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Figure 5.15 Procedure used for the extraction of hexavalent chromium : ICP-MS, inductively coupled plasma-mass spectrometry.
Methods for Environmental Trace Analysis
Figure 5.16 A colorimetric method, based on diphenylcarbazide, used for the analysis of hexavalent chromium.
5.7 Selective Extraction Methods
5.7.1 Plant Uptake Studies
Extraction procedures for plant uptake studies have existed for many years. These species are defined by their function, e.g. plant-available forms. The species defined in this way, however, are unlikely to be distinct chemical forms but may include a range of chemical entities that share the same function, e.g. are all available to plants. This type of functionality, using selective chemical extractants, has been widely employed in soil and agricultural laboratories to diagnose or predict toxicity deficiencies in crops and in animals eating such crops. Table 5.7 illustrates the diversity of this type of selective extraction. It is noted that the methods, evolved over many years on an empirical basis, are both element-specific and crop-specific.
5.7.2 Soil Pollution Studies
In the case of metal-polluted soils, a different approach for extraction has been identified. Extractants have been studied that allow isolation of a particular metal-containing soil phase, e.g. exchangeable materials. The information can then be used to identify the potential likelihood of metal release, transformation, mobility or availability as the soils are exposed to weathering, to pH changes, and to changes in land use, with the implications for environmental risk assessment that may result. As a result of the non-specific nature of most of these extraction methods, it is necessary to define limits in which the extractant will operate. Table 5.8 identifies a variety of reagents or methods of isolation for the removal
Table 5.7 Selective-extraction methods which are diagnostic of plant uptake . Reprinted from Sci. Total Environ., 178, Ure, A. M., ‘Single extraction schemes for soil analysis and related applications’, 3-10, Copyright (1996), with permission from Elsevier Science
Extractant Element Correlated plant content
Water Cd, Cu, Zn Wheat, lettuce
EDTA“ (0.05 moll-1) Cd, Cu, Ni, Pb, Zn Arable crops
EDTA“ (0.05 moll-1) Se, Mo Greenhouse crops
DTPA6 Cd, Cu, Fe, Mn, Ni, Beans, lettuce, maize, sorghum,
Acetic acid (2.5 vol%) Cd, Co, Cr, Ni, Pb, Zn Arable crops, herbage
Ammonium acetate, pH 7 Mo, Ni, Pb, Zn Herbage, oats, rice, sorghum,
(1 moll-1) Swiss chard
Ammonium acetate:EDTA“ Cu, Fe, Mn, Zn Wheat
(0.5 moll-1:0.02 moll-1)
CaCl2 (0.05 moll-1) Cd, Pb Vegetable(s)
NaNO3 (0.1 moll-1) Cd, Pb Vegetable(s)
Ammonium nitrate Cd, Pb Vegetable(s)
a EDTA, ethylenediaminetetraacetic acid (diammonium salt).
bDTPA, 0.005 moll-1 diethylenetriaminepentaacetic acid + 0.1 moll-1triethanolamine + 0.01 moll-1 CaCl2.
Table 5.8 Various extraction procedures used to isolate nominal soil/sediment phases . Reprinted from Sci. Total Environ., 178, Ure, A. M., ‘Single extraction schemes for soil analysis and related applications’, 3-10, Copyright (1996), with permission from Elsevier Science
Phase isolated or extracted Reagent or method of isolation
Water-soluble, soil solution, Water, centrifugation,
sediment-pore water displacement, filtration, dialysis
Organically bound Na4P2O7
H2O2 (at pH 3)/NaOAC
Carbonate NaOAc at pH 5 (HOAc)
Mn oxide-bound NH2OH.HCl
Fe (amorphous) oxide (NH4)2C2O4 in dark
Fe (crystalline) oxide (NH4)2C2O4 in UV light
of metals from defined soil phases. It can be seen that a range of different phases and approaches are possible and these are now briefly discussed in the following.
• Water-soluble, soil solution, sediment-pore water. This phase contains the most mobile and hence potentially available metal species.
Methods for Environmental Trace Analysis
• Exchangeable species. This phase contains weakly bound (electrostatically) metal species that can be released by ion-exchange with cations such as Ca2+, Mg2+ or NH4+. Ammonium acetate is the preferred extractant as the complex-ing power of acetate prevents re-adsorption or precipitation of released metal ions. In addition, acetic acid dissolves the exchangeable species, as well as more tightly bound exchangeable forms.
• Organically bound. This phase contains metals bound to the humic material of soils. Sodium hypochlorite is used to oxidize the soil organic matter and release the bound metals. An alternative approach is to oxidize the organic matter with 30% hydrogen peroxide (acidified to pH 3), followed by extraction with ammonium acetate to prevent metal ion re-adsorption or precipitation.
• Carbonate bound. This phase contains metals that are dissolved by sodium acetate acidified to pH 5 with acetic acid.
• Oxides of manganese and iron. Acidified hydroxylamine hydrochloride releases metals from the manganese oxide phase with minimal attack on the iron oxide phases. Amorphous and crystalline forms of iron oxides can be discriminated between by extracting with acid ammonium oxalate in the dark and under Uv light, respectively.