Bioavailable trace metal fraction

The need to assess the speciation of trace metals and to estimate their bioavailability in the plant root environment is crucial in order to establish the extent of trace metal contamination in soils and to address their potential adverse effects. Although the bioavailability of metals is best predicted by their liquid phase speciation, the assessment of the trace metal fractionation in the solid phase is still essential because the replenishment rate of metals in the soil solution is controlled by solid phase metals (Minnich et al., 1987 Zhang et al., 2001 Krishnamurti et al, 2002). For instance, Zhang et al (2001) showed that Cu concentrations in plant parts correlated best with the effective Cu concentration, which was defined as the Cu concentration in the soil solution combined with a... 

As is the case with assessments of the toxicity of dissolved trace metals, the development of sediment quality criteria (SQC) must be based on the fraction of sediment-associated metal that is bioavailable. Bulk sediments consist of a variety of phases including sediment solids in the silt and clay size fractions, and sediment pore water. Swartz et al. (1985) demonstrated that the bioavailable fraction of cadmium in sediments is correlated with interstitial water cadmium concentrations. More recent work (e.g., Di Toro et al, 1990 Allen et al., 1993 Hansen et al, 1996 Ankley et ai, 1996, and references therein) has demonstrated that the interstitial water concentrations of a suite of trace metals is regulated by an extractable fraction of iron sulfides. 

Development of chemical speciation schemes which can be directly related to measures of bioavailability - This would allow the determination of which active trace element species merit the most intensive research from the standpoint of environmental perturbation. Some studies have attempted to correlate metal fractions determined by a particular technique (operationally defined speciation) with those that are bioavailable (functionally defined speciation) (Larsen and Svensmark, 1991 Buckley, 1994 Deaver and Rodgers, 1996). However, any correlation is only empirical and more research is required to achieve an understanding of the mechanisms involved in bioavailability and to develop rational predictive models. 

A simplified version of the scheme of Tessier et al. has been developed during the past years (40, 41). The scheme is made of three sequential extractions called A, B, C, which can leach respectively the exchangeable and carbonatic fraction (A), the reducible Fe-Mn hydroxides phase (B) and the organic matter together with the sulphides (C). The residual fraction is normally disregarded because total dissolution of the same samples is always performed, so that the residual fraction can be caleulated as the difference. Moreover, the residual fraction is the less important one from the viewpoint of bioavailability of trace metals. 

The ratio of residual and nonresidual associations in a sediment sample will allow early estimations of the bioavailability of trace metals. In Table II, we compare the metal concentrations in the total sediment samples and within the nonresidual phases, with the latter as average values from the grain size fractionated samples. It is shown that as the metal concentrations in the Rhine River sample are enriched because of pollution influences (except for manganese where diagenetic effects are probably involved), there is a distinct increase in the nonresidual metal fraction. This is mainly valid for copper, lead, and zinc, of which more than 90% in the Rhine River sample can be considered as being potentially remobilizable under natural conditions. Most readily available for biological processes are the metal cations in water-soluble and easily extractable forms copper is typically enriched in these chemical phases in both samples. 

It has been mentioned that the free ionic metal form is often the best predictor of bioavailability and toxicity to plants. However, some studies have shown that dissolved metal-DOC complexes can also effectively correlate with bioavailability to plants, as they can enhance metal concentrations in plant shoots and roots. The free metal ion would, therefore, not be the only chemical fraction readily adsorbed and translocated into plants. For instance, in a pot experiment, Antoniadis and Alloway (2002) found that DOC amendments in soils increased the amount of CaClj-extractable Cd, Ni and Zn, as well as enhancing the assimilation of these trace metals by ryegrass (L. perenne L.). Nigam et al. (2001) also found increased Cd assimilation by plants when combined with higher organic acid concentrations. These authors also mention that Cd + translocation in plants... 

Enzymatic extraction carried out using in vitro models of the gastrointestinal tract is cheaper, faster, more reproducible, and ethically easier than the alternative of studies involving people and animals. Estimation of nutrient bioavailability (including trace elements, both essential and toxic) is particularly important for nutritionists, pharmacists, and toxicologists. Application of sequential procedures allows analyte fractionation (metals usually), but enzyme selectivity allows leaching of certain speciation forms of the determined elements. Table 6.9 gives examples of application of enzymatic extraction procedures for trace element analysis and speciation analysis [71, 72]. 

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