Trace element extractants

The determination of total amounts in soil is valid for finding whether there are toxic levels of certain metals (e.g. after repeated slurry applications), and comparisons can be made with published tables of maximum recommended levels. Some typical and maximum values are shown in Table 4.5 (ADAS, 1987; DOE/NWC, 1981). Dutch values differ from those developed in the UK in that the intention is to allow the return of contaminated land to any potential use, rather than tailoring the level of remediation to the intended use of the land. The most recent values include general targets and intervention values (http://www.athene.freeserve.co.uk/sanaterre/guidelines/dutch.htm).

The soil sample is ground to pass a 0.5-mm sieve, and 2.5 g taken for the analysis. There are two possible extractants. Firstly 1:4 HClO4 (60% by weight perchloric acid):HNO3 (70% by weight nitric acid), of which 25 ml is added to the sample. It is allowed to stand overnight, then heated at 100°C, next 180-200°C and finally at 240°C. The residue is dissolved in 6 N HCl, boiled, cooled, made to 50 ml and filtered before analysis by atomic absorption spectrophotometry. This can be a dangerous procedure with a risk of explosion, and the full details should be carefully followed as given in the original reference (MAFF/ADAS, 1986, p. 31). An alternative acid mixture,

Table 4.5. Typical and maximum recommended levels for some trace elements in soil.

Metal

Typical value in

Maximum

Earth/sediment

uncontaminated soil

recommended level

(mg kg-

dry matter)

(mg kg-1)

(kg ha-1)a

(mg kg-1)

(kg ha-1)a

Target

Intervention

value

value

Zinc

80

160

300

600

140

720

Copper

20

40

135

270

36

190

Nickel

25

50

75

150

35

210

Cadmium

0.5

1

3

6

0.8

12

Lead

50

100

250

500

85

530

aAssumes 2000 t ha-1 to depth of 15 cm.

aAssumes 2000 t ha-1 to depth of 15 cm.

aqua regia, is now suggested, and although a highly corrosive reagent, there should be no risk of an explosion.

The availability of the trace metals is easily determined without any of the above risks, and the results used to assess both deficiencies and toxicities. The metals need to be removed from the sites where they are bound to the soil particles by use of an even stronger binding agent than the soil. This is achieved with two possible complexing reagents: EDTA and DTPA. They are a class of chemicals known as complexones, which form complex molecules with metals in a cage-like structure called a chelate.

Fig. 4.4. A four co-ordinated Zn-EDTA complex ion.

Fig. 4.4. A four co-ordinated Zn-EDTA complex ion.

EDTA is ethylenediaminetetraacetic acid, also called (ethylenedinitrilo) tetraacetic acid, mol. wt 292.24. Although having four carboxylic groups, it behaves as a dicarboxylic acid with two strongly acidic groups. It is used as the disodium or ammonium salt, the latter being formed in situ. One mole of the EDTA salt reacts in all cases with one mole of the metal irrespective of its valency state. A four co-ordinated zinc EDTA complex is shown in Fig. 4.4, and a six co-ordinated cobalt EDTA complex in Fig. 4.5. EDTA is known as hexa- (or sexa-) dentate, having up to six active metal-complexing sites per molecule.

DTPA is diethylenetriaminepentaacetic acid, also known as diethylen-etrinitrilopentaacetic acid, mol. wt 393.36. It is octo-dentate, having eight active metal-complexing sites per molecule. A diagrammatic representation of the DTPA molecule is shown in Fig. 4.6.

The amount of metal extracted from the soil by both EDTA and DTPA is dependent on the pH, the metal being extracted, the soil:solution ratio, the concentration of chelating agent, the shaking time, the temperature, and the sample preparation procedure. Clearly, the methodology used should be clearly described and closely followed if repeatable work is to be possible, and comparison of results is to be meaningful.

Fig. 4.5. A six co-ordinated Co-EDTA complex ion.

co ch2

Fig. 4.5. A six co-ordinated Co-EDTA complex ion.

hooc-ch

ch2-cooh

0 0

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