Fjölrit RALA - 15.06.2004, Síða 61

BCR sequential extraction of trace elements in COST-622 soils
M. Espino-Mesa., J.l. Rodriguez, and J.M. Hernandez-Moreno
Dept. Edafología-Geología, Universidad de La Laguna (ULL)
In soils and other natural systems, the mobility, transport, and partitioning of trace elements
are dependent on their chemical form. Chemical extraction is employed to assess
operationally defined metal fractions, which can be related to chemical species, as well to
potentially mobile, bioavailable, or ecotoxic phases. Sequential extraction has been applied
using extractants with progressively increasing extraction capacity. The selectivity of many
extractants is weak or not sufficiently understood, and it is questionable as to whether specific
trace metal compounds actually exist and can be selectively removed from multicomponent
systems. For purposes of comparability and quality control, the Community Bureau of
Reference (BCR, now Standards, Measurements and Testing Program) has launched a
program to harmonize sequential extractions schemes for the determination of extractable
trace metals in sediments. BCR has proposed a standardized 3-step extraction procedure
(BCR EUR 14763 EN). This procedure is currently used and evaluated also as an extraction
method for soils (1,2).
In this work, the BCR sequential extraction scheme was studied in some COST-622 profiles
from Italy (IT), Portugal (AZ), Iceland (IS) and Spain (TFE). A modified BCR sequential
extraction (1) was applied: extraction-1: 0.11 mol L"1 Acetic Acid, extraction-2: 0.5 mol L"1
Hydroxylammmonium Chloride, extraction-3: 8.8 mol L 1 Hydrogen peroxide digestion and
extraction with 1 mol L"1 Ammonium Acetate. Residue from the third step, extraction-4:
Aqua Regia (AR). The sum of extractions 1, 2, and 3 are considered the potentially
mobilizable (PM) pool of an element. An independent extraction with AR was performed to
determine the “pseudototal” content. A certified reference material (BCR) was used for
quality control purposes.
Total-AR metal contents were compared with total values (FRX) from the COST-622 soil
database. Strong correlations were observed for Cu, Ni and Cr, total-AR representing about
70% of the total content for Cu and Ni and only about 10% for Cr. Total metal contents
generally agreed with soil lithology. Values greater than “Maximum Allowable
Concentrations” were observed in IT samples. High values of Ni and Cr were also observed in
some AZ and TFE samples.
The metal distribution (mg Kg"1) in the four extractions are shown in fig 1 for Zn and Cu.
Only for these elements a significant amount was obtained in extractions 1 and 2. In the case
of Cu the PM pool was more important in relation to the residual form than for Zn. In IT
samples, PM forms of Cu were predominant; this, together with the high total Cu values and
soil lithology points to anthropogenic contamination. Chromium was only detected from
extraction 3; Ni and Pb were mostly residual.
The relative metal distribution also varied with horizon type for each element. Only in the
case of Mn (Ext-1), a monotonic decrease with depth was observed.
An important dissolution of Al, Fe, and Mn occurred already from the first extraction (e.g.,
A1 in extraction-1 ranged from 3000 mg Kg"1 in AZ to 180 mg Kg'1 in IT). The amount of A1
in the first three extractions exceeded Al0 values. This is a peculiar behaviour of the Andic
soils which has to be considered when interpreting sequential extractions; for example, less
bioavailability of the “exchangeable” forms can be expected in relation to other soil types.
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