Abstract
The protective effect of reactive sulfide against AgNO~3~ toxicity to Daphnia magna neonates was studied. Acute (48βh) toxicity tests were performed in the absence (<5 nM) and presence of low (βΌ25 nM) and high (βΌ250 nM) concentrations of zinc sulfide clusters under oxic conditions. In both the presence and the absence of sulfide, lower mean lethal concentration (LC50) values were observed when measured as opposed to nominal silver concentrations were used in calculations. This reflected the fact that measured total silver concentrations were lower than nominal concentrations due to losses of silver from solution observed during the experiment. High concentration (βΌ250 nM) of sulfide completely protected against toxicity up to the highest silver concentration tested (2 ΞΌg/L [19 nM]) with measured silver data. In the presence of environmentally realistic levels of sulfide (βΌ25 nM) in receiving waters, acute silver toxicity was reduced by about 5.5βfold. However, when filtered (0.45 ΞΌm) silver concentrations alone were considered, toxicity (48βh LC50) was similar in the absence (0.22 ΞΌg/L) and presence (0.28 ΞΌg/L) of sulfide. The difference between measured total and filtered silver was attributed to chemisorption of the metal sulfide onto the membrane filter and provides evidence that the toxic fraction of silver is that which is unbound to sulfide. Accumulation of silver was greater in daphnids exposed to silver in the presence of sulfide than in its absence, even though a toxic effect was not observed under these conditions. In this case, silver appears to be incorporated by daphnids rather than merely adsorbed on the surface. Our results point out the need to incorporate sulfide into the acute biotic ligand model and to assess its potentially large role in preventing chronic toxicity.