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 AgClx(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 x 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 AgClx(x -1)- species irradiated under sunlight conditions contributes to the formation of nanosized silver (Ag) in the environment.