Copper and Cadmium Binding to Fish Gills: Estimates of Metal–Gill Stability Constants and Modelling of Metal Accumulation

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Abstract
Fathead minnows (Pimephales promelas) were exposed to 17 μg Cu∙L−1or 6 μg Cd∙L−1in synthetic soft water in the presence of competing ligands. Measured gill metal concentrations correlated with free metal ion concentrations, not with total metal. Langmuir isotherms were used to calculate conditional metal–gill equilibrium constants and the number of binding sites for each metal. Log KCu-gillwas estimated to be 7.4 and the number of Cu binding sites on a set of gills (70 mg, wet weight) was ~2 × 10−10mol (~30 nmol∙g wet weight−1). Log KCd-gillwas ~8.6, and the number of Cd binding sites on minnow gills was ~2 × 10−10mol (~2 nmol∙g wet weight−1). Stability constants for H+and Ca interactions at metal–gill binding sites and for metal interactions with dissolved organic carbon (DOC) were estimated using these metal–gill constants. All stability constants were entered into the MSNEQL+aquatic chemistry program, to predict metal accumulation on fish gills using metal, DOC, and Ca concentrations, and water pH. Calculated metal accumulation on gills correlated well with measured gill metal concentrations and with LC50values. Our approach of inserting biological data into an aquatic chemistry program is useful for modelling and predicting metal accumulation on gills and therefore toxicity to fish.