Ecotoxicology of metals in aquatic sediments: binding and release, bioavailability, risk assessment, and remediation

Major metal-binding phases in the aerobic layer of sediments are iron and manganese oxyhydroxides (FeOOH and MnOOH) and particulate organic carbon (POC). The acid-volatile sulfide (AVS) model proposed for predicting nontoxicity from metals-contaminated sediments is only applicable to anaerobic sediments. In other sediments, normalization by POC or FeOOH and MnOOH may be predictive, but binding constants are not well understood. Metal mobilization is enhanced by ligand complexation and oxidation of anaerobic sediments. Free metal ion is the most bioavailable species, but other labile metal species and nonchemical variables also determine metal bioavailability; biotic site binding models have shown promise predicting toxicity for systems of differing chemistry. Hazard identification and ecological risk assessment (ERA) depend on determining bioavailability, from water (overlying, interstitial) and food, which can be done prospectively (e.g., normalized sediment chemistry, laboratory bioassays) or retrospectively (e.g., in situ bioassays, field studies). ERA of sediment-bound metals requires primary emphasis on toxicity and consideration of the three separate transformation processes for metals in the aquatic environment, the differences between essential and nonessential metals, the complex interactions that control bioavailability, adaptation, which may occur relatively simply without appreciable cost to the organism, weight of evidence, and causality.