Roles of Silver-Chloride Complexations in Sunlight-Driven Formation of Silver Nanoparticles

In aerobic natural surface water, a silver ion (Ag⁺) exists in various Ag⁺-Cl^- complexes because of a strong affinity for a chloride ion (Cl^-); however, little information is available about the role of the Ag⁺-Cl^- complex in the formation of silver nanoparticles (AgNPs). This study demonstrates that soluble AgCl_x^(x-1)- species act as a precursor of AgNPs under simulated sunlight irradiation. The AgNP photoproduction increases with Cl^- levels up to 0.0025 M ([Ag⁺] = 5 × 10^-7 M) and decreases with continued Cl^- level increase (0.09 to 0.5 M). At [Cl^-] ≤ 0.0025 M (freshwater systems), photoproduction of AgNP correlates with the formation of AgCl_(aq), suggesting that it is the most photoactive species in those systems. Matching the ionic strength of experiments containing various Cl^- levels indicates that the trend in AgNP photoproduction correlates with Cl^- concentrations rather than ionic strength-induced effects. The photoproduction of AgNPs is highly pH-dependent, especially at pH > 8.3. The UV and visible light portions of the solar light spectrum are equally important in photoreduction of Ag⁺. Overall, we show evidence that AgCl_x^(x-1)- species irradiated under sunlight conditions contributes to the formation of nanosized silver (Ag) in the environment.