Speciation and Microalgal Bioavailability of Inorganic Silver

Silver accumulation in aquatic organisms is primarily attributed to the bioavailability of the free Ag ion (Ag+). Some reports suggest that AgCl(aq) is also available for biological uptake, but few studies of Ag bioavailability used the range of chloride concentrations over which AgCl0(aq) is the dominant Ag species. None used environmentally realistic, low Ag concentrations (10−200 pM). To assess the bioavailability of inorganic Ag species and the importance of the low polarity AgCl(aq) complex to biological uptake, we determined the octanol−water partition coefficient of Ag over a range of chloride concentrations (0−50 mM) representative of fresh to brackish waters and measured short-term Ag uptake rates in the euryhaline marine microalga Thalassiosira weissflogii exposed to a total silver concentration of 50 pM. Overall octanol−water partition coefficients (D ow) of inorganic silver ranged from 0.02 to 0.06. The K ow of AgCl(aq) calculated using D ow values measured at 0.5, 5, and 50 mM Cl- and the K ow of Ag+ (0.03, measured in the absence of Cl-) was 0.09. Silver D ow and uptake rate constants in phytoplankton were highest at the Cl- concentration (5 mM) where uncharged AgCl(aq) is the dominant (67%) silver species. Our results demonstrate that AgCl(aq) is the principal bioavailable species of inorganic silver in phytoplankton and suggest that direct uptake of AgCl(aq) is important to the overall accumulation of Ag in aquatic invertebrates.