Ozonation/UV irradiation of dispersed Ag/AgI nanoparticles in water resources: stability and aggregation

Proliferation of nanoparticles (NPs) as aqueous pollutants is a matter of growing concern today. The aggregation kinetics of colloidal bare silver (Ag, 20.5 nm) and silver iodide (AgI, 15.3 nm) NPs were investigated during ozone/ultraviolet (O-3/UV) oxidation. Dynamic light scattering was applied to monitor the aggregation of NPs, and the z-average of treated samples was considered aggregate diameter. The effect of temperature, pH, and initial concentration of NPs was investigated on the aggregation rate constant and stability ratio. At a short oxidation period of approximately 1 min, the lower stability ratio was achieved for Ag NPs (< 50) than AgI NPs (> 100). Under acidic conditions, the negative surface charge of both NPs was neutralized that resulted in faster aggregation. In contrast, the impact of temperature and initial concentration of NPs on the aggregation rate was different for both NPs, which was due to the type of O-3/UV interaction with the surface of NPs and the thickness of the electrical double layer surrounding the NPs. The aggregation behavior of Ag NPs obeyed diffusion-limited regime, while an intermediate regime between diffusion- and reaction-limited was observed for AgI NP aggregation. The resulting aggregate morphologies showed that the clusters were ramified for Ag and compressed for AgI NPs. Applying the O-3/UV oxidation process for water treatment purposes leads to a significant reduction in aggregation time for inherently unstable Ag and stable AgI toxic NPs from several hours or days to several minutes.

Automated summary

The aggregation kinetics of colloidal silver and silver iodide nanoparticles during ozone/ultraviolet oxidation were investigated in this study. The results showed that temperature, pH, and initial concentration of nanoparticles had an impact on the aggregation rate and stability of the nanoparticles. Acidic conditions led to faster aggregation due to the neutralization of the negative surface charge of the nanoparticles. Additionally, the type of interaction between ozone/ultraviolet and the nanoparticles' surfaces, as well as the thickness of the electrical double layer surrounding the nanoparticles, influenced the aggregation rate differently for each type of nanoparticles. Colloidal silver nanoparticles followed a diffusion-limited regime, while an intermediate regime between diffusion- and reaction-limited was observed for silver iodide nanoparticle aggregation.