It is well accepted that the total heavy metals contents in sediments cannot offer sufficient data concerning their impact on the ecosystem because the biogeochemical and the ecotoxicological significance of metals rely not only on their overall concentrations but also on the speciation in which they occur. (Quevauviller, 1998; Sahuquillo et al., 2003 Tessier et al., 1979). Strictu sensu, ‘speciation’ refers to determination of well-defined chemical species whereas the ‘extracted forms’ refers to the extractant used in operationally-defined procedures (IUPAC, Quevauviller and Sahuquillo). In practice, both terms are used as synonyms, which are in most instances sufficient for the purpose environmental studies. Speciation can be determined through selective extraction methods, either leaching or extraction procedures (e.g. single or sequential extraction) (Rao 2008). These procedures provide valuable information about the assessment of potential bioavailability, risk, and remobilization which makes the sequential extraction a powerful tool in metal pollution research in estuarine areas (Salomons and Forstner, 1984; Zimmerman and Weindorf, 2010).

EXEMPLOS DE SEP, CRIAÇÃO BCR

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The concept behind all selective extraction is that the most labile metals are released in the first step and continue to be extracted in order of decrescent mobility. A wide variety of sequential extraction procedures (SEP) has been developed with a different number of steps: 3 steps (Ure et al., 1993); 4 steps (Kiratli and Ergin, 1996); 5 steps (Tessier et al., 1979) to 6 steps (Kersten and Forstner, 1986). To harmonize these methods, the Bureau Communautaire de Reference (BCR) (now named Standard Measurements and Testing Program, SM&T) has proposed a 3 steps (acid soluble, reducible, oxidisable fractions) sequential extraction procedure (Rauret ?; Zimmerman and Weindorf, 2010). The major advantages of using the BCR method are to make the various environmental studies comparable and to validate the quality of measurements through the creation of suitable reference material to be certified according to the BCR-scheme, like BCR CRM 601 and 701 (citar refs.).

DETALHES DAS FRAÇÕES

In the ?rst fraction (F1), the bioavailable, exchangeable, or readily available fraction, the metals are adsorbed in sediment via weak electrostatic attraction coming from carbonates, sulfates and chlorides (ver refs). In Fraction 2 (F2), the reducible fraction, heavy metals are presented in sediment as cement, nodules, and concretions associated with Fe-Mn colloids. This fraction tend to be thermodynamically unstable in anoxic conditions, i.e., these metals are most bioavailable under low redox potential (Panda et al. 1995; Tessier  et  al., 1979). The third fraction (F3), the oxidizable fraction, the metals are bound to organic matter and sulfur that would be released into the environment if conditions became oxidative, degrading the organic matter (ref?). Residual fraction (F4), is the residual fraction derived from the parent rock, which contains primary and secondary minerals binding metals into their crystal structure, like as silicate ref?).