Modeling silver binding to gills of rainbow trout (Oncorhynchus mykiss)

Rainbow trout (Oncorhynchus mykiss, 1–3 g) were exposed to ∼0.1 μM silver (Ag) (∼11 μg.L−1 Ag) for 2 to 3 h in synthetic soft water (Ca, Na ∼ 300 μM, pH 6.5–7.5) to which was added Ca, Na, H+, dissolved organic carbon (DOC), CI, or thiosulfate (S2O3). Gills were extracted and gill Ag concentrations were measured using graphite-furnace atomic absorption spectrophotometry. The concentrations of cations (Ca, Na, H+) and complexing agents (DOC, CI, S2O3) needed to keep Ag off the gills were used to calculate conditional equilibrium binding constants (K) at the gills. Log K for Ag-gill binding was 10.0, with approximately 1.3 nmol Ag binding sites per fish. All experimentally determined log K values were entered into an aquatic chemistry equilibrium model, MINEQL+, to predict Ag binding at trout gills. For a series of natural waters, model-predicted gill Ag concentrations correlated well with observed gill Ag concentrations, with one exception, very hard city of Waterloo tapwater. This exception may indicate a kinetic constraint on the thermodynamic basis of the model.