Labile fraction

The partition index (IR, which will be discussed in details below) of Cd, Cu, Cr, Ni and Zn in both soils rapidly increased from time zero (calculated value) to one day and further to one year. This was especially true for Cr, Cu and to some extent Ni and Cd (Table 6.5). This result indicates that added trace metals are initially and rapidly transferred from the labile EXC fraction into the more stable fractions. Furthermore, IR of trace metals in native arid soils incubated under the saturated paste regime decreased at the end of year. This indicates mobilization of trace elements in these soils as saturation (Table 6.5). Also, it can be seen that IR decreased, for any given time, with an increase of the loading level (Table 6.5, Fig. 6.5). This means that higher additions of soluble metals result in higher metal content in the labile fractions and lower metal binding intensity in soils. 

Because of this complexation capacity, any standard addition performed at high pH will not return 100% of the spike, so a true value for the copper concentration cannot be calculated. Therefore, after an initial measurement at high pH the sample was acidified to pH 1.0 with 0.5 ml acid and another trace obtained. This compared the amount of copper released at low pH with the labile fraction at high pH. Standard additions were performed on the sample at low pH so almost all of the spike was returned. This allowed an estimate to be made of the percentage of total copper that was labile at high pH, and the quantification of this fraction in pg/1. This is illustrated graphically in Fig. 5.9. 

Aged contaminated samples (e.g., where contact times may have been months or years) can be enriched in the slow fraction (i.e., the fraction sorbed/ desorbed in the slow stage) owing to partial dissipation or degradation of more labile fractions before collection. The slow fraction of some pesticides was found to increase with contact time in the environment. 

Both the refractory and labile fractions of HMW DOM can be lost from seawater through formation of macrogels that aggregate into marine snow. The labile fraction that is known to participate in marine snow formation are the TEPs, such as mucopolysaccharides found in the mucus sheaths surrounding fecal pellets and plankton colonies. HMW DOM is also lost from seawater via (1) adsorption onto sinking POM and minerals, (2) conversion into POM at the sea surfece by turbulence associated with bursting bubbles, and (3) photochemical degradation. 

This model, in light of the discussion above, is clearly not representative of all of the kinetic processes which are occurring in sediment/water systems containing hydrophobic pesticides. However, it does include at least the more labile fraction of the sorbed pesticide in the overall kinetic model. Complications due to the inadequacy of this representation will be illustrated and discussed below. 

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]. 

We also carried out ELISA assays to determine the TGF-pj protein levels in FBC tissues. By measuring acid-stable and acid-labile fractions, we were able to measure the latent TGF-p i protein and the active TGF-p i protein. The total TGF-p i protein in day 7,21, and 48-55 FBC tissues was approximately 20,13, and 10 times as much as in control tissues, respectively. These increases were reflected approximately equally in both the active and the latent TGF-p i protein. These data indicate that TGF-Pi expression is elevated throughout FBC development and parallels closely the time course of the RNA transcripts. Specifically, we found that on day 7 both TGF-p mRNA and protein levels were found to be significantly higher than those at the other two time points (p < 0.05).16... 

Table 2.8 Determination of the labile fraction of total element content...

Anodic stripping voltammetry (ASV) is a very sensitive instrumental technique for the measurement of metals in solution. Of particular importance are determinations of reactive or ASV-labile metal concentrations. ASV-labile metal is defined as the fraction of the total metal concentration that is measured under a defined set of ASV and solution conditions. Labilities of metal species in natural water have been related to toxicities. Thus, one objective of speciation analysis carried out by ASV is to find conditions where the ASV-labile fraction is a close approximation to the toxic fraction of a metal. In experimental terms, the ASV-labile metal should be equal to the electroactive fraction of the metal. The latter... 

Therefore, steps 2 and 3 can be combined into a single step, and the overall kinetics can then be described in three compositional fractions the permanent (formed from insolubles), the resistant (in capillary pores and sorbed within aggregates), and the labile (in gravitational pores). The contribution of the permanent fraction (the insoluble portion) to the soluble portion in the leachate is proportional to its concentration. The release of the resistant fraction is represented by mass transfer, and the translocation of the labile fraction relates to convection flow and dispersion. 

Metal bioavailability is the fraction of the total metal occurring in the soil matrix, which can be taken up by an organism and can react with its metabolic system (Campbell, 1995). Metals can be plant-bioavailable, if they come in contact with plants (physical accessibility) and have a form which can be uptaken by plant roots (chemical accessibility). Soil metals become accessible for humans by ingestion, inhalation and dermal contact. Available forms of PTMs are not necessarily associated with one particular chemical species or a specific soil component. Main soil PTMs pools of different mobility, target organisms and routes of transfer are sketched in Fig. 9.2. The most labile fraction, corresponding to the soluble metal pool, occurs as either free ions or soluble complexed ions and is considered the... 

With careful selection of experimental conditions, the DPASV technique can be used to determine the inorganic forms of metals and weak organic complexes. Considering that the latter does not contribute substantially to the metal complexation because the concentration of ligands and the stability of complexes are low, the electroactive metal concentrations are constituted by ionic forms and inorganic complexes. In consequence of this, the ASV labile fraction can be equated to the conditional concentration [M ]. 

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