Sequential leaching procedures

The samples used in this work were selected from 147 soil samples of a former investigation made by Hess (1992). His work was focused on the determination of Hg concentrations in the soils of the mining area. The samples were taken in a 127 km sampling grid around Idrija. Additionally, Hg phases were investigated by using sequential leaching procedures. 

The results of Hess (1992), who applied a modificated sequential leaching procedure after Dttes (1987) proved that for all samples more than 90% of the Hg is bound to organic compounds or occurs in the "residual fraction (Fig. 4.6). 

Fig. 4.6. Hg phases determined by a sequential leaching procedure (Hess 1992)...

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. 

By applying methods of sequential leaching, direct chemical information can be obtained and a rough assessment of the remobilization potential of the sedimented heavy metals is possible. Because of the lack of specifity of the extraction procedures particular species cannot be identified (see also critical hints in [KHEBOIAN and BAUER,... 

In the case of solid environmental materials such as soils, sediments or indeed suspended particulates the use of selective chemical leaching methods can provide some basic information regarding the nature of the radionuclide species bound to the solid matrix. A typical sequential leaching might follow a procedure similar to that suggested for the study of trace metal binding to sediments by Gupta and Chen (1975) as shown in Fig. 13.2. 

Figure 13.2 A sequential chemical leaching procedure which may be used to investigate the binding of radionuclide species to soil or sediment matrices.

Sutherland R. A. (2002) Comparison between non-residual A1 Co, Cu, Fe, Mn, Ni, Pb and Zn released by a three-step sequential extraction procedure and a dilute hydrochloric acid leach for soil and road deposited sediment. Appl. Geochem. 17, 353 —365. 

In order to evaluate the maximum amount of metal that may be leached by urban sludge and the extracting action of different solutions on the same metal fraction, a future program based on a sequential extraction procedure on the same sludge sample must be carried out. 

In Chapter 3.4 a method has been presented for long-term prognosis of metal pollutant mobility, which combines column circulation leaching experiments at variable pH/E -conditions with sequential extraction procedures on the solid waste material before and after these experiments (Schoer FSrstner, 1987). Temporal release patterns are different for the individual elements (Figure 6-5) While at pH 5/400 mV release of cadmium seems to be completed within the experimental period mobilization of copper is still going on and the end point cannot be estimated from the data of the "kinetic" experiments. The same effect has been found for the examples of thallium and vanadium. For the other elements, the endpoint of release can be determined as approximately 10 mg cobalt, 0.6 mg cadmium, 600 mg zinc and 0.3 mg chromium, 2 mg barium and 20 mg lead (per 100 g of solid substrate treated with 140 L solu-... 

Sequential extraction procedures attack the sample by the use of consecutive leaching agents of increasing strength and ability of interaction with the sample matrix. Typical steps according to Ure et al. (1993) yield the following analyte fraction types ... 

The kinetics of dissolution of pure CaC03 and soil CaC03, as indicated by the volume of C02 released and Ca dissolved during extraction, are presented in Fig. 4.3. It shows that dissolution of both pure and soil CaC03 by the NaOAc-HOAc solutions at various pHs reached a plateau after two hours. This indicates that a certain acid dose reacts completely with the proper content of soil carbonate within two hours. Tessier et al. (1979) reported that after five hours of leaching sediments, there was no increase in the calcium concentration, thus indicating that it is unnecessary to allow 16 hours for extraction of the CARB fraction, as was originally done in this sequentially selective dissolution procedure. 

The Dutch Total Availability Leaching test (NEN 7341) was used to operationally quantify the elemental mass fraction available for leaching in the samples. The procedure involves two sequential extractions the first were conducted at a pH of 7.0 and LS of 100. For examination of mass fractions available for leaching, 8 g of sample was added to 800 mL of distilled, deionized water and stirred in a capped Teflon vessel. 

There are many review articles concerning application of USE in food technology [57] and for isolation of bioactive substances from herbs and other plant materials [58], as well as leaching of heavy metals from environmental and industrial samples [59]. Application of ultrasound during sequential extraction of trace elements significantly shortens the whole procedure however, for satisfactory efficiency it is necessary to increase the temperature and modify the matrix. 

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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