Metal fractions, bioavailability

Zhao K, Liu X, Zhang W, Xu J, Wang F (2011) Spatial dependence and bioavailability of metal fractions in paddy fields on metal concentrations in rice grain at a regional scale. J Soils Sediments 11(7) 1165—1177. doi 10.1007/s 11368-011-0408-6... 

Table 13.2 shows the concentrations of Cu2+ in tap water and lake water determined by the MPA-Gly-Gly-His modified electrode and compared with those of inductively coupled plasma mass spectrometry (ICP-MS). The measured concentration of Cu2+ in tap water when dilutions were accounted for using the MPA-Gly-Gly-His modified electrode was 0.55 +0.08 pM (95% confidence interval). This value is much lower than the copper concentration measured by ICP-MS of 1.70+0.15 pM (95% confidence interval). The peptide-modified electrode measures free Cu2+ or weakly bound copper complexes rather than the total metal-ion concentration measured by ICP-MS. Hence the use of peptides has the ability to measure metal-ion bioavailability for specific species. The results are consistent with others who have also shown that the labile fraction of copper is much less than the total copper [7]. 

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. 

Perez-Cid et al. (1996) compared Tessier and BCR sequential extractions for the speciation of Cu, Cr, Pb, Ni and Zn in sludge from an urban wastewater treatment plant and found similar performance for the two procedures. Zufiaurre et al. (1998) later demonstrated, using the Tessier method and PCA, that, since most metals in a sludge were bound to oxidisable and residual fractions, bioavailability was likely to be very low. 

Plant roots stabilize bioavailable soil metal fractions through several mechanisms. Precipitation at the rhizosphere of the solubilized fraction of heavy metals in the soil is the primary mechanism of most metal adsorption to root surfaces (Blaylock et al., 1997). Adsorption also occurs through the binding of free metal cations by pectins on root cell walls and in pectin and other polysaccharide combinations from root-secreted mucilage (Waisel et al., 1996). 

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. 

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... 

To understand the changes in metal fractionation within the rhizosphere, we need more insight into the key factors influencing metal fractionation. It has been assumed that the factors affecting metal fractionation and bioavailability in soil include root-induced pH changes, metal binding by root exudates, root-induced... 

Accurate assessment of human health risks associated with oral exposure to metals requires knowledge of the fraction of the dose absorbed into the blood. This information is important for As-contaminated environmental media, such as soil and mine waste, because metal contaminants exist in a variety of soluble and insoluble forms and may be contained within particles of inert matrix, such as rock or slag. Physicochemical properties such as these influence the enteric absorption fraction (bioavailability) of ingested metals. Therefore, site-speciflc data on metal bioavailability in the environmental media of concern will increase the accuracy and decrease the uncertainty in human health risk estimates. 

The behavior of elements (toxicity, bioavailability, and distribution) in the environment depends strongly on their chemical forms and type of binding and cannot be reliably predicted on the basis of the total concentration. In order to assess the mobility and reactivity of heavy metal (HM) species in solid samples (soils and sediments), batch sequential extraction procedures are used. HM are fractionated into operationally defined forms under the action of selective leaching reagents. 

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. 

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