Selective extraction of aquatic sediments

Sequential selective extraction procedures have been developed to determine the fate of potentially toxic trace metal pollutants in aquatic sediments. These multistep procedures operate through the application of reagents of increasing strength which solubilize different components of a sediment, thus releasing associated trace metals. Evaluation of the accuracy of these procedures is impossible with natural sediments because the location and concentration of trace metal pollutants are not known. Synthetic sediment standards must be made available to test extraction procedures. For this research, model aquatic sediments are produced by combination of individually synthesized geochemical phases. Each phase is doped with a trace metal contaminant, such as copper, lead, nickel, or zinc. Since the compositions of the model sediments are known, they are used to evaluate an extraction procedure. The results indicate that extraction procedures are susceptible to several deficiencies including inadequate reagent strength, nonselectivity of extracting reagents, and redistribution of extracted elements onto residual phases. The problem of redistribution is addressed by incorporating an ion-chelating resin, poly-dithiocarbamate, into the extraction procedure which competes with residual sediment phases for extracted elements. A model sediment is produced with a calcium carbonate phase which contains all the trace metal contaminants and with an undoped iron oxyhydroxide phase for scavenging extracted elements. The sediment is subjected to a sodium acetate extractant which can solubilize the contaminated phase. A portion of the resin is placed in the extractant to trap metals as they are extracted. The resin is successful in capturing some but not all of the extracted metals