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.