanding of the impact of “aging” processes on nanoparticle fate and transport in engineered and natural porous media. The objective of this study was to evaluate the mobility and dissolution of citrate-coated silver nanoparticles (nAg, 11 nm diameter) in water-saturated sand following ultraviolet (UV) irradiation with UVA (320–400 nm) or UVB (280–320 nm) light. A 3-day UV exposure resulted in up to a 5-fold increase in mean diameter, a 10 to 15 mV increase in zeta potential (i.e., less negative), red shifts in surface plasmon resonance, and up to a 25-fold increase in Ag⁺ release. The addition of a reactive oxygen species (OH˙) scavenger, tert-butyl alcohol, reduced aggregation and dissolution of nAg exposed to UV light up to 50%, indicating that free radical activity plays a central role in aging. Transport experiments conducted in columns packed with 40–50 mesh Ottawa sand revealed that 25 to 50% more UVA- and UVB-aged nAg were retained compared to freshly prepared (un-aged) nAg. Additionally, 35 to 50% of the applied UV-aged nAg mass eluted as Ag⁺, compared to less than 1% in experiments with fresh nAg. UVB exposure resulted in up to 4-fold greater Ag⁺ release and greater nAg retention compared with UVA exposure, consistent with the less negative zeta potential of UVB- compared to UVA-aged nAg (−31 vs. −37 mV). These findings demonstrate that exposure to UV light significantly enhances nAg retention and dissolution in porous media, and thus, oxidative aging of nAg is likely to enhance Ag⁺ release.