Soil extraction sequences

TABLE 6.3. BCR Extraction Sequence for 1 g of Soil. Extraction is at Room Temperature Unless Otherwise Noted11... 

Step 1 simulates the readily available soil fraction, steps 2-4 indicate potentially available soil fractions, and step 5 yields the unextracted residue and completes the mass balance. Note that the solvent used becomes inaeasingly nonpolar during the extraction sequence. Summary data for the she studied compounds are presented in Fig. 8.48. 

Table 2.2. Extraction sequences for the subdivision of the total metal content of soils...

The equipment and issues pertaining to this analysis are similar to those for soil chloride (see Method 5.5.1) using ion chromatography. The method is specific to sulfate, whereas ICPAES measures total S in the aqueous extracts. Accordingly, results will usually be lower for this analytical finish compared with Method 5.7.3. The only post-extraction preparation necessary, other than dilution, is the removal by filtration of particulate matter >0.20 pm. Since the concentration of SO -S can be influenced by biological transformations, aqueous soil extracts should be kept at 4°C if there is any delay (e.g. overnight) between completion of extraction and analysis. Under such conditions filtered aqueous samples should remain stable for at least 28 days (O Dell etal. 1984). The analysis can be undertaken in sequence with other anions (Dick Tabatabai 1979 O Dell etal. 1984). 

In this work a novel five-step leaching scheme for HM has been developed addressing exchangeable, acid soluble (carbonate), easily reducible (bound to Mn oxides), easily oxidizable (bound to humic and fulvic acids), and moderately reducible (bound to amorphous Ee oxides) fractions extractable by 0.05 M Ca(N03), 0.43 M CH3COOH, 0.1 M NH,OH-HCl (non-acidified), 0.1 M K/,03 (pH 11), and 0.1 M (NH4),C,04 (pH 3), respectively. The sequence of extractants was chosen according to recent studies on the selectivity of leachants toward dissolved phases of soils. 

Miller W.P., Martens D.C., Zelazny L.W. Effect of sequence in extraction of trace metals from soils. Soil Sci Soc Am J 1986 50 598-601. 

The Hot Air Vapor Extraction System (HAVE) is an ex situ commercial technology that uses a sequence of thermal, heap pile, and vapor extraction techniques to remove and destroy hydrocarbon contamination in soil. This technology is effective in cleaning soils contaminated with gasoline, diesel, heavy oil, and polycyclic aromatic hydrocarbons. 

The robustness of a sample preparation technique is characterized by the reliability of the instrumentation used and the variability (precision) of the information obtained in the subsequent sample analysis. Thus, variations in controlled parameters and sequences are to be avoided. In sample preparation methods employing supercritical fluids as the extracting solvents, it has been our experience that minimal variations in efficient analyte recoveries are possible using a fully automated extraction system. The extraction solvent operating parameters under automated control are temperature, pressure (thus density), composition and flow rate through the sample. The precision of the technique will be discussed by presenting replicability, repeatability, and reproducibility data for the extraction of various analytes from such matrices as sands and soils, river sediment, and plant and animal tissue. Censored data will be presented as an indicator of instrumental reliability. 

Figure 14.10. Principal component analysis of Py-FI mass spectra of (a) cold and (b) hot water extracts from the sequence of organic litter layers Oi-Oe-Oa in a beech stand (Fagus sylvat-ica) obtained before (-pre) and after (-post) aerobic incubation. The arrows indicate changes due to progressive decomposition top-down in the litter profile. Reprinted from Landgraf, D., Leinweber, P, and Makeschin, F. (2006). Cold and hot water extractable organic matter as indicators of litter decomposition in forest soils. Journal of Plant Nutrition and Soil Science 169,76-82, with permission of Wiley-VCH.

Various workers have questioned the ability of sequential extraction to provide accurate information on the mineralogical phases with which trace elements are associated in soil or sediments (e.g. Nirel and Morel, 1990). Problems, including non-selectivity of reagents and readsorption of analytes following release, are frequently reported. Hence, nowadays, most environmental analytical chemists accept that sequential extraction should be considered an operational form of speciation, in which the fractions isolated are defined purely by the sequence of reagents used, and not as a means to determine information on the specific mineralogical phases to which trace elements are bound. Modern sequential extraction procedures label the fractions obtained in terms of the type of chemical reaction used to isolate them, in order to emphasise this, e.g. reducible or oxidisable species. Unfortunately, this distinction is not always made clear in the wider environmental literature. 

An example of an extraction and reaction sequence of a soil matrix sample analysis is presented in Figure 1.3. 

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