| Summary: | Standard procedures designed for the determination of trace metal concentrations in sediments (and soils) largely overlook the main factors that control partitioning of metal ions and complexes into the solid phase, namely surface area and charge of the particles. These properties are directly proportional to the sediment's grain size, mineralogical composition, and organic matter content. The sediment's ability to accumulate and release metal contaminants is thus directly correlated with its grain size, mineralogy, and relative proportion of organic carbon. Four different standard procedures were used to extract the exchangeable metal fraction in a suite of siliciclastic sediments from the Larsemann Hills, East Antarctica. These include extraction using 1M MgCl2, 1M CH3COONH4, 1M NH4NO3, and 1M HCl. All metals in the leachates were analysed by ICP-MS (Cr, Co, Ni, Cu, Zn, As, Mo, Cd, Pb), with the exception of Mn (ICP-AES). Extractions by MgCl2 and NH4NO3 gave similar results, whereas extraction by CH3COONH4 gave considerably lower values (up to one order of magnitude) for all the metals analysed. Extraction with 1M HCl was considered to be too aggressive, and capable of dissolving metals that are not exchangeable and not bioavailable under normal conditions. Irrespective of the procedure used to extract metals, it was observed that metal concentrations are inversely proportional to grain size and strongly dependent on clay mineralogy. Bulk sediments were also leached with warm H2O (50 °C). Given the overall coarse grain-size of these sediments and their mineralogical composition, metals released using this procedure represent anthropogenic input and, at the same time, the exchangeable fraction. Values obtained following this procedure were close to detection limits. The same procedure applied to algal material yielded relatively high metal concentrations (in excess of 20 microg/g for some metals), thus suggesting that the metal sorption capacity of algal material is several orders of magnitude higher than that of sediments. Further leaching using NH4NO3 produced negligible metal concentrations in the sediment samples. These tests confirm that a rigorous mineralogical and sedimentological study of samples is absolutely necessary to determine the material's ability to accumulate metals, their potential bioavailability, and the nature (natural vs. anthropogenic provenance) of the potentially bioavailable metal.
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