The bioavailable fraction

For various purposes, the dissolved concentration (or even the free ion activity) in the pore water can be used to explain effects. In those cases available fraction and pore water concentration are equivalent. This is, however, only one of the definitions of bioavailability. Based on the pore water concentration (actual availability) it is possible to explain targets such as the 

There are already several methods to measure the availability of contaminants. Measured values of various indicators of the chemical availability results in a close correlation with effects. In this chapter, we do not want to give an overview of all these methods. In a more general way Alexander etal. (2003) distinguished between the following chemical measures of bioavailability  

Organic contaminants are almost completely adsorbed to the organic matter fraction. As long as the adsorption is reversible the partition between water and soil can be described using the Kot value. For several contaminants this function is known or can be derived from Kmv or the water solubility. This reversible partition, however, only occurs with freshly added contaminant and is true for freshly added pesticides. In this (rather special) case, effects can be explained from the total concentration. With the more soluble pesticides (not including the old DDT-like types) it is also possible to use the pore water concentration. After ageing occurs, which results in a significantly reduced availability, it becomes impossible to explain the effects from the total concentration. For relatively soluble contaminants, the pore water concentration can still be used to explain internal concentrations and (possibly) effects. 

For less soluble contaminants like PAHs and PCBs, it is not possible to measure pore water concentrations. Chemical availability has to be measured with a method that extracts a certain part from soil. Methods are available, such as a mild extraction for a short period of time. Relation with effects have been empirically established (certain strength and certain time). Methods are easy and cheap, but valid only after establishing the empirical relationship. Other methods are based on a strong adsorbent and stimulate the diffusion from the solid phase into the water phase and on to the adsorbent. This process is easier to understand (based on equilibrium between soil and water), but is also based on empirical relationships (Reid etal. 2000). 

Although methods are available, they are still in an experimental phase. The bioavailable fraction is currently not part of regulations. It is however to be expected that there will be an increasing interest in bioavailable fraction in the coming years. For analytical laboratories this means that special empirical extraction procedures will be introduced that may be combined with existing measurements. 

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As of the 1990s, the EPA recommends that the dissolved form of silver be used as a better estimate of the bioavailable fraction and recommends using 85% of the total recoverable quantity. Thus, in fresh water at hardnesses of 50, 100, and 200 mg/L CaCO, the concentration of dissolved silver should not exceed 1.0, 3.5, and 11 Fg/L, respectively. The concentration of dissolved silver in salt water should not exceed 1.9 Fg/L (46). 

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. 

GIT, is considered to be lost from the absorption site, as is metabolic clearance and sequestration in various cell types and membranes (72,14). It is clear from Scheme I that the relative rates of the various processes will define the bioavailable fraction of the dose and understanding those factors which control pulmonary absorption kinetics is obviously the key to enhancing bioavailability via the lung. In a recent book (75) the molecular dependence of lung binding and metabolism was considered alongside the parallel processes of absorption, clearance and dissolution in the lung (14). Some key features of this work will be repeated as it relates to the systemic delivery of polypeptides. 

Potential obstacles for the large-scale application of phytoremediation technologies, however, include the time required for remediation, the pollutant levels tolerated by the plants used, the disposal of the contaminated plants, and the fact that only the bioavailable fraction of the contaminants will be treated. This means that phytoremediation does not achieve 100% removal or reduction of the contaminants From the ecological, toxicological, and medical (health) points of view, the... 

Uptake occurs from the bioavailable fraction, which in almost all cases corresponds to the dissolved fraction. Sorption and binding to suspended solids, sediments, and DOM have a great effect on bioavailability [71,72] therefore the more hydrophobic surfactants tend to be less bioavailable. Thus, for the same initial concentration, the bioavailable fraction of C12LAS, compared with that of the Cn... 

Accordingly, sorption has received a tremendous amount of attention and any method or modeling technique which can reliably predict the sorption of a solute will be of great importance to scientists, environmental engineers, and decision makers (references herein and in Chaps. 2 and 3). The present chapter is an attempt to introduce an advanced modeling approach which combines the physical and chemical properties of pollutants, quantitative structure-activity, and structure-property relationships (i. e., QSARs and QSPRs, respectively), and the multicomponent joint toxic effect in order to predict the sorption/desorp-tion coefficients, and to determine the bioavailable fraction and the action of various organic pollutants at the aqueous-solid phase interface. 

When kdesorb is very slow (or even zero as when the compound is encapsulated in an authigenic mineral), [z]sorbed Ald[z]w so we can neglect the second term in the gradient driving transfer. In this case, we refer to the compound as experiencing sequestration. The parameter, (1 -fw), quantifies the extent of a compound s sequestration in a particular case of interest when we are justified to assume that the dissolved fraction is equal to the bioavailable fraction. Quantitative evaluation of desorb is taken up in Section 19.5. 

Owing to the low levels of metals in the bioavailable fraction, there was no correlation between their content and the observed toxic effect... 

The bioavailable fraction (responsible for the inhibition of bacterial bioluminescence) constituted only 3-8% of the total content of elemental sulfur in the sample... 

Gobas et al. [30] in 1989 investigated the bioconcentration potential of polychlorinated biphenyls (PCBs), polybrominated benzenes (PBBzs) and poly-brominated biphenyls (PBBs), and other super-hydrophobic chemicals, such as decachlorobiphenyl and Mirex. These authors also pointed out the importance of bioavailability for bio concentration of super-hydrophobic chemicals. Their study showed that the bioavailable fraction of the super-hydrophobic chemical decachlorobiphenyl can be as low as 3 % and of Mirex can be as low as 2.2 %. For decachlorobiphenyl, a BCF was found that was one to two orders of magnitude lower than the true BCF. 

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